The behaviour of physicochemical parameters in watercourses downstream of WWTPs under reduced flows

Significant changes in long-term hydrological values have been observed, resulting in an increase in the proportion of wastewater discharged from wastewater treatment plants (WWTPs) in watercourses. During periods of reduced flow, this proportion can exceed 50% of the flow in the receiving watercourse. For most parameters, concentrations increase in the watercourse downstream of the WWTP, thereby deteriorating water quality (nutrients, chlorides, conductivity, coliform bacteria, dissolved substances). However, the assessment is less clear-cut for some parameters. With decreasing flow, the differences in concentrations upstream and downstream of the WWTP discharge increase for most parameters. A notable increase in water temperature and pH downstream of the WWTP is observed during the warmer months, accompanied by a more rapid decomposition of organic matter. This results in smaller differences for ammonium nitrogen, and conversely, larger differences in its concentrations during the colder months.

In addition to the well-known problems with nutrients, the situation with increasing salinity of watercourses may also worsen with more frequent occurrences of low flow periods. Unlike nutrient parameters, salinity parameters do not have targets for good ecological status set in the Czech Republic, so they are not included in the assessment of the status of water bodies and no measures to improve their ecological status need to be proposed. Phosphorus and chloride concentrations exhibit a marked increase downstream of the WWTP throughout the whole year.

The paper will present the impacts of discharges from WWTPs that have been selected as significant for the Czech Republic in the Plan for Managing Drought and Water Scarcity in the Czech Republic. These examples are used to find flow rates that would eliminate the combined impact of drought and wastewater treatment plant discharges on the ecological status of the affected surface water bodies. The results of an assessment of the impact of the first stage of reconstruction of the largest WWTP in the Czech Republic, which treats wastewater from the capital city of Prague, on improving water quality in the Vltava River will also be presented.