Anaerobic digestion of rabbit wastes
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Bioresource Technology 35 ( 1991 ) 95-98
Anaerobic Digestion of Rabbit Wastes D. Trujillo, J. F. P6rez & F. J. Cebreros
Esc. Unix,. de Ingenier/a T6cnica Agr/cola de La Laguna, Carretera de Geneto no. 2, 38200 La Laguna, Tenerife, Spain
(Received 27 May 1990; revised version received 20 July 1990; accepted 25 July 1990)
This paper describes the study of the continuous anaerobic digestion of rabbit wastes diluted with water. Three series of experiments, at 37C, with different feedstock VS concentrations for several hydraulic retention times were tested. A maximum value of methane production was estimated and the values of the organic load reduction were obtained.
Key words: Anaerobic digestion, biomass energy, rabbit wastes, methane production, biogas, bio- energy.
Recently, the treatment of animal wastes of high moisture content has led mainly to energy pro- duction by anaerobic digestion. However, this process is also beginning to be recognised as a form of pol!ution control, which can be carried out in two ways: by discontinuous or continuous fermentation.
It is obvious that if a waste is digested for a longer time the extent of degradation will be greater; however, good conversions are obtained for almost any wastes after the first 20 days. From that moment, in order to get higher conversions it is necessary to increase appreciably the digestion time. This increase in time has to be considered not only because of the economic aspects related to the volume of the digester but because of the necessity to deal with the volumes of waste produced in the intensive farming unit.
The purpose of this research was to carry out an anaerobic treatment of rabbit wastes to obtain a certain volume of combustible gas that could partly solve the energy demands of the farm and to obtain an effluent with a lower polluting power and a higher fertiliser value than the fresh waste
(Huang & Shi, 1981; Aubart & Fauchille, 1982; Hills & Ravishanker, 1984).
The chemical characteristics of rabbit wastes, the analytical methods and the laboratory digesters used for the experiments of anaerobic digestion have been described in earlier works (P6rez, 1984, 1987; Trujillo etal., 1987a, 1988).
Spherical glass digesters of one litre capacity were used and were kept in a thermostatic water bath at 37C. They were mechanically stirred for 15 min every 30 min. The gas produced was stored and measured in a gas burette.
The digesters were started up by discontinuous fermentation of a suspension of rabbit wastes with the appropriate initial concentration of volatile solids in a helium atmosphere. After 15 days the continuous process was started. Digesters were loaded manually at the same time every day. The criterion for the steady-state was a constant gas production and after it was reached 20 samples were taken and the average values were used for the results.
Three series of continuous experiments (DCA, DCB and DCC), differing in feedstock VS con- centration, were carried out. Earlier studies on a discontinuous process (Trujillo et al., 1988, 1989) led to the choice of feedstock concentrations of 4, 5"5 and 7% volatile solids (VS0). The hydraulic retention times (HRT) used were in the range of 45 to 5 days, decreasing gradually.
RESULTS AND DISCUSSION
The values of methane production (B), the reduc- tion percentages of the chemical oxygen demand (PcoD) and of the volatile solids (Pvs) obtained in
95 Bioresource Technology 0960-8524/91/S03.50 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
96 D. Trujillo, J. F. P&ez, F. J. Cebreros
the steady-state periods of each experiment are presented in Tables 1, 2 and 3.
The plot of the pH against the HRT, for the three series of experiments, is shown in Fig. 1 and
Table 1. Continuous digestion: Series DCA
HR T B P vs P cot) (days) (litres CH4/kg VS) (%) (%)
45 100"81 70.20 47.11 40 91.87 69.48 80.41 35 101.85 66"73 55-42 30 101.89 69.85 64"16 25 67.60 64.63 66.01 20 88.68 76.83 65-93 15 71.77 77.30 70"08 10 68"87 65"18 74.10 5 47.06 67.24 81.32
VS0 = 40 g/kg. COD() = 37"58 g/kg. Subscript '0' indicates feed value. p Indicates degradation.
Table 2. Continuous digestion: Series DCB
HR T B P vs P cod (days) (litres CHu/kg VS) (%) (%)
a nonlinear relation is obtained. It can be seen that the pH has a constant value of about 7"1, when the HRT tends to infinity and, for the range of the HRT used a minimum pH value of about 6.6 can be estimated. Both values lie within the general range of digester pH values (McCarty, 1964; Cheremirinoff & Morresi, 1976; Grundey, 1982; Trujillo et al., 1987b), and show the systems were self-stabilising.
In Fig. 2 the plots of the methane content of the biogas (% by volume) against the HRT employed are presented. The results seem to indicate, at least qualitatively a similar tendency to that observed in the pH (Fig. 1 ). A minimum value of methane concentration of about 43% for the shortest HRT is obtained for all the series but the values are asymptotic at high HRT, to 54, 51 and 48% for the series DCA, DCB and DCC, respect- ively.
Comparing these values with those obtained by Trujillo et al. (1987b) for poultry manure, with similar concentrations in (VS0), a good agreement is observed. However, these values are slightly lower than those obtained by Pdrez (1984) for
45 121-12 79"64 70.73 40 114.35 64.25 71.08 35 124.87 76.95 72.45 30 97.97 64.93 64.41 25 101.99 65.44 71.32 20 99"43 73.27 55.41 15 102"60 72.84 74.08 10 73.42 66'69 77-03 5 55'73 67.43 75"82
VS0 = 55 g/kg. COD0 = 55'92 g/kg. Subscript '0' indicates feed value. p Indicates degradation.
Table 3. Continuous digestion: Series DCC
HR T B P vs P cot) (days) (litres CHu/kg VS) (%) (%)
8 .40 .
~P- -K" . . . .
I I I I
~-- - -A - -- -A - - - -A - - _
[ I [ [
45 112"47 70"69 68"09 40 98"85 72"27 78"98 7.20- 35 107"44 67"99 66"03 30 113"92 71"86 72'70 ~6.00- 25 127'00 63-69 61.46 20 102'05 61-66 57"35 15 70.94 73"90 88"98 6.40- 10 99"90 55"60 60-19 5 42"30 76"40 77"40 o.oo
VS o = 70 g/kg. COD o = 77"74 g/kg. Subscript 'o' indicates feed value. Fig. 1. p Indicates degradation.
I I I I l
~0_ _ -$ - - - -gF - - -Q- -
I I I q
~ - - I - m
I I I I I I I I f 5 10 15 20 25 30 35 40 45 50
pH at different HRT: (a) Series DCA; (b) Series DCB; (c) Series DCC.
Anaerobic digestion of rabbit wastes 97
I i I i [
I @ i
I I I
@ __ P
I I I I I I
J - - -
I I I I I I I I I 5 I0 15 20 25 30 35 40 45 50
Fig. 2. Methane concentration against HRT: (a) Series DCA; (b) Series DCB; (c) Series DCC.
~ 100 75
r j 5O
~t ) 100 -
'~ " /5 - O ~ 50-
I I I I , I L
~ 75- ~'~ 50-
J . . . . m- -
0 I I I I I I I I I 0 5 10 15 zo 25 30 ~5 40 45 50
Fig. 3. Methane production against HRT: (a) Series DCA; (b) Series DCB; (c) Series DCC.
sewage with feed concentration (VS0) about 2.5% and by Aubart and Bully (1983) for rabbit wastes.
The methane production (B) obtained for each series of experiments has been plotted against the HRT in Fig. 3. Minimum values of 47.1, 55"7 and 42.3 litres of methane/kg VS for the series of experiments DCA, DCB and DCC respectively, were obtained at low HRT, but all the values tend to the same volume of about 110 litres of methane/kg VS at high HRT. From plots of B against 1/HRT a production of about 140 litres methane/kg VS at infinite HRT was obtained.
There are no significant differences between the maximum and minimum methane production values obtained in the present work and the experimental values obtained for the anaerobic digestion of poultry manure in similar conditions. However, they are about 50% lower than those obtained by Aubart and Bully (1983) for rabbit wastes (5% VS0), although the digester volume was six times larger than the one used in this research. The difference could be caused by a possible scale effect and by the types of anaerobic
digester used, which had completely different mixing systems.
It seems interesting to point out an apparent greater variation in the experimental results when the content of (VS0) in the feed increases. This could be due to problems related to mixing.
The results in Tables 1, 2, 3 show reductions in VS to oscillate around a value of 70%, whatever the initial feedstock concentration, and a similar percentage reduction in COD is also shown.
The values for the COD reduction are rather higher than that found by Aubart and Fauchille (1982), but they agree with the higher value found by Aubart and Bully (1983) at 20 days HRT.
The experiments confirm previous results and show that rabbit wastes can be digested. The extent of digestion was relatively independent of HRT, so that a short HRT (about 15 days) could be used in practice; the volume of digester needed for a given volume of wastes would thus be quite small. Although there was some decrease in methane percentage in the biogas with increased feed VS, the results show that gas of about
98 D. Trujillo, J. F. POrez, F. J. Cebreros
45-50% methane content could be obtained. However, gas of this methane content could cause combustion problems in boilers or engines.
This investigation is part of the project 'Energy recovery from rabbit wastes by anaerobic diges- tion' supported by General Direction of Energy Politics of the Canary Government.
The authors are grateful to S. L6pez Casariego for his collaboration in supplying all the necessary rabbit wastes and to Dra. A. Brito for her help in the translation.
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