Nitrogen and phosphorus removal from urban wastewater by the microalga Scenedesmus obliquus

The removal of phosphorus and nitrogen by the freshwater alga Scenedesmus o bliquus, cultured in urban wastewater, previously submitted to secondary se wage treatment, was studied under different conditions of stirring and temp erature. In all cases, the amount of NH3 lost, as well as biomass productiv ity and its biochemical composition, were evaluated. The specific growth ra tes proved greatest in the stirred cultures, the highest mu value being 0.0 438 h(-1) at 30 degrees C. The stirring increased biomass productivity (P-B ) in the linear growth phase after exponential growth, with the optimum app earing at 25 degrees C. For the temperatures studied stirring was not neces sary to provide the highest percentage of P elimination (%P-max), but did r educe the time needed to reach that percentage (t(max)). The highest %P-max value, 98%, within the shortest time period, t(max) = 94.33 h, was found i n the culture with stirring at 25 degrees C. Ammonium removal was determine d by two factors - the consumption of ammonium for growth and elimination b y desorption as ammonia. The highest percentage of ammonium removal (%N-max ), 100%, resulted at the final culture time (t(f)) of 188.33 h, in the stir red culture at 25 degrees C. The biochemical composition of the biomass gav e the normal values for this microalga reported by other authors. The prote in content was notably low, around 11.8% by weight, and the polyunsaturated -fatty-acid content was high. The N:P ratio of the culture medium was 12.9. Finally, we proposed a dilution factor for the treated wastewater (f) to b e dumped in order to regulate operation conditions and time for an optimal removal of nitrogen and phosphorus. (C) 2000 Elsevier Science Ltd. All righ ts reserved.