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Waste Management 27 (2007) 406–414

Methane yield in source-sorted organic fraction of municipal solid waste

Asa Davidsson a, Christopher Gruvberger b, Thomas H. Christensen c,Trine Lund Hansen c, Jes la Cour Jansen a,*

a Water and Environmental Engineering at Department of Chemical Engineering, Center for Chemistry and Chemical Engineering, Lund University,

P.O. Box 124, SE-221 00 Lund, Swedenb Malmo Water and Sewage Works, S-205 80 Malmo, Sweden

c Environment and Resources DTU, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark

Accepted 14 February 2006Available online 18 April 2006

Abstract

Treating the source-separated organic fraction of municipal solid waste (SS-OFMSW) by anaerobic digestion is considered by manymunicipalities in Europe as an environmentally friendly means of treating organic waste and simultaneously producing methane gas.Methane yield can be used as a parameter for evaluation of the many different systems that exist for sorting and pre-treating waste.

Methane yield from the thermophilic pilot scale digestion of 17 types of domestically SS-OFMSW originating from seven full-scalesorting systems was found. The samples were collected during 1 year using worked-out procedures tested statistically to ensure represen-tative samples. Each waste type was identified by its origin and by pre-sorting, collection and pre-treatment methods. In addition to thepilot scale digestion, all samples were examined by chemical analyses and methane potential measurements. A VS-degradation rate ofaround 80% and a methane yield of 300–400 Nm3 CH4/ton VSin were achieved with a retention time of 15 days, corresponding to�70% of the methane potential.

The different waste samples gave minor variation in chemical composition and thus also in methane yield and methane potential. Thisindicates that sorting and collection systems in the present study do not significantly affect the amount of methane produced per VStreated.� 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Many municipalities in European countries are consid-ering anaerobic digestion as a means of treating thesource sorted organic fraction of municipal solid waste(SS-OFMSW). This new method not only allows recoveryof energy and nutrients but also responds to regulationsand taxes that are presently being levied on conventionaltreatment methods, such as incineration or landfilling.Unfortunately, digestion plants for this purpose are inoperation only in few countries, and the capacity of theplants is still limited compared to the organic wastepotential.

0956-053X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2006.02.013

* Corresponding author. Tel.: +46 46 222 8999; fax: +46 46 222 4526.E-mail address: jes.la_cour_jansen@vateknik.lth.se (J.l.C. Jansen).

Two main types of digestion systems have been pre-ferred for digestion of SS-OFMSW coming from house-holds. In some countries, the household waste is mixedwith park and garden waste or other waste types with alow water content that makes a ‘‘dry’’ process with drymatter content in the process higher than 25%. In Scandi-navia, park and garden wastes are normally compostedand the household waste is digested in a ‘‘wet’’ process withdry matter content less than 10%. Due to the high watercontent, the preferred digestion process in Scandinavia isbased on one stage, totally mixed reactors.

Several different sorting and collection systems are usedand the waste is pre-sorted in different pre-sorting systemsprior to digestion in order to reduce technical problems inthe digestion systems and to secure a good quality of theresidues from the digestion. From 2000 to 2003, a com-bined Danish and Swedish project was carried out in order

A. Davidsson et al. / Waste Management 27 (2007) 406–414 407

to evaluate the present full-scale collection systems fororganic household waste together with the existing pre-treatment systems. The waste was collected from seven dif-ferent full-scale systems and pre-treatment was performedwith five different pre-treatment methods. A detailed chem-ical characterization of the waste was carried out togetherwith methane potential measurements by batch laboratorytests (Hansen et al., accepted-a). Further, a limited numberof samples were selected for pilot scale digestion in order toevaluate the practical possibilities for digestion and to findthe methane yield under conditions similar to full scaledigestion.

The methane production – the methane yield – can beused to evaluate different waste types and thereby differentsystems for sorting out, collecting and pre-treating theorganic waste. In addition, methane yield can be comparedwith the theoretical methane yield based on the composi-tion of the waste.

The methane yield from anaerobic digestion of OFMSWhas been studied in recent years, e.g., Gunaseelan (1997).However, in most examinations where methane yield fromOFMSW has been studied, a wider characterization of thewaste tested has not been included and only in few studiesseveral different types of OFMSW have been compared.

The paper presents the methane yield results from ther-mophilic pilot scale digestion of 17 types of SS-OFMSW.The samples were selected as a subset of 40 collected sam-ples in order to reduce the workload but still covering dif-ferent housing types, sorting, collection and pre-treatmentsystems with known characteristics. All 17 types were sam-pled during 1 year using worked-out procedures, whichwere tested statistically to ensure representative samples(Jansen et al., 2004).

Table 1Characteristics of the 17 waste samples in the study

Pre-treatment Collection place Origin (single familyhouse orapartment)

Disc screen Copenhagen SFCopenhagen SFCopenhagen ApVejle ApKolding ApAalborg SFKolding SFKolding SF

Screw press Copenhagen SFAalborg SFAalborg SFAalborg ApVejle Ap

Shredder + magnet Grindsted SF

Food waste disposer Malmo ApMalmo Ap

Piston press Malmo Ap

2. Materials and methods

2.1. Waste types

Each waste type studied was identified by its origin, pre-sorting, collection and pre-treatment system. The wasteshad different origins (single family houses and apartmentblocks in different municipalities), kitchen wrappings (plas-tic bags, paper bags or none), and sack type in refuse bin(paper sack or none) and had been subjected to differentpre-treatment methods (screw press device, disc screen,shredder + magnetic separation, piston press device orfood–waste-disposer-system). Detailed descriptions of thewaste types and the composition of the waste can be foundin Hansen et al. (accepted-a). The waste types originatedfrom five Danish municipalities with large-scale collectionof SS-OFMSW, representing existing Scandinavian sys-tems. In addition, waste from two new systems for collec-tion of SS-OFMSW from a newly built district inMalmo, Sweden was included. Table 1 presents the charac-teristics of the 17 samples. The test material included fourpairs of waste samples (SF_Pap_1/SF_Pap_2, SF_Pla_7/SF_Pla_8, SF_Pla_10/SF_Pla_11 and Ap_No_15/Ap_No_16) where the same waste types were sampled attwo different occasions.

2.2. Sampling, processing and analyses

Each sampling occasion included weighing of: the wasteload before pre-treatment, the organic fraction after pre-treatment and the reject fraction after pre-treatment.

Special procedures were developed for field sampling inorder to achieve representative samples from the quite

Kitchen wrapping(paper, plasticor no bags)

Tested in round Sample name

Pap 1 SF_Pap_1Pap 2 SF_Pap_2Pap 2 Ap_Pap_3Pla 2 Ap_Pla_4Pla 3 Ap_Pla_5Pla 3 SF_Pla_6Pla 3 SF_Pla_7Pla 4 SF_Pla_8

Pap 1 SF_Pap_9Pla 1 SF_Pla_10Pla 4 SF_Pla_11Pla 2 Ap_Pla_12Pla 4 Ap_Pla_13

Pap 2 SF_Pap_14

No 3 Ap_No_15No 4 Ap_No_16

Pap 4 Ap_Pap_17

408 A. Davidsson et al. / Waste Management 27 (2007) 406–414

inhomogeneous SS-OFMSW. Different procedures wereused in field depending on the actual pre-treatmentmethod. As an example, the procedure for sampling SS-OFMSW pre-treated by disc screen is presented in Fig. 1.About 10% of the pre-treated waste was taken out fromthe pile by random sampling for further disintegration ina shredder. The disintegration makes the waste morehomogeneous and easier to mix, which enhances the possi-bility of taking out a representative final sample of 20–30 kg (Jansen et al., 2004).

The procedure used for further processing in the labora-tory was adjusted for each pre-treatment method as thatinfluences the homogeneity and consistency of the sample.The most extensive processing procedure was used for disc-screened waste (Fig. 2) (Jansen et al., 2004).

The quality of the waste sampling procedures and chem-ical analyses were statistically evaluated, as described inJansen et al. (2004). The analyses showed that no singlestep in the sampling or laboratory processing procedurecontributed significantly to sampling errors. Sampling pro-cedure and analyses were found to be reasonably represen-tative of the waste contained in a single truck and theuncertainty of the data was reasonably low, with a relative

10 %

Load of OFMSW

Disc screen

20-30 kg

Pre-treated OFMSW

Shredder

Fig. 1. Procedure for field sampling OFMSW pre-treated by disc screen.

Chemical analyses

Batch laboratory methane potential tests; pH, VFA andN-analyses

Dry blending

Wet blending

Dry matter and volatile solids analyses

High speedblending

Drying at 80ºC

Hammer mill

Physical characterizationand pilot-scale digestion

Fig. 2. Laboratory processing procedure used for waste pre-treated bydisc screen.

standard deviation between 3% and 10% for most of theanalytical parameters.

2.3. Chemical characterization

Standard environmental and feedstuff analytical proce-dures were used to characterize the waste. An extensivechemical analysis program was applied including the fol-lowing parameters: dry matter, volatile solids, crude fat,crude protein, crude fiber, starch, sugar, EDOM (enzymedigestible organic matter), K, Tot-P Tot-N, C, H, S-Tot,calorific value and C:N ratio. Standardized methods wereused and a detailed description of the chemical character-ization can be found in Hansen et al. (accepted-a).

2.4. Methane potential tests

The methane potential of the sampled wastes was testedin triplicate by laboratory-scale anaerobic batch testsdescribed in Hansen et al. (2003). The tests were performedin 2-l reactors that contained an amount of high-speed-blended waste (Fig. 2) representing 10 g of volatile solidsas well as 400 ml of thermophilic inoculum. The reactorswere kept at 55 �C and methane production was monitoredduring 50 days by a gas chromatograph (Hansen et al.,2003). The method was developed to be an easy-to-operateand fast method of measuring methane potentials of solidwaste samples generating high amounts of methane.

2.5. Pilot-scale digestion

2.5.1. Set-up

To enable an evaluation of the gas production from awell-defined organic waste, each load of waste was homog-enized (shredding + blending) and divided into 1-day-por-tions. This handling makes it possible to leave out thevariations of the waste that occur in reality at a full-scaleplant. The pilot-scale equipment resembles a full-scale bio-gas plant including heating, feeding once a day, stirring andgas collection. The equipment is described in Jansen et al.(2004b). Each set of test equipment included a cylindrical35-l digester connected to a 77-l gas-collection-tank. Thedigesters were kept at thermophilic temperature, 55 �C. Atop-mounted mechanical stirrer ensured a totally mixedtank.

2.5.2. Operation

Feeding and residue removal were carried out manuallyonce every day. The retention time was chosen to be 15days. Before feeding, the 1-day-portions of waste werediluted to a dry matter content of 5% corresponding toan organic loading rate of �2.8 kg VS/m3 Æ day, in orderto minimize the risk of organic overload and in order toensure sufficient mixing of all tested waste types. Tracerswere added during two test rounds to check the retentiontime and thereby the function of the mixing. Leakagecontrol was carried out during all test rounds. Digested

y = 0.3532x - 0.0524R2 = 0.9986

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Acc. volatile solids (kg)

Acc

. met

han

e (N

m3 ) Evaluation period

(10 days)

Fig. 4. Accumulated methane production versus added volatile solidsduring the whole test period for one of the 17 tested waste types. Theevaluation period of 10 days is highlighted.

A. Davidsson et al. / Waste Management 27 (2007) 406–414 409

residues from two different full-scale thermophilic digestionplants (Kalmar biogas plant and Vanersborg biogas plant,both Sweden) were used during the experimental period asinoculum for starting up the process.

Each test round lasted for 2–3 min including a start-upperiod, a full operation period (including a period of stableprocessing) and a post-digestion period (gas collectionwithout feeding), see Fig. 3. To measure the total residualmethane potential, samples of digested residues were setup in laboratory methane potential batch tests (Hansenet al., 2003).

2.5.3. Analyses

Analyses of produced gas and digested residues werecarried out every day. Gas composition (CH4 and H2S)was analyzed in the gas from the gas holding tank by aGas Surveyor 431 Portable Gas Detector, GMI Gas mea-surement Instruments Ltd, Scotland, UK. For the digestedresidue, temperature and pH were controlled daily. In addi-tion, HCO3, VFA, TS, VS, COD, P-tot, N-tot and NH4-Nwere analyzed once a week. Standard methods for the anal-yses where applied (APHA, 1989).

2.5.4. Calculations of methane yield

Fig. 4 shows the accumulated methane production ver-sus the accumulated volatile solids added during a wholetest period for 1 of the 17 waste samples tested (discscreened waste SF_Pla_7). The methane yield is calculatedas the slope of the linear regression line of accumulatedmethane per accumulated added VS for 10 days of stableprocess. The evaluated period of 10 days for the waste sam-ple in Fig. 4 is highlighted. The process was considered sta-ble a couple of weeks after complete feeding started, whenthe effect of the inoculum was largely reduced, and no oper-ational disturbances were occurring. The 10-day-periodwithin stable operation with the highest accumulated meth-ane yield was chosen for evaluation in order to have a com-mon measure for the stable methane production withoutinfluence from day to day variations in the operation. Only

0

0.01

0.02

0.03

0 10 20 30 40 50 60

days

CH

4 (N

m3 p

er d

ay)

start-up post-digestionfull-operation

Fig. 3. Daily methane production for a test period including start-up, full-scale digestion and post-digestion for one of the 17 tested waste types.Daily methane production was not read exactly at the same time everyday. Therefore, day-to-day variations can be seen.

minor variation in the yield was experienced when stableoperation was established.

3. Results and discussion

Four rounds of pilot-scale digestion were performedwith 3–5 waste types tested in each round. The data fromthese test rounds, which can be found in tables and figuresbelow, is arranged by pre-treatment method. A system ofnotation for the waste samples by their characteristics,explained in Table 1, is used in the tables and figures.

3.1. Chemical composition

The variations in composition between the differentwaste types were in general small (Table 2). TS-content var-ied between 17% and 37% for all types except waste gener-ated by the food-waste disposer system, which included agreat deal of water for transportation of the waste and rins-ing of the disposer. Since this waste type differs from theothers in many ways, it is not included in the ranges givenin Table 2.

3.2. Measured and theoretical methane potential

Both theoretical and measured (in laboratory batchtests) methane potentials for the 17 tested waste samplesare shown in Table 3. On average, measured potentialsachieved in laboratory tests represented 74% of the theoret-ical value: (1) based on element composition and 87% ofthe theoretical value, and (2) based on component compo-sition calculated by Buswells formula (Buswell and Neave,1930). Methane potentials measured in the laboratorybatch test varied from 300 to 570 Nm3 CH4/ton VSin. Inspite of the fact that the measurements showed great vari-ations, the results show no systematic variations amongdifferent housing areas, kitchen wrapping materials orpre-treatment methods. The results show great differenceseven in methane potential for pairs of the same waste typessampled at different occasions. Standard deviations for

Table 2Environmental and feedstuff analysis results for the 17 waste samples (range do not include Ap_No_15 and Ap_No_16)

Pre-treatment Sample TS(% w/w)

VS(% ofTS)

Crude fat(% ofTS)

Crudeprotein(% of TS)

Crudefibre(% of TS)

Starch(% ofTS)

Sugar(% ofTS)

EDOM(% ofTS)

K(% ofTS)

Tot P(% ofTS)

Tot N(% ofTS)

C(% ofTS)

H(% ofTS)

S tot(% ofTS)

Calorificvalue(MJ/kg TS)

C:Nratio

Disc screen SF_Pap_1 31 87 15 17 15 – – 80 1 0.5 2.8 48 7.2 0.2 20 17.1SF_Pap_2 26 91 11 13 23 13 9 82 1.1 0.4 2.6 49 7.2 0.2 21 18.8Ap_Pap_3 28 87 12 13 23 12 7 77 1 0.6 2.5 49 7.1 0.2 20 19.6Ap_Pla_4 30 80 10 10 26 13 5 69 0.9 0.5 2.3 46 6.7 0.2 19 20.0Ap_Pla_5 34 87 15 16 15 17 9 78 1 0.5 2.6 48 7.2 0.2 20 18.5SF_Pla_6 31 84 13 15 15 16 8 75 1 0.5 2.2 45 6.4 0.2 19 20.5SF_Pla_7 33 82 15 15 13 17 9 75 0.9 0.5 2.4 46 6.9 0.2 19 19.2SF_Pla_8 30 81 16 18 18 10 1 70 1 0.6 2.7 50 7.2 0.2 19 18.5

Screw press SF_Pap_9 29 92 18 17 13 – – 86 1.1 0.3 2.9 50 7.6 0.2 21 17.2SF_Pla_10 17 85 15 17 8 – – 81 1.3 0.4 2.8 47 7.1 0.2 19 16.8SF_Pla_11 25 87 17 16 12 14 4 82 0.9 0.3 2.9 52 7.8 0.2 20 17.9Ap_Pla_12 22 87 17 16 10 19 6 82 1.1 0.3 2.6 48 7 0.2 20 18.5Ap_Pla_13 37 85 14 15 13 15 4 78 0.8 – 2.7 49 7.4 0.2 20 18.1

Shredder + magnet SF_Pap_14 28 92 17 15 23 11 – 85 1 0.4 2.5 50 7.3 0.2 22 20.0

Food wastedisposer

Ap_No_15 4 92 18 16 10 15 6 86 0.2 0.3 4.6 58 8.5 0.4 26 12.6Ap_No_16 6 95 34 26 14 2 – 68 1 0.3 2.8 51 7.5 0.2 21 18.2

Piston press Ap_Pap_17 32 87 13 17 12 – 10 79 1 0.3 3.1 48 7 0.2 20 15.5

Range 17–37 81–92 10–18 10–18 8–26 10–19 1–10 69–86 0.8–1.3 0.3–0.6 2.2–3.1 45–52 6.4–7.8 0.2 19–22 15.5–20.5

SF, single-family houses; Ap, apartment blocks; Pap, paper bags in kitchen; Pla, plastic bags in kitchen; No, no bags.

410A

.D

avid

sson

eta

l./

Wa

steM

an

ag

emen

t2

7(

20

07

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06

–4

14

Table 3Theoretical and measured methane potentials for the 17 waste samples

Pre-treatment Sample Theoretical methanepotential (1)(Nm3 CH4/ton VSin)

Theoretical methanepotential (2)(Nm3 CH4/ton VSin)

Measured methanepotential(Nm3 CH4/ton VSin)

Disc screen SF_Pap_1 623 532 489SF_Pap_2 594 498 298Ap_Pap_3 635 503 500Ap_Pla_4 658 495 515Ap_Pla_5 612 534 404SF_Pla_6 583 525 464SF_Pla_7 629 539 573SF_Pla_8 734 543 388

Screw press SF_Pap_9 611 545 –a

SF_Pla_10 627 540 –a

SF_Pla_11 697 544 566Ap_Pla_12 605 545 380Ap_Pla_13 658 530 454

Shredder + magnet SF_Pap_14 591 537 495

Food waste disposer Ap_No_15 703 548 445Ap_No_16 651 653 472

Piston press Ap_Pap_17 620 519 556

(1) Theoretical methane potential based on element composition (C, H, N, O).(2) Theoretical methane potential based on component composition (fat, protein and carbohydrate).SF, single-family houses; Ap, apartment blocks; Pap, paper bags in kitchen; Pla, plastic bags in kitchen; No, no bags.

a No results obtained for SF_Pap_9 and SF_Pap_10 because of experimental failure.

Table 4Methane yield measured by pilot-scale digestion for the 17 waste samples

Sample Methane yield(Nm3 CH4/ton VSin)

Average CH4

content (%)VS degraded(%)

Residual methanepotential(Nm3 CH4/ton VSin)

Disc screen SF_Pap_1 347 61 82 41SF_Pap_2 340 60 74 53Ap_Pap_3 349 64 76 25Ap_Pla_4 311 60 85 35Ap_Pla_5 353 62 80 –a

SF_Pla_6 328 61 80 –a

SF_Pla_7 353 62 89 –a

SF_Pla_8 322 58 79 43

Screw press SF_Pap_9 275 66 77 52SF_Pla_10 367 64 75 51SF_Pla_11 410 60 80 –a

Ap_Pla_12 400 64 80 35Ap_Pla_13 319 62 81 –a

Shredder + magnet SF_Pap_14 289 63 83 72

Food waste disposer Ap_No_15 – –a

Ap_No_16 – –a

Piston press Ap_Pap_17 284 59 89 –a

All samples average 336 62 81

Disc screen (average); N = 8 338

Screw press (average); N = 5 354

Paper (average); N = 6 314

Plastic (average); N = 9 351

Paper and disc screen (average); N = 3 345

Paper and screw press (average); N = 1 275

Plastic and disc screen (average); N = 5 333

Plastic and screw press (average); N = 4 374

SF, single-family houses; Ap, apartment blocks; Pap, paper bags in kitchen; Pla, plastic bags in kitchen; No, no bags.a The residual methane potential was only measured in part of the samples.

A. Davidsson et al. / Waste Management 27 (2007) 406–414 411

412 A. Davidsson et al. / Waste Management 27 (2007) 406–414

these wastes (SF_Pap_1/SF_Pap_2, SF_Pla_7/ SF_Pla_8and Ap_No_15/ Ap_No_16) were in the range of 19–135 Nm3 CH4/ton VSin. The standard deviation for allsamples was 76 Nm3 CH4/ton VSin.

3.3. Methane yield

The pilot-scale continuous digestion was possible tostart-up and to operate for producing methane for mostwaste types. The methane yield was found to be between275 and 410 Nm3 CH4/ton VSin, with an average value of336 Nm3 CH4/ton VSin (Table 4). There were no statisti-cally significant differences between different housing areatypes, kitchen wrapping materials or pre-treatment meth-ods. One type, waste from apartment blocks with foodwaste disposers, did not reach stable operation. Therefore,no results on this waste type could be presented. Residualmethane potential measured in batch test on the digestedresidue is shown in Table 4. The results suggest that thereis an additional potential for about 50 Nm3/ton VS origi-nally added, which represents 10–15% of the yield. Table4 also shows that the average methane content during thestable period was 62% and that the degradation of organicmaterial represented by VS-content was on average 81%.

Only few results from previous studies on methane yieldfor OFMSW are directly comparable with the results fromthe present study. Very often only brief characterization ofthe waste is provided and in many cases the origin of the

Table 5Some results from previous studies on methane yield for OFMSW

Substrate and operational parameters The

This study Source sorted OFMSW, 5%TS, 81–92%VS, 15 days HRT, 2.8 kg VS/m3, d

T

Bolzonella et al. (2003) Mechanically sortedOFMSW + Putrescible fraction ofOFMSW, Semi-dry, 20%TS, 62%VS,13.5 days HRT, 9.2 kg VS/m3, d

T

Fruteau de Laclos et al. (1997) Source sorted OFMSW(food + garden waste), 30% TS, 20–55 days HRT

M

Gallert and Winter (1997) Manually sorted OFMSW, 18% TS,90% VS, 19 days HRT, 9.65 kg VS/m3, d

T

Gunaseelan (1997) Compilation of different studies:Mechanically sorted OFMSW,various thermophilic processes (oftenlow VS-content)

T

Pauss et al. (1984) Manually sorted OFMSW, 3-5.6%TS, 82–87% VS, 14-20 days HRT,OLR 2 resp 4 kg VS/m3, d

M

Rintala and Ahring (1994) Various batch and continuous testson source-sorted OFMSW

T

Weiland (2000) Monodigestion of municipal waste inMunster: wet, 1-stage

M

Weiland (2000) Monodigestion of municipal waste inQuarzbichl: dry, 1-stage, 19 daysSRT.VS calculated to 58%

M

(Values have been recalculated to Nm3/ton VSin).a The yield in references are given in different unities not always specifying

waste differs from the present waste types from full-scalecollection systems with insignificant contribution from gar-den waste. Further the process parameters for the digestionprocess may differ with respect to temperature, retentiontime and moisture content. Table 5 contains reported yieldsfrom a number of examinations where comparable infor-mation can be extracted. It is seen that reported methaneyields are in the range of 200–600 Nm3 CH4/ton VSin. Itis also seen that the yields from the present study are inthe upper range and close to the results presented in Wei-land (2000) from full scale systems very similar to the sys-tem used in the present examination.

3.4. Operational experiences

Operating the pilot-scale equipment for more than a yeargave practical experiences and revealed difficulties withanaerobic digestion of OFMSW. Most important, it waspossible to treat exclusively OFMSW without significantcontribution from garden waste by anaerobic digestionwithout many disturbances. However, the waste pre-treatedwith the disc screen and originally wrapped up in plasticbags created difficulties in feeding and outflow as well asin stirring since it contained a lot of plastic material. Detailsof the effect of the pre-treatment technologies on the qualityof the waste can be found in Hansen et al. (accepted-b). Theplastic material accumulated in the tank since it formed atop layer in the digester while residue was taken out from

rmo/Meso Yield a CH4 (%) Degradation(% of VS)

275–410 Nm3 CH4/tonVSin 62% 81

230 m3/ton VSin 68.7% n.a.

210–290 Nm3 CH4/tonVSin n.a. n.a.

Calc. 350 Nm3 CH4/tonVSin 59 65

128–319 Nm3 CH4/tonVSin n.a. 36–50

430 Nm3 CH4/ton VSin for both n.a. n.a.

400–590 Nm3 CH4/tonVSin n.a. n.a.

420 m3/ton VS n.a. 55

380 m3/ton VS n.a. 50

if measured in methane or biogas and at STP. (n.a., no data available).

A. Davidsson et al. / Waste Management 27 (2007) 406–414 413

the bottom. Even the full-scale plant treating this waste typehas had serious problems with heavy built-up of plastics inthe reactor. Another practical experience with the digestiontests was that in some reactors the alkalinity went downduring operation, especially during start-up. This was com-pensated for by adding small amounts of NaHCO3. Theammonium content in digested residue did not inhibit theprocess. Fig. 5 shows the measured values of ammoniumin weekly samples of digested residue.

The highest ammonium content (2600 mg/l) occurred inthe beginning of the second digestion round when the inoc-ulum used was rich in ammonium. However, the ammo-nium content was less than 1000 mg/l for all waste typesin their evaluation period (waste from food waste disposersdisregarded). The decreasing ammonium is a result oflower nitrogen content in the actual organic waste thanin the waste used at the plant that supplied the inoculum.As the inoculum is diluted by the new waste the ammoniumdecreases correspondingly. The pH was around 7 withmeasured values from 6.60 to 7.50. At this pH and ammo-nium level, ammonia is not expected to inhibit the metha-nogenic activity. Previous research has shown that the

Waste pre-treated by disc screen

0

1000

2000

3000

0 2 4 6 8 10 12

NH

4-N

(m

g/l)

14

SF_Pap_1SF_Pap_2Ap_Pap_3Ap_Pla_4Ap_Pla_5SF_Pla_6SF_Pla_7SF_Pla_8

Waste pre-treated by screw press

0

1000

2000

3000

0 2 4 6 8 10 12

Weeks after start-up

Weeks after start-up

NH

4-N

(m

g/l)

14

SF_Pap_9SF_Pla_10SF_Pla_11Ap_Pla_12Ap_Pla_13

Waste pre-treated by shredder + magnet (14), food waste disposers (15, 16) or piston press (17)

0

1000

2000

3000

0 2 4 6 8 10 12

Weeks after start-up

NH

4-N

(m

g/l)

14

SF_Pap_14

Ap_No_15

Ap_No_16

Ap_Pap_17

Fig. 5. Ammonium in samples of digested residue from thermophilicpilot-scale digestion.

methane production reduction at this level is less than10% (Gallert and Winter, 1997; Lay et al., 1997).

3.5. Relationship between measured methane yield, measured

methane potential and theoretical methane potential

Measured and theoretical values are compared in Fig. 6.Both theoretical values follow the same pattern but are ondifferent levels. It should be noted that the component-based theoretical value differs very much for Ap_No_16,waste from food waste disposers. This is caused by unusualhigh content of fat and protein. Measured yields make upon average 63% of the component-based theoretical poten-tials. No clear relationship is found between the theoreticaland the measured methane yield. Measured potentials areon the same level as component-based theoretical potentialwith some irregularities.

3.6. Relationship between methane yield, methane potential

and chemical composition

Generally, the measured yields and potentials of thewastes show no relationship with physical or chemicalcomposition. The lack of relationship with chemical com-position is also verified by comparing the theoretical valuesto the measured values, since the theoretical values for eachwaste type are calculated based on chemical composition.This similarity is not surprising since the chemical compo-sition is very much alike for the main part of the samples.

The fact that the chemical composition does not varysignificantly for the tested waste types shows that the ori-gin, sorting systems and collection systems have little influ-ence on the chemical composition as found in Hansen et al.(accepted-a).

The results indicate that the choice of system for sorting,collection and pre-treatment of SS-OFMSW is not veryimportant for the methane yield on VS-basis (on average336 Nm3 CH4/ton). This conclusion applies foremost towaste pre-treated by disc screen or screw press, since the

0

100

200

300

400

500

600

700

800

SF

paP_

1_

SF

2_paP_ pA

3_paP_

_alP_pA

4

A

_alP_p

5

FS_

_alP6

FSP_

_al7

FS

_alP_

8

S_FP

9_pa

FSP_

1_al0

FS

alP_

1_1

_alP_pA

12 1_alP_pA

3

FS_

paP

41_

AN_p

_o51

AN_p

61_o_pA

1_paP

7

mN

3H

C 4

not repV

Sni

Theor. meth. potentialelement based

Theor. meth. potentialcomponent based

Measuredmethanepotential

Measuredmethaneyield

Piston press

Shredder+ magnet

Foodwastedis-posers

Screw pressDisc screen

Fig. 6. Theoretical and measured values on methane potential and yield.

414 A. Davidsson et al. / Waste Management 27 (2007) 406–414

most extensive studies were carried out using these meth-ods. However, these values could be hard to achieve in real-ity for some of the waste types, especially the ones wrappedin plastics and pre-treated by disc screen, which causedsevere operational problems with plastic materials in thepilot-scale digestion. Similar experience with operationalproblems for this waste is known from a full-scale plant.

The measured methane yields seem to agree well withprevious research. However, comparisons are difficult tocarry out since it was hard to find previous research whereboth the type of OFMSW and the type of process werethe same as those used in this study. Most research foundis based on mechanically sorted OFMSW, which has alower VS-content.

4. Conclusions

The main conclusions from the study on methane yieldsin source-sorted organic fraction of municipal solid wastefrom households are:

� Anaerobic digestion of exclusively source-sorted organicfraction of organic waste from households without sig-nificant contribution from garden waste is possible.� A methane yield of 300–400 Nm3 CH4/ton VSin can be

expected in thermophilic wet digestion with a 15-dayretention time, corresponding to around 70% of thepotential measured during 50-days of batch digestionin lab-scale.� A VS-degradation of around 80% can be achieved in

thermophilic digestion with a 15 day retention time.� The present study showed that different parameters

identifying each waste type (origin, sorting system, col-lection system and pre-treatment system) gave smallvariations in both methane yields per VS, methanepotential and chemical composition.

Acknowledgements

The research project was partly financed by the DanishEnvironmental Agency and partly by Malmo City. Bothare greatly acknowledged.

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