Minimising combined sewer overflow, FHNW School of Life Sciences

In combined sewer systems, rainwater and wastewater from households are collected together and make their way to wastewater treatment plants. Heavy rainfall can cause these canals to overflow before the water has been treated.

To counter this, Prof. Dr. Michael Thomann’s team is participating in the European project StopUP, which aims to reduce pollution in receiving waters. The team is working closely with canton Basel-Landschaft’s Office for Industrial Operations (AIB), which is responsible for sewers, wastewater treatment plants (WWTP) and landfills.

Although AIB monitors the amount of storm water discharged from their sewers, little is known about its quality and thus the pollution load released into the environment during combined sewer overflow spills. Sewer networks and runoff dynamics are so complex that representative sampling and lab analysis are challenging to implement in everyday operation.

“Online sensors can collect sufficient data points to capture the dynamics of events. We aim to correlate parameters measured with our sensors with toxicologically relevant pollutants such as heavy metals or certain organic compounds” explains Michael Thomann. First sampling campaigns and chemical analyses showed promising results. Ecotoxicology testing is planned in 2024 to further understand environmental impacts of untreated wet weather runoff.

The project aligns with the Swiss concept to minimise the ecological impact on receiving waters through integrated management of the sewer network and WWTP, including optimal utilisation of the hydraulic and biochemical capacity of the WWTP. The WWTP Birs uses sequencing batch reactors to treat wastewater. During dry weather, these reactors execute full cleaning cycles. The plant switches into wet weather mode when heavy rainfall is expected, so the reactors can partially treat a greater quantity of water.

“Our hypothesis is that it’s probably beneficial to perform a shorter treatment, even if that treatment is a bit less effective, than having untreated water spill over” says Michael Thomann. “Modelling will help us understand how we could further increase the inflow to the WWTP during rainy weather without unduly compromising treatment efficiency.”