A simulation of the fate of nitrogen in an on-site wastewater treatment system

An experiment to study and to build a mathematical model to reproduce the behavior of nitrogen in a residential wastewater treatment process was performed. A pilot unit receiving grey and black water was used. The pilot consisted in a septic tank followed by a fixed-film, partly aerated bioreactor with effluent recirculation operated under two different scenarios: normal operating conditions and increased influent flow. Modeling was performed with the GPS-X™ software. Following a sensitivity analysis, the model was calibrated by comparing results from the pilot experiment and those of the simulation predictions. Obtained results show that the studied pilot unit is able to eliminate most of the ammonia contained in the influent, except for days with exceptionally high influent concentrations. Chemical oxygen demand (COD) and total suspended solids (TSS) reduction is also near complete and a partial denitrification is present. The calibrated model shows a good agreement with results obtained in the pilot unit, although work remains to be done for nitrates. Modeling of residential wastewater treatment appears to be a useful tool for understanding, optimizing and predicting the expected performances of such a system.