BIOGAS PRODUCTION FROM SOLID-WASTES REMOVED FROM FISH FARM EFFLUENTS

An experimental small scale partial recirculating system for rainbow t rout was assembled. The system components were two 1.3-m(3) fish tanks with sloping bottoms, each connected to a sedimentation column and co ntaining 50 kg rainbow trout biomass, an anaerobic up-flow digester (t otal volume 0.424 m(3), available volume 0.382 m(3)) connected to the funnel shaped bottom of the sedimentation column by means of a perista ltic pump, an aerobic submerged plug-flow filter (total volume 1 m(3); filled with 0.83 m(3) plastic rings with a specific surface of 194 m( 2). m(-3)) and a submerged pump. Aeration was provided through porous stones. The anaerobic digester was filled with 35 mm cubes of expanded polyurethane foam (25 pores . cm(-2), specific surface 1.375 m(2). m( -3) filtering volume 0.291 m(3)) and kept at a temperature of 24-25 de grees C using an electric heater. The gas chamber at the top of the an aerobic digester was connected to a gas meter and to an infrared conti nuous gas analyser. Measures on system performance with a recirculatio n rate of 60 % were done following three feeding levels (1, 1.5 and 2 % live weight). At the highest feeding rate, 2.8 L of faecal sludge co llected from the trout tanks were pumped every four hours in the anaer obic digester. Slurry characteristic were: total N 0.197 g . L-1, TAN 0.014 g . L-1, volatile solids (VS) 16.91 g . L-1, suspended solids (S S) 21.39 g . L-1 and pH 6.9. Biogas production was 144 L . d(-1) (mean value) with a methane content higher than 80 %. Methane volumetric pr oduction was 0.3 m(3). m(-3) d(-1) and methane daily yield was 0.4 and 0.32 m(3). kg(-1) VS and SS respectively. After passing through the a naerobic digester, effluents were characterized by a total N content o f 0.243 g . L-1, TAN 0.222 g . L-1, VS 1.1 g . L-1 SS 1.32 g . L-1 and pH 6.8. The anaerobic digester was able to significantly reduce VS an d SS content of wastewater and the zeolite ton-exchange column signifi cantly improved water quality of effluent produced by the digester. Th e aerobic biofilter significantly reduced the ammonia content of the w ater leaving the fish tanks. (C) Ifremer/Elsevier, Paris.