Full-scale anaerobic co-digestion of organic waste and municipal sludge

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    1 L

    Article history:

    Received 8 May 2006

    Received in revised form

    6 July 2007

    with municipal sludge. In recent years, such research has treatment is not very imaginative. Anaerobic digestion is




    rate (OLR) is up to 3.7 kg m3 d1 of volatile suspended solids


    B I OMA S S A ND B I O E N E R G Y 32 ( 2008 ) 162 167Corresponding author. Tel.: +386 1 4760 249; fax: +386 1 4760 300.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

    (VSS) [3], to two-stage digesters. Sosnowski et al. [6] and

    Gomez et al. [7] reported successful operation with an OLR of

    3.34.3 kg m3 d1 of VSS. Gallert et al. [8] reported operating a

    0961-9534/$ - see front matter & 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.biombioe.2007.07.006

    E-mail address: gregor.zupancic@ki.si (G.D. Zupancic).past, OW of domestic refuse (swill) has usually been a food

    source for domestic animals, mostly pigs. As a food source it

    ways of successfully digesting OW of any kind, ranging from

    conventional mesophilic digestion, where the organic loadingreceived 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

    therefore the most cost-effective way to efficiently pr

    wet OW for energy recovery [2]. Many authors have condu

    research in this field in recent years. There are many pos1. Introduction

    Anaerobic digestion has the potential for treatment of many

    kinds of organic waste (OW) mixtures, mostly in combination

    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 suchAccepted 10 July 2007

    Available online 20 August 2007


    Anaerobic digestion

    Biogas production

    Organic waste

    Sludge digestionA 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 m3 d1 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 d1 of volatile suspended solids. Biogas

    quantity increased by 80% and specific biogas production increased from 0.39 m3 kg1

    volatile suspended solids inserted prior to the experiment to over 0.60 m3 kg1 volatile

    suspended solids inserted, peaking at 0.89 m3 kg1 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


    & 2007 Elsevier Ltd. All rights reserved.a r t i c l e i n f o a b s t r a c thttp://www.elsevie

    Full-scale anaerobic co-digestimunicipal sludge

    Gregor D. Zupancica,, Natasa Uranjek-ZevaaNational Institute of Chemistry, Hajdrihova 19, PO Box 660, SI-100bMunicipality of Velenje, Koroska 37/b, 3320 Velenje, Sloveniam/locate/biombioe

    n of organic waste and

    b, Milenko Rosa

    jubljana, Slovenia

  • anaerobic digestion as well as reduction of CO2 emission by

    was fed to the digester according to OLR two to three times


    GYreplacement 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 l1, total suspended

    solids (TSS) concentration from 20 to 30 g l1, and TCOD of

    sludge between 18,000 and 30,000 mg 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 issingle-stage digester with OLR values as high as 8.5 kg m3 d1

    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 kg m3 d1 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


    BPR biogas production rate, m3 per m3 of the digesterper day (m3 m3 d1)

    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 m3 d1)

    B I OMA S S AND B I O E N E Rcollected 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 digesterper 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 m3 d1 of VSS (0.9 kg m3 d1 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 isoverflow 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

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

    inserted (m3 kg1)

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

    32 ( 2008 ) 162 167 163confirmed 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 0.9 kg m3 d1 of TCOD (0.76 kg m3 d1 of

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

    the OLR to 1.44 kg m3 d1 of TCOD (1.01 kg m3 d1 of VSS).

    After the end of the experiment OLR decreased below

    0.6 kg m3 d1 of TCOD (0.5 kg m3 d1 of VSS). The specific

    biogas productivity (SBP) prior to the experiment was

    0.39 m3 kg1 VSS inserted. According to the OLR, biogas

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

    increased to over 0.60 m3 kg1 (peaking in January 2005 at


    G YB I OMA S S A ND B I O E N E R1640.89 m3 kg1). BPR increased from 0.32 m3 m3 d1 prior to the

    experiment to 0.67 m3 m3 d1 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

    Table 1 Characteristics of OW influent (average monthly)

    Date COD(mg l1)

    Averagequantity (m3 d1)


    January 2004 199,600 1.00

    February 2004 196,950 1.03

    March 2004 298,800 1.17

    April 2004 189,000 1.29

    May 2004 144,500 1.30

    June 2004 298,500 1.30

    July 2004 309,550 1.07

    August 2004 290,800 1.48

    September 2004 268,800 1.83

    October 2004 219,800 1.80

    November 2004 239,750 2.01

    December 2004 239,150 2.03

    January 2005 146,150 1.93

    February 2005 223,300 2.08

    March 2005 184,350 1.08

    Fig. 1 Digesters an32 ( 2008 ) 162 167was 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

    SS loadkg d1)

    TSS(g l1)

    VSS(g l1)

    Ratio VSS/TSS(%)

    187 197 187 95

    173 221 206 93

    277 247 237 96

    228 188 177 94

    150 125 115 92

    212 230 220 96

    254 248 237 96

    253 178 171 96

    401 240 219 91

    310 184 172 93

    384 224 191 85

    369 194 182 94

    185 102 96 94

    343 176 165 94

    185 182 171 94

    d power set-up.








    GYTable 2 Average monthly total VSS and TCOD load of slud

    Date TotalVSS

    inserted(kg d1)


    inserted(kg d1)

    Date TV


    Jan-03 1399 1588 Jan-04 1

    Feb-03 1668 1892 Feb-04 1

    Mar-03 1366 1550 Mar-04 1

    B I OMA S S AND B I O E N E Ruse 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

    Apr-03 1563 1774 Apr-04 1608

    May-03 1761 1998 May-04 2012

    Jun-03 1516 1720 Jun-04 1703

    Jul-03 1481 1680 Jul-04 1964

    Aug-03 1598 1855 Aug-04 1881

    Sep-03 1445 1640 Sep-04 1569

    Oct-03 1307 1483 Oct-04 1445

    Nov-03 1680 1989 Nov-04 1538

    Dec-03 1440 1919 Dec-04 1891

    Fig. 2 Influent and effluent VS(PS+WAS+OW)




    inserted(kg d1)

    Date TotalVSS

    inserted(kg d1)


    inserted(kg d1)

    2124 Jan-05 1434 2362

    2157 Feb-05 1835 3491

    2273 Mar-05 1455 1742

    32 ( 2008 ) 162 167 165previous 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.

    2257 Apr-05 1792 2726

    2574 May-05 1891 2682

    2873 Jun-05 1262 1553

    3350 Jul-05 992 1184

    2842 Aug-05 750 868

    2271 Sep-05 1020 1233

    2158 Oct-05 1434 1597



    S quantity and composition.


    G Y16OW









    B I OMA S S A ND B I O E N E R6is disposed of (virtually removed) and it is regenerated as

    ergy very efficiently.


    full-scale experiment on co-digestion of OW of domestic

    use (swill) with municipal sludge is presented. Results have

    own that anaerobic digestion is the solution to handling

    (swill) and above all it is very beneficial with little adverse

    pacts on the environment. The experiment gave the

    lowing 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%.

    Fig. 3 OLR and monthly av

    Fig. 4 Daily biogas produc









    tio32 ( 2008 ) 162 16780% increased biogas quantity. BPR increased from 0.32 to

    0.67 m3 m3 d1. SBP increased from 0.39 to a peak of

    0.89 m3 kg1 VSS inserted.

    Electrical energy production increased by 130% and heat

    energy production increased by 55%.

    he authors hope that this experiment will encourage such

    ctice in handling OW in the future.


    e authors would like to thank all co-workers at the

    nicipality of Velenje who helped in arranging and conduct-

    the co-digestion experiment. The authors would also like to

    nk Dr. Anthony Byrne for revising English and grammar.

    ge biogas production.

    n and power output.

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    B I OMA S S AND B I O E N E R G Y 32 ( 2008 ) 162 167 167

    Full-scale anaerobic co-digestion of organic waste and municipal sludgeIntroductionMaterials and methodsResults and discussionConclusionsAcknowledgementsReferences


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