Indirect discharges: from blind spot to clear picture

Introduction

When thinking of industrial discharges, people often see the classic image of a factory pipe that discharges directly into a river. In reality, that is only part of the story. A significant share of industrial emissions takes place via indirect routes: wastewater that goes via municipal sewers or industrial systems to wastewater treatment plants (WWTPs) and from there still ends up in surface water.

These indirect discharges are often less visible. They run via many different companies and come together in complex networks of pipes and installations. That makes it harder to get a complete overview. The result: a route that greatly influences water quality, but which requires extra attention to properly visualize.

Why indirect discharges are difficult to see

There are several reasons why indirect industrial discharges are less visible:

• Historical focus on direct dischargers — For decades the emphasis in permits and monitoring was on large companies that discharge directly. Indirect routes were more fragmented and therefore less systematically visible.
• Dispersed information — Data are spread across parties: municipalities, water boards, provinces and environmental agencies, each managing valuable pieces. Bringing that information together is complex, but offers opportunities for new insights.
• Permits needing updating — Some indirect discharges still fall under older or generic permits, where new substances such as PFAS or other ZZS are less explicitly included.
• Complex streams — Wastewater from dozens of smaller companies often comes together in one pipe, making it difficult to trace exactly where a particular substance comes from.

These factors make indirect discharges challenging to make fully visible. But precisely here there are great opportunities to develop more control through smart analyses and cooperation.

The role of WWTPs and industrial estates

WWTPs play a central role. They are designed to reduce nutrients and organic load, but not to completely break down persistent substances or heavy metals. For ZZS they therefore often act as a transfer station: the substances pass the treatment and end up in the effluent.

Industrial estates require extra attention. Hundreds of small companies discharge collectively via one industrial sewer system. Individually the emissions seem limited, but together they form a considerable load. A WWTP mainly sees the sum, without it being directly visible which companies contribute the most.

The governance puzzle

Beyond the technical challenges, there is governance complexity. Different parties each have an important role, holding crucial information and expertise. The challenge is to bring that knowledge together and create an integrated picture. Through cooperation and data sharing, blind spots can be reduced and supervision made even more effective.

Consequences and urgency

• Environmental risks — ZZS accumulate in sediment, soil and food chains. Even low emissions can have major effects in the long term.
• Health impact — Many ZZS are carcinogenic, hormone-disrupting or neurotoxic. Invisible emissions increase the chance that risks are recognized too late.
• Strengthening cooperation and trust — Achieving the WFD goals in 2027 is a joint task. By smartly linking information and intensifying cooperation, authorities can further strengthen their role as reliable stewards of water quality—improving policy and supervision effectiveness and building trust among citizens and companies.

Conclusion

Indirect industrial discharges are one of the greatest challenges in today’s water management. They are fragmented, diffuse and embedded in complex technical and governance structures, making them less visible. That is precisely why attention to this route is so important. By connecting data, sharing information and working together, authorities can gain more grip on these discharges. This not only creates better insight into ZZS and other harmful substances, but also a stronger foundation for future-proof water management. The urgency is clear: by jointly working on insight and transparency, authorities can act more effectively and work with confidence towards achieving the WFD goals.