Experiments were conducted with pilot small-scale biogas digesters designed for the family level in developing countries. Four 3.5 m 3 ferrocement digesters equipped with manually operated mixers were fed a mixture of nightsoil, water hyacinth (Eichhornia crassipes) and rice straw and operated semi-continuously at hydraulic retention times (HRT) of 30, 50 and 70 days, corresponding to organic loadings of 1.2, 0.75 and 0.53 kg m -3 day -1 of total volatile solids (TVS), respectively. Although more biogas was produced at a HRT of 30 days with respect to the unit weight of TVS, the rates of biogas and methane production within these loading rates were similar and approximately 0.283 and 0.186 m 3 (kg TVS) -1 day -1 , respectively. A major operational problem was excessive scum accumulation in the digester due to the low specific gravity of the plant matter substrates. A parallel batch experiment using 20-1 aspirator glass bottles yielded biogas production rates of 0.400 and 0.406 m 3 (kg TVS) -1 at the end of 30 and 50 days, respectively. Manually operated biogas plants using heterogenous substrates in which plant matter is a major component may have little potential for poor farmers in developing countries.
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Polprasert et al. (1986) studied this question.