Modelling orthophosphate evolution in a high rate algal pond

High rate algal ponds (HRAPs) are an efficient treatment for controlling wastewater pollution by reducing the organic matter and the inorganic nutrient content. An experimental HRAP was set up in Mèze (France) and sampled over 24 months. A model simulating orthophosphate (PO4) evolution 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 processes 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. ted PO4 concentrations over a two-year period show that about 10% of total input is removed by precipitation whereas about 30% is removed by absorption. These results are of critical importance for the management of this type of wastewater treatment system.