Gas production parameters in sanitary landfill simulators

Waste Management & Research (1987) 5, 2 7 -39

GAS PRODUCTION PARAMETERS IN SANITARY LANDFILL SIMULATORS M . A. Barlaz,t M . W . Milke t and R . K . Ham t (Received 2 October 1986)

The decomposition of shredded municipal refuse was studied in 19 drums (208 1) . Gas production and composition were monitored for two years . The addition of old, anaerobically degraded refuse as a seed of anaerobic bacteria and the neutralization of the refuse were the two techniques which stimulated methane production . Yields of 80-150 1 of methane per kilogram of dry, grindable, volatile solids were measured . The addition of anaerobic sewage sludge, acetate, and the initial removal of oxygen from a drum, were not stimulatory . Initially, it was the development of the methanogen population and not polymer hydrolysis which limited methane production . Key Words-Methane, solid waste, landfill, refuse, anaerobic digestion .

1 . Introduction Methane production from municipal refuse represents a source of energy which is rapidly developing but underutilized at the present time (Stearns 1980) . One reason for the low utilization is the small amount of methane which is typically collected relative to the methane generation potential of refuse . The objective of this study was to define parameters which affect the onset of methane production, methane production rates and methane yields in sanitary landfills . Ultimately, it is desirable to develop methods for the operation of landfills which enhance their methane production potential . The ability to promote rapid refuse decomposition to methane has several advantages in addition to the recovery of methane . After the onset of methane production, a reduction in leachate strength is expected . This reduction will lead to lower leachate treatment costs and a reduced risk of groundwater contamination . A second advantage to rapid methane production is the reduced pay-back period for the investment associated with gas recovery facilities . Finally, most of the settlement of a landfill can be expected by the end of the refuse decomposition period . Thus, enhanced refuse decomposition will lead to a reduction in long-term care requirements for gas migration and cover maintenance . The effects of several variables which may be expected to stimulate the refuse ecosystem are reported here . These variables include the use of old, anaerobicallydegraded refuse as a diluent and a seed of methanogenic bacteria, the addition of anaerobically-digested sewage sludge as a seed of anaerobic bacteria, the use of sterile cover soil, the addition of acetate to refuse, leachate recycle and neutralization . §

*Based on a paper presented at the Ninth Annual Madison Waste Conference, 9-10 September 1986, Department of Engineering Professional Development, University of Wisconsin-Madison . f Department of Civil and Environmental Engineering, University of Wisconsin, Madison, WI, U .S .A. I Department of Civil Engineering, Carnegie Mellon University, Pittsburgh, PA, U .S .A . §Editors note : this work parallels that of Kinman et al ., the preceding article in this issue . 0734-242X/87/010027 + 13 $03 .00/0

© 1987 ISWA



28

M. A . Barlaz et al . TABLE I Summary of variables investigated

Block

Initial moisture (%)

At

1

42 .0

B

1

43 .2

C D

I 1

45 .0 44 .3

E F G

I 1 I

41 .8 40 .9 56 .5

H* I§,I' J

1 1 1

45 .3 41 .8 42 .8

25% Old sterile refuse 50% Sterile soil 25% Old sterile refuse 50% Sterile soil 50% Old sterile refuse 50% Old sterile refuse 100% Sterile soil Control 100% Sterile soil 50% Old sterile refuse field capacity 50% Old sterile refuse Control Control

Kc L',I N $ OI P Q R S T

2 2 2 2 2 2 2 2 2

55 .5 55 .9 55 .9 55 .9 55 .9 55 .9 59 .3 54 .1 55 .9

50% Old sterile refuse Acetate addition 100% Sterile soil Control Nitrogen purge Control 50% Old refuse 50% Old refuse Anaerobic sludge seed

Drum

Description

* Drum H was placed in an incubation room at 37C on day 110 . t Drum A was turned on its side and rotated every several days from days 105 to 292 . § An acetate solution was added on day 89 . I Leachate recycling and neutralization was in iated on day 220 in Drum I and day 99 in Drums L and O . T A buffer solution was added on day 113 .

2 . Experimental methods 2 .1 . Experimental design and literature review The study was executed in two phases which will be referred to as Blocks I and 2 . The test conditions for the refuse used in each experiment are presented in Table 1 . Block I served to evaluate the use of old sterile refuse as a diluent, the use of sterile cover soil and high moisture . A second objective of Block 1 was to evaluate the experimental apparatus . All drums in Block 1 were tested at about 40% moisture with the exception of Drum G, which was initiated at a moisture content of field capacity or 56 .5% moisture . Drum G was the only drum in Block I to produce methane over the first three months of the experiment . All drums in Block 2 were tested at about 55% moisture . The higher moisture content used in Block 2 did not stimulate methane production . The relationship of the variables listed in Table 1 to the physical, chemical and biological processes which occur in a sanitary landfill are discussed below . General reviews of the microbiology of anaerobic digestion have been provided by others (Wolfe 1979 ; McInerney & Bryant 1981) .



Gas production in sanitary landfills

29

2 .1 .1 . Moisture content Early studies on moisture content (Merz & Stone 1964 ; Farquhar & Rovers 1973) indicated that a higher initial moisture content helped accelerate methane formation, but a full scale experiment by EMCON (1975) showed no benefit to higher initial moisture . Moisture movement has also been shown to be a factor in stimulating gas production (Hartz & Ham 1983 ; Klink & Ham 1982) . The potential for moisture movement is much greater when the refuse is above field capacity . Water has many effects on the refuse ecosystem including the solubilization and distribution of substrates and nutrients and the dilution of toxic substances .

2.1 .2 . Old degraded refuse Anaerobically decomposed refuse can serve as a seed of bacteria acclimatized to anaerobic refuse decomposition . If the time required for the establishment of a viable population of anaerobic bacteria is what limits the onset of methane production, then the use of an acclimatized seed should decrease the time to the onset of methane production . (Drums R and S .) 2 .1 .3 . Old sterile refuse In addition to the value of old refuse as a seed as discussed above, it may also serve to dilute the accumulation of carboxylic acids or some other inhibitory compound (Stegmann 1983) . To separate these two effects, fresh refuse was mixed with both sterile and non-sterile refuse . (Drums A-D, G, K .)

2 .1 .4 . Sterile cover soil The source of methanogenic bacteria in sanitary landfills has not been identified . Tchobanoglous et al. (1977) have suggested that the soil used for daily and final cover is the principle source of organisms responsible for the decomposition of refuse . However, Ham & Bookter (1982) found that anaerobic decomposition occurred more rapidly when no cover soil was used . To treat this conflict, refuse was covered in test cells with sterilized cover soil in Drums D, F and N . 2 .1 .5 . Nitrogen purge to remove oxygen The presence of oxygen in freshly buried refuse will prevent the growth of bacteria which are obligate anaerobes such as the methanogens . It was thought that the removal of oxygen from refuse at the onset of the experiment would reduce the time to the onset of methanogenesis . Oxygen was removed by purging Drums L and P with nitrogen . 2.1 .6 . Acetate addition In anaerobic sewage sludge digestors, about 70% of the methane produced originates from acetate, a direct precursor of methane (Mah 1978) . If polymer hydrolysis is the rate limiting step, the addition of acetate to Drum L would be expected to lead to rapid methane production . Oxygen was also removed from the refuse in order to allow anaerobic processes to begin immediately .

2.1 .7 . Anaerobically-digested sewage sludge Anaerobically-digested sewage sludge was added to Drum T to evaluate its ability to act as a seed of anaerobic bacteria . Sewage sludge is also a good source of nutrients such as nitrogen and phosphorus (Buivid et al. 1981) .



30

M . A . Barlaz et al .

2 .1 .8 . Leachate recycle and neutralization Leachate recycle has been reported to enhance the production of methane from refuse (Pohland 1975) . The mechanism by which this enhancement is thought to occur is similar to that described for moisture content . Leachate accumulations sufficient for recycling were induced by the addition of deionized, deaerated water to Drums I, L and O . After an additional five weeks, the pH of the leachate had not increased from its initial value of 5 .0, whereupon a neutralization step was included in the recycle procedure for Drums I, L and O . In Drum K, the refuse was neutralized by the addition of buffer in one dose . 2 .2 . Equipment Tests were conducted in epoxy-coated steel drums (208 1) at room temperature (near 24 °C) . Modifications to the drum were made for installation of a leachate sampling port, temperature probe, water addition port and gas probe . Drums were sealed with a 100% silicone rubber general purpose sealant (Dow Corning, Midland, MI) and duct tape . Gas production in the drum passed through a gas collection tube prior to the gas measurement device . A septum was installed on the gas collection tube so that it was possible to withdraw a sample by syringe for composition analysis . Gas production was measured by one of three methods : a wet test meter (Precision, Chicago, IL) ; gas bags (Pollution Measurement Corp ., Chicago, IL) ; or a water displacement method (Ripley 1984) . used for the administration of intravenous fluids (Travenol Inc ., Deerfield, Il) . Leachate was neutralized by adjusting the pH to between 7 .0 and 8 .0 with a 30 g 1 - ' solution of sodium carbonate . 2 .3 . Materials Shredded refuse from the Madison Energy Recovery Plant was used for the study . Refuse at this facility is separated into a light, high heating value stream, a heavy, low heating value stream, and a metals stream . The three streams were combined in proportions representative of domestic Madison refuse for use in this study . Old refuse was excavated from a test lysimeter used by Ham & Bookter (1982) in the mid-1970s ; the refuse was known to have produced methane . Refuse or soil was sterilized, as called for in the experimental design, in an autoclave for 30 min . The biological activity of the refuse or soil after autoclaving was tested and found to be negligible . Soil from the Dane County Landfill, Verona, WI, was used to cover the refuse in each drum for Blocks I and 2 . 2 .4 . Analytical methods Gas composition was analysed on a Fischer Model 1200 Gas Partitoner . Leachate samples were analysed for carboxylic acids using an HP 402 gas chromatograph with a Supelco 100/120 chromosorb wow column . 3 . Results Data on the methane composition of each drum and methane production rates and yields are presented in Tables 2 and 3 . The results of each variable evaluated in this study will be presented in this section .



31

Gas production in sanitary landfills TABLE 2 Comparison of methane production rates between drums Average

7-Day maximum

Cumulative

Drum ¶

Rate *

Period (days)

Rate *

Period (days)

S A G R 0 D L K**

1 .0 0 .5 1 .22 0 .94 0 .85 0 .24 0 .14 0 .05

163-170 419-426 83-90 197-204 184-191 472-479 258-265 262-269

0 .22 0 .23 0 .22 0 .35 0 .17 0 .02 0 .04 0 .002

40-718 269-849 49-523 197-400 162-718 353-696 158-355 171-178

Yield f ,l I 150 .1 133 .5 103 .1 81 .7-122 .4§ 94 .1 7 .2 5 .5 0 .3

Days 718 849 523 718 718 754 718 649

* Units are litres of methane per day per kilogram of dry grindable volatile solids. t Units are litres of methane per kilogram of dry grindable volatile solids . $ Yields for Drums A, D, G, R and S are corrected for the methane which can be attributed to the old refuse in the drum . § The average rate for Drum R is computed beginning on day 197 and the yield is presented as a range due to problems with gas measurement . II To convert from litres per kilogram of dry grindable volatile solids to litres per kilogram of dry solids multiply by 0 .647 for Drums A, D and G and 0 .589 for Drums K, L, O, R and S . ¶ No methane production was measured in drums not listed here . ** Methane production not measured accurately, see text. TABLE 3 Maximum hydrogen and methane concentrations in the drums Drum

Maximum methane (%)

S A G R 0 D L K H B C E F I J N P Q T

63 .4 57 .8 61 .3 65 .6 60 .4 68 .4 62 .1 67 .6 57 .9 16 .7 38 .4 25 .0 18 .9 58 .6 17 .8 ND 20 .1 3 .1 9 .9

Day 387 460 45 387 387 512 318 318 156 114 93 500 222 408 100 359 167 318

Maximum hydrogen (%) ND ND 3 .0 ND 19 .3 ND 10 .9 5 .7 ND ND ND 4 .2 12 .3 3 .0 6 .9 16 .2 14 .4 15 .0 8 .0

Day

6 26 26 12

16 16 6 16 31 26 26 12

* Data presented in order of yield . ND, not detected .

3 .1 . Moisture content Drum G was the only drum in Block 1 to undergo anaerobic decomposition to methane in the first 60 days of the experiment . The higher moisture content in Drum G was thought to be the factor which stimulated methane production and water was added to



32

M . A . Barlaz et al .

the other drums in Block 1 on day 60, and again on day 88 . These later water additions did not stimulate the production of methane in any of the Block I drums . Methane production was not stimulated by the higher moisture content used in Block 2 . 3 .2 . Sterile refuse Ideally, the effectiveness of old refuse as a diluent should be based on the time up to the onset of methane production and the volume of methane produced . After 88 days, Drum G was the only drum in Block I to produce methane so such an analysis was not possible . A less accurate parameter on which to base the evaluation of old refuse as a diluent is the methane concentration in each drum . Statistically, there was not a significant difference at the 95% confidence level between the mean methane concentration in Block I drums containing 0 and 50% old sterile refuse based on the methane concentration on day 79 . After day 79, the experimental conditions of Drums A and I were modified and comparisons are no longer possible . Drum K was not included in this comparison because it was not part of Block l . There is a large amount of variation in the methane concentration data used for the above analysis and this is part of the reason that statistically significant differences cannot be demonstrated . Nevertheless, drums with higher amounts of old sterile refuse seem to have higher methane concentrations and further research on the use of diluents is needed . 3 .3 . Sterile cover soil

Drums D, F and N were the only drums tested with 100% sterile cover soil . The gas phase of Drums D and F, but not Drum N, contained methane . This demonstrates that the cover soil used in this study was not the only source of methanogens in the drums . There was no significant difference between the mean of the methane concentrations in drums with 100% fresh soil and those with 50 or 100% sterile soil on day 79 . The presence of methanogens in the cover soil was not the factor controlling methane production in this study . 3 .4 . Modifications to Block I Methane production began in Drum A almost 100 days after the drum rotation was initiated ; thus it is difficult to attribute methane production to this procedure . Drum I contained 19 .4% methane on day 88, and it was thought that the addition of acetate, an immediate precursor of methane would stimulate methane production . No stimulation was measured although the methane concentration in the drum increased . The methane composition in Drum H was 48 .7% prior to the temperature elevation on day 110 . The methane production was measured in Drum H ; however, there is evidence to show that Drum H leaked (Barlaz et al. 1986) . It is not possible to determine whether the elevated temperature in Drum H or the use of old sterile refuse is the factor which stimulated methane production . 3 .5 . Old refuse Old refuse was effective as a seed of bacteria acclimatized to anaerobic refuse decomposition . Methane production began immediately in Drums R and S, and the methane concentrations increased steadily to values of 57 .5% and 47 .2% after 47 days in Drums





Gas production in sanitary landfills

33

1200

1000

goo

~~

• • Y• goo L' C„

400

•

• •

,

• (..

•

0

(~

•'•

200

Gas production not measured due to a malfunctioning gas meter

(4•

010,

•• •

L

•

too(,*

200

300

400

•

500

600

700

500

Days

Fig . 1 . Methane production rate in Drum S (average weekly rate given in 1000 x litres of methane/day per kilogram of dry grindable volatile solids) .

R and S, respectively . The appropriate control drums, O and Q, did not produce any methane over this period . Methane production-rate data for Drum S is presented in Fig . 1 . 3 .6 . Anaerobic sewage sludge The use of sewage sludge as a seed of anaerobic bacteria was not successful . Over the first two weeks of the study less than 1 1 of methane was produced in Drum T and the methane concentration never exceeded 10% . On day 53 the pH of the leachate from Drum T was 6 .3 . In previous work by Buivid (1981), refuse seeded with sludge and with calcium carbonate to control the pH, exhibited enhanced methane production . The refuse used in Buivid's work contained a high paper content (77%) . This means that there was relatively less food waste which is more likely to contain bacteria and soluble substrates . Pohland (1975) has shown that the addition of sludge to refuse will cause an increase in carboxylic acid concentrations . Stamm et al. (1985) found that the addition of sludge to refuse which had been incubating in 0 .76 m 3 (1 yd 3 ) test lysimeters for three years stimulated methane production . 3 .7 . Nitrogen purge The removal of oxygen from Drum P did not stimulate the production of methane . The presence of oxygen in refuse initially was not the factor which limited the onset of methane production . It is expected that there is a population of bacteria present in refuse which is capable of consuming the oxygen initially present in the refuse ecosystem . However, there is not sufficient data from this study to demonstrate that rapid oxygen depletion will occur in freshly buried refuse . 3 .8 . Acetate addition The initial addition of acetate and the removal of oxygen from Drum L did not stimulate methane production . If acetate utilizing methanogens were present and environmental conditions were suitable for their growth, then the availability of acetate should have stimulated methane production .



34

M. A . Barlaz et al . 3 .9 Leachate recycling and neutralization and refuse neutralization

Drums I, L and O were selected for leachate recycling after it became apparent that these drums were not about to begin methane production without some stimulation . The initiation of leachate recycling did not stimulate methane production or increase the methane concentration of the head space in the drums . It was decided therefore to neutralize the leachate on a daily basis in Drums I, L and O . Methane production began about 55 days after the initiation of leachate neutralization in Drums L and O . Although the methane concentration increased, there was no methane production from Drum I . The methane concentration as a function of time for Drums K, L and O is presented in Fig . 2 . 70

o 60

ao-

•

O-_ O

50 c o

•

°~

40

U 0

30

C•

/

O

O

• C •

20

7E

0

^ /

10

'∎ ∎ • /

o, ∎-0 --0

0 60

60

100

120

140

160

ISO

200

220

Days

Methane concentration for Drums K, L and O . Leachate neutralization was initiated on day 99 in Drums L and O and buffer was added to Drum K on day 113 . The methane concentration remained above 50% after day 206 in all three drums. - •-, Drum L ; -0-, Drum K; -∎-, Drum O . Fig . 2 .

In Drum K the refuse was neutralized by the addition of a sodium carbonate solution in one dose . Methane production began 10 days after the sodium carbonate addition in this drum . The volume of methane produced in Drum K was not measured accurately (Barlaz et al . 1986) so that the degree of stimulation cannot be determined . However, as shown in Fig . 2, there was no methane in the gas phase of Drum K prior to the

TABLE 4 Comparison of buffer addition to Drums I, K, L and O

Drum 1 K L 0

Total buffer added (g CO3 -)

Calculated sodium concentration * (g 1 - ')

Total leachate recycled (1)

75 .1 47 .6 20 .4 146 .8

3 .16 0 .37 1 .10 1 .62

33 .8 25 .80 148 .9

* Based on the sodium added to the drum as sodium carbonate and sodium acetate (Drums I and L) and the calculated moisture content of the drum .



Gas production in sanitary landfills

35

neutralization step . Thus buffer addition, and not the use of 50% old sterile refuse in Drum K, is the parameter which stimulated methane production . The rate and yield of methane production in Drum O was much greater than that in Drum L . More buffer was added to Drum O than to the other drums, Table 4, and it is likely that a greater fraction of the refuse was neutralized . The absence of methane production in Drum I cannot be explained conclusively . Henson et al. (1986) have shown that a sodium concentration of 8 .7 g 1 -' is not inhibitory to M. barkeri, an acetate utilizing methanogen . 4. Discussion 4.1 . Methane composition and methane production rates In this study, the methane composition of a drum was not always indicative of the methane production rate of the drum . Drums D, I and L all contained more than 50% methane . However, there was very little methane production in Drums D and L and no significant gas production in Drum I . Clearly methanogens must have been present in the drums with high methane concentrations . However, environmental conditions in Drums D, L and I were not favourable for the production of gas . The methane concentration did not correlate with the onset of methane production in a drum . In Drums G and O, measurable gas production began after the drums reached 50% methane . In Drums R and S, gas production began immediately, although the methane concentration was only 2% . The methane production rates are presented in Table 2 . The seven-day maximum rate for Drums G, O, R and S are all within 30% of each other, while the maximum rate measured for Drum A is significantly lower . The yields reported in Table 2 are similar to the yields reported by others (Halvadakis et al . 1983) . 4.2 . Replication Two drums from Block 2, R and S, contained old refuse which served as a source of bacteria acclimatized to the degradation of refuse . Both drums produced significant volumes of methane so that it is possible to make comparisons and obtain some understanding of the variability involved in working with 50 kg samples of refuse . The methane concentrations of both drums rose steadily over the first several weeks of the experiment until each had reached a methane concentration of 50% . However, there was considerable variation in the rates of methane production between Drums R and S . Over the first 139 days of the experiment, Drum S produced 31 1 of methane per kilogram of dry grindable volatile solids, as compared to 61 in Drum R . The maximum rates for Drums for R and S are within 3% of each other, yet Drum R seemed to lag behind Drum S by 30-60 days . Drum G was initiated in Block 1 while Drum K was a part of Block 2 . Otherwise, conditions in the drums were similar . Drum G reached a methane concentration of 60 .4% after 38 days and began to produce significant volumes of methane after 49 days . There was no methane production in Drum K after 113 days at which time the experimental conditions in Drum K were modified so that further comparison is not appropriate . The behaviour of Drum G was not repeated in Drum K . There is evidence that the initial temperature in Drum G was higher than that in Drum K . At the time of filling, the temperature of the refuse used for Drum G was



36

M . A . Barlaz et al .

noticeably warmer than the refuse used for the other Block 1 drums . Drum K was not noticeably warmer than the other Block 2 drums . This temperature difference can probably be attributed to aerobic activity and may explain the difference between Drums G and K . The first substrates to be degraded in refuse are the readily available soluble carbohydrates . Under aerobic conditions, these substrates will be oxidized to carbon dioxide and the temperature of the refuse will increase . Under anaerobic conditions, these substrates will be fermented to carboxylic acids and alcohols . Fermentation, and not oxidation, of the soluble substrates present in refuse initially could result in a rapid decrease in pH . It is possible that the soluble substrates in fresh refuse were completely oxidized in Drum G but only fermented in Drum K . After consumption of the soluble substrates in Drum G, polymer hydrolysis would be necessary for refuse decomposition to proceed . Polymer hydrolysis is slow relative to the utilization of soluble substrates ; this may have allowed time for the methanogenic population to develop sufficiently in Drum G and prevent an inhibitory accumulation of carboxylic acids . In Drum K, where it is suggested that the soluble substrates were fermented to carboxylic acids, the pH of the refuse may have decreased rapidly to a point where the methanogens were inhibited . This would lead to the souring of Drum K and account for the difference in methane production between Drums G and K . This scenario suggests that, initially, the presence of oxygen in refuse may help to prevent the refuse from souring and would thereby stimulate anaerobic decomposition to methane . Of course, in time the oxygen in a landfill would be depleted .

4 .3 . Moisture content and water flux The effects of moisture content and water flux have been observed through the initially high moisture content in Drum G and the Block 2 drums, the water additions to Block 1, and the leachate recycling and neutralization program . Water has many effects on the refuse ecosystem, as presented earlier . This study has demonstrated that initiation of a drum of shredded refuse at 55% moisture will not assure methane production . A high initial moisture content appears to have stimulated the hydrolysis of cellulose to a greater extent than it stimulated the methanogens and the acidogens . This may be due to the lower initial population of methanogens relative to hydrolytic bacteria in fresh refuse . Rapid polymer hydrolysis led to an accumulation of carboxylic acids and a drop in the pH which inhibited biological activity . The results of the leachate recycling and neutralization work confirm the trend described above . With the initiation of leachate recycling, carboxylic acid concentrations increased and the leachate pH was about 5 .0 . Without leachate neutralization it is unlikely that there would have been any methane production . The carboxylic acid concentrations in Drum O began to decrease only after the initiation of leachate neutralization, the onset of methane production and the utilization of acetic acid by the methanogens . The carboxylic acid concentration in Drum O as a function of time is presented in Fig . 3 . Regression analysis showed that there was a significant decrease over time in the concentration of each acid, except propionate . There was a significant increase in the acid levels in Drum L over time which is as expected since the pH of the leachate in Drum L did not increase and there was little methane production . The methanogens, in effect, drive the reactions in which butyrate and propionate are converted to acetate by the acidogenic bacteria (Mah 1982) . This demonstrates the importance of an established methanogen population prior to the stimulation of the



Gas production in sanitary landfills

37

7000 000 5000

o

\ 0 o O OoÔ

0

300

u 2000 Q

0-, _O

0

'

~/O

1000 ∎~ • `∎"I M-0 o 50 100 150 200 250 Days

300

350

400 0

M

450

Fig . 3 . Carboxylic acid concentrations in Drum O . Onset of methane production was on day 154 . Leachate recycling and neutralization was initiated on day 99 . - •- , Acetate ; -O-, propionate; -∎-, isobutyrate ; -O-, butyrate .

refuse ecosystem as such stimulation is likely to lead to rapid production of carboxylic acids . If these acids are allowed to accumulate, the decrease in pH will inhibit methanogenesis . 4 .4 . The role of hydrogen gas in refuse decomposition The model proposed by Farquhar & Rovers (1973) for the decomposition of refuse is borne out by this study . Increased understanding of the process of anaerobic decomposition makes it possible to clarify the importance of hydrogen in an anaerobic system . Hydrogen concentrations of up to 20% were measured in this study (Table 3) . Hydrogen was typically detected within two weeks of the initiation of each block of drums . Hydrogen is produced by both the hydrolytic and acetogenic bacteria and it is a substrate for the methanogenic bacteria . However, measurable net hydrogen production is neither a necessary or sufficient condition for methane production from refuse . No hydrogen was detected in Drums R and S where methane production began immediately . The accumulation of hydrogen is an indication of an imbalance in the microbial population . Many key intermediate reactions in the formation of methane from cellulose are not thermodynamically favourable in the presence of even low concentrations of hydrogen . For example, the partial pressure of hydrogen must be less than 10 -6 atm for the production of acetate from butyrate to be thermodynamically favourable (Mah 1982) . The physiology of acetogenic bacteria is such that they must produce hydrogen in order to convert butyrate into acetate . Thus, hydrogen-utilizing bacteria are essential for butyrate utilization . Hydrogen serves as an important electron sink for the acidogenic bacteria . If cellulose fermentation is to proceed under conditions of a hydrogen accumulation, then relatively less acetate and higher amounts of other electron sinks such as lactate and succinate will be formed . The pathways leading to the production of electron-sink products other than hydrogen result in less energy generation for the microbes involved . Ultimately, this will reduce the rate of cellulose fermentation and thus, the rapid consumption of hydrogen by the methanogenic bacteria actually drives a complex series of reactions which lead to the production of methane from cellulose .



38

M . A . Barlaz et al . 4 .5 . Rate limiting step

The results reported here make it possible to address the question of the rate limiting step in the production of methane from refuse . Other researchers (Schink & Zeikus 1982) have suggested that the biodegradation of polymers is the rate limiting step in methane production . However, the addition of acetate to Drum L was not stimulatory, thus suggesting that acetate utilization is rate limiting . The application of such seemingly conflicting information to a landfill requires an understanding of the conditions under which each conclusion was drawn . Halvadakis et al. (1983) have suggested that both of the above hypotheses are correct . At the time of burial, there will be some soluble material in refuse which may be converted rapidly to carboxylic acids . At this stage it is the absence of a methanogen and acidogen population which is limiting the production of methane . In the second stage of refuse decomposition the easily degradable materials have been consumed and cellulose is the primary substrate from which methane may be produced . The very low levels of carboxylic acids in the drums which were producing methane (Barlaz et al. 1986) suggest that at this tage the methanogenic and acidogenic bacteria could handle more substrate ; in other words, a faster rate of cellulose hydrolysis would stimulate methane production . It is important to remember that in order for cellulose hydrolysis to become rate limiting, refuse decomposition must proceed to the second stage identified above .

5 . Conclusions (1) Old, anaerobically-degraded refuse was an effective seed to initiate anaerobic decomposition in containers of shredded refuse although it did not increase the maximum rate of methane production above that measured for shredded refuse without a seed . (2) The addition of acetate did not stimulate the production of methane in refuse at 55% moisture . (3) Leachate recycle and neutralization and the addition of buffer in one dose stimulated the production of methane . (4) There is much variation in the rate of methane production from refuse even when the methane concentration is greater than 50% . (5) It was difficult to duplicate methane production in replicate containers . Acknowledgements The authors would like to acknowledge Getty Synthetic Fuels Inc . for their support of this project .

References Barlaz, M . A ., Milke, M . W . & Ham, R . K . (1986), Parameters affecting refuse methanogenesis and the solids decomposition of anaerobically degraded refuse, Proceedings of the Ninth Annual Madison Waste Conference, 9-10 Sept . 1986 . University of Wisconsin, Madison, WI, p . 37 . Buivid, M . G ., Wise, D . L ., Blanchet, M . J ., Jenkins, B . M ., Boyd, W. F . & Pacey, J . G . (1981), Fuel gas enhancement by controlled landfilling of municipal solid waste . Resources and Conservation, 6, 3-20 . DeWalle, F . B . et al . (1978), Gas production from solid waste in landfills . ASCE-EE, 104 : EE3, p . 415 .



Gas production in sanitary landfills

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