~ Pergamon Wal. Sci. T~ch. Vol. 33. No.3. pp. 169-175. 1996.Copyright C 1996 \AWQ. Published by Elsevier Science Ltd

Pnnled in Oreal Britain. All right. reserved.0273-1223/96 SI5'OO +0-00

pn: S0273-1223(96)00310-1

ANAEROBIC TREATMENT OFMUNICIPAL SOLID WASTE

G. Plaza*, P. Robredo**, O. Pacheco* andA. Saravia Toledo**

• FaeuItad de Ingenierla. Instituto de Investigaeion en Energla No CoveneionalConsejo de Investigacian de la UNSa, Universidad Nacional de Salta. Buenos Aires177. CP 4400. Salta, Argentina•• Faeultad de Ciencias Exactas,lnstituto de Invesligacian en Energla NoConveneional Consejo de Investigacian de la UNSa. Universidad Naeional de Salta.Buenos Aires 177, CP 4400, Salta. Argentina

ABSTRACT

This paper discusses the present state of Municipal Solid Waste (MSW)disposal in Argentina, focusing on the particular situation of the cityof Salta, and suggests the future trend of the anaerobic treatment of thisresidue.

Source-sorted household solid waste was determined by means of astatistical design. Municipal solid waste produced 147,103 kg per day +/•6.7 t, the major portion corresponds to the biodegradable organic fraction(55.4 tl . The plastic, metal, paper, glass, and other material production

was also evaluated.

The organic municipal waste was chemically and biologically characterized,in order to study its behaviour during anaerobic digestion. The rationormally found in organic fractions from MSW in Salta, Argentina, was inthe optimum range which is 126:7:1 (C:N:P, w/wl. The stability of theanaerobic process was analized in a 18 liter batch reactor. Its pH, biogasproduction, alkalinity, and volatile fatty acid production was determined.

Although the work has been carried out at a small scale it appears to besufficiently promising to encourage further work at a larger scale.Copyright @ 1996 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS

Anaerobic digestion, biogas, municipal solid waste, reactor perfomance,source-sorting of household solid waste, statiscal sampling.

169

170

INTRODUCTION

G. PLAZA ~I at.

Environmental pollution as a result of waste production has been foundsince human communities exist.

The actual and current widely - used method of landfilling employed forultimate treatment of this waste is either unacceptable or just atemporary solution to its disposal. However, there are a lot of illegalsites where the waste is deposited, e.g. river banks, especially inrural regions. This situation occurs allover the country.

Landfilling and incineration are not definitive solutions, as they do noteliminate completely the waste materials, however, from an economic pointof view both methods are still attractive, despite their severaldisadvantages. These disadvantages are mainly the possibility of watertable contamination by the leachate, the low potentiality of biogasutilization and the difficulty of finding suitable areas near the siteswhere the waste is being generated.

The composition of municipal solid waste is affected by various factors,including regional differences, climate, collection frequency, season,cultural practices, as well as changes in technology.

For an efficient integral solution of the MSW, including recyclingpractices, data collection on the total amount of every kind of componentas well as on the economical feasibility of different treatments of suchwaste is necessary.

An attractive alternative for aerobic composting of the organic fractionof MSW (OFMSW) as biotechnological stabilization process could beanaerobic digestion. This method offers an advantage over aerobiccomposting: the production of biogas. However, the design and operationof an adecuate system for the anaerobic treatment requires a goodunderstanding of the effects of the composition on the perfomance of thereactor.

The methanogenic treatment processrelationship between interacting butpopulations: acidogenic bacteria and1990) .

relies on a balanced symbioticmetabolically distinct microbialmethanogenic bacteria {De Lorme,

High molecular weight compounds are converted into intermediate volatilefatty acids, mainly propionic and butyric acids by acidogenic bacteria.Acetic acid, the major intermediate in methanogenesis is formed throughthe degradation of propionic and butyric acids and through the oxidationof hydrogen, process involving distinct acetogenic populations. Methanecan only be formed by specific methanogenic bacteria which utilize aceticacid or hydrogen.

The anaerobic digestion process has been studied since the sixties, tryingto develop technologies that sum up advantages for energy and resourcesrecovering and for volume and organic mass reduction. There isconsiderable interest in applying this technology to the MSW problem,leading to a number of systems which demonstrate the feasibility of theprocess . These include process like RefCoM (Pfeffer, 1987), SOLCON(Biljetina, 1989), BIOMET (Szikriszt, 1988), VALORGA (Bonhomme, 1988),DRANCO (De Baere, 1987), BioCell (ten Brummeler, 1990), SEBAC (Chynoweth,1990). The aim of the present work is to analyze at a small scale, theOFMSW anaerobic digestion, in order to implement it as a Salta's wastetreatment.

Anaerobic tre.1.tment of municipal solid waste

STATISTICAL SAMPLING

171

As there was no information about Salta's and Cerrilos's MSW composition,a "Pilot Statistical Experience" was carried out in order to evaluate thestatistical parameters.

salta, a subtropical city, situated in northwestern Argentina, has apopulation of 353,209 inhabitants.

The statistical study was designed according to the National PopulationCensus (1991).

208 houses were surveyed (Plaza, 1994). During one week they classifiedtheir waste in organic, paper, metals, glass, plastic and other materialsusing different plastic bags which were daily collected and weighted. Theorganic fraction was chemically analized to determine its composition. Thesame evaluation was carried out in rural conditions (Cerrillos, 15 km awayfrom Salta, has 7,500 inhabitants). The households selected for the pilotexperience received information on how to sort the waste through brochuresas well as in community meetings.

In the two communities, separate collection of MSW has proved to besuccessful by the pilot experience and the results are shown in Fig.1.

40

f~20

10

oe-..L,~-C+-~...L.+:Jo-..L,LJ..-C+-.l-...L.~Orgllnic GIIlOO Pllper Othera

Fig 1 Composition of MSW inSalta and Cerrillos today

Salta'S household MSW produced 147,103 kg/day +/- 6.7 \, and Cerrillo's2,767 kg/day +/- 13.2 \ for winter.

The total organic fraction produce in Salta (household plus greatgenerators) will be aerobically - anaerobically digested at a full scaleprocess. Previously, a pilot scale will be implemented in Cerrillos.

THE SUBSTRATE

As discussed above, 55.4 t of the total weight of MSW in Salta consistsof organic material. This fraction constitutes the biodegradable part ofthe MSW.

In relation to the composition of the organic waste, the C:N:P ratio ofthe substrate is an important parameter which has to be considered. Anindication of the optimum nutrient ratio can be derived from the ratio of

172 G. PLAZA el al.

the nutrients present in the biomass of bacteria, which is 126:7:1 (C:N:P,w/wl.

The C:N:P ratio data for Salta and Cerrillos are plotted for a week.It shows that the ratio normally found in our community is in the optimumrange. However , it does not necessarily mean that the nutrients areavailable to the microorganisms.

30

~25 ~::::: ----.r ~....~

20 -------- ---15

10...... -

~-- -- ......... ~ -5 ...............0

Mon Tu.... Wed Thur Fr. Sat

1-- C : N Sa/ra -- N: P Salta -- C : N Cerr --N:PCerr ~Fig. 2 Carbon N1trogen and N1trogen

Phosphorus ratios

The composition of the particles of organic matter depends greatly on thesource of the organic fraction. Roughly, it reflects the composition ofligno-cellulosic biomass, but also contains lipid, starch and sugarcompounds.

MATERIALS AND METHODS

Materials

Household municipal solid waste was obtained from Salta. Representativesamples were analyzed according to the results discussed above. They wereshredded to a size range of 2-6 mm.

Table 1 gives figures of OFMSW composition in terms of parameters thatgenerally indicate the biodegradability of complex substrates.

TABLE 1: MEAN CHARACTERISTICS OF THESUBSTRATE USED IN THE EXPERIMENT

Total solids % w/wTotal volatile solids % STTotal carbon % STTotal nitrogen % STTotal phosphorus % STTotal sodium % STTotal calcium % STTotal magnesium % STTotal VFA mg acetic acid/lpH

8.481. 748.13.42.683.961.43

3,1207.2

Anaerobic treatment of municipal solid waste

The seed sludge was obtained from a manure digester.

Anaerobic Digestion and Operation

173

The experiment was carried out in a batch reactor made of PVC with aninternal diameter of 0 .27 m and a total height of 0.37 m. Thereactor was maintained at a temperature of 3SoC (+\- 1°C).

The reactor was filled manually with fresh OFMSW and inoculum in arelation of 6 % w/w. At the beginning the total solid concentration of theinoculum plus substrate mixture was 13.2 % (w/w).

Digester perfomance was monitored daily by measuring pH and gasproduction. Alkalinity (ALK) and volatile fatty acid production (VFA) wasalso determined

The composition parameters were determined directly on the sample takenfrom the reactor according to the Standard Method (1,9aS).

RESULTS AND DISCUSSION

A balance between acid production and acid consumption is essential fora stable anaerobic process running at the highest possible rate.

It is well known that the process stability strongly depends, on thestart-up of the reactor, which is a period when the VFA concentrationmust not become too high, so an equilibrium between the acid phaseand the methane generation is achieved. Figure 3 shows an increase of VFAwhich causes a drop in pH. It was controlled by the addition of sodiumbicarbonate as a buffer at a minimum buffer/substrate solid ratio of 0.06kg kg-l TS.

25

20

>'? 15

~~ 10<

5

0

6.5

+-__-++- ---l6

~~-2r---3r--4..--~5-..,.6-....7-.....e-..,9r---'or---,r,-,"'3-'-5··5

1-'- AlII (gIl) -- VFA (gil) -- Gas Vol (I) -- pH ~

Fig. 3 Perfomance data for a reactor using OFMSWas the feedstock

Adequate alkalinity, or buffer capacity, is necessary to maintain a stablepH in the digester for optimal biological activity. Alk 5.75 provides amore accurate estimation of theoretical true bicarbonate alkalinity,(Cobb, 1990). The alkalinity ranged from 1,000 to S,OOO mg CaC03/1, asMetcalf and Eddy (1979) recommended.

The production of 1.2 1 biogas/l digester with 39 % of methane wasobserved in the third day. Production improved in following days.

174 G. PLAZA el al.

The biogas composition and production indicated a balanced start-up of thedigestion, i. e. a rapid start of the methanogenesis without hydrogenproduction in the biogas.

As it is shown in the Fig. 3 biodigestion of MSW is feasible but highresidual VFA is generated and consequently lower biogas conversion isobtained.

Wujcik and Jewell (1,979) reported maximum concentrations of 0.33 Mvolatile fatty acids and simultaneous methane production in dry mesophilicanaerobic batch digestion of cellulosic wastes.

The organic acids concentration increased up to a value of 16,200 mgacetic acid/I, in eight days but rapidly decrease to 13,000 mg aceticacid/l within a period of ten days. This level is mantained during thefollowing days. Changing the operative conditions of the reactor, loweracetic acid levels could be obtained.

The relative large particles of organic material may hinder a closecontact between methanogenic bacteria and substrate. This situation couldbe also improved.

CONCLUSIONS

The first pilot-statistical experience in Salta and Cerrillos showed thatthe households can perform a proper sorting of the solid waste.

The organic fraction of the municipal solid waste of Salta can be treatedin an anaerobic digestor. The high residual VFA concentration could bedecreased, and enhanced the contact between methanogenic bacteria andparticulate substrate in order to obtain higher biogas production.

According to the results, the work carried out at small scale appears tobe sufficiently promising to encourage further work at a larger scale.

An anaerobic processing of MSW has the advantage of energy recovery, andit is also a suitable system for the reduction of amount of waste to betreated, by the recycling materials, as well as by the reduction of theorganic fraction.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge Prof. Horacio Di Veltz for hisstatistical assistance, and Bioq. Elsa Scaroni, Lie. Marla RitaMartearena, Orlando Jerez Lie. M6nica Camacho, Eng. Eduardo Corvalan, Dr.C. Cuevas for their technical assistance.The authors also wish to thank Eng. O. Sivori, Senator from Cerrillos whois helping us obtaining the necessary funds for the project.

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Bonhmme M. (1988). The VALORGA process for the recycling of urban waste by methanization.Energy from Biomass and Wastes XI, D. L. Klass (Ed). Institute of Gas Technology,

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Anaerobic treaunent of municipal solid waste 175

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