Wastewater discharges and microbial variability in a surface water source

Licentiate thesis, 2008

Planning for drinking-water safety in surface waters includes a systematic assessment of hazards in the catchment, and microbial contamination is considered a major health risk. The aim of this thesis was to identify and characterize microbial hazardous events reported for River Göta älv, Sweden. Moreover, the magnitude and variability of the microbial load from municipal sewer systems was determined for varying weather conditions. The raw water intake to Göteborg at River Göta älv, regularly closed to avoid contaminants in the raw water, act as a barrier against various hazardous substances. Occasionally, bacterial peak concentrations penetrate the raw water intake, as not registered in time. Present results show that data on upstream precipitation, turbidity and real-time reporting of wastewater discharges may give valuable guidance to predict high microbial concentrations at the raw water intake. Wastewater treatment plants and sewer networks with about 110,000 persons connected give a substantial pathogen load to the river. Monte Carlo simulations showed that wastewater treatment plants dominated the pathogen load to River Göta älv during dry weather conditions. During wet weather conditions, including sewer overflows and emergency discharges, total microbial loads from the municipal sewer systems were for E. coli about 13 log10 CFU/day and for Cryptosporidium about 8 log10 oocysts/day. Microbial river water concentrations due to municipal wastewater discharges were approximated by simulations. Wastewater contaminations may explain the magnitude and variability of river water sample concentrations of indicator bacteria (E. coli, clostridia and somatic coliphages) and Giardia. Risk reduction options are discussed based on sensitivity analyses for dry and wet weather conditions.