Anaerobic digestion of organic fraction of municipal solid wastes — digester performance
Post on 10-Nov-2016
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 -
F. CECCHI, P.G. TRAVERSO and P. CESCON
Dipartimento di Scienze Ambientali - Dorso Duro 2137 - Venezia - Italy
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.
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
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
MATERIAL AND METHODS
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.
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
Representative] ~ .~ area I Trgvis 0 City I
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.
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).
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
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
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
The analyses were carried out according to the procedure suggested in (12,13).
RESULTS AND DISCUSSION
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
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
T-COD 80 (g-t -1) 70
T.-VS 0.90 TS 0.80 i
T-VS 55 (g-r 1) 45
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).
process, thus permitting a more precise identification of the optimal control
parameters, and will also give indications about developments in anaerobic
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.
From analysis of outlet sludge at the bottom of the reactor, it was observed
that the feed contains a small quantity (. 4 t~ "0
c o g. 2 u
"0 o L
a . 1
~~ ~, , , ,~~~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,
so the increased production of biogas has no influence on the production of
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.
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.
Analysis of data in Tab.3 shows that the soluble fraction of the feed is almost
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
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
~ 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
~ 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.
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
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
i lOOO 900 80(] coo
400 o > 300
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
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).
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
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
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.
=z 7.08 7.04
600 e SO0 > 400
- I I I I I I I I
I I I I I
I I I I I
7 8 9
concentrations, pH values vs. time
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
Mean values, after the burst
concentrations in the digester.
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).
- 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
- 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
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
Population served: 60-100 inhabitants/(m3digester) (according to 730 g MSW
produced/(inhabitant, d) 50% OFMSW with 15-25% dry matter and 90% TVS).
- 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
- 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.
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
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.