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

Abstract

Operational data over 2 years from three large Austrian wastewater treatment plants (WWTPs) with design capacities of 4 million, 950,000 and 110,000 population equivalent (PE) were examined. Salt peaks, due to thawing road 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 plants' behaviour in a controlled environment, NaCl was dosed into the inflow of a laboratory-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 activated sludge activity. Particle size distributions of the activated sludge floc sizes were measured using laser diffraction particle sizing and showed a decrease of the floc size by approximately two-thirds. The floc structure was examined and documented using light microscopy. At salt concentrations below 1 g/L, increased respiration was found for autotrophic biomass, and between 1 and 3 g NaCl/L respiration was inhibited by up to 30%. HIGHLIGHTS Yearly, 7–12 thawing road-salt events of up to 3 g NaCl/L were detected at WWTPs.; Approx. 25% of the salt applied yearly in public areas was found in the WWTPs' inflow.; Respiration increased for concentrations <1 g NaCl/L and decreased above them.; Autotrophic activity OURA (+42%) was enhanced by more than heterotrophic OURH (+20%).; Sludge floc particle size decreased by two-thirds after salt shocks.;

Keywords

• activated sludge
• nacl shock loading
• oxygen uptake rate
• particle size
• sludge volume index