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Anaerobic digestion of rabbit wastes and pig manure mixed with rabbit wastes in various experimental conditions
Agricultural Wastes 10 (1984) 1-13
Anaerobic Digestion of Rabbit Wastes and Pig Manure Mixed with Rabbit Wastes in Various Experimental Conditions Ch. A u b a r t & F. Bully Laboratoire de Recherchessur Les Fermentations, Soci6t~ Commercialedes Potasses et de L'Azote, 68700 Aspach Le Bas, France ABSTRACT Experiments on anaerobic digestion of rabbit wastes and pig manure mixed with rabbit wastes were made in 6-litre digesters at 37°C with manual loading once a day and mixing for 3 min each hour. The results show the optimum Total Solids ( TS) content as 6 % and the retention time as 7.5 days. The optimum methane yield was about 215 litres k g - x VS but this is of little use for an industrial biogas plant because the wastes volume is low on a rabbit farm. A possible use of this kind of agricultural waste is in mixing rabbit wastes with pig manure to increase the pig manure VS content, which is generally low. With retention times between 15 and 10 days the methane production increases from 0"20 to 0"25 litres of methane per litre of digester per day without affecting pollution control.
INTRODUCTION In France and certain other countries, such as Belgium, the Netherlands and Denmark, the size of 'zero grazing' stock farms is not always commensurate with the area available for the spreading of effluents (liquid manure, poultry wastes, etc.). It has therefore been necessary to develop treatment systems, and many pig farms, for example, have turned to methane fermentation of effluents. Among the many techniques for the transformation of biomass (Aubart & Reisinger, 1981), methane fermentation has, in recent years, won favour for the treatment of wet substrates. I
Agricultural Wastes 0141-4607/84/$03.00 © ElsevierApplied Science Publishers Ltd,
England, 1984. Printed in Great Britain
2
Ch. Aubart, F. Bully
Other types of animal breeding are becoming increasingly industrialized; among them are rabbit farms. France is the world's leading producer of rabbits, with an output of 175000 metric tonnes in 1980 (Henaff & Sinquin, 1981). There are many industrial breeding units that have sets of fattening cages with automatic waste removal systems (excrement with no straw litter). It seemed that it would be worth while to study the biogas production potential of this waste and to examine the reduction in organic pollution achieved by anaerobic digestion under various experimental conditions. Research on the anaerobic digestion of pig manure (Hobson & Shaw, 1973; Hobson et al., 1977, 1979; Van Velsen, 1977, 1979a, b) has shown that this technique gives good results with regard to pollution control and deodorization, and has provided precise data about biogas production. Our own research (Aubart, 1981b) using laboratory digesters and our industrial-scale (200 and 250m 3) digesters has shown that there is a problem with the critical profitability threshold. Biogas production from pig manure is sometimes not enough to pay back the initial cost of a plant in a sufficiently short time. This low production is explained by the relatively low percentage of Total Solids in the pig manures analysed (of the order of 3 to 4 ~). Such results have led us to consider mixtures of animal wastes, especially as it is common in France for a breeder to have two or three types of animal. Thus, for example, the Total Solids (25 ~o) in the solid manure from an industrial-type rabbit farm might be used to increase the Total Solids content of pig manure. In this paper, we present laboratory experiments on mixtures of pig manure and solid rabbit wastes. Using these experimental results it is possible to consider their application to an actual farm and the optimization of biogas plants for pig manure enriched with a substrate having a higher concentration of Total Solids, so lowering the economic threshold and the minimum size of breeding operation for which a biogas plant is profitable. METHODS
Experimental design Two types of experiment were conducted to investigate both the energy potential and the pollution control aspects of anaerobic digestion.
Digestion of rabbit and pig wastes
(i)
Anaerobic digestion of rabbit wastes diluted with water, with two Total Solids concentrations--6 and 7.5 % - - b o t h below the technological limit of pumpability, at five different retention times. (ii) Anaerobic digestion of a mixture of pig manure and rabbit wastes, with pig manure of two Total Solids concentrations--2.5 and 4.5%--giving final mixtures containing 3.7 and 6-1% Total Solids.
Digesters The digesters used for these experiments were designed and built in our laboratory. They had a capacity of 6 litres and were installed in several lines of five digesters each to make use of a single heat source. The cylindrical digesters were made of transparent PVC and heated by hotwater circulation in an external jacket with a feed tube entering the top and opening near the bottom, and an internal pipe from the bottom acting as an overflow weir. Mechanical stirring for 2 min each hour, by two propellers on a shaft turning at 50 rpm, was used~ The gas evolved was stored and measured in a PVC bell gasometer.
Substrates The rabbit wastes came from a litterless, caged, industrial-type farm; feeding and veterinary care were closely controlled. On this type of farm, the urine is separated from the solid portions of the wastes which alone were used for the experiments. The pig manure was from a farm with 3000 meal-fed pigs on a grating floor. Deliveries of the wastes to the laboratory were at short intervals to forestall any decomposition that might be caused by storage, even though storage was at 4 °C. At each delivery and throughout the test, analyses were made to confirm the constancy of the test material. The compositions of the two products are given in Table 1.
Experimental procedure Digesters were fed once a day, at the same time of day. Having large quantities of Total Solids in suspension the manures were well stirred before feeding, but not oxygenated. To start up the digesters, they were inoculated with effluents from pig manure digesters. Earlier studies
TS ( ~ ) VS ( ~ of TS) C O D ( m g O 2 per litre) Supernatant TS (~o) VS ( ~ of TS) Cellulose (g litre- 1) Lignin (g litre- 1) N r (glitre- 1) P (mg litre- 1) K (mg litre- 1) Ca (mglitre- 1) Na (mg litre- 1) Mg (mg litre- 1) NH4 (g litre- 1)
Analysis
0-94 58.11 2.3 1.22 2.66 735 1 190 685 300 365 2.24
1.01 51.69 --3.64 1 020 1 950 1 460 410 520 2.39
1.36 53.94 5.68 2.3 4.38 1 120 2 292 1 240 555 590 3-98
1.11 56-19 8-2 4.72 4 1 445 1 625 1 945 460 552 3.46
1.46 55.34 7.64 3.49 5-4 1 275 2 325 1 825 387 540 4.42
Pig manure with rabbit wastes mixed
1-08 57.73 3 1.32 --1 647 825 363 -2.24
1.58 53.53 10-6 4.44 4.65 1 290 2 570 1 595 619 675 3.98
1.26 58-44 --4.1 1 525 1 860 2045 460 585 3.46
3"7 6 6.1 77.37 77-36 73"86 42505 65376 59844
Wastes
2"5 3'7 4"32 4"66 6"05 72"53 68"74 72.59 72.31 76-93 34586 39484 51 172 52013 76 117
Pig manure
TABLE 1 Wastes Composition
1.25 53.33 20.4 7.72 1.78 890 1 967 1 515 323 387 1.3
6"1 83"51 61 068
Rabbit wastes dilution 6 % TS
Digestion of rabbit and pig wastes TABLE 2
Inoculum Composition Used for Start Up of Laboratory Digesters Analysis
Inoculum 1
lnoculum 2
TS (%) VS (% of TS) COD (mg 0 2 per litre) Supernatant TS (%) VS (% of TS) N x (g litre- 1) P (mg iitre-1) K (mg litre- i) Ca (mg litre-1) Na (mg litre-1) Mg (mg litre- 1) NH 4 (g iitre-1)
4.54 79-73 50 534
3 67.50 41 014
1.20 51.26 2.64 825 2 069 1 308 372 419 1.87
0.97 52.00 2.65 765 2 155 1 180 370 444 2.63
Inoculum 1: 50 % 50 % Inoculum 2:50 % 50 %
effluent of anaerobic digestion of pig manure, rabbit wastes diluted to 6 % TS. effluent anaerobic digestion of pig manure, pig manure.
(Aubart, 1981a) led to the choice of a ratio of 50/50 inoculum/test material. The compositions of the inocula are given in Table 2. The retention times used for the investigation of diluted rabbit wastes were 20, 13, I0, 7-5 and 5 days. For investigation of the pig/rabbit mixture and of pig manure alone, the retention times were 15 and 10 days. The various digesters were started up with much longer retention times to enable a steady state to be reached before each change in retention time (Fig. 1). For the retention times used in these investigations, each experiment lasted at least 2 months. The temperature of the digesters was 37 °C. Gas production was recorded daily and the composition of the gas determined once a week. The input material was analysed once a week on a sample taken at random from the week's inputs. The output material was analysed once every 2 weeks on a specimen representing two samplings of effluent a week.
Analytical methods The same analyses were carried out on the input and output materials and on their supernatants (centrifugation at I l 000 rpm for 15 min).
Ch. Aubart, F. Bully
¢u "=10 u~
Period
B
t;
10 d
7.5 d
A
Retention time 10 d TS 6%
,.s%
8
A
I--
I
,/
s t.w z
w
1~12
1~6
1=40
1=54
168
1~12
1~6
2110
TIME ---Days
Fig. 1. Methane production from rabbit wastes at different loading rates with a laboratory digester (capacity, 6 litres). Total Solids (TS) were determined by evaporation at 75 °C for 36 h and Volatile Solids (VS) by the difference after combustion at 600 °C for I h. Total nitrogen and phosphorus were determined by a Technicon autoanalyser (model P2, second generation). Calcium and magnesium were determined by atomic absorption. Potassium and sodium were determined with a flame emission spectrometer (Pinta, 1962). The ammonia content was determined by distillation from an alkaline medium (NF T90-015 in Degremont, 1978). Cellulose was determined after sulfuric acid and alkaline treatment (Recueil d'Analyses des Communaut6s Europ6ennes, 1976). The determination of lignin was based on the same principle, with an additional 3 h of treatment with sulfuric acid. COD was measured by oxidation in an acid medium using potassium dichromate (NF T90-101 in Degremont, 1978, with a few changes made in dilutions). CH 4, CO 2 and HzS concentrations were measured using a Girdel catharometer chromatograph (model 3000). The stainless steel column (0.32 × 400cm) was packed with Porapack Q.
Digestion of rabbit and pig wastes
RESULTS AND DISCUSSION
Energy production Rabbit wastes All the results presented here, in the Tables or Figures, are the means of many analyses over the entire duration of the tests (one year, on ten digesters operating in parallel). However, the continuous anaerobic digestion of materials that are highly heterogeneous in composition sometimes results in errors of analysis due to sampling errors. Furthermore, although the source of our materials was controlled as closely as possible, their biological nature and their origin (a livestock operation on industrial scale, and run on conventional lines) resulted in greater or lesser variations with time in gas production and, even more so, in the reduction of organic content of the wastes. The methane content of the biogas produced by anaerobic digestion of rabbit wastes was 58.03 _+ 2.57 ~o; with pig manure it was 68-74 _+ 1.10 ~o. The same result was obtained with the mixture of pig manure and rabbit wastes, 68.08+ 1.16~. The HzS content of the gas from the two substrates and of the mixture seemed to be identical. It had a 95 probability of being in the range from 0.185 ~o to 0.193 ~o. Table 3 shows results obtained with rabbit wastes diluted to 6 ~ Total Solids at various retention times. Retention times of more than 20 days are incompatible with adequate production, and so are not included in the results. Figure 2 plots the specific production of methane (P: litres o f C H 4 per litre of digester per day) in terms of retention time (T: days) and TS content. The relationship between specific methane production and retention time can be fitted to a curve of the following equation for 6 ~o Total Solids: P = 2.265/10°'°3~2r; r = -0"9859. Table 3 also shows the results obtained with a dilution of 7"5~oTS at various retention times. The relation between the specific methane production and retention time (Fig. 2) can be fitted to a curve of the following equation for 7.5~o Total Solids: P=21-7/TI'2828; r = -0"9975. The specific production for the dilutions was identical at a retention time of 10.5 days. From 10.7 to 7.5 days specific methane production was higher with waste diluted to 7.5 ~ Total Solids. Rabbit waste diluted to 6~o TS appears to be the most interesting dilution because the biological yields were higher than those obtained
Ch. Aubart, F. Bully TABLE
3
Results O b t a i n e d with R a b b i t Wastes
TS in input 6 ~o
7.5 ~
Retention time (Days) Technological yield (litres o f m e t h a n e per litre o f digester per day) Biological yield (iitres of m e t h a n e per kg o f VS input) Technological yield (litres o f m e t h a n e per litre o f digester per day) Biological yield (litres o f m e t h a n e per kg o f VS input)
20
13
10
0-56
0.88
0.97
219
230
--
--
0.82
172
216
!.10
177
7"5
1.44
213
1.66
201
5
4
1.62
1.70
151
142
--
--
--
--
TABLE4
R e s u l t s ~ o m Mixed Pig M a n u ~ a n d R a b b i t Wastes
--
Per cent TS pig manure
Per cent TS pig manure with rabbit wastes mixed
Retention time (Days)
6"05* 4"66 4"66*
-6-1 --
10 10 10
1'08 1'08 0'84
224 240 249
6-26* 4-32 4"32* 3"7* 2"5 2"5*
-6 --3'7 --
15 15 15 15 15 15
0"84 0'83 0-58 0'47 0'59 0'39
262 268 277 277 309 323
* ( A u b a r t , 1982). N o r a b b i t waste added.
Technological yield Biological yield (litres of methane (litres of methane per litre of digester per kg of VS) per day)
Digestion of rabbit and pig wastes 1.7
1,5
ol "o
"~ =~ 1 . 0 -
~,J
~( 0 . 5 D r.=
:=
~
6
%TS
7.5
% TS
u.J t-.
,~ ~
7.15
110 RETENTION TIME
1S
20 Days
Fig. 2. Technological yield in terms of retention time and Total Solids content.
with rabbit wastes diluted to 7.5 % TS. The retention time of 7.5 days gives a yield of 213 litres CH 4 per kilogram of VS. Mixed substrates." Pig manure and rabbit wastes (Table 4) Pig manure was enriched by the addition of rabbit wastes from 4-66 to 6.1% TS (Table 1). The results showed an increase in efficiency of 22 % or 0-24 litres of methane per litre of digester per day with a retention time of 10 days. The biological yield of 240 litres of CH 4 per kilogram of VS obtained with pig manure mixed with rabbit wastes is slightly inferior to the yield obtained with pig manure alone at 4.66% TS. Summers & Bousfield (1980), at a retention time of 10 days with pig manure alone, found the optimum biogas yield at TS contents between 3.1 and 5.5 %. Two pig manures (see Table 1 for composition) were tested at a retention time of 15 days and the results are shown in Table 4. A 28 o/ /o enrichment in TS (to 6 %) with rabbit wastes resulted in an increase in efficiency of 30 % or 0.25 litres of methane per litre of digester per day.
Ch. Aubart, F. Bully
10
The biological yield on VS obtained with a retention time of 15 days is always greater than the yield obtained with a retention time of I0 days and it decreases when TS content increases (Aubart, 1982). At lower TS the increase in yield caused by the addition of rabbit wastes was 0.20 litres of methane per litre of digester per day. Whatever the Total Solids content, the mixture of rabbit wastes and pig manure is advantageous as regards energy production. However, at the equivalent loading rate (VS) the mixture fails to produce the quantity of gas expected from pig manure alone. The difference between methane production per kg VS obtained with the mixture (3-7 ~o Total Solids) and pig manure alone (3.7 ~ Total Solids) is due to the difference in VS content (Tables 1 and 5). Pollution control R a b b i t wastes
Table 5 shows the results of determinations of COD, TS and VS on inputs and outputs of digesters fed with 6 ~o TS rabbit waste. At a retention time of 20 days good overall reduction in pollutants was obtained in both total input and soluble fraction. Ten- and 7-5-day retention times did not give as satisfactory reductions, although these were better at 10 days than the reductions (16.2 ~o in TS, 18.2 ~o in VS, 34-2 ~ in COD) obtained by van Velsen (1981 ) for 6 ~o TS pig waste. Reductions in TS, VS and COD (44.0, 45.0, 54"0) and soluble TS and VS (37.2, 51-0) found for 6 ~o TS poultry waste at 10-day retention time were more comparable (Aubart & TABLE5 Pollution Control--Results Obtained w i t h 6 ~ TS D i l u t i o n o f R a b b i t Wastes
Retention time (Days)
Total input decrease ~o decrease ~o decrease Soluble fraction in input ~o decrease decrease
20
13
10
7.5
5
4
TS VS COD
61.5 69.1 70.2
43-1 49.4 56.9
26.6 31.7 37.2
25.3 29.7 19.5
18.7 22.1 28.1
18.1 22.0 23-6
TS VS
58.6 77.3
48.6 68.6
41-9 54-7
39.3 54.3
22.7 37.9
22.5 35.0
Digestion of rabbit and pig wastes
11
TABLE 6 Reduction in Pollutants--Comparison of Pig Manure with Pig Manure and Rabbit Wastes Mixed Retention time (Days)
15
10
Pig manure
TS in input Total input TS decrease VS decrease COD decrease TS in input Total input TS decrease VS decrease COD decrease Soluble fraction TS decrease VS decrease
6.05
3'7
23.54 28.87 27.80 6.05
43-6 48.64 54.43
Pig manure with rabbit wastes mixed
2.5 16"03 25.28 47-42 4.66
6"0
3-7
22"32 20'21 27"00 32-32 31-93 44.04 6.1
23.12 28.87 31.99
21.31 25"57 32-28
28.33 33"03 35"07
21.38 31.26
24'53 35.48
31'00 41.71
All figures are percentages. Fauchille, 1982). With 7.5 % TS rabbit wastes, decreases in C O D , VS and TS of 25 to 35 %, depending on retention time, were obtained. These results were unsatisfactory for pollution control. However, with pig slurry of high TS (9 %), van Velsen (1981) found reductions o f T S , VS and C O D were only 19.0, 24.2 and 31.7 %, respectively. M i x t u r e s o f substrates: P i g m a n u r e with rabbit wastes
Table 6 shows results at 15-days retention time. Reduction in pollutants is better with a pig manure having a Total Solids content of 3.7 ~o than with 6-05 ~o Total Solids. There were no significant differences in pollution control with pig waste having different Total Solids contents at 10-day retention time. The pig and rabbit waste mixture gives similar results (Table 6). DISCUSSION This investigation has served to assess the energy potential of a waste that is widely available in F r a n c e - - r a b b i t wastes. The optimum percentage of
12
Ch. Aubart, F. Bully
Total Solids in the feedstock, and the retention time, have been experimentally determined for the objective to which priority is given-energy production or pollution control. The experiments carried out on mixtures of rabbit wastes and pig manure, with the objective of higher gas production, have shown that the theoretical production, calculated as the sum of the individual productions, is not achieved. With rabbit wastes alone it is best to operate at 7.5-day retention time when, with 6 ~o Total Solids, the optimum yield is obtained (213m a of methane per tonne of organic material fed into the digester). The theoretical example of an operation with 8000 rabbits being fattened will help to show the point of this investigation of mixtures of substrates. This operation produces about 104 kg of Total Solids per day, or 1-73 m 3 of rabbit wastes at 6 ~o Total Solids. The digester will have a volume of 12.2m 3 for a retention time of 7.5 days. Mean biogas production will be 30.4 m 3 per day, or 6431 m 3 of methane per year. Since roughly a third of the production is used for the needs of the biogas plant itself, 4244.5m 3 of methane remain available for other uses: this is equivalent to 36 078 kcal year-1, or 3798 litres of domestic fuel oil. Methane fermentation would seem to be of little interest for a livestock operation of this size, except perhaps for heating. On the other hand, the results of one of our experimental digesters, which produces 145 m 3 of methane a day from a piggery waste containing 3.7 ~ Total Solids, point to the advantage of adding rabbit wastes. The gross annual output of this installation is 45 tonnes oil equivalent (Aubart & Farinet, 1983). Also available on site are 217 tonnes per year of rabbit wastes at about 25 Total Solids which could be used to increase the loading rate of the manure. Experimental results obtained by mixing this rabbit waste with the piggery waste in the proportion used in the present experiments suggest that an increase in biogas production could be obtained with the full-scale plant.
ACKNOWLEDGEMENTS The authors appreciate the technical assistance of J. P. Birling and J. L. Farinet. Review and comments by Dr O. Reisinger have been highly appreciated.
Digestion of rabbit and pig wastes
13
REFERENCES Aubart, C. (1981 a). La fermentation m6thanique des d6chets agricoles. Principe Recherches Industrielles-Avenir de cette technique. S~minaire International Combustibles de Replacement, Lidge, 25-27 mai, 13"1- 13.11. Aubart, C. (1981b). La fermentation m&hanique des d6chets agricoles. R6alisations--Perspectives. Energie et Agriculture. Cahiers du CENECA No. 48, 21-30. Aubart, C. (1982). Recherches sur la fermentation m&hanique des d6chets d'~levages. Rapport COMES--Contrat No. 8075306, p. 106. Aubart, C. & Reisinger, O. (1981). Biomasse--Bioconversion: Quelques applications. Agriculture (450), 113-17. Aubart, C. & Fauchille, S. (1983). Anaerobic digestion of poultry wastes. Part 1 : Biogas production and pollution decrease in terms of retention time and total solids content. Process Biochemistry, 18(2), 31-4. Aubart, C. & Farinet, Jl. (1983). Anaerobic digestion of pig and cattle manure in large scale digesters and power production from biogas. IGT Symposium ~Energy from Biomass and Wastes VII', Orlando, 24-28 January. Degremont. (1978). M~mento technique de l'eau. (French 8th edn.), 925-7, 929-30. Henaff, R. & Sinquin, Jp. (1981). La production et la commercialisation du lapin. B.T.I., No. 358-359, 179-93. Hobson, P. N. & Shaw, B. G. (1973). The anaerobic digestion of waste from an intensive pig unit. Water Research, Pergamon Press, Vol. 7, 437-49. Hobson, P. N., Summers, R., Bousfield, S., Mills, P. J., Clouston, D. and Auld, I. (1977). Notes on anaerobic digestion and anaerobic digestion based on experimental plant in Aberdeen. Rowett Institute, July 1977. Hobson, P. N., Bousfield, S. & Summers, R. (1979). The anaerobic digestion of piggery and poultry wastes. Proc. Int. Conf. on Anaerobic Digestion, Cardiff, September. Pinta, M. (1962). Recherche et dosage des 616ments traces. Dunod, 142, 144-5. Recueil d'Analyses des Communaut+s Europ6ennes (1976). Bipea, September, p. 51.. Summers, R. & Bousfield, S. (1980). A detailed study of piggery-waste anaerobic digestion. Agricultural Wastes, 2(1), 61-78. van Velsen, A. F. M. (1977). Anaerobic digestion of piggery wastes. 1. The influence of detention time and manure concentration. Neth. J. Agric. Sei., 25, 151-69. van Velsen, A. F. M. (1979a). Anaerobic digestion of piggery wastes. 2. Start-up procedure. Neth. J. Agric. Sci., 27, 142-52. van Velsen, A. F. M. (1979b). Anaerobic digestion of piggery wastes. 3. Influence of temperature. Neth. J. Agric. Sci., 27, 255-67. van Velsen, A. F. M. (1981). Anaerobic digestion of piggery waste. Thesis. Agricultural University, Wageningen.
Anaerobic Digestion of Rabbit Wastes and Pig Manure Mixed with Rabbit Wastes in Various Experimental Conditions Ch. A u b a r t & F. Bully Laboratoire de Recherchessur Les Fermentations, Soci6t~ Commercialedes Potasses et de L'Azote, 68700 Aspach Le Bas, France ABSTRACT Experiments on anaerobic digestion of rabbit wastes and pig manure mixed with rabbit wastes were made in 6-litre digesters at 37°C with manual loading once a day and mixing for 3 min each hour. The results show the optimum Total Solids ( TS) content as 6 % and the retention time as 7.5 days. The optimum methane yield was about 215 litres k g - x VS but this is of little use for an industrial biogas plant because the wastes volume is low on a rabbit farm. A possible use of this kind of agricultural waste is in mixing rabbit wastes with pig manure to increase the pig manure VS content, which is generally low. With retention times between 15 and 10 days the methane production increases from 0"20 to 0"25 litres of methane per litre of digester per day without affecting pollution control.
INTRODUCTION In France and certain other countries, such as Belgium, the Netherlands and Denmark, the size of 'zero grazing' stock farms is not always commensurate with the area available for the spreading of effluents (liquid manure, poultry wastes, etc.). It has therefore been necessary to develop treatment systems, and many pig farms, for example, have turned to methane fermentation of effluents. Among the many techniques for the transformation of biomass (Aubart & Reisinger, 1981), methane fermentation has, in recent years, won favour for the treatment of wet substrates. I
Agricultural Wastes 0141-4607/84/$03.00 © ElsevierApplied Science Publishers Ltd,
England, 1984. Printed in Great Britain
2
Ch. Aubart, F. Bully
Other types of animal breeding are becoming increasingly industrialized; among them are rabbit farms. France is the world's leading producer of rabbits, with an output of 175000 metric tonnes in 1980 (Henaff & Sinquin, 1981). There are many industrial breeding units that have sets of fattening cages with automatic waste removal systems (excrement with no straw litter). It seemed that it would be worth while to study the biogas production potential of this waste and to examine the reduction in organic pollution achieved by anaerobic digestion under various experimental conditions. Research on the anaerobic digestion of pig manure (Hobson & Shaw, 1973; Hobson et al., 1977, 1979; Van Velsen, 1977, 1979a, b) has shown that this technique gives good results with regard to pollution control and deodorization, and has provided precise data about biogas production. Our own research (Aubart, 1981b) using laboratory digesters and our industrial-scale (200 and 250m 3) digesters has shown that there is a problem with the critical profitability threshold. Biogas production from pig manure is sometimes not enough to pay back the initial cost of a plant in a sufficiently short time. This low production is explained by the relatively low percentage of Total Solids in the pig manures analysed (of the order of 3 to 4 ~). Such results have led us to consider mixtures of animal wastes, especially as it is common in France for a breeder to have two or three types of animal. Thus, for example, the Total Solids (25 ~o) in the solid manure from an industrial-type rabbit farm might be used to increase the Total Solids content of pig manure. In this paper, we present laboratory experiments on mixtures of pig manure and solid rabbit wastes. Using these experimental results it is possible to consider their application to an actual farm and the optimization of biogas plants for pig manure enriched with a substrate having a higher concentration of Total Solids, so lowering the economic threshold and the minimum size of breeding operation for which a biogas plant is profitable. METHODS
Experimental design Two types of experiment were conducted to investigate both the energy potential and the pollution control aspects of anaerobic digestion.
Digestion of rabbit and pig wastes
(i)
Anaerobic digestion of rabbit wastes diluted with water, with two Total Solids concentrations--6 and 7.5 % - - b o t h below the technological limit of pumpability, at five different retention times. (ii) Anaerobic digestion of a mixture of pig manure and rabbit wastes, with pig manure of two Total Solids concentrations--2.5 and 4.5%--giving final mixtures containing 3.7 and 6-1% Total Solids.
Digesters The digesters used for these experiments were designed and built in our laboratory. They had a capacity of 6 litres and were installed in several lines of five digesters each to make use of a single heat source. The cylindrical digesters were made of transparent PVC and heated by hotwater circulation in an external jacket with a feed tube entering the top and opening near the bottom, and an internal pipe from the bottom acting as an overflow weir. Mechanical stirring for 2 min each hour, by two propellers on a shaft turning at 50 rpm, was used~ The gas evolved was stored and measured in a PVC bell gasometer.
Substrates The rabbit wastes came from a litterless, caged, industrial-type farm; feeding and veterinary care were closely controlled. On this type of farm, the urine is separated from the solid portions of the wastes which alone were used for the experiments. The pig manure was from a farm with 3000 meal-fed pigs on a grating floor. Deliveries of the wastes to the laboratory were at short intervals to forestall any decomposition that might be caused by storage, even though storage was at 4 °C. At each delivery and throughout the test, analyses were made to confirm the constancy of the test material. The compositions of the two products are given in Table 1.
Experimental procedure Digesters were fed once a day, at the same time of day. Having large quantities of Total Solids in suspension the manures were well stirred before feeding, but not oxygenated. To start up the digesters, they were inoculated with effluents from pig manure digesters. Earlier studies
TS ( ~ ) VS ( ~ of TS) C O D ( m g O 2 per litre) Supernatant TS (~o) VS ( ~ of TS) Cellulose (g litre- 1) Lignin (g litre- 1) N r (glitre- 1) P (mg litre- 1) K (mg litre- 1) Ca (mglitre- 1) Na (mg litre- 1) Mg (mg litre- 1) NH4 (g litre- 1)
Analysis
0-94 58.11 2.3 1.22 2.66 735 1 190 685 300 365 2.24
1.01 51.69 --3.64 1 020 1 950 1 460 410 520 2.39
1.36 53.94 5.68 2.3 4.38 1 120 2 292 1 240 555 590 3-98
1.11 56-19 8-2 4.72 4 1 445 1 625 1 945 460 552 3.46
1.46 55.34 7.64 3.49 5-4 1 275 2 325 1 825 387 540 4.42
Pig manure with rabbit wastes mixed
1-08 57.73 3 1.32 --1 647 825 363 -2.24
1.58 53.53 10-6 4.44 4.65 1 290 2 570 1 595 619 675 3.98
1.26 58-44 --4.1 1 525 1 860 2045 460 585 3.46
3"7 6 6.1 77.37 77-36 73"86 42505 65376 59844
Wastes
2"5 3'7 4"32 4"66 6"05 72"53 68"74 72.59 72.31 76-93 34586 39484 51 172 52013 76 117
Pig manure
TABLE 1 Wastes Composition
1.25 53.33 20.4 7.72 1.78 890 1 967 1 515 323 387 1.3
6"1 83"51 61 068
Rabbit wastes dilution 6 % TS
Digestion of rabbit and pig wastes TABLE 2
Inoculum Composition Used for Start Up of Laboratory Digesters Analysis
Inoculum 1
lnoculum 2
TS (%) VS (% of TS) COD (mg 0 2 per litre) Supernatant TS (%) VS (% of TS) N x (g litre- 1) P (mg iitre-1) K (mg litre- i) Ca (mg litre-1) Na (mg litre-1) Mg (mg litre- 1) NH 4 (g iitre-1)
4.54 79-73 50 534
3 67.50 41 014
1.20 51.26 2.64 825 2 069 1 308 372 419 1.87
0.97 52.00 2.65 765 2 155 1 180 370 444 2.63
Inoculum 1: 50 % 50 % Inoculum 2:50 % 50 %
effluent of anaerobic digestion of pig manure, rabbit wastes diluted to 6 % TS. effluent anaerobic digestion of pig manure, pig manure.
(Aubart, 1981a) led to the choice of a ratio of 50/50 inoculum/test material. The compositions of the inocula are given in Table 2. The retention times used for the investigation of diluted rabbit wastes were 20, 13, I0, 7-5 and 5 days. For investigation of the pig/rabbit mixture and of pig manure alone, the retention times were 15 and 10 days. The various digesters were started up with much longer retention times to enable a steady state to be reached before each change in retention time (Fig. 1). For the retention times used in these investigations, each experiment lasted at least 2 months. The temperature of the digesters was 37 °C. Gas production was recorded daily and the composition of the gas determined once a week. The input material was analysed once a week on a sample taken at random from the week's inputs. The output material was analysed once every 2 weeks on a specimen representing two samplings of effluent a week.
Analytical methods The same analyses were carried out on the input and output materials and on their supernatants (centrifugation at I l 000 rpm for 15 min).
Ch. Aubart, F. Bully
¢u "=10 u~
Period
B
t;
10 d
7.5 d
A
Retention time 10 d TS 6%
,.s%
8
A
I--
I
,/
s t.w z
w
1~12
1~6
1=40
1=54
168
1~12
1~6
2110
TIME ---Days
Fig. 1. Methane production from rabbit wastes at different loading rates with a laboratory digester (capacity, 6 litres). Total Solids (TS) were determined by evaporation at 75 °C for 36 h and Volatile Solids (VS) by the difference after combustion at 600 °C for I h. Total nitrogen and phosphorus were determined by a Technicon autoanalyser (model P2, second generation). Calcium and magnesium were determined by atomic absorption. Potassium and sodium were determined with a flame emission spectrometer (Pinta, 1962). The ammonia content was determined by distillation from an alkaline medium (NF T90-015 in Degremont, 1978). Cellulose was determined after sulfuric acid and alkaline treatment (Recueil d'Analyses des Communaut6s Europ6ennes, 1976). The determination of lignin was based on the same principle, with an additional 3 h of treatment with sulfuric acid. COD was measured by oxidation in an acid medium using potassium dichromate (NF T90-101 in Degremont, 1978, with a few changes made in dilutions). CH 4, CO 2 and HzS concentrations were measured using a Girdel catharometer chromatograph (model 3000). The stainless steel column (0.32 × 400cm) was packed with Porapack Q.
Digestion of rabbit and pig wastes
RESULTS AND DISCUSSION
Energy production Rabbit wastes All the results presented here, in the Tables or Figures, are the means of many analyses over the entire duration of the tests (one year, on ten digesters operating in parallel). However, the continuous anaerobic digestion of materials that are highly heterogeneous in composition sometimes results in errors of analysis due to sampling errors. Furthermore, although the source of our materials was controlled as closely as possible, their biological nature and their origin (a livestock operation on industrial scale, and run on conventional lines) resulted in greater or lesser variations with time in gas production and, even more so, in the reduction of organic content of the wastes. The methane content of the biogas produced by anaerobic digestion of rabbit wastes was 58.03 _+ 2.57 ~o; with pig manure it was 68-74 _+ 1.10 ~o. The same result was obtained with the mixture of pig manure and rabbit wastes, 68.08+ 1.16~. The HzS content of the gas from the two substrates and of the mixture seemed to be identical. It had a 95 probability of being in the range from 0.185 ~o to 0.193 ~o. Table 3 shows results obtained with rabbit wastes diluted to 6 ~ Total Solids at various retention times. Retention times of more than 20 days are incompatible with adequate production, and so are not included in the results. Figure 2 plots the specific production of methane (P: litres o f C H 4 per litre of digester per day) in terms of retention time (T: days) and TS content. The relationship between specific methane production and retention time can be fitted to a curve of the following equation for 6 ~o Total Solids: P = 2.265/10°'°3~2r; r = -0"9859. Table 3 also shows the results obtained with a dilution of 7"5~oTS at various retention times. The relation between the specific methane production and retention time (Fig. 2) can be fitted to a curve of the following equation for 7.5~o Total Solids: P=21-7/TI'2828; r = -0"9975. The specific production for the dilutions was identical at a retention time of 10.5 days. From 10.7 to 7.5 days specific methane production was higher with waste diluted to 7.5 ~ Total Solids. Rabbit waste diluted to 6~o TS appears to be the most interesting dilution because the biological yields were higher than those obtained
Ch. Aubart, F. Bully TABLE
3
Results O b t a i n e d with R a b b i t Wastes
TS in input 6 ~o
7.5 ~
Retention time (Days) Technological yield (litres o f m e t h a n e per litre o f digester per day) Biological yield (iitres of m e t h a n e per kg o f VS input) Technological yield (litres o f m e t h a n e per litre o f digester per day) Biological yield (litres o f m e t h a n e per kg o f VS input)
20
13
10
0-56
0.88
0.97
219
230
--
--
0.82
172
216
!.10
177
7"5
1.44
213
1.66
201
5
4
1.62
1.70
151
142
--
--
--
--
TABLE4
R e s u l t s ~ o m Mixed Pig M a n u ~ a n d R a b b i t Wastes
--
Per cent TS pig manure
Per cent TS pig manure with rabbit wastes mixed
Retention time (Days)
6"05* 4"66 4"66*
-6-1 --
10 10 10
1'08 1'08 0'84
224 240 249
6-26* 4-32 4"32* 3"7* 2"5 2"5*
-6 --3'7 --
15 15 15 15 15 15
0"84 0'83 0-58 0'47 0'59 0'39
262 268 277 277 309 323
* ( A u b a r t , 1982). N o r a b b i t waste added.
Technological yield Biological yield (litres of methane (litres of methane per litre of digester per kg of VS) per day)
Digestion of rabbit and pig wastes 1.7
1,5
ol "o
"~ =~ 1 . 0 -
~,J
~( 0 . 5 D r.=
:=
~
6
%TS
7.5
% TS
u.J t-.
,~ ~
7.15
110 RETENTION TIME
1S
20 Days
Fig. 2. Technological yield in terms of retention time and Total Solids content.
with rabbit wastes diluted to 7.5 % TS. The retention time of 7.5 days gives a yield of 213 litres CH 4 per kilogram of VS. Mixed substrates." Pig manure and rabbit wastes (Table 4) Pig manure was enriched by the addition of rabbit wastes from 4-66 to 6.1% TS (Table 1). The results showed an increase in efficiency of 22 % or 0-24 litres of methane per litre of digester per day with a retention time of 10 days. The biological yield of 240 litres of CH 4 per kilogram of VS obtained with pig manure mixed with rabbit wastes is slightly inferior to the yield obtained with pig manure alone at 4.66% TS. Summers & Bousfield (1980), at a retention time of 10 days with pig manure alone, found the optimum biogas yield at TS contents between 3.1 and 5.5 %. Two pig manures (see Table 1 for composition) were tested at a retention time of 15 days and the results are shown in Table 4. A 28 o/ /o enrichment in TS (to 6 %) with rabbit wastes resulted in an increase in efficiency of 30 % or 0.25 litres of methane per litre of digester per day.
Ch. Aubart, F. Bully
10
The biological yield on VS obtained with a retention time of 15 days is always greater than the yield obtained with a retention time of I0 days and it decreases when TS content increases (Aubart, 1982). At lower TS the increase in yield caused by the addition of rabbit wastes was 0.20 litres of methane per litre of digester per day. Whatever the Total Solids content, the mixture of rabbit wastes and pig manure is advantageous as regards energy production. However, at the equivalent loading rate (VS) the mixture fails to produce the quantity of gas expected from pig manure alone. The difference between methane production per kg VS obtained with the mixture (3-7 ~o Total Solids) and pig manure alone (3.7 ~ Total Solids) is due to the difference in VS content (Tables 1 and 5). Pollution control R a b b i t wastes
Table 5 shows the results of determinations of COD, TS and VS on inputs and outputs of digesters fed with 6 ~o TS rabbit waste. At a retention time of 20 days good overall reduction in pollutants was obtained in both total input and soluble fraction. Ten- and 7-5-day retention times did not give as satisfactory reductions, although these were better at 10 days than the reductions (16.2 ~o in TS, 18.2 ~o in VS, 34-2 ~ in COD) obtained by van Velsen (1981 ) for 6 ~o TS pig waste. Reductions in TS, VS and COD (44.0, 45.0, 54"0) and soluble TS and VS (37.2, 51-0) found for 6 ~o TS poultry waste at 10-day retention time were more comparable (Aubart & TABLE5 Pollution Control--Results Obtained w i t h 6 ~ TS D i l u t i o n o f R a b b i t Wastes
Retention time (Days)
Total input decrease ~o decrease ~o decrease Soluble fraction in input ~o decrease decrease
20
13
10
7.5
5
4
TS VS COD
61.5 69.1 70.2
43-1 49.4 56.9
26.6 31.7 37.2
25.3 29.7 19.5
18.7 22.1 28.1
18.1 22.0 23-6
TS VS
58.6 77.3
48.6 68.6
41-9 54-7
39.3 54.3
22.7 37.9
22.5 35.0
Digestion of rabbit and pig wastes
11
TABLE 6 Reduction in Pollutants--Comparison of Pig Manure with Pig Manure and Rabbit Wastes Mixed Retention time (Days)
15
10
Pig manure
TS in input Total input TS decrease VS decrease COD decrease TS in input Total input TS decrease VS decrease COD decrease Soluble fraction TS decrease VS decrease
6.05
3'7
23.54 28.87 27.80 6.05
43-6 48.64 54.43
Pig manure with rabbit wastes mixed
2.5 16"03 25.28 47-42 4.66
6"0
3-7
22"32 20'21 27"00 32-32 31-93 44.04 6.1
23.12 28.87 31.99
21.31 25"57 32-28
28.33 33"03 35"07
21.38 31.26
24'53 35.48
31'00 41.71
All figures are percentages. Fauchille, 1982). With 7.5 % TS rabbit wastes, decreases in C O D , VS and TS of 25 to 35 %, depending on retention time, were obtained. These results were unsatisfactory for pollution control. However, with pig slurry of high TS (9 %), van Velsen (1981) found reductions o f T S , VS and C O D were only 19.0, 24.2 and 31.7 %, respectively. M i x t u r e s o f substrates: P i g m a n u r e with rabbit wastes
Table 6 shows results at 15-days retention time. Reduction in pollutants is better with a pig manure having a Total Solids content of 3.7 ~o than with 6-05 ~o Total Solids. There were no significant differences in pollution control with pig waste having different Total Solids contents at 10-day retention time. The pig and rabbit waste mixture gives similar results (Table 6). DISCUSSION This investigation has served to assess the energy potential of a waste that is widely available in F r a n c e - - r a b b i t wastes. The optimum percentage of
12
Ch. Aubart, F. Bully
Total Solids in the feedstock, and the retention time, have been experimentally determined for the objective to which priority is given-energy production or pollution control. The experiments carried out on mixtures of rabbit wastes and pig manure, with the objective of higher gas production, have shown that the theoretical production, calculated as the sum of the individual productions, is not achieved. With rabbit wastes alone it is best to operate at 7.5-day retention time when, with 6 ~o Total Solids, the optimum yield is obtained (213m a of methane per tonne of organic material fed into the digester). The theoretical example of an operation with 8000 rabbits being fattened will help to show the point of this investigation of mixtures of substrates. This operation produces about 104 kg of Total Solids per day, or 1-73 m 3 of rabbit wastes at 6 ~o Total Solids. The digester will have a volume of 12.2m 3 for a retention time of 7.5 days. Mean biogas production will be 30.4 m 3 per day, or 6431 m 3 of methane per year. Since roughly a third of the production is used for the needs of the biogas plant itself, 4244.5m 3 of methane remain available for other uses: this is equivalent to 36 078 kcal year-1, or 3798 litres of domestic fuel oil. Methane fermentation would seem to be of little interest for a livestock operation of this size, except perhaps for heating. On the other hand, the results of one of our experimental digesters, which produces 145 m 3 of methane a day from a piggery waste containing 3.7 ~ Total Solids, point to the advantage of adding rabbit wastes. The gross annual output of this installation is 45 tonnes oil equivalent (Aubart & Farinet, 1983). Also available on site are 217 tonnes per year of rabbit wastes at about 25 Total Solids which could be used to increase the loading rate of the manure. Experimental results obtained by mixing this rabbit waste with the piggery waste in the proportion used in the present experiments suggest that an increase in biogas production could be obtained with the full-scale plant.
ACKNOWLEDGEMENTS The authors appreciate the technical assistance of J. P. Birling and J. L. Farinet. Review and comments by Dr O. Reisinger have been highly appreciated.
Digestion of rabbit and pig wastes
13
REFERENCES Aubart, C. (1981 a). La fermentation m6thanique des d6chets agricoles. Principe Recherches Industrielles-Avenir de cette technique. S~minaire International Combustibles de Replacement, Lidge, 25-27 mai, 13"1- 13.11. Aubart, C. (1981b). La fermentation m&hanique des d6chets agricoles. R6alisations--Perspectives. Energie et Agriculture. Cahiers du CENECA No. 48, 21-30. Aubart, C. (1982). Recherches sur la fermentation m&hanique des d6chets d'~levages. Rapport COMES--Contrat No. 8075306, p. 106. Aubart, C. & Reisinger, O. (1981). Biomasse--Bioconversion: Quelques applications. Agriculture (450), 113-17. Aubart, C. & Fauchille, S. (1983). Anaerobic digestion of poultry wastes. Part 1 : Biogas production and pollution decrease in terms of retention time and total solids content. Process Biochemistry, 18(2), 31-4. Aubart, C. & Farinet, Jl. (1983). Anaerobic digestion of pig and cattle manure in large scale digesters and power production from biogas. IGT Symposium ~Energy from Biomass and Wastes VII', Orlando, 24-28 January. Degremont. (1978). M~mento technique de l'eau. (French 8th edn.), 925-7, 929-30. Henaff, R. & Sinquin, Jp. (1981). La production et la commercialisation du lapin. B.T.I., No. 358-359, 179-93. Hobson, P. N. & Shaw, B. G. (1973). The anaerobic digestion of waste from an intensive pig unit. Water Research, Pergamon Press, Vol. 7, 437-49. Hobson, P. N., Summers, R., Bousfield, S., Mills, P. J., Clouston, D. and Auld, I. (1977). Notes on anaerobic digestion and anaerobic digestion based on experimental plant in Aberdeen. Rowett Institute, July 1977. Hobson, P. N., Bousfield, S. & Summers, R. (1979). The anaerobic digestion of piggery and poultry wastes. Proc. Int. Conf. on Anaerobic Digestion, Cardiff, September. Pinta, M. (1962). Recherche et dosage des 616ments traces. Dunod, 142, 144-5. Recueil d'Analyses des Communaut+s Europ6ennes (1976). Bipea, September, p. 51.. Summers, R. & Bousfield, S. (1980). A detailed study of piggery-waste anaerobic digestion. Agricultural Wastes, 2(1), 61-78. van Velsen, A. F. M. (1977). Anaerobic digestion of piggery wastes. 1. The influence of detention time and manure concentration. Neth. J. Agric. Sei., 25, 151-69. van Velsen, A. F. M. (1979a). Anaerobic digestion of piggery wastes. 2. Start-up procedure. Neth. J. Agric. Sci., 27, 142-52. van Velsen, A. F. M. (1979b). Anaerobic digestion of piggery wastes. 3. Influence of temperature. Neth. J. Agric. Sci., 27, 255-67. van Velsen, A. F. M. (1981). Anaerobic digestion of piggery waste. Thesis. Agricultural University, Wageningen.