Although it is prohibited to discharge scrubber water and blackwater into the Baltic Sea, it is still allowed to release metals, microplastics, and other organic materials that affect the ecosystem. This concerns wastewater from showers, sinks, kitchens, and laundry facilities. A new report from Lighthouse and the Swedish Transport Administration (Trafikverket) evaluates the economic and environmental performance of alternative strategies for managing greywater.

Since 1 July 2025, discharging scrubber water in Swedish territorial waters has been prohibited, while passenger ships have been banned from discharging blackwater (toilet water) throughout the Baltic Sea since 2021, according to IMO regulations. Currently, however, there is no international or Swedish regulation prohibiting the discharge of greywater.

– In an earlier study, we found that greywater contained high levels of various nutrients, such as nitrogen and phosphorus. In short, it is polluting, says Jenette Tifuh Mujingni, who led the research project Cost-Benefit Analysis of Alternative Greywater.

For the first time, the project conducted an integrated cost–benefit analysis of how greywater from ships can be managed through port treatment, land-based purification, and reuse in the Baltic Sea. Ten different scenarios were analyzed, ranging from direct discharge into the sea (which is still common today) to advanced land-based treatment with water reuse. Trelleborg port, which has long received and treated ship wastewater, was used as a representative case. Unsurprisingly, all scenarios were more expensive than direct discharge into the sea.

– Among the scenarios focusing solely on treating greywater, none achieve a positive net socio-economic benefit. The environmental gains are too limited relative to the cost for it to be sustainable.

Sustainability is only achieved in scenarios that include reuse. In the most advanced scenarios, treated greywater is, for example, used for toilet flushing on board the ships, reducing the need for freshwater. These options provide significantly greater environmental benefits and lower socio-economic losses than scenarios without reuse. The best outcome is achieved with full reuse, although even this option is currently marginally unprofitable—mainly due to the low economic value of freshwater in Northern Europe.

– The processes are costly. To enable water reuse, for example, Trelleborg port needs to upgrade its treatment system to ensure sufficiently high quality, says Jenette Tifuh Mujingni.

The question is who should bear the costs. The study points to a clear imbalance among stakeholders. Shipping companies bear the largest costs, while ports are relatively little affected, and municipalities mainly benefit from reduced strain on wastewater treatment plants. If the most environmentally beneficial solutions are to be implemented, policy instruments in the form of cost-sharing mechanisms and new regulations will be required.

– I believe it will be implemented. In several countries, it is under discussion, and Finland has already decided to ban greywater discharges in its territorial waters from 1 January 2030.

The report Cost-Benefit Analysis of Alternative Greywater – Management Scenarios from “Cradle to Grave” was authored by Jenette Tifuh Mujingni (Chalmers), Fredrik Hedman (IVL Swedish Environmental Research Institute), and Elias Sonnsjö Lönegren (IVL Swedish Environmental Research Institute).