Modelling the effect of salt from road runoff on nitrification of a wastewater treatment plant

Salt (NaCl) that is being dispersed on the roads to prevent the formation of ice and snow can have positive and negative effects on nitrification rates in wastewater treatment plants (WWTPs). Based on experimental data, a numerical model has been derived to describe these effects. The numerical model has been successfully implemented in the SIMBA# simulation software and tested on a real case study, the Freistadt WWTP, located in Upper Austria. A number of parameters impacting nitrification have been investigated: inflow salt concentration, duration of the salt loading, temperature during salt loading, and increasing volumetric inflow to the WWTP during salt loading events. Simulation results revealed that salt concentration lower than 1 g NaCl/l brought improvement in nitrification rates. However, when this threshold was exceeded, inhibition of nitrification occurred. Furthermore, prolonged salt dosing exposure times brought amplification of both positive and negative effects on removal rates. Results show that salt concentration and salt load have the biggest impacts on nitrification.