Nitrogen and Phosphorus Reclamation from Municipal Wastewater through an Artificial Food-Chain System

Scenedesmus sp. cultivated under laboratory conditions in secondarily-treated urban wastewater attained a mean productivity of 20.1 g (dry wt) m-2 day-1 and the complete removal of nitrogen and phosphorus from the effluent in a 24 h cycle. A Daphnia magna population inoculated into the Scenedesmus sp. suspension exhausted the microalgae in 48-72 h and achieved productivity levels of up to c. 3 g (wet wt) litre-1 week-1, a maximal food conversion efficiency of 0.44 mg mg-1 and efficiencies of nitrogen and phosphorus incorporation varying from 40.6 to 49.5% and from 69.5 to 85.2%, respectively. When the D. magna cultures were kept under continuous illumination instead of under light-dark cycles or in darkness, the biomass yield, the food conversion efficiency and the efficiency of incorporation of nutrients increased significantly, showing that, in the light, Scenedesmus sp. cells were able to use the daphnid excreta as a source of nutrients for growth.