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HIGH LOADED ANAEROBIC MESOPHILIC DIGESTION OF SEWAGE SLUDGE
An evaluation of the critical organic loading rate and hydraulic retention time for the anaerobic digestion process at Käppala Wastewater Treatment Plant (WWTP).
IBRAHIMA SORY GÄRDEKLINT SYLLA
School of Business, Society and Engineering Course: Degree Project in Energy Engineering Course code: ERA403 Credits: 30 hp Program: Master of Science in Engineering in Energy Systems
Supervisor at Mälardalens University: Monica Odlare Supervisor at Käppalaförbundet: Jesper Olsson and Sofia Bramstedt, Examiner: Eva Thorin Costumer: Käppalaförbundet Date: 2020-08-21 Email:
ABSTRACT
Käppala wastewater treatment plant (WWTP) has, during a few years, observed an increase
in organic loading rate (OLR) in the mesophilic anaerobic digester R100, due to an increased
load to the WWTP. The digestion of primary sludge at Käppala WWTP is today high loaded,
with a high organic loading rate (OLR) and low hydraulic retention time (HRT). This study
aims to evaluate the effect of the maximum OLR and the minimum HRT for the anaerobic
digestion of sewage sludge and to investigate further actions that can be taken into
consideration in case of process problems in the digestion. The study consists of (a) a
practical laboratory experiment of 6 pilot-scale reactors to investigate how the process
stability is affected when the OLR increases and the HRT decreases. (b) A mass balance
calculation based on the energy potential in the feeding sludge and the digested sludge. (c) A
study of the filterability of the digested sludge. (d) The construction of a forecasting model in
Excel, to predict when digester R100 will reach its maximum OLR and minimum HRT. The
result of the study shows that the maximum OLR for Käppala conditions is 4.9 g VS dm-3 d-1,
meaning that R100 will reach its maximum organic load around the year 2031. An OLR of
4.5-4.9 and an HRT of 12 days is optimal for R100, according to the present study. Keeping
the anaerobic digestion process in balance is vital when it comes to the outcome of energy in
the anaerobic digestion process. Pushing the process to produce more gas can become
counterproductive since a high OLR can lead to process imbalance, which in turn leads to low
biogas production. Imbalance in the digestion process can occur fast; therefore, the margin
for overload in the anaerobic digestion process must be significant. The methane
concentration in the converted biogas and the pH level in the reactor are the best stability
parameters for the conditions at Käppala. Ammonia is the less efficient stability parameter
since it did not predict or detect any instability during the experimental process.
Furthermore, the OLR and HRT have a significant impact on the needed quantity for
dewatering polymer. The higher digestion of organic material in the sludge, the bigger the
need for the polymer to take care of the rest material.
Keywords: Dewaterability, Mesophilic digestion, Methane, Organic load rate, Renewable
energy source, Sludge filterability, Wastewater treatment.
PREFACE
This report is a master thesis, part of the master’s program Sustainable Energy Systems at
Mälardalen University, Västerås, Sweden, written by Ibrahima Sory Gärdeklint Sylla during
the spring semester of 2020. The study focusses on investigating the effect of high organic
loading rate and short hydraulic retention time on mesophilic anaerobic digestion of sewage
sludge at Käppala Wastewater Treatment Plant. The maximum organic loading rate and the
minimum hydraulic retention time for a stable digestion process are also evaluated.
The study was conducted at Käppala WWTP in Stockholm, and I would like to thank my
supervisors, Sofia Bramstedt and Jesper Olsson, development engineers at
Käppalaförbundet, for their guidance and support during this period. A great thanks to the
supervisor for this thesis, Monica Odlare, professor at the Department of Environmental
Engineering and Energy Processes at Mälardalen University, and the examiner, Eva Thorin,
professor at the Division of Environmental Engineering and Energy Processes at Mälardalen
University, for guidance and support.
Käppalaverket, Stockholm, Västerås, June 4th 2020
Author: Ibrahima Sory Gärdeklint Sylla
SAMMANFATTNING
Avloppsslam bildas i reningsprocessen vid avloppsreningsverk, då vattnet renas. Slammet
behandlas traditionellt som avfall, men eftersom det innehåller förnybara råvaror som kol,
som kan omvandlas till biogas, och fosfor och kväve som kan användas som gödningsmedel
för odling, betraktas slammet i dag som en förnybar resurs. För att vara användbart som
resurs måste dock slammet stabiliseras för att minska biologisk aktivitet och dålig lukt.
Anaerob rötning (AD) är en vanlig behandlingsmetod för att stabilisera slammet, genom att
omvandla det organiska materialet till brännbar biogas som innehåller 60–70% metan.
Eftersom biogas är en förnybar energikälla är utvecklingen av biogasproduktionssystem ett
viktigt bidrag till den globala omvandlingen från fossila bränslen till förnybar energi.
Käppalaverket i Stockholm tar hand om kommunalt avloppsvatten från 550 000 invånare i
norra Stockholm, genom en mekanisk, biologisk och kemisk reningsprocess. Käppalaverket
har under de senaste åren sett en ökning av den organiska belastningen i rötkamrarna,
troligen på grund av det ökade inflödet av avloppsvatten. Rötningen vid Käppalaverket har
idag hög organisk belastning (OLR) och korta uppehållstider (HRT). I framtiden riskerar
processen att bli överbelastad om inga förändringar görs, detta på grund av
befolkningstillväxten i regionen.
Syftet med examensarbetet är därför att utvärdera maximal belastning och kortast möjliga
uppehållstid för en stabil anaerob rötprocess vid Käppalaverket, samt vad som ytterligare kan
göras vid processproblem i rötningen. Fokus i arbetet är att eftersträva en stabil rötprocess,
något som i sin tur är avgörande för effektiv energikonvertering. Studien består av fyra delar:
• Ett praktiskt laboratorieexperiment, med syftet att i sex modeller av rötkammaren R100 vid
Käppalaverket, undersöka hur processtabiliteten påverkas när den organiska belastningen
(OLR) ökar och uppehållstiden (HRT) i rötkammaren minskar.
• En massbalansberäkning baserad på den specifika metanproduktionen och på
energipotentialen i matningsslammet och i det rötade slammet.
• En studie av filtrerbarheten i det rötade slammet, för att undersöka hur slamavvattningen
påverkas av förändringar i OLR och HRT i de olika rötkamrarna.
• Byggandet av en prognosmodell i Excel, för att förutsäga när rötkammaren R100 vid
Käppalaverket når maximal organisk belastning (OLR).
Resultatet av denna studie visar att maximal OLR för Käppalaverket är 4,9 g VS dm-3 d-1. Det
innebär att reaktor R100 vid Käppalaverket når maximal organisk belastning omkring år
2031. Då måste en tredje reaktor finnas på plats för att avlasta R100. Det är därför av yttersta
vikt att belastningen i Käppalaverkets reaktorer inte går över detta värde.
Att hålla den anaeroba rötprocessen i balans är avgörande för en effektiv energiomvandling
vid anaerob rötning. Obalans i systemet leder omedelbart till nedgång i produktionen av
biogas. Att driva på processen för att producera mer gas kan bli kontraproduktivt, eftersom
hög OLR kan leda till obalans, vilket i sin tur leder till låg biogasproduktion och i
förlängningen risk för processkollaps. Men resultatet visar också hur komplicerat det är att
hålla OLR på en konstant nivå i den anaeroba rötprocessen, på grund av variationer i
torrsubstansen (TS) och det organiska innehållet, glödförlust (VS) i slammet. En annan viktig
observation är att när det finns en obalans i rötprocessen, går det mycket snabbare än väntat
att nå en processkollaps. Detta betyder att marginalen för överbelastning i den anaeroba
rötprocessen måste vara betydande för att undvika stopp i processen. Det visar också på
vikten av regelbundna mätningar av stabilitets-parametrarna vid en högbelastad process.
Baserat på resultatet för reaktor R3 under denna studie är en OLR på 4,5–4,9 och en HRT på
12 dagar optimala värden för R100 vid Käppalaverket.
Resultatet visar även att metankoncentration i den konverterade biogasen och pH-nivån i
reaktorn är de mest effektiva stabilitetsparametrarna för den anaeroba processen vid
Käppalaverket. VFA och förhållandet VFA/TA visade också på instabilitet i processen, men i
en mycket långsammare takt. Ammoniakvärdet visade inte alls på någon instabilitet under
experimentprocessen, vilket tyder på att ammoniak inte fungerar som processindikator för
Käppalaverkets förhållanden. Processbehandling med kemiska tillsatser visade sig inte räcka
för att uppnå stabilitet i processen under det aktuella experimentet.
Experimentet visar också att OLR och HRT påverkar avvattningsegenskaperna i processen i
hög grad. Ju högre nedbrytning av det organiska materialet i slammet, desto större blir
behovet av polymer, för att ta hand om det rötade slammet, i slutet av processen.
TABLE OF CONTENT
1 INTRODUCTION ................................................................................................................................................................ 1
1.1 Background ............................................................................................................................................................... 1
1.2 Purpose ..................................................................................................................................................................... 2
1.3 Research questions .................................................................................................................................................. 2
1.4 Delimitation ............................................................................................................................................................... 2
2 THEORETICAL FRAMEWORK ......................................................................................................................................... 4
2.1 Sewage sludge stabilization ..................................................................................................................................... 4
2.2 The anaerobic digestion processes ......................................................................................................................... 5
2.2.1 Anaerobic digestion process affecting parameters ......................................................................................... 6
2.2.1.1. PH, ALKALINITY, AND VOLATILE FATTY ACIDS ...................................................................................... 6
2.2.1.2. TEMPERATURE............................................................................................................................................ 7
2.2.1.3. HYDRAULIC RETENTION TIME, ORGANIC LOADING RATE, AND PROCESS AID .................................. 7
2.2.2 Inhibition of the anaerobic digestion process .................................................................................................. 8
2.3 Importance of sludge recycling for the environment and society ......................................................................... 9
3 METHODS AND MATERIALS ......................................................................................................................................... 10
3.1 Experimental set-up and operational protocol ...................................................................................................... 10
3.1.1 Automatic Methane Potential Test SYSTEM (AMPTS) ................................................................................. 11
3.1.2 Set up of the system .................................................................................................................................... 12
3.1.3 Feeding process .......................................................................................................................................... 13
3.2 Substrates and inoculums ..................................................................................................................................... 13
3.2.1 The outcome of the experimental setup and experiment period ................................................................... 15
3.3 Analysis parameters and methods ........................................................................................................................ 16
3.3.1 pH, TS, and VS ............................................................................................................................................ 16
3.3.2 Alkalinity ...................................................................................................................................................... 16
3.3.3 VFA, NH4-N, COD and CODs ...................................................................................................................... 17
3.3.4 Methane concentration................................................................................................................................. 17
3.3.5 Free ammonium NH3-N ............................................................................................................................... 18
3.3.6 Nitrogen mineralization ................................................................................................................................ 18
3.3.7 Degree of degradation ................................................................................................................................. 19
3.3.8 Process stability ........................................................................................................................................... 19
3.4 Theoretical methane potential................................................................................................................................ 19
3.5 Mass balance .......................................................................................................................................................... 21
3.6 Forecasting of the future OLR at Käppala ............................................................................................................. 21
3.7 Process aid .............................................................................................................................................................. 22
3.8 Dewaterability study of digested sludge ............................................................................................................... 22
3.9 Description of the anaerobic digestion at Käppala WWTP .................................................................................. 23
4 RESULT ........................................................................................................................................................................... 26
4.1 Substrate analyses ................................................................................................................................................. 26
4.2 The anaerobic pilot-scale experiment ................................................................................................................... 26
4.2.1 OLR and HRT in the six pilot-scale reactors ................................................................................................. 26
4.3 Stability analysis of the process ............................................................................................................................ 28
4.3.1 CH4 and pH ................................................................................................................................................. 28
4.3.2 VFA and ratio VFA/TA ................................................................................................................................. 29
4.3.3 Free ammonia NH3-N (FAN) ........................................................................................................................ 31
4.4 Energy potential and performance of the experiment .......................................................................................... 31
4.4.1 Methane yield in the pilot-scale experiment .................................................................................................. 31
4.4.2 The degree of degradation ........................................................................................................................... 33
4.4.3 Chemical oxygen demand COD ................................................................................................................... 34
4.4.4 Mass balance ............................................................................................................................................... 35
4.5 Result of the Chemical additive ............................................................................................................................. 37
4.6 Dewaterability study of the digested sludge ......................................................................................................... 38
4.7 Result of the excel prediction model ..................................................................................................................... 39
5 DISCUSSION ................................................................................................................................................................... 40
5.1 Suitability of the substrate for biogas production ................................................................................................ 40
5.2 Maximum organic loading rate (OLR) .................................................................................................................... 40
5.3 Stability .................................................................................................................................................................... 41
5.4 Gas production ....................................................................................................................................................... 42
5.5 Process aid .............................................................................................................................................................. 43
5.6 Dewaterability study ............................................................................................................................................... 44
5.7 Error sources .......................................................................................................................................................... 44
5.8 Sewage sludge and sustainability ......................................................................................................................... 45
6 CONCLUSIONS ............................................................................................................................................................... 47
7 SUGGESTIONS FOR FURTHER WORK ......................................................................................................................... 48
REFERENCES ......................................................................................................................................................................... 49
APPENDIX 1 EXPERIMENT PROTOCOL
APPENDIX 2 EXTRA DATA ON METHANE PRODUCTION
LIST OF FIGURES
Figure 1 Stages of anaerobic digestion process for biogas production Kumar & Samadder, (2020). .......................... 6
Figure 2: The laboratory-scale biogas production process set up. .............................................................................. 12
Figure 3: Description of the feeding process. ............................................................................................................... 13
Figure 4: The sludge dewatering thickening process with the aid of a compact moisture analyzer (first object from
the left) and a high molecular filter (second object from the left) ......................................................... 15
Figure 5: Alkalinity analyses of the digested sludge by the titration robot. 50 mL digested sludge liquid in the
different test tubes are being analyzed with a blue pH-meter, a white mixer, a nitrogen gas tube, and
a 0.05 M hydrochloric acid (HCl) tube. ................................................................................................... 17
Figure 6: Measurement of methane concentration in the produced biogas, using the membrane gas sampling port
and the NaOH solution containing the pH indicator, in an Einhorn pipe meter. ................................. 18
Figure 7: CST measurements. ....................................................................................................................................... 23
Figure 8 Process chart over Käppala WWTP used with permission Kappala (2011) ................................................. 25
Figure 9 Organic loading rate (OLR) [g VS/day] and hydraulic retention time (HRT). ............................................ 27
Figure 10 pH and CH4 of the digesters during the experiment................................................................................... 29
Figure 11 The VFA level and the ratio VFA/TA during the pilot-scale experiment. .................................................. 30
Figure 12 Ammonia in the different digesters during the pilot-scale experiment ...................................................... 31
Figure 13 Specific methane production (methane yield) and the accumulated methane produced .......................... 33
Figure 14 Degree of degradation for reactors R1, R2, R3, and R6. ............................................................................. 34
Figure 15 CODs, and total COD in primary and digested sludge during the experiment. .......................................... 35
Figure 16 Theoretical methane content converted into methane gas ......................................................................... 36
Figure 17 Comparison of the theoretical and the specific methane yield .................................................................... 36
Figure 18 Effect of the Sodium Carbonate on the pH and CH4 of R4 ......................................................................... 37
Figure 19 Result of the CST analyses. ...........................................................................................................................38
Figure 20 Result of the prediction model. .................................................................................................................... 39
Figure 21 Accumulated methane production ................................................................................................................. 2
Figure 22 Daily methane production per day ................................................................................................................. 2
Figure 23 Methane production flow per day .................................................................................................................. 3
LIST OF TABLES
Table 1: The scenarios that, according to the first plan, were to be studied in the six lab-scale reactors. The organic
loading rates and hydraulic retention times are written as OLR and HRT ............................................ 11
Table 2: Expected values and real outcome in the six pilot-scale reactors between February 17th and March 1st.
(Experimental period 2) ........................................................................................................................... 15
Table 3 The scenarios applied in the six lab-scale reactors during the different periods of the experiment ............ 16
Table 4 Theoretical methane yield assumption........................................................................................................... 20
Table 5 Substrate composition in the primary sludge and the digested sludge .......................................................... 26
Table 6 The needed quantity of polymer per mass TS sludge. ....................................................................................38
Table 7 Weekly protocol of the pilot-scale experiment .................................................................................................. 1
NOMENCLATURE
Symbol Description Unit
𝐵𝐴 Bicarbonate alkalinity [mgBasic ions L−1]
%𝐶𝐻4 Methane concentration [%]
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐶𝐻4 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 Specific methane production/ Methane yield
[𝑁𝑚𝐿 𝑔−1𝑉𝑆−1]
d Number of days [d]
HRT Hydraulic Retention Time [d]
NH3 − N The free ammonium in the digested sludge
[%]
𝑀𝐿 Nitrogen mineralization [%]
𝑚𝑁𝑎2𝐶𝑂3 Mass of Sodium Carbonate [g]
𝑀𝑁𝑎2𝐶𝑂3 The molar mass of Sodium Carbonate
[g mol-1]
𝑁𝐶𝑢𝑠𝑡𝑜𝑚𝑒𝑟 Number of people connected to the Käppala WWTP
[-]
Qin Organic feeding load measured in volume or mass
[g d-1]
OLR Organic Loading Rate [g VS dm-3 d-1] or [kg VS m-3 d-1]
𝑉𝐹𝐴s Volatile Fatty Acids [mg L-1]
𝑇𝐴 Total Alkalinity [mgBasic ions L−1]
TSDesired_PS Desired total solids [%]
𝑉 Digester’s working volume [dm3]
𝑉𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 The desired volume of the collected primary sludge
[dm3]
𝑉𝑆𝐴𝑠𝑠𝑢𝑚𝑒𝑑 The assumed volatile solid [%]
𝛼 Degree of degradation [%]
ABBREVIATIONS
Abbreviation Description
AD Anaerobic digestion
ALK Alkalinity
BMP Biochemical methane potential
COD Chemical Oxygen Demand
GWP Global warming potential
HRT Hydraulic Retention Time
NL Normal liter
OLR Organic Loading Rate
TAN Total Ammonia Nitrogen
TKN Total Kjeldahl Nitrogen
TS Total Solids
VS Volatile Solids
WWTPs Wastewater treatment plants
DEFINITIONS
Definition Description
Anaerobic digestion
The biotechnological oxygen-free process to degrade organic material extracting biogas
Acetogenins A key enzyme in energy metabolism
Acetoclastic methanogens
Microorganisms produce methane by fermenting acetate and H2-CO2 into methane and carbon dioxide
Free ammonia nitrogen (FAN)
NH3-N, the undissociated form of ammonia, free ammonia nitrogen (FAN)
Ammonium ion NH4-N is the dissociated form of ammonia
Biogas Gaseous product from fermentation consisting of methane and carbon dioxide, and depending on the substrate used, ammonia, hydrogen sulfide, and water vapor
Biogas yield Quantity of biogas produced per quantity of substrate feed
Biogas formation potential
Maximum possible biogas yield from a defined quantity of substrate
Total biogas production
The quantity of biogas formed in units of volume
Biogas production rate
Biogas quantity produced per unit of time
Total ammonia nitrogen (TAN)
Total ammonia nitrogen (TAN) is the sum of FAN and Ammonium in water
Chemical Oxygen Demand (COD)
The measure of the content of oxidizable compounds in a substrate
Colloidal particles Microscopic solid particles suspended in a fluid
Degree of degradation
Reduction in the concentration of organic substance due to anaerobic degradation
Dewaterability The ability in digested sludge to let go of water
Fermentation Anaerobic process in which a product, in this case, biogas is produced by the activity of microorganisms
Global warming potential (GWP)
Amount of heat a greenhouse gas traps in the atmosphere up to a specific time horizon, relative to carbon dioxide
Hydraulic Retention Time (HRT)
Average time for which the substrate remains in the fermenter
Inflow load Mass fed daily into the fermentation installation
Definition Description
Inhibition Hindering of fermentation due to damage to the active micro-organisms or reduction in the effectiveness of enzymes
Loading rate per unit volume
The ratio of the daily load to the fermenter volume
Methane formation potential
Maximum possible methane yield from a defined quantity of substrate
Methane yield Quantity of methane produced per quantity of substrate feed
Methane production rate
Methane quantity produced per unit of time
Organic Loading Rate (OLR)
The amount of organic material per unit reactor volume, which is subjected to the anaerobic digestion process in the reactor in a given unit period
Solids retention time (SRT)
The time the solid fraction of the wastewater spends in a treatment unit
Total solids (TS) TS is the substance contained in the sludge that is left after dying the sludge at 105 degrees for 24 hours.
Volatile solid (VS) The amount of organic solids in water
1
1 INTRODUCTION
The volume of wastewater in the world is expected to increase during the coming decades due
to the continuously growing world population and increasing industrialization (Duan et al.,
2012). According to the Swedish Environmental Protection Agency, about 1 million tons of
sewage sludge is produced every year in municipal wastewater treatment plants in Sweden
(Arthurson, 2008). The optimal management of sewage sludge is, therefore, an important
issue worldwide, since the sludge can cause severe damage to both the environment, animals,
and humans (Chen et al., 2020). However, since the sludge contains organic matter, it can
serve as a renewable resource. It also contains un-degradable particles and living organisms,
and in order to reduce biological activity and odor, it must be stabilized (Chen et al., 2020).
Municipal wastewater treatment plants (WWTP) generate sewage sludge from mechanical,
biological, and chemical treatment. A common treatment used to stabilize the sludge is
anaerobic digestion (AD). It is an efficient and well-studied process that biologically converts
the chemical energy of sewage sludge into combustible biogas that contains 60-70% methane
(Appels et al., 2008), and makes it a carbon-neutral alternative to fossil fuels.
Simultaneously, it reduces dangerous pathogens and odor in the process (Zhen et al., 2017).
Thus, transforming a waste problem into an essential renewable energy resource. As biogas is
a renewable energy source, the expansion of biogas production systems is an essential
contributor to the global conversion from fossil fuels to renewable energy systems
(Tchobanoglous & Burton, 2014).
To meet the increasing volume of sewage sludge, upgrading existing wastewater treatment
plants and the sludge digesters is essential. This is the case at Käppala WWTP in Stockholm,
which purifies wastewater from approximately 550,000 inhabitants northeast of Stockholm
(Käppala Association, 2020).
1.1 Background
Käppala WWTP has, during the last years, observed an increase in organic loading rate
(OLR) in the mesophilic anaerobic digestion process, due to the increased load to the WWTP.
This has resulted in a decrease in alkalinity and pH in the digested sludge. Since pH and
alkalinity are two of the anaerobic process stability parameters, this shows a risk for
instability in the digestion process. The digestion at Käppala WWTP is today high loaded,
with a high organic loading rate and low hydraulic retention time (HRT). In the future, this
problem will be noticeable due to the increased population in the region. R100 at Käppala
WWTP is currently operating with an organic loading rate of 3.9 [kg VS m-³, d-1], and a
hydraulic retention time of 13,2 days. The experience from Käppala show that this is working
well, but how close to the process limits it is possible to run a stable anaerobic digestion
2
process? If nothing is done, the digester R100 will be overloaded, which will lead to process
imbalance and process failure. Studies by, for example, Halalsheh et al., (2005); Duan et al.,
(2012); Olsson et al., (2018) show that the operational experience with similar process
conditions exists from other wastewater treatment plants. However, process conditions differ
from facility to facility, and local conditions are usually governing. Evaluating the mesophilic
digestion conditions for Käppala WWTP has never been done before.
1.2 Purpose
For Käppala to encounter the higher load of sewage sludge in future sludge strategies, this
thesis aims to evaluate the effect of the maximum organic loading rate and the minimum
hydraulic retention time for the anaerobic digestion of sewage sludge. Moreover, to
investigate further actions that can be taken in case of process problems in the digestion. The
focus of this work is to strive for a stable digestion process, which is vital for optimal biogas
production.
1.3 Research questions
The study is aiming at answering the following research questions:
1. Maximum load: What is the maximum organic loading rate (OLR) and the minimum
hydraulic retention time (HRT) to achieve a stable digestion process?
a. Which range of process parameters is acceptable?
b. What happens to the process parameters when the process is overloaded?
c. What measures can be taken in case of overload or risk of overload in the mesophilic
anaerobic digestion process? Could chemical additives be used?
d. When will R100 at Käppala WWTP reach maximum load, and a third anaerobic digester
will be needed?
2. Energy potential: How is the gas production affected when the organic loading rate
increases and the hydraulic retention time in the digesters decreases? How is the gas
production affected by instability in the digester?
3. Dewatering process: How are the sludge dewatering properties affected by changes in the
organic loading rate and hydraulic retention time in the digester?
1.4 Delimitation
The work in this master thesis is limited to a laboratory-scale experiment, studying six pilot-
scales anaerobic digestion reactors. Each one of the six reactors is representing the full-scale
reactor R100 at Käppala WWTP, with continued stirring of sewage sludge, in the mesophilic
conditions of 37±1 ˚C temperature. The study only deals with the sludge digestion for biogas
3
production at Käppala WWTP, not the treatment of wastewater. No economic analyses or life
cycle analyses of the process are included in the study.
4
2 THEORETICAL FRAMEWORK
2.1 Sewage sludge stabilization
Sewage sludge is produced while treating wastewater in municipal wastewater treatment
plants. It contains water, degradable organic material, living organisms, and un-degradable
particles. The sludge is generally treated as a waste product. However, since it contains
renewable resources such as carbon, that can be converted into biogas, and phosphorus and
nitrogen, that can be used as fertilizer for cultivation (Wawrzynczyk, 2007), it has become
more and more useful as a resource. Nevertheless, to be useful, it needs to be stabilized to
reduce biological activity, odor production, and the release of harmful chemicals substances
into the environment (Boušková et al., 2005).
Anaerobic digestion (AD) is an efficient method for sludge stabilization. Mainly because of
the high efficiency in organic matter degradation when producing biogas with 60-70%
methane content, which can be upgraded to biofuel (Cha & Noike, 1997; Chen et al., 2020). It
reduces greenhouse gas (GHG) and provides clean and renewable energy in the process.
According to Chen et al., (2008b), anaerobic digestion is one of the most promising sludge
stabilization methods. Mainly because it involves both controlling the pollution from
industrial and agricultural waste and a way to recover energy in the process. Other
advantages using anaerobic digestion for sludge stabilization are, for example:
• High reduction of volume in the process, between 30-50%.
• Reduction of offensive odors in the sludge.
• High rate of pathogen destruction in the sludge when using the thermophilic digestion for
sludge stabilization (Rubia et al., 2005).
Anaerobic digestion can be performed at two different temperatures, the mesophilic
digestion process at 30-40˚C, and the thermophilic digestion process at 45-60˚C (Cha &
Noike, 1997; Kumar & Samadder, 2020). Mesophilic digestion is the method used at Käppala
WWTP, investigated in the present study. However, thermophilic digestion is becoming more
and more popular due to its potential when it comes to the reduction of pathogens in the
sludge (Watanabe et al., 1997).
However, there are some obstacles to consider using anaerobic digestion. For example,
several studies have reported of foaming and low efficiency in the degradation of the volatile
solids in the anaerobic digestion process (Li & Noike, 1992; Halalsheh et al., 2005). Low rates
of VS degradation of colloidal particles in the waste have been reported due to the physical
limitations of low biodegradability (Elmitwalli et al., 2001). The degradation of insoluble
substances has also been mentioned as rate-limiting steps for the anaerobic digestion
(Eastman & Ferguson, 1981). For a high degradation of colloidal particles, there is a need for
long retention times, up to 20-30 days according to Parawira et al., (2004), even 35 days in
some full-scale operations for waste stabilization. Here thermophilic digestion has been
mentioned as more advantageous than mesophilic digestion, with higher VS degradation
5
efficiency, higher biogas production, less foaming, and better dewaterability (Rimkus et al.,
1982).
2.2 The anaerobic digestion processes
The anaerobic digestion process is a complex microbiological process that occurs in an
oxygen-free environment. The process involves a series of metabolic reactions that oxidize
organic matter into biogas and organic fertilizers (Kumar & Samadder, 2020). Sewage
sludge, produced from wastewater treatment plants, is a well suited organic material as a
substrate for a stable anaerobic digestion process since its nutrient content varies very little
(Bramstedt, 2015). According to Kumar & Samadder, (2020), the whole anaerobic process is
divided into four different stages (see Figure 1):
• Hydrolysis is the first stage in which complex organic compounds like carbohydrates,
proteins, and fats are broken down into soluble organic molecules such as amino acids, sugar,
fatty acids, and other related compounds. This stage is the slowest step due to the large size of
the molecules, the volatile fatty acid formation, and other toxic by-products (Zhang et al.,
2014).
• Acidogenesis or fermentation is the stage during which the produced organic compounds
from the previous stage are further broken down into intermediate products, such as short-
chain fatty acids along with hydrogen, carbon dioxide, and other by-products. Acid formation
under this stage takes place with the help of acidogenic bacteria. This stage is sometimes
divided into two stages. If the breakdown of fatty acids goes slowly, they accumulate and can
lead to a decrease in pH and instability in the process (Tchobanoglous et al., 2014).
• Acetogenesis is the third stage during which the organic acids formed in the acidogenesis
stage gets converted into acetic acid as well as hydrogen and carbon dioxide.
• Methanogenesis is the fourth stage during which two different groups of methanogens
produce methane. One group splits the acetic acid into methane and carbon dioxide, while the
other uses the intermediate products, H2 and CO2, for the methane formation (Appels et al.,
2008).
6
Figure 1 Stages of anaerobic digestion process for biogas production (Kumar & Samadder, 2020).
2.2.1 Anaerobic digestion process affecting parameters
Several essential parameters affect the speed of the different stages of the digestion process in
the anaerobic environment, namely pH, alkalinity, temperature, and the hydraulic retention
time.
2.2.1.1. pH, alkalinity, and volatile fatty acids
The pH level is one of the most crucial anaerobic process stability parameters. Every
microorganism group has a different optimal pH range. Methanogenic bacteria responsible
for methane formation in the anaerobic digestion process are susceptible to the pH level, with
an optimum pH for methane formation between 6.5 and 7.2 (Kanokwan, 2006; Kumar &
Samadder, 2020). When it comes to the fermentation microorganisms, they are less sensitive
and can function within a broader range of pH, between 4.0 and 8.5. Acetics are the main
products at a low pH level, while acetic and propionic acids are mainly produced at a high pH
value (Kanokwan, 2006).
The production of volatile fatty acids (VFA) in the anaerobic digestion process tends to lower
the pH level. This pH reduction is usually countered by the activity of methanogenic bacteria,
due to their capacity also to produce alkalinity in the form of carbon dioxide, bicarbonate,
and ammoniac (Appels et al., 2008; Kumar & Samadder, 2020). Together the two create a
balance in the reactor. The pH level in the anaerobic process varies depending on in which
state the process is. The process pH usually increases when the ammonia concentration
7
increases due to the reduction of proteins present in the substrate and decreases when the
VFA concentration increases (Kumar & Samadder, 2020).
2.2.1.2. Temperature
A stable operating temperature in the digester is essential for the anaerobic digestion process
since it is one of the most critical parameters in the anaerobic reactor. It affects the
performance in general, but especially the methanogenesis (Turovskiy & Mathai, 2006;
Kumar & Samadder, 2020). The temperature also has a significant effect on the
physicochemical properties of the components in the digestion substrate. Besides the growth
rate and metabolism of micro-organisms, the whole population dynamic in the anaerobic
reactor is affected. Sharp or frequent fluctuations in the temperature affect the bacteria,
especially the methanogens, and process failure can occur if the changes are more than 1 o
C/day. Any changes in temperature of more than 0,6 o C/day are to be avoided (Appels et al.,
2008).
2.2.1.3. Hydraulic retention time, organic loading rate, and process aid
According to Svenskt Vatten, (2019), hydraulic retention time is the most crucial process
parameter for the anaerobic digestion process. The digestion result depends primarily on the
HRT, followed by the OLR and the temperature (Svenskt Vatten, 2019). Stabilization in the
digestion process also depends on the HRT, since an HRT less than ten days (10 d) under
mesophilic conditions can give rise to wash-out of the organisms and thus inhibit the process
according to Forkman, (2014). Each time the digested sludge is withdrawn from the digester,
a fraction of the bacterial population is also removed. The cell growth must then compensate
for the removed bacteria from the digester to ensure a steady-state in the process and avoid
process failure (Appels et al., 2008).
The OLR is, as mentioned, another essential parameter in the anaerobic digestion, due to the
process imbalance risk connected to an overload in the digesters. Increasing the OLR in the
anaerobic digestion process can lead to operational disruptions due to process imbalances.
They are commonly detected as disruptions in gas production, an increase in the CO2 level in
the produced biogas, and a decrease in the pH level (Svenskt Vatten, 2019). However, high
organic loaded digestion can result in the reduction of the needed digester tank volume and
improved process stability and process efficiency if all the conditions are in place (Appels et
al., 2008). According to Svenskt Vatten, (2019), a well-stirred anaerobic digester can be
loaded with up to 2-3 kg VS (organic material) per m3 and day. However, raw feeding sludge
should not exceed 6% TS. At higher TS level in the sludge, there is a risk of poisoning the
process with mainly ammonium ions or ammonia gas. High TS content also makes
mechanical stirring difficult.
In order to regain the process balance while keeping the high organic load in the digester,
conceivable measures to consider are, among other things, the use of chemicals. According to
Svenskt Vatten, (2019), the following chemicals are commonly used:
• Sodium hydroxide (NaOH) can be used in most cases.
8
• Calcium hydroxide (Ca (OH) 2) should be used when the sludge contains a high content of
inorganic material (a residue level higher than 60%).
• Sodium carbonate (Na2CO3) and sodium hydrogen carbonate (NaHCO3) should only be used
when the pH level is higher than five, due to high CO2 production, which can lead to foaming
problems in the digester.
Kasali et al., (1989) managed to fully recover a failed anaerobic digestion process, using
sodium hydrogen carbonate (NaHCO3) as a pH controller. Sodium carbonate (Na2CO3),
recommended by Svenskt Vatten, (2019), is used in the present study to recover a failed
overloaded pilot-scale digester at Käppala WWTP.
2.2.2 Inhibition of the anaerobic digestion process
Several studies on the anaerobic digestion process have shown considerable variations in the
inhibition levels reported for most substances. Some of these inhibitory substances are
ammonia, organic compounds sulfide, light metal ions, and heavy metals. The complexity of
the anaerobic digestion process, where mechanisms such as antagonism, synergism,
acclimation, and complexing could affect the inhibition phenomenon significantly, is the
primary reason for these variations in the inhibition levels (Chen et al., 2008a). The two most
common inhibitory substances reported by most studies are ammonia and volatile fatty acids
(VFA).
• Ammonia exists in two primary forms, ammonium ion (NH4) and ammonia nitrogen (FAN),
often called free ammonia (Chen et al., 2008a). Combined in water, the two becomes total
ammonia (TAN). Ammonia is a significant inhibitor of microbial activities in the anaerobic
reactor (Akindele & Sartaj, 2018). It can support the system, acting as a buffer, but it can also
become a problem if the level of concentration is too high. Then it becomes toxic to the process
and reduces methane production (Browne et al., 2014; Rajagopal et al., 2013; Sprott & Patel,
1986). The outcome depends, for example, on pH, temperature, C/N ratio, and the type of
substrate and inoculum. Yenigün & Demirel, (2013) concludes that a FAN value higher than
100 mg L-1 is the threshold value for ammonia in the anaerobic digestion process. Inhibition
of the anaerobic process has been reported by Zhang & Angelidaki, (2015) to start at a TAN
level of 1.5 g-N L-1.
• VFA:s are, like ammonia, one of the main in-between compounds in the metabolic pathway of
methane fermentation and can cause microbial stress if they are present in high
concentrations in the anaerobic digestion process (Buyukkamaci & Filibeli, 2004). The VFA
level in the process generally indicates the metabolic state of the obligate hydrogen-producing
acetogenins and the acetoclastic methanogens. Accumulation of VFA occurs when the
methanogens are unable to break down the VFA to methane, but the fermenting bacteria
continue to form VFA. As a result, the methanogens are held back even more. Monitoring the
evolution of the VFA level is thus key to detecting process imbalance (Aymerich et al., 2013). A
high concentration of VFA can also result in a pH decrease, which can lead to process failure.
The VFA concentration is regularly monitored in the present study to examine the optimal
conditions and the efficiency of the pilot digesters.
9
2.3 Importance of sludge recycling for the environment and society
To reach the global sustainable development goals (SDG) (The Global Goals, 2015),
systematic procedures for the treatment and recycling of sewage sludge, converting it into
energy and other renewable resources, is crucial (Arthurson, 2008). Anaerobic digestion of
sewage sludge is an efficient way of transforming the challenge into an opportunity. Not only
producing biofuel and methane-rich biogas that can be utilized as fuel but also to produce
offset heat and electricity for the wastewater treatment sector itself. Renewable energy
sources that reduce the need for externally produced heat, electricity and fosil fuel in the
process significantly (Cao & Pawłowski, 2012). The produced heat can also serve as an
essential contributor to the local energy demand, which reduces the need for non-renewable
energy sources in the district heating system, warming up houses, and producing hot water.
In Käppala WWTP, the sewage sludge-to-energy process results in several different energy
outcomes. The purified wastewater is lead to a heat pump to recover heat from it to use
internally in the WWTP to heat the sludge in the digester. The excess part is then delivered to
the district heating system (see point 13 in Figure 8). In the vehicle gas plant (see point 10 in
Figure 8), the carbon dioxide is removed, upgrading the biogas into vehicle fuel with a
minimum of 97% methane, used in the local busses (SL) (see point 12 in Figure 8). Biogas
can also be used to produce electricity via a water-to-steam-system that makes turbines turn,
thus creating energy. However, this is not the case in Käppala WWTP (Käppala 2018).
10
3 METHODS AND MATERIALS
The study consists of four parts:
• A practical laboratory experiment using six pilot-scale anaerobic digesters of 2.5 dm3, to
investigate how the process stability in the digested sludge is affected when the OLR increases
and the HRT decreases.
• A mass balance calculation based on the specific methane yield and the energy potential in the
feeding and the digested sludge.
• A study of the digested sludge’ filterability, to investigate how the sludge dewatering properties
are affected by changes in OLR and HRT in the digester.
• The construction of a forecasting model in Excel that can predict when the studied digester
R100 at Käppala WWTP will reach its maximum OLR and minimum HRT.
3.1 Experimental set-up and operational protocol
The pilot-scale experiment contained six pilot-scale anaerobic digesters of 2.5 dm3, running
parallelly. They were filled with 2 dm3 of mesophilic inoculum from the full-scale anaerobic
digester R100. The reactors were then operated at the same HRT as the full-scale reactor
R100, during one retention time (13.2 days). OLR and HRT were then changed in reactor R2
to R6, while reactor R1 was maintained at the same conditions and operated as a reference.
In digester R2, the change was made gradually from the conditions in R3 to R6. Each OLR
and HRT in R2 was maintained for at least two retention times. In R3 to R6, the conditions
were changed directly. The HRT for R2 to R6 was assumed as presented in Table 1, and the
feeding sewage quantity was determined using equation 2. The OLR for digesters R2 to R6
was determined using equation 1. VS% was assumed before each primary sludge collection to
calculate the desired TS (see equation 5) and then measured using the standard method
(APHA et al., 1995). The incoming proportion of TS% into the digester was controlled by
thickening the sludge to keep the OLR and HRT constant in the different reactors.
𝑂𝐿𝑅 =𝑄𝑖𝑛∗𝑇𝑆∗𝑉𝑆
𝑉 Equation 1
𝑄𝑖𝑛 =𝑚
𝐻𝑅𝑇 Equation 2
𝐻𝑅𝑇 =𝑚
𝑄𝑖𝑛 Equation 3
• 𝑂𝐿𝑅 is the organic loading rate in the digesters [g VS dm−3d−1].
• 𝑇𝑆 is the total solids matter in the sludge [%]
• 𝑉𝑆 is the volatile solids in total solids matter the sludge in [%].
• V is the working volume of the reactors, which is the part of the 2.5 𝑑𝑚3 that is filled with
mesophilic inoculum from the full-scale anaerobic digester R100 in [𝑑𝑚3]
• 𝑄𝑖𝑛 is the hydraulic flow into the digestor per day, in [g d−1]. 𝑄𝑖𝑛 was measured in grams (as a
mass) to ensure that the same amount of solid matter was obtained in the daily feeding sludge
(see Figure 3). It was assumed that 1L = 1dm3 = 103cm3 = 103g = 1kg. And the sludge density
was also assumed to 1000 [𝑔 𝑑𝑚−3]
• HRT is the hydraulic retention time in [d].
11
• 𝑚 is the quantity of the mesophilic inoculum from the full-scale digester R100 in the pilots scales
reactors [g]
Table 1 presents the original scenario protocol that was planned to be investigated in this
study, where the presented parameters were used to determine the volume and TS content of
the incoming sludge to the reactors. The VS content of the incoming feeding sludge to the
reactors was assumed to 80%.
Table 1: The scenarios that, according to the first plan, were to be studied in the six lab-scale reactors. The organic loading rates and hydraulic retention times are written as OLR and HRT
Digester OLR [g VS dm-3 d-1] HRT [d] Feeding Sludge (𝑄𝑖𝑛) [g d-1]
R1 3.9 13.2 151.52
R2 4.3-5.5 12-9.5 166.67-210.53
R3 4.3 12 166.67
R4 4.7 11 181.82
R5 5.2 10 200
R6 5.5 9.5 210.53
3.1.1 Automatic Methane Potential Test SYSTEM (AMPTS)
An Automatic Methane Potential Test System (AMPTS II) is an analytical instrument for
estimating biochemical methane potential and anaerobic biodegradability (Bioprocess
Control, 2020). The AMPTS II was used in the present study for the experimental system that
consisted of the below components. AMPTS II was not chosen because it is the best system,
but because it is the system available at Käppala WWTP. The system is not adapted to
experimenting with high organic loaded digestion.
The incubation unit was the main component of the experiment. It included six 2.5 dm3,
continuously stirred, glass tank reactors, with one stirring device, one feeding tube, one
membrane gas sampling port, and one outlet valve each. The stirring device rotated
continuously in the sample to favor an optimal mix of the sample during the experiment. The
six stirred glass tanks were gathered in a water bath of 37-38˚C, to keep the same
temperature conditions in the reactors all the time.
The CO2 removal unit consisted of six 0.4 dm3 bottles that were filled with 0.4 dm3 each
of the NaOH solution containing the pH indicator. This solution was prepared, mixing 2.5
dm3 of NaOH solution and a 0.4% Thymolphthalein pH-indicator solution, according to the
instruction manual Bioprocess Control, (2020). As shown in Figure 2, each CO2 absorber
bottle had two connectors connecting the CO2 absorber bottle to the reactor and the gas
collection unit. The purpose of this unit was, as the name indicates, to absorb the carbon
dioxide CO2 in the produced biogas before releasing the methane gas to the gas collection
unit. According to the operation manual of the bioprocess control, the CO2 removal unit
removes up to 98% CO2 from the produced biogas.
12
The gas collection unit consisted of fifteen flow cells with an embedded server. Six of the
fifteen flow cells were in operation during the present study. The gas flow and volume
measurements were performed with real-time temperature and pressure compensation.
3.1.2 Set up of the system
Finally, all the experimental system components were gathered (see Figure 2) and flushed
with nitrogen gas to create an anaerobic environment (free from oxygen). The reactors were
also pressure controlled before starting the experiment. Then the reactors were filled with 2
dm3 of mesophilic inoculum from the full-scale anaerobic digester R100. The feeding sludge
was prepared (see section 3.2) and kept in the fridge to avoid microbiological activity in the
sludge. See (Figure 2), where the generated biogas from the digesters goes through the CO2
absorber. The produced methane goes through the gas collector, to be monitored in the
Bioprocess Control software in the computer.
Figure 2: The laboratory-scale biogas production process set up.
13
3.1.3 Feeding process
The feeding was performed daily according to the scenarios presented in Table 1. To avoid
feeding during weekends, but still keeping the load for one week, the digesters were only fed
five days a week, Monday to Friday, using the average quantity of primary sludge measured
to cover for seven days (see equation 4). The gas flow and the mixer’s rotation were stopped
during the feeding process in order to avoid that the digested sludge became mixed with the
feeding sludge. If not, this could affect the analyses of the digested sludge. The feeding sludge
was then put in the feeding tube. After that, the outlet valve was opened, and the sludge was
pushed down using nitrogen gas flow from a 50 mL syringe connected to the tube at the
feeding tunnel. In the end, the digested sludge was siphoned out from the open outlet valve
and collected for analyses. See (Figure 3), where the feeding sludge is weighed on a scale, put
in the feeding tube, and pushed into the reactor using a nitrogen gas ball’ pressor.
𝑄𝑖𝑛(5𝑑𝑎𝑦𝑠) =𝑄𝑖𝑛(7𝑑𝑎𝑦𝑠)∗7
5 Equation 4
• 𝑄𝑖𝑛(5𝑑𝑎𝑦𝑠) is the amount of feeding sludge for five days/week in [𝑔 d−1].
• Qin(7days) is the amount of feeding sludge during seven days/week according to the experiment
protocol in [𝑔 𝑑−1].( see Table 7 in Appendix 1).
Figure 3: Description of the feeding process.
3.2 Substrates and inoculums
The study consisted of six pilot-scale digesters with mesophilic inoculum from the full-scale
anaerobic digester (R100) for primary sludge, at Käppala WWTP in Stockholm. Sewage
sludge from Käppala WWTP, collected from the primary sludge, was used as substrates.
The digester R100 at Käppala operates with an HRT of 13.2 days and an OLR of 3.9 kg VS m-3
d -1. The same OLR and HRT values were obtained in the pilot-scale experiment, assuming
the VS to be between 80% to 87% and determining the desired TS (see equation 5) of the
collected primary sludge for the feeding of the digesters.
14
𝑄𝑖𝑛 ∗ 𝑇𝑆𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 ∗ 𝑉𝑆𝐴𝑠𝑠𝑢𝑚𝑒𝑑 = 𝑂𝐿𝑅 ∗ 𝑉𝐷𝑖𝑔𝑒𝑠𝑡𝑒𝑟 ↔ 𝑇𝑆𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 =𝑂𝐿𝑅 ∗ 𝑉𝐷𝑖𝑔𝑒𝑠𝑡𝑒𝑟
𝑄𝑖𝑛 ∗ 𝑉𝑆𝐴𝑠𝑠𝑢𝑚𝑒𝑑
𝑇𝑆𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 =𝑂𝐿𝑅∗𝐻𝑅𝑇
𝑉𝑆𝐴𝑠𝑠𝑢𝑚𝑒𝑑 Equation 5
• TSDesired_PS is the desired TS into the digester [%].
• 𝑉𝐷𝑖𝑔𝑒𝑠𝑡𝑒𝑟 is the working volume of the reactors in [𝑑𝑚3].
• 𝑉𝑆𝐴𝑠𝑠𝑢𝑚𝑒𝑑 is the assumed volatile solids in [%].
• 𝑄𝑖𝑛 is the quantity of primary sludge for feeding measured in volume [𝑔 𝑑−1].
The needed TS in the primary feeding sludge was calculated through the above equation 5.
This made it possible to determine the thickness of the primary sludge. The sludge was then
thickened through a dewatering process with a high molecular filter, Ytteknik QP3, (2020), to
obtain the desired total solids (TS) amount for the study.
During this step, the initial TS of the collected primary sludge was measured using a compact
moisture analyzer (see the first object to the left in Figure 4) (Ohaus MB45, 2020). This was
done to determine to what extent the sludge needed to be filtered to contain the desired TS.
▪ Knowing the initial volume of the collected sludge, the initial TS of the collected primary
sludge, and the desired TS of the primary sludge, the desired volume of the sludge was
calculated as follows:
𝑉𝐼𝑛𝑖𝑡𝑖𝑎𝑙_𝑃𝑆 ∗ 𝑇𝑆𝐼𝑛𝑖𝑡𝑖𝑎𝑙_𝑃𝑆 = 𝑉𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 ∗ 𝑇𝑆𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆
𝑉𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 =𝑉𝐼𝑛𝑖𝑡𝑖𝑎𝑙_𝑃𝑆∗𝑇𝑆𝐼𝑛𝑖𝑡𝑖𝑎𝑙_𝑃𝑆
𝑇𝑆𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 Equation 6
• 𝑉𝐷𝑒𝑠𝑖𝑟𝑒d_PS is the desired volume of the collected primary sludge in [dm3].
• 𝑉𝐼𝑛𝑖𝑡𝑖𝑎𝑙_𝑃𝑆 is the known initial volume of the collected primary sludge in [dm3].
• 𝑇𝑆𝐼𝑛𝑖𝑡𝑖𝑎𝑙_𝑃𝑆 is the measured initial TS value of the collected primary sludge in [%].
• 𝑇𝑆𝐷𝑒𝑠𝑖𝑟𝑒𝑑_𝑃𝑆 is the desired TS value of the collected primary sludge, determined in the next
section in [%].
▪ The collected sewage sludge was filtered to obtain the desired volume, determined by equation
6, using a high molecular filter (see the second object from the right in Figure 4).
▪ The thickened sludge was then stored at a temperature of 4˚C for feeding the reactors.
15
Figure 4: The sludge dewatering thickening process with the aid of a compact moisture analyzer (first object from the left) and a high molecular filter (second object from the left)
3.2.1 The outcome of the experimental setup and experiment period
The experimental outcome could be divided into four periods. The first period (see Table 3)
of the pilot-scale experiment started on February 3rd with all six reactors running at the same
OLR and HRT for one retention time (13.2 days), before the OLR and HRT were changed, on
February 17th. Due to calculation errors and false assumptions of the VS values during the
start-up of the experiment, the outcome did not follow the expected original protocol. Thus,
the obtained OLR and HRT in all the digesters (R1 to R6) turned out differently than
expected for the second period (February 17th to March 2nd), especially in R4, R5, and R6, the
values deviated significantly from the expected (see Table 2). Table 2 presents the expected
and the real OLR and HRT for the period.
Table 2: Expected values and real outcome in the six pilot-scale reactors between February 17th and March 1st. (Experimental period 2)
Expected outcome Real outcome
Digester OLR [g VS dm-3 d-1] HRT [d] OLR [g VS dm-3 d-1] HRT [d]
R1 3.9 13.2 4.2 13.1
R2 4.3 12 4.7-4.5 12
R3 4.3 12 4.7-4.5 12
R4 4.7 11 12.6 - 8.3 4.4 – 6.5
R5 5.2 10 8.8 - 6.7 6.3 - 8
R6 5.5 9.5 6.5 – 5.4 8.6 – 9.9
The errors were corrected, resetting the new OLR and HRT values as close as possible to the
original protocol during period 3 (see Table 3). The experiment could then continue following
a new plan (see Table 3). The affected reactors were expected to recover during the third
period (March 2nd to April 16th) after the correction of the calculation errors. During period 3,
the chemical additive Na2CO3 was also used once a week to investigate if the anaerobic
digestion in reactor R4 could recover, while still keeping the high OLR and short HRT.
The last experimental period (period 4) (see Table 3), starting on April 17th, is the period most
affected by instability in all the reactors. The feeding of the digesters was then stopped twice,
16
in an attempt to allow the digesters to recover and avoid process failure. Between April 17th
and 21rst, the feeding was stopped entirely because of total process failure. On April 22nd, the
feeding started again, but with only half of the OLR level. Between May 1st and 12th, the
feeding was stopped again because of process imbalance. Table 3 presents how the OLR and
HRT varied in the reactors during all four periods.
Table 3 The scenarios applied in the six lab-scale reactors during the different periods of the experiment
Period Period (1)
2020-02-03
2020-02-16
Period (2)
2020-02-17
2020-03-01
Period (3)
2020-03-02
2020-04-16
Period (4)
2020-04-17
2020-05-12
R1 OLR [g VS dm-3 d-1] 3.9 4.2 4.1-4.5 3.5-1.8-3.5-0.0
HRT [d] 13.1 13.2 13.2 16.5-32.9-16.5
R2 OLR [g VS dm-3 d-1] 3.9 4.7- 4.5 4.5-5.6 4.6-2.9-4.9
HRT [d] 13.1 12 12 12.5-19.9-11.9
R3 OLR [g VS dm-3 d-1] 3.9 4.7- 4.5 4.5-4.9 3.9-1.9-3.9
HRT [d] 13.1 12 12 15-29.9-15
R4 OLR [g VS dm-3 d-1] 3.9 12.6 - 8.3 4.9-5.3 4.2-2.6-4.2-0.0
HRT [d] 13.1 4.4 - 6.5 11 13.8-22-13.7
R5 OLR [g VS dm-3 d-1] 3.9 8.8 - 6.7 5.3-5.9 4.6-2.9-4.6-0.0
HRT [d] 13.1 6.3 - 8 10 12.5--20-12.5
R6 OLR [g VS dm-3 d-1] 3.9 6.5 - 5.4 5.6-6.2 4.9-3-4.9-0.0
HRT [d] 13.1 8.6 - 9.9 9.5 11.9-19-11.9
3.3 Analysis parameters and methods
During the experiment, primary sewage sludge from Käppala WWPT was collected and
thickened once every two weeks, to be analyzed for relevant parameters for the anaerobic
digestion process, like TS, VS, COD, CODs, pH, NH4-N, Kjeldahl-N. The first feeding and the
last digested sludge were also analyzed for fat and proteins to determine the fat and protein
content in the primary sludge. The raw sludge was kept at a temperature of 4˚C for the
feeding of the digesters. The digested sludge was daily analyzed for pH. The methane
concentration in the produced biogas was also daily measured. The digested sludge was
weekly analyzed for TS, VS, COD, CODs, NH4-N, VFA, and ALK.
3.3.1 pH, TS, and VS
The pH, TS, and VS were analyzed according to the standard method APHA et al., (1995)
before and after the digestion, to evaluate the physicochemical change in the sludge
characteristics.
3.3.2 Alkalinity
The alkalinity of the digested sludge was analyzed once a week through a titration robot
connected with an 896 Compact Sampler Changer (Metrohm, 2020). The digested sludge
was first centrifuged at 4000 rpm for 20 minutes (Thermo Scientific, 2020) to separate the
17
liquid part of the sewage sludge from the substantial part, and then analyzed by the titration
robot (see Figure 5). The titration robot mixed the sample with 0.05 M hydrochloric acid
(HCl) to reduce the pH of the sample to 5.4 to determine the BA. The pH of the sample was
reduced to 4.5 to determine the TA. BA and TA were then calculated using the following
equations 7 and 8 (SS-EN ISSO 9963-1 & SS-EN ISSO 9963-2 ) (Jarvis & Schnurer, 2009).
𝐵𝐴 = 380 ∗ 𝑉𝐻𝐶𝑙 Equation 7
𝑇𝐴 = 380 ∗ 𝑉𝐻𝐶𝑙 Equation 8
• BA is the bicarbonate alkalinity in [mgHCO3− dm−3].
• VHCl is the volume of the hydrochloric acid in [ dm−3].
• TA is the total alkalinity in [mgBasic ions dm−3].
Figure 5: Alkalinity analyses of the digested sludge by the titration robot. 50 mL digested sludge liquid in the different test tubes are being analyzed with a blue pH-meter, a white mixer, a nitrogen gas tube, and a 0.05 M hydrochloric acid (HCl) tube.
3.3.3 VFA, NH4-N, COD and CODs
The VFA of the digested sludge was analyzed, filtering the sludge with a suction filter with a
pore size of 0.45 μm Tisch Scientific, (2020). The filtered sludge was then analyzed for VFA
using LCK 365 cuvette test from HACH LANGE, which was later measured by a
spectrophotometer. The total chemical oxygen demand COD and the filtered CODs were
analyzed the same way as the VFA, except that the sample of the sludge analyzed for the total
COD was not filtered but diluted to a specific volume. The total COD and the filtered CODs
were analyzed using LCK 114 cuvette test from HACH LANGE, which was later measured by a
spectrophotometer. The ammonium NH4-N was analyzed the same way as the COD, with an
LCK 303 cuvette test from HACH LANGE.
3.3.4 Methane concentration
The methane concentration in the produced biogas was analyzed every day before the feeding
process. This was done taking 5 milliliters of the produced gas (with a 5 mL syringe) through
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the membrane gas sampling port, and injecting it into the NaOH solution containing the pH
indicator in an Einhorn pipe meter (see Figure 6). The percentage of the methane contained
in this biogas was then calculated using equation 9 below:
%𝐶𝐻4 =𝑉𝐶𝐻4
𝑉𝐵𝑖𝑜𝑔𝑎𝑠 Equation 9
• CH4 is the methane content in the produced biogas [%].
• 𝑉𝐶𝐻4 is the volume of methane read on the Einhorn pipe meter [mL].
• 𝑉𝐵𝑖𝑜𝑔𝑎𝑠 is the total volume of the sampled biogas [mL].
Figure 6: Measurement of methane concentration in the produced biogas, using the membrane gas sampling port and the NaOH solution containing the pH indicator, in an Einhorn pipe meter.
3.3.5 Free ammonium NH3-N
The amount of free ammonium NH3-N in the primary and the digested sludge was
determined from the measured NH4-N in the sludge, according to equation 10 Gallert &
Winter, (1997); Olsson et al., (2018).
𝑁𝐻3 −𝑁 = 𝑁𝐻4
+−𝑁∗10𝑝𝐻
𝑒(6344273+𝑇)+10𝑝𝐻
Equation 10
• 𝑁𝐻3 −𝑁 is the concentration of free ammonia in the digested sludge in [g L-1].
• 𝑁𝐻4+ −𝑁 is the concentration of free ammonium [g L-1].
• 𝑇 is the temperature in [oC].
• 𝑝𝐻 is the value of pH.
3.3.6 Nitrogen mineralization
The nitrogen mineralization was determined using the equation below:
𝑀𝐿 = ((𝑁𝐻4
+−𝑁)𝐷𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒
−(𝑁𝐻4+−𝑁)
𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒
(𝑁𝑂𝑟𝑔)𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝑠𝑙𝑢𝑑𝑔𝑒) ∗ 100 Equation 11
19
• 𝑀𝐿 is the nitrogen mineralization [%].
• (𝑁𝐻4+ − 𝑁)Digested sludge is the concentration of free ammonium in the digested sludge [g L-1].
• (𝑁𝐻4+ − 𝑁)Primary sludge is the concentration of free ammonia in the primary sludge [g L-1].
• (𝑁𝑂𝑟𝑔)Primary sludge is the organic N in the primary sludge [g L-1].
3.3.7 Degree of degradation
The degree of degradation was determined according to Schnurer & Jarvis, (2017).
𝐷𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 = 𝛼 = (1 − (𝑇𝑆𝑂𝑢𝑡∗𝑉𝑆𝑂𝑢𝑡
𝑇𝑆𝐼𝑛∗𝑉𝑆𝐼𝑛)) ∗ 100 [%] Equation 12
• 𝛼 is the degradation degree in [%].
• 𝑇𝑆𝑂𝑢𝑡 is the TS amount in the digested sludge in [%].
• 𝑉𝑆𝑂𝑢𝑡 is the amount of VS in the digested sludge in [%].
• 𝑇𝑆𝐼𝑛 is the TS amount in the primary sludge in [%].
• 𝑉𝑆𝐼𝑛 is the amount of VS in the primary sludge in [%].
3.3.8 Process stability
The process stability was examined, looking at the ratio between VFA and TA as proposed by
Svenskt Vatten, (2019).
• 𝑉𝐹𝐴
𝑇𝐴< 0.3 stable process.
• 𝑉𝐹𝐴
𝑇𝐴= 0.3 𝑡𝑜 0.5 small instability in the process.
• 𝑉𝐹𝐴
𝑇𝐴> 0.5 real instability in the process.
• 𝑉𝐹𝐴
𝑇𝐴> 1 There is a significant risk for a sudden reduction of gas production.
• 𝑉𝐹𝐴 is the volatile fatty acid in [mg L-1].
• 𝑇𝐴 is the total alkalinity in [mg L-1].
3.4 Theoretical methane potential
For the estimation of the theoretical methane yield in the primary feeding sludge and the
digested sludge, the substrates were analyzed for lipids, protein, and carbohydrates. The
protein content was determined using the Kjeldahl method for organic nitrogen analysis,
according to Väänänen & Koivistoinen, (1996). The Kjeldahl method for organic nitrogen
analysis consists of multiplying the deducted nitrogen content by 6.25, which is the
conversion factor used for the protein determination in food samples.
The primary purpose of this was to evaluate how the different components in the primary
feeding sludge had contributed to the production of the generated methane gas. The purpose
was also to investigate the wasted methane potential, in connection with the OLR and HRT,
in the digesters. According to the German Standard Verein Deutscher Ingenieure, (2006),
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lipids in primary sludge produce 1000.8 NmL g-1 VS-1 methane, protein produce 480 NmL g-1
VS-1 methane, and carbohydrates generate 375 NmL g-1 VS-1 methane. The theoretical
methane production values from the different substrates were determined from the given
theoretical biogas yield (see Table 4).
Table 4 Theoretical methane yield assumption
Substrate type
Theoretical biogas
yield [NL kg-1 VS-1]
Theoretical CH4/CO2 composition [%/Volume]
Carbohydrate 750 50%CH4 50%CO2
Fats (Lipids) 1390 72%CH4 28% CO2
Proteins 800 60%CH4 40% CO2
The carbohydrates, estimated as the remaining portion of organic material in each substrate,
were determined using equation 13, the theoretical methane potential in the sludge using
equation 14, and the methane yield (or specific methane production) using equation 15.
𝐶𝑎𝑟𝑏𝑜ℎ𝑦𝑑𝑟𝑎𝑡𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 [𝑊%] = 100[𝑊%] − 𝐻2𝑂[𝑊%] − 𝐼𝑛𝑜𝑟𝑔𝑎𝑛𝑖𝑐 𝑐𝑜𝑛𝑡𝑒𝑛𝑡[𝑊%] − 𝐿𝑖𝑝𝑖𝑑𝑠[𝑊%] − 𝑃𝑟𝑜𝑡𝑒𝑖𝑛𝑠[𝑊%]
𝐶𝑎𝑟𝑏𝑜ℎ𝑦𝑑𝑟𝑎𝑡𝑒 𝑐𝑜𝑛𝑠𝑡𝑒𝑛𝑡 [𝑊%] = 𝑉𝑆 − 𝐿𝑖𝑝𝑖𝑑𝑠[𝑊%] − 𝑃𝑟𝑜𝑡𝑒𝑖𝑛𝑠[𝑊%] Equation 13
𝐶𝐻4_𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙_𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 = 𝐿𝑖𝑝𝑖𝑑𝑠[𝑊%] ∗ 𝐵𝑖𝑜𝑔𝑎𝑠𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙𝑖𝑛𝐿𝑖𝑝𝑖𝑑∗ 72%+ 𝑃𝑟𝑜𝑡𝑒𝑖𝑛𝑠[𝑊%] ∗ 𝐵𝑖𝑜𝑔𝑎𝑠𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙𝑖𝑛𝑃𝑟𝑜𝑡𝑒𝑖𝑛
∗
60%+ 𝐶𝑎𝑟𝑏𝑜ℎ𝑦𝑑𝑟𝑎𝑡𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 [𝑊%] ∗ 𝐵𝑖𝑜𝑔𝑎𝑠𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙𝑖𝑛𝐶𝑎𝑟𝑏𝑜ℎ𝑦𝑑𝑟𝑎𝑡𝑒∗ 50% Equation 14
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐶𝐻4 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 =𝑉𝐶𝐻4
𝑄𝑉𝑆_𝑖𝑛 Equation 15
𝑄𝑉𝑆_𝑖𝑛 = 𝑄𝑖𝑛 ∗ 𝑇𝑆 ∗ 𝑉𝑆 Equation 16
• 𝐶𝑎𝑟𝑏𝑜ℎ𝑦𝑑𝑟𝑎𝑡𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 is the carbohydrate content in the sewage sludge [%]
• 𝑉𝑆 = 100[𝑊%] − 𝐻2𝑂[𝑊%] − 𝐼𝑛𝑜𝑟𝑔𝑎𝑛𝑖𝑐 𝑐𝑜𝑛𝑡𝑒𝑛𝑡[𝑊%] is the volatile solids content.
• 𝑃𝑟𝑜𝑡𝑒𝑖𝑛𝑠[𝑊%] = 𝑁𝑂𝑟𝑔 ∗ 𝐹 is the protein content in the sludge. And 𝐹 here is the conversion
factor used for protein determination in food samples.
• Lipids[W%] is the fat content in the sludge.
• 𝑁𝑂𝑟𝑔 is the organic nitrogen content in the substrate
• CH4_Potential_Theoretical is the total theoretical methane potential in the sludge [NmL g-1 VS-1].
• BiogasPotentialinLipid is the theoretical biogas potential in the lipid in the sludge [NmL g-1 VS-1].
• BiogasPotentialinProtein is the theoretical biogas potential in the proteins in the sludge [NmL g-1 VS-1].
• BiogasPotentialinCarbohydrate is the theoretical biogas potential in carbohydrates in sludge [NmL g-1 VS-1].
• 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐶𝐻4 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑 is the specific methane production per gram fed VS [𝑁𝑚𝐿 ∗ 𝑔−1𝑉𝑆−1]
• VCH4 is the volume of the produced methane in [𝑁𝑚𝐿]
• QVS_in is the quantity of primary sludge for feeding the digesters in 𝑔 𝑉𝑆 𝑑−1
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3.5 Mass balance
The mass balance between the fed sewage sludge, the produced methane, and the remaining
organic matter in the digested sludge was determined by analyzing the substrate composition
in both the primary sludge (at the beginning of the experiment) and the digested sludge (at
the end of the experiment).
[𝑘𝑔] 𝑉𝑆𝑖𝑛 = [𝑘𝑔] 𝑉𝑆𝐶𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 𝑖𝑛𝑡𝑜 𝐶𝐻4 + [𝑘𝑔] 𝑉𝑆𝑖𝑛 𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒 →
[𝑘𝑔]𝑉𝑆𝐶𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 𝑖𝑛𝑡𝑜 𝐶𝐻4 = [𝑘𝑔] 𝑉𝑆𝑖𝑛 − [𝑘𝑔] 𝑉𝑆𝑖𝑛 𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒 Equation 17
• [𝑘𝑔]𝑉𝑆𝐶𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 𝑖𝑛𝑡𝑜 𝐶𝐻4 is the theoretical methane yield converted to methane through the
anaerobic digestion.
• [𝑘𝑔] 𝑉𝑆𝑖𝑛 is the theoretical methane yield in the primary feeding sludge [kg].
• [𝑘𝑔] 𝑉𝑆𝑖𝑛 𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒 is the theoretical methane yield in the digested sludge in [kg].
The mass balance between the theoretical methane yield in the feeding sludge and the
average specific methane potential achieved by the different reactors was also determined to
investigate how much of the theoretical methane yield in the feeding sludge that was actually
produced by the digesters (see equation 18).
𝐶𝐻4𝑇𝑟𝑎𝑛𝑠𝑓𝑜𝑟𝑚𝑒𝑑 = 1 −𝐶𝐻4𝑡ℎ𝑒𝑜_𝑃𝑆−𝐶𝐻4𝑆𝑝𝑒𝑐𝑖𝑝ℎ𝑖𝑐
𝐶𝐻4𝑡ℎ𝑒𝑜 Equation 18
• 𝐶𝐻4𝑇𝑟𝑎𝑛𝑠𝑓𝑜𝑟𝑚𝑒𝑑 is the transformed methane yield in [%].
• 𝐶𝐻4𝑡ℎ𝑒𝑜_𝑃𝑆 is the theoretical methane yield in the feeding sludge in [N-L kg-1 VS-1].
• 𝐶𝐻4𝑆𝑝𝑒𝑐𝑖𝑝ℎ𝑖𝑐 is the average specific methane potential of the reactors in [N-L kg-1 VS-1].
3.6 Forecasting of the future OLR at Käppala
To find out when a third digester will be needed at Käppala, in order to be able to digest the
future high OLR, a forecasting model was used. The model, which was based on Käppala’s
prediction model, was built in excel, assuming the future increase in the number of people
connected to the Käppala WWTP. The assumption was based on the increase in the number
of people during recent years. With this data, the specific amount of primary sludge produced
per person could be calculated using the equation below:
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐_𝑃𝑆𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑠/𝑝 =𝑇𝑜𝑡𝑎𝑙_𝑃𝑆𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑠
𝑁𝐶𝑢𝑠𝑡𝑜𝑚𝑒𝑟 Equation 19
𝑇𝑜𝑡𝑎𝑙𝑃𝑆𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑠 =𝑇𝑆𝑡ℎ𝑖𝑐𝑘𝑒𝑛𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒
100∗ 𝑄𝑖𝑛 Equation 20
• 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐_𝑃𝑆𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑠/𝑝 is the specific amount of sludge produced per person a day in [g d-1].
• 𝑇𝑜𝑡𝑎𝑙𝑃𝑆𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑠 is the total produced primary sewage sludge at Käppala per day in [g d-1].
• 𝑁𝐶𝑢𝑠𝑡𝑜𝑚𝑒𝑟: is the number of people connected to the Käppala WWTP.
• 𝑇𝑆𝑡ℎ𝑖𝑐𝑘𝑒𝑛𝑒𝑑 𝑠𝑙𝑢𝑑𝑔𝑒 is the TS amount in the thickened sludge of this pilot experiment in [%].
• 𝑄𝑖𝑛: is the inflow of the thickened sludge into the full-scale digester R100 [g d-1].
• OLR, HRT and 𝑄𝑖𝑛 in the model were determined as described in equations 1 and 4.
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3.7 Process aid
To determine which measures can be taken to utilize the existing digester R100 to the
maximum under maximum load at Käppala WWTP, an experiment was done on pilot reactor
R4. R4 was the reactor that showed a significant instability in the process. The idea was to
test R5 and R6 as well, but due to process failure in these reactors, this was not possible. The
test was done, letting R2 proceed with the same ORL and HRT as R4. Then the chemical
additive Sodium Carbonate (Na2CO3) was used once a week to investigate if the anaerobic
digestion in reactor R4 could recover even if the high OLR and short HRT were kept. Sodium
carbonate Na2CO3, also known as washing soda or soda ash, has a strong bitter taste and
gives an alkaline solution in water Rajesh et al., (2019). According to Svenskt Vatten, (2019),
𝑁𝑎2𝐶𝑂3 can be used as an anaerobic process aid if the process pH is not less than 5. In the
digester, the sodium carbonate is decomposed as presented in Reaction 1 and the free 𝐶𝑂32−
reacts with the 𝐶𝑎2+ in the anaerobic digestion to form calcium carbonate 𝐶𝑎𝐶𝑂3 (see Reaction
2), which raised the alkalinity and the pH of the digester.
𝑁𝑎2𝐶𝑂3𝐻2𝑂→ 2𝑁𝑎+ + 𝐶𝑂3
2− Reaction 1
𝐶𝑎2+ + 𝐶𝑂32− → 𝐶𝑎𝐶𝑂3 Reaction 2
1000 milligram 𝐶𝑎𝐶𝑂3 per liter of the substrate was added to digester R4 once a week. The
needed amount of Sodium carbonate for this was determined as follows:
𝑚𝑁𝑎2𝐶𝑂3=𝑀𝑁𝑎2𝐶𝑂3∗(𝑚𝐶𝑂3
2−)𝑖𝑛 𝑅𝑒𝑎𝑐𝑡.2
𝑀𝐶𝑂3
2−=𝑀𝑁𝑎2𝐶𝑂3∗(
𝑚𝐶𝑎𝐶𝑂3∗𝑀
𝐶𝑂32−
𝑀𝐶𝑎𝐶𝑂3)
𝑀𝐶𝑂3
2− Equation 21
• 𝑚𝑁𝑎2𝐶𝑂3 is the amount in the mass of Sodium Carbonate in [g].
• (𝑚𝐶𝑂32−)𝑖𝑛 𝑅𝑒𝑎𝑐𝑡.2 is the mass of the free carbonate ion in the reaction 2 [g].
• 𝑀𝑁𝑎2𝐶𝑂3 is the amount in molar mass of Sodium Carbonate in [g mol-1].
• 𝑀𝐶𝑂32− is the amount in molar mass of the free carbonate ion in [g mol-1].
• 𝑀𝑁𝑎2𝐶𝑂3 is the molar mass of sodium carbonate in [g mol-1].
• 𝑀𝐶𝑎𝐶𝑂3 is the molar mass of calcium carbonate in [g mol-1].
3.8 Dewaterability study of digested sludge
At the end of the experiment, a dewaterability study was performed to investigate the
filterability of the digested sludge. The aim was to evaluate how the sludge dewatering
properties were affected by a change in OLR and HRT in the digesters. According to
Tchobanoglous et al., (2014), the measuring of the filterability index in the digested sludge
helps determine the amount of polyelectrolyte required for the conditioning of sewage
sludge.
This filtration test was done measuring the sludge capillary suction time (CST) for four
different samples of digested sludge from four reactors with different OLR and HRT values.
The instrument Triton Electronics, type multi-CST with Whatman No. 17 filter paper, was
used for the filtration test. When performing the test, 2 grams of the polymer was mixed with
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1000 mL water for one hour. During the experiment, different amounts of the polymer
solution were applied on the digested sludge to investigate the optimal volume that
performed well with the studied sludge, making the sludge lump together and leave water
behind. The test resulted in the chosen amount of 22 mL of polymer solution for the analyses
of 100 mL sludge. The CST was measured after vigorous mixing of the above polymer and
sludge solution for 40, 60, and 100 seconds. Each test was repeated five times for each
sample (see Figure 7).
In order to reduce the volume of the sludge after digestion, the sludge is dewatered. Above
all, this is done to decrease transport expenses, a process that requires energy. At the time of
purchase of the dewatering centrifuges, it was stated that the energy used is 28 kWh ton-1TS_in
Médoc & Sobrio, (2015). In (Figure 7) below, the Triton Electronics instrument is connected
to five cylinders containing a mix of sludge and polymer, and underneath five Whatman No.
17 filter papers. The mixer in the top picture to the right was used for the mixing. The bottom
pictures show the filter papers after the CST measurement.
Figure 7: CST measurements.
3.9 Description of the anaerobic digestion at Käppala WWTP
The Käppala wastewater treatment plant (WWTP) at Lidingö in Stockholm purifies
wastewater from approximately 550,000 inhabitants in the northeast of Stockholm. In the
various stages of the wastewater treatment process, different types of sludge are collected,
with varying content of organic material. The accumulated sludge is digested, producing
biogas through a mesophilic anaerobic digestion process (Käppala Association, 2020). The
produced biogas is then upgraded to vehicle fuel for busses in the local traffic since the
production of vehicle fuel is one of the most environmentally friendly ways of using biogas. It
not only replaces fossil fuel that contributes to global warming through greenhouse gas but at
the same time, it takes care of the increasing volume of wastewater sludge. Furthermore, the
24
expansion of biogas production systems is an essential contributor to the global conversion
from fossil fuels to renewable energy systems (Tchobanoglous & Burton, 2014).
The plant started operating in 1969. With a 65-kilometer tunnel system carrying wastewater
to the plant, it is the third-largest wastewater plant in Sweden. Approximately 50 million
cubic meters of wastewater, from 11 municipalities, are treated at Käppala every year
(Käppala Association, 2020). Käppala WWTP has a conventional purification process, with
both mechanical, chemical, and biological treatment (Käppala 2018).
The wastewater treatment process consists of five steps; the pretreatment, the primary
sedimentation, the biological treatment, the secondary clarifier, and the sand filtration (see
Figure 8):
• During the pretreatment, debris is removed from the sewage sludge with rotating step screens.
Sand and grit are settled on the bottom of the aerated grit chambers. (see point 1 in Figure 8).
Parallelly the odor is treated, collecting the ventilation air from the screens, the grit chambers,
and the primary sedimentation tanks, treating it with UV light and activated carbon filters.
The treated air is then released through a 149,5 m high chimney (see point 2 in Figure 8).
• In the primary sedimentation tank (see point 4 in Figure 8), the produced primary sludge is
collected with sludge scrapers and thickened, and then sent to the digester for biogas
production.
• The wastewater continues into the biological treatment tank (see point 5 in Figure 8), where
the remaining organic material in the wastewater is consumed by microorganisms to produce
phosphorus bounded biological sludge. The process is done controlling the living conditions
for the microorganism’s nutrients, removing nitrogen and phosphorus. The nitrogen is
released to the atmosphere transforming it into nitrogen gas, and the phosphorus is bound to
the biological sludge. In this step, iron sulfate is also added to chemically precipitate
phosphorus. The precipitate is then separated in the next step.
• In the secondary clarifier basin (see point 6 in Figure 8), the produced biological and chemical
sludge is settled to the bottom of the basin and collected to be sent partly back to the biological
treatment tank. A part of the biological sludge, the excess sludge, is pumped to the anaerobic
digesters to produce biogas. At point 8 in Figure 8, part of the phosphorus that is not removed
in the biological treatment is precipitated with ferrous sulfate.
• Finally, iron sulfate is added to precipitate the last phosphorus, and the water continues into
the sand filter basin (see point 7 in Figure 8), where the remaining particulate matter is
filtered from the water before the clean water is released at a depth of 45 meters in the
Stockholm archipelago.
• The sludge from the pre- and post-sedimentation tanks is pumped into the digesters (see point
9 in Figure 8) to produce biogas that is upgraded to vehicle fuel. There are thee digesters at
Käppala WWTP today, R100, R200, and R300, which generate biogas parallelly. The primary
sludge is digested in the first digester, R100, until 2020. Then R300 was put into operation as
the first digester, and R100 was emptied for renovation. R200 digests the already digested
sludge from R100/R300 together with the excess sludge.
• The produced biogas contains 60-65% methane gas and 30-35% carbon dioxide. In the vehicle
gas plant (see point 10 in Figure 8), the carbon dioxide is removed, using a water scrubber,
upgrading the biogas into vehicle fuel with a minimum of 97% methane.
• The digested sludge is dewatered in a chemical process where it is treated with polymer (see
point 11 in Figure 8). The sludge is then quality checked and used as agricultural fertilizer.
25
• A part of the purified wastewater is lead to a heat pump to recover heat from it to use
internally in the WWTP to heat the sludge in the digester. The excess part is then delivered to
the district heating system (see point 13 in Figure 8).
Figure 8 Process chart over Käppala WWTP used with permission Kappala (2011)
26
4 RESULT
4.1 Substrate analyses
Table 5 presents the composition of the substrates in the feeding, and the digested sludge,
where the total ratio of carbon and nitrogen C/N is 14.8. The feeding sludge in 6.64 kg ton-1 in
lipid, which was calculated to 14.66% kg-1 VS-1, 20.70 % kg-1 VS-1 in protein, and 64.64% kg-1
VS-1 in carbohydrates. The theoretical methane potential in the substrate used in this
experiment, when it comes to the content of carbohydrates, proteins, and lipids, was
determined to 488.48 NmL g-1 VS-1 for the primary sludge (see section 3.4).
Table 5 Substrate composition in the primary sludge and the digested sludge
Parameters Primary Sludge Digested sludge Unit
R1 R2 R3 R4 R5 R6
Dry Substance TS 5.2 - 6.72 4.80 5.10 4.90 4.10 8.80 6.70 [%]
VS 87.10-89.09 73.68 85.99 82.74 79.24 83.22 82.27 [%]
pH 5.37-5.55 6.15 5.18 4.99 - 4.93 5.45 [-]
Total Nitrogen N-
Tot
1.60 2.70 2.60 2.80 2.60 3.60 3.0 [kg ton-1]
Organic Nitrogen
Norganic
1.50 1.40 1.30 1.50 1.30 2.60 1.50 [kg kg-1]
Ammonium
Nitrogen (NH4-N)
0.10 1.30 1.30 1.30 1.30 1 1.50 [kg ton-1]
Total carbon 24.20 23.50 24.60 24.10 20.40 44 32.30 [kg ton-1]
Tot-C / Tot N 14.80 8.60 9.40 8.60 8 12.3 10.7 [-]
Total phosphorus 0.37 0.63 0.54 0.59 0.57 0.83 0.63 [kg ton-1]
Total potassium 0.16 0.18 0.16 0.15 0.16 0.17 0.16 [kg ton-1]
Total magnesium 0.11 0.12 0.11 0.11 0.11 0.15 0.13 [kg ton-1]
Total calcium 0.80 0.99 0.9 0.93 0.90 1.37 1.14 [kg ton-1]
Total Sodium 0.09 0.08 0.08 0.08 0.78 0.10 0.09 [kg ton-1]
Total sulfur 0.28 0.42 0.41 0.40 0.37 0.72 0.47 [kg ton-1]
Fat 6.64 1 0.91 1.03 1 1.95 1.42 [kg ton-1]
CODs 1, 813 – 5,603 22,800 23,733.33 27,800 23,633.33 29,700 23,466.67 [mg L-1]
COD 70,633 – 92, 000 70,700 38,700 41,433.33 65,133.33 93,100 75,300 [mg L-1]
NH4-N 52.60 1,593.33 1,246.67 1,353.33 1,403.33 1,046.67 1,383.33 [mg L-1]
4.2 The anaerobic pilot-scale experiment
4.2.1 OLR and HRT in the six pilot-scale reactors
Figure 9ab illustrates the performance when it comes to OLR and HRT for digesters (R1 to
R6) during the whole experiment. In the beginning, during period 1, all the reactors were
performing at the same OLR and HRT values. The slight increase in OLR on February 13th is
27
due to the TS and VS levels in the new feeding sludge taken from Käppala WWTP on
February 13th. Between February 17th and March 2nd, the reactors performed at a higher OLR
and a shorter HRT than the original experiment protocol, as mentioned earlier (see section
3.2.1). This resulted in a process imbalance in digesters R4, R5, and R6 (see Figure 10ab).
The OLR and HRT were then changed in reactors R4 to R6, with an ambition to fulfill the
plan for the experiment. During the 4th period, the OLR was decreased from April 13th to see
if the stability could be regained, but it did not succeed. Between April 17th and 21st, the
feeding was stopped entirely because of total process failure. On April 22nd, the feeding
started again, but with only half of the OLR level. Between May 1st and 12th, the feeding was
also stopped because of process imbalance (see Figure 9a). According to this result (see
Figure 9a around April 13th) the OLR decreases from 4.4 [g VS dm-3 d-1] in R1, 5.6[g VS dm-3
d-1] in R2, 4.9 [g VS dm-3 d-1] in R3, 5.3[g VS dm-3 d-1] in R4, 5.8[g VS dm-3 d-1] in R5 and 6.1[g
VS dm-3 d-1] in R6.
1.5
3.5
5.5
7.5
9.5
11.5
13.5
15.5
OLR
[g
VS
dm
-3d
-1]
Time [d]
(a) OLR [g VS dm-3 d-1]
R1 R2 R3 R4 R5 R6
0.2
5.2
10.2
15.2
20.2
25.2
30.2
35.2
40.2
[d]
Time [d]
(b) HRT [d]]
R1 R2 R3 R4 R5 R6
Figure 9 Organic loading rate (OLR) [g VS/day] and hydraulic retention time (HRT).
28
4.3 Stability analysis of the process
The anaerobic digestion process stability data shows how the stability of the digesters was
affected by variations in OLR and HRT during the experiment.
4.3.1 CH4 and pH
The extremely high OLR and short HRT in digesters R4, R5, and R6 (during period 2), at the
beginning of the change in OLR and HRT, caused process imbalance in the reactors. This
imbalance in the digestion process was easily noticed due to the low pH value and the low
methane concentration in reactors R4 and R5 during the second period (see Figure 9ab,
Figure 10ab). The pH level dropped immediately when the OLR increased on February 17th to
a level of 5.45 in reactors R4 and R5 (see Figure 10a). The same thing happened to the CH4
concentration, dropping to a level of 25% in both reactors (see Figure 10b). During this
period, the OLR level in R4 was 12.6 g VS dm-3 d-1 and in R5 8.8 g VS dm-3 d-1. This drop in
pH and CH4 concentration took place during only three to four days (see Figure 10ab). In R6,
a similar development took place, only at a slower pace. The difference in reactor R6 was that
it maintained stability for ten days before the stability indicator values started to drop. The
pH started dropping after February 24th when the pH value was 7.29 (see Figure 10a). Four
days later, it was down to 5.38. The same thing happened with the CH4 concentration. On
February 24th, the CH4 concentration was 74.67%. Two days later, the concentration was only
50.67% (see Figure 10b).
In digester R2, which increased in OLR gradually during the experiment, the pH and the
methane concentration started to decrease around March 23rd and 25th. During this time, the
pH level in R2 decreased from 7.25 to 5.79, and the methane concentration from 76.53% to
40% (see Figure 10ab). An interesting thing to notice here is that R2 increased in OLR from
4.9 to 5.4 already a week earlier (see Figure 9a).
During Period 4, an extreme imbalance was observed in all the reactors. This general process
instability was also seen in the low pH value in all digesters except R4, which increased
slightly in pH due to the addition of chemical process aid (see Figure 10a). The methane
concentration in all the reactors also indicated a digestion imbalance during period 4 (see
Figure 10b). An important observation to notice here is that the pH and the methane
concentration for R1 and R3 followed each other during the whole experiment. The pH and
methane concentration in digesters R1 and R3 also decreased simultaneously, showing
imbalance in the two digesters around April 7th and April 17th.
29
4.3.2 VFA and ratio VFA/TA
Figure 11 presents the VFA and the VFA/TA ratio development during the experiment for
reactors R1, R2, R3, and R6. They were the only four reactors analyzed continuously during
the whole experiment, due to a lack of project resources. The VFA levels remained stable at a
low level in all digesters at the beginning of the process, when all the reactors had the same
OLR and HRT. The VFA/TA ratio that has been used in several studies to investigate the
anaerobic digestion process stability was also on a stable level (less than 0.3) for all the
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
pH
pH
R1 R2 R3 R4 R5 R6
20%
30%
40%
50%
60%
70%
80%
90%
CH
4 [
%]
Time [d]l
CH4
R1 R2 R3 R4 R5 R6
Figure 10 pH and CH4 of the digesters during the experiment.
30
digesters. On March 5th, the VFA levels suddenly raised rapidly in R6 (see Figure 11a). It can
be seen in Figure 11ab that the digester R6 managed to keep the balance for ten days before
the imbalance started, even with a high OLR and short HRT. Since measurements were made
on February 27th, it is known that the VFA levels were still in balance then. It is somewhere
between February 27th and March 5th that the imbalance started and raised rapidly up to a
VFA level of 6780 mg L-1 and a VFA/TA level of 3.2, which is far beyond recommended levels
by (Svenskt Vatten, 2019) to keep a stable process (see section3.3.8). In R2, the same thing
happened around March 19th when R2 increased to an OLR of 5.4 March 16th (see Figure 9ab
and Figure 11a). The VFA and VFA/TA levels increased in digesters R1 and R3 after April 7th,
at the end of the experiment. This can also be seen in the pH and CH4 diagram since their
levels dropped as the level of organic acids in the process increased. An imbalance in the
digestion process was observed here in all the reactors.
0
5 000
10 000
15 000
20 000
25 000
30 000
35 000
40 000
03/02/2020 03/03/2020 03/04/2020 03/05/2020
[mg
L-1
]
Time [d]
(a) VFA
R1 R2 R3 R6
0
1
2
3
4
5
6
7
Time [d]
(b) Ratio VFA/TA
R1 R2 R3 R6
Figure 11 The VFA level and the ratio VFA/TA during the pilot-scale experiment.
31
4.3.3 Free ammonia NH3-N (FAN)
Figure 12 illustrates how the anaerobic process instability parameter, ammonia (FAN),
performed evaluating the stability in the digesters during the experiment. During period 1,
when all reactors performed at the same OLR and HRT, the FAN levels differed despite this
in all reactors. They kept doing so during the whole experiment, except during period 4 at the
end, when all the reactors crashed. An important observation to notice here is that the FAN
level decreased to zero when there was a process imbalance or process failure. In R2, the
ammonia started decreasing from a FAN level of 26.53 to 1.26 mg L-1 during March 16th and
23rd (see Figure 12). A similar observation was also made initially and at the end of the
experiment when the FAN level in R6 decreased on February 24th and in R1 and R3 on March
30th.
Figure 12 Ammonia in the different digesters during the pilot-scale experiment
4.4 Energy potential and performance of the experiment
4.4.1 Methane yield in the pilot-scale experiment
The methane yield and the theoretical methane yield in the primary sludge (488.48 [NL kg-1
VS-1]) were determined in this study to investigate how much of the theoretical energy
potential that was converted to methane by the pilot digesters.
The diagrams in Figure 13a present the specific methane production per gram VS, which
shows how the different reactors in the pilot-scale experiment and the full-scale reactor R100
performed when it comes to gas production. All the six reactors performed well, compared to
the methane yield in the full-scale reactor R100, until the start of the change in OLR and
HRT during period 2. During period 2, when the digestion process instability was observed in
reactors R4, R5, and R6 due to the raise in OLR (see section 4.3), the accumulated methane
production in reactors R4 and R5 immediately stopped and remained almost constant (see
0
10
20
30
40
50
60
70
NH
3-N
[m
g L-
1]
Time [d]
R1 R2 R3 R6
32
Figure 13b). The accumulated methane production in R6 raised at first to a level of 71 760
NmL and then remained almost constant from February 23rd. Reactor R3, on the other hand,
raised in accumulated methane production steadily during the whole process, up to levels
beyond 190, 569 NmL, followed by R1, until all reactors failed at the end of the experiment
(April 12th to 13th). Methane yield and accumulated methane produced in R1 and R3 were
also affected by the process failure during period 4. The methane yield in R1 and R3
decreased around April 13th to 19th. Moreover, the accumulated methane produced by these
two reactors also stopped increasing around this time.
The average methane yield was also determined for R1 (322.78 NmL g-1 VS-1), R2 (236.04
NmL g-1 VS-1), R3 (308.0 NmL g-1 VS-1), and R6 (89.96 NmL g-1 VS-1) for periods 2 and 3.
Period 1 was excluded here since all the digesters had the same OLR and HRT during this
period. Moreover, the purpose was to investigate how the methane production was affected
by the different OLR and HRT values in the digesters. Including the 4th period during which
the digestion process failed, and no methane was produced, the average methane yield would
be 247.77 NmL g-1 VS-1 for R1, 180.74 NmL g-1 VS-1 for R2, 234.83 NmL g-1 VS-1 for R3, and
77.3 NmL g-1 VS-1 for R6.
33
4.4.2 The degree of degradation
The degree of degradation for reactors R1, R2, R3, and R6 was determined to evaluate the
effect of the changes in OLR and HRT values during the experiment. The result of this is seen
050
100150200250300350400450
Spec
ific
CH
4 [
Nm
L g-
1 V
S-1
]
Time [d]
(a) CH4 METHANE YIELD
R1 Specific CH4 production [NmL g-1 VS-1] R2 Specific CH4 production [NmL g-1 VS-1]
R3 Specific CH4 production [NmL g-1 VS-1] R4 Specific CH4 production [NmL g-1 VS-1]
R5 Specific CH4 production [NmL g-1 VS-1] R6 Specific CH4 production [NmL g-1 VS-1]
R100 Specific CH4 (Aver.) [NmL g-1 VS-1]
0
50000
100000
150000
200000
250000
[Nm
L]
Time [d]
Accumulated Methane Production
Reactor 1 Volume [NmL] Reactor 2 Volume [NmL] Reactor 3 Volume [NmL]
Reactor 4 Volume [NmL] Reactor 5 Volume [NmL] Reactor 6 Volume [NmL]
Figure 13 Specific methane production (methane yield) and the accumulated methane produced
34
in Figure 14, where the degree of degradation decreases with the HRT value, as the OLR
value increases. This result follows the same tendency observed in the results of the early part
of this study, namely that the increase in OLR causes process imbalance (see R4 to R6),
which in turn affects the degree of degradation negatively. According to this result, the degree
of degradation in R2 decreased from 58.06% to 22.84% between March 19th and April 2nd.
Moreover, the degree of degradation in R1 and R3 also followed R2 at the beginning of period
4.
Figure 14 Degree of degradation for reactors R1, R2, R3, and R6.
4.4.3 Chemical oxygen demand COD
Total and soluble chemical oxygen demands were analyzed for both the primary and the
digested sludge during the experiment. The measurement of the COD content indicates the
amount of oxygen required to break down a certain amount of organic material. The result, as
presented in Figure 15, shows how the chemical oxygen demand (COD) value in both the
feeding sewage sludge and the digested sludge varies during the experiment period. All the
reactors’ CODs curves follow the same tendency at the beginning before the OLR and HRT
values were changed on February 17th. After that, the CODs value in R6’s digested sludge
increased dramatically, followed by the CODs value of R2’s digested sludge later, when R2
increased in OLR value up to 5.4 g VS dm-3 d-1 during period 3. On April 12th, the CODs value
in R1 and R3’s digested sludge followed the same pattern, due to the anaerobic process
imbalance in all the digesters (period 4). This same observation was made for the total COD
diagram too. These results illustrate how the chemical oxygen demand in the reactors
depends on the digestion process stability. The COD level in the digestion process increases
during process instability or process failure in the digesters. This can be confirmed by the
result of the degree of degradation, which increases as the COD decreases and vice versa (see
Figure 14).
0%
10%
20%
30%
40%
50%
60%
70%
[%/W
eek]
Time [d]
R1 R2 R3 R6
35
4.4.4 Mass balance
The mass balance was determined based on the theoretical methane yield in both the feeding
sludge and the digested sludge. The purpose was to investigate how much of the theoretical
methane yield in the feeding sludge that remained in the digested sludge from the different
digesters. The diagram below illustrates in percentage how much of the theoretical methane
yield in the feeding sludge that was converted into methane (see Figure 13 Figure 16).
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
CO
D [
mg
L-1]
Time[d]
(a) COD(s) for PREMARY & DIGESTED SLUDGE
R1 R2 R3 R6 CODs of PS
21,000
31,000
41,000
51,000
61,000
71,000
81,000
91,000
101,000
CO
D [
mg
L-1]
Time [d]
(b) Total COD for PREMARY & DIGESTED SLUDGE
R1 R2 R3 R6 COD of PS
Figure 15 CODs, and total COD in primary and digested sludge during the experiment.
36
Figure 16 Theoretical methane content converted into methane gas
To investigate how much of the theoretical methane yield in the feeding sludge that was
produced by the digesters, another mass balance calculation was performed. The calculation
made a comparison between the theoretical methane yield in the feeding sludge and the
methane yield achieved by the different reactors (see Figure 17). The diagram below
illustrates in percentage how much of the theoretical methane yield in the feeding sludge that
was converted into methane by the digesters. This result also shows that digesters R1 and R3
performed better than the rest. Reactors with lower OLR performed better than the ones with
higher OLR. R4 generated less methane than the others because it had the highest OLR at the
beginning of the experiment (period 2).
Figure 17 Comparison of the theoretical and the specific methane yield
0%
2%
4%
6%
8%
10%
12%
14%
16%
18%
20%
R1 R2 R3 R4 R5 R6
Co
nve
rted
CH
4 [
%]
Digesters
R1 R2 R3 R4 R5 R6
0%
10%
20%
30%
40%
50%
60%
70%
R1 R2 R3 R4 R5 R6
Co
mp
arat
ion
of
CH
4_t
heo
an
d C
H4
_sp
ecif
ic [
%]
R1 R2 R3 R4 R5 R6
37
4.5 Result of the Chemical additive
The chemical additive Sodium Carbonate (Na2CO3) was used once a week to investigate if
the anaerobic digestion in reactor R4 could recover, still keeping the high OLR and short
HRT. Around 2.13 g L-1 of the process aid, Na2CO3 was added on the 7th, 13th, and 28th of
April, during periods 3 and 4. The result shows that the pH increased in R4 on April 7th and
then decreased around April 10th to 11th, to increase again on April 13th and April 28th (see
Figure 18a ). A similar observation was also made on the methane concentration in R4 that
increased and decreased around the same dates (see Figure 18b).
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
pH
Time [d]
pH of R4 (a)
R4
20%
30%
40%
50%
60%
70%
80%
CH
4 [
%]
Time [d]
CH4 of R4 (b)
R4
Figure 18 Effect of the Sodium Carbonate on the pH and CH4 of R4
38
4.6 Dewaterability study of the digested sludge
The dewaterability of the digested sludge was analyzed to understand how the sludge
dewatering properties were affected by changes in OLR and HRT in the digesters. The result
showed that when the TS content in the digested sludge was low, the quantity of the polymer
needed for dewatering in the sludge was high (see Table 6). When the TS content in the
digested sludge was high, the needed polymer level in the sludge was low. This applies to the
digested sludge from all the six reactors (see Table 6). When organic material has been well
digested in the reactor, like in the case for R1 and R3 (see Figure 19), the polymer needed for
dewatering increases. The reason for this is that the need for polymer gets higher, the lower
the TS value is. Overloaded reactors with anaerobic digestion process imbalance result in a
low degradability of the sludge, but with better filterability.
Table 6 The needed quantity of polymer per mass TS sludge.
Reactors TS in the digested sludge [%] M_Polymer/M_ST [g Kg-1]
R1 3.59% 12.256
R2 6.06% 7.265
R3 4.57% 9.629
R6 5.02% 8.766
The CST was measured after vigorous mixing of the above polymer and sludge solution for
40, 60, and 100 seconds. The result shows that in the digested sludge from reactor R1, a long
mixing time used resulted in a need for a long filtration time as well. In R2 and R3, it was
different since the filtration time decreased from the mixing time 40 to 60 seconds and then
increased from a mixing time of 60 to 100 seconds. The digested sludge from reactor R6
reacted oppositely from R2 and R3 (see Figure 19).
Figure 19 Result of the CST analyses.
318.94
1158.24
2024.1
319.…
45.8486.42100.58 32.2 187.42
302.…403.42 351.5
0
500
1000
1500
2000
2500
40 60 100
CST
AV
ERA
GE
VA
LUE
[s]
Digest 1 Digest 2 Digest 3 Digest 6
39
4.7 Result of the excel prediction model
The built forecasting model of the organic loading rate was applied on forecasting data for the
coming thirty years, based on the increase in the number of customers during recent years.
The result shows how the OLR in the full-scale reactor R100 will increase during the coming
thirty years, and how the HRT will decrease during the same period (see Figure 20).
R1 and R3 are the two reactors that managed to keep the balance in the digestion process
during periods 1-3. Following their results, that are very similar throughout the experiment,
it can be seen that R1, which was the reference reactor for the real reactor R100 at Käppala
WWTP, could have had the same OLR and HRT as R3 during the whole period, still keeping
the balance in the process. R3 performed stably during periods two and three, keeping an
OLR of between 4.5 and 4.9 [g VS dm-3 d-1], with a similar performance as R1 but with a one-
day shorter HRT. This makes R3 with an OLR of 4.5-4.9 [g VS dm-3 d-1], with a stable and
productive process, the one to determine the optimal value for R100 according to the present
study. Following R2 that increases in OLR and decreases in HRT gradually during the
experiment, it shows that it managed to keep stability up to an OLR of 5.4 [g VS dm-3 d-1],
before reaching process failure. (see section 4.2 and 4.3). When R2 reaches the OLR of 5.4 [g
VS dm-3 d-1], it took just a few days before the process instability was observed in the digester,
which means that R2 could not manage this high OLR value (see Figure 10, Figure 13). Since
the OLR of 5.4 [g VS dm-3 d-1] was the level that created an imbalance in the system in R2,
according to the result, this is the value R100 should avoid reaching. Moreover, the
maximum OLR that should be kept at Käppala WWTP, to keep a stable digestion process, is
the last stable value for both R2 and R3, which is 4.9 [g VS dm-3 d-1].
Combining these results on the maximum OLR and minimum HRT investigated in the pilot
experiment (see Figure 10, Figure 13 and section 4.2 and 4.3), the maximum organic load, 4.9
[g VS dm-3 d-1], will be reached around the year 2031 in R100.
Figure 20 Result of the prediction model.
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
4
5
6
7
8
9
10
11
12
13
14
2015 2020 2025 2030 2035 2040 2045 2050 2055
OLR
[g
VS/
m3
d]
HR
T [d
]
Time [Years]
HYDRAULIC RETENTION TIME [g VS/day] ORGANIC LOAD RATE [g VS/day]
40
5 DISCUSSION
5.1 Suitability of the substrate for biogas production
It is important to always evaluate the biogas potential of a substrate in order to be able to
theoretically compare how suitable different substrates are for biogas production. The
substrate should contain easily degradable nutrients in the right proportions and amounts to
promote microorganisms and be easily accessible in sufficient quantities (Nordberg, 2006).
According to Verein Deutscher Ingenieure, (2006), the methane potential in fat is higher
than the methane potential in protein, and the methane potential in protein is higher than in
carbohydrates. However, substrates with a high content of fat lead to a slower digestion
process as the fats are more difficult to degrade than proteins and carbohydrates (Sialve et
al., 2009). The primary sludge used for the feeding in the present study contained 6.64 kg
ton-1 in lipid, which was evaluated to 14.66% kg-1 VS-1 in fat, 20.70 % kg-1 VS-1 protein, and
64.64% kg-1 VS-1 in carbohydrates (See section 4.1).
In order to maintain the right balance between nutrients in the biogas process, a correct C/N
ratio is also essential (Sialve et al., 2009). If the ratio is too low, there is too little carbon in
relation to nitrogen, and if the ratio is too high, there is too little nitrogen in relation to
carbon. The microorganisms that break down the organic material are most efficient at a
certain balance between the nutrients, which should correspond with the organisms' nutrient
proportions (Forkman, 2014). The lower the values of C/N in the process becomes, the less
efficient the biogas production conditions become. A low C/N value also raises the risk of
ammonia toxicities in the digestion process, which in turn increases the imbalance in the
system, and leads to low gas production. The substrate analyses in this report result in the
total C/N ratio of 14.8 in the feeding sludge, which is within the optimal condition span for
efficient gas production in the anaerobic digestion process according to Mshandete et al.,
(2004) and Olsson et al., (2018). In previous studies on biogas production, there is
disagreement on which ratio is best for anaerobic digestion. A ratio between 15 and 20 is
suggested by Schwede et al., (2013). Sialve et al., (2009) recommend a C/N ratio higher than
20. Mata-Alvarez et al., (2011) think a ratio between 20 to 70 is best, but that ratios between
12 to 16 can create a balance between carbon and nitrogen.
5.2 Maximum organic loading rate (OLR)
The result of the experiment shows how challenging it is to keep the OLR constant in the
anaerobic digestion process, due to TS and VS variations in the feeding sludge. This was
particularly challenging in the current experiment that was fed semi-continuously, only once
a day. Every time the feeding sludge was thickened and prepared, the sludge composition
ended up differently. In full-scale conditions, the inflow of feeding sludge is continuous and
more regular compared to the experiment, which makes the process less affected by the
challenge of keeping the OLR constant. Another challenge that was observed, both in lab-
41
scale and full-scale conditions, is that the composition of the organic material in the feeding
sludge differed, which in turn affected the thickening process. The thickness, in turn, affected
the OLR and made it differ connected to the composition of the feeding sludge.
To answer the question of which year a third anaerobic digester will be needed at Käppala
WWTP. According to the pilot-scale experiment, the maximum organic load for the current
digestion process is 4.9 [g VS dm-3 d-1]. This result is based on the experiment data from
periods 1-3 (67 days), since all the reactors crashed at the end of the experiment, during
period 4. However, even if period 4 is considered, the result would be the same, since all the
reactors, R1-R6, represented the full-scale reactor R100, but with different OLR and HRT
values (see section 3.1 and 4.2). This was ensured by letting all the reactors operate with the
same OLR and HRT during one HRT of 13.2 days, operating as reference reactors for R100.
When the reactors crashed, they did so no matter what load they had. In other words, the
load could not have been the cause of the crash; otherwise, the crash would have taken place
step by step from the reactor with the highest OLR and down. This means that R3 should
have crashed before R1. The result is therefore drawn based on the time the process worked,
67 days during period 1-3. Thus, the main conclusion of the experiment is that the Käppala
reactor R100 will reach its maximum organic load around the year 2031.
5.3 Stability
As confirmed by previous studies by Kanokwan, (2006); Kumar & Samadder, (2020) (see
2.2.1.1), the methanogenic bacteria needed for methane production are extremely susceptible
to the pH level in the process. As can be seen in Figure 10, a low pH level affects the
methanogenic bacteria negatively, which results in a low production of methane in the
produced biogas. The decreasing pH value is seen in the curve for digester R2, which
decreased first on February 17th, then on March 25th, and then on April 7th, precisely at the
same time as the OLR value increased (see Figure 9ab, Figure 10ab). A clear connection that
can be seen during the whole experiment, especially following digester R2, is that as the OLR
increases and the HRT decreases, the process stability is negatively affected, and the methane
yield in the produced biogas becomes low.
On March 5th, the VFA levels suddenly raised rapidly in R6 (see Figure 11a), possibly because
the methanogens stopped producing methane due to process imbalance caused by the high
OLR. This, in turn, made the VFA levels rise rapidly, as reported in previous studies by
Aymerich et al., (2013) (see section 2.2.2). It can be seen in Figure 11ab that digester R6
managed to keep the balance for ten days before the imbalance started, even with a high OLR
and short HRT. Since measurements were made on February 27th, it is known that the VFA
levels were still in balance then. It is somewhere between February 27th and March 5th that
the imbalance started and raised rapidly up to a VFA level of 6780 mg L-1 and a VFA/TA level
of 3.2, which is far beyond recommended levels by Svenskt Vatten, (2019) to keep a stable
process (see section3.3.8). In R2, the same thing happened around March 19th.
Figure 12 illustrates how the anaerobic process instability parameter, ammonia (FAN),
performed evaluating the stability in the digesters during the experiment. According to
previous studies by Yenigün & Demirel, (2013); Zhang & Angelidaki, (2015), ammonia levels
higher than 100 mg L-1 can have an inhibitory effect on anaerobic digestion. Thus, FAN
42
values higher than 100 mg L-1 are showing a risk of imbalance in the process. However,
according to the result of the ammonia analyses in the present study, there was no sign of
process imbalance or risk of process imbalance in the analyses of the ammonia levels, and no
sign of inhibition in the process. All the ammonia values were still below 100 mg/L in the
digesters R1, R2, R3, and R6 (see Figure 12), even when there was a process failure. During
period 4, when there was a total process imbalance in all digesters, the FAN value was below
10 mg L-1 (see Figure 12).
The result of the analyzed anaerobic process stability parameters in this study (see section
4.3) confirms that all the parameters indicate a stable digestion process in the six reactors
during the first period of the experiment. During the second period of the experiment, a
considerable process instability was observed in digesters R4, R5, and R6, due to the increase
in OLR and decrease in HRT. The process imbalance was detected by the pH value, the
methane concentration, the VFA, and the ratio VFA/TA, but not by the ammonia level (NH3-
N) (see section4.3.3). Thus, ammonia does not indicate an imbalance in the studied process
as it was supposed to. This means that the conclusions of Yenigün & Demirel, (2013); Zhang &
Angelidaki, (2015) do not apply for the mesophilic condition at Käppala WWTP (see section
2.2.2). The question arises: Would this be the case in real conditions in Käppala WWTP, in
reactor R100 as well? In that case, ammonia is not useful as a stability indicator under the
studied conditions. This would be interesting to look into during further studies on the
subject.
The VFA levels and the ratio VFA/TA, on the other hand, were beneficial in detecting process
imbalance (see section 4.3.2). The raise of VFA levels at this time is interesting since the OLR
in R6 was changed more than two weeks earlier. This means that a stable process can endure
high levels of OLR for a shorter period. This study has also confirmed that controlling the
VFA levels regularly in the high organic loaded anaerobic digester is vital for a balanced
process. The accumulation of organic acids in the digester is fatal for the digestion process. In
case of imbalance problems, the levels often increase rapidly, as seen in Figure 11a, why
continuous and regular measurements of the acid levels are of great importance (Aymerich et
al., 2013; Andersson, 2015).
The monitoring of levels in the process was made only once a week during the experiment.
With more frequent measurements, the imbalance could have been detected earlier, saving
the process from failing in reactor R6. With the frequency used in the experiment, it would
have been too late to under real conditions save the process in R100. On the other hand, the
instability in R6 was detected earlier, already on February 26th, by the pH level and the
methane concentration in the produced biogas (see Figure 10 and section 4.3.1).
5.4 Gas production
Comparing the average specific methane production with the full-scale production at Käppala
WWTP (320 NmL g-1 VS-1), it is clear that R1 performed very well as a reference for R100, as
it was supposed to. The average specific methane production was determined for R1 (320.978
NmL g-1 VS-1), R2 (242.137 NmL g-1 VS-1), and R3 (307.005 NmL g-1 VS-1) for periods 1, 2, and
3. According to this result, a very low percentage of the theoretical methane potential (488.48
43
[NL kg-1 VS-1]) was really produced by the pilot reactors, even while operating under stable
conditions. This is the same for the full-scale reactor R100 at Käppala WWTP.
R3, with a higher OLR than R1, but with a one-day shorter HRT, still managed to produce
only 13 NmL g-1 VS-1 less than the average specific methane produced by R100. Reactor R3
increased in methane production steadily during the whole process, up to levels beyond 190
569 NmL. R3 was followed by R1 that also had a steady production raise throughout a long
period in the process (Periods 1 to 3), reaching levels of more than 193 853 NmL until all
reactors dropped in methane production at the end of the experiment (see Figure 13b). This
indicates that a higher load can be used, while still performing well and without process
imbalance. However, increasing the OLR can also lead to a decrease in both biogas and
methane yield and a decrease in the degree of degradability (see section 4.4.1). A similar
result has been reported by Nges & Liu, (2010) in their study where the HRT was reduced
from 35 to 3 days, and the OLR increased from 1.6 to 19 kg VS m-3. In their experiment, the
increase in OLR led to an increase in biogas production under stable mesophilic anaerobic
digestion. However, at an HRT of 9 days and an OLR of 7 kg vs m-3, the process crashed.
It is visible in the experiment how vital the stability in the digestion process is for optimal gas
production. As shown in the study, all six reactors performed well as long as they were stable.
The process instability was observed by a decrease in the specific methane production in
three reactors (R4, R5, and R6). It could also be observed in the degree of degradability for
the same reactors (see section4.4). This shows how contra-productive it can be to overload
the digestion process without considering the balance. Imbalance in the anaerobic digestion
leads to almost zero methane production, as shown by R4, R5, R6 in Figure 13.
The result of the chemical oxygen demand in the digested sludge shows that the CODs value
increases when the OLR value increases and the degree of degradation decreases (see Figure
14 and Figure 15a). This was confirmed by observing R2 during period 3 and at the end when
all the digesters became overloaded and crashed. The reason for this is that the COD level
increases when there is an imbalance in the reactors since the reactors do not have time to
digest the organic material in the daily fed sludge. This leads to an accumulation of organic
material in the reactors, which in turn leads to an increase in the COD level. Note here, that
during process imbalance, the sludge clumps together in the digesters which leads to an
increase in the water content in the discharged digested sludge. This in turn leads to a
decrease in the water content in the reactors. The total volume of the reactor does not
increase, but the water content in the reactor decreases in relation to the total volume.
5.5 Process aid
During the experiment, it turned out that treatment with chemical additives, adding around
2.13 g L-1 of the Na2CO3 once a week, was not efficient enough to repair an overloaded
digestive process. The positive effect on the digestion process did not become continuous.
One reason could be that the high OLR in R4 during period 2 lead to a washout of all the
methanogenic bacteria from the reactor. The remaining bacteria were then not enough to
recover the digestion process with the help of Na2CO3. Another possible reason could be that
44
the added quantity of Na2CO3 was not enough or that the treatment frequency was too low.
This was only tested in reactor R4, though, due to process failure and the unexpected ending
of the experiment.
It would be interesting for future studies on the subject to experiment with different forms of
process aid with chemicals, to investigate which chemicals would suit the Käppala conditions
best. During the experiment, stopping the feeding, rather than treating with chemicals, tested
twice during period 4, turned out to give a better result during the process. A pause gave the
bacteria time to digest the overloaded sludge in the digester, preventing it from failure. An
action that could be used for full-scale conditions as well, in combination with frequent
measurement of the stability parameters, when increasing the OLR. However, the risk for a
similar fast increase in OLR during full-scale conditions in R100 is low, due to the more
controlled and regular process.
5.6 Dewaterability study
The dewatering of sludge is one of the highest operational costs in the wastewater treatment
cycle (Capodici et al., 2016). An interesting question when it comes to dewaterability of the
sludge to investigate further is: Is it economically beneficial for Käppala to break down the
sludge to a greater extent? Reducing the volume of the sludge and producing more biogas
even though this means that the cost for polymer increases.
5.7 Error sources
The most crucial possible error source in the present study is that the result analysis is based
on only three out of four periods of the experiment, because of an unexpected failure in all
reactors during the beginning of period 4. However, based on the fact that reactors R1, R2,
and R3 had a stable digestion process for a full 67 days, periods 1, 2, and 3 between February
3rd and April 10th, this is enough to be able to conclude from the results. Moreover, since the
result is based on three reactors running stably during three periods and were not affected by
the initial calculation errors at the beginning of the experiment. Possible reasons for the
process failure during period 4 might have been the content in the new feeding sludge used
during period 4. It could also be the long period of high OLR values in the reactors. This is
unlikely, though, since the reactors with the highest load did not be the ones to crash first.
Another vital error source is that the result is based on pilot-scale digesters, with semi-
continuous feeding, compared to the full-scale digester R100, which is continuously fed with
primary sludge. Thus, the conditions in the pilot-scale experiment do not fully represent the
full-scale reactor R100. The fact that process imbalance can occur extremely fast in case of
process instability in the reactor, according to the result of this study, might not be the same
in full-scale conditions, due to the volume of the full-scale reactor that could mitigate the
effect.
45
There are also possible error sources when it comes to the prediction on when R100 at
Käppala WWTP will reach its maximum OLR and minimum HRT. The built forecasting
model in this study is only based on assumptions from Käppala on how the number of
customers will increase in the future and how much sludge will be produced per person. The
quantity data for the produced sludge per person in the study is calculated based on this data,
which affects the reliability of the prediction result. However, using external data would not
have been useful for the study because it would have made it challenging to compare
obtained results with the Käppala system.
The mass balance calculation, which was determined based on the theoretical methane
potential in both the feeding sludge and the digested sludge, is not reliable since it does not
reflect the result of the gas production (see Figure 16). The reactor that produced most
methane should have digested the most methane potential in the feeding sludge. The
substrate analysis result, however, indicates that the digesters that produced less methane
still digested a big part of the methane potential in the feeding sludge. Unrealistic, since these
reactors had stopped producing biogas early in the process. The lack of reliability in the mass
balance result is due to the time for the substrate analyses. It was done too late and,
therefore, not representative of the experiment. The feeding in the studied reactors was
already stopped for two weeks when the samples for analyses were taken (see section 4.4.4).
However, the mass balance calculation that compared the theoretical methane yield in the
feeding sludge and the specific methane potential achieved by the different reactors was
representative of the conditions in the reactors. The reactors that produced the most
methane digested the most methane potential in the feeding sludge as they were supposed to
(see Figure 17). This mass balance calculation was not affected by the failure in the reactors.
The reliability of the test with process aid is also low in the present study. The test was only
performed on one reactor, R4, which was not enough to draw reliable conclusions from the
results. More frequent additions of the used chemicals might have helped, but also tests on
several of the reactors, to be able to compare different results.
5.8 Sewage sludge and sustainability
Optimal management of sewage sludge is vital for sustainable development worldwide,
according to the global UN Agenda 2030 goal number 6 for clean water and sanitation. The
development of efficient methods to convert the sludge into a renewable resource, accessible
worldwide, is also an essential contribution to the Agenda 203o goal number 7 of affordable
and clean energy to everyone (United Nations Sustainable Development, 2015).
The anaerobic digestion process is, as shown in this report, an efficient and well-studied
process that biologically converts the chemical energy of sewage sludge into combustible
biogas and makes it a carbon-neutral alternative to fossil fuels. However, the method needs
close monitoring in order to become more efficient. One of the keys to optimal biogas
production through anaerobic digestion of sewage sludge is, as shown in this study, to keep
the anaerobic digestion process stable. The report gives some of the answers on how to run a
46
stable, and thus efficient, anaerobic process. In the experiment, the development in reactor
R3 also shows that higher process limits than used today, with higher load and shorter
retention times, and thus a less energy-demanding process, could be possible without process
failure.
Parallelly there is an ongoing discussion in society on the hazards of using digested sludge as
fertilizers in the agricultural sector. Anaerobic digestion reduces some of the pathogens in the
sludge, but not all. Although not investigated in this report, how to more efficiently reduce
harmful substances in the sludge would be something to further investigate in future studies
(Larshans & Finnson, 2020a, 2020b; Wijkman et al., 2020).
47
6 CONCLUSIONS
Käppala WWTP has, during the last years, observed an increase in organic loading rate in the
digester. The digestion at Käppala WWTP is today high loaded with a high organic loading
rate (OLR) and low hydraulic retention time (HRT). The experimental set up of reactor R100
at Käppala WWTP in this report was done to evaluate the effect of the maximum OLR and
the minimum HRT for the anaerobic digestion of sewage sludge in the mesophilic conditions
at Käppala WWTP. The result of this study shows that the maximum OLR for Käppala
conditions is 4.9 g VS dm-3 d-1. This means that R100 at Käppala WWTP will reach maximum
organic load around the year 2031. At this point, additional reactors need to be in use to
relieve R100.
Keeping the anaerobic digestion process in balance is vital when it comes to the outcome of
energy in the process. Imbalances in the system immediately lead to a significant decline in
the production of biogas. Pushing the process in order to produce more gas can become
counterproductive. However, the result also shows that it is challenging to maintain the OLR
at a constant level in the anaerobic digestion process, due to the variations in the total solids
(TS) and the volatile solids (VS) in the feeding sludge. Another challenge that can be
observed, both in lab-scale and full-scale conditions, is that the composition of the organic
material in the feeding sludge differs, affecting the thickening process. The thickness affects
the OLR/HRT and makes it differ connected to the composition of the feeding sludge.
Another important observation is that when there is an imbalance in the digestion process, it
goes much faster than expected to reach process failure. This means that the margin for
overload in the anaerobic digestion process must be significant to avoid sudden process
failures. This shows the importance of continuous measurements of the stability parameter
levels while having a high loaded digestion process. Measurements once a week, as
performed in this study, are not frequent enough when there is a risk of instability.
Based on the performance of reactor R3 during the experiment, an OLR of 4.7-4.9 g VS dm-3
d-1 and an HRT of 12 days is the optimal value for R100 at Käppala WWTP. Though during
the studied lab-scale conditions, one of the reactors, R2, managed to keep stability up to an
OLR of 5.4 for nine days, before reaching process failure.
The result shows that the methane concentration in the converted biogas and the pH level in
the reactors are the most efficient stability parameters for the anaerobic digestion conditions
at Käppala WWTP. During the experiment, they were the first parameters to detect the
imbalance in the digesters. VFA and the ratio VFA/TA also showed instability in the process
but at a much slower pace. Ammonia as a stability parameter did not predict or detect
instability during the experimental process, which indicates that this parameter does not
work as a process indicator for Käppala conditions. Chemical additives are often not enough
to treat an overloaded digestive process. More frequent additions of chemicals might have
helped, but this was not tested in the study.
The OLR and HRT have a high impact on the needed quantity for dewatering polymer. The
higher the digestion of the organic material in the sludge, the bigger the need for polymer in
the process, in the end, to take care of the rest material.
48
7 SUGGESTIONS FOR FURTHER WORK
It would be interesting to perform the same experimental setup as for the current study, but
under large-scale but not full-scale conditions, at Käppala WWTP. Large-scale conditions
would permit a more regular and continuous feeding of sludge and thus give a more reliable
result that represents more precisely the full-scale reactor R100.
In future studies, it would be interesting to investigate if the unexpected ending of the
experiment could have been avoided. During period 4 in the experiment, all the reactors,
including the stable reactors R1 and R3, crashed. The reason for this might have been the
content in the new feeding sludge used during period 4. It could also be due to the long
period of high OLR in the reactors. This is unlikely, though, since the reactors with the
highest load did not be the ones to crash first.
It would also be interesting to do further studies on the fact that ammonia did not indicate
process instability for Käppala conditions during the current study as it was supposed to
according to the literature study. Would this be the case under real conditions in reactor
R100 as well?
Another suggestion for future studies on the subject would be to experiment with different
process aid chemicals to investigate which chemicals would suit the Käppala conditions best.
Interesting for further studies would also be: Is it economically beneficial and efficient when
it comes to the usage of energy versus the energy conversion, to save one day of HRT while
digesting more organic material (OLR) in the process? R3 performs well during the
experiment, with a higher organic load (4.5–4.9 g VS dm-3 d-1 ) and shorter hydraulic
retention time (12 [d]) while still producing well and without process imbalance.
Furthermore, a study of dewaterability for Käppala conditions would be interesting. Would it
be economically beneficial for Käppala to break down the sludge to a greater extent and
produce more biogas, even though the need for polymer increases?
49
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APPENDIX 1: EXPERIMENT PROTOCOL
Table 7 Weekly protocol of the pilot-scale experiment
Volym (cm3)
Måndag Tisdag Onsdag Torsdag Fredag Antal prov och budget
Temperatur X X X X X
pH - X X X X X -
Metan X3 - X X X X X -
VFA (HAC) rötslam X3
2 ml /prov X 270 kr/prov – Totalt 270 prover = 73 000 SEK
ALK rötslam X3
150 ml/prov
X 18*15 = 270 prov
COD rötslam X3
1 ml/prov X 18*15 = 270 prov
CODs rötslam X3
1 ml/prov X 18*15 - 270 prov
COD råslam X3 1 ml/prov X 3*8 = 270 prov
CODs råslam X3
1 ml/prov X 3*8 = 24 prov
GR/TS rötslam X3
15-20 g/prov
X 18*15 = 270 prov
GR/TS råslam X3 varannan vecka
15-20 g/prov
X 3*8 = 24 prov
NH4-N råslam varannan vecka
1000 ml X 8 st – 300 kr/prov = 2 400 SEK
Kjeldahl-N råslam varannan vecka
1000 ml X 8 st – 300 kr/prov = 2 400 SEK
NH4-N rötslam
? X 15 prover - körs internt av labbet
HPLC- fettsyror
100 ml/prov Eventuellt mindre
Slam från fredagens prover samlas och fryses. 10 g slam blandas upp med 40 ml vatten, centrifugeras, klarfasen filtreras först genom ett 0,45 mikronfilter och sedan genom ett 0,2 mikronfilter. Provet sparas i 10 ml behållare och fryses. Skulle rötkamrarna börja ackumulera VFA skickas valda prover för analys. Eventuell budget 1 gång/månad 1000 kr/prov = 24 000 SEK
Fett, proteiner och gödselprov – råslam – Gödselprov
1200 kr /prov- Genomförs av Agrilab
Gödselprov på råslam 3 prover 1200 kr/prov = 3600 kr
Total budget 105 400 SEK – Osäkerhet för eventuellt fler VFA prover
APPENDIX 2: EXTRA DATA ON METHANE PRODUCTION
The following Figures present extra data on the methane production obtained during the
experiment.
Figure 21 Accumulated methane production
Figure 22 Daily methane production per day
0
50000
100000
150000
200000
250000
30/01/2020 00:00 29/02/2020 00:00 31/03/2020 00:00 30/04/2020 00:00 31/05/2020 00:00
[Nm
L]
Time [day]
Reaktor 1 Volume [NmL] Reaktor 2 Volume [NmL] Reaktor 3 Volume [NmL]
Reaktor 4 Volume [NmL] Reaktor 5 Volume [NmL] Reaktor 6 Volume [NmL]
0.00
2,000.00
4,000.00
6,000.00
8,000.00
10,000.00
12,000.00
[Nm
L/d
]
Time [d]
Reaktor R1 Volume [Nml] Reaktor R2 Volume [Nml] Reaktor R3 Volume [Nml]
Reaktor R4 Volume [Nml] Reaktor R5 Volume [Nml] Reaktor R6 Volume [Nml]
Figure 23 Methane production flow per day
0
20000
40000
60000
80000
100000
120000
[Nm
L/d
]
Time [d]
Reaktor R1 Flow [Nml/day] Reaktor R2 Flow [Nml/day] Reaktor R3 Flow [Nml/day]
Reaktor R4 Flow [Nml/day] Reaktor R5 Flow [Nml/day] Reaktor R6 Flow [Nml/day]
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