Figure 10 shows the gas production from water hyacinth in four slots of hydraulic retention time. In first slot from 0-10 days about 12% gas is produced. From 11-20 days gas production increases to about 27%. In the third slot from 21-30 days the maximum gas is produced which is about 46%. Finally, from 31-36 days gas production reduces to about 13%and gas production almost ceases after the 36th day. Constituents of biogas from water hyacinth were found very similar to that from cow dung (Fig. not shown here).