Reuse of treated domestic sewage and duckweed (Lemna gibba) in tilapia aquaculture

110 Reuse of treated domestic sewage and fresh duckweed (Lemna gibba) harvested from duckweed sewage stabilisation ponds has been applied in culture of Nile tilapia (Oreochromis niloticus). Fresh duckweed was compared with local fishmeal-based diet, while using treated sewage and de-chlorinated city tap water as a water source. The daily feeding rate in duckweed-fed fishponds was 25 g fresh duckweed per 100 g of live fish weight. The fishponds fed commercial feed received equal amounts of protein as in the duckweed-fed ponds. The growth performance of juvenile tilapia showed no significant ( https://www.w3.org/1998/Math/MathML" display="inline"> p > 0 . 05 ) difference in the Specific Growth Rate (SGR) of tilapia reared in treated sewage and fed either fresh duckweed or commercial feed. In freshwater ponds, the juveniles of tilapia fed on commercial feed had significant ( https://www.w3.org/1998/Math/MathML" display="inline"> p < 0 . 01 ) higher SGR than those fed on fresh duckweed. In case of duckweed fed ponds, the SGR of tilapia reared in treated effluent was significantly ( https://www.w3.org/1998/Math/MathML" display="inline"> p < 0 . 01 ) higher than in freshwater ponds. Similar results were obtained with commercial feed. The SGRs of tilapia were 0.97 and 1.01 in the treated sewage-duckweed-fed pond (TDP) and treated sewage-commercial feed-fed pond (TCP), respectively. In the freshwater ponds the values were 0.63 and 0.79 for the freshwater-duckweed-fed pond (FDP) and freshwater-commercial feed-fed pond (FCP), respectively. Extrapolation of results revealed that the fishponds fed on fresh duckweed provided a net fish yield of 15.7 ton/ha/y in the TDP and 8.5 ton/ha/y in the FDP. In case of commercial feed-fed ponds the net fish yields were, 16.8 ton/ha/y and 11.2 ton/ha/y in the TCP and FCP, respectively. The feed conversion ratios (FCRs) were 1.2, 1.4, 1.6 and 1.5 in, TDP, TCP, FDP and FCP, respectively. The N balance in the fish ponds indicated that, the percentages of the nitrogen recovery from the total nitrogen input, represented 13.4% and 13.7% in treated sewage fed ponds, and, 24.2% and 27.9% in the freshwater fed ponds. The results show that reuse of treated sewage and sewage-grown duckweed in aquaculture produces high fish yield and efficient nutrients recovery at low feeding costs.