Integrated Biogas Technology in the Tropics. 2. Use of Slurry for Fish Culture

Abstract

The composite slurry from four 3.5-m3 ferrocement digesters fed a mixture of nightsoil, water hyacinth (Eichhornia crassipes) and rice straw was loaded into four 200-m2 earth fish ponds at organic loading rates of 25, 50, and 100 kg chemical oxygen demand (COD) ha-1 day-1. Impressive extrapolated net fish yields of about 10 kg ha-1 day-1 (3.7 ton ha -1 year-1) were obtained at the highest organic loading rate, equivalent to loading rates of about 120 kg total solids and 4.5 kg total Kjeldahl nitrogen ha-1 day-1. There may be potential for even greater yields from biogas slurry fed fish ponds because the relationship between net fish yield and slurry loading rate was linear. Constraints in the development of integrated biogas technology for poor farmers in the tropics are likely to be associated with the digester rather than with the reuse of slurry for fish culture.