Stormwater treatment in a changing regulatory landscape

Across much of Western Europe, wastewater utilities are under growing pressure to manage complex weather challenges. More frequent periods of intense rainfall, continued urban development and ageing sewer infrastructure are placing greater demands on treatment plants. While tighter environmental standards are raising expectations around the protection of rivers, lakes and coastal waters.

The revised Urban Wastewater Treatment Directive (UWWTD) reinforces the need for a more integrated approach to urban wastewater management. It encourages utilities to strengthen resilience, reduce pollution from stormwater and combined sewer overflows, and make better use of existing treatment infrastructure. 

Here, Andrea Maggio, Global Product Manager, Process Intensification at Xylem, explains how wastewater operators can increase resilience while balancing capital investment and ongoing operational costs, and why innovative treatment technologies will play an increasingly important role in meeting future regulatory and environmental demands.

The revised UWWTD raises expectations

Managing stormwater is not a new problem for wastewater treatment plants. But as our climate continues to change, the impact of weather events is evolving. While the picture is different country to country, Europe is the fastest-warming continent in the world. This means heatwaves are becoming more severe and heavy rainfall is causing more flooding.

Now the spotlight is firmly on taking control of the issue. The revised UWWTD places greater emphasis on protecting receiving waters from pollution associated with stormwater and combined sewer overflows. It encourages EU Member States to better assess urban hardstanding runoff, improve monitoring, and implement measures to reduce pollution entering the environment during wet weather. It also requires the monitoring of storm discharge frequency and the development of sewer management plans. Together, this information is expected to provide a clearer picture of peak flow discharges from combined sewers during rainfall events.

For wastewater operators, this means increasing treatment resilience and demonstrating that infrastructure can respond to changing climate conditions while maintaining environmental performance. This is likely to be achieved by increasing the peak flows treated during storm events.

EU Member States now have until 31 July 2027 to transpose the revised UWWTD into national law. This gives utilities a crucial window to understand future obligations, develop investment strategies and begin preparing for the operational changes ahead. As implementation progresses across Europe, many utilities are evaluating technologies that can increase hydraulic capacity, improve nutrient removal and support compliance without requiring extensive new construction. 

Increasing capacity and minimising footprint

Alongside cost, one of the biggest constraints facing existing wastewater treatment plants is space. Many facilities have little opportunity for physical expansion, making it essential to extract greater performance from existing infrastructure.

Advanced ballasted clarification technologies offer an effective way to increase treatment capacity within existing treatment set-ups. By accelerating solids separation, these systems enable significantly higher hydraulic throughput while maintaining consistent effluent quality during both normal and peak flow conditions.

This approach can help utilities improve resilience during storm events while minimising the disruption, cost and environmental impact associated with major civil engineering projects.

Dual-purpose innovation

Modern ballasted clarification systems also provide benefits beyond stormwater management.

Xylem’s CoMag^™ system use magnetite to enhance floc formation, enabling solids to settle significantly faster than conventional clarification processes. The result is increased treatment capacity alongside improved removal of suspended solids and phosphorus, supporting compliance with increasingly stringent discharge standards.

Because this technology can be retrofitted into existing tanks, utilities can enhance plant performance while making better use of existing assets. The process also recovers almost all of the magnetite for reuse, reducing operating costs and supporting wider sustainability objectives.

The ability to operate as both a storm flow treatment process and an advanced tertiary treatment stage allows operators to maximise equipment utilisation throughout the year, improving both operational efficiency and investment value.

Importantly, this type of process intensification is not only about increasing capacity within the same footprint. By consolidating multiple treatment objectives within a single system, it can simplify plant operations, reduce the need for additional treatment processes and infrastructure, and maximise the value of existing assets.

This approach is already being demonstrated in mainland Europe. Just one example is a municipality in Norway which selected the CoMag system for a first-of-its-kind storm flow application in the region. Originally specified to provide treatment during wet weather events, the project evolved to incorporate tertiary treatment during normal operation, allowing a single system to perform two critical treatment functions.

The installation has been designed to achieve high levels of phosphorus removal to help protect the Oslofjord while providing additional hydraulic resilience during periods of elevated flow. By combining stormwater management with enhanced treatment quality, the project illustrates how utilities can gain greater value from investment while preparing for future regulatory requirements.

A more resilient future

For many operators, enhancing the performance of existing treatment assets will be one of the fastest and most practical routes to improving compliance and reducing the environmental impact of stormwater. 

Technologies, such as ballasted clarification, that increase capacity, improve treatment performance and support regulatory compliance without requiring major expansion, will play an increasingly important role in building resilient wastewater infrastructure across Europe.