thermophilic anaerobic digestion of solid waste for fuel gas production

17
BIOTECHNOLOGY AND BIOENGINEERING, VOL. XVII, PAGES 1119-1135 (1975) Thermophilic Anaerobic Digestion of Solid Waste for Fuel Gas Production CHARLES L. COONEY , Department of Sutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 0.21 39, and DONALD L. WISE, Dynatech R I D Company, Cambridge, Massachusetts Summary Anaerobic digestion offers a potential means of converting organic solid waste into fuel gas and thereby provide a supplemental and readily utilizable source of energy. We are particularly interested in the use of thermophilic digestion over a mesophilic operation for it can achieve higher rates of digestion, greater con- version of waste organics to gas, faster solid-liquid separation, and minimization of bacterial and viral pathogen accumulation. Our results comparing meso- philic (37°C) and thermophilic (65°C) anaerobic digestion of domestic solid waste confirm the increased rate and conversion of waste to methane. In addi- tion, utilizing radioactive labeling of glucose and acetic acid, we have measured the volumetric rates of volatile acid production and disappearance under both mesophilic and thermophilic conditions. INTRODUCTION Although methane gas production from organic materials has been known for many decades, only recently has it been considered as a viable means of converting waste organic material into a readily utilizable fuel.lJ The early work on anaerobic digestion focused on its use as a means of waste treatment for the disposal of organic material.3 Its attractiveness was based on the ability of micro- organisms to reduce the amount of both solid and liquid waste by its conversion to a gas (CH, + COz), which was readily disposed of by venting and/or burning. The net result was a significant decrease in the amount of waste and the biological stabilization of waste prior to final disposal. Two factors which have limited the greater application of anaerobic digestion are relatively long digester resi- dence time (i.e., greater than 15 days) and system instability to shock loadings. 1119 @ 1975 by John Wiley & Sons, Inc.

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Page 1: Thermophilic Anaerobic Digestion of Solid Waste for Fuel Gas Production

BIOTECHNOLOGY AND BIOENGINEERING, VOL. XVII, PAGES 1119-1135 (1975)

Thermophilic Anaerobic Digestion of Solid Waste for Fuel Gas Production

CHARLES L. COONEY , Department of Sutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 0.21 39, and DONALD L. WISE, Dynatech R I D Company, Cambridge,

Massachusetts

Summary Anaerobic digestion offers a potential means of converting organic solid waste

into fuel gas and thereby provide a supplemental and readily utilizable source of energy. We are particularly interested in the use of thermophilic digestion over a mesophilic operation for it can achieve higher rates of digestion, greater con- version of waste organics to gas, faster solid-liquid separation, and minimization of bacterial and viral pathogen accumulation. Our results comparing meso- philic (37°C) and thermophilic (65°C) anaerobic digestion of domestic solid waste confirm the increased rate and conversion of waste to methane. In addi- tion, utilizing radioactive labeling of glucose and acetic acid, we have measured the volumetric rates of volatile acid production and disappearance under both mesophilic and thermophilic conditions.

INTRODUCTION

Although methane gas production from organic materials has been known for many decades, only recently has it been considered as a viable means of converting waste organic material into a readily utilizable fuel.lJ The early work on anaerobic digestion focused on its use as a means of waste treatment for the disposal of organic material.3 Its attractiveness was based on the ability of micro- organisms to reduce the amount of both solid and liquid waste by its conversion to a gas (CH, + C O z ) , which was readily disposed of by venting and/or burning. The net result was a significant decrease in the amount of waste and the biological stabilization of waste prior to final disposal. Two factors which have limited the greater application of anaerobic digestion are relatively long digester resi- dence time (i.e., greater than 15 days) and system instability to shock loadings.

1119

@ 1975 by John Wiley & Sons, Inc.

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1120 COONEY AND WISE

Recently, there has been a renewed interest in the use of anaerobic digestion as a means not only to reduce and stabilize organic waste, but also to convert these wastes into a readily usable fuel. Even though many organic wastes, and especially those rich in cellulose, can be directly burned as a fuel, the physical collection, handling, storage, and shipment problems are difficult and expensive. Meth- ane gas, however, can readily be used in existing equipment and and stored and shipped by existing methods for natural gas distri- bution; for this reason conversion of organic waste into fuel gas is a particularly attractive means of providing a supplemental fuel.

With the foregoing rationale, our objective was to examine the use of solid organic waste as a raw material for conversion to fuel gas via anaerobic digestion, with particular attention given to the biologi- cal and engineering problems. One approach was to employ thermo- philic culture conditions as a means of imptroving the process.

Thermophilic digestion offers several advantages over traditional mesophilic digestion. These are : 1) increased rates of digestion, 2) decreased fluid viscosity, 3) decreased biomass formation, 4) in- creased conversion of waste to gas, and 5) absence of bacterial and virus pathogen accumulation. The primary drawback to this approach is the need to supply heat to maintain the elevated tem- perature. Capitalizing, however, on the positive attributes, we were interested in attempting to minimize the retention time in a thermophilic digester, while at the same time increasing the solids loading.

The first portion of this paper is a review summarizing the revelant work done on thermophilic anaerobic digestion and provides the rationale underlying thermophilic operation. The second part of the paper presents results from an experimental program on the production of fuel gas from domestic solid waste under thermophilic conditions.

LITERATURE REVIEW ON THERMOPHILIC ANAEROBIC DIGESTION

The literature on thermophilic growth of microorganisms is quite extensive, describing the isolation of new organisms, attempting to elucidate the molecular basis for thermophiles, and, in a few cases, discussing the practical advantages of thermophilic growth. Prior reviews include an early, but excellent, one by McBee4 discussing anaerobic thermophilic cellulolytic bacteria. Farrell and Rose5 have reviewed the literature on temperature effects on microorganisms

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ANAEROBIC DIGESTION OF WASTE FOR FUEL 1121

through 1966, and Egorova6 has briefly reviewed the Russian litera- ture through 1965. More recently, Campbell and Pace' have discussed growth at higher temperatures, and Singleton and Amelun- xed have reviewed the biochemical and structural basis for the increased stability of enzymes from thermophiles.

Early investigations into the effect of temperature on anaerobic digestion of waste presented conflicting results. From the work of R ~ d o l f , ~ Hatfield et a1.,l0 and Heukelikean" there does not appear to be any advantage in raising the temperature of the process above 25°C. More recently, Malina12 found that gas production at 32°C was greater than a t 52°C. These results are in direct contrast to the classic work of Fair and Moorel3-15 in which they demonstrated increasing gas production with increasing temperature up to 60°C. In fact, the gas production a t 60°C was 1.25 times that at 25"C.15 The most complete and recent examination of the effect of tempera- ture on anaerobic digestion is the work of PfeffeF who looked at the digestion of organic solid waste a t temperatures from 3040°C. His results show two temperatures optimum for digestion, one a t 42°C for mesophilic digestion and one greater than 60°C (the highest temperature examined). Overall, in Pfeff er's system, thermophilic operation performed better than mesophilic operation.

In retrospect, with our current knowledge of microbiology and microbial adaptation and evolution, it is possible to understand many of these apparently conflicting results. Most microorganisms possess the ability to grow reasonably well only over a temperature range of 20-30°C.17 Therefore, a microbial population operating optimally a t 30°C should not be expected to show an increased rate of activity a t 60°C. It is reasonable, however, to assume that a new population of organisms can be selected which will optimally function at 60"C, although not necessarily do well a t 30°C. Evidence for this statement is provided by the many successful efforts in isolating thermophilic microorganisms.6s18-20

Interestingly, much of the work on the thermophilic methane fermentation has been done by Russian, Latvian, Japanese, and Czechoslovakian workers. Fukui et a1.21*22 have studied the forma- tion of vitamin B12 in a continuous methane fermentation a t 52°C of distillage from acetone-butanol fermentations. Methane-forming bacteria are well known for their ability to produce vitamin B12 and, for this reason, considerable effort has been devoted towards the use of waste material in the production of Biz. Using acetone-butanol and ethanol fermentation wastes as substrates for methane formers, Pantskhava and B y k h ~ v s k i i ~ ~ have produced over 5,000 pg of BIZ

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1122 COONEY AND WISE

per liter in a thermophilic methane fermentation. Other studies on the synthesis of vitamin B12 by thermophilic methane formers have been done by Zeile and Kaleja,24 Sarma,25 Zeile,26 Pantskhava and P ~ h e l k i n a , ~ ~ Apinis,28 and P a n t s k h a ~ a . ~ ~ The biomass from these fermentations also show antibiotic activity, and extracts of these thermophilic methane formers have been evaluated for their poten- tial use as an animal feed ~upplernent.~0-32

Neither aerobic nor anaerobic thermophilic treatment of waste are completely novel processes; however, except for the process described by Popova and B ~ l o t i n a , ~ ~ no one currently uses thermophilic proc- esses for waste treatment. Furthermore, investigations on the feasibility of this approach are few in number. Results from early studies were conflicting and only the studies of Fair and Moore13-15 were encouraging. The most recent attempt to closely examine thermophilic anaerobic digestion is reported by Maly and Fadrus34 and P f e f f ~ r . ~ ~ In batch culture work by Maly and F a d r u ~ , ~ ~ the ultimate overall performance of the digesters at 20, 30, and 50°C were about the same, although initial rates of gas formation were much faster at the higher temperatures. This report, like the several before it, was not completely conclusive in that the overall performance of the thermophilic process was not much better than the mesophilic process. The main reason for this is felt to be the result of the manner in which the thermophilic microbial population was developed or actually was not developed. In the work by Maly and F a d r u ~ , ~ * as in earlier studies by other workers, a digester acclimated to one set of conditions was simply shifted to new tem- peratures for evaluation. A more rational approach is to adapt and select a microbial population which is suited for growth at the higher temperatures.

The effect of temperature on specific growth rate is described reasonably well by the Arrhenius relationship with a typical activa- tion energy of 15 kcal /m01.~~J~ While the meaning of this activation energy is not clear and the relationship only holds over relatively small temperature ranges, it is a useful correlation for predicting the effect of temperature on cell growth. An Arrhenius plot presented by Brock@ demonstrates the general nature of this relationship for a variety of bacteria over a broad temperature range and shows specific growth rate to generally increase with temperature. Thus, growth rates and, hence, substrate utilization rates may be expected to be greater in a thermophilic process in comparison to a similar process under mesophilic conditions. It is important to remember, however, that even though the microbial populations of mesoPhilic

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ANAEROBIC DIGESTION OF WASTE FOR FUEL 1123

and thermophilic processes are probably quite different, the net result will be greater bioconversion of waste to methane a t thermo- philic temperatures. Of particular relevance is the observation by

that the growth rate for methanogenic bacteria in a sewage sludge digestion process is almost doubled between 30 and 50°C. Over this same temperature increase, the rate of gas production increased from 63 to 100 ml/g/day.

One of the advantages of anaerobic digestion over aerobic treat- ment of waste is the decreased quantity of biological materials formed in the process. Furthermore, the cell yield, expressed as mass of cells formed per mass of substrate (waste) consumed, is a function of temperature. Marr et al.40 have shown that as the growth temperature is increased, the amount of energy needed for maintenance of the culture is increased; therefore, less of the sub- strate consumed is converted into biomass. Similar results have been observed by Levine and C ~ o n e y ~ ~ and Snedecor and C ~ o n e y ' ~ for cells grown on methanol a t a constant growth rate while the tem- perature was raised as an independent variable. In anaerobic digestion, increased maintenance means that the ratio of organic carbon incorporated into cells and utilized for energy production will also decrease with increasing temperature. The result is greater production of methane per mass of waste consumed a t the expense of cell mass synthesis. In the utilization of solid waste, this benefit of thermophilic operation is small, since most of the residual solids are undegraded solids and not biomass.

One of the most costly bottlenecks in waste treatment processes is the problem of solid-liquid separation after digestion is complete; therefore, any means for facilitating this step is desirable. As tem- perature is increased, water viscosity is decreased such that the viscosity of water a t 55°C is half that a t 20°C.

In this light, it is useful to examine the data of Garber43 on the vacuum filtration of sludge from an anaerobic digester operated a t 49 and 29°C. The respective filtration fluxes were 6.3 and 1.7 lb/hr/ft2. From the relative viscosity of water a t these temperatures one would expect an increase of 1.5 times, while a 3.7 times increase was observed. It is tempting to speculate that the additional increase in filtration flux is attributable to the characteristics of the sludge; this point, however, will require additional examination.

Ther- mophilic processes will, in general, be more sanitary and, hence, are likely to be more publicly acceptable. Nonspore-forming pathogens usually have temperature optima close to 37°C and fail to grow when

The advantage of increased pathogen destruction is clear.

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1124 COONEY AND WISE

the temperature rises to 4245°C. Furthermore, death of these organisms is a time-temperature phenomenon. At 55"C, a nominal sludge residence time of only 1 hr is sufficient to reduce the coliform count by eight orders of magnitude (this calculation is based on a death rate constant at 55°C of 0.3 min-l, taken from Aiba et al.44). In this light, Popova and B ~ l o t i n a ~ ~ commented that one of the prime advantages of the thermophilic anaerobic process in Moscow is the sanitary quality of the effluent from the digestion process.

An important additional consideration concerning pathogen accumulation is the deactivation of viruses during the digestion process. Rate constants for thermal deactivation of viruses are typically greater than 0.1 min-1 a t 55°C (see refs. 4.5-47). As a consequence, a 1 hr residence time will result in greater than a 4 X lo2 reduction in virus count. Grigor'eva et al.48 examined the survival of both Escherichia coli and bacteriophage in thermophilic and mesophilic digesters. Temperatures are not given. The phage survival time was determined for lo5 phage/ml added to sludge. In a mesophilic digester, the survival time was four months, while in the thermophilic digester it was only nine days. Although human viruses may be transmitted through wastewater, they do not replicate in wastewater; therefore, the thermal destruction of viruses at thermophilic digester temperatures is more effective than that of bacteria, which if not completely inactivated have the potential to regrow and accumulate.

GENERAL ASSESSMENT OF THERMOPHILIC DIGESTION

The literature on thermophilic microbiology is extensive, yet relatively few efforts have been made to utilize the unique advantages of thermophiles for practical purposes. It has been noted that the previous efforts on such processes have been somewhat inconclusive, yet not discouraging. As a result, it is difficult to fairly judge the feasibility, economic or technical, of thermophilic waste treatment. In much of the work to date on thermophilic waste treatment, it appears that insufficient effort has been given to the selection and adaptation of microbial populations a t thermophilic temperatures before initiating studies on the performance of a thermophilic treatment process.

In light of developments in thermophilic microbiology, the demon- strated feasibility of thermophilic methane fermentation, and the potential advantages of a thermophilic process, we feel that the development of this approach warranted further consideration.

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ANAEROBIC DIGESTION OF WASTE FOR FUEL 1125

The purpose of this paper is to present results on the development and characterization of a process for the conversion of domestic solid waste to fuel gas a t thermophilic temperatures.

MATERIALS AND METHODS

Organisms The mixed populations of microorganisms used in these studies

were selected from natural sources (e.g., anaerobic digester effluent, rumen juice, mud, compost, etc.) for their ability to convert solid waste to methane and carbon dioxide a t the desired temperatures of operation. All cultures were maintained in continuous digesters fed with solid waste and sewage sludge.

Media Growth and Gas Production The feedstock for gas production consisted of 90% (by wt) of

shredded solid waste from the Office of Solid Waste Management, Cincinnati, Ohio, and 10% domestic sewage sludge from the primary settling tanks of the Nut Island Sewage Treatment Plant, Boston, Mass. The concentrations of this 90/10 mixture added to the digesters range from 2.5 to 5.0% on a dry solids basis and are specified for each experiment presented in the results. A typical analysis of the solid waste used for these studies showed it to contain 35% vola- tile solids. The solid waste was 93% total dry solids and 7% mois- ture. The sewage sludge was typically a pH of 5.6 and contained 4.8y0 total solids of which 20% was ash and 80% was volatile solids. The digesters were fed once per day. The nominal residence time of each of the digesters is given in the Results section.

Experimental Equipnient The anaerobic digesters were 50 liter polypropylene carboys with

a 2.5 in. opening at the top. The top of the vessel was fitted with a rubber plug through which passed a stirring shaft and a gas exit line. A 1 in. diameter fitting was installed near the top for feeding and withdrawal of digester effluent. The contents of the vessel were agitated a t 100 rpm. The impeller was a 0.5 in. stainless steel shaft to which three 8 in. crossbars, equally spaced along this vertical shaft, were bolted. Gas production was measured with a wet test gas meter or by collection in water displacement bottles. The digesters were kept in constant temperature rooms to maintain temperature control.

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1126 COONEY AND WISE

Assay Procedures

Total solids, volatile solids, and total volatile acids were deter- mined by the procedure outlined in Standard Methods for the Exami- nation of Water and Wastewater.49

Total gas production was measured by means of a wet test meter and values are reported at room temperature (70°F) and pressure. Samples of gas for composition analysis were taken either from the digester head space, or from the exit gas line a t a point immediately as it left the digester. A model 250 Fisher gas partitioner with a standard 30 x t in. aluminum column packed with 30% HMPA (hexamethyl phosphoramide) on 60/80 mesh Chromosorb P and a 6.5 in. aluminum column packed with 40/60 mesh molecular sieve 13 x was used for gas analysis. Helium was the carrier gas and a standard mixture of methane and carbon dioxide was used for calibration.

Radioactive Tracer Studies

A digester operating under steady-state conditions was pulsed with a charge of labeled material. The digester was allowed to mix and continue normal operation. Samples were withdrawn at regular time intervals and the concentration of radioactive tracer was determined by conventional radioassay techniques.

The two types of radioactive labeled material used in the experi- ments were glucose (H3 labeled) and acetate (or sodium acetate, C14 labeled in the C1 position) ; both labeled materials were obtained from New England Nuclear Corp. Two 52.5 liter, mesophilic digesters (numbers 4 and 10) with a 667 hr (28 days) residence time were pulsed with glucose and acetate. One thermophilic digester with the same volume and residence time was pulsed with labeled acetate and a second thermophilic digester, which was 35 liters and had a 445 (18.5 days) residence time, was pulsed with glucose.

RESULTS AND DISCUSSION

A characterization of some of the potential benefits of thermo- philic anaerobic digestion in the conversion of solid waste to fuel gas has been attempted. The initial literature review indicated that thermophilic operation could lead to increased rates of gas produc- tion, increased conversion of feed to gas, increased settling rates, and prevention of microbial and viral pathogen accumulation.

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ANAEROBIC DIGESTION OF WASTE FOR FUEL 1127

Selection of a Thermophilic Anaerobic Population of Microorganisms

The development of a thermophilic anaerobic digestion system was achieved by a simultaneous selection and acclimation of micro- organisms to thermophilic temperatures. For this purpose, we chose 65°C as a temperature which was well into the range of thermophilic growth and, further, has been frequently found as an optimum tem- perature for many thermophiles. The selection process was begun by preincubating potential sources of the desired organisms (e.g., sewage sludge, rumen juice, mud, compost, etc.) at 54, 60, and 65°C in 1 gal jars containing solid waste and sewage sludge in a 9: 1 ratio. After several days, gas production was observed as well as a thinning of the solid waste slurry. Four 1 gal jars wwe maintained a t each temperature and, every other day, one half the contents of one jar at each temperature was fed to one of the 50 liter agitated digesters. The digesters were simultaneously acclimated to temperature by increasing the temperature approximately 3°C every three weeks until 65°C was reached. This temperature was chosen as one representative of true thermophilic operation. Also, every other day, the laboratory thermophilic digesters were fed solid waste and raw sewage sludge, thus completing their daily feed schedule. On a random basis the thermophilic digesters were seeded with anaerobic cultures grown up on marsh mud, manure, compost, and rumen juice.

The results from this selection process are shown in Table I as total gas production and total volatile acids for a 50 liter digester operated with a 30 day nominal residence time; this residence time was chosen to ensure a high probability of selecting a raised population of thermo- philes which would later be evaluated at shorter residence times.

When the temperature was raised above 37"C, gas production fell. This is reasonable in light of the general observation that mesophilic organisms are unable to grow well above 3740°C. It is noteworthy, however, that gas production did not fall to zero, but as the tempera- ture was increased further, stayed at about the same level observed a t 45°C. The low but continued gas production is most likely the con- tribution from thermophilic organisms present initially in the di- gester, as well as those organisms selected separately in small digesters and then added routinely to the 50 liter digester to facilitate the enrichment process. Interestingly, volatile acids continued to accumulate in the vessel as the temperature was raised above 37"C, thus suggesting that the portion of the mixed population responsible for the initial breakdown of cellulose to volatile acids was still quite

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1128 COONEY AND WISE

TABLE I Development of a Thermophilic Anaerobic Digestion Process on Solid Waste-

Average gas production Total volatile acids Temperature ("C) (ft3/day) (mg/liter)

Average performance of mesophilic digesters 37 0.41

Temperature adaption of digester No. 12

42 0.12 45 0.067 49 0.079 54 0.036 57 0.043

After three months of operation 65 0.51

After six months of operation 65 0.72

3000 3800 3600 3200 2700

1300

a Digester feed: 2.5y0 HGSW (90/10).

active. Once steady state was established, the pool size of volatile acids went down from above 3000 to 1300 mg/liter, thus indicating a different balance of the two populations at the higher temperature. This transition is completely consistent with the need to establish different populations of bacteria which are best suited for either mesophilic or thermophilic operation but not both. This residual level of volatile acids is quite high relative to most anaerobic di- gesters. Digesters on sewage sludge would be considered close to failure if operated at 1300 mg/liter of volatile acids. In this light, it is interesting to note that satisfactory performance was achieved in this case even at the high acid levels.

After three months of operation a t 65"C, the gas production in- creased to 0.51 ft3/day and then to 0.72 ft3/day after another six months. This represents a 75% increase over the steady-state rate of gas production at 37°C. Most of this increase is attributable to an increase in conversion of volatile solids to gas, as indicated by the 47% increase in conversion of volatile solids to gas a t 65°C operation. The increase in conversion is at least partially attributable to the increased demand for maintenance energy, which is characteristic of growth at elevated temperature^.^^^^^

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ANAEROBIC DIGESTION OF WASTE FOR FUEL 1129

Comparison of Mesophilic and Thermophilic Digestion From the point of view of fuel gas production from solid waste, it is

essential to consider the system productivity and net conversion of organic solid waste Lo methane. Results for 30 day retention time digesters are shown in Table 11, comparing the performance of three 50 liter mesophilic (37°C) and three 50 liter thermophilic (65°C) digesters operating on solid waste. The average gas production rate a t 65°C is 0.54 ft3/day, which is 50% greater than the mesophilic digesters which averaged 0.36 ft3 of gas per day. This value is lower than the results shown for the single digester in Table I and represents an average value for three digesters; in all cases, the thermophilic digesters performed better than the parallel mesophilic digesters. The reactor volume was 50 liter or 1.75 ft3. Thus, thc productivities of the digesters are 0.21 and 0.31 ft3 gas/ft3 reactor/day, respectively, for the mesophilic and thermophilic processes. The conversion of volatile solids to gas is 7.5 and 11 ft3 gas/lb volatile solids for the mesophilic and thermophilic digesters, respectively. This is based on 58% of the total dry solids being volatile solids. The net conversion of waste to gas is also greater under thermophilic conditions.

The above comparison was made with 30 day retention time di- gesters. In retrospect, one might consider shorter retention times more appropriate for this comparison, especially taking into account the results of Pfeffer16 and Cooney and Ackerman151 which show maxi- mum productivities of thermophilic systems to be less than 10 day retention times. However, the longer retention time provides an opportunity to measure the effect of temperature under conditions of extreme degradation.

TABLE I1 Gas Production in Mesophilic and Thermophilic Anaerobic

Digesters( fed 2.5% solid wastea on a 30 day retention time)

Conversion of volatile solids to fuel gas

Gas productionb (ft3 gas/lb volatile Temperature No. digesters (ft3/day 1 solids fed)

37 3 0.36 65 3 0.54

7.5 11

* 2.5 wt yo feed of solid waste and sewage sludge in a 9: 1 ratio. Average performance over 1 to 2 months operation of a digester a t steady

state.

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1130 COONEY AND WISE

Still another basis for comparison is the amount of gas produced per pound of solid waste fed to the reactor. The results from mesophilic and thermophilic operation are 4.2 and 6.2 f t3 gas/lb dry waste fed, respectively.

Kinetics of Solid Waste Digestion: Radioactive Tracer Studies The previous results show that overall conversion of solid waste to

fuel gas proceeds faster under thermophilic conditions than under mesophilic conditions. In light of this observation, as well as the general belief that the rate of gas production is usually limited by the rate of methane formation, it was decided to use labeled glucose and acetate in both mesophilic and thermophilic digestion to attempt to elucidate the rate limiting step in the conversion of solid waste to fuel gas. Ideally, one would use labeled cellulose in the examination; however, it was not available a t the time of this study. Thus, glucose was utilized to elucidate the acid formation step. The experi- ments were conducted by introducing the labeled compounds into the digesters, and then samples were withdrawn for analysis of the disappearance of label. Acetic acid was chosen as the intermediate in this study since it is the most common organic acid found in digesters, and it is readily converted to methane.

In each case, the data are plotted as the ratio of the label in the broth, a t any time, to the initial level. The results shown in Figure 1 show that a t thermophilic temperatures, the rate of disappearance of acetic acid is first order for a t least the first 175 hr and considerably faster than under mesophilic conditions. All three digesters were operating with the same nominal residence time of 667 hr; this cor- responds to a dilution rate of 0.00149 hr-l, The washout curve on Figure 1 illustrates the rate of loss of label at this dilution rate if there were no metabolic uptake of the acetic acid. The rate constant over the linear range for the thermophilic digester is 0.015 hr-I and the mesophilic digester is 0.0047 hr-l. Thus, the rate of acetic acid consumption is 3.2 times faster under thermophilic conditions. The last point on the curve a t 330 hr showed a deviation from the first order washout. By this time, less than 5% of the initial level was remaining, and it is likely that the residual radioactivity had become incorporated into cell mass or some other nonvolatile form. The decay curve from 175 to 330 hr parallels the washout curve, which further supports this hypothesis. The results of the labeling experiments using tritiated glucose were not so clear-cut as the acetic acid labeling work. The rate of tritiated

The results of these experiments are shown in Figures 1 and 2.

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ANAEROBIC DIGESTION OF WASTE FOR FUEL 1131

1.0

0.9 0.1 0.7

0.6

0.5

0.4

0.3 - .)

-I

1 0 . 2 - - 5 - 2 r F'

p 0.1

0

.O!

.O l

; .oi .O'

O!

K

.O*

.01

.O

Fig. 1. Disappearance of 1*Glabeled acetate after its pulse addition to meso- philic (0,o) and thermophilic anaerobic digesters operating at 37 and 65"C, respectively.

glucose disappearance, which reflects the rate of action of the acid- forming bacteria, is shown in Figure 2 for two 37°C mesophilic digesters, both with a residence time of 667 hr (dilution rate of 0.00149 hr-l), and a 65°C thermophilic digester with a residence time of 445 hr (dilution rate of 0.00224 hr-1). The respective wash- out curves for the two dilution rates are also shown in this figure. The disappearance of glucose label has two phases; phase one is a first order loss over the first 150 hr, phase two over the next 150 hr, is more rapid. However, the precise nature of the curve is not clear from the limited data available. For these reasons, the kinetic rate constants are calculated for the first 150 hr. The mesophilic digesters

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1132 COONEY AND WISE

- -

; .T

:: .6 -

- 0 A - c 0

- L

2, - .? . 5 - 0 0

0 - L

\ c 0 . 4 - 'A - 0 Melophllic Digeo tor No. 10

0 MoroPMlic Digelter No.4 - 0

0 L A Thormophllic Digr l ter No. 12

.3 I 1 1 1 I I

100 I50 200 2 50 300 50

l i m e After Addltion of Lobeled Olucor8 (hr 1

Fig. 2. Disappearance of tritiated glucose after pulse addition to mesophilic (0,o) and thermophilic ( A ) anaerobic digesters operating at 37 and 65"C, respectively.

have rate constants of 0.0070 and 0.0034 hr-' for digesters 4 and 10, respectively. These compare with a rate constant for the thermo- philic digester of 0.0037 hr-l. In all cases, the constants have been corrected for the appropriate washout effects. Thus, within experi- mental accuracy, the rate constants are equivalent.

Measurement of Biological Mass The assessment of active biological cell mass in a complex and

heterogeneous system is a difficult problem. Direct methods of measurement are impossible; for this reason we have attempted to measure cell mass indirectly in order to obtain a relative measure of the cell mass under mesophilic and thermophilic conditions. We chose the, analysis of DNA in digester samples as a marker for cell mass. The amount of DNA entering the digester is very small, coming mostly from the sewage sludge. The sewage sludge was found to contain 150 mg of DNA per liter. Since 10% of the solids in the feed to the digester was sewage sludge, the contribution of DNA in the digester by the feed was 8 mg/liter. Results from the analysis of DNA in mesophilic and thermophilic digesters are given in Table

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ANAEROBIC DIGESTION OF WASTE FOR FUEL 1133

111. Based on the average of three samples taken on separate days over the course of six weeks of steady-state operation, the DNA level in the mesophilic digester is approximately twice the level in the thermophilic digester. This indicates a lower concentration of bacteria under thermophilic operation.

The rate constants measured by our techniques are volumetric rate constants and represent the product of the specific rate constant for the bacteria times the bacterial concentration. As a consequence, it is not possible to separate the specific rate constant from the overall rate constants measured in our experiments. Utilizing the DNA measurements above, we can quantitatively estimate the relative values of the specific rate constants. The results from our DNA analysis indicate that the level of bacteria in the thermophilic di- gester is approximately half that in the mesophilic digester. Com- bining this observation with the comparison of overall metabolic rates of the two types of digesters, which show increased rates a t 65"C, one can even further support the conclusion that thermophilic di- gestion can proceed a t faster rates than mesophilic digestion.

In summary, the literature on the effect of temperature on anaer- obic digestion suggests several potential benefits of thermophilic over mesophilic operation. The experimental work described here was done to explore and quantitate these benefits in the anaerobic con- version of domestic solid waste to fuel gas. The results on gas pro- duction and from tracer studies support the expected increase in gas production and conversion of volatile solids to gas. While the use of labeled glucose is not necessarily directly coupled to the hydrolysis of cellulose, these observations, coupled with the advantages of in- creased ease of solid-liquid separation a t elevated temperature demonstrated by G a ~ - b e r ~ ~ and the prevention of bacterial and viral

TABLE I11 Assessment of Biological Mass in Anaerobic Digesters

by Means of DNA Analysis

DNA concentration (mg/liter) Sample Mesophilic digester Thermophilic digester

37°C 65°C

1 40 2 41 3 32 Average 38

28 15 17 20

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1134 COONEY AND WISE

pathogen accumulation, make thermophilic anaerobic digestion an attractive alternative process for the production of fuel gas.

The support of Consolidated Natural Gas Service Co., Inc. for this work is acknowledged. The authors wish to recognize the direction provided by Dr. Robert C. Weast, Vice President, Research, and Mr. Howard W. Scott, Research Engineer. Publication No. 2527 from the Department of Nutrition and Food Science, Massachusetts Institute of Technology.

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Accepted for Publication March 28, 1975