Water Softener Regeneration Effects on the Operation of Domestic Wastewater Treatment Plants: A Preliminary Study

There has been a large amount of scientific research carried out to date on the impact of salty backwash brine from domestic water softeners (WS) on domestic wastewater treatment plants (DWTPs). Experts and practitioners agree that the impact is harmful and there is still a need to look for new technologies. The study of the effect of an increased sodium chloride (NaCl) concentration after softener regeneration is important from the point of view of the operation of DWTPs and soil properties. This paper presents the results of a field study of the concentration of NaCl at the septic tank (ST) drainage point, into which the grey water from the regeneration of the water softener flowed. During the six-month measurements (recorded every 1 min), an increase in NaCl concentration was observed in the septic tank outflow, from an average NaCl concentration of 1.5 g·L−1—between regenerations—to an average concentration of 4.5 g·L−1—after water softener regeneration. The increased NaCl concentration decreased significantly up to 2 days after the water softener regeneration. Temperature changes in the treated wastewater were also measured—during the winter period, temperature differences of up to 10 °C per day were recorded. In the second part of the study, conducted on a semi-technical scale, the effect of brine from the regeneration of the water softener on the hydraulic conductivity (Ks) of the soil from the infiltration drain of the DWTPs studied was assessed. The Ks was determined by analysing the time it took the water to soak into the soil, using the Van Hoorn equation. The results and statistical analysis indicate an increased salt content in the soil absorbing the brine, which may have been influenced by the reduced absorption and capacity of the drain due to adverse physico-chemical changes.