coupling a-stage technology with dissolved air flotation

COUPLING A-STAGE TECHNOLOGY

WITH DISSOLVED AIR FLOTATION

(DAF) FOR INCREASED ORGANICS

REMOVAL AND COMPACT SLUDGE

PRODUCTION AT PILOT SCALE Aantal woorden: 22531

Stijn Decru Stamnummer: 01202687

Promotor: Prof. Dr. ir. Korneel Rabaey Prof. Dr. ir. Bart De Gusseme

Copromotor: dr. ir. Jo de Vrieze, ir. Cristina Cagnetta

Masterproef voorgelegd voor het behalen van de graad Master of Science in de bio-

ingenieurswetenschappen: milieutechnologie

Academiejaar: 2016 - 2017

i

Copyright

“The author and the promoter give the permission to use this thesis for consultation and to copy

parts of it for personal use. Every other use is subject to the copyright laws, more specifically

the source must be extensively specified when using results from this thesis.”

“De auteur en de promotor geven de toelating deze scriptie voor consultatie beschikbaar te

stellen en delen ervan te kopiëren voor persoonlijk gebruik. Elk ander gebruik valt onder de

beperkingen van het auteursrecht, in het bijzonder met betrekking tot de verplichting de bron te

vermelden bij het aanhalen van resultaten uit deze scriptie.”

Gent, juni 2017

De promotoren, De auteur,

Prof. dr. ir. Korneel Rabaey Stijn Decru

Prof. dr. ir. Bart De Gusseme

Dr. ir. Jo de Vrieze

Cristina Cagnetta

iii

Acknowledgements

The year that lies behind me is one in which I learned a tremendous amount of things. The

experience I gained during the HRAS-DAF pilot work told me that you should never expect

anything to work properly from the beginning and that really anything can fail. The problem

solving was challenging but kept everyone closely involved to the project. Working in the lab

gave me insights in the chemical analyses that are necessary to run a biotechnological plant.

First of all I would like to thank Cristina for her enthusiasm and patience she showed towards

me. She was always available for questions and if something wasn’t clear from the first time,

she would eagerly explain it to me twice. She steered me in the good way and gave me the

critical insights that were necessary to build the story of my thesis. I am grateful to Jo Devrieze

for the advice he gave me on the anaerobic digestion of A-stage sludge and for reading my

thesis. He was also the one who supplied me with vacutainers without which I couldn’t have

brought a single gas sample safely back to the faculty for analysis.

I remember feeling like an engineer for the first time when I attended Professor Rabaey courses

on environmental technology during the third year. It was during these courses that he

awakened my interest in biotechnological processes for wastewater treatment and made me

choose one of his topics. His advice was always to the point and helped a lot to keep the focus

and define the paths to take. My first encounter with professor De Gusseme was during the

course ‘microbial resource recovery’. I admired his practical approach and strong link with the

industry and his involvement was an enriching complement to the story of the thesis.

To the people from Aquafin. Thank you Bart & Francis for sustaining the HRAS-DAF pilot

together with me and answering my questions and Marjolein for guiding this project. Special

thanks to Francis for bringing samples several times from Aartselaar to the lab in Gent. Thank

you Marc, Solo (Souleymane), Kris (Christiaan) and Eric who were there to come up with fast

and practical solutions if there was a mechanical problem with the HRAS-DAF pilot.

To the people from Nijhuis for making this thesis possible by providing the DAF unit and doing

necessary adaptions to make everything work.

iv

Further, I would like to thank everyone from CMET. The people I could chat or have fun with

during my work in the lab, have a coffee together or provide me with answers to my questions.

My fellow leaders at the youth movement (chiro), I would like to thank you for letting me do

one extra year, knowing that I would be less closely involved because of the thesis. Once again

we had fantastic moments, and you tolerated the many times I was absent. It’s like Timmy

Simons once said, it’s better to quit to late than to early.

Finally, to my parents, sister and smaller brother, I am grateful that you pushed me through the

last weeks of the thesis. You supported me when I needed it and gave me courage to go further.

v

Abstract

Current wastewater treatment processes such as the conventional activated sludge process

(CAS) focus solely on the removal of organics and nutrients and are therefore not sustainable.

The organics present in the wastewater possesses an abundant resource potential and the focus

should shift from solely environmental protection to the recovery of energy and materials.

High rate activated sludge (HRAS) systems such as the A-stage or high rate contact stabilisation

(HiCS) process are able to capture a significant fraction of these organics and transfer them to

the sludge. Although some energy is lost via microbial respiration, most of it is captured in the

sludge. Part of the energy stored in the sludge can be recovered under the form of biogas (CH4)

via anaerobic digestion (AD). The high rate activated sludge is more biodegradable and

conversion efficiencies during AD are up to 2.5 times higher than for excess sludge produced

during CAS. Unfortunately, the high food to microorganism ratios specific to these high rate

systems lead to a poor settling sludge.

Solid/liquid separation of HRAS via conventional settling generates diluted sludge (± 10 g L-

1). Further thickening is thus required prior to AD. Moreover, a considerable part of the organics

(up to 50 %) are not separated in the settler and leave with the effluent. In the view of optimal

energy recovery, a more efficient technique for solid/liquid separation is needed. During this

thesis dissolved air flotation (DAF) was applied for solid/liquid separation and it was shown

that the HRAS-DAF combination is feasible at pilot scale but needs close monitoring.

The HRAS-DAF pilot was able to obtain high organics removal and produce concentrated

sludge (21 – 47 g COD L-1). The sludge was anaerobically digested to recover the energy

present in the sludge under the form of biogas. When a single polymer was used to assist

flocculation, conversion efficiencies to methane were 58 – 68 %. This is comparable to a

conventional HRAS where settling is used for the solids separation. When a dual polymer

system was used conversion efficiencies were lower, 40 – 42 %. This was likely due to

phosphorous or trace elements depletion or to the polymers interfering with hydrolysis and

digestion of sludge during AD.

vi

Nederlandse samenvatting

De huidige technieken voor waterzuivering zoals het actief slib proces (CAS) richten zich enkel

op de verwijdering van organisch materiaal en nutriënten en zijn daarom niet duurzaam. De

aanwezige organische stoffen in het afvalwater beschikken over een overvloedig potentieel

voor grondstof recuperatie en de focus moet overgaan van enkel milieubescherming naar het

terugwinnen van energie en materialen.

Hoog belast geactiveerd slib (HRAS) systemen, zoals de A-trap (A-stage) of het hoog belast

contact stabilisatie proces (HiCS), kunnen een significante fractie van deze organische stoffen

opnemen en overbrengen naar het slib. Hoewel sommige energie via microbiële respiratie

verloren gaat, wordt het grootste deel in het slib vastgelegd. Een deel van de energie die in het

slib wordt opgeslagen, kan vervolgens worden teruggewonnen onder de vorm van biogas (CH4)

via anaerobe vergisting. Het hoog belaste geactiveerd slib is beter biologisch afbreekbaar en de

omzettingsefficiënties tijdens anaerobe vergisting zijn maximaal 2,5 keer hoger dan voor slib

geproduceerd tijdens CAS. Helaas leiden de hoge ‘feed tot micro-organismen’-verhoudingen

(F/M) die specifiek zijn voor deze hoog belaste systemen tot een slecht bezinkbaar slib.

Vast/vloeistof scheiding van HRAS slib via conventionele sedimentatie zorgt voor laag

geconcentreerd slib (± 10 g L-1). Verdere verdikking is dus vereist voorafgaand aan anaerobe

vergisting (AD). Bovendien wordt een aanzienlijk deel van de organische stoffen (tot 50%) niet

gescheiden in de bezinkingstank maar meegenomen in het effluent. Met het oog op optimale

energieherwinning is er een efficiëntere techniek voor vast/vloeistof scheiding nodig. Tijdens

deze thesis werd opgeloste lucht flotatie (DAF) toegepast voor vast/vloeistof scheiding en het

bleek dat de HRAS-DAF combinatie haalbaar is op pilootschaal mits nauwkeurige opvolging.

De HRAS-DAF piloot kon hoge organische verwijdering verkrijgen en geconcentreerd slib

produceren (21 - 47 g COD L-1). Het slib werd anaeroob vergist om de aanwezige energie in

het slib in de vorm van biogas te recupereren. Wanneer enkel anionisch polymeer werd gebruikt

voor flocculatie, was de conversie efficiëntie naar methaan 58 – 68%. Dit is vergelijkbaar met

een conventionele HRAS, waar bezinking gebruikt werd voor de vaste-stofafscheiding.

Wanneer een dubbel polymeer systeem werd gebruikt, was de conversie-efficiëntie lager, 40 –

42%. Dit was waarschijnlijk te wijten aan de uitputting van fosfor of sporenelementen of aan

de polymeren die interfereren met hydrolyse en vertering van slib tijdens AD.

vii

Table of contents

Copyright ..................................................................................................................................... i

Acknowledgements ................................................................................................................... iii

Abstract ...................................................................................................................................... v

Nederlandse samenvatting ......................................................................................................... vi

Table of contents ...................................................................................................................... vii

List of abbreviations .................................................................................................................. xi

PART 1: LITERATURE REVIEW ...................................................... 1

1 Municipal wastewater treatment ........................................................................................ 2

Characteristics of Municipal wastewater ..................................................................... 2

Conventional Activated Sludge (CAS) process ........................................................... 3

Drawbacks related with the CAS process .................................................................... 4

Achieving sustainability through energy recovery ...................................................... 5

2 High rate activated sludge processes .................................................................................. 7

The A/B-process .......................................................................................................... 7

Benefits and drawbacks from A-stage technology ...................................................... 9

The high rate contact stabilisation process ................................................................ 10

3 Coagulation and flocculation ........................................................................................... 11

4 Dissolved air flotation (DAF) .......................................................................................... 13

Working principle ...................................................................................................... 13

Applications of DAF at MWTP’s .............................................................................. 15

5 Anaerobic digestion of high rate sludge ........................................................................... 16

The role of anaerobic digestion at the modern WWTP ............................................. 16

General process principles ......................................................................................... 17

6 Research questions ........................................................................................................... 19

viii

PART 2: MATERIALS & METHODS ......................................... 20

1 Pilot scale HRAS-DAF set-up .......................................................................................... 21

Configuration 1: A-stage ........................................................................................... 24

1.1.1 Treatment 1: dosing of FeCl3 and A130hp ......................................................... 24

1.1.2 Treatment 2: dosing of FeCl3, C492 and A130hp .............................................. 24

Configuration 2: HiCS operation (treatment 3) ......................................................... 25

MLSS characteristics ................................................................................................. 25

Digesters .................................................................................................................... 26

Sampling .................................................................................................................... 27

2 Jar testing .......................................................................................................................... 28

Experiment 1: Optimal polymer selection for flotation ............................................. 28

Experiment 2: Optimal flocculant dosage (T1) ......................................................... 28

Experiment 3: Optimal flocculant dosage (T2) ......................................................... 29

Experiment 4: Optimal coagulant & flocculant dosage (T3)..................................... 29

3 BMP tests ......................................................................................................................... 29

4 Fed batch digesters ........................................................................................................... 32

5 Chemical analyses ............................................................................................................ 32

COD analysis ............................................................................................................. 32

Solids analysis ........................................................................................................... 33

VFA analysis ............................................................................................................. 33

Gas analysis ............................................................................................................... 34

Anion analysis ........................................................................................................... 34

Nitrogen analysis ....................................................................................................... 34

ix

PART 3: RESULTS ........................................................................................... 35

1 Pilot scale HRAS-DAF .................................................................................................... 36

Organics removal ....................................................................................................... 36

1.1.1 A-stage-DAF T1 ................................................................................................. 38

1.1.2 A-stage-DAF T2 ................................................................................................. 38

1.1.3 HiCS-DAF T3 ..................................................................................................... 38

DAF separation efficiency ......................................................................................... 39

1.2.1 A-stage-DAF T1 ................................................................................................. 41

1.2.2 A-stage-DAF T2 ................................................................................................. 41

1.2.3 HiCS-DAF T3 ..................................................................................................... 41

1.2.4 Effluent characteristics ...................................................................................... 41

Sludge characteristics ................................................................................................ 44

COD balance .............................................................................................................. 46

Digester performance ................................................................................................ 47

2 Jar tests ............................................................................................................................. 48

Experiment 1: optimal polymer selection for flotation ............................................. 48

Experiment 2: optimal flocculant dosing (T1) .......................................................... 48


Experiment 4: optimal FeCl3 and C492 dosing (T3) ................................................. 51

3 BMP tests ......................................................................................................................... 52

BMP T1 ..................................................................................................................... 52

BMP T2 ..................................................................................................................... 52

BMP T3 ..................................................................................................................... 53


PART 4: DISCUSSION ................................................................................. 55

1 Pilot scale HRAS-DAF .................................................................................................... 56

x

Organics removal ....................................................................................................... 57

1.1.1 A-stage-DAF T1 ................................................................................................. 57

1.1.2 A-stage-DAF T2 ................................................................................................. 59

1.1.3 HiCS-DAF T3 ..................................................................................................... 60

DAF performance ...................................................................................................... 60

1.2.1 A-stage-DAF T1 ................................................................................................. 61

1.2.2 A-stage-DAF T2 ................................................................................................. 61

1.2.3 HiCS-DAF T3 ..................................................................................................... 61

1.2.4 Effluent characteristics ...................................................................................... 61

Comparison between HRAS-DAF and HRAS-settling ............................................. 62

Sludge characteristics ................................................................................................ 64

COD balance .............................................................................................................. 65

2 Jar tests ............................................................................................................................. 66

Experiment 1: optimal polymer selection for flotation ............................................. 66



Experiment 4: optimal FeCl3 and C492 dosing (T3) ................................................. 67

3 BMP tests ......................................................................................................................... 67

BMP T1 ..................................................................................................................... 67

BMP T2 ..................................................................................................................... 67

BMP T3 ..................................................................................................................... 69


5 General conclusions ......................................................................................................... 70

6 Future work and perspectives ........................................................................................... 72

PART 5: REFERENCE LIST .................................................................. 74

xi

List of abbreviations

A/B Adsorption / Bio-oxidation

AD Anaerobic Digestion

BOD Biological Oxygen Demand

CAS Conventional Activated Sludge

cCOD colloidal COD

CD Charge Density

CEPT Chemically Enhanced Primary Treatment

CHP Combined Heat & Power

COD Chemical Oxygen Demand

DO Dissolved Oxygen

EPS Exo Polymeric Substances

F/M Food to Micro-organism

HRAS High Rate Activated Sludge

HRT Hydraulic Retention Time

MLSS Mixed Liquor Suspended Solids

MO Micro Organism

MW Molecular Weight

PAC Poly Aluminium Chloride

pCOD particulate COD

PE People Equivalents

PFF Plug Flow Flocculator

RAS Return Activated Sludge

xii

sCOD soluble COD

SRT Sludge Retention Time

SVI Sludge Volume Index

TKN Total Kjeldahl Nitrogen

TP Total Phosphorous

TSS Total Suspended Solids

VSS Volatile Suspended Solids

WAS Waste Activated Sludge

1

PART 1: LITERATURE REVIEW

2

1 Municipal wastewater treatment

The United Nations World Water Development Report from 2015 states that global water use

is increasing and potable water is becoming scarcer (Connor, 2015). Population growth,

industrialisation and a higher consumption per capita are the main reasons for this. In the

meantime, the production of wastewater is increasing. According to the 2030 Water Resources

Group, the world will only have 60% of the water it needs by 2030 without significant global

policy change (Boccaletti, 2009). In industrialised countries where there is enough potable

water, there is a competition between the households and the industries. Also in Flanders, the

high pressures on the groundwater reserves has led to stricter regulations for the industry and

to incentives for water recovery from used waters (Verstraete et al., 2016).

Wastewater treatment consumes considerable amounts of energy and materials to meet

discharge standards. Instead, wastewater should be looked at as a source of valuable resources.

An important step towards self-sufficient wastewater treatment is controlling and managing the

carbon flows to minimize oxidation and maximise sludge harvest (Rahman, 2016).

Characteristics of Municipal wastewater

Municipal wastewater discharged from houses, agriculture and industries contains

contaminants that need to be removed. These contaminants are organics (carbohydrates, lipids,

proteins), nutrients (nitrogen and phosphorous) and other chemicals. Medium strength

wastewater contains about 500 mg L-1 chemical oxygen demand (COD), 40 mg L-1 total

Kjeldahl nitrogen (TKN) and 8 mg L-1 total phosphorous (TP) (Tchobanoglous & Burton,

1991). The ratios of COD:N:P are thus in the order of 10:1:0.2.

The organic matter can be divided in a settable (i.e. particulate, pCOD) and non-settable fraction

(i.e. soluble, sCOD and colloidal, cCOD). For municipal wastewater, these three fractions seem

to be quite constant and are on average 45 %, 31 % and 24% respectively (Guellil et al., 2001).

3

Conventional Activated Sludge (CAS) process

Treatment of municipal wastewater is depending on microorganisms (MO) that are

implemented in biotechnological processes. These processes are of great importance for human

health, food provisioning and protection of natural ecosystems. One process in particular, the

‘activated sludge process’, is very widely spread (Modin et al., 2016).

“There is not a single process that has made such a tremendous impact on

human health as the activated sludge process” (Rabaey, 2014)

Activated sludge systems for wastewater treatment consist of a biological aerated reactor with

a microbial community of heterotrophic and autotrophic MO (Ekama & Wentzel, 2008).

Municipal wastewater is brought into contact with this community and organic matter is

removed. Part of the organics are transformed into CO2 and water to provide energy for

catabolic processes (Henze et al., 2001). The other part is incorporated in the cell components

of the MO’s or stored as intra- and extracellular energy source.

The activated sludge process is only a part of the whole municipal wastewater treatment

process. It can be divided in three different stages: primary, secondary and tertiary treatment

(Tchobanoglous & Burton, 1991). Before primary treatment, large particles ( > 1 cm) are

removed by a grit and grease and fats can be skimmed off to protect downstream equipment.

Figure 1: Process configuration for a typical CAS treatment plant with primary settling

During primary treatment, the wastewater passes a primary settling tank in which 60 – 65 % of

the suspended solids (TSS) are removed (Constantine et al., 2012), so the organic load on the

activated sludge reactor is lowered and smaller reactor volumes and less aeration are required.

This was an incentive to investigate chemically enhanced primary treatment (CEPT). Using

chemical coagulants (i.e. iron chloride and aluminium sulphate), high removal efficiencies for

4

TSS (90 %) and TP (80 – 90 %) could be achieved but the technique has to cope with high

chemical costs and excess sludge production (Xu et al., 2009).

Secondary treatment is typically a CAS system. Its main components are an aerated biological

reactor, a sedimentation tank and a recycle system (Figure 1) for returning part of the

solids/biomass back to the biological reactor (Ekama & Wentzel, 2008). Additional aeration

and a sludge retention time (SRT) of 8 – 20 days are needed for bacterial nitrification

(Loosdrecht et al., 1997). Operating at long SRT means that most biodegradable organics will

be oxidised. The energy content of the organic carbon is lost rather than recovered (Verstraete

& Philips, 1998), and electrical energy (0.45 – 1.25 kWh kg-1 O2) is needed for aeration

(Rabaey, 2014). Additional organic carbon (e.g. methanol) is often added for bacterial

denitrification in the anoxic part of the reactor.

The MO grow in flocs which allows solid/liquid separation by gravitational settling in the

secondary settler. Part of the sludge from the settler is recirculated and the remainder is wasted.

Exopolymeric substances (EPS) play a crucial role in the floc formation. They are formed by

active cell secretion, cell lysis and adsorption processes. Sludge settles less well with increasing

EPS content (Y. Liu & Fang, 2003).

Tertiary treatment is typically applied to meet effluent standards commissioned by the

government. Residual suspended solids can be removed by ultrafiltration and also disinfection

is sometimes applied (Tchobanoglous et al., 1998).

Drawbacks related with the CAS process

Large areas are needed for settling of primary and secondary sludge. The settling behaviour of

activated sludge can be measured based on the sludge volume index (SVI). A good working

secondary settler concentrates the incoming MLSS (1.5 – 3.5 g TSS L-1) to secondary sludge

having a content of 5 – 15 g TSS L-1 (Higgins & Novak, 1997). Doing so 5 – 15 m2 secondary

settling area per 1000 Person Equivalents (PE) is needed (Henze et al., 2008). The sludge needs

to be further concentrated before anaerobic digestion. This is typically don by gravity

thickening and the secondary sludge concentration is raised from 5 – 15 to 60 g TSS L-1 (EPA,

2003).

5

Current wastewater treatment processes do not focus on energy recovery, and are not

sustainable (Verstraete et al., 2009). The CAS process consumes considerable amounts of fossil

fuel-derived energy and chemicals. During oxidation, the chemical energy present as COD is

lost as metabolic heat. According to Garrido (2013), the energy use for CAS treatment lies

between 0.3 – 0.8 kWh m-³ of which most is needed for aeration (60 – 70 %) and pumping. In

addition to the high energy use, the processes themselves also generate potent greenhouse gases

such as CH4 and N2O (Sheik et al., 2014).

The energy potential of medium strength wastewater with a COD content of 500 mg L-1 is

around 1.9 kWh m-³ (McCarty et al., 2011). This exceeds the energy needed for treatment by a

factor four. A systematic evaluation of the energy content in wastewaters is needed since there

is no straight correlation between the COD content and the energy content (Heidrich et al.,

2010).

Achieving sustainability through energy recovery

The focus of municipal wastewater treatment has shifted from solely protection of downstream

users to environmental protection. Currently it is considered as a source of three valuable types

of resources: water, nutrients and energy (Mo & Zhang, 2012). Municipal wastewater is of

specific interest due to its availability in urbanized regions.

Two main strategies to achieve higher sustainability during wastewater treatment are (1) energy

efficiency improvement and (2) integrated resource recovery. Measures that reduce the energy

consumption such as bubble aeration and aeration control systems are being implemented in

wastewater treatment plants to improve the efficiency of energy usage (Frijns et al., 2013).

Methods for energy recovery are (1) on-site energy generation through combined heat and

power (CHP) based on the organic load and/or thermal heat of the wastewater, (2) nutrient

recycling which offsets the environmental load associated with producing the same amount of

fertilizer and (3) water reuse (Mo & Zhang, 2013). On-site energy generation must deal with a

high one-time investment cost in the order of 2000 € kW-1 installed power. High amounts of

biogas are needed to cover this investment cost which is not in favour of smaller treatment

plants.

6

Wastewater treatment plants are currently large energy consumers. About 3 % of electricity

consumption in 2004 in US was used for wastewater treatment and this is similar for other

developed countries (Cams, 2005). Achieving energy neutrality or even contributing energy to

the society would be beneficial for multiple reasons (Constantine et al., 2012):

i. Energy produced by wastewater facilities is sustainable.

ii. Achieving energy neutrality is important in terms of risk management to deal with

fluctuating energy prices.

iii. On-site energy production enables the WWTP’s to cope with power outages.

Both capturing the energy in the dissolved organics and meeting the effluent standards would

lead to high energy saving (McCarty et al., 2011). Production of biogas is the main source of

energy at a municipal CAS treatment plant. The biodegradable organics present in the sludge

are converted to CH4 and CO2 and new biomass. Since direct anaerobic treatment of municipal

wastewater is not applicable for low to medium strength wastewater, the recovery of chemical

energy can be maximised by concentration of the organic carbon in the wastewater and

maximised sludge digestion (Frijns et al., 2013).

Concentration of municipal wastewater followed by anaerobic digestion of organics and

maximal reuse of the mineral nutrients and water is estimated to have a total cost of 0.9 € m-3

which is similar as for CAS treatment with little or no reuse. (Verstraete et al., 2009). Other

techniques for energy recuperation from sludge like incineration, gasification and pyrolysis are

less commonly used (Tyagi & Lo, 2013).

7

2 High rate activated sludge processes

High rate activated sludge processes (HRAS) are characterized by short sludge retention times

(SRT) (< 1 day), low dissolved oxygen conditions (0.5 mg L-1) and a high food to micro-

organism (F/M) ratio (> 2 g BOD g-1 VSS day-1) (Smitshuijzen et al., 2016). These systems are

able to maximise carbon harvesting from wastewater (Rahman, 2016). Most important process

parameters are the HRT, SRT and DO. The organic matter is removed through non-oxidative

processes rather than being oxidized to CO2. The major mechanisms for non-oxidative removal

of carbon in the HRAS are assimilation, storage and sorption (Modin et al., 2016) and will

further be referred to as ‘biosorption’. Applications of HRAS processes are hampered by poor

settling properties of the high-rate sludge (Bisogni & Lawrence, 1971).

In the search for energy efficiency improvements at the wastewater facility, there is also

renewed interest in employing two-stage biological treatment in which carbon and nitrogen

removal are decoupled. Aeration energy savings could be around 15 – 20 % and 25 – 40 %

more biogas could be produced when applying a two-stage treatment compared with the CAS

(Constantine et al., 2012).

The A/B-process

Two stage processes with HRAS in the first step are referred to as the A/B-process. It is an old

process that was reintroduced in the 70’s by Boehnke et al. (1998) as an answer to the higher

treatment standards commissioned by the government and the desire for a cost-effective

technology to reduce the micro pollutants and nutrients in the wastewater. A/B is an

abbreviation for ‘Adsorption-Belebungsanlagen’ and is typically translated as ‘Adsorption-

activated sludge’ or ‘Adsorption-Biooxidation system’. Several plants were installed in

Germany and the Netherlands (de Graaff et al., 2016). Even more stringent effluent standards

led to reconfiguration of existing A/B-processes for better nitrogen and phosphorous removal.

The possibility to use anammox bacteria for autotrophic nitrogen removal combined with higher

energy recovery renewed the interest in the A/B-process (Kartal et al., 2010).

8

The A/B process is a two-stage dual sludge system that consists of a highly loaded Adsorption-

stage (A-stage) and a lowly loaded Bio-oxidation-stage (B-stage). In the A-stage the F/M ratio

is high compared with the CAS system, 2 – 10 g BOD g-1 VSS day-1 and 0.3 – 0.6 g BOD g-1

VSS day-1 respectively. HRT is about 30 minutes and SRT is less than 1 day (Jimenez et al.,

2015). The second stage, the B-stage, has a lower F/M ratio (less than 0.1 g BOD g-1 VSS day-

1) combined with higher SRT (8 – 20 days).

Figure 2: Process configuration of an A/B system, adapted from Modin et al. (2016)

The first stage, the A-stage, is the most innovative element in the process. No primary treatment

(or CEPT) is applied in the A/B-process, so the A-stage can be seen as a biologically enhanced

primary treatment (Constantine et al., 2012). Removal of the organic load in this stage happens

fast and is based on physico-chemical processes. Colloidal and particulate matter can be

removed by biosorption on the activated sludge flocs. Biosorption capacity increases with

increasing SVI, and is negatively affected by long periods of anoxia (Pujol & Canler, 1992).

Next to the physico-chemical removal of organics, also intracellular storage occurs. Removal

efficiencies for COD and BOD in existing A-B WWTPs are 50 – 60 % and 40 – 80 %

respectively (de Graaff et al., 2016). Lower F/M and longer SRT values yielded sludge in

which carbon storage was the main process for carbon capture (Lim et al., 2015).

Operational conditions under which the A-stage performs best were investigated by de Graaff

et al. (2016). Based on results from four operational A/B wastewater treatment plants in the

Netherlands, it was shown that the A-stage could more effectively remove COD than a primary

settler when the influent contained a high fraction (> 25 %) of sCOD. Sludge production was

found to be maximal at an SRT of about 0.3 days, and sufficient aeration (> 2 mg O2 L-1) seemed

essential for a good sCOD conversion (de Graaff et al., 2016). Removal efficiencies of pCOD

and cCOD increase linearly as the EPS production increases until an EPS production of

9

approximately 80 mg COD g-1 VSS (Jimenez et al., 2015). EPS production also increased at

higher specific sCOD removal rates.

Microbial communities of A-stage reactors share more core genera with the influent municipal

wastewater than CAS communities (Gonzalez-Martinez et al., 2016). The influent microbial

community entering the A-stage only has a short time to shift and it leaves the bioreactor almost

unchanged. The longer SRT in the CAS bioreactors impacts microbial community structure

more profoundly.

In the B-stage, which resembles a CAS reactor, nitrification and denitrification and

mineralisation of the residual COD occurs. Enough BOD leaves with the effluent of the A-stage

to provide the denitrification in the B-stage. Another approach for nitrogen removal is

autotrophic nitrogen removal from low strength wastewaters (< 100 mg N L-1) at low

temperatures (< 20 °C) in the main stream via the cold anammox process. This process is still

under investigation for application in the main stream (Hendrickx et al., 2012).

Benefits and drawbacks from A-stage technology

The A-stage technology is promising for achieving maximal energy recovery from wastewater

with minimum energy expenditure. More COD is preserved in sludge and the sludge has a

higher digestibility than CAS sludge (Bolzonella et al., 2005). The lower HRT requires smaller

reactor volumes and lowers the systems footprint. The A-stage is able to handle shock loads

and high variations in the influent and guarantees stable operation of the B-stage (Boehnke et

al., 1998). The A-stage technology can be implemented in existing installations as was shown

at the Strass wastewater treatment plant in Austria. This plant proves that it is possible to

achieve energy neutrality. Indeed, 11% of the calorific energy in influent organics, i.e. 54 Wh

PE-1, could be converted in electricity and this was sufficient to supply the energy needed for

the entire plant (Wett et al., 2007).

The high loading of A-stage systems, which coincides with high F/M ratios ( 2 – 10 g BOD g-1

VSS d-1) leads to poor settling sludge with an SVI > 250 mL g-1 (Bisogni & Lawrence, 1971).

Under these conditions low concentrated sludge (± 10 g COD L-1) is yielded and a large part of

10

the COD (40 – 50 %) leaves with the effluent (De Graaff & Roest, 2012). Other techniques

should be looked at to separate the sludge from the water.

Since the A-stage only removes a minor part of the nutrients via the sludge, the remaining

nutrients need to be removed in the B-stage. The A stage favours the processes in B stage by:

buffering shock loads (thus creating a more stable influent for the B-stage) and increasing the

relative abundance of easily degradable COD (Boehnke et al., 1998). Denitrification is possible

in the A-stage but only if nitrates are present in the influent.

The high rate contact stabilisation process

High-rate contact stabilization (HiCS) was defined by Meerburg et al. (2015) as a high-rate

system in a contact stabilization configuration, with a minimal sludge-specific loading rate

of 2 g BOD g-1 VSS d-1 and a maximal SRT of 2 days. In the HiCS system, municipal

wastewater is brought into contact with the activated sludge in a contact tank at low HRT (< 30

min) and under moderate DO conditions (ca. 1 mg O2 L-1) (Gujer & Jenkins, 1975). Removal

of organics happens mainly through sorption on the activated sludge flocs.

The sludge flocs are then separated from the water in a secondary settler. Part of this sludge is

wasted and part of it is sent to a stabilisation tank that is aerated to regenerate the sludge (Figure

3). The stabilizer stabilizes and oxidizes extracellular (particulate and colloidal) and

intracellular (soluble) carbon from the returned activated sludge (RAS) which is rich in carbon

(Rahman et al., 2016). This induces a feast and famine regime and improves the biosorption

capacity and bioflocculation capacity of the sludge (Rahman et al., 2016). This way the

adsorption area and the storage capacity of the RAS is increased (Meerburg et al., 2015).

Figure 3: Process configuration of a contact stabilisation system, adapted from Modin et al.

(2016)

11

3 Coagulation and flocculation

An efficient solid/liquid separation between the sludge and the effluent from the A-stage is vital

for good plant operation. The first step to achieve this separation is coagulation/flocculation

which is a commonly used process in wastewater treatment (John Bratby, 2006). Coagulant and

flocculant are added to the mixed liquor suspended solids (MLSS) to agglomerate the smaller

sludge particles into larger flocs. The performance can be checked based on turbidity

measurements expressed in nephelometric turbidity units (NTU) (Aziz et al., 2007).

The coagulation working principle is based on destabilization of stable suspensions. Sludge

particles bear a negative surface charge which can be measured based on the zeta potential.

These surface characteristics are strongly linked with the EPS content of the sludge (Y. Liu &

Fang, 2003). Charged particles repel each other and cannot grow into a larger floc. Coagulation

counters the negative surface charges of the sludge flocs by applying chemicals bearing positive

charges (coagulants). Three types of inorganic coagulant are mainly used: iron chloride (FeCl3),

aluminium chloride (AlCl3) and poly aluminium chloride (PAC) (John Bratby, 2006).

Flocculation is the subsequent process during which the destabilised particles grow into larger

flocs. This can happen spontaneously (perikinetic flocculation) but will be enhanced by

agitation (orthokinetic flocculation) (Verliefde, 2014). Anionic as well as cationic

polyacrylamide polymers are being applied as flocculant to enhance floc formation by binding

the flocs stronger together and allowing them to grow larger. The flocculant plays a role in

dewatering the sludge and lowers the need for coagulant (Aguilar et al., 2002). The flocculant

molecule consists of a polymer backbone with positively or negatively charged functional

groups and its most important characteristics are molecular weight (MW) and charge density

(CD) (John Bratby, 2006). The increase of the MW and CD will lead to better flocculation but

is limited by decreasing solubility of the polymer and increasing viscosity of the polymer

solution.

12

Two main mechanisms of flocculation in wastewater treatment are (Bolto & Gregory, 2007):

i. Polymer bridging: the polymer is adsorbed on two or more particles thus holding them

together. There should be enough unoccupied space on the particle for polymer

attachment.

ii. Charge neutralisation: particles are most often negatively charged in which case

cationic polyelectrolytes are dosed to neutralize the charge. Their application leads to

the formation of electrostatic patches on the particle surface.

Phosphorous present as orthophosphate is also removed during coagulation/flocculation. This

means that the phosphate content in the wastewater should be accounted for when optimizing

the coagulant dosage based on TSS concentration. Removal can happen the following ways

(Aguilar et al., 2002):

i. Incorporation of the phosphate into the solids in suspension.

ii. Removal as phosphate precipitate with the metal ion used as coagulant.

The mixing regime is important since coagulant needs to be uniformly distributed and the

growing flocs need to ‘meet’ each other. Mixing intensity is expressed as the mean velocity

gradient G (s-1). A rapid mixing phase should be followed by a slow mixing phase (Adachi,

1995). Too vigorous agitation will lead to flocs disruption. Next to basins with mixers, also

plug flow flocculators (PFF) can be used. Velocity gradients in this setup are induced by the

continuous flow reversal in the bends of the pipes.

13

4 Dissolved air flotation (DAF)

Dissolved air flotation (DAF) is a solid/liquid separation process that can be used as an

alternative for settling. The suspended solids are separated from the water by producing low

density bubble-floc aggregates that float to the top of the DAF unit. The DAF process is already

used at large scale for drinking water treatment (Haarhoff, 2008) and to treat several industrial

wastewaters (Poh et al., 2014). It is also used for sludge thickening and could increase the

sludge concentration to 30 – 40 g TSS L-1 (De Rijk & den Blanken, 1994) or even to 60 g TSS

L-1 (EPA, 2003). The main incentives to use DAF are the lower footprint, higher removal

efficiencies and denser sludge production compared with conventional settling (Ødegaard,

2001).

Working principle

The DAF process takes place in a tank (Figure 4) which is divided in two zones by a baffle. In

the first zone, the contact zone, air microbubbles (10 – 100 µm) are introduced in the influent

and aggregate with the flocs (Agarwal et al., 2011). Turbulent as well as laminar flow conditions

occur in the contact zone and it’s configuration is of great importance for the removal

efficiency. The height of the contact zone should be higher than 0.8 m and the contact zone

loading should not exceed 100 m h-1 to assure good mixing but meanwhile prevent floc

disruption (Lundh et al., 2002). In the second zone, the separation zone, the bubble-floc

aggregates are separated from the water phase by floating to the top where they form a floating

layer. This layer is scraped off mechanically (Broeders, Menkveld, et al., 2014).

Part of the effluent, called subnatans, is recycled and brought to a pressure of 400 – 600 kPa in

a pressurizing vessel called the saturator (J. Edzwald, 2010). Air is dissolved in this recycle

water by adding pressurized air. The principle of microbubble generation is based on the higher

solubility of air in water under high pressure, according to Henry’s law. When depressurized,

the recycle flow becomes supersaturated with air and the microbubbles are formed.

14

Figure 4: DAF unit with a contact (left) and a separation (right) zone, adapted from Broeders,

Menkveld, et al. (2014)

The recycle flow is most often described in terms of the recycle ratio R which equals the recycle

flow QR divided by the influent flow Q, and is typically in the order of 10 – 20 % (Lundh et al.,

2002). By depressurizing the recycle water through pressure reduction nozzles in the contact

zone the microbubbles are generated with most bubbles ranging from 40 to 80 µm (Leppinen

& Dalziel, 2004). The microbubbles give the water a milky appearance so that this water is

often referred to as ‘white water’. Increasing saturator pressure yields finer micro-bubbles but

their generation must cope with high energy consumption (0.020 – 0.040 kWh m-3) (Schofield,

2001). Efforts to reduce the energy consumption by oscillating the air stream are currently under

investigation (Brittle et al., 2015).

Bubble-bubble and bubble-particle interactions are affected by several forces: London-van der

Waals, electrostatic and hydrophobic interactions and hydrodynamic retardation. There are two

ways for formation of bubble-particle aggregates (De Rijk & den Blanken, 1994):

i. Inclusion of the air bubble into the sludge floc

ii. Adsorption of the bubbles on the outside of the flocs

A coagulation and flocculation treatment prior to the DAF affects it’s performance in a strong

way. Without coagulation both bubbles and particles carry negative zeta potentials. Successful

15

bubble attachment or adhesion to particles requires charge reduction and also the creation of

hydrophobic spots on particle surfaces (James K. Edzwald, 1995).

Rising velocities of the bubbles and bubble-floc aggregates under optimal conditions are about

20 m h-1 (Haarhoff & Edzwald, 2004). The separation zone hydraulic loadings (𝑣ℎ𝑙) of

conventional DAF units range from 5 to 15 m h-1 which still allows the aggregates to reach the

top of the DAF unit. 𝐴𝑠𝑧 is the footprint area of the separation zone.

𝑣ℎ𝑙 =𝑄 + 𝑄𝑅𝐴𝑠𝑧

High-rate systems also exist with loadings from 15 to 30 m h-1. The hydraulic loading exceeds

the rising velocities of the bubbles. These systems obtain a higher nominal surface area using

lamella’s (James K Edzwald et al., 1999). In contrast, the surface overflow rate of a secondary

clarifier is limited by the settling velocity and is in the order of 1.2 m h-1 . Compared to DAF

with a an average surface loading of 12 m h-1 the footprint of a settler is a factor 10 higher and

the retention time is brought down from several hours to about 30 minutes (Ding et al., 2015).

Applications of DAF at MWTP’s

A DAF was investigated at pilot scale in Finland as tertiary treatment of municipal wastewater

(Koivunen & Heinonen‐Tanski, 2008). High removal efficiencies of enteric microbes (90 – 99

%), COD (55 – 81 %) and TP (29 – 39 %) were obtained at a PAC coagulant dose of 10 mg

Al3+ L-1. The treatment of primary wastewater effluents was investigated to assess the

performance of DAF in the treatment of by-pass wastewaters during WWTP overloading. The

removal efficiency of COD and TSS was 47 % and 77 % respectively (Koivunen & Heinonen‐

Tanski, 2008).

In the captivator system® the DAF is used to provide solid/liquid separation after a biosorption

process. The DAF functions as a combined liquid separator and sludge thickener (Ding et al.,

2015).

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5 Anaerobic digestion of high rate sludge

The role of anaerobic digestion at the modern WWTP

Sludge treatment is a major issue at municipal wastewater treatment plants and its disposal

represents up to 50 % of the operating costs (Davis & Hall, 1997). Anaerobic digestion (AD) is

a microbial technology that is often applied at WWTPs to simultaneously stabilize and

minimize the amount of sludge that has to be disposed of (Appels et al., 2008). The AD process

converts the organics in sludge into renewable energy in the form of biogas which is a mixture

of 60 – 70 % CH4, 30 – 40 % CO2 and can contain traces of H2O, N2, H2S, NH3 and H2 (Shen

et al., 2015). At WWTP level, the primary and secondary sludge are combined and thickened

before entering the digesters (Figure 5). The supernatant from the digester is denitrified or sent

back to the influent and the residual sludge is dewatered.

Figure 5: Flow chart of the anaerobic digestion process in municipal wastewater treatment

systems (Appels et al., 2008)

17

The AD process is considered as an essential part of the modern WWTP as it removes most of

the pathogens present in the sludge and reduces the sludge volume and odour problems, (Appels

et al., 2008). There are some limitations as the organic fraction is only partially decomposed

and reaction rates are quite slow. The process is complex and vulnerable to inhibition. Next,

the biogas contains not only CH4 but also other constituents that need to be removed for

applications like grit injection or car fuel (Persson et al., 2006).

Biogas can be upgraded and injected into the gas grit or used as vehicle fuel, but the main

application of biogas worldwide is direct use through combined heat and power production via

a combined heat and power (CHP) unit at the WWTP itself (Adachi, 1995). Upgrading the

biogas implies removing CO2, H2S and H2O. Removal is mostly done via ad- or absorption

processes. Also cryogenic condensation and membrane separation are possible techniques

(Deublein & Steinhauser, 2011).

General process principles

During AD, which takes place in absence of oxygen, organics are transformed into digestate

and biogas. This complex process can be divided in four phases (Batstone & Virdis, 2014) that

are shown in Figure 6. These phases are (1) hydrolysis during which polymeric organic

compounds (polysaccharides, proteins and fat) are hydrolyzed by extracellular enzymes into

soluble organic substances; (2) acidogenesis, during which the smaller hydrolysis products are

converted into hydrogen gas, acetate and other volatile fatty acids (VFA); (3) acetogenesis,

where short-chain organic acids and alcohols produced by acidogenesis are metabolized into

acetic acid and hydrogen by acetate forming bacteria; and (4) methanogenesis, during which

biogas is produced from hydrogen, formate, and acetate by two groups of methanogenic

archaea. Acetoclastic methanogens produce methane in syntrophic relation with acetate

producing bacteria while hydrogenotrophic methanogens use hydrogen gas as electron donor

and CO2 as electron acceptor.

Of these four steps, hydrolysis is considered as the rate limiting step due to the slow breakdown

of polymeric substances (Tiehm et al., 2001).

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Figure 6: The different microbial steps in the AD process (Appels et al., 2008)

Methanogens are sensitive to pH changes and thrive best between pH 6.5 and 7.2 (Boe &

Angelidaki, 2006). Anaerobic digestion is carried out at a pH in this range. Typically AD of

sludge is carried out at mesophilic temperatures (35 °C) so that more organic compounds are

solubilized and the metabolic rates are increased compared to standard temperatures. Also

pathogens are destroyed at a higher pace (Rehm & Winter, 1999). On the other hand ammonia

and VFA toxicity increases with temperature and high fluctuations (> 1 °C day-1) should be

avoided (Boe & Angelidaki, 2006).

Several compounds are known to inhibit the AD process and they can either be present in the

substrate or be generated during the digestion process (Kroeker et al., 1979). Free ammonia

(NH3) damages the MO’s by passing through the cell membrane and causing proton imbalances

(Chen et al., 2008). Higher temperatures and increasing pH values result in a shift from ionised

(NH4+) to free (NH3) ammonia species, and this increases the toxicity.

Sulphide inhibition can have two reasons. First, there is competition for substrate between

sulphate reducing bacteria (SRB) and the fermentative bacteria and methanogens. Second, there

is sulphide toxicity which affects acetogens and methanogens the most (Boe & Angelidaki,

2006). Third, the formation of metal sulphides may lead to trace element deficiencies. Due to

process imbalances, situations can occur wherein the methanogens are not able to convert the

19

VFA fast enough to CH4 and CO2. This results in an accumulation of VFA, and the pH

decreases, so there is a shift to the non-dissociated form of the VFA. These can pass freely

through the cell membrane and cause pH reduction in the cell.

The cell growth must compensate for the biomass loss when digestate is removed. Stable

digestion can only be achieved at SRTs > 10 days (Miron et al., 2000). Typical loading rates

are in the order of 1 – 2 kg VSS m-3 day-1 for WAS digestion and specific gas production of

WAS decreases as the SRT in the CAS process is raised. Bolzonella et al. (2005) observed a

decreasing specific gas production from 0.18 to 0.07 m³ kg-1 VSfed when the SRT in the CAS

process was raised from 8 to 35 days. This stresses the potential of high rate systems like the

A-stage or HiCS process to produce better digestible sludge.

Iron rich A-stage sludge has been used in co-digestion with kitchen waste and it improved the

stability of the methane production (De Vrieze et al., 2013). The iron content could be correlated

linearly with methanogenesis during single digestion of A-stage sludge (Cagnetta et al., 2016).

6 Research questions

In short the research questions are as follows: what are the maximum achievable removal

efficiencies of a HRAS-DAF system for TSS, VSS and tCOD? How much of the influent

organics can be captured in the sludge? How well concentrated is HRAS-DAF sludge?

Is there an influence of flotation on the anaerobic digestion of HRAS-DAF sludge?

20

PART 2: MATERIALS & METHODS

21

1 Pilot scale HRAS-DAF set-up

A pilot-scale HRAS system, coupled with DAF separation in place of a settler, was built in

collaboration with the companies Aquafin and Nijhuis Water Technologies. The pilot HRAS-

DAF consisted of a contact tank with a working volume of 2 m3 (diameter 1.6 m) and a 0.66

m3 DAF unit for solids/liquid separation. Municipal wastewater (characteristics are given in

Table 1) was collected from the WWTP of Aartselaar (Belgium) after grit removal and sand

trap, filtered in a drum filter (1 mm pore size) to remove coarse particles (Figure 8, nr.1), and

fed to the contact tank at a flowrate of 2 m³ h-1.

Sludge recirculation was set to keep the MLSS concentration in the A-stage contact tank at 1 g

TSS L-1. This was done by manually changing the on/off time of the sludge return pump. The

A-stage contact tank was equipped with a pH-probe (Hach, Germany), a DO-sensor (Optical

Dissolved Oxygen Probe LDO® Model 2, Hach) and a TSS-sensor (Suspended Solids InSitu

Sensor, Hach). The feeding of the wastewater was based on level control in the A-stage tank.

The MLSS were transferred from the A-stage tank to the DAF by means of a cavity pump. For

optimal mixing, the coagulant was added via a peristaltic membrane pump directly before the

cavity pump. The polymer(s) were dosed directly in the pipes used to transfer the MLSS from

the contact tank to the DAF unit (further details in the next sections). The polymers were

prepared as a 0.1 % solution and stored for 1 – 3 days.

The DAF unit (Figure 8, nr.4) had a volume of 0.66 m³ and an HRT of 20 – 25 min. The

saturator pressure was 0.7 MPa and an air flow between 0.5 – 0.7 L min-1 was used. The water

level in the DAF could be adjusted manually. A scraper removed the floating sludge, which

was then pumped to a stirred collection tank. The scraper speed could be altered between 10

and 70 Hz and also the on/off time could be adjusted. From the collection tank (shown

schematically on Figure 7), which was equipped with a DO, TSS and level sensor (Hach,

Germany), the sludge was recirculated to the A-stage contact tank to maintain a solids

concentration of 1 g L-1 or wasted. The DAF unit was equipped with an online turbidity sensor

(SOLITAX sc200 Turbidity Analyzer, Hach, Germany) that was used to measure the turbidity

22

of the effluent. At the start of each treatment and also on day 64 of treatment 1, the collection

tank was filled with fresh CAS sludge (8.5 ± 1.5 g TSS L-1) from the WWTP at Aartselaar.

Figure 7: Schematic representation of the HRAS-DAF pilot plant during A-stage-DAF

operation

Figure 8: The components of the HRAS-DAF pilot: 1. Drum filter, 2. A-stage tank, 3. Plug flow

flocculator, 4. DAF unit

23

An automated sand drain at the bottom of the DAF unit periodically removed settled

particulates. During the operation of the HRAS-DAF pilot, there were two different system

configurations, resulting in three different treatments (these are described further in Table 2).

Treatment 1, 2 and 3 will be referred to as T1, T2 and T3, respectively.

Table 1: Overview of the influent characteristics during T1, T2 and T3

A-stage - DAF HiCS - DAF

Treatment 1 Treatment 2 Treatment 3

Total chemical oxygen demand, tCOD (g L-1) 0.36 ± 0.12 0.16 ± 0.04 0.18 ± 0.05

Soluble chemical oxygen demand, sCOD (%) 28 31 33

Total suspended solids, TSS (g L-1) 0.17 ± 0.09 0.08 ± 0.02 0.08 ± 0.02

Volatile suspended solids, VSS (%) 81 85 87

Total Kjeldahl nitrogen, TKN (mgN L-1) 52 ± 11 40 ± 10 28 ± 10

Total ammonia nitrogen, TAN (mgN L-1) 32 ± 10 16 ± 4 22 ± 6

Phosphate, P (mg L-1) 1.9 ± 0.5 1.3 ± 0.7 1.3 ± 0.5

Some days of operation were confronted with foam formation in the A-stage tank (Figure 9)

and in the DAF unit.

Figure 9: Foam formation in the A-stage tank

The overall A-stage-DAF removal efficiency 𝜂𝑝𝑖𝑙𝑜𝑡 was calculated for TSS, VSS, tCOD, using

the influent (summarized in Table 1) and the effluent concentrations.

24

𝜂𝑝𝑖𝑙𝑜𝑡 = 1 −𝐶𝑒𝑓𝑓

𝐶𝑖𝑛𝑓

The removal efficiency of the DAF itself 𝜂𝐷𝐴𝐹 was calculated accordingly based on the effluent

concentrations and the incoming MLSS concentrations.

𝜂𝐷𝐴𝐹 = 1 −𝐶𝑒𝑓𝑓

𝐶𝑀𝐿𝑆𝑆

Configuration 1: A-stage

The HRAS-DAF pilot was operated in an A-stage configuration for 106 days during T1 and for

67 days during T2. In this configuration, the A-stage contact tank was mixed and aerated (DO

set point of 3.5 mg L-1), while the collection tank was mixed but not aerated.

1.1.1 Treatment 1: dosing of FeCl3 and A130hp

Treatment 1 lasted 106 days. 50 mg L-1 of FeCl3 (Kemira) and 3 mg L-1 anionic polymer

(A130hp, Kemira Superfloc) were dosed to assist coagulation and flocculation, respectively.

The coagulant was prepared as a 10 % solution in a 0.2 m³ tank. During the first 64 days of

operation the FeCl3 was dosed to the MLSS and was then pumped through 4.5 m pipes.

Thereafter the polymer was added and the MLSS was transported through another 1.5 m of

pipes before entering the DAF unit. Also, the pressurized water was added at the entrance of

the DAF unit.

After 64 days of operation, a 17.5 m plug flow flocculator (PFF) was taken in operation to

increase the flocculation time and enhance flocculation. The FeCl3 was added to the MLSS and

pumped through pipes of a 4.5 m length, after which roughly half of the pressurized water was

added to the pipes to increase flocs stability. Subsequently the MLSS was pumped through the

17.5 m PFF before entering the DAF unit. A130hp polymer was dosed in the PFF after 0.5 m.

1.1.2 Treatment 2: dosing of FeCl3, C492 and A130hp

Treatment 2 was operated for 67 days and 50 mg L-1 of FeCl3 and a combination of 2 mg L-1

cationic polymer (C492, Kemira) and 0.5 mg L-1 A130hp were added to assist coagulation and

flocculation. The addition of FeCl3 and white water was performed as from day 65 to day 106

25

of T1. The C492 polymer was dosed in the PFF after 0.5 m. The A130hp polymer was dosed

0.5 m further.

Configuration 2: HiCS operation (treatment 3)

Before starting T3, the configuration of the pilot was changed from A-stage to high rate contact

stabilisation (HiCS). The tank which was previously used as a collection tank for the sludge

was now aerated (DO set point of 3.5 mg O2 L-1) and served as stabilization tank. The aerated

A-stage contact tank was changed into an unaerated contact tank. The adaptations are

schematically visualized in Figure 10.

Treatment 3 lasted 46 days. The use of the PFF and the dosing points of coagulant, white water

and flocculant were the same as during T2. 30 mg L-1 of FeCl3 and a combination of 1.5 mg L-

1 cationic polymer (C492) and 0.5 mg L-1 anionic polymer (A130hp) polymer were added

during T3.

Figure 10: Schematic representation of the pilot during HiCS operation

MLSS characteristics

The chemical dosing as well as the MLSS characteristics from each treatment are given in Table

2. During T1 the molar ratio Fe:PO43- was 5.03. The T2 was characterized by the highest Fe:P

ratio (7.52) since the same amount of FeCl3 was dosed as during T1 but the influent contained

less phosphate

26

The F/M ratio of T2 and T3 were similar since the influent was similar. Although less iron was

dosed during T3, the ash content was higher (lower VSS:TSS ratio). In any treatment, the molar

ration of Fe to P was higher than 4.5. This is high compared to other A-stage systems where the

Fe:P ratios are between 0.24 – 0.79 (De Graaff & Roest, 2012).

Table 2: Overview of the chemical dosing and MLSS characteristics during T1, T2 and T3



Chemical dosing

FeCl3 50 mg L-1

(10 %; 1 L h-1)

50 mg L-1

(5 %; 2 L h-1)

30 mg L-1

(3 %; 2 L h-1)

Cationic polymer (C492, Kemira) - 2 mg L-1

(0.1 %; 4 L h-1)

1.5 mg L-1

(0.1 %; 3 L h-1)

Anionic polymer (A130hp, Kemira) 3 mg L-1

(0.1 %; 6 L h-1)

0.5 mg L-1

(0.1 %; 1 L h-1)

0.5 mg L-1

(0.1 %; 1 L h-1)

MLSS characteristics



Food to Microorganisms ratio, F/M

(kg COD kg-1 MLVSS d-1)

13.9 8.6 7.9

mol Fe/ mol P 5.03 7.52 4.51

Digesters

Two 60 L pilot-scale digesters were operated at a HRT of 18 days. These were inoculated with

digestate from a full-scale anaerobic digester from Aquafin, Leuven. The digesters were mixed

mechanically, and operated at mesophilic conditions (35 °C). The sludge feeding was done

continuously by a peristaltic pump with a flow rate of 170 mL h-1. The gas flow was measured

by a MGC-10 Ritter® flow meter. Each of the two digesters had its own sludge buffer tank

from which the sludge was fed. The buffer tanks were filled manually with fresh sludge on

Monday, Wednesday and Friday. Each buffer tank was equipped with a mechanical mixer and

a level sensor. For the A-stage-DAF configuration, A-sludge generated from the pilot was used

solely to feed the digesters, and these were run as duplicates. For the HiCS-DAF configuration,

27

one of the digesters was fed with HiCS-sludge, while the second digester was fed with a mixture

of WAS sludge (Aartselaar, Belgium) and HiCS-sludge at a TSS ratio of 3:7. Digestion of

HRAS-sludge (A-stage or HiCS) generated during different treatments was carried out for 54

days (3 x 18 days) for each treatment.

Figure 11: The digesters at Aquafin, Aartselaar: 1. Buffer, 2. Digester, 3. Peristaltic pump, 4.

Gas flow meter

Sampling

Samples from the influent wastewater (before and after the drum filter), MLSS, sludge and

effluent taken collected three times per week, and analysed for TSS, VSS, tCOD and sCOD.

Total Kjeldahl nitrogen (TKN) and Total ammonia nitrogen (TAN) were analysed every two

weeks. Phosphate content was analysed for influent and effluent samples.

28

Gas samples from the digesters’ head spaces were collected twice a week using 10 mL

vacutainers. The biogas composition was determined using a Compact GC. The digestate from

both digesters was sampled on the same days as the gas samples. The pH, conductivity, TSS,

VSS, tCOD and VFA content were determined for each sample.

2 Jar testing

Experiment 1: Optimal polymer selection for flotation

At the start of T1, jar tests were performed to select which polymer to use to assist flocculation.

1 L flasks were filled with 500 mL MLSS at the pilot installation and 50 mg L-1 FeCl3 was

added in combination with a cationic polyacrylamide (PAM) polymer (C492 or C494 or C496,

Kemira) or an anionic PAM polymer (A130hp, Kemira). Polymer concentrations ranging from

0.5 to 4 mg L-1 were tested. The flasks were stirred for 10 minutes at 270 rpm using a magnetic

stirrer. Then 200 mL white water was added to each flask and the flotation performance was

evaluated visually.

The cationic polymers are characterized by a medium (C492, 105–106 g mol-1) to high (C494

and C496, >106 g mol-1) molecular weight. The charge density (CD) as well as the viscosity

increases from low (C492, CD 20 %, 80 mPa.s for 0.1 % solutions) to medium (C494) and high

(C496, 130 mPa.s for 0.1 % solutions) (Kemira, 2010). A130hp has a higher molecular weight

(> 107 g mol-1) than the previous three cationic polymers, and has a CD of 30 %.

Experiment 2: Optimal flocculant dosage (T1)

The 1 L flasks were filled with 500 mL MLSS sampled at the HRAS-DAF pilot plant. FeCl3

(50 mg L-1) was added in combination with different concentrations of anionic polymer

(A130hp) (0.75 – 6.5 mg L-1). The mixtures were stirred subsequently for a few seconds at 500

rpm. Then 200 mL of white water was added. After 20 minutes of flotation, a subnatans sample

was taken from each beaker, and was analysed for TSS and for optical density (OD) at 610 nm.

29

Experiment 3: Optimal flocculant dosage (T2)

The 1 L flasks were filled with 500 mL MLSS sampled at the HRAS-DAF pilot plant. The

FeCl3 (50 mg L-1) was added in combination with different concentrations of cationic (C492)

(0.8 – 3.8 mg L-1) and anionic (A130hp) (0.1 – 0.9 mg L-1) polymers. The same procedure as

in experiment 2 was followed.

Experiment 4: Optimal coagulant & flocculant dosage (T3)

1 L flasks were filled with 500 mL MLSS sampled at the HRAS-DAF pilot plant. A130hp

polymer (1 mg L-1) and different combinations of cationic C492 (2 – 3 mg L-1) and FeCl3 (5 –

45 mg L-1) were added. The same procedure as in experiment 2 was followed.

3 BMP tests

Biochemical methane potential (BMP) tests were performed in batch reactors, each with a

working volume of 80 mL and a headspace of 40 mL. The experiments were operated for 18

days at mesophilic conditions (35 °C). The inoculum was collected from a full-scale anaerobic

digester in Leuven WWTP (Belgium) or from the pilot-scale digesters in Aartselaar. To each

flask, first, a specific amount of inoculum was added to obtain a final VS concentration of 10 g

VSS L-1. Second, A-sludge, A-sludge with addition of M9 minimal medium (modified with no

glucose addition, Table 3) or A-sludge with addition of vitamins and trace elements (Table 4)

was added (except for the negative controls) to obtain a substrate to inoculum ratio of 0.5 g

COD g-1 VSS. Finally, tap water was added to acquire a total liquid volume of 80 mL in each

bottle, irrespective of the selected inoculum or substrate. For each inoculum triplicate negative

controls were run, containing the selected inoculum at a concentration of 10 g VSS L-1, to

estimate endogenous methane production.

After inoculum and substrate addition, the serum flasks were sealed to avoid air intrusion, and

connected to air-tight gas columns by means of an air-tight needle. These gas columns were

placed in a water bath containing distilled water at pH < 4.3 to avoid CO2 in the biogas from

dissolving. The serum flasks were incubated in a linear shaking water bath (Aqua 12 Plus,

30

Novolab, Geraardsbergen, Belgium). Volumetric biogas production was evaluated by means of

water displacement in the gas columns. Biogas production was measured on daily basis for 7

days, until the biogas production was below 1 – 3 % of total production in all treatments for 3

consecutive days. Biogas volumes were reported at standard temperature (273 K) and pressure

(101325 Pa) (STP). Biogas composition was evaluated at the end of the experiment. Methane

yield was expressed as COD yield as the fraction of substrate COD converted to methane.

During T1, two BMP tests were performed on A-sludge from day 14 and 76 with inoculum

from full scale anaerobic digesters from Leuven. Another BMP was performed on A-sludge

from day 92, this time with inoculum from the pilot scale digesters in Aartselaar.

During T2, a BMP test was performed on A-sludge from day 54 with inoculum from Aartselaar.

One additional BMP was performed on the A-sludge with addition of M9 minimal feeding

medium. A second BMP was done on A-sludge with addition of vitamins. Last, a BMP in which

a combination of M9 and vitamins was added, was performed.

During T3, BMP tests were performed on HiCS-sludge from day 21 and 38 with inoculum from

the digesters in Aartselaar.

31

Table 3: Composition of M9 minimal medium

Component Volume %

M9 salts solution 20

Glucose (20%; Sigma-Aldrich)a 2

MgSO4 (1 M; Fisher Scientific)b 0.2

CaCl2 (1 M; Fisher Scientific)b 0.01

H2O 78

aFilter-sterilize and store at 4°C.

bAutoclave and store at room temperature. cM9 salts solution composition Concentration (g L-1)

Na2HPO4.7H2O 64

KH2PO4 15

NaCl 2.5

NH4Cl, 5

Table 4: Composition of trace element and vitamin solution

Trace element solution Vitamin solution

Nitrilotriacetic acid 1.50 g Biotin 2.00 mg

MgSO4.7H2O 3.00 g Folic acid 2.00 mg

MnSO4.H2O 0.50 g Pyridoxine-HCl 10.00 mg

NaCl 1.00 g Thiamine-HCl.2H2O 5.00 mg

FeSO4.7H2O 0.10 g Riboflavin 5.00 mg

CoSO4.7H2O 0.18 g Nicotinic acid 5.00 mg

CaCl2.2H2O 0.10 g D-Ca-pantothenate 5.00 mg

ZnSO4.7H2O 0.18 g Vitamin B12 0.10 mg

CuSO4.H2O 0.01 g p-Aminobenzoic acid 5.00 mg

KAl(SO4)2.12H2O 0.02 g Lipoic acid 5.00 mg

H3BO3 0.01 g Distilled water 1000 ml

Na2MoO4.2H2O 0.01 g

NiCl2.6H2O 0.03 g

Na2SeO3.5H2O 0.30 mg

Na2WO4.2H2O 0.40 mg

Distilled water 1000 ml

32

4 Fed batch digesters

Two 2 L scotch bottles were filled both with 890 mL inoculum from a full-scale anaerobic

digester in Leuven WWTP (Belgium) and 110 mL HRAS-sludge from the HRAS-DAF pilot in

Aartselaar summing up to an operational volume of 1 L. They were fed in batch mode on

Monday, Wednesday and Friday and operated at mesophilic conditions (35 °C). The inoculum

had a VS content of 25 g L-1. To impose an SRT of 18 days, each Monday and Wednesday 110

mL of digestate was removed, and 110 mL fresh HRAS-sludge was added. On Friday 168 mL

digestate was removed, and 168 mL fresh sludge was added. At day 55, 1 ml of the trace

elements and vitamins solutions (Table 4) was added together with 2.68 g L-1 Na2HPO4 and

0.94 g L-1 KH2PO4.. Gas production was followed by connecting each fed batch digester to a

water column. Digestate was sampled and analysed for pH, conductivity and VFA content each

time the digesters were fed.

5 Chemical analyses

COD analysis

The Chemical Oxygen Demand (COD) is the amount of oxygen required to oxidize organic

carbon completely to CO2 by chemical means. Determination of total and/or soluble COD of

aqueous (wastewater, effluent) or slurry-like (MLSS, sludge) samples quantifies the

concentration of organic constituents.

The COD analysis on MLSS and sludge samples was performed according to Standard Methods

for the examination of water and wastewater (5220-C, APHA, 1992). This is the classical

oxidation method, using an excess of potassium dichromate (K2Cr2O7) in an acid environment

and high temperature (148 °C) in the presence of silver sulphate (AgSO4) as catalyst. After

oxidation, the COD is determined by a back titration in which the excess of unreduced K2Cr2O7

is titrated with an iron ammonium sulphate (FeNH3SO4) solution with ferroin as indicator.

Mercuric sulphate (Hg2SO4) is added in order to precipitate the chlorides to minimise their

33

interference. Samples with a COD content higher than 900 mg L-1 were diluted as the required

concentration range of this method is between 100 and 900 mg L-1.

The COD analysis of non-coloured samples (influent & effluent) was measured using

Nanocolor® kits (CODE; Macherey-Nagel) of the type COD - 160 with an analysis range of

15 – 160 mg COD L-1. For determination of sCOD, the samples were centrifuged and filtered

with 0.45 m pore diameter filters, and then also analysed using Nanocolor® kits. Starting from

day 19 of T2, it was decided not to use the titration method anymore and use the Nanocolor®

kits type COD - 1500 with an analysis range of 100 - 1500 mg COD L-1 for COD determination

of the MLSS and sludge samples. In a comparison experiment, only minor differences (± 3 %)

were observed between the titration and the colorimetric method.

Solids analysis

Total suspended solids (TSS) and volatile suspended solids (VSS) analysis was performed

according Standard Methods 2540D and E (APHA, 1997). The TSS measurements included

filtering the samples over a 0.45 µm filter, and drying them overnight at 105°C. The filters were

weighed before and after. For VSS measurements, the dried filters were placed in a furnace

(Nabertherm LE6/11/B150, Germany) at 550 °C for 1.5 h, and afterwards they were weighed

again. The calculation of the TSS was done by distracting the clean filter weight from the dried

filter weight and dividing by the applied sample volume. The VSS was calculated by distracting

the muffled filter weight from the dried filter weight and dividing by the applied sample volume.

VFA analysis

The C2 – C8 fatty acids (including isoforms C4 – C6) were measured by gas chromatography

(GC-2014, Shimadzu®, The Netherlands) with a DB-FFAP 123-3232 column (30m x 0.32 mm

x 0.25 μm; Agilent, Belgium) and a flame ionization detector (FID).. The 2 mL liquid samples

were conditioned with 0.5 mL concentrated sulphuric acid (H2SO4) and 0.4 g sodium chloride

(NaCl) and 400 µL 2-methyl hexanoic acid as internal standard for quantification. Extraction

of the VFA was done by adding 2 mL diethyl ether. The samples were than rotated (2 min) and

centrifuged (3 min, 1000 g), and the diethyl ether phase was moved to a GC vial. The prepared

sample (1 μL) was injected at 200 ºC with a split ratio of 60 and a purge flow of 3 mL min-1.

34

The oven temperature increased by 6 ºC min-1 from 110 ºC to 165 ºC where it was kept for 2

min. The FID temperature was 220 ºC. The carrier gas was N2 at a flow rate of 2.49 mL min-1.

Gas analysis

The gas phase composition was analysed with a Compact GC (Global Analyser Solutions,

Breda, The Netherlands), equipped with a Molsieve 5A pre-column and Porabond column

(CH4, O2, H2 and N2) and a Rt-Qbond pre-column and column (CO2, N2O and H2S).

Concentrations of gases were determined by means of a thermal conductivity detector, which

had a detection limit of 100 ppmv.

Anion analysis

Phosphate (PO43-) in influent and effluent collected from the HRAS-DAF pilot plant was

determined on a 761 Compact Ion Chromatograph (Metrohm, Switzerland), equipped with a

conductivity detector.

Nitrogen analysis

Kjeldahl nitrogen (TKN) and total ammoniacal nitrogen (TAN) were analysed using steam

distillation, according to standard methods (Greenberg et al., 1992). The TKN includes the

organic nitrogen and TAN. The organic nitrogen can be determined by the subtraction of the

TAN (total ammoniacal nitrogen) from the TKN. During Kjeldahl analysis, the distillation is

preceded by a destruction phase. The organic nitrogen present in the sample is transformed into

ammoniacal nitrogen (NH4)2SO4 by means of destruction at 380°C with sulphuric acid (H2SO4)

(98 %) and potassium and copper sulphate (K2SO4, CuSO4) as a catalyst. During steam

distillation, 20 mL of sample (for TAN analysis) or destructed sample (for TKN analysis) is

distilled, and the NH3 is captured in a boric acid indicator with an initial pH of 5.3. The NH3

that is captured in that acid solution (as (NH4)3BO3) was titrated with hydrochloric acid (HCl,

0.02N). The titration was carried out with a pH meter.

35

PART 3: RESULTS

36

1 Pilot scale HRAS-DAF

Results that were obtained during the three different HRAS-DAF pilot treatments will be

presented in this chapter. On each time plot describing the HRAS-DAF pilot results, the

transitions between the different treatments are indicated with black vertical dotted lines.

Another grey vertical dotted line was added on day 65 to indicate the day the PFF was taken in

use.

The primary goal of the HRAS-DAF pilot plant was to achieve high organics removal from the

influent. Next, a good solid/liquid separation in the DAF unit was pursued as well for good

effluent quality as for maximal preservation of the influent organics in the sludge. Since the

DAF was desired to work as a combined settler and thickener, sludge concentration was

analysed, and is also presented in this chapter. Finally, the results obtained from the pilot scale

digesters are given.

Organics removal

The overall HRAS–DAF pilot removal efficiency 𝜂𝑝𝑖𝑙𝑜𝑡 was calculated for TSS, VSS and tCOD

, based on the influent and the effluent concentrations. The average removal efficiency for each

treatment is given in Table 5. A timeline showing the removal during T1, T2 and T3 is shown

in Figure 12 .

Table 5: Overview of the HRAS-DAF TSS, VSS and tCOD removal during T1, T2 and T3

Period TSS VSS tCOD

T1 (day 1 - day 64) 51 ± 17 % 53 ± 19 % 50 ± 13 %

T1 (day 65 - day 106) 77 ± 14 % 81 ± 10 % 62 ± 13 %

T2 71 ± 10 % 73 ± 10 % 58 ± 10 %

T3 67 ± 10 % 68 ± 9 % 54 ± 9 %

37

Figure 12: Timeline of the HRAS-DAF pilot removal efficiencies for TSS, VSS and tCOD during T1, T2 and T3. The grey dotted line at day 65

indicates the implementation of the plug flow flocculator.

0

20

40

60

80

100

0 20 40 60 80 100 120 140 160 180 200 220

Rem

oval

eff

icie

ncy

(%

)

Time (days)

TSS

VSS

CODTSS: 51 ± 17 %

VSS: 53 ± 20 %

tCOD: 50 ± 13 %

TSS: 77 ± 14 %

VSS: 81 ± 10 %

tCOD: 62 ± 13 %

TSS: 71 ± 10 %

VSS: 73 ± 10 %

tCOD: 58 ± 10 %

TSS: 67 ± 10 %

VSS: 68 ± 9 %

tCOD: 54 ± 9 %

A-stage-DAF treatment 1 A-stage-DAF treatment 2 HiCS-DAF treatment 3

38

1.1.1 A-stage-DAF T1

During T1 the influent wastewater had a COD concentration of 0.38 ± 0.12 g L-1 and could be

classified as a medium-strength wastewater. The first 64 days of operation featured a short

distance between polymer addition and the entrance of the DAF and the inherent short

flocculation time could only support the growth of small flocs. The A-stage-DAF pilot removed

51 ± 17 % of the influent TSS, 53 ± 19 % of the VSS and 50 ± 13 % of the tCOD, and no stand-

alone operation could be achieved for longer than 2 – 3 days. After day 64 and the

implementation of the PFF, the removal was increased to 77 ± 14 % of the TSS, 81 ± 10 % of

the VSS and 62 ± 13 % of the tCOD.


During T2, the wastewater had a COD concentration of 0.18 ± 0.04 g L-1, and could be classified

as a low-strength wastewater. The A-stage-DAF operation was stable, and the overall removal

was quite constant. An average removal efficiency of 71 ± 10 % was obtained for the influent

TSS, 73 ± 10 % of the VSS and 58 ± 10 % of the tCOD.

1.1.3 HiCS-DAF T3

During T3, the wastewater had a concentration of 0.16 ± 0.03 g COD L-1, and could be classified

as a low-strength wastewater. The DAF operation was stable, and no process failures occurred

during this treatment. Average removal was 67 ± 10 % of the influent TSS, 68 ± 9 % of the

VSS and 54 ± 9 % of the tCOD.

39

DAF separation efficiency

To evaluate the performance of the DAF unit, the TSS, VSS and tCOD separation efficiency

𝜂𝐷𝐴𝐹 was calculated based on the effluent concentrations and the incoming MLSS

concentrations. The separation efficiencies of the DAF unit during each treatment are given in

Table 6. A timeline showing the DAF unit separation efficiency during T1, T2 and T3 is shown

in Figure 13 .

Table 6: Overview of the DAF unit TSS, VSS and tCOD separation during T1, T2 and T3

Period TSS VSS tCOD

T1 (day 1 - day 64) 90 ± 7 % 88 ± 7 % 83 ± 8 %

T1 (day 65 - day 106) 97 ± 2 % 96 ± 2 % 88 ± 7 %

T2 96 ± 3 % 95 ± 3 % 90 ± 3 %

T3 95 ± 4 % 93 ± 4 % 86 ± 8 %

The microbubble quality was evaluated visually each day by filling a bottle with pressurized

water from the DAF. Good microbubble quality was assumed when the water had a milky

appearance and when the bubbles stayed in suspension at least 2 minutes. Based on this

determination it was decided whether or not to change the air flow (always between 0.5 – 1 L

min-1) or to flush the pressurized water tubing to remove impurities from the recycle stream.

40

Figure 13: Timeline of the DAF unit separation efficiencies for TSS, VSS and tCOD during T1, T2 and T3. The grey dotted line at day 65 indicates

the implementation of the plug flow flocculator

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180 200 220

Rem

oval

eff

icie

ncy

(%

)

Time (days)

TSS

VSS

tCOD


41


It can be observed from Figure 13 that the operation of the DAF unit was not stable before day

64. For several days, only 60 – 70 % tCOD removal was achieved. Average removal efficiencies

during the first 64 days were 90 ± 7 % of the TSS, 88 ± 7 % of the VSS and 83 ± 8 % of the

tCOD respectively. After day 64, the DAF performance became more stable, and the removal

efficiency of TSS, VSS and tCOD was increased to 97 ± 2 %, 96 ± 2 % and 88 ± 7 %,

respectively.


The DAF unit operation was stable during the whole period of T2. Average separation

efficiencies were 96 ± 3 % of the TSS, 95 ± 3 % of the VSS and 90 ± 3 % of the tCOD.

1.2.3 HiCS-DAF T3

The tCOD removal was low (± 70 %) on day 14, 15, 31 and 41. The main reason for the low

performance of the DAF was the inferior microbubble quality at those days. The white water

had no milky appearance and the bubbles were relatively large and didn’t stay in suspension for

longer than a minute. Impurities in the recycle stream were presumably causing the inferior

microbubble quality, and effluent results for these days were not considered. Average removal

efficiencies were 95 ± 4 % of the TSS, 93 ± 4 % of the VSS and 86 ± 8 % of the tCOD.

1.2.4 Effluent characteristics

The average HRAS-DAF effluent composition for each treatment is included in

42

Table 7. During T1, the influent had a higher strength than during T2 and T3. The effluent

produced during T1 (0.17 ± 0.06 g COD L-1) also had a higher strength compared to T2 (0.07

± 0.03 g COD L-1) and T3 (0.09 ± 0.03 g COD L-1). This explains why the overall HRAS-DAF

pilot organics removal remained roughly the same over the three treatments.

The VSS:TSS ratio was 75 % for T1, 82 % for T2 and 79 % for T3. This is slightly lower than

for the influent. The sCOD:tCOD ratio was low during T1 (36%) but increased for T2 (56 %)

and T3 (53 %).

43

Table 7: Overview of the effluent characteristics during T1, T2 and T3





Total chemical oxygen demand, tCOD (g L-1) 0.17 ± 0.06 0.07 ± 0.03 0.09 ± 0.03

Soluble chemical oxygen demand, sCOD (%) 36 56 53

Total Kjeldahl nitrogen, TKN (mgN L-1) 44 ± 9 42 ± 10 28 ± 12

Total ammonia nitrogen, TAN (mgN L-1) 27 ± 8 17 ± 1 22 ± 1

Phosphate, P (mg L-1) 1.2 ± 0.4 0.01 ± 0.03 0.2 ± 0.3

44

Sludge characteristics

The HRAS-DAF was expected to produce good digestible sludge with concentrations up to 60

g COD L-1. Table 8 contains the average sludge composition during the different treatments.

Figure 14 displays how the sludge composition varied over the different treatments.

Table 8: Overview of the HRAS-DAF sludge composition (TSS, VSS, tCOD and sCOD) during

T1, T2 and T3

Period TSS (g L-1) VSS (g L-1) tCOD (g L-1) sCOD (%)

T1 37 ± 8 25 ± 6 47 ± 9 0.8

T2 27 ± 5 16 ± 3 21 ± 6 1.1

T3 28 ± 5 27 ± 3 25 ± 6 1.2

The high concentration potential of the DAF unit was shown during the first 64 days of T1 as

the average sludge concentration was 50 ± 8 g COD L-1. On two days (day 24 and 50) a

concentration even higher than 60 g L-1 was observed. After day 64 until the end of T1, the

concentration decreased to 39 ± 8 g COD L-1. In general the sludge from T1 had the following

characteristics: 37 ± 8 g TSS L-1, 25 ± 6 g VSS L-1 and 47 ± 9 g COD L-1. Further, the T1 sludge

was characterised by a VSS:TSS ratio of 69 % and a tCOD:TSS ratio of 1.28. The average SRT

(1.11 d) and the aerobe SRT (0.40 d) were both lower than two days.

Throughout the whole operation of T2, the sludge concentration remained rather constant. The

average composition was 27 ± 5 g TSS L-1, 16 ± 3 g VSS L-1and 21 ± 6 g COD L-1. Maximum

sludge concentration measured during T2 was 32.2 gCOD L-1 (day 51). The sludge had a

VSS:TSS ratio of 57 % and a tCOD:TSS ratio of 0.77. The SRT (1.39 d) and the aerobe SRT

(0.39 d) were again lower than the maximum of two days.

The contact stabilisation sludge that was produced during T3 was similar to the sludge from

T2. The highest sludge concentration during T3 was 33.6 g COD L-1 (day 36). Average

composition was 28 ± 5 g TSS L-1, 17 ± 3 g VSS L-1 and 25 ± 6 g COD L-1.

45

Figure 14: Timeline of the sludge TSS, VSS and tCOD composition for T1, T2 and T3. The grey dotted line at day 65 indicates the implementation

of the plug flow flocculator.

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140 160 180 200 220

Rem

oval

eff

icie

ncy

(%

)

Time (days)

TSS

VSS

COD


46

Figure 15: Comparison of sludge TSS, VSS and tCOD for T1, T2 and T3

Sludge concentration was negatively affected by the amount of water that was scraped off

together with the sludge. Minimising this amount of water could be done by lowering the

scraper speed and lowering the level of the water in the DAF unit. Meanwhile, over

accumulation of sludge at the DAF surface had to be avoided.

COD balance

In the search for optimal energy preservation, it was important that respiration of COD to CO2

was minimalised and that influent organics were preserved in the sludge. A COD balance

(Figure 16) was made with data of the dates on which the pilot was running in a steady state

and not recovering from a recent failure.

The influent COD has three possible routes to follow. It can end up in the effluent, in the waste

sludge or be mineralised to CO2. For every treatment, the amount of COD that leaves with the

effluent was around 40 %. There are large differences however in the proportion of removed

COD that is mineralised. During the first 64 days of T1, 61 % of the COD that didn’t leave with

the effluent was left in the sludge. The highest proportion of COD (84%) was captured in the

sludge from day 64 until the end of T1. Sludge capture was lower for T2 (65 %) and T3 (72 %).

Considering the quasi identical influent during T2 and T3, the HiCS process obtained a higher

entrapment of organics.

0

10

20

30

40

50

60


A-stage-DAF HiCS-DAF

TS

S,

VS

S,

tCO

D (

g L

-1)

TSS VSS tCOD

47

Figure 16: COD balance for the three treatments. The proportion between the COD captured in

the sludge and the mineralised COD is given in a pie chart next to each bar plot

Digester performance

The normalised methane content of the biogas for the different treatment is given in Figure 17.

Similar biogas (66 ± 1 % CH4) was produced in both digesters during T1. Higher percentages

but also higher fluctuations of methane content were determined for the biogas during T2 (74

± 3 % CH4 for digester 1 and 75 ± 3 % CH4 for digester 2). Under the HiCS operation (T3) the

digesters were fed differently. Digester 1 was fed HiCS sludge (72 ± 2 % CH4), while digester

2 (73 ± 4 % CH4) was fed a combination of CAS sludge and HiCS sludge in a 3:7 ratio on TSS

basis. CAS sludge was co-digested with HiCS sludge as it was believed that the CAS sludge

could provide the nutrients and trace elements that could be deficient in the HiCS sludge. The

pH of both digesters (7.4 ± 0.1) remained stable throughout the entire experimental period and

for both digesters, the conductivity varied between 5 – 10 mS cm-1.

The COD conversion efficiency during T1 was 30 – 40 %. The gas productions of T2 and T3

could not be obtained from Aquafin before the end of this thesis.

48

Figure 17: normalised methane content of the biogas during T1, T2 and T3

2 Jar tests

Experiment 1: optimal polymer selection for flotation

Before T1, an optimal polymer had to be selected for flotation. The C492 polymer was tested

during the start-up, but no flotation occurred. Other polymers were tested via jar tests. These

polymers were cationic C494 and C496 polymer and anionic A130hp polymer. Visually,

A130hp polymer showed the best and fastest flotation, and, thus, it was selected over the C492,

C494 and C496 polymers.

Experiment 2: optimal flocculant dosing (T1)

At the beginning of T1 (day 7 and 14), the optical density (OD) at 610 nm and TSS were

determined on the subnatans from jar tests performed on MLSS. The FeCl3 concentration was

kept constant at 50 mg L-1. A130hp polymer concentrations ranging from 0.75 to 6.5 mg L-1

were applied. An A130hp concentration of 3.5 mg L-1 resulted in the clearest effluent (OD of

60

65

70

75

80

Digester 1 Digester 2 Digester 1 Digester 2 Digester 1 Digester 2

A-stage sludge T1 A-stage sludge T2 HiCS sludge

T3

30 % CAS +

70 % HiCS

sludge

No

rmal

ised

m

etha

ne co

nten

t C

H4

N(%

)

49

0.3 and TSS 0.08 g L-1) and a TSS removal of 95 %. As can be seen in Figure 18 the TSS

concentrations increases again for polymer dosages higher than 3.5 mg L-1.

Based on the advice from the experts from Nijhuis and via trial and error, a dosing of 3 mg L-1

A130hp was applied during T1. Under these conditions, the sludge floated and large flocs were

formed after the addition of polymer. This jar test justified the dosing of A130hp

Figure 18: OD and TSS measurements on subnatans for constant FeCL3 (50 mg L-1) and

different A130hp concentrations (n=2)


At a constant concentration of FeCl3 (50 mg L-1) and C492 (2.3 mg L-1), the A130hp polymer

concentration was varied between 0.1 and 0.9 mg L-1 (Figure 19). The OD kept decreasing for

higher polymer concentrations, while a minimum TSS concentration (0.045 g L-1) was observed

for 0.5 mg L-1 A130hp. This concentration was selected to operate the pilot during T2.

0

0.05

0.1

0.15

0.2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.75 1.5 2.5 3.5 4.5 5.5 6.5

TS

S (

g L

-1)

OD

at

610 n

m (

-)

A130hp (mg L-1)

OD TSS

50

Figure 19: OD and TSS measurements on subnatans for constant FeCl3 (50 mg L-1) and C492

(2.3 mg L-1) and different A130hp (0.1 – 0.9 mg L-1) concentrations during T2

At a constant FeCl3 (50 mg L-1) and an optimal A130hp (0.5 mg L-1) concentration, the C492

polymer concentration was varied between 0.8 and 3.8 mg L-1 (Figure 20) The minimal OD

was observed at 3.05 mg L-1 C492. The minimal TSS concentrations were measured at 0.8 and

3.8 mg L-1.

Figure 20: OD and TSS measurements on subnatans for constant FeCl3 (50 mg L-1) and

A130hp (0.5 mg L-1) and different C492 (0.8 – 3.8 mg L-1) concentrations during T2

0

0.02

0.04

0.06

0.08

0.1

0.1 0.3 0.5 0.7 0.9

0

0.05

0.1

0.15

0.2

0.25

TS

S (

g L

-1)

A130hp (mg L-1)

OD

at

610 n

m (

-)

OD TSS

0

0.02

0.04

0.06

0.08

0.1

0

0.05

0.1

0.15

0.2

0.25

0.8 1.55 2.3 3.05 3.8

TS

S (

g L

-1)

OD

at

610 n

m (

-)

C492 (mg L-1)

OD TSS

51

Experiment 4: optimal FeCl3 and C492 dosing (T3)

At an optimal A130hp (0.5 mg L-1) concentration and two different C492 polymer

concentrations (2 – 3 mg L-1) the FeCl3 concentration was varied between 5 and 45 mg L-1

(Figure 21) The minimal TSS concentration was measured for 3 mg L-1 C492 and 35 mg L-1

FeCl3. An increase in TSS concentration in the subnatans could be seen from 5 to 25 mg L-1

FeCl3 after which a minimum was found at 35 mg L-1.

Figure 21: TSS measurements on subnatans for constant A130hp (0.5 mg L-1) and different

C492 (2 – 3 mg L-1) and FeCl3 (5 – 45 mg L-1) concentrations during T3

0

0.01

0.02

0.03

0.04

0.05

0.06

0 10 20 30 40 50

TS

S (

mg L

-1)

FeCl3 (mg L-1)

C492 2 mg L-1 C492 3 mg L-1

52

3 BMP tests

During the three different treatments, BMP tests were performed on the sludge from the pilot

to determine the biodegradability and the potential methane yield. The sludge characteristics

were given earlier this chapter. The BMP tests were carried out until the volume of biogas

produced did not increase for 2 consecutive sampling points.

BMP T1

The results for the BMP tests on the sludge that was obtained during T1 are shown in Figure

22. The COD conversion efficiency to methane was between 58 and 68 %. Conversion

efficiencies were similar to these reported for conventional A-stage systems where settling is

used for solid/liquid separation (De Vrieze et al., 2013). Whether the inoculum was from the

full scale digester in Leuven of form the pilot scale digesters in Aartselaar had no influence on

the BMP results.

Figure 22: Conversion efficiencies for the BMP tests during T1

BMP T2

A BMP test was performed on A-sludge from day 54 of T2 with inoculum from Aartselaar. A

low COD conversion to methane (24 ± 2 %) was observed.

6858

67

0

20

40

60

80

100

Day 14 Day 76 Day 92

Inoculum Leuven Inoculum Aartselaar

Conver

sion e

ffic

iency

to C

H4

(% C

OD

CH

4C

OD

-1fe

d)

53

The BMP tests with addition of M9 and/or vitamins or trace elements were performed to

investigate if the low conversion efficiencies to CH4 for sludge generated during T2 were due

to a lack of phosphorous and/or nutrients. Conversion efficiencies were similar for the control

and the test where M9 was added, at 24 % and 28 %, respectively. When trace elements and

vitamins were added, in combination with M9 or solely, conversion efficiencies were higher,

at 37 % and 36 %, respectively (Figure 23).

Figure 23: Conversion efficiencies for the BMP tests during treatment 2

It was investigated if the polymer or the iron were inhibiting the AD by doing a BMP test on

MLSS where neither FeCl3 nor polymers were added yet (except the one coming from the

recirculation). Further, to test whether the incoming wastewater contained compounds that

could inhibit AD of the A-sludge generated during the second treatment of the A-stage-DAF

system, a BMP test was performed on waste activity sludge (WAS) generated in a CAS system

in Aartselaar treating the same wastewater. The COD to CH4 conversion efficiencies for MLSS

and WAS were 26 ± 1 % and 23 ± 3 %. Respectively.

BMP T3

Poor COD conversion to methane (± 42 %) was observed for the sludge yielded during T3

(Figure 24). The COD conversion to methane was higher compared to T2 but lower compared

to T1

24

28

37 36

0

10

20

30

40

50

BMP BMP M9 BMP M9 +

vitamin

BMP tap water +

vitamin

Conver

sion e

ffic

iency

to C

H4

(% C

OD

CH

4C

OD

-1fe

d)

54

Figure 24: Conversion efficiencies for the BMP tests during treatment 3


During the first 18 days (= 1 SRT) a COD to methane conversion efficiency between 40 – 70

% was observed (Figure 25). Afterwards, the conversion went down to about 15 %. On day 55

there was an addition of trace elements and phosphorous salts leading the a minimal recovery

of the methanogenesis (± 20 % conversion). On day 62, there was a switch from A-stage-DAF

sludge to HiCS-DAF sludge.

Figure 25: Time plot showing the conversion efficacy of the fed batch digesters. The dotted line

on day 55 indicates the addition of phosphorous salts.

42 41

0

10

20

30

40

50

Day 21 Day 38

Conver

sion e

ffic

iency

to

CH

4(%

CO

DC

H4

CO

D-1

fed)

0

20

40

60

80

100

0 20 40 60 80 100 120

Conver

sion e

ffic

iency

to C

H4

(% C

OD

CH

4C

OD

-1fe

d)

Time (days)

A-stage-DAF (T2) HiCS-DAF (T3)

55

PART 4: DISCUSSION

56

1 Pilot scale HRAS-DAF

The HRAS-DAF pilot was designed to resemble the A-stage of an A/B-process in terms of

SRT, HRT and DO. A DAF was used with the objective to obtain better solid-liquid separation,

higher organics removal and more compact sludge in comparison with conventional settling.

The A/B-process is a dual stage wastewater treatment system in which the first stage is a high

rate reactor designed for carbon and phosphorous removal carbon removal whereas the second

stage serves for removal of nitrogen and residual COD. The high rate operation of the first

stage, ensures that the influent sCOD, cCOD and pCOD are concentrated to the sludge with

minimal energy input and at a small footprint (Jimenez et al., 2015).. .The dual stage operation

of the A/B-process makes it possible to optimise the organics and nitrogen removal separately.

By incorporation in the high rate sludge, part of the nutrients are already removed in the first

stage (Jetten et al., 1997)

There are different scenarios for nutrient removal in the B-stage and these are determined by

the effluent characteristics from the A-stage. A/B-systems were described where enough BOD

(100 – 150 mg L-1) remains in the A-stage effluent to have a BOD:N ratio of 4 or higher

(Boehnke et al., 1998). A BOD:N ratio of 4 is considered the minimum to support conventional

nitrification/denitrification (Tchobanoglous & Burton, 1991). The HRAS stabilises shock loads

for the B-stage and the HRAS effluent organics are more biodegradable compared to CAS

effluent organics. It was shown that recalcitrant compounds are disproportionally more

extracted in HRAS systems compared to CAS because of the adsorption processes (Sorensen

et al., 1994). This effect is even increased by the high sludge production in the first stage.

For effluent from the first stage having a BOD:N ratio lower than 4, complete

nitrification/denitrification in the B-stage is not possible without the addition of an external

carbon source (Loosdrecht et al., 1997). A second option for nitrogen removal in the B-stage is

via the partial nitrification/annamox process (Kartal et al., 2010). However this process is still

under lab scale investigation for application on cold (< 20 °C) and diluted streams (< 100 mg

N L-1) (Hendrickx et al., 2012).

A second goal of the HRAS-DAF pilot was to produce more concentrated sludge compared to

conventional A-stage systems with a settler where sludge is concentrated to ± 10 g COD L-1

57

(De Graaff & Roest, 2012). Producing concentrated sludge should be possible due to the

coupling with DAF. It has already been proven on lab scale that an A-stage-DAF system was

able to produce sludge twice as concentrated (16 – 20 g COD L-1) (De Saedeleer, 2016). The

higher concentration ability of the DAF makes the use of a thickener before AD unnecessary.

In this perspective, the DAF can be seen as a combined settler and thickener. The higher energy

use of a DAF compared to a settler sets the higher operating costs for DAF solid/liquid

separation.

Organics removal

The organics removal in the HRAS-DAF pilot plant during the three treatments will be

discussed in the following sections. Three processes regulate the organics removal:

1. Biological growth

2. Adsorption, coagulation and (bio)flocculation

3. Solid/liquid separation in the DAF

Fast biological growth occurs in both A-stage and HiCS process and part of the sCOD.is

consumed or stored. During every treatment the ratio of sCOD:tCOD ration in the influent was

higher than 25 %. Research done by de Graaff et al. (2016) shows that in this case an A-stage

is better suited as primary treatment than a primary settler. In the HiCS system the sludge with

the best biosorption capacity is selected via the feast and famine regime and should result in the

highest pCOD and cCOD removal via biosorption. Bioflocculation is key for optimal sludge

floc formation and chemicals are dosed to enhance the flocculation. The performance of the

HRAS-DAF pilot is inherently related to the performance of the DAF unit. A good solid/liquid

separation in the DAF unit strongly defines the overall removal efficiency of the HRAS-DAF

pilot. The performance of the DAF unit itself relies on effective coagulation and flocculation

and on the quality of the microbubbles. Both proved to be critical for stand-alone operation.


Before day 64 and the implementation of the PFF, T1 faced several days of operation with low

TSS removal (< 30 %) and also the overall solids removal was low (50 %). This had several

causes.

58

First of all, on the days with low TSS removal the sludge flocs after coagulation and flocculation

appeared to be small and weak (based on subjective visual criteria). Whereas the coagulant

could be dosed just before the screw pump for optimal mixing, this could not be done for the

flocculant as this would destroy the polymer structure (D. H. Liu & Liptak, 1999). It is likely

that the flocculant was not enough dispersed into the MLSS to create stable flocs because of

the short distance between the polymer addition and the entrance of the DAF. Unstable flocs

are more prone to disruption when entering the turbulent contact zone of the DAF unit. The

importance of stable flocs for flotation was shown by Klute et al. (1995) and flocs that are

broken by shear do not regrow easily under less turbulent conditions (Yoon & Deng, 2004).

Secondly, it was assumed that the bubbles floc interaction was hampered due to inefficient

charge neutralisation. Bubbles are hydrophobic and for bubble – floc adhesion it is needed that

the negative charges on the particles are neutralised effectively via addition of FeCl3. The

membrane pump that pumped the FeCl3 solution to the screw pump was working near its

minimum flow rate.. This led to intermittent pulses being dosed instead of a more continuous

flow of coagulant solution. This could be countered partly by operating the membrane pump in

slow operation mode, which made the plunger movement smaller but faster. To tackle this

problem, during T2 the concentration of the FeCl3 solution was decreased from 10 % to 5 %.

Thirdly, as a result of the inefficient solid/liquid separation, the sludge production on days with

low TSS removal was not sufficient to keep the MLSS concentration in the A-stage tank at 1 g

L-1 via sludge recirculation. This caused the MLSS concentration to drop below the desired

concentration and that in turn led to an overdosing of coagulant and flocculant since the dosages

were optimised for ± 1 g MLSS L-1. Overdosing of coagulant leads to charge inversion and re-

stabilisation of the particles in suspension. Overdosing of A130hp polymer should be avoided

since bridging flocculation is hampered. The optimal dosage for polymer bridging is reached

when roughly half of the surface of the particle is covered with polymer as then the chance of

bridge formation is at its largest (Bolto & Gregory, 2007). In addition, overdosing of polymer

causes the flocs to become too heavy for flotation partially due to the high molecular weight of

the polymers used to assist flocculation (particularly in case of A130hp with molecular weight

(MW) higher than 107 g mol-1).

59

The performance of the A-stage-DAF system improved when the PFF was connected to the

system and the polymers were dosed directly in the PFF. Organics removal was increased from

± 50 % for all three parameters (TSS, VSS and tCOD) during the first 64 days to 77 ± 14 % for

TSS, 81 ± 10 % for VSS and 62 ± 13 % for tCOD from day 64 until day 106.

The improvement in solid/liquid separation was likely due to the implementation of the PFF,

which increased turbulence and shear forces and resulted in the formation of stable sludge flocs

more suitable for flotation. Under non-turbulent conditions the polymers in solution are

randomly coiled, but when subjected to high shear, the polymer chains are considerably

extended (Bolto & Gregory, 2007). Further, covalent bonds around the middle of the polymer

chain can rupture, especially in the case of high MW polymers, leading to a reduction in MW

(Scott et al., 1996). This causes a reduction in viscosity and is important for flocculation under

turbulent conditions. The turbulent conditions in the PFF likely increased dispersion of

coagulant and flocculant into the MLSS. In addition, roughly half of the pressurized white water

was added directly in the PFF. This likely further increased turbulence, increased flocs

buoyancy and led to stronger bubbles-floc adhesion. Finally, the PFF provided more time for

flocculation and the dosage points of bubbles and flocculant could be optimised.


During T2, FeCl3 was dosed with the same concentration as in T1, but it was chosen to change

from using solely anionic A130hp polymer to a combination of cationic C492 and A130hp

polymer to assist flocculation. This combination is called ‘dual polymer’ and the combination

of a cationic flocculant and a high MW (> 107 g mol-1) anionic flocculant creates stronger and

more compact flocs (Petzold et al., 2003). By using the dual polymer system, the microbubbles

should be more firmly incorporated into the floc.

First the cationic polymer is dosed and it reduces the charge on the particles via charge

neutralisation. Cationic patches arise according to the electrostatic patch mechanism: because

of the higher charge density of cationic polymer compared to the surface of the particles, it is

not possible that each surface charged site is neutralised by a cationic polymer segment (Bolto

& Gregory, 2007). Further in the PFF the high MW anionic A130hp polymer is dosed and the

cationic patches act as anchor points for anionic polymer binding. Since the number of patches

is limited, the anionic polymer cannot bind several times to the same particle and will adopt a

60

more extended configuration, which enhances the flocculation via ion bridging. It was shown

by Petzold et al. (2003) that the addition of cationic polymer followed by anionic polymer is

the most effective order for flocculation. The dual polymer system assures a thoroughly

neutralisation of the charge on the particles which improves the floc attachment to the

hydrophobic flocs.

Due to an almost constant effluent concentrations in terms of TSS, VSS and tCOD,

independently from the influent concentration, it was assumed that the system would be able to

further improve removal efficiencies if medium-strength or high-strength wastewater would be

treated.

1.1.3 HiCS-DAF T3

The HiCS process was chosen as earlier studies showed that HiCS could increase carbon

capture compared to the A-stage (Meerburg et al., 2015). In this thesis similar removal

efficiencies were obtained for the HiCS-DAF system as for the A-stage-DAF system.

Nevertheless the amount of coagulant and flocculant dosed in HiCS-DAF configuration were

lower than in the previous treatments. Therefore, the presence of an aerated stabilization tank

seemed to enhance bioflocculation and floc formation, so that a lower amount of chemicals was

necessary to obtain similar outputs. T3 was characterized by a lower F/M ratio (4.5) compared

to T2 (7) and this increases the share of carbon storage for carbon capture (Lim et al., 2015).

DAF performance

Adequate aggregation of the particulates is crucial for efficient flotation (Klute et al., 1995).

This stresses the importance of successful coagulation and flocculation. During operation, the

DAF unit needed to be drained several times to remove settled sludge. To cope with this

problem, the sand drain valve was set to open more regularly.

A sensitive parameter regarding the microbubble quality is the air flow (J Bratby & Marais,

1976). The optimal flow was between 0.5 – 0.7 L air min-1 and higher air flows did not yielded

better flotation. Further, impurities in the recycle stream were harmful for the microbubble

quality. The small tubing, specific to a pilot scale plant, clogged easily and had to be flushed

several times a week. These clogging problems would less likely be encountered at larger scale.

61

It can be assumed that process stability would improve by installing a filter before the recycle

tubing. This however comes along with an additional investment cost and higher operating costs

due to a higher pressure drop and the need for filter backwashing/replacement.

The performance of the DAF unit during the different treatments will be discussed in the

following sections.


Via sludge recirculation, it was tried to maintain a MLSS concentration of 1 g TSS L-1 in the

A-stage/contact tank but due to several reasons there was a wide spread in MLSS concentrations

(1.08 ± 0.42 g TSS L-1). As mentioned before MLSS dropped when problems with the DAF

occurred. Sludge production was too low to assure enough recirculation to the A-stage tank and

so the MLSS concentration went down several times during T1. Next, the sludge recirculation

was fixed via an on/off setting of the screw pump. When the sludge characteristics changed but

recirculation remained the same, this influenced the MLSS concentration. The WWTP receives

wastewater from a combined sewer system so rain events diluted the wastewater and this also

had an impact on the MLSS concentration. Al these factors led to suboptimal dosing of

coagulant and flocculant and thus to suboptimal flotation.


During T2, not enough solids could be recirculated to keep the MLSS at the desired

concentration of 1 g L-1 (0.75 ± 0.23 g L-1). Nevertheless, the separation efficiency was excellent

for TSS and VSS (> 95 %) and tCOD (90 %).

1.2.3 HiCS-DAF T3

The conversion from A-stage to HiCS was not a problem for flotation. Similar separation

efficiencies as during the two A-stage-DAF treatments were obtained.

1.2.4 Effluent characteristics

The effluent characteristics are discussed to show to which extent the organics could be

removed by the HRAS-DAF systems. The HRAS-DAF pilot performance was the main factor

defining the effluent characteristics, but also the influent strength influenced the effluent

62

concentration as the influent was less concentrated (but not significantly) during T2 and T3

compared to T1. The influent vs effluent strength is compared in Figure 26.

Figure 26: Comparison of influent and effluent TSS, VSS and tCOD for T1, T2 and T3. The

error bars show 1 standard deviation.

Different DAF operation strategies can be chosen in function of the desired effluent. In this

thesis, optimal organics removal was pursued through optimization of coagulant and flocculant

addition. Under these circumstances, the BOD:N ratios are low and the effluent cannot support

complete denitrification in the B-stage. Further, due to the high Fe:PO43- ratios (4 – 7) almost

no PO43- was left in the effluent which is detrimental for biological growth in the B-stage

(especially the vulnerable nitrifiers). At a Fe:PO43- ratio of 1, about 80 % of PO4

3- is removed

(Broeders, Menkveld, et al., 2014). A phosphate concentration of 1.5 mg L-1 is advised for good

microbial growth. This could be achieved by applying a strategy in which the dosing of FeCl3

is controlled by on online monitoring of the effluent phosphate concentration. In contrast, the

DAF could also be operated to have an effluent with a BOD:N ratio that allows denitrification

in the B-stage (Broeders, Menkveld, et al., 2014).

Comparison between HRAS-DAF and HRAS-settling

To evaluate the HRAS-DAF systems on their solid/liquid separation efficiency and their sludge

production and to evaluate whether it could be useful to implement DAF in existing A/B-

0.0

0.1

0.2

0.3

0.4

0.5

Influent Effluent Influent Effluent Influent Effluent


A-stage-DAF HiCS-DAF

TS

S, V

SS

, tC

OD

(g

L-1

)

TSS VSS tCOD

63

processes, the HRAS-DAF system performances obtained during this thesis are compared with

four operational A/B systems in the Netherlands (De Graaff & Roest, 2012). The removal of

TSS and tCOD for four full scale A-stage systems in the Netherlands where settling is used for

solid/liquid separation is given in Table 9.

Table 9: Removal of TSS and tCOD for four full scale A-stages in the Netherlands (De Graaff

& Roest, 2012)

WWTP TSS tCOD

Nieuwveer, Breda 59% 53%

Dokhaven, Rotterdam 68% 74%

Utrecht - 60%

Garmerwolde, Groningen 67% 55%

The tCOD removal for the HRAS-DAF systems was 50 – 62 % and this is comparable to what

is achieved at the large scale A-stages in the Netherlands . Only the A-stage at Dokhaven

achieved a considerably higher tCOD removal (74 %).

The TSS removal obtained via conventional settling can be compared to the TSS separation

efficiency obtained with the DAF. Whereas the settlers were able to remove 59 – 68 % of the

solids, the HRAS-DAF in this thesis could remove 90 – 97 % of the solids. This shows that

DAF is a better solid/liquid separation technique. The lower solid/liquid separation efficiency

of the settler can be contributed to the high F/M ratio (Bisogni & Lawrence, 1971). On the other

hand, this does not seem to influence the flotation potential.

The best flotation results were obtained using the dual polymer system. Apart from selection

the optimal combinations of coagulant and flocculant, also the costs of these chemicals should

be accounted for. Market prices are €4 kg-1 polymer (Broeders, Menkveld, et al., 2014) and

FeCl3 comes at a cost of 0.25 – 0.3 € kg-1 (Yonge, 2012). Both the efficacy of flotation and the

cost of coagulant and flocculant use should be taken into consideration. A higher flotation

efficacy must outweigh the higher costs related an increased chemical use. Further the influence

of the coagulant and flocculant on the downstream AD cannot be neglected.

64

DAF The different energy consuming components of the HRAS-DAF installation are:

1. The influent pump

2. The mixers for the making of the polymer solutions

3. The pumps for the dosing of coagulant and flocculant

4. The pumps for removal of floated sludge and for the sand drain

5. The scraper system

6. The recirculation pump

7. The compressor

The recirculation pump is the largest energy consumer and the total energy use of a large scale

DAF installation sums up to ± 0.033 kWh m-3 (Broeders, Flameling, et al., 2014). The energy

consumption of a primary settler for running influent and recirculation pumps is considerably

lower and amounts to 1% of the total energy use at a WWTP (Wendland, 2005). Although a

DAF unit has larger operational costs compared to a settler, it saves the capital expenditures of

a thickener and saves a considerable amount of space due to its ten times lower footprint.

Sludge characteristics

Sewage sludge is a complex heterogeneous mixture of active and dead biomass, undigested

organics, plant residues, oils, or faecal material, inorganic materials and moisture (Tyagi & Lo,

2013). Nearly 10 million tons of dry sludge are produced in the EU each year (Commission,

2010). Sludge treatment is expensive (€35 m-³ sludge, 22% solids) (Broeders, Menkveld, et al.,

2014) and can amount up to 50 % of the operating costs of a WWTP. The better digestion

properties of HRAS sludge should result in lower overall sludge production compared to single

stage nutrient removal processes with primary sedimentation (Loosdrecht et al., 1997).

The HRAS-DAF sludge concentration was between 47 (T1) and 21 (T2) g COD L-1 which is

high compared to a normal A-stage-settler in which sludge is only concentrated to ± 10 gCOD

L-1 (De Graaff & Roest, 2012). The lower organic loading during T2 and T3 supported less

biomass growth and thus less sludge production. This can explain why the HRAS-DAF sludge

was less concentrated compared to T1. Another reason could be that a considerable amount of

water was scraped off together with the sludge. This is mainly determined by the level in the

DAF and the scraper speed.

65

The digestibility of the HRAS-DAF sludge was tested on pilot as well as on lab scale and these

results will be discussed further.

COD balance

The ratio of COD that left via the effluent was similar for all treatments (± 40 %). The

proportion of removed COD that was transferred to the sludge lay between 61 – 84 %. To

compare with, the COD balance for a conventional CAS system (Ding et al., 2015) is shown in

Figure 27. In the CAS system, 59 % of the removed COD ends up in the sludge. In this thesis

a higher proportional COD capture was achieved using the HRAS-DAF processes. High DAF

separation efficiency and a medium influent strength resulted in the highest COD capture (84

%) during the last part of A-stage-DAF (T1) operation. Probably because of the lower influent

strength during A-stage-DAF (T2) and a higher SRT (1.39 d vs 1.11 d) compared to T1, only

65 % of the removed COD ended up in the sludge. Being confronted to a similar strength

wastewater as T2, the HiCS-DAF (T3) performed slightly better with 72 % of the organics

being sent to the sludge. The higher biosorption capacity of the HiCS sludge might be an

explanation.

Figure 27: COD balance for a conventional CAS system. From the COD that is not left via the

effluent, the proportion is given between COD in the sludge and COD that is mineralised.

66

2 Jar tests

Experiment 1: optimal polymer selection for flotation

The optimal polymer for achieving the best flotation was selected visually. A130hp polymer

was seen to have the best flocculation and flotation abilities although it has a higher molecular

weight than C492. The anionic nature of the polymer sets the mechanism of flocculation to be

via ion bridging. The coagulant Fe3+ ions are bound on the negative charges of the flocs and the

negative charged groups of the anionic polymer link to the positively charged Fe3+ ions. This

way the anionic polymer can form a bridge between different flocs.


A dosage of 3 mg L-1 A130hp polymer was selected for the A-stage-DAF operation during T1

and the jar tests confirmed that this is the optimal dose. This is similar to a pilot study wherein

DAF was investigated as wastewater pre-treatment technique and 1.8 – 2.8 mg polymer L-1 was

dosed for maximal organics removal (Broeders, Menkveld, et al., 2014).

The average MLSS concentration was 1.08 g L-1 resulting in 2.8 mg polymer g-1 TSS being

dosed. This is high compared to settling where 1 mg polymer g-1 TSS or less is applied (Bolto

& Gregory, 2007). This test also showed that there is no benefit related to dosing more than 3

mg L-1 of the high MW A130hp polymer. Higher concentrations hamper bridging flocculation

and the high MW of the polymer makes the flocs less susceptible for flotation.


In the first series of jar test, an optimal A130hp dosing of 0.5 mg L-1 was found. Further jar

tests suggested that at this A130hp concentration in combination with either a low (0.8 mg L-1)

or high (3.05 mg L-1) provided the best solid liquid separation. The low C492 concentration

was attempted to run the A-stage-DAF pilot but no effective flotation was observed. So the

C492 concentration was increased to 2 mg L-1. Although this is not the optimal concentration

67

as could be derived from the jar test, this concentration assured good flocculation and flotation

at the A-stage-DAF pilot. This experiment showed that dosing more than 2 mg L-1 of C492 does

not yield higher separation efficiencies for TSS. At a total dosage of 2.5 mg polymer L-1 during

T2, with an average MLSS concentration of 0.75 g L-1, 3.3 mg polymer g-1 TSS was dosed,

which is even higher compared to T1. From an economical viewpoint, the higher separation

efficiency of the DAF should justify the high polymer consumption.

Experiment 4: optimal FeCl3 and C492 dosing (T3)

This experiment proved that is was advantageous to lower the FeCl3 concentration from 50 mg

L-1 (as was applied during T1 & T2) to 35 mg L-1. Eventually 30 mg L-1 was applied to the

HiCS-DAF pilot during T3. At a total dosage of 2 mg polymer L-1, with an average MLSS

concentration of 1.17 g L-1, 1.7 mg polymer g-1 TSS was dosed. This was the lowest polymer

dosing of the three treatments.

3 BMP tests

BMP T1

Conversion efficiencies to CH4 of 58 – 67 % were observed during BMP tests on A-stage-DAF

sludge yielded during T1. This was comparable to A-sludge generated from a conventional high

rate sludge where settling is used for solid/liquid separation (Bolzonella et al., 2005; De Vrieze

et al., 2013).

BMP T2

Conversion efficiencies to CH4 (24 ± 2 %) indicated the lower (compared to T1) digestibility

of the A-stage-DAF sludge yielded during T2. It was assumed that methanogenesis could be

inhibited by a lack of phosphorous and other (micro)nutrients in the sludge, toxicity caused by

components in the wastewater or a toxic effect of the coagulant or the flocculant.

68

The additional BMP that were performed showed that AD could not be restored to its original

state by addition of M9 minimal feeding solution (containing phosphorous) or vitamins plus

trace elements or a combination of these. Although there was an improvement for M9 addition,

the positive effect on AD was the largest for addition of vitamins plus trace elements (39 %

conversion).

The BMP that was performed on the WAS from Aartselaar had conversion efficiencies (23 %)

that were comparable to other WAS (Bolzonella et al., 2005). This meant that the influent

wastewater can be excluded from being a reason for the failing AD.

A possible explanation for the hampered methanogenesis is a toxic effect of the coagulant or

flocculant. No toxic effect is to be expected from the FeCl3 coagulant since iron is an important

trace element for methanogens and stabilizes the AD process (Zhang & Jahng, 2012). That is

because iron acts as cofactor in several enzymatic reactions occurring during methanogenesis,

and it disproportionally stimulates the methanogens over the fermentative bacteria thus

enhancing AD.

No polymer toxicity data were found for bacteria nor archaea. Generally synthetic polymers

tend not to be readily absorbed by organisms whereas the monomers are more toxic. Cationic

polymers are rated more toxic to aquatic organisms than anionic polymers (Hamilton et al.,

1994). Further, algae are sensitive to anionic polymers (order 1 mg L-1) because of the chelation

of nutrient metal cations (Murgatroyd et al., 1996). The same mechanisms might hinder the

methanogens. However no AD inhibition was seen during T1 when only anionic polymer was

applied. Besides, no inhibition was seen neither for AD of high rate sludge that was treated with

cationic polymer (De Saedeleer, 2016). The AD problems arose during T2 and T3 so it seems

that the combination of cationic and anionic polymer is causing an obstacle for the

methanogens.

It is hypothesized that first easily biodegradable organics are consumed, after which the PAMs

are consumed by the MO’s. PAMs can both be used as a carbon or as a nitrogen source (Dai et

al., 2014). When PAM is used an a N-source, this is accompanied by an accumulation of

ammonium. No increase in ammonium was found which suggests that the PAMs were solely

used as carbon source which in turn leads to accumulation of acryl monomer (Dai et al., 2014).

69

Eventually, when most of the PAM is used, the methanogenesis almost ceased. The reason for

this could be an improved stability of the flocs could leading to a mass transfer resistance (Chu

et al., 2003) so that the less biodegradable organics in the flocs are not degraded by the MO’s.

BMP T3

Conversion efficiencies to CH4 were 40 ± 2 % and thus lower compared to T1 of the HiCS-

DAF configuration. The low conversion efficiency could be explained as for T2 by the

combination of the anionic and cationic polymer.


40 – 70 % conversion was obtained during the first SRT, but then dropped to 15 %. Addition

of phosphorous salts on day 55 and micronutrients on day 57 only slightly improved the process

(20 % conversion). It seemed that the nutrients in the inoculum were exhausted or no longer

available for the MOs. With fresh inoculum, methanogenesis proceeded unhindered but after

18 – 20 days (equal to 1 SRT), the process was inhibited. It is likely that the HRAS-DAF sludge

is the reason for the failing methanogenesis.

70

5 General conclusions

In this thesis, the feasibility of combining an A-stage or HiCS process at pilot scale for

municipal wastewater treatment with DAF for solid-liquid separation was demonstrated. High

removal efficiencies of TSS and VSS (> 70 %) and tCOD (58 – 62 %) were obtained during A-

stage-DAF operation, even when confronted to low strength (< 0.2 g COD L-1) wastewater.

During HiCS-DAF operation the removal of TSS (68 %), VSS (67 %) and tCOD (54 %) was

comparable to the A-stage-DAF but nevertheless the HiCS performed better than the A-stage

as less coagulant and polymer were dosed.

The high surface loadings (20 m h-1) determine the low footprint of the DAF installation

compared to a settler (1.2 m h-1) and the ease of implementation make it a promising technique

for the conversion or expansion of existing A/B-systems or the building of new plants. However

the DAF process showed to be vulnerable to failure. In the view of standalone operation, there

are some critical aspects that require intensive surveillance.

First of all, optimal coagulation and flocculation and good bubble floc attachment are crucial

for the flotation process. Using FeCl3 (30 – 50 mg L-1) in combination with either solely anionic

polymer (2.8 mg g-1 TSS) during T1 or a dual polymer system combining cationic and anionic

polymer (1.7 – 3.3 mg g-1 TSS) during T2 and T3, flotation was proven to be successful.

Introducing part of the microbubbles in the MLSS – coagulant – polymer mixture under

turbulent conditions in a plug flow flocculator enhanced bubble – floc attachment and also

flotation efficiency.

Second, the quality of the recycle stream water was crucial for qualitative microbubble

generation since impurities disturb the generation of the microbubbles. A deteriorating effluent

quality can lead to process failure.

This thesis showed that HRAS-DAF processes are capable of capturing a significant proportion

(61 – 84 %) of the removed organics and diverting it to AD for biogas production. DAF proved

to be an excellent alternative compared to settling in the view of energy recovery since less

organics were washed out with the effluent and more concentrated sludge (21 – 47 g COD L-1)

71

was produced. The latter eliminates the need a of a thickener which further lowers the footprint

and the capital expenditures of a WWTP.

The high rate operation of the HRAS-DAF pilot plant lead to the expectation for high

digestibility of the HRAS-DAF sludge. High potential COD to CH4 conversion (58 – 67 %)

was seen during BMP tests on A-stage-DAF sludge from T1 but AD was inhibited during T2

and T3 (24 – 42% conversion). The use of a combination of cationic and anionic polymer was

assumed to be one of the main reasons why AD failed. Via addition of vitamins and trace

elements it was possible to restore AD only partially.

72

6 Future work and perspectives

Further research of the HRAS-DAF system should focus on the biological activity and its role

in uptake and release of sCOD. Next, the biosorption capacity of the HRAS-DAF sludge should

be investigated and related to the biosorption capacity of activated sludge (40 – 100 mg COD

g-1 TSS) (Guellil et al., 2001). It was shown that high rate activated sludge microbial

communities differ significantly from conventional activated sludge communities (Gonzalez-

Martinez et al., 2016). An interesting future research question is whether there is also a

significant difference in bacterial community between floated high rate sludge and settled high

rate sludge. This could be useful for inoculation of new HRAS-DAF systems.

Effective coagulation and flocculation is crucial for flotation. The dual polymer system (Petzold

et al., 2003) that was applied during T2 and T3 guaranteed successful flotation, but still different

polymer combinations with differing charge densities and molecular weights should be

investigated. Further optimisation of the coagulation and flocculation could be achieved by

dosing the polymer proportionally to the MLSS solids content, which varies with varying

influent conditions. This makes sense since polymer bridging efficiency is at its maximum when

half of the solids surface is occupied and the surface increases linearly with the solids content.

During this thesis, the HRAS-DAF sludge was seen not to be concentrated until is potential

maximum of 60 g COD L-1. Further maximisation of the HRAS-DAF sludge concentration

lowers the costs of sludge handling and reduces reactor volumes for AD. Based on TSS

measurement in the small sludge storage basin of the DAF unit, the scraper speed and the water

level in the DAF could be controlled to prevent large amounts of water being scraped off

together with the sludge and thus diluting it.

The stability of the flocs might make it impossible for the MO’s to break down the organics in

the flocs during AD. Sludge pre-treatment techniques before AD should be investigated to

destabilize the flocs and make the highly biodegradable organics accessible for the MO’s.

Further, the availability of nutrients and trace elements in the sludge should be investigated.

73

As the HRAS-DAF was built to resemble the A-stage of an A/B process, the focus should now

be on the development of the B-stage and its coupling to the A-stage. The carbon limited

effluent brings new challenges for nitrogen removal in the B-stage. There are two possible

pathways that require investigation. Conventional nitrification/denitrification or partial

nitrification/annamox via the cold annamox process (Kartal et al., 2010). Batch tests already

were performed on denitrification in the effluent. The building of a continuous reactor should

be the next step.

74

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