Waste Management & Research (1996) 14, 163–170

FULL-SCALE MESOPHILIC ANAEROBIC CO-DIGESTION OFMUNICIPAL SOLID WASTE AND SEWAGE SLUDGE:

METHANE PRODUCTION CHARACTERISTICS

Jukka A. Rintala1 and Kimmo T. Jarvinen2

1Institute of Water and Environmental Engineering, Tampere University of Technology,P.O. Box 527, FIN-33101 Tampere, Finland, and 2Nordic Envicon Oy,

Kanslerinkatu 8, FIN-33720 Tampere, Finland

(Received 26 November 1993, accepted 4 February 1995)

The methane production characteristics of a full scale anaerobic biogas digesterprocessing the putrescible fraction of municipal solid waste (PFMSW) and sewagesludge (SS) were studied by using the operation data of the digester and laboratoryexperiments. The batch assays indicated methane yields of 300 l kg volatile solids(VS)−1 for PFMSW and 220 l kg VS−1 for SS. The full scale digester showed methaneproduction yields up to 90% of the methane yield potential of the added feed. Therates of methane production and the specific methanogenic activities varied duringthe cycle of feeding. Apparently, the methanization in the digestion process wassubstrate-limited, especially during weekends when no feed was added. The batchstudies indicated that the propionate and butyrate conversions were no more limitingthan acetate methanization. The sludge from the digester was more active than thesludges from digesters processing sewage sludge alone. 1996 ISWA

Key Words—Anaerobic digestion, full scale, methane production, municipal solidwaste, sewage sludge, biodegradable, putrescible waste.

1. Introduction

The anaerobic digestion of the putrescible fraction of municipal solid wastes (PFMSW)has been intensively studied and both dry [total solids (TS) content 30–40%] and wet (TSaround 10%) anaerobic process modifications have been developed and demonstrated tobe feasible to treat PFMSW (e.g. Cecchi et al. 1992). Though the number of full-scaleapplications is still limited, anaerobic digestion is likely to be increasingly used inmunicipal solid waste (MSW) management (Cecchi et al. 1992; Fouhy 1993).

In the anaerobic process, almost all the energy content of the sorted waste is convertedto methane, a valuable source of energy to replace fossil fuels in heat and electricitygeneration. Methane production also reflects the performance of the anaerobic digestionprocess and is frequently used to control and to characterize anaerobic industrialwastewater systems. However, little research has been done on the methane productioncharacteristics of anaerobic PFMSW digesters; customarily only daily methane pro-duction rates and methane yields have been reported.

This study sought to evaluate the methanogenic characteristics of a mesophilicanaerobic digester processing PFMSW. The process under study was the Stormossenbiogas digester in Finland (Westerqard & Akers 1992) which, when operational in 1990,was one of the first full-scale PFMSW processing anaerobic digesters in the world. The

0734–242X/96/020163+08 $18.00/0 1996 ISWA

J. A. Rintala & K. T. Jarvinen164

study consisted of measurements and sampling at the plant as well as of laboratoryexperiments.

2. Materials and methods

2.1 The biogas process

The volume of the mesophilic (37°C) digester at Stormossen is 1500 m3 with a liquidvolume of about 1400 m3. The contents of the digester are stirred by means of gasrecirculation. The feed is pumped from the feed tank into the digester each workingday (five days a week) during a 2–5 h period. An equivalent amount of the digestercontents is removed prior to adding waste.

2.2 The waste and feed characteristics

At the time of this study, MSW was received from an area where paper and glass werecollected separately. In the plant, the MSW was mechanically sorted into three fractions:PFMSW, a refuse-derived fuel fraction, and metals (Westerqard & Akers 1992). Thedigester feed consisted of PFMSW, mixed with thickened sewage sludge (SS) containingprimary and waste activated sludge from the local municipal wastewater treatmentplant, and with supernatant (S) from the dewatering of the digested material. The feedwas prepared in the feed tank with a hydraulic retention time (HRT) of approximately3 days during weekends and 1 day during the rest of the week.

According to long-term analyses, the average TS content was 45% and 14% and theVS content 34% and 8% for the PFMSW and SS, respectively (Teir pers. comm. 1992).During the study period, the feed was 38–64 m3 d−1, containing approximately equalvolumes of PFMSW (density 600 kgm−3), SS (density 1100 kgm−3), and S (density1000 kgm−3) and resulting in approximate loading rates of 2.5–4.1 kgVSm−3 d−1 (perday of addition). The digester was considered to be in normal operational mode duringthe study period.

2.3 The origin of the data and the sampling at the plant

The plant personnel provided data on routine gas production and methane content ofthe full scale digester (Akers pers. comm. 1992). The methane content of the gas wasconsidered constant at 65% with a variation of less than 2%. The data presented isfrom one cycle of a normal feeding routine, viz. from Monday 0:00 h to Sunday24:00 h (5–11 October 1992).

The digester contents were sampled between 5–8 October 1992, and the samples weretaken from the bottom, middle and top of the digester, once or twice daily, before,during, and after a new load of feed. The digester sludge samples (from here on referredto as DS) were refrigerated until used.

Reference sludge samples were obtained from two mesophilic municipal digesters(MDI and MDII) processing sewage sludge. MDI was the Viinikanlahti treatment plantand MDII the Rahola treatment plant, both in Tampere, Finland.

2.4 The batch assays

The batch assays were performed using 120 ml vials with 5 ml of DS as substrate or asinoculum. To determine the methane yields from the digester feed fractions, 0.5 g of

Co-digestion of MSW and sewage sludge 165

PFMSW or 4.0 g of SS were added per vial. Distilled water was added up to totalliquid volumes of 45 ml. In the specific methanogenic activity (SMA) assays, 40 ml ofmedia containing either acetate [2.8 g chemical oxygen demand (COD)l−1] or a mixtureof volatile fatty acids (VFA) (2.9 gCODl−1; acetate:propionate:butyrate 1:1:1 by weight)were added to the vials with the sludge. The sludge samples were distributed into thevials under O2-free N2/CO2 (4:1), and the head-space was flushed with the same gas.The vials were then sealed with butyl rubber stoppers and aluminum crimps. Na2Sx9H2O(1.0% vv−1) was added from an anaerobic stock solution to give a final concentrationof 0.25 gl−1. The bottles were statically incubated at 37°C. The experiments wereperformed with 2–4 replicates.

2.5 Calculations

In the batch assays, the methane yields per gram of added substrate VS were calculatedas cumulative methane production for a period of 20–30 days until methane productionhad stopped. The SMA was calculated from the slope of the cumulative methaneproduction curve, divided by the amount of VS in the inoculum in the vial. The meanswere considered significantly different in paired comparison when P was <0.01, asdetermined by Student’s t-test.

The data from the biogas digester was analysed by calculating the maximumvolumetric methane production rate (VMR) and the maximum SMA from the maximumslope of the cumulative methane production curve. The mean values were calculatedfrom the cumulative methane production between two successive feedings. The VSvalues of the DS samples served to estimate the biomass in the digester.

2.6 Analyses

Volatile acids (VA) were analysed by the method described by DiLallo & Albertson(1961). COD, TS, VS and alkalinity were determined according to Standard Methods(APHA 1985). COD, VA and alkalinity were determined for samples obtained throughfiltration with GF/A filters. In the batch assays, the methane content of the gas sampleswas determined by a Perkin Elmer Sigma 300 gas chromatograph. The pH was measuredimmediately upon sampling in order to avoid pH changes due to CO2 loss from theliquid.

3. Results and discussion

3.1 The characteristics of the digester sludge

The digester contents were sampled in order to determine the characteristics of thedigester sludge along the digester height and during the feeding cycle. The COD values(Fig. 1) and the other parameters showed no trends along the digester height or in thesampling period (Fig. 1), which suggested that the contents of the digester were wellmixed. The results of all the samples are summarized in Table 1.

3.2 Methane yields

The methane yields in the batch assays were 310±5 and 220±0 l kgVS−1 for thePFMSW and for the SS, respectively. Accordingly, 1 tonne of PFMSW (34% VS) and

J. A. Rintala & K. T. Jarvinen166

5

05 October

Date

CO

D (

gO2

l–1)

4

3

2

1

6 October 7 October 8 October

Fig. 1. Soluble COD at various heights of the full scale digester. Ε, top; Β, middle; Μ, bottom.

TABLE 1Characteristics of the full scale digester sludge samples.Samples are from the bottom, middle and top of the digester.

The sampling times are indicated in Fig. 2

Mean±..∗

pH 7.6±0.1Total solids (%) 5.33±0.33Volatile solids (%) 2.91±0.23Volatile solids/total solids 0.55±0.02CODSoluble (mgO2l−1) 3400±450Alkalinity (mgl−1) 7600±450Volatile acids (mgl−1) 285±200

∗ n=21, except for COD when n=20..., standard deviation.

1 tonne of SS (8% VS) would produce 105±1.7 m3 and 17.6±0 m3 of methane,respectively. The methane yields reported in the literature vary depending on, forexample, the actual composition of the waste and specific test conditions: methaneyields from 200–400 l kgVS−1 have been obtained for PFMSW (reviewed by Cecchi etal. 1988).

Approximately 45 tonnes of PFMSW and 77 tonnes of SS were fed into the digesterbetween 5–8 October, and approximately 5500 m3 of methane was produced within 24 h

Co-digestion of MSW and sewage sludge 167

11 October

200

05 October

Date

Gas

pro

duct

ion

(m

3 h–1

)

100

50

7 October 9 October

150

38

52

64

52

47

Fig. 2. Rate of gas production in the full scale digester. No data was available from 9 October 18:00 h to10 October 15:00 h. The approximate methane content of the gas was 65%. The arrows indicate the times

and amounts of new feed (in tonnes).

TABLE 2Methane yields from the digester sludge samples in

the batch assays

Sample Methane yield(CH4 kgVS−1)

6 October: Top 20.0±∗6 October: Middle 21.4±1.16 October: Bottom 25.1±0.98 October: Top 32.3±∗8 October: Middle 34.1±3.68 October: Bottom 41.1±1.0

∗No replicates.

of these feedings (Fig. 2). According to the methane yields obtained in the presentedbatch assays, the waste should have produced approximately 6100 m3 of methane. Theestimations indicated that the full scale digester performed up to 90% of its methaneyield potential.

The digester sludge samples of 6 October and 8 October were assayed to determinetheir methane yields. The yields differed on those two days (Table 2), suggesting anaccumulation of biodegradable organics in the digester during the cycle of feeding.Most likely the increase in the concentration of the organics was not significant enough

J. A. Rintala & K. T. Jarvinen168

24

3000

0

Time (h)

Cu

mu

lati

ve g

as p

rodu

ctio

n (

m3 )

1000

2500

2000

1500

500

4 8 12 16 20

Fig. 3. Cumulative methane production in the full scale digester between two consequent feedings.Μ,5 October; Β, 6 October; Ο, 7 October; Φ, 8 October.

to be detected by analysis of the VS. The methane yields were the highest for thesamples from the bottom of the digester, which could mean minor differences in sludgecharacteristics along the digester height (Table 2).

The methane yields of the DS samples (Table 2) were less than 10–15% comparedto the mean yield of the PFMSW and the SS found in the batch assays. Consequently,up to 90% of the added biodegradable feed had been converted to methane, as estimatedabove.

3.3 The rate of methane production

The rate of methane (gas) production (m3 h−1) was highest immediately upon feedingand decreased thereafter (Fig. 2), as found also in a pilot digester fed twice a day(Cecchi et al. 1991). During the feeding cycle, the methane production rates (Fig. 2)and the cumulative methane production yields (Fig. 3) increased as did the maximumand the mean VMR and SMA (Table 3). The increases were apparently caused bygrowing substrate levels from more frequent feeding during the week or from increasedloads (Fig. 2). However, the results suggested that the anaerobic process was substrate-limited, especially during the unfed period over the weekend. Semi-continuous feedinghas been proposed in place of a ‘‘once a day’’ feeding pattern to utilize anaerobic sludgedigesters’ capabilities for higher loading rates (Puhakka et al. 1992). This feeding schemeis a possibility worth examining in the full scale digester studied.

Mean daily volumetric methane production rates up to 3.0 m3 m−3 d−1 have been

Co-digestion of MSW and sewage sludge 169

TABLE 3The maximum (maximum slope in Fig. 3) and the mean volumetricmethane production rate (VMR) and the specific methanogenic

activity (SMA) in the full scale digester

VMR (m3 m−3 d−1) SMA (CH4 kgVS−1d−1)

Day Maximum Mean Maximum Mean

1 0.80 0.40 29.7 15.22 0.93 0.81 34.6 30.53 1.45 1.16 53.1 42.14 1.72 1.26 64.2 46.9

VS, volatile solids.

TABLE 4The specific methanogenic activities (SMA) (mean±..) ofthe digester sludge (DS) samples and the reference digester

(MDI and MDII) samples

SMA∗ (CH4 kgVS−1d−1) on

Sample VFA Acetate

6 October: Top 33.1±1.7 34.7±1.06 October: Middle 39.0±1.0 43.9±2.36 October: Bottom 38.3±1.9 42.6±0.88 October: Top 46.4±3.4 45.7±2.88 October: Middle 42.3±1.4 43.9±1.08 October: Bottom 42.1±1.9 46.7±4.9MDI na 23.4±0.1MDII na 23.5±4.3

∗Methane production by the blanks were subtracted; na, notanalysed; VS, volatile solids; VFA, volatile fatty acids.

reported on mesophilic anaerobic digesters processing PFMSW as reviewed by Cecchiet al. (1988).

3.4 SMAs on acetate and VFA

Several DS samples were examined to determine the SMA on acetate and VFA. Nostatistically significant differences could be found in the SMAs between acetate andVFA (Table 4), indicating that the propionate and butyrate conversion did not limitthe anaerobic degradation process.

The unvaried SMA along the digester height (Table 4) showed that methanogenicactivity was uniformly distributed in the digester.

The DS samples had a higher SMA than the MDI and MDII sludges (Table 4), aresult suggesting that with a mixture of PFMSW and SS, more active sludge coulddevelop than with SS alone. Also the different process conditions in the studied fullscale digester and reference digesters (not shown) could be the reason for the developmentof sludges with different methanogenic activities.

J. A. Rintala & K. T. Jarvinen170

4. Conclusions

The waste fractions studied were readily digestible: the methane yield was 310 l kgVS−1

for the putrescible fraction of municipal solid waste and 220 l kgVS−1 for the sewagesludge, respectively. The studied full scale digester methanized up to 90% of the feed,which consisted of about 50% (by volume) of both the putrescible fraction of municipalsolid waste and sewage sludge. The methane production rates of the digester variedbecause of the feeding pattern (once a day on weekdays and no feed on Saturdays andSundays) and the varying amounts of feed. The possibility of raising the loading rateby more continuous feeding deserves further research.

Acknowledgments

This study was funded by the Nordic Ministerial Council (J.A.R.), Avecon ConsultingEngineering Ltd (K.T.J.), and Nordic Envicon Oy (K.T.J.).

References

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Cecchi, F., Traverso, P. G., Mata-Alvarez, J., Clancy, J. & Zaror, C. (1988) State of the art ofR & D in the anaerobic digestion process of municipal solid waste in Europe. Biomass 16,257–284.

Cecchi, F., Mata-Alvarez, J., Marcomini, A. & Pavan, P. (1991) First order and step-diffusionalkinetic models in simulating the mesophilic anaerobic digestion of complex substrates.Bioresource Technology 36, 261–269.

Cecchi, F., Mata-Alvarez, J. & Pohland, F. G. (eds) (1992) Anaerobic digestion of solid waste.Proceedings of the International Symposium on Anaerobic Digestion of Solid Waste, Venice,Italy, 14–17 April.

DiLallo, R. & Albertson, O. E. (1961) Volatile acids by direct titration. Journal Water PollutionControl Federation 33, 356–365.

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