Experiment 1 (Exp 1) was a completely randomized design under in vitro conditions with six treatments and three replicates 10-L flasks. Treatments were the replacement of pig manure by water hyacinth juice (WHJ) at 0%, 20%, 40%, 60%, 80%, and 100% (dry mass). The experiment period lasted 42 days. Biogas production between days 8 and 21 significantly differed between treatments (P < 0.01). Methane production was 0.261, 0.223, 0.201, 0.190, 0.171, and 0.155 m3/kg OM (P < 0.001). The Exp 2 was an in vitro factorial design with the first factor being plant materials (rice straw, water hyacinth [Eichhornia crassipes] and Para grass [Brachiaria mutica]) and the second being the replacement ratios of plants to pig manure from 10, 20, 30, 40, and 50 (dry matter basis). The experimental period was 60 days. The methane concentration of rice straw on day 37 reached the methane concentration standard of biogas (61.6%), whereas that of water hyacinth and Para grass was 58.2% and 16.6%, respectively. In Exp 3, a 50-m3 bio-digester was used to evaluate biogas and electricity production with 3 levels of pig manure loaded daily (40, 60, and 80 kg fresh material). Methane production in the digester
was high (61.2%) and sufficient for use as an energy source for electricity and cooking.