Mesophilic anaerobic digestion of dairy cow slurry on a farm scale: Economic considerations

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Abstract Two identical continuously stirred tank reactors (CSTR) of 125 m3 capacity were used to digest anaerobically dairy cow slurry of about 7% total solids (TS). One digester was operated at 25°C and the other at 40°C. Both were run at 15 d retention time from 1 February to 20 March 1985, and then at 20 d retention time until 20 May. During the first period, the biogas was used to fuel a combined heat and power (c.h.p.) unit and the recovered heat was used to heat both digesters via a common hot water circuit. During the second period, digester heating was achieved by burning biogas in individual boilers. Daily gross energy production was calculated from the volume and calorific value of the biogas produced by each digester. Net energy production was determined by subtracting from the gross energy produced the energy required to counteract losses through the digester walls plus that required to heat the feed slurry. During the second period, a direct measure of net energy was obtained by recording the volume of biogas burnt in the boilers. There were wide variations in daily gross energy production but mean values were approximately 30% greater at 40°C than at 25°C, and approximately 20% greater at 15 d retention time than at 20 d. Mean net energy production was greater during the period 1 February to 20 March, when the digesters were operated at 20 d retention time, than from 21 March to 20 May when the retention time was 15 d. However, when the mean net energy values were corrected for the differences in ambient temperature between the two periods and for the electrical energy required to operate the pumps and mixers, maximum values were obtained by operation at either 25°C and 20 d retention or at 40°C and 15 d retention time. The ratio of heat required to counteract losses through the digester walls to that required for heating the feed slurry ranged from 0·44 to 0·71 whilst the ratio of mean net energy production to mean gross energy production ranged from 0·15 to 0·53. The overall efficiency of the c.h.p. i.e. the energy input as biogas compared with the energy output as electricity and hot water averaged 75% but the unit proved to be unreliable in operation.