Grey becomes blue – greywater for irrigation and industrial processes

Historically, the Swedish perception has been that access to drinking water is unlimited. However, recent years with several dry summers have shown that access to drinking water can become a challenge.

The availability of raw water for use in drinking water production varies across Sweden, with some areas more vulnerable than others. In Stockholm, this is primarily due to the capacity of water treatment plants being unable to provide drinking water for irrigation during dry periods. This has raised awareness that we must manage water responsibly.

For the Loudden sub-area in Stockholm Royal Seaport, a source-separated wastewater system is planned, with separate collection and treatment of greywater from households and businesses. When Loudden is completed, approximately 1,000 m³ of greywater will be generated daily. Water that, after treatment, could be used to replace drinking water for purposes such as irrigating green spaces, instead of being discharged directly into the recipient.

In Stockholm Royal Seaport, there are significant water needs that do not necessarily require drinking water quality. For example, water for irrigating green spaces during dry periods, dust suppression during street sweeping, and industrial operations such as those at the Mass Consolidation Centre, Ports of Stockholm, Stockholm Exergi, and a concrete plant. In the research project "Grey Becomes Blue", these needs have been the basis for the work.

Purpose and goal

The project's purpose is to investigate how greywater can be used to reduce the use of drinking water in Loudden in both the short and long term by:

• Contributing to increased knowledge about greywater as a water resource in urban environments
• Enhance understanding of the advantages and limitations of various technical solutions (treatment and distribution).

The project's goal is to explore the possibility of replacing drinking water with treated greywater for irrigating green spaces and to investigate the potential for other uses in industrial activities.

Results and experiences

The project started in December 2022 and has contributed to the following results:

• The significant need for irrigation of urban greenery during summer could be met with treated greywater. A low-energy treatment solution using a filter bed followed by polishing in a constructed wetland could also be used as a design element in parks.
• The industrial need in the area is mainly found at Stockholm Exergi's facility. The need is much greater than the volumes that would be generated in Loudden and requires drinking water quality.
• The project has helped raise issues regarding water use within the City's administrations and has generated great interest from the City District Administration of Northern inner city and the Traffic Administration for the use of treated greywater, as well as for nutrients from toilet water.
• Through the project, documentation—volumes, availability over time, and areas of use—has been collected for further planning in Loudden.
• The legal conditions for Stockholm Vatten och Avfall as the main water and waste utility to handle greywater and provide water of a different quality than drinking water have been clarified.

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