temperature effects on anaerobic fermentation of domestic refuse

BIOTECHNOLOGY AND BIOENGINEERING VOL. XVI, PAGES 771-787 (1974)

Temperature Effects on Anaerobic Fermentation of Domestic Refuse

JOHN T. PFEFFER, Department of Civil Engineering, University of Illinois, Urbana, Illinois 61801

Summary Anaerobic fermentation of organic solid waste can provide a significant source

of fuel gas (methane). Application of this process requires a better understand- ing of the kinetics of the biological system. The literature is replete with kinetic studies of this process as applied to waste solids from water pollution control systems. Much of this work has been conducted in the mesophilic temperature range. Increased temperatures yield higher reaction rates that will improve the economics of the process. The rate limiting step in the fermentation of refuse is the hydrolysis of the complex organic solids, in particular cellulose. Cellulose is a major component of the refuse. A laboratory study employing domestic refuse has shown the effect of temperature on the rate of methane fermentation. The optimum mesophilic temperature was found to be 42"C, while the optimum thermophilic temperature was a t least 60°C. No data was obtained beyond the 60°C temperature. Reaction rate constants are presented for anaerobic fermentation of domestic refuse. Because of the characteristics of the substrate it was not possible to obtain the necessary measurements for evaluation of constants in the Monod model. An overall system constant was developed.

INTRODUCTION

Two concurrent problems facing this country are the disposal of solid wastes generated by society and the need for new sources of fuel to supply the energy needs of society. Solid wastes, especially the combustible portion, has been considered as a fuel for production of energy in the form of steam and electricity. Incineration of refuse with heat recovery is practiced in a number of large communities. A demonstration project to determine the practicality of using a refuse-coal mixture as fuel for the generation of electricity is being conducted. Pyrolysis of refuse has also been extensively researched with the hope of reclaiming energy from refuse.

Since refuse does contain a significant amount of energy, 7000 to 8000 BTU per lb of dry solids, it is logical to attempt to utilize this

771 @ 1974 by John Wiley di Sons, Inc.

772 PFEFFER

energy while processing the refuse for final disposal. The urban refuse production in 1967 was estimated to be 256 million tons.’ The energy contained in this refuse is about 3.6 X 1015 BTU. Refuse production continues to increase due to population increases as well as increased per capita production. The energy consumption in 1967 was approximately 60 X 1015 BTU.2 The natural gas consumption accounted for about 19 X 1015 BTU. Reclamation of a portion of the energy in refuse in the form of methane can add a significant supplement to existing sources of natural gas. Of course, this process offers a possibility of producing major quantities of natural gas (methane) from “energy crops.” This provides a mechanism for converting solar energy into a usable fuel.

REFUSE COMPOSITION

Refuse contains any .and all solid material rejected by society. The major portion of the refuse from a municipality originates from residential and commercial areas. While the refuse composition is location and time dependent, the composition of a typical domestic refuse is as shown in Table I. A major portion of the organic material is of plant origin, i.e., paper, leaves, garbage, and wood. As would be expected, cellulose is a major constituent in refuse.

The chemical composition of the refuse used in this study is shown in Table 11. This refuse was obtained from the Environmental Pro- t#ection Agency’s Cent,er Hill Research Laboratory in Cincinnati,

TABLE I Composition of a Typical Domestic Refuse

yo of Total by Weight

Component (wet wt) (dry wt)

Paper Leaves Wood Synthetics Cloths Garbage

Combustibles Glass Metal Ashes, stone, etc. Total moisture content

48.0 9.0 2 .0 2.0 1.0

16.0 78.0 6.0 8.0 8 .0

-

-

35.0 5.0 1.5 2.0 0.5 8.0

52.0 6.0 8.0 6 .0

28.0

-

ANAEROBIC FERMENTATION OF REFUSE 773

TABLE I1 Composition of Refuse

Number of Determination Determinations Average Range

Volatile total solids 3 81.8% 81.6-82.1 Lipids 3 6.2% 5.98-6.36 Chemical oxygen .i 0.982 g/g 0.943-1.021

demand Calorific value 2 7510 BTU/lb 7410-7610

Carbon 2 44.170 43.6844.50 Hydrogen 2 ,i.7% 5.625-5.750

dry solids

- Nitrogen 2 0% (CHN analyzer)

(wet Kjeldahl) Nitrogen 4 0.68% 0.666-0.700

- Phosphorus 5 0% Total carbohydrates 3 52.7570 51.5-53.5 Cellulose 5 35.870 35 .O-37.0

Ohio. The refuse was shredded at this laboratory prior to transport- ing it to Urbana. A sample for analysis was selected using routine solid wastes sampling procedures. After drying, the glass, metal, ceramics, stones, etc. were carefully separated by hand, with the remaining material used for characterization. The analyses were conducted according to Standard method^.^ Carbohydrate and cellulose were determined using the Anthrone method as described by Viles and Silverman14 Gaudy,5 Snell and Snell16 and Koehler.' The calorific value was measured in a Parr-Bomb Calorimeter as per manufacturer's instructions.

The use of this material as a microbial substrate requires the addition of supplemental nutrients. The phosphorus content was essentially zero. The carbon to nitrogen ratio was approximately 65 to 1 which would indicate nitrogen deficiency. Supplemental nutrients in the form of ammonium chloride and dibasic potassium phosphate were used. Sufficient nitrogen was added to maintain the reactor ammonium nitrogen between 300 and 500 mg/liter. Phosphorus was added to maintain a nitrogen to phosphorus ratio of 5 to 1.

Initial experiments indicated that the refuse fermentation was aided with the addition of raw sewage sludge. Suf€icient sludge was added such that the dry sludge solids accounted for 3 percent of the solids added to the reactors. The sludge probably contributed many

774 PFEFFER

of the trace nutrients required for good fermentation. It was also a continued source of inoculum of various microorganisms.

The composition of the gas produced by fermentation of refuse will be dependent upon the chemical composition of the refuse. In the conversion of carbohydrates to carbon dioxide and methane, equal volumes of each gas are produced. This reaction is shown in eq. (1).

However, all of the carbon dioxide is not released as a gas but enters into reactions with water and hydrogen ions. Microorganisms can deaminate biodegradable protein, producing ammonia which reacts with water according to eq. ( 2 ) . The hydroxide produced by this reaction,

(CtjH1oO5)~ x H ~ O -+ XCsH&6 -+ 3xCHI + 3xCO2 (1)

iSH3 + HOH S NH4+ + OH-

reacts with carbon dioxide to form bicarbonate ion as shown in eqs. (3) and (4). Therefore, the protein content of the substrate will significantly affect the quantity of carbon dioxide actually released from solution as well as the bicarbonate buffer capacity of the system. Since the refuse contains very little nitrogen, the natural buffer formed will be insufficient to maintain the pH in the desired range.. It will be necessary to add caustic to control the pH. The quantity of caustic added will influence the composition of the gas in the same manner as described above.

COz + HOH S HzC03 S H+ + HCO3-

&C03 + OH- + HCO3- + HOH

( 2 )

(3)

(4)

The carbon dioxide incorporated in the bicarbonate ion is removed from the reactor in the liquid phase rather than the gas phase. There are two factors related to this washout of carbon dioxide. The concentration of alkalinity will control the washout rate a t a given retention time. The retention time, or the liquid throughput rate, will also influence the washout rate. Therefore, for a given substrate, digestion a t shorter retention times will produce a gas having a higher methane content.

The bicarbonate ion is not the only ion contributing to the alkalinity or buffer capacity. The organic acids and acid salts also con- tribute to the alkalinity. Equation (5), originally developed by Pohland and Bloodgood* and modified by M ~ C a r t y , ~ can be used to calculate the bicarbonate alkalinity.

BA = TA - (0.85)(0.833)TVA (5)


where

BA = bicarbonate alkalinity, mg/liter as CaC03 TA = total alkalinity, mg/liter as CaC03

TVA = total volatile acid concentration, mg/liter as acetic acid

The milligrams/liter of acetic acid is converted to the equivalent alkalinity as CaC03 by the 0.833 multiplier. The 0.85 factor accounts for the fact that only 85 percent of the volatile acid alkalinity is measured by titration of total alkalinity to pH 4. This equation also assumes that there is no significant concentration of other materials such as phosphates, silicates, or other acid salts which also produce alkalinity.

The above analysis permits an accurate evaluation of the actual bicarbonate ion concentration in a digesting system. Since the normal operating pH in these systems is 7.0 or less, the carbon dioxide- bicarbonate equilibrium can be combined with the solubility of carbon dioxide to predict the dry gas composition under various operating conditions. One must also include the effect of the vapor pressure of water. This increases substantially a t the thermophilic temperatures. These data are shown in Table 111. The added water vapor in the gas reduces the partial pressure of carbon dioxide which in turn changes the bicarbonate alkalinity equilibrium.

For a given pH, say 7.0, the ratio of bicarbonate ion to carbonic acid (carbon dioxide) increases from 4.9 at 30°C, to 5.19 at 60°C. These values are shown in Table IV. When the effect of temperature on the solubility of carbon dioxide as described by Henry's Law is considered, the effect is compounded. Equation (6) shows this solubility relationship. For a given mole fraction in solution, an increase in temperature will increase the partial pressure.

Temperature also influences this equilibrium.

(6) P partial pressure (mmHg) X mole fraction

K = - =

TABLE I11 Vapor Pressure of Water at Various Temperat.uresa

Temperature 30 3 5 40 4.3 30 5.5 60

Vapor Pressure 31.8 42.2 55.3 71.9 92.5 118 149.4 ("C)

(mmHg)

Source: Handbook of Chemistry and Physics, 40th ed., CltC Publishing Company.

776 PFEFFER

TABLE IV Effect of Temperature on K1 of Carbonic Acida

Temperature ("C) 30 40 50 60 K~ x 107 4 . 9 .5.04 5.13 5.19

a Source: Handbook of Chemistry and Physics, 40t,h ed., CRC Publishing Company.

Table V lists Henry's constant for carbon dioxide a t various temperatures. The temperature effect on this equilibrium results in a significant reduction in alkalinity a t higher temperatures. A decrease in the solubility of carbon dioxide by a factor of 0.85 in going from 30 to 60°C in conjunction with an increase in the vapor pressure of water by a factor of 4.7 produces a significant reduction in the carbon dioxide in solution. This effect is not offset by the change in the carbon dioxide-bicarbonate relationship with increasing temperature. Therefore, a much lower bicarbonate alkalinity will be asso- ciated with a given pH at the elevated temperatures. Since more carbon dioxide is lost in the gas phase, the caustic addition required to maintain a desired pH would be less.

A similar analysis for gas composition would suggest that the carbon dioxide content of the gas would increase with increasing temperatures. The decrease in the amount of carbon dioxide remaining in the liquid phase results from more carbon dioxide in the gas phase. This fact is complicated by the increased vapor pressure of water a t higher temperatures. At 60°C the gas contains 19.6 percent water vapor. This changes the percentage of methane and carbon dioxide in the dry gas from what i t was as produced.

Based on eq. ( l ) , 6 moles of dry gas jvill be produced for each 162 g of cellulose fermented. The gas production would be 0.829 liter/g of cellulose decomposed. If an estimate of the biodegradable material in the refuse can be made, the gas production (in the liquid and gas phase) can serve as an indicator of the kinetics of refuse diges-

When the gas is dried, this water is removed.

TABLE V Henry's Constant for Various Temperat.uresa

Temperature ("C) 30 40 50 60 K x 10-7 0.139 0.173 0.217 0.258

Source: Handbook of Chemistry and Physics, 40th ed., CIiC Publishing Company.


tion. Golueke'O reported that kraft paper, which accounts for 40 percent of the organic urban refuse is 91 percent digestible. News- print is about 33 percent of the organic urban refuse and about 50 percent is destroyed in the digestion process. Therefore, paper, which constitutes about 73 percent of the organic refuse, can be destroyed to the extent of 72 percent. This means that 52.5 percent of the volatile solids can be digested as cellulose type materials. Complete digestion of the paper fraction would produce 0.435 liters of gas per gram of volatile solids added.

The remaining 27 percent of the organic refuse is composed of garbage, wood, lawn trimmings, plastics, cloth, etc. The destruction of this combination of materials in anaerobic digestion could be expected to be about 50 percent. Therefore, of the total organic refuse digested measured in terms of volatile solids, it could be expected that 66.0 percent of this material is biodegradable. If one assumes that cellulose and other complex carbohydrates are the only materials of significance that are fermented, then the maximum gas production per gram of volatile solids can be estimated as 0.547 liters. This figure can be used as the gas value of the substrate a t time zero. It must be remembered that this is the total gas production including not only the measured gas flow, but also the carbon dioxide contained in the liquid phase as carbon dioxide and bicarbonate ion.

EXPERIMENTAL APPARATUS

The experiments were conducted in completely mixed reactors housed in a constant temperature room. These reactors had a liquid volume of 15 liters. The gas production was measured by Precision wet test meters and has been corrected for water vapor and temperature. All gas data are reported as dry gas a t standard temperature and pressure. All reactors received 50 g per day of refuse plus 50 ml of raw sewage sludge a t 3 percent solids. The residence time was varied by increasing the volume of tap water added to the dry refuse. The system was batch fed daily with an equal volume of reactor contents withdrawn.

The reactors were monitored for pH, alkalinity, gas production and composition, volatile acids, and ammonia nitrogen. The pH was controlled with the addition of sodium hydroxide. Adequate nitrogen and phosphorus were maintained. Initial runs were a t the 35°C temperature. Seed sludge was obtained from the digester of the local water pollution control plant. However, a period of accli-

778 PFEFFER

mation was required to achieve substantial destruction of the feed material. The organic acids remained low, but the gas production was less than anticipated. After about a 30 day period, the gas production significantly increased indicating that the culture was acclimated to the refuse substrate.

A transition period of approximately four weeks was provided when increasing the temperature to a new operating level. The temperature was increased approximately 1.0 to 1.5"C each week until the new operating temperature was reached. In all cases except from 43 to 45"C, the operating parameters were stable and the gas production increased. When the 43°C was achieved, the gas production decreased significantly. In order to verify this effect, the temperature was lowered to 40°C. The gas production increased and the operating parameters returned to normal. After allowing the systems to achieve equilibrium a t 40"C, the temperature was again increased a t 0.5 to 1.O"C intervals. The system was operated a t each temperature for one week. Again the gas production began to decrease when the 43°C was reached. The temperature was grad- ually increased to 45"C, which was one of the operating temperatures tested.

Operating temperatures investigated were at 5°C intervals.

RESULTS

After the reactors became acclimated to the operating temperature, data was collected to determine the operating conditions and gas production when operating at what could be considered a steady state condition. Such a condition could not be maintained because of the variability in the feed material. However, the only variable was the feed composition resulting from the heterogeneous nature of the shredded refuse and the small size of daily input, i.e., 50 g dry weight. The average reactor operating conditions for approximately 60 days a t each temperature are given in Table VI. The most significant variation in these data is the decrease in alkalinity and caustic re- quirements with increasing temperature. This decrease occurs while the pH remains constant or increases slightly.

This table includes the measured gas production as well as the calculated carbon dioxide loss from the reactors in the liquid phase. The total gas production is expressed in terms of liters per day as well as liters per gram of volatile solids added to the reactors. This gas production can be

The gas production is shown in Table VII.


TABLE VI Reactor Operating Conditions

Volatile Retention Alkalinity Acids Caustic

Reactor Time (mg/liter (mg/liter Added Number (days) PH CaC08) acetic acid) (meq/day)

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

4 -1 6 8

10 15 20 30

4 4 6 8

10 15 20 30

4 4 6 8

10 15 20 30

4 4 6 8

10 15 20 30

Temperature, 35°C

6.77 1400 6.78 1460 6.78 1700 6.75 1840 6.75 1900 6.77 2300 6.81 2600 6.87 3000

Temperature, 40°C

6.77 1244 6.77 1319 6.78 1595 6.78 1780 6.78 1982 6.80 2356 6.83 2554 6.87 3087

Temperature, 45°C

6.73 1269 6.67 1214 6.74 1312 6.78 1612 6.78 1752 6.81 2002 6.80 2335 6.82 2410

Temperature, 50°C

6.75 1156 6.78 1154 6.83 1251 6.81 1394 6.82 1544 6.85 1778 6.86 2082 6.89 2343

420 420 380 324 278 282 279 330

278 240 222 298 405 642 733 866

143 531 116 81

171 282 220 22 1

292 210 187 257 218 325 302 402

93 93 59 56 56 38 38 38

80.3 80.3 70.3 52.0 49.0 36.0 23.0 19.0

77.3 78.6 57.0 50.7 42.1 35.7 27.0 20.0

92 87 54.7 52.5 45.0 22.5 15.0 12.5 (Continued)

580 PFEFFER

TABLE VI (Continued)

Volatile Retention Alkalinity Acids Caustic

Reactor Time (mg/liter (mg/liter Added Number (days) PH CaC03) acetic acid) (meq/day)

3 4 6 8

10 1 5 20 30

3 4 6 8

10 15 20 30

Temperature, 55°C

6.85 1050 6.85 1300 6.85 1395 6.85 1515 6.85 1435 6.85 1545 6.90 1490 7.00 1885

Temperature, 60°C

6.8 1030 6.8 1140 6.8 1530 6.9 1600 6.9 1920 6.8 1900 6.8 1900 6.8 2030

112 63.7 58 58.5 64 37.4 79 30.0

208 23.7 215 10.7 298 3.7 332 1.1

9.14 7.85 6.43 5.57 3.00

- 1.86 0.86 0.29

-

- -

- -

- -

used as a measure of the degree of substrate utilization. The calculated carbon dioxide loss in the reactor effluent is determined by the carbon dioxide incorporated in the bicarbonate ion as a result of caustic addition and the natural production of hydroxide ion from protein deamination. It also includes the free carbon dioxide that is present based upon the carbonic acid-bicarbonate equilibrium. This value can be calculated if the pH, alkalinity, and temperature are known.

DETERMINATION OF THE RATE CONSTANT

An attempt to fit the Monod model to this system was an immedi- ate failure. Determination of the mass of organisms was impossible. All of the standard approaches were fruitless because of the large amount of biologically inert material and the difficulty of achieving a mass balance for the system. After analyzing the data, it was found that the best approach would be to establish an overall rate


TABLE VII Dry Gas Production (standard temperature and pressure)

Total Gas

Reactor Measured Calc. C02 in liter/g Number Gas (Ipd) Liquid eff. (Ipd) IPd vol. sol. Add.

1 4.81 2 4.81 3 6.92 4 7.86 .5 9.06 6 9.93 7 10.08 8 10.34

1 8.35 2 8.35 3 9.89 4 10.11 5 11.05 6 11.77 7 12.56 8 13.65

1 7.18 2 7.18 3 4 8.23 .5 9.51 6 7 10.94 8 11.39

-

-

- 1 2 7.52 3 4 11.62 .5 13.20 6 13.65 7 14.10 8 14.70

-

Temperature, 35°C

1.55 1.62 1.25 1.02 0.84 0.68 0.58 0.48

Temperature, 40°C

1.30 1.39 1.12 0.93 0.83 0.66 0.54 0.46

Temperature, 45°C

1.41 1.34 0.97 0.89 0.78 0.59 0.52 0.36

Temperature, 50°C

1.29 1.29 0.92 0.77 0.68 0.53 0.46 0.35

6.36 6.43 8.17 8.88 9.90

10.61 10.66 10.82

9.65 9.74

11.01 11.04 11.88 12.43 13.10 14.11

8.59 8.52

9.12 10.29

11.4F 11.75

-

-

-

8.81

12.39 13.88 14.18 14.56 15.05

-

0.169 0.171 0.217 0.236 0.263 0.282 0.284 0.288

0.257 0.259 0.293 0.294 0.316 0.331 0.348 0.375

0.228 0.230

0.243 0.278

0.305 0.313

-

-

-

0.234

0.335 0.369 0.377 0.387 0.400

-

(Continued)

782 PFEFFER

TABLE VI I (Continued)

Total Gas

Reactor Measured Calc. COz in liter/g Number Gas (lpd) Liquid eff. (lpd) 1Pd vol. sol. Add.

6.66 12.56 13.65 13.99

-

15.72

4.7 15.86 15.98 14.59 15.75 15.49 16.28 16.96

Temperature, 55°C

1.55 1.44 1.03 0.83 0.63 0.46 0.33 0.28

Temperature, 60°C

1.52 1.26 1.13 0.88 0.85 0.56 0.42 0.30

8.21 14.00 14.68 14.82

- 16.00

6.22 17.12 17.11 15.47 16.60 16.55 16.70 17.24

0.218 0.372 0.390 0.394

- 0.426

0.165 0.455 0.455 0.411 0.441 0.440 0.444 0.459

constant based on the total gas production. The gas data are more indicative of the degree of stabilization than any other measurable parameter in this system.

The original substrate level, So, was assumed to be the gas yield previously discussed, 0.547 liters per g of volatile solids added. The standard first order rate expression given in eq. (7) can be employed to obtain the system constant for fermentation of refuse. While this is not as sophisticated a model as might be desired, it will provide a constant that will have applicability when dealing with such a complex system as that involved in the fermentation of refuse.

_ - - - K S dS dt (7)

For a completely mixed reactor not employing recycle, a mass balance for substrate can be written in the form of eq. (8).

dS - V = QSo - QS - KSV clt


Under equilibrium conditions, dS/dt is equal to zero. eq. (8) can be rearranged and simplified as given by eq. (9).

Therefore,

so - s s ~- - KO (9)

The data in Table VIII show the relationship between residence time, 0, and substrate removal. A plot of (So - S) /S versus fl will yield a straight line with the slope of the line defining the value for K . This plot is shown in Figure 1.

TABLE VIII Substrate Removal as Residence Time Inweaves

S so - s, - s

5

4 4 6 8

10 15 20 30

4 4 6 8

10 15 20 30

4 4 6 8

10 15 20 30

0.169 0.171 0.217 0.236 0.263 0.282 0.284 0.288

0.257 0.259 0.293 0.294 0.316 0.331 0.348 0.375

0.228 0.230

0.243 0.278

0.305 0.313

-

-

Temperature, 3.5"C

0.378 0.376 0.330 0.311 0.284 0.265 0.263 0.259

Temperature, 40°C

0.290 0.288 0.254 0.253 0.231 0.216 0.199 0.172

Temperature, 45'C

0.691 0.687 0.603 0.569 0.519 0.484 0.481 0.473

0.530 0.527 0.464 0.463 0.422 0.395 0.364 0.314

0.319 0.583 0.317 0.580

0.313 0. .32 0.269 0.492

0.242 0.442 0.234 0.428

- -

- -

0.447 0.455 0.658 0.7.59 0.831 I . 064 1.080 1.112

0.886 0.899 1.154 1.162 1.368 1.532 1.749 2.180

0.715 0.725

0.776 1.033

1.260 1.338

-

-

(Continued)

784 PFEFFER

TABLE VIII (Continued)

e so - s S S so - s - (days) (liter/g) (liter/g) so 5

3 4 6 8

10 15 20 30

3 4 6 8

10 15 20 30

3 4 6 8

10 1 -5 20 30

-

0.234

0.335 0.369 0.377 0.387 0.400

-

0.218 0.312 0.390 0.394 -

-

-

0.426

0.165 0.4.55 0.455 0.411 0.441 0.440 0.444 0.4.59

Temperature, 50°C

0.313 0.572

0.212 0.388 0.178 0.325 0.170 0.311 0.160 0.293 0.147 0.269

- -

Temperature, 55 "C

0.329 0.601 0.235 0.430 0.157 0.287 0.153 0.280

- - -

0.121 0.221

Temperature, 60°C

0.382 0.092 0.092 0.136 0.106 0.107 0.103 0.099

0.698 0.168 0.168 0.249 0.194 0.196 0.188 0.161

- 0.748

1.580 2.073 2.218 2.419 2.721

-

0.663 1.328 2.484 2.575 -

-

-

3.521

0.432 4.946 4.946 3.022 3.877 4.112 4.311 4.636

An acceptable fit of the data is obtained for all temperatures except for the shorter retention times in the 60°C test. It is not known \I hy this poor fit occurred. Additional studies are being conducted to refine this data. As the temperature increases, the ratc constant increases as would be expected. However, there appears to be two ratc constants, one a t the short rctention times and another one for the longer retention times. These two constants are not pronounced a t 35OC, but increasing temperatures show a definite break in the curve. The significance of this break is not known a t this time except that the low rates a t the longer times may be nothing more than endogenous respiration. It is more likely that these rates re-

ANAEROBIC FER.MENTATION OF REFUSE: 785

Aect the very stable nature of a fraction of the refuse. Tests for cellulose remaining showed that even under the high temperature and long retention time, 20 percent of the cellulose as measured by the Anthrone technique remained.

The reaction rates as determined from Figure 1 are given in Table IX. The effect of temperature on this rate is interesting in that shifting of the temperature from the mesophilic to thermophilic range causes a reduction in the rate of stabilization. Increasing the temperature from 35 to 40°C produces the expected increase in reaction rates. However, a t 45°C the rate is less than at 35"C, indicating an adverse response to increased temperature.

Opera- tion a t 50°C produces a rate a little better than at 40"C, but not double as might be expected. However, at 55°C and 60"C, the rates greatly increase. Therefore, i t is essential that one is sure that the

A similar relationship exists in the thermophilic range.

D O 1

0 '0 0 5 10 15 20 25 30

Residence Time, e - Days

Fig. 1 . Rate of substrate removal at. various temperatures.

786 PFEFFER

TABLE IX Reaction Rate Constant

Temperature Initial Final (“C) (day-’) (day-9

3.5 0.055 0.003 40 0.084 0.043 45 0.052 0.007 ,iO 0.117 0.030 .5,5 0.623 0.042 60 0.990 0.040

system is operating a t the optimum temperature for the population of microorganisms desired, i.e., either mesophilic or thermophilic. Temperatures in between the optimum for these ranges will inhibit rather than enhance the biological system.

The final rates achieved at the longer retention times are excep- tionally low at all temperatures. The 40°C run shows a significant rate of stabilization at longer retention times, but this is not the case with the remaining temperatures. The data in Figure 1 suggest that the increase in refuse fermentation a t the longer retention times is marginal a t essentially all temperatures and may simply be the endogenous respiration rate. These data show that maximum retention times should not be greater than 10 days a t 35°C or 6 days a t the 60°C temperature.

The results from this study are not as precise as might be desired. This is because of the nature of the substrate, i.e., shredded domestic refuse. However, it is not necessary to extrapolate the results of a study on a single chemically defined substrate to actual refuse. These data can be applied directly to a system for fermenting refuse for methane production.

This research was supported by the U.S. Environmental Protection Agency (;rant No. EPA-It-800776.

References 1. R. Eliassen, “Solid waste management-A comprehensive assessment of

rolid waste problems, practices, and needs,” Office of Science and Technology, Executive Office of the President, Washington, D. C., May 1969.

2. G. A. Mills, Environ. Sci. Technol., 5 (12), 1178 (1971). 3. Standard Methods for the Examination of Water and Wastewater, 12th ed.,

4. F. J. Viles, Jr., and L. Silverman, Anal. Chem., 21, 950 (August 1949). 5. A. F. Gaudy, Jr. Ind. Water Wastes, 7 , 17 (January-February 1962).

.liner. Public Health Assoc., New York, 1965.


6. F. D. Snell and C. T. Snell, Colorimetric Methods of Analysis, vol. 3,

7. L. H. Koehler, Anal. Chem., 24, 1576 (October 1952). 8. F. G. Pohland and D. E. Bloodgood, J . Water Pollut. Contr. Fed., 35, 11

9. P. L. McCarty, Public Works, 95. 123 (October 1964).

Van Nostrand, Princeton, N. J., 1961.

(1963).

10. C. G. Golueke, “Comprehensive studies of solid wastes management. Third annual report,” Univ. of California, Berkeley, SERL Rep. No. 70-2, June 1970.

Accepted for Publication February 2, 1974

temperature effects on anaerobic fermentation of domestic refuse

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