BIODEGRADABILITY OF THE ORGANIC FRACTION OFMUNICIPAL SOLID WASTE IN A HIGH-SOLIDS ANAEROBIC

DIGESTER

Masoud Kayhanian

Department of Civil and Environmental Engineering, University of California at Davis, Davis,California 95616, U. S. A.

(Received 25 October 1993, accepted in revised form 18 March 1994)

Three methods were used to estimate the ultimate biodegradability of the organicfraction of municipal solid waste. These methods included: long-term batch digestionstudies, measurement of lignin content, and chemostat studies. The ultimate bio-degradability values obtained from these methods were compared to a field operationusing a pilot scale, high-solids, complete-mix, thermophilic, anaerobic digestionprocess. The biodegradability obtained from the pilot study, at a mass retentiontime of 30 days, was approximately 83 and 81% of the estimated values obtainedfrom the lignin content and the batch study, respectively. In addition, it has beenshown that the contents of the biodegradable volatile solids affects the prediction ofbiogas production rate, the computation of the organic loading rate, and feedstockC/N ratio.

Key Words—Biodegradability, biodegradable volatile solids (BVS), biodegradableorganic fraction of municipal solids waste (BOF/MSW), high-solidsanaerobic digestion process.

1. Introduction

Biological transformations can generally be classified as either aerobic or anaerobicprocesses. While each organic waste may contain a constant ultimate biodegradablefraction, practical biodegradability for an aerobic process may be different than for ananaerobic process under similar conditions. Additionally, in practice, factors such asparticle size, time, and environmental conditions (i.e. temperature, nutrient requirements,etc.) will influence the final outcome of biodegradation. For example, because the rightconditions do not exist in most landfills, the biodegradability estimated from analyticaltests will usually over-estimate the real biodegradation that occurs in landfills. It isclear that the issue of biodegradability is broad, and many factors must be consideredto completely describe all substrates and various biological transformation processes.The aim of this paper is not to completely describe all aspects of biodegradability. Forpractical purposes, this paper was prepared to deal with the biodegradability of theorganic fraction of municipal solid waste in an in-vessel, high-solids, anaerobic digestionprocess.A typical organic fraction of municipal solid waste (MSW) in the U.S.A., consisting

primarily of paper, yard waste, food waste, and other organic waste, including plastics,comprises approximately 70% of the waste stream. Paper, yard waste, and food wastemake up the majority of the organic fraction, comprising approximately 53% of thewaste stream which is collectively called the biodegradable organic fraction of MSW

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(BOF/MSW). A BOF/MSW feedstock for a biological degradation process can beproduced using a source separation programme or a sorting facility.Many communities are now in the process of using aerobic or anaerobic biological

treatment processes for the management of BOF/MSW. The anaerobic digestion processis an increasingly attractive method for the stabilization of the BOF/MSW, since it

produces energy, reduces the volume of the waste, and creates a stable by-product inthe form of a humus. The technical feasibility of source separated or processed BOF/MSW digestion has been proven in both pilot and large scale operations (Kayhanianet al. 1991a & b, Six & De Baere 1992, Saint-Joly 1992).For anaerobic digestion of BOF/MSW to be economically viable, attention must be

focused on feedstock biodegradability. The importance of feedstock biodegradabilityon the economic feasibility of biological conversion processes is considered by Gossettet al. (1974) and Kispert et al. (1975). Substrate biodegradability is of special importancein an in-vessel, anaerobic digestion process, where the production of energy is ofconcern (Chandler et al. 1980).While attempts have been made to determine the ultimate biodegradability of

agricultural wastes and biomass crops, to date, the estimation of biodegradability forthe organic fraction of BOF/MSW in an in-vessel, high-solids, anaerobic digestionprocess has not been evaluated thoroughly. The study reported upon in this paper wasundertaken (1) to estimate the ultimate biodegradability of BOF/MSW using variousmethods, (2) to conduct a pilot study and determine the biodegradability of a mixedBOF/MSW under pilot process operation, and (3) to assess the application of estimatedbiodegradability on the design and performance of a high-solids, anaerobic digestionprocess.

’

2. Biodegradable volatile solids of an organic substrate

Dry organic substrates consist of volatile solids (VS) and ash. Taken together, thesetwo components comprise the total solids (TS) of a substrate. The fraction of ashtypically depends on the nature of the organic substrate. Volatile solids is measured asloss on ignition. Only the biodegradable volatile solids (BVS) fraction of the VS hasthe potential for bioconversion, largely because of the presence of refractory volatilesolids (RVS) which, in most digester feedstocks, is mostly lignin. Lignin is a complexorganic material which is not easily degraded by anaerobic bacteria, and normallyrequires a long period of time for complete degradation. The characterization of atypical organic substrate is illustrated in Fig. 1. From Fig. I, it is clear that organicsubstrates with high RVS and ash contents have low biodegradabilities.The RVS in the organic fraction of MSW consists of the lignin content which is

associated with cellulose in plant materials and thermo-plastic materials. The lignincontent of an unsorted organic fraction of MSW will fluctuate as the percentage ofcellulosic and thermo-plastic materials will vary with season, socio-economic conditionsand geographical locations. Knowing the BVS fraction of VS in the individual organicfractions of MSW will enable estimation of the biodegradability of any composite BOF/MSW.

3. Methods to estimate biodegradability

Several methods can be used to estimate the BVS fraction of an organic substrateincluding (1) long-term batch digestion studies, (2) measurement of lignin content, and(3) chemostat studies. These methods are described below.

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Fig. 1. Characterization of a typical organic substrate. TS, total solids; A, ash; VS, volatile solids; RVS,refractory volatile solids; BVS, biodegradable volatile solids.

3.1 Long-term batch digestion

Batch digestion studies, designed to simulate a specific anaerobic digestion process, canbe used to predict the biodegradable fraction of an organic substrate. One method,developed by Jewell & McCarty (1971), involves graphical analysis of weight loss overtime, and is commonly used to predict the ultimate biodegradability of energy cropsand agricultural wastes (Jewell et al. 1987). This method is based on a linear regressionplot of the remaining VS concentration as retention time approaches infinity. Regularweight measurements of the batch digester throughout the course of the study arerequired to apply this method.An alternative method for determining the biodegradable fraction of an organic

substrate, using batch digesters, is to determine and compare the initial and final drymasses. Initial measurements are made of the mass and the percentage of total solidsof active reactor mass of each mixture to be tested. Each mixture includes a percentageof seed, material taken from an active digester to provide suitable micro-organisms.One batch mixture is always 100% seed. At the end of the batch study, the dry massin each individual reactor is measured. The mass loss in each unit, corrected for seedbiodegradation, is due to conversion of the biodegradable portion of the substrate tobiogas.

3.2 Measurement of lignin content

A commonly-used analytical method for determining the BVS fraction of an organicsubstrate is based on the measurement of the crude lignin content of the substrate’svolatile solids. Chandler et al. (1980) correlated the biodegradability of various agri-cultural residues and newsprint, as determined by long-term batch digestion studies,with the substrate’s crude lignin content, as determined by sequential fibre analysis,developed by Robertson & Van Soest (1981). The following empirical relationship wasdeveloped to estimate the biodegradable fraction of an organic substrate from lignintest results:

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Biodegradable fraction = 0.83 - 0.028LC [1] ]

where the biodegradable fraction is expressed as a fraction of the volatile solids, andLC is the lignin content, expressed as a percentage of the volatile solids.

3.3 Chemostat studies

The BVS fraction of an organic substrate can also be estimated using chemostattechniques. The true digestible organic matter (TDOM) technique, developed by animalscientists, is used to assess the digestibility of animal feeds (Van Soest & Robertson

1986). The test involves the digestion of a sample in vitro in rumen fluid for 48hours. A second chemostat technique, the biochemical methane potential (BMP) assay,developed by Owen et al. (1979), is used to characterize the biochemical methane

potential and biodegradability of many organic substrates (Smith et al. 1988). In theBMP assay, the substrate biodegradability is determined by monitoring the cumulativemethane production from a slurry sample, which is anaerobically incubated (typicallyfor 30 days at 35°C) in a chemically defined media and innocula.The biodegradability of energy crops as estimated using BMP assay tends to be

3-10% greater than the corresponding BVS value, as estimated using the TDOMtechnique (Richards et al. 1991 a & b). The BMP assay appears to be more suitable fordetermining ultimate biodegradability.

4. Experimental study

4.1 Experimental facilityA pilot scale, high-solids, anaerobic digester (Fig. 2) was used to conduct experimentsfor BVS mass removal and computation of BVS fraction using biogas volume. Thepilot digester used in this study was designed and constructed by Microgen Corporationin Ithaca, New York. The reactor is equipped with several important features including(1) a mechanical agitator to mix the contents of the reactor, (2) a platform scale underthe entire reactor to measure the daily loss in mass, and (3) an electrical control panel,which controls the temperature and mixing period of the anaerobic reactor system.The control panel is designed so that the reactor can be operated in manual or automaticmode. A summary of the physical and operational characteristics of the pilot-scale,high-solids, anaerobic digester is reported in Table 1. Additional information about thepilot reactor can be obtained elsewhere (Kayhanian et al. 1991a & b, Kayhanian &

Tchobanoglous 1993a & b).

4.2 Feedstock material

The feedstock used in this study was a simulated biodegradable organic fraction ofMSW, comprised of mixed office paper, newsprint, food waste, and yard waste. Thepaper materials were shredded to a size of approximately 25 mm. The yard waste wasmainly grass clippings, dried in the field and shredded to the same size. Food wasteswere donated by a local restaurant. The particle size of the feedstock was reduced toless than 25 mm to insure a uniform feedstock preparation for all tests.

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Fig. 2. View of the high-solids anaerobic digester. Note mixer drive motor and feed trough located on topof the reactor, platform scales on which the reactor is set, discharge port for digested solids located at thelower right hand side of the reactor, digester control panel located in foreground, and wet-test meter used

to measure gas production located on desk in front of the reactor.

4.3 Analytical methods

Test methods used to determine TS, VS, and ash were performed according to

ASTM Standard Methods (1989). Bulk density and elemental analysis were determinedaccording to procedures specified in the annual book of ASTM Coke and Coal Methods(1980). The fibre analysis was conducted according to the procedure outlined byRobertson & Van Soest (1981).

4.4 Experimental procedures to estimate Bus fractionsA batch digestion study was conducted to estimate the BVS of the organic fractionsof MSW under simulated, high-solids, anaerobic digester operating conditions. Duplicatereactors were used for each sample. Eight 1 litre reactors were used to estimate theBVS of newspaper, office paper, yard waste and food waste. Two additional reactorswere used to test a mixed blend consisting of 19% newspaper, 53% ofhce paper, 15%yard waste, and 13% food waste. The particle size of all feedstock fractions was reducedto less than 25 mm. The feedstock for each reactor was prepared by pre-mixing withseed, provided from an operating, high-solids digester. Water was added to bring thetotal solids concentration to the same level as the seed control (approximately 25%).

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TABLE 1

Summary of the physical and operational characteristics of the pilot-scale,high-solids anaerobic digestion process*

* Partially adapted from Kayhanian and Tchobanoglous (1993b).z MRT, nominal mass retention time based on the total wet mass fed each day.+ SRT, solid retention time based on the total wet effluent mass discharged each

day.

The digesters were operated under thermophilic conditions, and mass loss was monitoredfor a period of 75 days.The lignin analyses were performed and applied into Equation 1 to estimate the

biodegradability of various organic fractions of MSW. It is important to note that forprediction of substrate biodegradability, Equation I is limited to a maximum value of83%. Equation I is appropriate for estimating the biodegradability of a typical mixedblend of the materials comprising the organic fraction of MSW because the BVSfraction of such materials is normally less than 83%. However, when Equation 1 isused for highly-biodegradable organic substrates, the ultimate biodegradable fractionwill be under-estimated.

5. Results

The feedstock characteristics and the results obtained from each of the three methodsused to estimate the BVS are presented in this section. The characteristics of the

feedstock materials are summarized in Table 2. The empirical formula for the organicfeedstock used in this study was determined to be C46H73031N. The estimated BVSfractions of the individual organic wastes and of the mixed blend, as determined bylong-term batch digestion, are reported in Table 3. The results of the lignin contentanalyses and estimated BVS fractions are reported in Table 4.

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TABLE 2Characteristics of a typical BOF/MSW used as the feedstock*

* Adapted from Kayhanian and Tchobanoglous (1993a & b).’~ Obtained based on a typical BOF/MSW mixed blend consists of 19%

newsprint; 53% ofhce paper; 15% yard waste; and 13% food waste (dry basis).

TABLE 3Estimated biodegradable fraction of organic waste components of MSW

based on long-term batch studies

* Mixed blend consists of 1 9%> newsprint; 53% ofhce paper; 15% yard waste;13% food waste (dry basis).

t The value reported for mixed blend is based on the actual organic waste notthe value that can be computed from each individual component.

.

> .

6. Discussion .

As shown above, several methods can be used to estimate the biodegradability of anorganic substrate. The values obtained from these methods are considered to be ameasure of ultimate biodegradability. However, in practice, not all of the ultimatelybiodegradable materials are available for biological degradation. How much of the

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TABLE 4Estimated biodegradable fraction of organic waste components of MSW

based on lignin content*

* The values for mixed paper and mixed blend are based on the actual organicwaste, not the value that can be computed from each individual component.

t Computed using Equation 1.

I Mixed blend consists of 19% newsprint; 53% office paper; 15% yard waste;and 1 3’%o food waste (dry basis).

estimated ultimately biodegradable material can be biologically converted in an in-vessel high-solids, anaerobic digestion process is the major discussion topic presentedbelow. Additional aspects of biodegradability to be discussed include comparison ofBVS values and impact of BVS on process design and performance.

6.1 Estimation of BT~S using the data obtained from field operation

In practice, the BVS can often be estimated from the quality and quantity of the dailygas production of a field operation. To predict the BVS removed, the daily volume ofgas produced must be corrected to standard temperature and pressure, and the watertaken up in the production of biogas must be calculated.

6.1.1 Biogas volume at standard conditionsThe first step in estimating BVS removal, using measured gas production, is to correctthe volume of gas produced for its moisture content, and to normalize the dry gasvolume to standard conditions. To normalize the gas volume for standard conditions,the pressure and ambient temperature when the gas volume was measured must beknown. The gas, corrected for water vapour content and standardized at 0°C and I

atmosphere, is by definition called dry biogas (Richards et al. 1991 b ). The dry biogasvolume is then used to correct for water uptake as discussed below.

6.1.2 Correction for water uptakeThe second step in the process is to account for the water taken up in the productionof biogas. Buswell & Muller (1952) developed an expression to predict the quantity ofmethane and carbon dioxide gas produced under anaerobic conditions, based on aknowledge of the chemical composition of the waste. The expression developed byBuswell & Muller was later modified by Richards et al. (199 1 b). The modified equationis:

C,H,O~N, + [n -0.25a -0.5b + 1.75c] H20 ~ [O. Sn + 0.125a -- 0.25b - 0.375c] CH4[2]+ [O.Sn -0.125a + 0.25b -0.625c] CO, + cNH4 + cho- 3

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In this equation, it is assumed that the ammonia released is retained in solution and isbalanced by bicarbonate derived from product carbon dioxide. In an anaerobic digestionprocess, it is important to note that the mass of gas formed is larger than the mass ofthe organic substrate removed by the process (Voznaya 1979). Due to the water

requirement, the actual BVS mass removed is some fraction, f’, of the measured, drybiogas mass. For a given organic waste, the correction factory, can be estimated froman empirically derived formula (Richards et al. 1991 b) using the variables in Equation2:

f =1 I - (I 8n - 4.5a - 9b + 12.5c)/(30n -3.5a +7b -33.5c) [3]

The value of f’ normally ranges from 0.7-1, depending largely on the type of organicsubstrate (e.g. f = for glucose). Using the computed value for f , the mass of BVSremoved can be estimated from the dry biogas volume measurements.

6.1.3 Estimation of BVS massThe following expression can be used to estimate the BVS mass from an actual pilotor full scale operation.

BVS mass=f’ x VSTI X p [4]

where f’ = correction factor for water uptake ( < 1 ); VS-l.P = dry biogas volume at O’Cand 1 atmosphere, m3 and p=biogas density, kg m-3

As indicated in Equation 4, it is necessary to compute the dry biogas volume in orderto estimate the BVS mass. Using the ideal gas law and assuming that the biogasmeasured is not under pressure, Equation 4 can be written as follows:

BVS mass =f’ x p x V,(273/273 + 7)[P -p,,)IP [5]

where VT = measured biogas volume, m3; P = atmospheric pressure, 101.325kPa; p, =vapour pressure at temperature T, kPa (2.34 kPa at 20°C); 7&dquo;= temperature of biogasat measured conditions, °C.

The biogas density can be computed using the molecular weights of carbon dioxideand methane, and the molar volume of an ideal gas at STP (Table 5). The biodegradabilityof input feedstock using the data obtained from a pilot study is presented below.

6.1.4 Estimation of feedstock biodegradability from pilot studyThe experimental results from the pilot study based on a 1 year study are summarizedin Table 5. As reported in Table 5, the average biogas volume, measured at 20°C andincluding the water taken up in production of the biogas, is about 10.4 m3 day-’ witha methane content of approximately 52%. The corrected volume of biogas removed atSTP (0°C at 1 atmosphere) and free of moisture is about 9.5 m3. Using the empiricalformula for the feedstock, discussed previously, and Equation 2, the following re-lationship can be developed: .

_

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TABLE 5Experimental results obtained from a pilot study with procedures to estimate the BVS fraction

of VS using the measured biogas volume*

* The loading rate and biogas production reported are based on a 30 day retention time.t The volume of dry biogas at standard temperature and pressure is computed using the following

expression: ~p= ~273/273+7)[(P-~,)/P].I p = [( 16 x CH4)1100+(44 x C02/100)]/22.413, g I ‘.§ Computed from Equation 3 using the variables in Equation 6.II Computed from Equation 5.1 Percent of input VS loading rate (10.1/17.8 x 100).

C~Hy~O~ N +14 H~O -~ 24 CH4 + 21 CO, + NH4 + HCO- 3 [6]

Using the variables expressed in Equation 6 and Equation 3, the correction factor, f’,for the feedstock used in this study was found to be 0.82. These values are then usedto compute the BVS mass. The computed BVS mass is then compared to the originalinput VS mass for computation of actual biodegradability. The results of the step-by-step computation procedures described above are shown in Table 5.

6.2 Comparison of BVS values

The estimates of the BVS, as a percentage of VS, of a typical organic fraction of MSWbased on long-term batch studies, lignin content, and a pilot study are compared inFig. 3. As can be seen, the average BVS fractions calculated using either the lignincontent or the long-term batch study are essentially the same. The BVS fraction obtainedfrom the pilot study, using a complete mix reactor and mass retention time (MRT) of30 days, is approximately 83% and 81% of the estimated values obtained from lignincontent or the batch study, respectively. Similarly, Richards et al. ( 1991 a) were onlyable to remove 83% of the BVS of sorghum, using a thermophilic, high rate, high-solids reactor with a MRT of 45 days.The values of the biodegradable fraction for various organic substrates using different

methods are compared in Table 6. It is important to note that the estimated BVS valuesbased on field operations are normally less than 83% of the values estimated bychemostat studies. Use of BMP values, for example, would predict a larger biogasproduction rate than is realistic in actual practice. Using Equation 4 and the operation

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Fig. 3. Comparison of the biodegradable volatile solids, (BVS) fraction of a typical mixerd biodegradableorganic fraction of municipal solid waste (BOF/MSW), obtained using three methods of analysis.

TABLE 6Biodegradability of various organic substrates using batch digestion, chemostat, and field

operation methods*

* All field operations were conducted in a thermophilic, high-rate, high-solids reactor.data for field operations were obtained from Richards et al. (1991b).number in parentheses are nominal mass retention times, in days.

data reported in Table 5, it can be estimated that the maximum achievable dry biogasproduction rate, under standard conditions, for BOF/MSW is about 13 m3 day-’ usingthis pilot unit. This volume of gas production would not be possible at MRTs of about30 days. If the retention time is extended to nearly 60 days, it may be possible toremove up to 90% of a material’s ultimate biodegradability. It is clear, however, thatcomplete removal of BVS as estimated from batch digestion and analytical or chemostatmethods at a practical range of MRT (20-40) is not possible. For an in-vessel, high-solids, anaerobic digestion process, the BVS can be defined as that fraction of VS thatcan be biodegraded under optimum environmental and nutritional conditions in a

period of 20-40 days. ,

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6.3 Impact of BVS on process design and performanceThe common use of TS or VS in describing the organic loading rate and feedstockC/N ratio, and in estimating gas production from the BOF/MSW is misleading, assome of the organic compounds are low in biodegradability (Tables 3 & 4). Use ofBVS allows meaningful comparisons to be made between the different anaerobic

digestion processes reported in the literature. The impact of BVS on the organic loadingrate, (2) the computation of C/N ratio, and (3) the prediction of biogas production areconsidered below.

6.3.1 Impact of BVS on organic loading rateTraditionally, loading rates for biological processes have been based on the VS contentof the substrate feedstock. When handling substrates with variable biodegradabilities,estimations of the methane production potential and digester performance can not bemade accurately using a VS loading rate. Loading rates based on the BVS fraction ofthe substrate should be used when estimating the conversion of organic wastes tomethane. The BVS organic loading rate can be expressed as the input BVS mass peractive reactor mass per day. The BVS mass of an organic substrate, can be computedusing total inflow VS mass multiplied by the BVS fraction of volatile solids.

6.3.2 Impact of BVS on feedstock CIN ratioCustomarily, the C/N ratio is determined using the total dry mass of the organic matterand the corresponding percentage concentrations of carbon and nitrogen. This methodof determining the C/N ratio may not be appropriate for the BOF/MSW because notall of the organic carbon is biodegradable and/or available for biological decomposition.However, it appears that almost all of the nitrogen in the organic material is availablefor conversion to ammonia via microbial metabolism (Mindermann 1993). As theavailable nitrogen in the organic feedstock can be converted to ammonia, the C/N ratioshould be based on the nitrogen content of the total organic mass and the carboncontent of the BVS mass (Kayhanian & Tchobanoglous 1992)

6.3.3 Impact of BVS on prediction of gas production ’

Accurate biogas volume estimates can be obtained using the BVS mass, rather thanTS or VS mass. Based on the results of this study, the maximum volume of biogas thatcan be produced using a high-solids, anaerobic digestion process can be estimated fromthe BVS organic loading rate using Equation 4. The BVS mass used in Equation 4 isbased on the value of the biodegradability as estimated from a pilot study. However,when the BVS mass is computed using the lignin content or the results of long-termbatch studies, a correction factor of approximately 0.82 must be used.

7. Summary and conclusions

The issue of biodegradability is a broad one. Biodegradability cannot be generalizedfor all substrates and biological transformation processes. The biodegradability of asubstrate of interest must be determined under the conditions which apply to theprocess to be used. In this paper, the biodegradability of the organic fraction of MSWwas assessed as it affects the design and performance of an in-vessel, high-solids,anaerobic digestion process.

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The biodegradability of BOF/MSW can be determined by three methods, namelylong-term batch studies, measurement of lignin content and chemostat studies. Basedon the results reported on in this paper, the estimated biodegradability of BOF/MSWwas similar using the results of batch tests or lignin content. Compared to lignin contentand batch studies, only 82% of the biodegradable mass was removed during a pilotinvestigation. Also, the percentage of BVS removed during field operation is normallyless than 82% of the value estimated by chemostat studies. While the three methodsmentioned above determine the ultimate biodegradability of BOF/MSW, only fieldoperation will determine the practical biodegradability.Where energy recovery is an important consideration, the amount of gas to be

expected should be computed from the BVS as estimated using field observations, ifpossible. If field observations of BOF/MSW are not possible, it must be assumed thatonly 83% of the ultimate biodegradability is available for biotransformation in a high-solids, anaerobic digestion process. In addition, the estimated BVS mass has an impacton the computation of organic loading rate and feedstock’s C/N ratio, which, in turn,affects digester design and performance.

Acknowledgments

The work reported upon in this paper was supported by a grant from the CaliforniaEnergy Commission (CEC) and Prison Industry Authority (PIA) of the State ofCalifornia. The assistance of Sharla Hardy and Karl Lindenauer for daily operationand laboratory tests is acknowledged gratefully. The author is indebted to ProfessorGeorge Tchobanoglous for his valuable suggestions in the initial preparation of thismanuscript. The valuable comments made by the reviewers are also appreciated.

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