In the Netherlands, there is approximately 35,000 km of stormwater drainage system. Some of these systems are separated, while others are improved separated (VGS in Dutch). In practice, municipalities often struggle with these systems. Improved systems sometimes discharge large amounts of relatively clean water to the wastewater treatment plants, leading water authorities to question whether they can turn off the pump or close the valve. However, this can result in the deterioration of surface water quality due to issues such as illicit connections or other pollution sources. Is detecting and addressing those illicit connections cost-effective, or would converting to a VGS 2.0 system be more beneficial? Or are there alternative solutions available?

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On behalf of STOWA, we are developing a "guideline for managing existing stormwater systems". The goal of this guideline is to structure and standardize the decision-making process regarding how to manage an existing stormwater system. The aim is not to prescribe standard solutions.