Agricultural Wastes 7 (1983) 1-12
Wastewater Recycling in Anaerobic Digestion of Beef Cattle Wastes
A. C. Chang, W. C. Fa i rbank , T. E. Jones & J. E. Warneke
Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521, USA
A BSTRA C T
Six bench top mesophilic anaerobic digesters were used to determine the feasibility of recirculating the digester's wastewater effluent during the anaerobic fermentation of beef cattle wastes. The results indicated that wastewater recycling in anaerobic fermentation of beef cattle wastes increased the pH and salinity levels of the digester contents. At high digester loading (> 11.6g Total Solids per litre of digester volume per day) and high rates of wastewater recycling (> 80 %), there was also significant reduction in the percentage of volatile solid destruction and in gas generation.
INTRODUCTION
The production of methane gas (e.g. biogas) through anaerobic fermentation of selected biomass and organic wastes has attracted widespread attention in recent years. In addition to the benefit of energy recovery, it was believed that residues from anaerobic fermentation of orgarAc wastes could also yield a high-protein livestock feed ingredient (Meckert, 1978; Hashimoto et al., 1978).
Since 1978, a pilot plant utilising wastes from a beef cattle feedlot to produce biogas has been operating near Brawley, California. The project was jointly funded by Pacific Gas and Electric Co. (San Francisco, California) and Southern California Gas Co. (Los Angeles, California).
1
Agricultural Wastes 0141-4607/83/$03"00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
2 A.C. Chang, W. C. Fairbank, T. E. Jones, J. E. Warneke
The 67.4M 3 (active volume) experimental digester received 4-3 M 3 of digester feed daily consisting of 2361 kg beef cattle manure (approxi- mately 7 days old, collected at a nearby feedlot) and 3045 kg water. The facility has the capacity of handling wastes from 200 head of cattle (weighing from 300-460 kg). The operating parameters and performance measurements of the pilot digester are summarised in Table 1.
TABLE 1 Operating Parameters and Performance Characteristics of the Pilot Digester
Active volume Hydraulic retention time Operating temperature Solid reductions
Total Solids Volatile Solids
Digester gas Volume Composition (by volume)
67.4M 3 20 days 37"8 °C
25~o 30%
42.5-51.0 M3/day CH 4 58°/o, CO 2 38~o, other 4~o
The digester liquor withdrawn each day was centrifuged to extract the solid fraction. The recovered fibre solids were sun dried, then blended with other feed ingredients. The centrate, which has high pollution potential, requires proper disposal (Table 2). In a previous investigation, the constraints and alternatives for disposing of this wastewater in the Imperial Valley were analysed (Chang & Fairbank, 1981). It was concluded that wastewater disposal would be the greatest limiting factor in scaling up the pilot digester to a full operation. All of the wastewater treatment alternatives considered (e.g. joint treatment with domestic wastes, land disposal, sand bed drying and evaporation ponds) had serious limitations.
Water was added daily to liquefy the manure, so it was logical to consider using the centrate in preparing the digester feed slurry. If recycling of wastewater back into the digester proved successful, it could eliminate the need for wastewater disposal. Even a partial recycling should reduce the amount of effluent requiring final disposal. However, reintroducing the centrate to the digester might affect the performance of the anaerobic fermentation process. This paper summarises the results of a bench top experiment set up to examine the effects of effluent recycling on the anaerobic digestion of beef cattle wastes.
The microbiology and biochemistry of anaerobic degradation of
Wastewater recycling of beef cattle wastes
TABLE 2 Characteristics of Digester Feed and Wastewater Effluent (Centrate)
Generated by the Pilot Digester
Parameter Digester feed Effluent
pH 6"0-6-5 8.0-8"6 Electrical conductivity (mmho cm- 1) 11.5 11"5 Total Solids (~o) 5.0 3.3-4.1
Volatile Solids ( ~o TS) 66 58 Fixed Solids ( ~o TS) 34 42
Biochemical Oxygen Demand (mg litre- 1) _ 1 600-2 200 Chemical Oxygen Demand (mg litre- 1) - - 31 000-45 000 Total Organic Carbon (mg litre- 1) 6 000
organic wastes have been studied in recent years (Kirsch & Sykes, 1971; Hobson et al., 1974). Based on the microbial kinetics, anaerobic fermentation with feedback of the reactor effluent should help to overcome the slow growth rate of anaerobic bacteria and to increase the efficiency of biochemical conversion. Anaerobic fermentation reactors based on this concept have been mathematically analysed and experimentally demonstrated (Herbert, 1961; Fencl, 1966; Pirt & Kurowski, 1970). There have also been examples of successful recycle in full-scale treatment systems (Torpey & Melbinger, 1967; Schroepfer et al., 1955). However, the purpose and nature of recycling the effluent in this case was entirely different. Biogas production using beef cattle manure produces wastewater which gives a difficult disposal problem. If effluent can be used for make-up water without interfering with the performance of the anaerobic fermentation process, the wastewater volume can be greatly reduced.
METHODS
The experiment was initiated by reproducing the operating conditions of the pilot-plant digester (Table 1) in six bench top anaerobic digesters. The anaerobic fermentation reactors were made from polyethylene tubing, 14 cm inside diameter and 38 cm in height. They were partially submerged in a water bath to maintain 35 °C _+ 1 °. Each reactor was filled with 2 litres of digester contents obtained from the pilot digester and then fed daily with the feed prepared for the pilot digester until its performance stabilised (approximately 15 days). Digesters were slow mixed for 15 min
TA
BL
E 3
T
otal
Sol
ids
(TS)
and
Vol
atil
e So
lids
(V
S) L
oadi
ng (
g of
sol
ids
per
litr
e of
dig
este
r vo
lum
e) o
f E
fflu
ent R
ecyc
ling
Ana
erob
ic D
iges
ters
t,.
Dig
este
r P
er c
ent
of c
entr
ate
recy
cled
lo
adin
g st
age
0 20
40
60
80
10
0
TS
V
S
TS
V
S
TS
V
S
TS
V
S
TS
V
S T
S
VS
I 7'
10
5"00
7"
30
5.10
7-
60
5"30
8"
20
5-60
8.
70
5.80
9.
20
6.10
II
8.
70
5.70
9"
00
7-90
9"
30
6"00
9-
80
6"30
10
.40
6'60
11
"20
7"
00
III
11.6
0 8"
30
11'6
0 8'
10
11.8
0 8"
20
13.8
0 8"
80
13"8
0 9"
40
14.3
0 9.
50
IV
15.7
0 10
.80
15.6
2 10
.80
16.7
0 11
.40
17.9
0 12
.00
18-0
2 12
-00
18.2
6 12
.00
e~
e~
Wastewater recycling of beef cattle wastes 5
every 2 h. The mixing actions were synchronised by fastening each centre shaft on to a jar test apparatus drive.
Once the digesters were normalised to the laboratory environment, each was converted to operate at an effluent recycling condition of 0, 20, 40, 60, 80 or 100 % daily. Based on the data collected in the field, as much as 75 ~o of daily water input would be recovered as wastewater effluent during the centrifugation. At 100% recycling, the entire amounts of wastewater were re-turned into the digester. In the laboratory, the effluents for recycling were simulated by centrifuging the daily draw-off of each digester at 1500 rpm for 15 min. The daily digester feed was prepared by mixing the waste solids with given amounts ofwastewater. The remaining daily water requirement of the digester feed was made up from tap water.
The digesters underwent four loading stages with gradually increased feeding at each stage. The Total Solids and Volatile Solids inputs for each loading stage are summarised in Table 3. To maintain the hydraulic loading time at 20 days, the Total Solids input at each loading stage was increased but the volume of the digester feed remained unchanged. For a digester without any effluent recycling, the Total Solids input rose from 7.10g litre -1 of digester volume per day at stage I to 15.70gday -1 at stage IV. There were additional increments in Total Solids and Volatile Solids inputs due to effluent recycling. The digesters were allowed to equilibrate with the current input loading (greater than 20 days) before any performance data were collected.
Electrical conductivity, pH and gas volume (expressed as volume at 1 atmosphere and 0 °C)were recorded daily. The composition ofbiogas was analysed once a week using gas chromatography (Model 25V Fisher Gas Partitioner, Fisher Scientific Co., Pittsburgh, Pennsylvania, USA). Total Solids, Volatile Solids and Fixed Solids in each digester were routinely analysed when the digester reached the equalised phase of each loading cycle by procedures outlined in the Standard Methods for the Examination of Water and Wastewater.
RESULTS AND DISCUSSION
Effects of wastewater recyding on digester operation
After laboratory normalisation, each bench top digester was set to operate at a given level of wastewater effluent recycling. Figure 1 shows
6 A .C . Chang, I4/. C. Fairbank, T. E. Jones, J. E. Warneke
14
6
24 rr
f (.9 12
UJ -i-tO
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Fig. 1.
14
0% CENTRATE 12 RECYCLED , / ~ I0
°, / ±,/~ ± 8
± I" . .I . . ~ / I]I T ]3Z 6
CENTRATE "" 20% RECYCLED . ~
. . . . " ° °
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14
40% CENTRATE / ° 12 RECYCLED ~ I0
L ± ~# f . r I T ~ 7 , ~ T ~ " 8
• ~ 6
16
80% CENTRATE " ~ 14 RECYCLED .~.. • ~2
"" " '" I0
[ • 8
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• l ' L i L I k l l l l 4
JAN MAR MAY JUL SEP FEB AM dUN AUG OCT
• °
60% CENTRATE / " RECYCLED ~ . o
. ! . : s / ~ , m" rv
_,S'" _ L I I I I I I L
I 0 0 % CENTRATE RECYCLED
i m
./"~ " ". •
JAN MAR MAY JUL FEB
f
L I
SEP APR JUN AUG OCT
Total Solids content of anaerobic digesters during wastewater recycling. (For Total Solids loadings during Stages I IV in each digester, see Table 3.)
the Total Solids content o f each digester as it responded to four digester loading rates. Each dot represents an actual determination of the digester's solids content. The horizontal portion of the line indicates the mean Total Solids content after the digester had stabilised for a given loading rate. There was a transition period between each equilibrated phase. Data used for evaluating the performance at each loading rate were taken from the equilibrated phase of each recirculation cycle. Effluent recycling introduced additional amounts of solids into the digester input. As a result, the Total Solids content of digesters at a given loading rate increased with the level of effluent recycling.
TA
BL
E 4
pH
an
d E
lect
rica
l C
on
du
ctan
ce (
mm
ho
cm
i at
25°
C)
of
Dig
este
r L
iquo
r U
nd
er V
ario
us F
eed
Loa
ding
an
d C
entr
ate
Rec
ycli
ng
Dig
este
r P
er c
ent
of
cent
rate
rec
ycle
d fe
edin
g st
age
0 20
40
60
80
I0
0
pH
E
C
pH
E
C
pH
E
C
pH
E
C
pH
E
C
pH
E
C
2'
I 7.
1 11
'0
7"2
12.6
7"
3 14
.5
7"2
17-8
7-
3 21
'7
7.4
26.4
II
7'
1 13
"8
7"2
15.7
7.
3 17
.3
7.4
19-9
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5 23
'5
7.6
29"1
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I 7-
2 15
-0
7.3
17.1
7.
4 19
"8
7.5
23"8
7.
6 28
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7.7
32.4
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4 19
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7.4
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.6
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7.
6 32
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7.6
34-1
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8 A .C . Chang, W. C. Fairbank, T. E. Jones, J. E. Warneke
The most noted change in the digester (besides the increased Total Solids contents) was the rising salinity level (as measured by electrical conductance in mmho/cm at 25°C) with effluent recycling (Table4). When the digester was loaded at 7.10 g of TS per litre, the electrical conductivity rose from l l -0mmho cm-1 at 0 ~ effluent recycling to 26.4 mmho cm - 1 at 100 ~o effluent recycling. The highest salinity level for a digester occurred at the heaviest load rate and with complete effluent recycling. There was also a slight increase in pH due to both recycling and loading (Table 4).
Solids destruction
The percentages of the input Total Solids (TS) and Volatile Solids (VS) destroyed during digestion are summarised in Table 5. When the effluent
TABLE 5 Per Cent of Total Solids (TS) and Volatile Solids (VS) Destruction of Centrate Recycling
Anaerobic Digesters
Digester Per cent ofcentrate re~Te&d loading
0 20 40 60 80 100
TS VS TS VS TS VS TS VS TS VS TS VS
I 31 38 26 31 25 32 26 32 21 26 22 26 II 30 33 29 34 28 30 19 24 20 24 21 24
III 30 39 24 33 20 27 20 30 22 32 17 25 IV 25 36 23 36 21 33 21 33 17 30 14 28
was not recycled, the Total Solids and Volatile Solids reductions (25 30 ~o TS, 35-40 ~o VS) were comparable with those obtained from the pilot digester. They also agreed well with figures reported by other researchers (Varel et al., 1977; Hashimoto et al., 1978; Morris et al., 1978; Smith, 1978). Statistical analysis indicates that effluent recycling has a significant effect on the percentage of solid destruction. In fact, there was an approximately 30 ~o decrease in the percentage of solids destruction as the level of effluent recycling increased from 0 ~ to 100 ~o. At 100 ~ recycling, there appeared to be effects on the percentage of solids reduction due to increased digester loading.
Wastewater recycling of beef cattle, wastes 9
Effluent recycling introduces additional solids into the digester feed, so it is misleading to compare the percentage of solids reduction directly. Instead, the net solids destruction in each digester (values in Table 3 multiplied by corresponding values in Table 5) would be a more appropriate measurement. Data summarised in Table 6 indicate that the net destruction of Volatile Solids in each digester did not appear seriously affected by effluent recycling. It seemed that the recycled solids were less
TABLE 6 Amounts of Volatile Solids Destroyed in Centrate Recycling Anaerobic Digesters
(grams of Volatile Solids per litre per day)
Digester Per cent of centrate recycled loading
0 20 40 60 80 100
| 1"90 1"60 1.75 1"80 1.50 1'60 II 1 '84 2"00 1 "80 ! "50 1 "60 1 '70
I I I 3'00 2-70 2-20 2.80 3-00 2'90 IV 3.88 3-86 3"76 3.96 3"60 3"30
degradable than the original solids. Therefore, the apparent decrease in the percentage Total Solids and Volatile Solids destruction shown in Table 5 was due largely to the additional solids introduced through effluent recycling. At higher loading rates ( > 11.6 g of TS per litre digester volume per day) and high wastewater recycling (100 ~o), the net Volatile Solids destruction was reduced by approximately 15 ~.
Gas production
The gas production data of this experiment are summarised in Table 7. During anaerobic fermentation of organic wastes, the volume of gas generated is usually directly related to the amount of Volatile Solids destroyed. At a given condition (i.e. retention time, digester loading, etc.), it is also a function of the Volatile Solids input. When wastewater effluent of the digester was not recycled, the anaerobic digestion process produced an average of 0.13 litres of biogas for each gram of Volatile Solids added per day. The digester input, ranging from 7.10 to 15-70 g of Total Solids per litre of digester volume per day, did not affect the unit gas production (i.e. volume of gas per unit VS added). With the recirculation of effluent,
10 A. C. Chang, W. C. Fairbank, T. E. Jones, J. E. Warneke
TABLE 7 Gas Production of Wastewater Recycling Anaerobic Digesters
% Centrate recycled
Digester loading Gas production % CH 4
Feed Feed Volume VS VS and (litre per unit per unit
centrate litre- 1 added destroyed (g litre 1 day-l) day- l ) (litre g-1 day- l )
0 7'10 7.10 0'62 0.12 0.33 58 8"70 8.70 0-82 0"14 0'45 55
11"60 11.60 1'10 0-13 0.37 57 15"70 15.70 1-49 0"14 0"38 58
20 7.10 7"30 0'61 0.12 0.38 57 8"70 9"00 0"81 0.10 0"41 56
11-60 11.60 1.11 0'14 0"41 57 15-70 15"62 1"56 0"14 0"40 58
40 7-10 7.60 0.62 0" 12 0-35 57 8"70 9"30 0" 81 0" 14 0"45 55
11-60 11'80 1'11 0"14 0-50 56 15"70 16-70 1"57 0'14 0.42 58
60 7' 10 8.20 0.61 0.11 0.34 57 8.70 9.80 0.81 0"13 0.54 55
11"60 12.80 1.12 0"13 0"40 56 15'70 17.90 1.48 0.12 0"37 58
80 7' 10 8'70 0'63 0" 11 0'42 56 8"70 10"40 0'80 0"12 0'50 56
11'60 13"80 1"07 0'11 0"37 56 15'70 18-02 1"38 0"12 0"38 57
100 7"10 9"20 0"53 0"09 0"33 58 8"70 11"20 0"70 0"10 0"41 54
11"60 14-30 0"87 0-09 0"36 55 15"70 18-26 1"29 0"11 0'39 56
the gas volume was lowered to less than 0.1 litre per gram VS added per day at 100 % recycling. Again, it was believed that the apparent decrease in gas production was due to the recycling of less degradable organic solids into the digester. Judging by the gas production per unit of digester volume, which reflects the true organic solids input, there was little indication of reduction in gas production until the digester was operated at 100 % effluent recycling. The gas volume produced, measured in terms
Wastewater recycling of beef cattle wastes 11
of unit Volatile Solids destruction, also did not show any reduction. Throughout the entire experiment, the methane content of the gas varied from 55 ~ to 60 ~ . The average methane content for each operating condition is also given in Table 7.
A C K N O W L E D G E M E N T
Financial support of this study by a grant from Pacific Gas and Electric Co., San Francisco, California, USA, is gratefully acknowledged.
R E F E R E N C E S
Chang, A. C. & Fairbank, W. C. (1981). A study on the treatment and disposal of waste water generated by methane producing anaerobic digesters. Report prepared for Pacific Gas and Electric Co., San Francisco, California. January, 1981.60 pp.
Fencl, Z. (1966). A theoretical analysis of continuous culture systems. In: Theoretical and methodological basis of continuous culture of micro- organism (Malek, I. & Fencl, Z. (Eds)). Academic Press.
Hashimoto, A. G., Chen, Y. R. & Prior, R. L. (1978). Thermophilic anaerobic fermentation of beef cattle residue. In: Symposium Papers Energy from Biomass and Wastes. Institute of Gas Technology, 14-18 August, 1978. Washington, DC, pp. 379-402.
Herbert, D. A. (1961). A theoretical analysis of continuous culture systems. In: Continuous culture." Monograph No. 12. Soc. Chem. Ind., London, p. 21.
Hobson, P. N., Bousfield, S. & Summers, R. (1974). Anaerobic digestion of organic matter. CRC Crit. Rev. Environ. Contr., 4(2), 131-92.
Kirsch, E. J. & Sykes, R. M. (1971). Anaerobic biological waste treatment. Progress in Industrial Microbiology, 9, 155.
Mah, R. A., Ward, D. M., Baresi, N. & Glass, T. C. (1977). Biogenesis of methane. Ann. Rev. Microbiol., 31, 309-41.
Meckert, Jr., G. W. Commercial SNG production from feedlot wastes. In: Symposium Papers Energy from Biomass and Wastes. Institute of Gas Technology, 14-18 August, 1978. Washington, DC, pp. 431-47.
Morris, G. R., Jewell, W. J. & Leohr, R. C. (1978). Anaerobic fermentation of animal wastes: A kinetic design evaluation. In: Proc. 32nd Indu Waste Conference, Purdue University. Ann Arbor Science Publishers, p. 689.
Pirt, S. J. & Kurowski, W. N. (1970). An extension of the theory of the chemostat with feedback of organisms, its experimental realization with a yeast culture. J. Gen. Microbiol., 63, 357.
12 A. C. Chang, W. C. Fairbank, T. E. Jones, J. E. Warneke
Schroepfer, G. J., Fullen, W. J., Johnson, A. S., Ziemke, N. R. and Anderson, J. J. (1955). The anaerobic contact process as applied to packing house wastes. Sewage Ind. Wastes, 27, 4.
Smith, K. D. (1978). Operation of an anaerobic digester at the Washington State Dairy Farm. In: Symposium Papers, Energy from Biomass and Wastes. Institute of Gas Technology, 14-18 August, 1978. Washington, DC, pp. 403-29.
Standard Methods for the Examination of Water and Wastewater (1980). 15th edition, American Public Health Association, Washington, DC.
Torpey, W. N. & Melbinger, N. R. (1967). Reduction of digested sludge volume by controlling recirculation. Jour. Water Poll. Contr. Fed., 39, 1464.
Varel, V. H., Isaacson, H. R. & Bryant, M. P. (1977). Thermophilic methane production from cattle wastes. Appl. and Environ. Microbiol., 33(2), 298.
