Tackling sewer inflow and infiltration with cost-effective strategies in Trollhättan

Data-driven planning supports compliance and long-term environmental sustainability

Trollhättan Energi AB, a municipal utility serving the Swedish city of Trollhättan, has long worked to manage sustainable water and wastewater services in a region known for its waterways and frequent rainfall. Despite years of effort, the utility faced ongoing challenges with inflow and infiltration (I/I) in its sewer system, causing overflows and environmental stress. When authorities required a reduction of I/I to 60%, Trollhättan Energi partnered with DHI to explore more effective solutions. Using advanced modelling with DHI CityFlow Live Wastewater Network (formerly Future City Flow), the project shifted focus from percentage-based I/I targets to actual environmental impact. This enabled the development of cost-effective strategies aimed at reducing pollution and overflow incidents. The new approach proved equally or even more effective, showing how data-driven planning supports compliance and long-term environmental sustainability.

Challenge

For several years, Trollhättan Energi had been grappling with a high proportion of inflow and infiltration (I/I) in its sewer system — a challenge the utility was well aware of. In response, the company developed an action plan and implemented a range of measures aimed at reducing both I/I volumes and the frequency of sewer overflows. Despite these efforts, the proportion of I/I remained high at around 70%. This prompted the County Administrative Board to issue a requirement to lower the rate to a maximum of 60%. Before investing in additional measures, Trollhättan Energi turned to DHI to better understand the underlying causes of the persistently high proportion of I/I and identify the most effective way forward.

Solution

Since 2018, DHI has been supporting Trollhättan Energi in mapping the current situation and identifying the causes behind the high proportion of sewer inflow and infiltration (I/I). Conducted with support from DHI CityFlow Live Wastewater Network, the process combined in-depth interviews and detailed sewer network modelling. A comprehensive status overview was developed, enabling rapid simulation of a wide range of scenarios, which would otherwise have taken months using traditional methods.

DHI CityFlow Live Wastewater Network also served as an educational and decision-support tool, clearly visualising the costs and impacts of various alternatives. This proved valuable in both internal discussions and communications with the County Administrative Board. One scenario, which involved reducing the I/I proportion to 60 percent, was found to require substantial investment from the municipality.

Based on these findings, the focus shifted from simply reducing the I/I proportion to mitigating its environmental impact. Rather than treating I/I as the core issue, attention turned to rainfall-induced inflows – the component responsible for peak flows and sewer overflows that elevate pollutant levels in receiving waters.

Alternative action plans were then evaluated, targeting a reduction in sewer overflows and bypasses at the wastewater treatment plant. While these measures did not lower the I/I proportion below 60 percent, they achieved comparable or greater reductions in environmental impact and overflow incidents, and at a significantly lower cost to Trollhättan Energi.

Results

The following year, DHI delivered two alternative action plans to Trollhättan Energi: one focused on reducing the proportion of inflow and infiltration (I/I), and the other on lowering environmental impact by mitigating rainfall-induced overflows. Both were presented to the County Administrative Board, and a revised permit application was submitted.

In response, the County Administrative Board imposed an investigation condition requiring the utility to determine both the appropriate performance indicator for follow-up and the corresponding target value within three years. DHI acted as advisors throughout this process. Trollhättan Energi has since supplemented its submission and is currently awaiting the final terms and conditions.

By shifting focus from meeting a strict I/I percentage to reducing the actual environmental impact, the project identified more cost-effective and targeted solutions. The results demonstrate that a flexible approach — one that considers both environmental outcomes and socio-economic factors — can deliver equal or even greater benefits than traditional percentage-based targets, while also reducing strain on municipal resources.