Anaerobic digestion of organic fraction of municipal solid wastes — digester performance

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  • The Science of the Total Environment, 56 (1986) 183--197 183 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

    ANAEROBIC DIGESTION OF ORGANIC FRACTION OF MUNICIPAL SOLID WASTES -

    PERFORMANCE

    F. CECCHI, P.G. TRAVERSO and P. CESCON

    Dipartimento di Scienze Ambientali - Dorso Duro 2137 - Venezia - Italy

    DIGESTER

    ABSTRACT

    The anaerobic digestion of source separated organic fraction of municipal solid wastes was studied by using a stirred 3 cubic meter working volume reactor fed on a semicontinuous basis. The hehaviour and performance of the digester at various organic loading rates was illustrated with particular regard to turnover of the gas production rate during a cycle in the digester feed.

    INTRODUCTION

    In recent years solid waste management and disposal has become one of our most

    important environmental concerns. Unfortunately the ultimate disposal techniques

    are only a few and our solid wastes are almost always directly disposed of on

    the land, thus causing environmental damage (i). The recent upward trend in

    energy costs has also created renewed interest in previously uneconomical energy

    production techniques; many recent studies, in particular, show that the energy

    production from municipal solid waste (MSW) is an important applied research

    topic (2-4).

    Conversion of the organic fraction of municipal solid wastes (OFMSW) to methane

    provides at least a partial solution to energy saving and to the enviro~ental

    impact of municipal solid waste disposal as the nature of final residues is such

    that they can be disposed of on agricultiral land (5).

    Some previous reports by these authors have shown the digestion of source

    separated OFMSW can be performed without mixing with primary sewage sludge, and

    have proved that the process performance is about double when the feed is 100%

    OFMSW (6-8). The aim of this paper is to show the performance of a mixed phase

    anaerobic digester (no cell recycle) in mesophilic conditions, fed by source

    separated OFMSW.

    MATERIAL AND METHODS

    Apparatus

    The flow-sheet of the pilot plant is shown in Fig. I. The experimental runs were

    conducted in a 3 cubic meter working volume completely mixed reactor maintained

    0048-9697/86/$03.50 1986 Elsevier Science Publishers B.V.

  • 184

    at the optimal mesophilic temperature range: 352C (9). The digester was fed

    (2-6 times a day) by a volumetric pump. The mixing device was an armed anchor

    stirrer rotating at 70 r.p.m.. The pressure of the gas in the digester top

    (150-180 mm w.c.) was controlled by a hydraulic valve. The gas production was

    carefully measured by a wet gas-meter. Further details on the pilot plant are

    in (6,8).

    Representative] ~ .~ area I Trgvis 0 City I

    "1 ,,

    Moisture I Adiusternent I

    Feedstock ~ Ana lyses

    D igester i Gas Meter ~.~ ~ Effluent Stock ~-~ ' ~-j_ _~Outlet Sludge

    Fig. I. Flow sheet of the pilot plant.

    Biomass feed

    The source separated OFMSW used in this study was collected daily from a

    representative area in Treviso city. The characteristics of the MSW collected

    were found to be consistent with those in N.W. Italy (I0).

    The biomass was pretreated by shredding and diluted before being stored in the

    feedstock tank. The mean chemical characteristics of the feed are in Tab. i.

    TABLE 1 Mean chemical characteristics of the biomass feed.

    Parameters Mean Value St. Deviation Sample Numbers

    Moisture, (%) 93.6 0.7 41 STS, (%TS) 32.8 4.1 29 TVS, (%TS) 89.9 1.9 41 SVS, (%TVS) 28.6 2.8 24 TCOD/TVS 1.6 0.07 28 SCOD, (%TCOD) 30.0 3.4 27 TC, (%TS) 48.0 2.3 27 SC, (%TC) 30.1 5.6 27 N, (%TS) 3.2 0.7 25 P, (%TS) 0.4 0.I 21

    The scattering of the characteristics of the feed is typical of the MSW.

    Further details of the collecting procedure and processing methods to shred

    OFMSW are in a previous report (6,7).

    the

  • 185

    Experimental operations and analyses

    The experimental runs (carried out during the period Feb-Aug "85) were planned

    to test biogas and methane production, biodegradation efficency and system

    stability for a large range of hydraulic retention times (HRT).

    The data referred to in this paper are from simulated steady state conditions

    (defined as digester operations for a period of one or more HRTSs during which

    time the variations of gas yield did not exceed 5% (ii)).

    The digester reached the same production of gas as steady state gas production

    in one or two days, but one HRT, at least, was allowed to pass before

    considering the collected experimental data (11).

    The ~eed, the reactor content and the effluent material were analysed to

    determine the total solid (TS) and the total volatile solid (TVS)

    concentrations, the total chemical oxygen demand (TCOD), the total carbon (TC)

    and the related soluble fractions: STS, SVS, SCOD, SC (filtered through 0.45 ~m

    membrane filters) for the material balance. An example to illustrate the

    analytical plan of the parameters used for the mass balance (tested parameters

    and testing frequency) during one period after the acclimatisation time is in

    figure 2.

    To monitor the system stability, the total alkalinity (TA), the pH in the

    reactor and the percentage of CO 2 in the gas were analysed two or three times a

    week and volatile fatty acids (VFA) at least one or two times during the steady

    state conditions.

    The ultimate analysis (TC, N, P) was carried out on the feed material, once a

    week, to control the presence of the main nutrients. The N-NH 4 concentration

    analysis in the reactor material was justified by the necessity to test if a

    toxic level had been reached.

    The gas production rates were expressed at ambient temperature and atmospheric

    pressure (20C and 760 mm Hg).

    Furthermore careful observations of pH, of VFA, of soluble substrate and TA

    concentration patterns in the reactor, of biogas production and of CO 2

    percentage in the gas were carried out in the time between one feed and the

    subsequent one.

    The analyses were carried out according to the procedure suggested in (12,13).

    RESULTS AND DISCUSSION

    Reactor performance

    The high concentration value of soluble substrate in the feed (see Table I) and

    its high biodegradation rate (shown by some preliminary measurements) indicated

    that in order to obtain more precise information about their digestibility it

    would be advisable to study both the total substrate and the soluble substrate.

    It is believed that this iformation will provide more detailed knowledge of the

  • 186

    CH 4 65 or) (%) 55

    (,~ Biogas 200 ., , I I -h - l l 100

    ~--TC 1.5 (g'1-11 0.5

    S-COD 2.0 (~ r 1) 1.0

    SWS 2.0 (g-1-1 } 1.0

    S-TS 4.0 i~. (g'1-1 ) 2.0

    O T--COD 30 (g.1-1) 20

    VS 0.75 o~5

    UJ~ T-VS 17 (g.1-1) 15

    S-TC 0.35 T C 0.25

    TC 0.50 TS 0.40

    S-COD 0.35 T-C(~ 025

    ("t S-VS 0.35 LU T-VS 0.25 LU LL S-TS 0.40

    TS O.30

    T-COD 80 (g-t -1) 70

    T.-VS 0.90 TS 0.80 i

    T-VS 55 (g-r 1) 45

    t

    .:

    v

    18 22 26 2 6 10 14 18 22 26 30 Feb. I March 1985

    Fig. 2. Example of tested parameters and testing frequency in a steady state

    period (OLR = 2.1KgTVS/(m3/day)). (e = effluent;u = reactor).

  • 187

    process, thus permitting a more precise identification of the optimal control

    parameters, and will also give indications about developments in anaerobic

    technology.

    The mean values of the parameters analysed in the 4 RRT conditions which were

    studied (in pseudo steady-state conditions) are shown in Table 2, together with

    the biogas production (m3/day) and its quality.

    TABLE 2 Summary of Steady-State data at various organic loading rates (OLR).

    Runs Parameters 1 2 3 4

    HRT (d) 25.0 17.8 13.6 8.9

    Mean feed TVS 52.5 57.0 57.1 61.4 material TCOD 77.5 83.7 83.0 89.9 characteristics TC 27.5 30.3 29.9 32.9 (g 1-1 ) SVS 15.0 16.0 16.3 17.8

    SCOD 22.5 24.9 24.5 26.7 SC 7.5 8.9 9.5 9.8

    Mean Digester TVS 16.3 18.2 18.8 22.7 and effluent TCOD 24.0 27.6 28.2 33.3 material TC(a) Ii.3 12.6 12.0 14.8 characteristics SVS 1.2 1.6 1.5 2.1 (g I -I) SCOD 0.9 1.2 1.2 (b)

    SC 0.8 0.8 0.9 2.2

    Gas Production Biogas 3.99 6.00 8.01 10.86 (m 3 d -1) Methane 2.52 3.69 5.40 6.09

    Methane (%) 63.0 61.5 62.5 56.0

    (a) Calculated from the mass balance (b) Lack of analyses

    The data on the effluent is also representative of conditions in the reactor.

    From numerous analyses carried out along the axis of the reactor and in the

    effluent (HRT constant), it was possible to establish that the TSV fluctuations

    were so small as to enter an acceptable range of experimental error.

    For instance in the test at HRT = 25 days with 4 series of analyses of samples

    taken at different points along the axis (see ref. 5) the TVS values were: 17.8

    (G=O.5); 16.8 (G=O.8); 16.3 (G=O.9).

    The same low level of fluctuation was found in the measurement of TS and the

    granular distribution of suspended solids.

  • 188

    From analysis of outlet sludge at the bottom of the reactor, it was observed

    that the feed contains a small quantity (. 4 t~ "0

    (,,).

    ~E 3

    c o g. 2 u

    "0 o L

    a . 1

    0

    ~~ ~, , , ,~~~athane 0 , [ , I , I , I i I , I ,

    2 3 4 5 6 7

    Organic Loading Rate, OLR (Kg T-VS.m-:~day -1)

    Fig. 3. Gas production rate vs. OLR.

    Fig 3 also showns that the energy recovery is constant up to an OLR of 4.2

    KgTVS/(m3d). In fact it doubles as the OLR is doubled. Interpolation of the

    experimental data indicates however that with loads over 5 KgTVS/(m3d), the

    production of methane remains constant and the energy recovery decreases. With

    large loads the system in the reactor changes and the percentage of CO 2 rises,

  • 189

    so the increased production of biogas has no influence on the production of

    energy.

    The data obtained partially substantiates the research carried out by Diaz (14)

    who observed a maximum load (for refuse digesters) of 4.8 g TVS/(m3d) wihout

    loss in efficiency.

    One notable difference was observed in loads over 5 RgTVS/(m3d). Diaz observed

    that the system broke down under a load of 6.4 KgTVS/(m3d); however in our case

    the gas production continued to increase up to a load of 6.9 KgTVS/(m3d),

    without any sign of breaking down. This leads us to believe that the feed with

    source separated OFMSW not only permits maximum energy production up to a load

    of 5 KgTVS/(m3d) but also a safety margin of about 40%. This permits variations

    in the organic loading which are typical of large-scale plants.

    It is important to note that with loads of > 5 KgTVS/(m3d) the TVS reduction is

    considerably diminished and the sludge is poorer in quality. This is quantified

    in Table 3 where the efficiency of the process with reference to soluble and

    insoluble materials is shown.

    It is possible to make a comparison with other research work on both similar and

    different organic wastes referring to the parameter biogas yield which is the

    most commonly found. From this comparison (see Tab. 4) it is possible to deduce

    that source separated OFMSW are particularly suitable for the process of

    anaerobic digestion: the results we obtained showed our biogas yield from a

    mesophilic process to be comparable with yields obtained by other researchers

    from a thermophilic process.

    TABLE 3 Efficiency of matter.

    the digester in the removal of various fractions of organic

    ORGANIC REMOVAL (%) run 1 2 3 4

    TVS 69 68 67 63 TCOD 69 67 66 63 TC (a) 59 59 60 62 SVS 92 90 91 88 SCOD 96 95 95 (b) SC 90 91 90 78 IVS 60 58 57 53 ICOD 59 55 53 (b) IC 55 56 53 55

    (a) Calculated from the mass balance (b) Lack of analyses.

  • 190

    TABLE 4 Comparison betwen the data of the present paper and that in other literature.

    Wastes HRT T OLR Gas Y. CH4 TVSR Ref. d C (a) (b) % %

    Source Separated OFMSW 14 35 4.2 0.64 63 63 (c) MSW/Sewage Sludge (9:1) 30 37 - 0.47 - - (15) MSW/Sewage Sludge (9:1) 30 65 - 0.69 - - (15) MSW 14 35 4.0 0.43(d) - 69 (16) Hyacinth/Grass/ MSW/Sludge Blend 12 35 1.6 0.34 63 34 (11) MSW/Sewage Sludge (8:2) 15 35 4.8 0.39 65 78 (14) MSW 20 37 1.3 0.59 60 - (17) MSW/Sludge Blend 20 37 3.2 0.32 60 - (17) MSW I0 60 - 0.46 56 - (18) Synthetic MSW 30 35 1.2 0.39 58 67 (19)

    "" 20 35 1.8 0.32 56 67 (19) "" 15 35 2.5 0.33 56 53 (19) "" 30 35 2.5 0.39 56 58 (19)

    Beff Cattle Manure 8 55 6.3 0.67 55 53 (20) Organic Wastes: (87.5% TVS) 0.62 - - (21) (83.0% TVS) 0.64 - - (21)

    (a) = Kg TVS/(m 3 d); (b) = m 3 biogas/Kg TVS added; (c) = this paper; (d) = refers to methane; TVSR = TVS removal.

    Gas Production

    Analysis of data in Tab.3 shows that the soluble fraction of the feed is almost

    completely digested.

    This leads us to believe that the substrate utilisation rate may vary in the

    period between feeds. This hypothesis is consistent with the information in

    Figs. 4 and 5.

    In Fig. 4 we observe that the experimental data from organic loading rates of

    between 2.1 and 4.2 is grouped into three sections.

    The first section from t=0 to t = I h is characterised by large variation in the

    gas production rate. In the second section from t=1 to t=6-7 h the gas

    production rate gradually drops from its maximum variation rate to its minimum.

    From Pig. 4 and Fig. 5, which indicates the variations in soluble substrate

    concentrations inside the reactor over a period of time after the feed, we

    deduce that a rapidly digested part of the soluble fraction of the feed (50%

    SVS) is used in the first hour. In the following 2 or 3 hours the soluble

    fraction is almost completely consumed, and the contribution of the more easily

  • 191

    hydrolized fraction cannot be ignored. This contribution is important up to the

    sixth or seventh hour, and then the process is governed by the solubilisation of

    the fractions which decompose more slowly.

    ,0% 0 L R; o= 21 KgT-VS m-3 day -1

    iP - m.e 60 e= 3 .2 4=4.2

    ~'500 o= 69

    4.1

    ~ 4oo "0

    o. 300 - A w w m200 - .2 mlO0 o o o o

    I I I I I I I I I I I , 0 1 2 3 4 5 6 7 8 9 10 11 12

    Time after feeding, (h)

    Fig. 4. Biogas production rate during every cycle in digester fed 2-6 times

    daily

    140~

    120C

    ,- 1000

    800 0

    5

    ~ S COD

    I I I I t 0 1 2 3 4

    Time after feeding, (h)

    Fig. 5. Evolution of soluble substrate concentration vs. time after the feed.

  • 192

    Bearing in mind that cellulose and hemicellulose are among the principal

    components of the wastes which we considered (ii) after the first hour this is

    compatible with the indicated pathway observed by Ghosh (11) under mesophilic

    conditions.

    It is difficult to interpret the behaviour of the reactor in the first hour

    after the feed, because the gas production is independent of the VFA

    concentration in the reactor (see Fig. 6).

    _ " F ,,ooF/.\ i "FI ~ 1600 I - -

    12oo I- :>

    OLR:6,9 KgTVS-m-3d -1

    " 1100

    i lOOO 900 80(] coo

    u. 500

    400 o > 300

    OLR: 4,2

    i i I I I I I I 0 1 2 3 4 5 6 7

    Time after feeding , (h)

    Fig. 6. Pattern of VFA concentration in the reactor after feeding.

    Similar behaviour was observed by Mackie and Bryant (22). They ascribed the

    large initial burst of fermentation after feeding to the presence in the fresh

    substrate of amounts of compounds (acetate, lactate, bacterial protein) which

    greatly increase the gas production from acetate which would not be found in

    continuous feeding.

  • 193

    The reactor's behaviour is quite different with a load of 6.9 KgTVS/(m3d).

    Compared to other loads the percentage of CO 2 is much higher (mean value 44%)

    and the pH value is 3 or 4 tenths lower. The gas production in the first hour is

    the same as that observed with other loads. After that, however, the gas

    production rate remains constant until the next feed.

    HEMICELLULOSE- CELLULOSE- LIGNIN

    MANNANS GALACTANS ARABANS XYLANS GLUCANS4

    1 MANNOSE GALACTOSE ARABINOSE XYLOSE GLUCOSE

    ?

    ~ CO 2, H 2 Pat h A, t C3--C 6 FATTY ACID

    ~ ~ - - ~ A C ETAT E

    H 2 C02 CH 4

    Fig. 7. Reaction steps in anaerobic digestion of complex polysaccharides (11).

    700

    6OO

    500

    400

    300

    oo 20~ l

    0 6 I I I I I 1 2 3 4 5

    Time Af ter Feedlng,(h)

    Fig. 8. Influence of temperature on gas production.

    This can be explained by the accumulation of VFA long chain (C3,C6) in the

  • 194

    digester due to an increased frequency in feeding (see Fig.6), so that the gas

    production remains constant but it decreases in quality. This is consistent with

    Ghosh's scheme showing the utilisation of complex substrates (ii) (see Fig. 7

    path A).

    The scattering of experimental points in Fig.4 can be justified both by

    characteristic variations in the substrate and by the wide range of the

    operating temperature. The influence of temperature on gas production is

    illustrated in Fig. 8, which shows the results at the two extremes of the range

    of temperature (33C and 37C) with other parameters constant.

    720-

    7.16

    7.12

    =z 7.08 7.04

    "F

    600 e SO0 > 400

    - 42!

    - I I I I I I I I

    v

    I I I I I

    I l

    :.

    I I I I I

    7 8 9

    concentrations, pH values vs. time

    m

    38- I I I i I I I 0 1 2 3 4 5 6

    T ime a f te r feed ing , (h)

    Fig. 9. Evolution of CO 2 percentage, VFA

    after feeding. (OLR = 3.2 Kg TVS/(m 3 d)).

    Stability of the system.

    Table 5 contains the various stability indices of the reactor (pH, TA, VFA)

    (23,24). These refer to conditions after the transition phase (about the first 2

    hours after feeding) and were taken during the pseudo-steady state periods.

    Furthermore Fig. 9 shows an example of the turnover of the stability parameters

    during a cycle (the TA value, which was always very high, did not vary during

    the cycle). Similar behaviour was observed when referring to the other organic

    loads.

  • TABLE 5

    Mean values, after the burst

    concentrations in the digester.

    195

    of the fermentation, of the pH and VFA, TA

    Parameters Run 1 2 3 5

    VFA, (mg CH3COOH/I) (a) 380 800 1650

    TA, (mg CaCO3/l) 3615 3780 4150 3540

    pH 7.23 7.29 7.25 6.98

    (a) Lack of analyses.

    The plots in Fig. 9 show that the system returns to stability a short time after

    feeding. This is evidently due to high TA values (see Tab. 5) which prevent

    large variations in the pH value. It must be underlined that the buffer capacity

    of the system is able to cope the higher organic load of 6.9 even when the

    concentration of VFA reaches levels higher than those in the transition zone of

    digestion success described by Kroeker (24).

    CONCLUSIONS

    - It is confirmed that anaerobic digestion of source separated OFMSW is feasible

    without external modifications (PH, nutrients) even for large organic loads (at

    least 6.9 KgTVS /(m 3 d)) which are greater than those normally processed in

    conventional digesters.

    - It was observed that up to IIRT = 14 days and OLR ffi 4.2 (which can be

    extrapolated up to OLR ffi 5 KgTVS/(m 3 d)) the methane recovery remains constant

    while with higher loads (6.9) the recovery and quality of the gas decreases.

    - It was deduced that the best operating conditions for digesting source

    separated OFMSW are: HRT ffi 14 d; OLR ffi 5 Kg TVS/m 3 d. These conditions warrant a

    safety margin of 40% OLR.

    - In the latter operating conditions the capacity of a full scale plant could be

    the following:

    Energy recovery: 1.8 m3CII4/(m3Reactor d) (the obtainable data of energy

    consumption from a pilot plant doesn't allow the extention to a full scale

    plant).

    Population served: 60-100 inhabitants/(m3digester) (according to 730 g MSW

    produced/(inhabitant, d) 50% OFMSW with 15-25% dry matter and 90% TVS).

  • 196

    - The efficiency of the substrate removal (soluble and insoluble) is nearly

    constant up to 4 .20LR and then decreases. The most notable drop is that of the

    soluble carbon fraction of the feed (from 90% to 78%).

    - The gas yield from source separated OFMSW compares favourably with that

    obtained from other wastes.

    The variations in the production rate are not dependent on the VFA

    concentration in the reactor (in the range 350-1600 mg CH3COOH/I). The greatest

    variation observed during the first hour after feeding seems to depend

    only on the quality of the feed and quantity of each feed.

    - The system is stable (possibly due to the high values of TA) up to a load of 7

    Kg TVS/(m 3 d). It can rapidly return to stability after the initial burst of gas

    production.

    - Our results show that it would be advisable to continue research concentrating

    on the following areas: a) variations in frequency of feed and their effect on

    reactor performance; b) detection of optimal temperature range; c) separate

    studies of the hydrolitic/acidogenic and the metanogenic phases.

    REFERENCES

    1 M. Reinard, N.L. Goodman, J.F. Barker, Occurrence and distribution of organic chemicals in two land-fill leachate, Environ. Sci. Technol., 18 (1984), 953-961.

    2 D.L. Wise, R.G. Kispert, E.W. Langton, A review of bioconversion systems for energy recovery from municipal solid waste.Part I: liquid fuel production, Resources and Conservation, 6 (1981), 101-115.

    3 D.L. Wise, R.G. Kispert, E.W. Langton, A review of bioconversion systems for energy recovery from municipal solid waste.Part ll:fuel gas production, ibidem, 117-136.

    4 D.L. Wise, R.G. Kispert, A review of bioconversion systems for energy recovery from municipal solid waste.Part III: Economic evaluations, ibidem, 137-142.

    5 L. De Baere, High dry anaerobic composting process for the organic fraction of solid wastes, Seventh Symposium on Biotechnology for Fuels and Chemicals, Gatlinburg, TN, May 14-17, (1985).

    6 P. Cescon, F. Cecchi, F. Avezzu, P.G. Traverso, Ottenimento di biogas dalla frazione organica di rifiuti solidi urbani, in A. Frigerio, Rifiuti urbani e industriali - Trattamento e smaltimento, nuovi aspetti tecnologici e normativi, Bi & Gi Ed.,Verona, (1986), p. 57-71.

    7 P. Cescon, F. Cecchi, F. Avezzu, P.G. Traverso, Anaerobic digestion of organic fraction of municipal solid waste - Preliminary investigation, in W. Palz, J. Coombs, D.O. Holl, Energy from Biomass, Elsevier, Amsterdam, (1985) p. 572-576.

    8 F. Cecchi, P.G. Traverso, Biogas dalla frazione organica di rifiuti solidi urbani e fanghi di supero - studio preliminare, Chimica Industria, 67, (1985), p.609-616.

    9 L.F. Diaz, F. Kurz, G. Trezek, Methane gas production as part of refuse recycling system, Compost Science, Summer (1974), 7-13.

    I0 G. Chiesa, R. Bonaiuti, I rifiuti domestici prodotti in Italia, Acqua Aria, 8 (1983), 807-812.

    II S. Ghosh, M.P. ~enry, R.W. Christopher, Hemicellulose conversion by anaerobic digestion, Biomass, 6 (1985), 257-269.

    12 A.P.H.A.-A.W.W.A.- W.P.C.F., Standard Methods For the Examination of Water

  • 197

    and Wastewater, 15th Ed., A.P.H.A., Washington, 1981, 1134 pp. 13 I.R.S.A., Metodi Analitici per le acque, Quaderno n. ii, C.N.R., Roma, 1979. 14 L.F. Diaz, G.J. Trezek, Biogas of a selected fraction of municipal solid

    waste, Compost Science, March-April (1977), 8-13. 15 C.L. Cooney, D.L. Wise, Thermophilic anaerobic digestion of solid waste for

    fuel gas production, Biotechnol. Bioeng., 12 (1975), 1119-1135. 16 A. Pauss, E.J. Nyns, H. Naveau, Anaerobic Digestion and Carbohydrate

    Hydrolysis of Wastes, Proceeding of the E.E.C. Symposium, Luxembourg May 8-10 1984, Elsevier.

    17 M.K. Stenstrom, A.S. Ng, P.K. Bhunia, S.D. Abramson, Anaerobic digestion of municipal solid waste, J. Environ. Eng. Div. Proc. Am. Soc. Cir. Eng., 109 (1982), 1148-1158.

    18 K.F. Fannin, J.R. Conrad, V. Srivastava, D.E. Jerger, D.P. Chynoweth, Anaerobic processes, J.W.P.C.F., 56 (1984), 586-593.

    19 C.G. Golueke, Comprehensive Studies of Solid Waste Management, Third Annual Report, U.S.E.P.A. (1971), pp. 201.

    20 A.G. Hashimoto, Conversion of straw-manure mixtures to methane at mesophilic and thermophilic temperatures, Biothecnol. Bioeng., 25 (1983) 185-193.

    21D. Hawkes, R. Horton, D.A. Stafford, The use of anaerobic digestion for the treatment and recycling of organic waste, Conservation & Recycling, 2 (1978), 181-195.

    22 R.I. Makie, M.P. Bryant, Metabolic activity of fatty acid-oxidizing bacteria and the contribution of acetate, propionate, butyrate, and CO 2 to methanogenesis in cattle waste at 40 and 60 C, Appl. Env. Microbiol., 41 (1981), 1363-1373.

    23 P.G. Stephen, J.F. Andrews, Stability and control of anaerobic digestion, J.W.P.C.F., 46 (1974), 667-683.

    24 E.J. Kroeker, D.D. Schulte, A.B. Sparling, H.M. Lapp, Anaerobic treatment process stability, J.W.P.C.F., 51 (1979), 718-727.

    P ~ S

    We wish to thank E.N.E.A. for financial support, Comune di Treviso for the interest and E. Vita, S. Badoer and Z. Vincenzi for their help in the experimental work.

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