MODELING ORTHOPHOSPHATE EVOLUTION IN A HIGH-RATE ALGAL POND

High rate algal ponds (HRAPs) are an efficient treatment for controlli ng wastewater pollution by reducing the organic matter and the inorgan ic nutrient content. An experimental HRAP was set up in Meze (France) and sampled over 24 months. A model simulating orthophosphate (PO4) ev olution was constructed using Stella II software. It was thought that deterministic modelling of the temporal evolution of PO4 might provide a rational basis for pond management policies. In hardwater, two proc esses have to be taken into consideration: PO4 uptake by algae and PO4 precipitation. The model also has to take into account the fact that the latter process is very sensitive to nycthemeral variations in pH. Simulated PO4 concentrations over a two-year period show that about 10 % of total input is removed by precipitation whereas about 30% is remo ved by absorption. These results are of critical importance for the ma nagement of this type of wastewater treatment system.