Determining the Fate of Drinking Water Sludge at the Rya Wastewater Treatment Plant in Gothenburg

Abstract

Regulations on effluent nutrient concentrations become increasingly stricter, which puts pressure on the treatment capacities of wastewater treatment plants (WWTPs). Many WWTPs receive and treat drinking water sludge (DW sludge) produced by drinking water treatments plants (DWTPs). The DWTPs use coagulants such iron- or aluminum sulfate to reduce the turbidity of the drinking water, producing sludge as a by-product. This sludge is sometimes discharged into the sewer system and reaches WWTPs with the influent wastewater. The DW sludge can potentially affect the treatment capacity of WWTPs; however, its fate in the wastewater treatment processes is uncertain. This thesis focuses on the Rya WWTP in Gothenburg, Sweden, which receives DW sludge rich in aluminum, from the DWTPs, Lackarebäck and Alelyckan. The main goal of the study was to determine the fate of DW sludge in the primary settlers at the WWTP. The methodology involved analyzing the metal contents in the incoming wastewater, the DW sludge, as well as other sludge samples from various locations in the WWTP. The metals were used as indicator substances to differentiate between the DW sludge and the wastewater sludge (WW sludge). Aluminum and arsenic were used as indicator substances for DW sludge. Based on the results from the specific study period, the DW sludge represented approximately 40% of the aluminum and the arsenic in the incoming wastewater. Metals such as calcium, copper, magnesium and zinc were underrepresented in the DW sludge, 6% or less, and therefore served as indicators of WW sludge. The fate of the two sludge fractions was determined by calculating mass balances of the metals in the WWTP. Based on the metals representative of the WW sludge, approximately 50% was removed in the primary settlers. When using the measured mass flow of incoming wastewater approximately 90% the DW sludge is removed. However, mass balances for many metals in the WWTP could not be closed, indicating uncertainties in the measurements. Therefore,calculations were conducted using a mass flow in the incoming wastewater based on the mass flows of waste activated sludge and primary sludge, which showed that approximately 50% of the DW sludge is removed. The effect of another uncertainty, the DW sludge flow, was evaluated by varying the flow ± 30 %, which showed that the calculated removal of DW sludge decreases if a larger amount of influent DW sludge is assumed and vice versa. The study concluded that the DW sludge is most likely removed in the primary settlers to the same extent as WW sludge, potentially more.