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Full-scale anaerobic co-digestion of organic waste andmunicipal sludge
Gregor D. Zupancica,�, Natasa Uranjek-Zevartb, Milenko Rosa
aNational Institute of Chemistry, Hajdrihova 19, PO Box 660, SI-1001 Ljubljana, SloveniabMunicipality of Velenje, Koroska 37/b, 3320 Velenje, Slovenia
a r t i c l e i n f o
Article history:
Received 8 May 2006
Received in revised form
6 July 2007
Accepted 10 July 2007
Available online 20 August 2007
Keywords:
Anaerobic digestion
Biogas production
Organic waste
Sludge digestion
nt matter & 2007 Elsevieioe.2007.07.006
hor. Tel.: +386 1 4760 249;[email protected] (G.D
a b s t r a c t
A full-scale experiment on the anaerobic co-digestion of organic waste from domestic
refuse (swill) and municipal sludge is described. In a wastewater treatment plant of 50,000
population equivalents, two conventional mesophilic digesters with a combined volume of
2000 m3 and 20 days hydraulic retention time were used. The digesters’ usual influent is
waste sludge from wastewater treatment plants (a mixture of primary sludge and waste
activated sludge) with an average organic loading rate of 0.8 kg m�3 d�1 of volatile
suspended solids. In the experiment, organic waste was added to the digester influent to
increase the organic loading rate by 25% to 1.0 kg m3 d�1 of volatile suspended solids. Biogas
quantity increased by 80% and specific biogas production increased from 0.39 m3 kg�1
volatile suspended solids inserted prior to the experiment to over 0.60 m3 kg�1 volatile
suspended solids’ inserted, peaking at 0.89 m3 kg�1 volatile suspended solids inserted. The
excess biogas was used in a boiler and a 50 kW combined heat and power engine. Electrical
energy production increased by 130% and heat production increased by 55%. Volatile
suspended solids degradation efficiency increased from 71% to 81% with no increase of
volatile suspended solids in the digester effluent. Virtually all of the organic waste was
degraded.
& 2007 Elsevier Ltd. All rights reserved.
1. Introduction
Anaerobic digestion has the potential for treatment of many
kinds of organic waste (OW) mixtures, mostly in combination
with municipal sludge. In recent years, such research has
received much attention [1–3] due to its potential for
increased output of biogas (renewable energy) in digestion
plants and some economic benefits in OW disposal. In the
past, OW of domestic refuse (swill) has usually been a food
source for domestic animals, mostly pigs. As a food source it
was recognised as a possible source of pathogenic hazard [4]
and was therefore banned for such use. This caused
accumulation of increased quantities of OW, which are
disposed of by landfilling. Such handling is prohibited in
r Ltd. All rights reserved.
fax: +386 1 4760 300.. Zupancic).
Slovenia by a decree on waste handling and pollution [5]. The
alternatives offered are processing by anaerobic digestion or
composting. Incineration is also an alternative, but due to the
high moisture content, energy recovery is poor and such
treatment is not very imaginative. Anaerobic digestion is
therefore the most cost-effective way to efficiently process
wet OW for energy recovery [2]. Many authors have conducted
research in this field in recent years. There are many possible
ways of successfully digesting OW of any kind, ranging from
conventional mesophilic digestion, where the organic loading
rate (OLR) is up to 3.7 kg m�3 d�1 of volatile suspended solids
(VSS) [3], to two-stage digesters. Sosnowski et al. [6] and
Gomez et al. [7] reported successful operation with an OLR of
3.3–4.3 kg m�3 d�1 of VSS. Gallert et al. [8] reported operating a
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Nomenclature
BPR biogas production rate, m3 per m3 of the digester
per day (m3 m�3 d�1)
CHP combined heat and power
TCOD total chemical oxygen demand (mg l�1)
HRT hydraulic retention time, d
OLR organic loading rate, kg of TCOD or VSS per m3 of
the digester per day (kg m�3 d�1)
OW organic waste
PE population equivalent
PS primary sludge
SBP specific biogas productivity, m3 per kg VSS
inserted (m3 kg�1)
TSS total suspended solids (mg l�1)
VSS volatile suspended solids (mg l�1)
WAS waste activated sludge
WWTP waste water treatment plant
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 1 6 2 – 1 6 7 163
single-stage digester with OLR values as high as 8.5 kg m�3 d�1
of total chemical oxygen demand (TCOD).
In the municipality of Velenje, about 1200 m3 of wet OW
(250 tonnes of dry matter) are collected annually. Instead of
dumping this waste on a sanitary landfill, its potential for
biogas production was quickly realised and a 15-month full-
scale pilot project was started to test the possibilities of OW
co-digestion with municipal sludge. Our digesters are de-
signed to process the OLR of 1.0–1.5 kg m�3 d�1 of VSS;
therefore we had plenty of deviation to multiply the load
with OW several times. The aim of the work was to
investigate the possibilities for increasing the portion of
renewable energy by adding the value to OW residues using
anaerobic digestion as well as reduction of CO2 emission by
replacement of fossil fuels (mostly natural gas) in the waste-
water treatment plant (WWTP) where the experiment was
conducted with biogas.
2. Materials and methods
The municipality of Velenje operates a WWTP of 50,000
population equivalents (PE) with two mesophilic anaerobic
digesters of a combined volume of 2000 m3. The digesters are
fed with municipal sludge from the WWTP semi-continu-
ously every 3 h from a sludge thickener. The VSS concentra-
tion in sludge ranges from 10 to 20 g l�1, total suspended
solids (TSS) concentration from 20 to 30 g l�1, and TCOD of
sludge between 18,000 and 30,000 mg l�1. Sludge is a mixture
of primary sludge (PS) and waste activated sludge (WAS). The
average ratio is 60% of PS to 40% of WAS. The hydraulic
retention time is 20 days. Biogas produced in the digesters is
collected in a biogas storage unit and used online in a biogas
boiler initially to cover all heat demands of all WWTP
premises, and any surplus is used in a 50 kW combined heat
and power (CHP) engine. The digesters and power set-up are
shown in Fig. 1.
TCOD, TSS and VSS of OW and municipal sludge (influent
and effluent) were monitored and analysed using standard
methods [9]. The average values of OW influent are shown in
Table 1. Total influent load is shown in Table 2 and influent
composition in Fig. 2. Biogas and pH were also continuously
measured and monitored. We also monitored the electrical
power output of the CHP engine and the heat power output of
the biogas boilers and CHP combined. The degradation
efficiency presented in this paper is calculated from influent
solids and dewatered effluent solids. Solids in the digester
overflow and water from dewatering are not accounted for in
the degradation efficiency. These unaccounted solids are
returned to the influent of the WWTP.
Normal digester operation is with municipal sludge only
(a mixture of PS and WAS). The experiment involving addition
of OW was conducted from January 2004 to March 2005. OW
from domestic refuse was collected from households in the
local area and brought to the WWTP two to three times
weekly. Our aim was to slowly raise the digester OLR to
achieve a steady state in 5–6 months. Therefore, 3 m3 of OW
was fed to the digester according to OLR two to three times
per week from January 2004 to August 2004. From August 2004
to March 2005 the digester was fed with more OW (up to 6 m3
per batch) to achieve a steady OLR, because the WWTP
produced less sludge. The OW was fed to the digester at once
in a batch. Prior to the experiment, the average OLR was
0.76 kg m�3 d�1 of VSS (0.9 kg m�3 d�1 of TCOD). We decided to
plan the OLR increase gradually by 40%, since a digester
overload and possible breakdown was just not affordable in a
fully operating WWTP plant.
3. Results and discussion
Fig. 2 shows the VSS content of influent and effluent in the
digester. We gradually increased the VSS load by 30% from the
start of the experiment in January 2004 until September 2004.
In the effluent there was no significant change; therefore, we
can conclude that practically all of the OW was degraded.
With such a low OLR (Fig. 3) such a result can be expected.
Table 1 shows that for most of the time over 90% of the OW
influent is volatile, most probably biodegradable, which is
confirmed by the degradation efficiency. In the year 2003 the
average degradation efficiency was 71%, while at the time of
the experiment with OW (January 2004–March 2005) it was
81%. After finishing the experiment the degradation efficiency
again decreased to 73.5%.
Fig. 3 shows the OLR and biogas production. The average
OLR in 2003 was 0.9 kg m�3 d�1 of TCOD (0.76 kg m�3 d�1 of
VSS). At the time of the experiment we gradually increased
the OLR to 1.44 kg m�3 d�1 of TCOD (1.01 kg m�3 d�1 of VSS).
After the end of the experiment OLR decreased below
0.6 kg m�3 d�1 of TCOD (0.5 kg m�3 d�1 of VSS). The specific
biogas productivity (SBP) prior to the experiment was
0.39 m3 kg�1 VSS inserted. According to the OLR, biogas
quantity increased on starting to add OW by 80%. SBP slowly
increased to over 0.60 m3 kg�1 (peaking in January 2005 at
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Table 1 – Characteristics of OW influent (average monthly)
Date COD(mg l�1)
Averagequantity (m3 d�1)
VSS load(kg d�1)
TSS(g l�1)
VSS(g l�1)
Ratio VSS/TSS(%)
January 2004 199,600 1.00 187 197 187 95
February 2004 196,950 1.03 173 221 206 93
March 2004 298,800 1.17 277 247 237 96
April 2004 189,000 1.29 228 188 177 94
May 2004 144,500 1.30 150 125 115 92
June 2004 298,500 1.30 212 230 220 96
July 2004 309,550 1.07 254 248 237 96
August 2004 290,800 1.48 253 178 171 96
September 2004 268,800 1.83 401 240 219 91
October 2004 219,800 1.80 310 184 172 93
November 2004 239,750 2.01 384 224 191 85
December 2004 239,150 2.03 369 194 182 94
January 2005 146,150 1.93 185 102 96 94
February 2005 223,300 2.08 343 176 165 94
March 2005 184,350 1.08 185 182 171 94
Fig. 1 – Digesters and power set-up.
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0.89 m3 kg�1). BPR increased from 0.32 m3 m�3 d�1 prior to the
experiment to 0.67 m3 m�3 d�1 in February 2005. Interestingly,
after finishing the experiment in March 2005, biogas values
did not return to the values before the experiment. SBP
increased dramatically and BPR decreased slightly, but it
remained significantly higher than the values in 2003 (by
60%). After we stopped feeding the digester with OW at the
end of March 2005, it took about 30 days for the biogas
production to start decreasing. At this point, all of the OW
was most probably degraded. However, it seems that
the activity of the digester biomass (which is reflected in
the SBP) needed an additional 5 months to decrease to the
initial value of 2003. Throughout the experiment, the pH in
the digester was monitored. The values were always between
7.1 and 7.5.
Fig. 4 shows the daily quantity of biogas produced and the
power output of the boiler and CHP engine. A 40% higher OLR
resulted in 80% more biogas. The power set-up is designed to
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Table 2 – Average monthly total VSS and TCOD load of sludge (PS+WAS+OW)
Date TotalVSS
inserted(kg d�1)
TotalCOD
inserted(kg d�1)
Date TotalVSS
inserted(kg d�1)
TotalCOD
inserted(kg d�1)
Date TotalVSS
inserted(kg d�1)
TotalCOD
inserted(kg d�1)
Jan-03 1399 1588 Jan-04 1226 2124 Jan-05 1434 2362
Feb-03 1668 1892 Feb-04 1805 2157 Feb-05 1835 3491
Mar-03 1366 1550 Mar-04 1788 2273 Mar-05 1455 1742
Apr-03 1563 1774 Apr-04 1608 2257 Apr-05 1792 2726
May-03 1761 1998 May-04 2012 2574 May-05 1891 2682
Jun-03 1516 1720 Jun-04 1703 2873 Jun-05 1262 1553
Jul-03 1481 1680 Jul-04 1964 3350 Jul-05 992 1184
Aug-03 1598 1855 Aug-04 1881 2842 Aug-05 750 868
Sep-03 1445 1640 Sep-04 1569 2271 Sep-05 1020 1233
Oct-03 1307 1483 Oct-04 1445 2158 Oct-05 1434 1597
Nov-03 1680 1989 Nov-04 1538 2370
Dec-03 1440 1919 Dec-04 1891 2687
Fig. 2 – Influent and effluent VSS quantity and composition.
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 1 6 2 – 1 6 7 165
use gas in the boiler first and surplus in the CHP engine.
Therefore, it is to be expected that in winter months,
electrical power would be rarely produced, as shown in
Fig. 4, with production occurring only in the warmer months.
During the experiment, 45% more heat energy and 130% more
electrical energy was produced. It is also observed that during
the period from June to November 2004 the CHP engine was in
operation over 95% of the time. It has never happened before
during WWTP operation that the CHP would be fully
operational for such a long period. Even after November
2005, the CHP engine was operating more often than in
previous winter seasons. After finishing the experiment,
electrical power production decreased to levels similar to
those prior to the experiment. There is, however, a break in
electrical power production in May 2004, which was the result
of engine maintenance.
On completing the experiment, our opinion, as well as that
of many other authors [10,11], is that anaerobic digestion is
the solution to handling OW. All the results clearly show that
digesting OW (swill) is very beneficial. There are almost no
residual solids and degradation of OW VSS is very close to
100%. This is also reflected in increased biogas production.
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Fig. 3 – OLR and monthly average biogas production.
Fig. 4 – Daily biogas production and power output.
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 1 6 2 – 1 6 7166
OW is disposed of (virtually removed) and it is regenerated as
energy very efficiently.
4. Conclusions
A full-scale experiment on co-digestion of OW of domestic
refuse (swill) with municipal sludge is presented. Results have
shown that anaerobic digestion is the solution to handling
OW (swill) and above all it is very beneficial with little adverse
impacts on the environment. The experiment gave the
following results:
�
Virtually complete degradation of OW. The results showedno increase in effluent VSS during the experiment and
degradation efficiency increased from 71% to 81%.
�
80% increased biogas quantity. BPR increased from 0.32 to0.67 m3 m�3 d�1. SBP increased from 0.39 to a peak of
0.89 m3 kg�1 VSS inserted.
�
Electrical energy production increased by 130% and heatenergy production increased by 55%.
The authors hope that this experiment will encourage such
practice in handling OW in the future.
Acknowledgements
The authors would like to thank all co-workers at the
municipality of Velenje who helped in arranging and conduct-
ing the co-digestion experiment. The authors would also like to
thank Dr. Anthony Byrne for revising English and grammar.
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