Harvesting daphnia magna grown on urban tertiarily-treated effluents

Abstract

The work undertaken was intended to measure the yield of Daphnia magna populations harvested under a semi-continuous mode and fed continuously a suspension of the green microalga Scenedesmus obliquus grown in urban wastewaters.The non selective harvesting of daphnid populations leads to the full expression of their growth potential and reduces the instability of size distribution, which corresponds to a lowered yield. After 2 months of harvesting at rates of 6, 18 and 32% every 3 days, average harvests of daphnids were respectively 465, 2065 and 3025 mg l−1 wk−1 (wet wt); corresponding maximum population densities were 12,500, 20,000 and 15,000 daphnids l−1. The daphnid/alga energy conversion ratio was 0.4 for the population harvested at a rate of 32%/3 days. Protein and energy contents of the harvested daphnid biomasses were respectively 59.5% and 20.0 kJ g−1 (dry wt).The results demonstrate the possibility of growing Daphnia magna populations on tertiarily-treated effluents with a high harvest rate and show the high potential of urban wastewaters to grow biomasses useful for animal feeding purposes.