The influence of winter road maintenance on the presence of chlorides in wastewater entering small treatment plants

During the appearance of the first snowfall, there is a revival of discussion on effective methods of protecting road surfaces and sidewalks against icing. In Poland and many other countries, so-called road salt, mainly sodium chloride (NaCl) with additives, is often used to lower the melting point of snow and ice. Using chemicals to protect road surfaces brings many negative side effects reported in the literature. Less frequently published research results indicate, and also alarm, that increased chloride concentrations can appear in wastewater flowing into sanitary (separate) sewers. In the case of small wastewater treatment plants, increased chloride concentrations can have a negative impact primarily on the biological processes of wastewater treatment and, after discharge from the wastewater treatment plant, on the biological life in the waters and the nearest recipient environment of the treated wastewater. The study aimed to determine the concentrations and loads of chlorides in wastewater flowing through the distribution sewer system to 4 small wastewater treatment plants located in Poland, in the Lesser Poland Province, during snowmelt and heavy rainfall in 2019–2023. The study showed a significant increase in concentrations and loads of chlorides in wastewater in February. Unit chloride load in raw sewage during snowmelt varied from 7 to 12 kg∙d–1 per 1 km length of separate sewer network. There was also a repeated, but much lower, increase in chloride con-centrations during summer and autumn precipitation. This is when the leaching of residual salt accumulated around the road surface occurred.