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: 35±2°C (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 ~ A n a l y s e s

D iges te r 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).

t he

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 (20°C 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 1 0 0

~--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.

1 8 8

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

that the feed contains a small quantity (<1% by volume) of inert substances with

a high specific weight (shells, glass, metal and stone). The TVS of these

materials is < 35% of TS.

The sieve analysis showed it was possible to collect most of the granular

material on a 5 mm sieve mesh; in the reactor this material settles to the

bottom (which is conical in shape) and does not go back into suspension.

This material, which is also deposited at the bottom of the feedstock tank, is

not taken in the sample for analysis of the feed.

This means that the outlet sludge can be ignored in the mass balance. This

sludge is periodically removed from the reactor to avoid a reduction in the

reactor's working volume.

The relationship between specific gas production or methane and the organic

loading rate is shown in fig. 3.

'T>. 4 t~

"0 (,,).

~E 3

c o g. 2 u

"0

o L

a . 1

0

~~ ~ , , , , ~ ~ ~ a t h a n e 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 6 0 e = 3 . 2 4=4.2

~'500 o= 6 9

4.1

~ 4oo "0

o. 3 0 0 - 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 af ter 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 a f te r 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 af ter 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).

7 0 0

6OO

5 0 0

400

300

oo 2 0 ~ l

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

Time A f t e r 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 (33°C and 37°C) with other parameters constant.

7 2 0 -

7.16

7.12

=z 7.08 7.04

"F

600 e SO0 > 4 0 0

- 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

3 8 - I I I i I I I 0 1 2 3 4 5 6

T i m e a f t e r f e e d i n g , (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.

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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).

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

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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

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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.