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Experiments were conducted with pilot small-scale biogas digesters designed for the family level in developing countries. Four 3.5 m3 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 m3 (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 m3 (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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