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Full-scale anaerobic co-digestion of organic waste and

municipal 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

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

2000m3 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 kgm3d1 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 m3d1 of volatile suspended solids. Biogas

quantity increased by 80% and specific biogas production increased from 0.39 m3kg1

volatile suspended solids inserted prior to the experiment to over 0.60 m3kg1 volatile

suspended solids inserted, peaking at 0.89 m3kg1 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%. Volatilesuspended 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 [13] 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

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 3.7kg m3d1 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

3.34.3kgm3d1 of VSS. Gallert et al. [8] reported operating a

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0961-9534/$ - see front matter&

2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.biombioe.2007.07.006

Corresponding author. Tel.: +3861 4760249; fax: +3861 4760300.E-mail address: [email protected] (G.D. Zupancic).

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
http://localhost/var/www/apps/conversion/tmp/scratch_10/dx.doi.org/10.1016/j.biombioe.2007.07.006mailto:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_10/dx.doi.org/10.1016/j.biombioe.2007.07.006


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single-stage digester with OLR values as high as 8.5 kg m3d1

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 1.01.5 kgm3d1 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 20gl1, total suspended

solids (TSS) concentration from 20 to 30gl1, and TCOD of

sludge between 18,000 and 30,000mg l1. 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 56 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

0.76kgm3d1 of VSS (0.9kgm3d1 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 2004March 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 0.9kgm3d1 of TCOD (0.76kgm3d1 of

VSS). At the time of the experiment we gradually increased

the OLR to 1.44kgm3d1 of TCOD (1.01kgm3d1 of VSS).

After the end of the experiment OLR decreased below

0.6kgm3d1 of TCOD (0.5kgm3d1 of VSS). The specific

biogas productivity (SBP) prior to the experiment was

0.39m3kg1 VSS inserted. According to the OLR, biogas

quantity increased on starting to add OW by 80%. SBP slowly

increased to over 0.60m3kg1 (peaking in January 2005 at

ARTICLE IN PRESS

Nomenclature

BPR biogas production rate, m3 per m3 of the digesterper day (m3m3d1)

CHP combined heat and powerTCOD total chemical oxygen demand (mg l1)HRT hydraulic retention time, dOLR organic loading rate, kg of TCOD or VSS per m3 of

the digester per day (kg m3d1)

OW organic wastePE population equivalentPS primary sludgeSBP specific biogas productivity, m3 per kg VSS

inserted (m3kg1)

TSS total suspended solids (mg l1)VSS volatile suspended solids (mg l1)WAS waste activated sludgeWWTP 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


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0.89m3kg1). BPR increased from 0.32 m3m3d1 prior to the

experiment to 0.67 m3m3d1 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 1 Characteristics of OW influent (average monthly)

Date COD(mgl1) Averagequantity (m3d1) VSS load(kgd1) TSS(g l1) VSS(g l1) 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 94February 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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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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Table 2 Average monthly total VSS and TCOD load of sludge (PS+WAS+OW)

Date TotalVSS

inserted(kgd1)

TotalCOD

inserted(kgd1)

Date TotalVSS

inserted(kgd1)

TotalCOD

inserted(kgd1)

Date TotalVSS

inserted(kgd1)

TotalCOD

inserted(kgd1)

Jan-03 1399 1588 Jan-04 1226 2124 Jan-05 1434 2362Feb-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.



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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 showed

no increase in effluent VSS during the experiment and

degradation efficiency increased from 71% to 81%.

80% increased biogas quantity. BPR increased from 0.32 to

0.67m3m3d1. SBP increased from 0.39 to a peak of

0.89m3kg1 VSS inserted. Electrical energy production increased by 130% and heat

energy 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.

ARTICLE IN PRESS

Fig. 3 OLR and monthly average biogas production.

Fig. 4 Daily biogas production and power output.



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[3] Stroot PG, McMahon KD, Mackie RI, Raskin L. Anaerobiccodigestion of municipal solid waste and biosolids undervarious mixing conditionsI. Digester performance. WaterResearch 2001;35(7):180416.

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[5] Decree on the input of dangerous substances and plantnutrients into the soil. Official Journal of Republic of Slovenia,No. 68, November 1996, Ljubljana.

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[7] Gomez X, Cuetos MJ, Cara J, Moran A, Garca AI. Anaerobic co-digestion of primary sludge and the fruit and vegetablefraction of the municipal solid wastes: conditions for mixing

and evaluation of the organic loading rate. Renewable Energy2006;31(12):201724.[8] Gallert C, Henning A, Winter J. Scale-up of anaerobic

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[9] APHA, AWWA, WEF. Standard Methods for the Examinationof Water and Wastewater, 20th ed. Washington, DC, 1998.

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