Why do we let our rainwater drain away through the sewage system?

We must manage water differently, given the current triple planetary crisis. This includes not discharging waste water into natural surface waters. Yet regional water authorities are sometimes forced to discharge untreated waste water from our sewage system. ‘It then ends up in a canal or a lake,’ computer scientist Dilek Düştegör explains. She collaborates with municipalities and regional water authorities to develop computer models to best manage wastewater flow.

FSE Science Newsroom | Text Charlotte Vlek | Images Leoni von Ristok

‘The biggest problem is that multiple actors are involved in the Dutch sewage system,’ Düştegör reports. Water boards are responsible for cleaning our waste water before it ends up in surface water. Their main goal is to make sure that the water quality in nature is up to standard. They have no influence on the inflow of water into their treatment facilities as this is managed by municipalities whose main goal is to quickly dispose of the waste water. These are conflicting goals. Düştegör: ‘It can be difficult to combine them. Moreover, the two organizations do not consult each other about this matter.’

That is why Düştegör builds digital models that simulate the sewage system: so-called digital twins. Using these models, both parties can explore the best way to achieve their individual goals. More specifically, Düştegör will build two separate digital twins: one that shows the water level at different places in the sewage pipes, and one that monitors water quality in nature for the regional water authorities. The idea is that these two models ultimately ‘talk’ to each other, allowing for optimal solutions.

A municipality full of sensors

Düştegör collaborates with the municipality of Heusden, near Den Bosch, where sensors were placed in various locations throughout the sewage system. ‘We use these sensor measurements and combine them with the groundwater level and the weather forecast,’ she explains. ‘This information is used to predict the wastewater levels in the sewage pipes, so we can test the best way to prevent overflow.’

‘For instance, the municipality can decide where to open a valve to make sure that the water doesn’t overflow based on our simulations.’ And sometimes, this means that water has to be discharged somewhere. That is undesirable for the water quality in nature, and the model for the regional water authorities will clearly show this effect. By bringing these two models together, Düştegör hopes to bring about structural improvements.

‘A big advantage of a digital twin is its ability to test out what-if scenarios,’ says Düştegör. What would be the effect of an entire afternoon of heavy rain? And what if we invest in a separate system to drain rainwater from the streets to avoid heavy rains overloading the sewage system? Would that be worth the investment? Düştegör: ‘For regional water authorities, it would be advantageous to separate wastewater and rainwater. But right now, they have little to say about the matter.’

On Friday 21 March 2025, the Wubbo Ockels School for Energy and Climate organizes an event on climate adaptation and water , one of the key focus areas of the school. The main questions of this day are: How can we emphasize the urgency and opportunities of regional, national, and international climate adaptation, and how can new or old means of communication help us.

Sign up for this event here!

This article is the third in a series on water research at the Faculty of Science and Engineering at the University of Groningen.

Read the other articles of this series here: