Influence of road salt thawing peaks on the inflow composition and activated sludge properties in municipal wastewater treatment

Abstract

Operational data of two years from three large Austrian wastewater treatment plants (WWTPs) with 4 Mio, 1 Mio and 110,000 population equivalents (PE) design capacity, respectively, were examined. Salt impulses due to street thawing salt were detected and quantified by electrical conductivity, temperature and chloride measurement in the inflow of the WWTPs. Daily NaCl inflow loads up to 1,147 t/d and PE-specific loads of 0.26–0.5 kg NaCl/(PE · y) were found. To mimic the plant's behaviour in a controlled environment, NaCl was dosed into the inflow of a lab-scale activated sludge plant. The influence of salt peaks on important activated sludge parameters such as sludge volume index, settling velocity and floc size were investigated. Influent and effluent were sampled extensively to calculate removal rates. Respiration measurements were performed to quantify the activated sludge's activity. Particle size distributions of the activated sludge flocs size were measured using laser diffraction particle sizing and showed a decrease of the floc size by approximately. 2/3. The floc structure was examined and documented by light micrographs. At salt concentrations below 1 g/L an increased respiration was found for autotrophic biomass, between 1 and 3 g NaCl/L the respiration was inhibited up to 30%.