BIOGASIFICATION OF MUNICIPAL SOLID WASTES

L. F. DIAZ, G. M. SAVAGE, G. J. TREZEK, and C. G. GOLUEKE Cal Recovery Systems, Inc.

Richmond, California

ABSTRACT

A series of experiments on the anaerobic diges­tion of the organic fraction of municipal refuse was performed. The refuse fraction used in the study was one of the portions segregated in a resource recovery system developed at the University of California, Berkeley.

The scale of experiments includes 4, 9, and 1600 I digesters. The refuse used as feed was en­riched by the addition of raw sewage sludge in various ratios, i.e., from 0-100 percent of the total volatile solids. No other sources of nutrients or chemicals for pH control were introduced into the reactors.

Organic loading rates ranged from 1.1-6.4 g of volatile solids/l·d. Typical hydraulic detention times were 15 and 30 days. Temperatures were kept within the range of 72-104 F (22-40 C). Digestion efficiency was based on energy conver­sion and gas production.

INTRODUCTION-STATE OF THE ART,

PAST AND PRESENT

Increasingly large energy demands and a de­creasing availability of fossil fuels are two very serious problems that confront not only the United States but also practically every country in the world. Hence, the necessity of efficiently utilizing all existing sources of energy as well as developing new ones is now more apparent than ever. A source hitherto largely untapped is in the form of organic

wastes, of which municipal solid waste (MSW) is an important example. Municipal solid wastes con­tain an abundance of chemical energy in the form of carbonaceous components, which if properly processed can provide a new energy source. Indeed, several processes are available for recovering this energy. These processes, which range from direct combustion to hydrolysis, are in varying degrees of development at present. One such process involves the biological conversion of the organic fraction of MSW into methane, carbon dioxide, and a relative­ly stable residue through anaerobic fermentation. In sewage treatment the process generally is known as "anaerobic digestion". In the more popular literature, it has become increasingly referred to as "biogasification" .

Large-scale biogasification has long been ex­tensively practiced in the treatment of wastewater solids. As of the present, with one exception bio­gaSification of MSW has been carried out only on a laboratory and a pilot scale. The exception is the demonstration project presently underway at Pompano Beach, Florida.

The first attempt to dispose of garbage through a wastewater treatment plant took place in Lebanon, PA, in 1923 [1]. However, little or no attention was paid to such an approach at that time, and it was not until the 1930's and 1940's that the feasibility of digesting the organic fraction of MSW received serious attention. The occasion for the attention was the introduction of the home garbage grinder [2-4] . The studies were confined to the digestion of the garbage fraction (food

403

wastes) of urban wastes, inasmuch as that is the only fraction entering the sewers. The main con­cern was the effect of garbage on the anaerobic digestion process, and hence on the performance of the existing treatment facilities.

bIle of the first studies to go beyond the gar­bage fraction and to include the entire organic con­tent of muniCipal solid waste was carried out at the Sanitary Engineering Research Laboratory (SERL) of the University of California -(Berkeley) as part of an extensive pioneering investigation on solid waste management [5-8] . The emphasis in the University's studies was not so much on energy production as on the reduction and stabilization of the organic fraction in urban solid waste via anaerobic fermentation. At the time, energy short­ages did not constitute an apparent problem in the United States; whereas the solid waste problem had begun to command serious attention. The SERL digestion studies led to the conclusion that with the exception of wood and to some extent, of printed newspaper, the organic components (Le., of biological origin) of municipal solid waste can be readily digested, and that at an organic loading rate of about 1.12 g volatile solids/ l-d (VS/lrd), a detention time of 30 days, and a temperature of 95 F (35 C), approximately 0.4-0.6 I of gas per gram of volatile solids intro­duced (l/g VS) can be expected.

In the early 1970's Pfeffer conducted investiga­tions to evaluate the feasibility of digesting mix­tures of sewage sludge with the entire organic fraction ofMSW [9-10]. Pfeffer observed that the total gas production fluctuated from 0.12-0.45 l/g VS added. The variation in gas production was a function of loading rate, temperature, and deten­tion time. Other experiments were conducted by

Klass and Ghosh [11-12] , as well as by the research team of Dynatech R&D Corp [13] . The studies by Klass and Ghosh were centered on the digesti­bility of sludge when mixed with the organic frac­tion of refuse. The researchers reported gas pro­ductions equivalent to 10-20 ft3 (280-570 1) of methane per person per day. On the basis of labora­tory experiments, the Dynatech research team came to the conclusion that biogasification was both technically and economically feasible.

The commercial feasibility of digesting the or­ganic fraction of refuse presently is being evaluated at Pompano Beach, Florida. The demonstration project is commonly referred to by the acronym "Ref Com" (Refuse Conversion to Methane). In the project, Waste Management Inc., under contract

with the Department of Energy, is to evaluate the technology and economics involved in converting up to 100 tons (90 t) of refuse per day into methane gas.

Aware of the pressing need to increase the gas yield as well as to lower production costs, Gossett and McCarty sought for suitable ways of enhancing the anaerobic biodegradability of refuse [14]. In

their search, they found that the biodegradability of digested refuse could be almost doubled when the material was exposed to a temperature of 271 F (133 C) and a pH of lover a 3 hr period. A likely drawback to such a heat treatment is that the addi­tional expenditure of energy required to attain the high temperatures may exceed the amount of energy resulting from the' enhancement .in gas pro­duction.

An important feature of all of the proceeding studies was the use of a feedstock that was essen­tially a mixture of MSW and sewage sludge in which the entire organic fraction of the MSW was includ­ed. As such, the mixture contained all the types of paper, textiles, and plastics present in urban wastes. In addition, most of the investigators found it necessary to add nutrients and to buffer for pH control.

EXPERIMENTATION

GENERALITIES

CONCEPT, RA TlONALE, AND OBJECTIVE

Despite the work done thus far, many questions remain to be answered. Subjects that could profit from further investigation are those related to the development of new approaches that would lead to a lowering of the cost of the digestion process, to an increase in its efficiency,-and to an improve­ment in its reliability. With this need in mind, a new approach was proposed by the authors which in essence was based on anaerobic digestion con­stituting a unit process in a waste treatment facility. Since in such a circumstance the refuse feed (substrate) would differ drastically from that used in previous research by others, the need for a detailed investigation seemed to be indicated, and accordingly, one was initiated. The authors con: fined their study to municipal wastes because.slich wastes are the ones from which the potential of resource recovery is greatest. Moreover, they were the wastes handled by the University's resource recovery system [15-17] . The study presented herein is based upon two basic premises: 1. the materials typically found in urban wastes have a

404

hierarchy of values; and 2. the organic fraction of

MSW consists primarily of pa�r products. These

premises allow for the integration of various unit

processes such that valuable materials (e.g. ferrous

metals and paper) can be recovered for reuse. Furthermore, each process can be selected such

that the residue from one becomes the input to a

second process. The latter practice of course has technical and economical limitations.

The major objective was to reduce the quantity

of waste destined to be landfilled, especially of

those wastes potentially or actually detrimental to public well-being and to the environment.

FEEDSTOCK

The air classified light fraction of MSW consists mostly of paper. As such this portion of the waste

stream contains a relatively high concentration of

carbohydrates, mainly in the form of cellulose.

This fraction may be as much as 90 percent of the

dry weight. The cellulose may be free or combined

with lignin and other binders.

About 40-50 percent of the organic fraction of MSW is digested in normal anaerobic digestion; the remainder becomes a residue. The digestibility of the cellulosic and of other materials is indicated by the data in Table 1. According to the table, kraft paper and garbage are easily digested while wood and newspaper are digested only with difficulty. The poor digestibility stems from the fact that al­though free cellulose may be almost completely digestible, cellulose associated with a ligno-cellu­

losic complex is not. lignin is practically inert to

conventional anaerobic digestion. The fact that a sizeable amount of the organic

material in urban refuse does not respond to nor­

mal digestion has three very undesirable effects on

TABLE 1 SOLIDS REDUCTION OF DIGESTION MIXTURES CONTAINING SOLID WASTE

COMPONENTSc

C�onent Ad Cled

GarbCIIge

Krctft Paper

�wspaper

Wood

Sewage Sludge

C�onent in Sludge (Percent)

100

60

30

60

Volat lle Solids

Reduction (Percent)

66.2

76.9

58.0

•

64.0

Gas Product ionb

(1/9 VS)

0.55

0.57

0.47

0.27

0.60

ill Rapid subsidence of wood in digestion mixtures interferes with sa"" 1 i n9

b 30 day detention time, 37wC, 1.249 VSfl-d c

Reference 4

the utility of the process. The effects are: 1. a need

for a large digester volume; 2. a need for intensive

mixing; and 3. the production of a sizeable quan­

tity of material (residue) that must be dewatered

and disposed. The treatment required for the

residue can have a tremendous impact on the economics of the process.

To escape the problems named in the preceding paragraph, the biogasification study described herein was centered on the utilization of the "residues" that remained after higher-valued ma­

terials had been removed. This residue has virtually

no value as a solid fuel and is highly digestible.

SPEE:I F ICS

MATERIALS AND METHODS

The conduct of the experimental part of the re­search was divided into two phases, namely,

laboratory scale and pilot plant scale. Only the

laboratory-scale experiments are reported in the

present paper. Information not presented herein

can be found in reference [ 16] . Four and nine-L glass vessels were used as diges­

ters. Each digester was sealed with a two-hole rub­

ber stopper. A gas outlet line was passed through one of the two holes, and a thermometer was in­

serted through the other. The gas line consisted of � in. (6.35 mm) inner diameter (ID) Tygon tubing.

One of the 9-1 digesters was especially designed for the digestion of refuse. It was constructed of plexi-

FIG.1 INCUBATOR CONTAINING 9-L DIGESTERS

405

glass and was equipped with a mechanically sealed stirrer driven by a variable speed doc motor. The stirrer was arranged such that duration and fre­quency of mixing of the digester contents could be controlled either manually or automatical­ly. The design of the 9-1 plexiglass digester is shown in Fig. 1.

Tygon tubing � in. (6.35 rom) ID was used to connect gas outlets with the gas collectors. The gas collectors consisted of glass bottles sealed with rubber stoppers and filled with a 2 percent sulfuric acid solution to which a few drops of a pH indicator solution had been added. The solution was made acidic to prevent absorption of CO2 by the liquid. As gas formed in the digester it entered the col­lector vessel and displaced the acid solution through an outlet glass tube into a calibrated con­tainer. The volume of the displaced gas was equi­valent to that of the gas generated in the digester. The volume of gas generated was normalized to standard temperature and pressure.

The digesters were placed in incubators to pro­vide a constant temperature environment for the digester contents.

PARAMETERS

Parameters used in the research were total solids (TS) and volatile solids (VS) concentration, pH level, volatile acids concentration, volume and

•

composition of gas produced, and heat of com-bustion of the digester inputs and outputs. Since the research was concerned with energy production, gas composition and energy conversion efficiencies were necessary parameters for process evaluation.

The TS and VS were measured as described in Standard Methods [18] . The hydrogen ion con­centration was determined with a pH meter while the concentration of volatile acids was determined

•

by liquid chromatography. The composition of the gas was analyzed by means of gas chromatography and according to Standard Methods [18]. The heating value of the solids in the feed as well as of those in the digested sludge was obtained with an oxygen bomb calorimeter.

FEED CHARACTERISTICS

Refuse Feed

The refuse fed to th,e digesters was a highly organic fraction of municipal solid waste segregated in a dry separation system developed at the Uni­versity of California (Berkeley). A combined schematic and flow diagram of the process is pre-

406

sented in Fig. 2. In the system, the refuse is size­reduced by means of a 10 tons/hr (9 t/h) Gruendler hammermill and then air-classified by means of a vertical air classifier. The air classifier separates the incoming stream into two fractions, the "lights" and the "heavies". The lights are composed of mainly paper and plastic wastes; while the heavies consist mostly of metals, glass, rocks, and dirt. The lights are passed through a trommel screen that has '" in. (13 mm) openings. Materials retained on the screen can be used as refuse derived fuel (RDF) or can be processed through a fiber recovery system. Those that pass through the screen are considered as being rejects. The rejects (i.e., the "light" re­jects), after further processing, constituted the feed to the digesters. The processing involved the segregation of the organic from the inorganic fractions in a manner that would require a mini­mal investmen t of energy, and if expanded to a full-scale operation would entail the least possible monetary expenditure.

CYCLONE

• 5. " . GRINDER

FIBER PRIMARY TROMMEL

FAACTI ON

LIGHT R[JECTS

MAG BELT I I SECONDAR'I' TROMMEL DOGESTI BLES

�A"IEs" TROMMEL LANDFI

HEAVIES (GLASS! STONE" DIGESTt8l.ES

FIG.2 DRY FRONT-END SYSTEM

100 ,-----------------------= ---, 8

6

4

II: Z -

'"

� 2 '­... z '" FEED RATE MOISTURE � 10 o 1.5 TPH 32 'Y. '" o. 8 o 2.3TPH 27% '" > 6 -

... :5 4

2

I�--������--�--���� 0.01 0.1 1.0 IN.

, ,

0.5 l.0 SCREEN SIZE

, ,

5.0 10.0 ,

FIG.3 SIZE DISTRIBUTION OF SAMPLES FROM LIGHT SCREEN REJECTS

The size distribution of the light rejects when the feed rates were 1.7 and 2.5 tons/hr (1.5 and 2.3 t/h) to the dry separation system is shown in Fig. 3. Particle size is important because, as other researchers have demonstrated, gas production rate and gas yield increase with decrease in particle size of the digester feed [12] .

The light rejects were subjected to a series of screenings in order to fmd a significant reject frac­tion that would have a high volatile to total solids ratio. As such, when used as digester feed, it would present a high concentration of digestible material and a low concentration of inert substances.

In Fig. 4 is shown the composition of the light rejects at the various size ranges obtained in the screening study. From the figure, one can readily see that the major portion of the fibrous materials was within the range 3/8 in. (9.5 mm) to 7-mesh; whereas the lint, grit, and dirt were concentrated· at sizes less than 7-mesh, (minus 7 mesh).

f­Z W U a:: w a..

46 .-----------------------------, BASED ON 100 LB. (DRY WT.l 4 MESH

SCREEN REJECTS

40 "" VOLATILE SOLIDS

30

20

10

0'----7 MESH

LUMINUM

PLA STIC

OTHER

, GLASS, WOOO"f TC

14 MESH

HER

GLASS ROCK

PASSING 14 MESH

FIG.4 COMPONENTS 4-MESH REJECTS­RESIDENTIAL REFUSE

From the data in Fig. 4 it is also apparent that the material retained by the 7 -mesh screen had a large volatile solid fraction. The results of the volatile and total solids determinations demon­strated that the plus 7-mesh light rejects had a higher volatile to total solids ratio than did rejects from other combinations. Consequently, the light refuse fraction that passed the 7l in. (12.7 mm)

screen and was retained by the 7-mesh screen was chosen as the digester feed. This material was ap­proximately 6.5 percent (by weight) of the in­coming refuse.

In addition to determining size distribution and composition of the 7l in. (12.7 rom), plus 7-mesh light rejects, an analysis was made of their total carbon, total nitrogen, and heavy metals content. The total carbon and total nitrogen analyses were carried out with the use of the Dumas and Pregl methods. The heavy metal concentrations were determined by atomic absorption [18]. The results of the analyses are shown in Table 2.

TABLE 2 CHEMICAL ANALYSES OF FEED --------------------------�--------

Materia 1 Carbond Ni trogena

Refuse 37.6 2.3

Sludge 31.8 2.6

• . t Percent dry welgh b

percent wet weight c .

Percent of total SOllds

Sludge Feed

SulfuP C/N

0.08 16.S

<0.1 12.1

86. 9

4.8

73.8

3.3

The primary (raw) sludge, also used as feed, was obtained from the City of Richmond, California, Water Pollution Control Plant. Six experimental

•

digesters were used in the study involving refuse. Two were 9-1, and four were 4-1 digesters. All the digester cultures were kept at 35 C. A 30 day re­tention time was maintained for hydraulic loading considerations. Three digesters were established as controls and received nothing but sludge during the entire experimental period.

PROCEDURES

The first set of experiments were conducted with the use of the 9-1 digesters. Only sewage sludge was used as feed during the first 12 days following initiation of the cultures to allow them to become acclimated to the reactors. Following the 12 day period of 100 percent sludge digestion, the concentration of refuse in one of the digesters was gradually increased from 5 percent* of the total volatile solids loading to 20 percent during the next 12 days. The 14-day data period for the 20 percent loading was begun immediately there­after. The refuse loading was subsequently increas-

'Henceforth, whenever percent of refuse or sludge loading

is used, it should be und erstood that the percentage re­

fers to total volatile solids.

407

ed to 30 percent and was continued at this rate over the following 22 day period. The same ex­perimental procedure was followed when the amount of refuse in the feed was increased to 40, 60, 80, and finally to 100 percent. The entire ex­periment was continued for a 24 month period.

No nutrients or buffering solutions were added to the digester cultures. The digesters were mixed before and after feeding. The withdrawn digested sludge was checked weekly for volatile solids and total solids content and pH. Gas volumes and di­gester temperatures were recorded immediately after the first mixing and prior to feeding. Gas analysis of the digester off-gas was performed periodically.

The second set of refuse digestion experiments involved two 4-1 digesters. The organic loading rate was 1.12 g VS/l· d and the detention time was maintained at 30 days. One reactor was fed only raw sludge, and was used as a control. The second reactor was fed mixtures of refuse and sludge ranging from 30 to 100 percent refuse. The second set of experiments was designed to verify the re­sults obtained with the 91 digesters. The pro­cedures and the data collection were the same as those described for the first set.

In the third series of experiments, the two remaining 4-1 digesters were utilized. The third set of experiments was carried out in the same man­ner as with the first two sets except that the load­ing rate was increased to 1.6 g VS/l· d, and the de: tention time shortened to 15 days. Refuse com­posed 40, 60, and 100 percent of the volatile solids in the feed. The main objective of the third set of experiments was to determine whether or not the digester would function at 100 percent refuse loading.

A fourth set of experiments was designed to ascertain the maximum loading rate that may be applied to a refuse digester without incurring a significant loss in efficiency. A 4-1 digester was used in the study. The temperature of the culture wa's maintained at 95 F (35 C) and the hydraulic detention time at 15 days. Refuse accounted for approximately 80 percent of the volatile soli<;ls in­troduced into the digester, while raw sewage sludge made up the remaining 20 percent. The combina­tion of 80 percent refuse and 20 percent sludge was maintained constant throughout the experi­ment. The organic loading rate was initially kept at. 3.2 g VS/I· d. Subsequently it was changed to 4.8 g VS/I· d and finally was increased to 6.4 g VS/I· d.

The digested sludge was analyzed periodically

408

for pH, volatile solids, total solids, and volatile acids. Daily gas production was recorded. The gas composition was checked weekly.

RESULTS

Typical results for the first two sets of experi­ments are listed in Table 3, and of the third set, in Table 4.

The data in Table 4 show that the optimum refuse loading for gas production was between 60 and 100 percent. The solids destruction that took place in the refuse digester was relatively high. Volatile solids destruction by the refuse digester increased from 53 percent at 40 percent refuse loading to 63 percent at 100 percent. Similarly, the TS destruction increased from 42 percent at 40 percent refuse loading to 63 percent at 100 per­cent. The pH of the cultures generally was slightly less than 7.0. The gas composition at the 40, 60, and 100 percent refuse loadings is also summarized in Table 4. The data indicate that at an organic loading rate of 1.6 g VS/I· d and a detention time of 15 days, the concentration of methane in the digester gas apparently was independent of refuse loading. The gas produced by the refuse digester consistently contained approximately 57 percent methane, 30 percent carbon dioxide, and 13 per­cent nitrogen, by volume.

TABLE 3 TYPICAL PERFORMANCE OF 9·LlTER DIGESTERS

-- Feed --- TS VS Refuse Sludge Reduction Reduction pH

(S) (S) (S)

20 eo 33

JJ 70 28

40 60 42

60 40 48

80 20 13

100 73

46

44

16

61

61

77

7,0

7.3

6.93

7.0

6.94

6.81

Gas PrOduction CH4

(1/9 VS .dded) (S)

0.483 61

0.473 16

0.410 60

0.111 60

0.471 17

0.339 12

TABLE 4 PERFORMANCE OF 4·LlTER DIGESTERS·

(ORGANIC LOADING RATE AT 1.6 G VS/L-DAY)

---- Feed - --- 15 VS pH Gas Product ion CH4 Refuse Sludge Reduction Reduction

(S) (S) (S) (1/9 VS.dded) (S)

40 60

60 40

100 0

42

46

63

13

61

63

6.8

6.8

6.6

• detention time _ 15 days, temperature • 35�C

0.20

0.27

0.24

17

17

17

The experiment showed that the refuse fraction used as feed could be digested at sizeable solids reduction, and would yield relatively high gas production.

The efficiency of anaerobic digestion of refuse­sludge mixtures was considered from the viewpoint of energy conversion since the degree to which anaerobic digestion can transform the energy con­tained in the feed into a more useful form, i.e., methane, is a measure of process performance.

The high heating value (HHV) of the components of the digester feed was determined by means of an oxygen bomb calorimeter. The energy conversion effiCiency was calculated by dividing the amount of energy contained in the methane by the amount of energy in the feed. The results of the energy conversion study are summarized in Fig. 5. As

shown in the figure, at 30 percent refuse loading, . the efficiency was approximately 37 percent. When the percentage of refuse in the feed was increased to 80 percent, the effiCiency rose to 42 percent. Finally, when only refuse was fed to the digester (100 percent refuse loading), the efficiency drop­ped to 26.4 percent. The data show that when refuse constituted 80 percent of the volatile solids in the feed, maximum energy conversion effiCiency was attained.

The results of the fourth set of experiments are given in Table 5. They show that the maximum organic loading rate was about 4.8 g VS/I' d. The trial at the loading rate of 6.4 g VS/I' d was begun immediately at end of the 4.8 g VS/l' d run. In the run, total gas production did not continue to rise

50.--------------------------------,

40

-

� -

30 >-u z w -

u

u. 20

u. w

10

• •

O.L.R . • 1.129 VS/I T • 350 C

t • 30 days

oL---J......----L.----::l:-----:'':--� w � 60 80 100

REFUSE IN FEED (%)

FIG.5 ENERGY CONVERSION EFFI CIENCY OF

9-LlTER DIGESTER AT VARIOUS REFUSE LOADINGS

TABLE 5

O.loR.

(gVS/I-dl

3.2

4.8

6.4

0.5

0.375

'" UJ '" Cl < <n > '"

--'

0.25 z 0 -

>-u ::::> '" 0 0: "-

<n < '"

0.125

0.0

PERFORMANCE OF 4-LlTER DIGESTERS

AT HIGH LOADING RATES

TS VS pH Reduction Reduction

(S) (S)

72 75 6.6

73 78 6.6

43 46 5.2

A"g. Gas Production

(1/g VS)

0.29

0.39

0.045

O.L.R . • 4.8

50

65

50

O.L.R . • 3.2

2 4

% REFUSE = 80 t =15DAYS T = 350 C

O.L.R . • 6.4

6 8 10 12 14

TIME - DAYS

FIG.6 DAILY GAS PRODUCTION FOR VARIOUS

LOADING RATES

to a relatively high value. Instead, after the third day of operation, gas production rapidly declined and reached a low of 0.028 l/g VS by day 7. From day 7 to day 20, the gas production remained lower than 0.0281/g VS. The daily gas productions at organic loadings of 3.2, 4.8, and 6.4 g VS/I' d are given in Fig. 6.

SUMMARY OF EXPERIMENTAL FINDINGS

The results of the study demonstrated that the organic fraction segregated from municipal solid waste by the sorting system developed at the Uni-

409

versity of California can be digested anaerobically. The results also show that the digestion process can be carried out without the addition of nutri­ents or buffers. At low organic loading rates, i.e., 1.12 g VS/l- d, and long detention times (i.e., 30 days) the total gas production was found to be independent of refuse loading. Over-all average gas production by the 4 and 9-1 digesters at a loading rate of approximately 1.12 g VS/l- d was about 0.42 l/g VS added, while the gas composition was approximately 57 percent CH4, and 32 percent CO2, and 11 percent N2. The pH did not vary con­siderably from 7.0. In general, the solids destruc­tion increased as the refuse loading was increased. At an 80 percent refuse loading, the TS destruction was 53 percent and the VS destruction was 65 per­cent. When the organic loading rate was set at 1.6 VS/l- d and the hydraulic detention time shortened from 30 to 15 days, the average total gas production declined to about 0.31 l/g VS added. The gas composition was surprisingly similar to that at the l.12 g/l- d organic loading rate. The average methane concentration was 57 percent, while those of CO2 and of N2 were 30 percent and 13 percent respectively. The pH fluctuated be­tween 6.5 and 6.8. At 100 percent refuse loading, the volatile solids destruction was 62.7 percent; and the total solids, 45.5 percent.

At a temperature of 95 F (35 C) and a hy­draulic detention time of 15 days, the maximum organic loading rate seemed to be approximately 6.4 g VS/ l·d.

In conclusion, it can be stated that the results of the study described herein demonstrate that the organic rejects from air classification not only can be digested, but can be done so efficiently and readily. Thus, instead of adding to the amount of the residues that must be landfilled, this Significant fraction of the municipal waste stream is put to good use. The overall effect of successfully digest­ing the organic rejects, therefore, would be to broaden the applicability of anaerobic digestion in resource recovery.

REFERENCES

(1) Fox, C. R., and W. S. Davis, "New Process of Garbage Disposal at Lebanon, Pa.," Engineering News­

Record, Vol. 82, May 1924, p. 857. (2) Reefer, C. E., and Kratz, H., "The Quantity of

Garbage that can be Digested with Sewage Sludge,"

Sewage Works Journal, Vol. 6, Mar. 1934, p. 250. (3) Bloodgood, D. E., "Digestion of Garbage with

Sewage Sludge," Sewage Works Journal, Vol. 8, Jan. 1936, p. 3.

(4) Wylie, G. F., "A Year's Experience in Digestion of Sewage and Garbage Solids," Sewage Works Journal,

Vol. 12, Aug. 1940, p. 760. (5) Golueke, C. G., and McGauhey, P. H., Compre­

hensive Studies of Solid Wastes Management, 2nd Annual SERL Report No. 69-1, Sanitary Engineering Research Laboratory, University of California, Berkeley, 1969.

(6) Chan, D. B., and E. A. Pearson, Comprehensive

Studies of Solid Wastes Management: Hydrolysis Rate of

Cellulose in Anaerobic Fermentation. SER L Report No. 70-3, Sanitary Engineering Research Laboratory, Uni­versity of California, Berkeley, 1970.

(7) McFarland, J. M. et al., Comprehensive Studies

of Solid Wastes Management, Final Report, SERI Report No. 72-3, Sanitary Engineering Research Laboratory, University of Californ ia, Berkeley, 1972.

(8) Klein, S. A., "Anaerobic Digestion of Solid Wastes," Compost Science, Vol. 13, No. 1, Jan.-Feb. 1971, pp. 6-11.

(9) Pfeffer, J. T., "Reclamation of Energy from Organic Refuse," Solid Waste Program, EPA Grant No. EPA-P-800776, Dept. of Civil Eng., Univ. of Illinois, Urbana, Illinois, April 1973.

(10) Pfeffer, J. T., "Anaerobic Processing of Organic Refuse," in Proceedin{Jf, Bioconversion Energy Research

Conference, Institute for Man and His Environment, Uni­versity of Massachusetts, Amherst, Mass. 1973.

(11) Klass, D. l. and Ghosh, S:, "Fuel Gas from Organic Wastes," Chemtechnology, Vol. 3, Nov. 1973, pp. 689-698.

(12) Ghosh, S., and Klass, D. l., "Conversion of Urban Refuse to Substitute Natural Gas by the Biogas Process," presented at Fourth Mineral Waste Utilization

Symposium, Chicago, Illinois, 1974. (13) "Fuel Gas Production from Solid Waste, 1974

Semi-Annual Progress Report," NSF Contract C-827, Dynatech Report Number 1151, 1974.

(14) Gossett, J. M., and McCarty, P. l., "Heat Treat­ment of Refuse for Increasing Anaerobic Biodegradability," NSF Grant GI-43504, Dept. of Civil Engineering, Stanford University, 1974.

(15) Diaz, l. F., Kurz, F., and Trezek, G. J., "Methane Gas Production as Part of a Refuse Recycling System," Compost Science, Vol. 15, No. 3, Summer 1974, pp. 7-13.

(16) Diaz, l. F., "Energy Recovery through Bio­gasification of Municipal Solid Wastes and Utilization of Thermal Wastes from an Energy-Urban-Agro-Waste Com­plex," Doctoral Dissertation, University of California, Berkeley, 1976,242 pp.

(17) Diaz, l. F., and Trezek, G. J., "Biogasification of a Selected Fraction of Municipal Solid Wastes," Compost Science, Vol. 18, No. 2, Mar.-Apr. 1977, pp. 8-13.

(18) Taras, M. J., et ai, eds., Standard Methods for

the Examination of Water and Wastewater, 13th ed., American Public Health Association, 1971.

Key Words: Biological, Digestion, Energy, Research

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