mesophilic digestion

Waste Management & Research (1990) 8, 33-44

MESOPHILIC DIGESTION OF THE ORGANICFRACTION OF REFUSE: PERFORMANCE AND KINETIC

STUDY

Franco Cecchi*, Antonio Marcomini*, Paolo Pavan*, Guido Fazzini* and JoanMata-Alvarezt

(Received May 1989, revised August 1989)

The anaerobic digestion of the organic fraction of municipal solid waste sorted byplant was investigated in a 3 m3 stirred digester, operating under mesophilic con-ditions . Yields of gas obtained at 7 kg total volatile solids per m 3 per day and 15-dayhydraulic residence time (about 10% biodegradeable solids, 20% total solids) were asmuch as 1.6 times the digester volume per day. Better sorting should improve theyields which already compare favorably with the existing literature data . A kineticstudy on the substrate utilization was performed by employing the "first order"model .

Key Words-Anaerobic digestion, municipal solid waste, biogas, refuse, kineticmodel, solid waste, dry digestion .

1 . Introduction

The organic fraction of municipal solid waste (OFMSW) constitutes a large renewableenergy resource which is of interest to the Council of European Communities (CEC)policy. In June 1978, the CEC decided to provide financial support for projects in thefield of alternative energy sources and energy savings . Biomass and energy from wastewas a part of the programme. Production and utilization of biogas from wastes was thetopic which covered most of the 153 projects supported by the CEC in November 1986 .

A recent publication has revised the work carried out in Europe in the field of biomassand energy from waste with particular regard to the anaerobic digestion of the organicfraction of MSW (Cecchi et al. 1988b). It was carried out at research, development,demonstrative and commercial levels. In the article cited, the qualitative and quantita-tive differences between European and North American wastes were outlined . Based onthis observation, the present discussion focuses also on the European situation .

Anaerobic digestion, first considered an inefficient and unreliable treatment processapplicable to a limited number of substrates, in actual fact offers a large potential as aMSW disposal technique . Semi-solid substrates (i .e . those with a total solid contentbetween 12-20%) can be digested using several technologies, employing one-phase andtwo-phase systems. One phase anaerobic digestion of the OFMSW, by itself or mixedwith sewage sludge, has been widely applied in Europe (Cecchi et al . 1988b). Vice-versa,the two-phase technology has only been used to a limited extent with vegetable wastes atthe laboratory and pilot plant level (Mata-Alvarez 1987 ; Rijkens et al. 1984;

*Dipartimento Scienze Ambientali, Università di Venezia . Calle Larga S . Marta 2137, 30123 Venezia, ItalytDepartament d'Enginyeria Quimica, Universitat de Barcelona . C. Marti i Franquès, 1, 6p., 08028

Barcelona, Spain

0734-242X/90/010033 + 12 303 .00/0

©1990 ISWA

34

F. Cecchi et al .

Mtz-Viturtia et al. 1989) . However, given its advantages (Verrier et al. 1987), thistechnology promises further developments .

Both technologies are being explored by members of the Universities of Venice (Italy)and Barcelona (Spain) using the experimental facility located in Treviso (Italy) . Thisfacility consists of a 3 m3 working volume completely stirred tank reactor, that can rununder mesophilic and thermophilic ranges of temperature, and a two-phase pilot systemformed by a hydrolyzer (plug flow reactor, with an inside screw, 0.08 m3) and amethanizer (a fluidized bed reactor, 0.06-0.12m3) (Cecchi & Mata-Alvarez 1989) .

In this paper we present the results obtained from the CSTR pilot plant operatingunder mesophilic conditions and carry out a kinetic study of the substrate degradation .The first order kinetic model was considered for this goal .

Although the application of a new model (step-diffusional) presented by the authors ina previous paper (Cecchi et al. 1988c) would be interesting further experimental dataneeds to be done to draw firm conclusions in extending the model to the anaerobicdigestion of the mechanically sorted OFMSW . The step diffusional model was in factcarried out taking into account that the anaerobic digestion of the source sortedOFMSW proceeds according to the following mechanism : the soluble fraction of thefeed is to a large extent digested during the first hour after feeding and almost completelyafter 2-3 hours; the contribution of the more easily hydrolyzable fraction becomesimportant during the fifth and sixth hours ; then the kinetic of the overall process iscontrolled by the solubilization of the fractions which are decomposed more slowly (upto the twelfth hour). After this time the substrate utilization rate is so slow that thecontribution of this period to the total gas production balance can be considerednegligible .

2. Materials and methods

2.1 Experimental device

Experiments were performed in a pilot plant with a stirred tank digester of 3 m3 workingvolume operating under mesophilic temperature range (37 ± 0.5°C) . A flow-sheet of thepilot plant is shown in Fig . 1 . This device, first used for the treatment of source sortedOFMSW (Cecchi et al. 1986, Traverso & Cecchi 1988), was modified in order to handlethe mechanically sorted OFMSW .

The digester was fed twice a day with variable total solid (TS) substrate concentration

Fig. 1 . Flow sheet of the pilot plant.

Mesophilic digestion

35

(up to> =20%). Feed was prepared in a separate homogenizing tank, transferred to astorage tank by a screw pump and then loaded into the reactor by means of a membranepump. As shown in Fig. 1, the biomass was diluted before being stored in the feedstocktank and digester effluent supernatant was used for dilution purposes as a method ofcontrolling digester stability (Cecchi et al. 1989a) . The hydraulic-solid retention time(HRT-SRT) and the organic loading rate (OLR) applied to the reactor varied accordingto the change of the solid content and the daily flow rate of the feed . The gas pressureinside the digester was maintained within the range of 200-250 mm water column (w.c .) .Gas production was measured by a wet gas meter . An armed anchor stirrer, rotating at70 r.p.m. was the mixing device .

2.2 Substrate

The substrate fed to the digester was partially composted OFMSW sorted by theindustrial plant of S . Giorgio di Nogaro (Udine, North-East Italy) . The flow-sheet of theplant is shown in Fig . 2. Table 1 shows the average characteristics of the substrateobtained from analyses of 22 samples over about one year . Each sample was a mixture of10 significative portions of substrate collected from an OFMSW mass of about 2000 kg .Table 1 also shows in detail the types of materials forming the substrate . Five fractionswere considered to characterize the OFMSW : putrescible fraction, paper, wood, plasticand inert matter . Each fraction, sorted by hand, was analysed for total solids (TS) andtotal volatile solids (TVS), in order to discover the distribution of the organic matter .The results obtained from these analyses were expressed in terms of the TS and the TVSthat are contributed by each fraction to the substrate . Since the separation of thefractions is a difficult task, the results obtained must obviously be considered only asgood approximation . Both the separation process (Fig. 2) and the types of materials(Table 1) are important when considering digester yields .

MSW

OFMSW

hammermill

magnet

magnet

glassseparator

2n 0 trommelscreen

I" trommelscreen

lightclassifier

cyclonehomogenizer compostingbuilding

air

bagfilter

RDF line

T

sludgepit

Fig . 2 . Flow sheet of the sorting and composting plant located at S . Giorgio di Nogaro (UD) (Italy) .

36

TABLE 1Average main characteristics of the organic fraction of municipalsolid waste sorted by the industrial plant located atS. Giorgio di Nogaro (Italy) . This refuse was used after dilution as

feed to the pilot plant

F. Cecchi et al .

2.3 Analysis

A complete set of analyses was performed in order to monitor the process . Volatile fattyacids (VFA) and alcohols were determined according to the gas chromatographicmethod reported by Cecchi et al . (1988a). The methane percentage was measuredcontinuously by means of an infra-red instrument . Gas production rate, biogascomposition, digester and external temperature were automatically recorded on acomputer at five minute intervals . The analyses of the alkalinity (TA) (TA 6 and TA4depending on pH end points 6.0 and 3 .8 respectively) . pH, solids, ammonia and totalnitrogen (N-NH4 , TKN), total phosphorus (P) and other parameters were carried outaccording to the Standard Methods (1985) . The TA parameter was measured at two pHend points to evaluate the contribution of the bicarbonate and VFA . TA 6 may in fact berepresentative of the bicarbonate alkalinity and TA 4 includes the titration of VFAalkalinity (Cecchi et al. 1987, Rozzi & Brunetti 1980) . The total organic and inorganiccarbon (TOC, TIC) analyses were carried out by means of a carbon analyser (LECOCR- 12 model) and the COD according to the potassium dichromate oxidation method .All of these were performed on solid samples .

Chemical characteristics :

TS, gkg - ' 763.0TVS, % TS 43.9TCOD, % TS 58.8TKN, % TS 2.2

P, % TS 0.1SCOD, % TS 10 .3STS, % TS 8 .1SVS, % TVS 9.6TOC, % TS 19 .3TIC, % TS 1 .3

Other characteristics:

Putrescible fraction, TS 58 .9TVs 77.9

Paper, TS 5 .1TVs 7.2

Wood, TS 1 .1TVS 2.2

Plastic, %TS 1 .8TVs 3 .4

Inert fraction, TS 33 .1TVs 9.3


37

3. Results and discussion

3.1 Reactor performance

On the basis of the evaluation of the digester behaviour over a ten-month period inwhich the pilot plant was run using OFMSW sorted by industrial plant, the reactorperformance can best be described by identifying three different operating periods . TheTS concentration in the feed was around 12-15% during the initial period (A), then,after installing the new mechanical elements of the pilot plant, this concentrationincreased to 20% (period C). Period B corresponds to the transition from A to C afterthe plant was shut down for modification . During period B, the HRT value wascontrolled at 32 days to facilitate the increase of OLR, and steady-state conditions werenot achieved. However, a short pseudo-stationary state during this period is assumed inthe following discussion on the digester performance because of the constant digesterperformance over about one HRT .

Figure 3 reports the evolution of the operating parameters during the periods B and Cas an example of the monitoring plan . Table 2 presents the mean values of feedcharacteristics and temperatures corresponding to the two periods (A and C) in whichthe steady state conditions were achieved, as well as to the transition period (B) .Furthermore Table 3 presents the correspondent mean values recorded inside thereactor. Digester solids measurements were carried out on samples taken from threesample ports situated along the axis of the reactor (top, medium and bottom), and thedata in Table 3 are the averages of these three measurements . Finally, Table 4 showsreactor yields expressed as percentages of substrate removal and as gas production rates .The organic matter removal was calculated taking into account the total organic carbon,the total volatile solids, and the total chemical oxygen demand strength in the feed (seeTables 1 and 2) and the biogas produced in the steady state conditions of each period .This was done to avoid problems of reactor homogeneity which are related to thevariable quality of the substrate . In fact, if the percentage of removal is computed fromthe data of Tables 2 and 3 (feed and digester), the results do not correspond to thosereported in Table 4 .Three OLR were tested, namely: 4.1, 3.4 (pseudo-stationary) and 6 .8 kgTVS

m - 'day- ' . Inspection of Table 4 shows that the performance of the reactor, in terms ofsubstrate removal and specific gas production rate, remains almost constant even whenit is operating at the more severe conditions tested (OLR=6 .8 kgTVS m-'day - ' periodC) and also when the steady state conditions are not achieved (period B) .

Under more severe operative conditions, the methane percentage decreases, but VFAconcentration maintains very low levels and the health of the process does not appearaffected by these conditions .The change of pH of the feed substrate from 7.8 to 7.0 going from period A to C

(Table 2) can be related to the higher VFA concentration in the feed (about five timeshigher in C) . This is due to the effects of the external temperature, which accelerates thedecomposition of more easily degradable fractions, as discussed in a previous paper(Cecchi et al. 1989b). This value of feed pH does not affect the reactor pH, which wasalways kept around 7 .0-7 .5, as a result of the high values of total alkalinity (TA) . Even ifthe feed alkalinity is rather small (see Table 2), the reactor alkalinity is high due to theuse of the effluent supernatant to dilute the waste and as a medium to control the reactorstability parameters (Cecchi et al. 1989a). The effluent recirculation ratio, defined as theratio between the recirculation flow rate and the feed flow rate, oscillated around thevalue 0.27. Figure 4 shows the TA evolution . Since TA4 decreases as a consequence of

38

39

36

0

12

8

4

0

2 .0

1 .0

0

O0 0

m

0°

00 a

Oe m0 O

% m ° OO moo 0 00 0

O

O•

0°°

13am°0 m ô 0 0 0 a 0

0

III0 20 40 60 80

am°0 1

Oa

IOP alb At- dIUIb

III I

i

1

120

20

40

O

O

IiI1

O

a0913

D j

IEPa Cp mCb°

a

0

20

40

40Days

60

60

60

F. Cecchi et al .

80

80

∎I1

°m0®

~0O

•

~ ddO

MU

0CPM0

wWIM

II80

oû

200

150

50

160

100

40

8 .0

6 .5

6,0

2400

1600

0

0

0

•

0 O 00 ° O

a 0 00 00 131 °0

0

+

+ ++ ++++

+ +t+'}+tF +++l+ I +II I0

20

40

60

80

a

m

m m]

On

®

,,~ 0m O

om

111I111

m

20

I 1I1I20

20

40

40

40Days

80

60

_a

a

•

° 013

130 00 0

CP °0001I1II

60

80

Fig . 3 . Operative parameters evolution during the transition period B and the steady state period C . Period B :

from day 0 to day 37 . Period C from day 38 to day 77, the las' 17 days of this period are representative of thesteady state conditions .

80

80

Group A: acetate and methanol alcohol ;Group B: Ethanol, and VFA > = C, .

the reduction of reactor VFA, whereas TA6 (mainly due to bicarbonates) is fairlyconstant and higher than the corresponding value in the feed, thus the stability of theprocess is guaranteed. TA 6 increases at the end of period C ; this can probably be ascribedto the high CO2 percentage in the biogas which will increase the bicarbonate concentra-tion .

The yields obtained are similar to those reported in the literature . For instance, themethane production rate per reactor volume and day is comparable to that reported by


39

TABLE 2Mean chemical characteristics of the reactor feed during the period studied

* Calculated according to the ratio TCOD/TVS = 1 .2 . This is the mean value for the periods B and C .

TABLE 3Reactor monitoring parameters observed during the periods studied

Period A Period B Period C

Tem_ . , àC 18 .9 12 .0 24.8Temin ,, àC 11 .9 3 .1 10 .9Reactor temperature, àC 37 .4 37 .3 36.4

TS, g kg - ' 143.73 215 .4 208.08TVS, g kg - ' 65.87 111 .2 106.67STS, g 1 - ' 22 .2 21 .75SVS, g 1- ' 11 .6 11 .85TCOD, g kg- ' 79.0 (*) 130 .7 121 .44SCOD, g 1 - ' 10 .7 15 .24

TVFA, mgCH,COOH 1 - ' 653 3180pH 7.8 7 .4 7 .0TA6 , gCaCO, 1 - ' 0.39 0.91TA41 gCaCO, 1 - ' 4.47 4.46


TS,g kg- ' 84.23 125 .2 144.40TVS, g kg - ' 36.18 59 .7 67.87STS, g 1 - ' 11 .4 10 .84SVS, g 1 - ' 5.6 4.94TCOD, g kg - ' - 41 .0 84.10SCOD, g 1 - ' 6.29

Soluble compounds:Group A mgCH,COOH 1 - ' 1047 217Group B mgCH,COOH 1 - ' 110 76

pH 7.5 7 .2 7 .1TA6, gCaCO, l- ' 13.38 4.52 4.32TAè gCaCO,1 - ' 18.23 13 .23 9.36TA; TA6 , gCaCO, 1 - ' 4.56 7.35 5 .04NH; N,mg 1 - ' 299 90

40

F. Cecchi et al .

TABLE 4Operative conditions and digester performance during the three periods studied . The mass balance

is carried out considering the gas production and the feed flow rate

30

i 20

oUoUo,

- D

D\D D<

\ \ D

DD

D

U

Dlo

D

-a- a--

DD 0-

+

- -D-~~

D

D\~~D+ GF -j-

- - +~ -+

-+T_+ - + --- - ++ + -F-_-+'+-'++ +++ ++

+I I I II II I IIi1i1i

0

10

20

30

40

50

60

70

Days

Fig. 4. Total alkalinity measured at pH 3 .8 (TA4 ) and 6 (TA6 ) evolution during the transition to the stationaryconditions of period C . Key: 11, TA 4; +, TA,

De Baere and Verstraete (1984) (Dranco process) or Klein & Rump (1987). However,specific gas production yields, or TVS removal, appear smaller in our case (Cecchi et al.1988b). The low conversion to biogas of the organic matter fed to the digester could bedue to the very poor quality of the substrate . In fact if the specific gas production rate, interms of m' (kg TVS removed) - ' reported in this paper (0 .8-0.9 m' is (kg TVSremoved)- ' matched with the results obtained by others 0.9 m' (kg TVS removed)- '"Warren Spring Laboratory data" ; 1 .0 "Biomet Process" ; 1 .0 "Freseinus Process" ; 0 .8"Dranco Process"; 0.5 "Valorga Process", Cecchi et al. 1988b) the data match very well


Q, Id - ' 184.8 92 192 .1OLR, kgTVS(m'd)- ' 4.1 3 .4 6.8HRT, days 16.2 32.0 15 .6

GP, m' d- ' 2.80 2.63 4.75GPR, m' (m'd) - ' 0.93 0.88 1.58SGPR, m' (kgTVSA) - ' 0.23 0.28 0.23CH4, % 63 .4 56.9 50.6

TVS removal, % 25 30 29TCOD removal, % 25 29 27TOC removal, % 27 34 31


41

and lead to the conclusion that the problem of the conversion level lies in the highpercentage of recalcitrant compounds present in the substrate. However, these sameyields are higher if compared with those reported by Szikriszt et al. (1988) (Biometprocess). The conclusion, from the comparison with the literature data, is that theEuropean results are quite comparable, but more work is necessary to understand theprocess of anaerobic digestion of these complex substrates .

As previously reported, the available data has been fitted to a first order kinetic model .It is simple and has been applied to wastes with a high solid content (Pfeffer 1974, Ripley& Boyle 1983) . The fitting was done considering the characteristic experimental pathwayof the gas production rate between one feed and the subsequent one of each steady-statecondition studied. Figure 5 shows an example of the biogas production profile duringthe steady-state conditions of the period C .

The results obtained with the kinetic study are presented in Table 5 . The value of k isconstant under different experimental conditions when using mechanically sortedOFMSW, but differs when the substrate composition changes (it is larger for the source-sorted OFMSW) . The difference is not surprising, because a larger degradation rate isquite normal when a more easily degradable substrate is treated. The different kineticbehaviour of source sorted and mechanically sorted OFMSW could be explained byapplying the step diffusional model (Cecchi et al. 1988c) which, although more complex,could provide kinetic constants of general application . Anyway, at the moment, theavailable data can not allow the possibility to draw any firm conclusion with this model .

4. Conclusions

The results presented in this work lead to the following conclusions :-The anaerobic digestion of the organic fraction of mechanically sorted MSW canproceed at high solid concentrations in the feed (more than 20% TS) in a conventionalreactor operating under mesophilic conditions (37àC) . No problems arise when the OLRis up to 7 kgTVS(m3day) - 'and HRT 15 days. The gas production which can be reachedis 1 .6 m3(m3day) - 'when the SGPR is 0.23 m' kgTVS fed .-The observed yields depend strongly on the quality of the OFMSW treated, in termsof the portion of the organic matter represented by non-degradable compounds . In fact,the specific yields (biogas production per kg TVS removed obtained in this work arequite similar to those reported in the literature for the treatment of better substrateswhereas the TVS removal and SGPR (m 3 (kgTVSA)- ') have low values. In order toimprove the latter parameters, more attention must be paid to the sorting processes .-The kinetic study performed applying the first order kinetic model gives a kineticconstant for the substrate utilization equal to 0.0003 min - ' when mechanically sortedOFMSW are treated. This value is about one order of magnitude lower than that whichwas observed in digesting source sorted OFMSW (0.002 min- ') . These results allow theconclusion that a more general model has to be studied . This is the aim of the work inprogress at the Universities of Venice and Barcelona .

5 . Acknowledgements

Authors acknowledge the "Istituto Trevigiano di Ricerca Scientifica del Comune diTreviso" for its support. Collaboration by Ing. L. Babos, CSR Bassa Friulana andfinancial support from NATO (grant No . 0178/87) is also gratefully acknowledged .

I.

<y

-71

7

t

00000000000000000~~0000O 01 ~D 1~ 0 o v m N O 0 w ,̀_ 0 YI v m N-

K) M N N N N N N N N N N -

-( 1_41) uol j3npo,id so6oi8

0Ucd

Mesophilic digestion 43

TABLE 5Results of the tested kinetic model using source sorted organic fraction of municipal solid waste

and sorted by plant

*OFMSW sorted by plant (present paper)tsource sorted OFMSW (Cecchi et al. 1988c)

Period A*

Period C*

OLR, kgTVS (mad)- '

4.1

6.8

2.1

First order model :k, (min) - '

0.00026

0.00030

0.00200R2

0.999

0.999

6. Notations

GP gas production (m3 day- ')GPR gas production rate (m 3(m3day)- ')HRT hydraulic retention time (days)k first order kinetic constant (min - ')OFMSW organic fraction of municipal solid wasteOLR organic loading rate (kg TVS (m3day)- ')P total phosphorus content (%TS)Q feed flow rate (1 day) - 'SCOD soluble chemical oxygen demand (%TS)SGPR specific gas production rate (m3(kgTVSA)- ')STS soluble total solids (%TS)SVS soluble volatile solids (%TS)TA total alkalinity (mg CaCO3 I- )TA4 total alkalinity measured at pH 3 .8 (mg CaCO3 1- ')TA6 total alkalinity measured at pH 6 .0 (mg CaCO3 l- ')TCOD total chemical oxygen demand (%TS)Temax max external temperature ( àC)Temiè min external temperature (àC)TIC total inorganic carbon (%TS)TOC total organic carbon (%TS)TKN total Kjeldhal nitrogen (%TS)TS total solids (g kg - ')TVFA total volatile fatty acids (mg CH 3000H 1 - ')TVS total volatile solids (%TS)TVSA total volatile solids added to the digester (kg day - ')VFA volatile fatty acids (mg CH3000H 1 - ')

ReferencesCecchi, F., Basei, R ., Fazzini, G., Traverso, P. G. & De Poli, F . (1988a), Determinazione gas-

cromatografica di VFA, etanolo, metanolo, acido lattico nel monitoraggio di processi didigestione anaerobica (Gas-chromatographic analysis of VFA, ethanol, methanol and lacticacid in the monitoring of the anaerobic digestion process), Acqua Aria 9, 993-997 .

Cecchi, F., Marcomini, A ., Fazzini G ., & Mata-Alvarez, J. (1989a), OFMSW anaerobic digestionenhancement by effluent recirculation . Biocycle (submitted) .

44

F. Cecchi et al .

Cecchi, F . & Mata-Alvarez, J . (1989), The anaerobic digestion of municipal solid waste . InLandfill Gas and Anaerobic Digestion of Solid Waste (Y . R. Alston & G . E . Richards, eds) .Harwell Lab . Pub. pp 550-557 .

Cecchi, F ., Mata-Alvarez, J ., Fazzini, G ., Vallini, G . & De Poli (1989b), Effects of externaltemperature on the anaerobic digestion of municipal solid waste, Journal Chemical ofTechnology and Biotechnology, (submitted) .

Cecchi, F ., Traverso, P. G. & Cescon, P . (1986), Anaerobic digestion of organic fraction ofmunicipal solid waste-digester performance, The Science of Total Environment, 56, 183-197 .

Cecchi, F ., Traverso, P . G ., Clancy, J ., Mata-Alvarez, J . & Zaror C . (1988b), State of the art ofresearch and development in the anaerobic digestion process of municipal solid waste inEurope . Biomass, 16, 257-284 .

Cecchi, F., Traverso, P . G., Mata-Alvarez, J ., Llabres, P . (1987), pH and CO, as monitoringparameters of the anaerobic digestion process of municipal solid waste, Ingegneria Sanitaria,XXXV, 339-344 .

Cecchi, F ., Traverso, P. G ., Mata-Alvarez, J ., Medici, F. & Fazzini, G . (1988c), A new approachto the kinetic study of anaerobic degradation of the organic fraction of municipal solid waste,Biomass, (in press).

De Baere, L. & Vestraete, W. (1984), High rate anaerobic composting with biogas recovery,Biocycle, 25, 30-31 .

Klein, M . & Rump, H. (1987), Anaerobic digestion of solids-example: organic fraction ofmunicipal solid waste. In Biomass for Energy and Industry 4th E . C. Conference, Grassi G.,Delmon B ., Molle J. F ., Zibetta H . Elsevier Appl . Sci ., London, pp 845-849 .

Mata-ALvarez, J. (1987), A dynamic simulation of a two-phase anaerobic digestion system forsolid wastes . Biotechnology and Bioengineering, 30, 844-851 .

Mtz-Viturtia, A., Mata-Alvarez, J ., Cecchi, F . & Fazzini, G . (1989), Two phase anaerobicdigestion of a mixture of fruit and vegetables wastes, Biological Wastes, 29, 189-199 .

Pfeffer, J . T . (1974), Temperature effects on anaerobic fermentation of domestic refuse . Biotechno-logy and Bioengineering, 16, 771-787 .

Rijkens, B . A ., Voetberg, J . W., Hofenk, G . & Lips, S . J . J . (1984), Two Phase Anerobic Digestionof Solid Organic Wastes Yielding Biogas and Compost . Final report . E.C. Contract no . : ESE-E-R-040-NL. IBVL Wageningen, The Netherlands .

Ripley, L . E . & Boyle, W . C. (1983), Anaerobic digestion models : Implications for the designengineer. Third International Symposium on Anaerobic Digestion, Boston, Mass, August .

Rozzi, A. & Brunetti A. (1980), Anaerobic process control by inorganic carbon analysis,Environmental Protection Engineering, 6, 113-116 .

Standard Methods for the Examination of Water and Wastewater (1985), 14th Edition, AmericanPublic Health Association-American Water Works Association-Water Pollution ControlFederation, 1193 pp .

Szikriszt, G ., Frostell, B ., Normann, J . & Bergstrom, R. (1988), Pilot scale anaerobic digestion ofmunicipal solid waste after a novel pretreatment. In Anaerobic Digestion 1988, Hall E. R . &Hobson P. N ., p 375. Pergamon Press, Oxford .

Traverso, P . G. & Cecchi, F . (1988), Anaerobic digestion of the shredded organic fraction ofmunicipal solid waste . Biomass, 16, 97-106 .

Verrier, D., Roy F. & Albagnac, G. (1987), Two-phase methonization of solid vegetable wastes .Biological Wastes 22, 163-177 .

mesophilic digestion

Documents