05 Feb 2020

Is rainwater from cloudbursts a health risk in green recreational areas?

Green recreational areas are increasingly being used for temporary storage of flood water after cloudbursts to reduce costs for drainage. DHI is investigating whether it poses a health risk to people using the areas after heavy rains.

The heavy rainfall and cloudbursts which we have experienced throughout recent years are putting pressure on urban sewer systems. Because of this, several municipalities and water utilities plan to direct rainwater into green recreational areas, such as football fields and parks, for temporary storage. However, in areas with combined sewer systems, the flood water is a mixture of rain and wastewater, which pose health risks. When playing in or visiting these areas after the flood, the public is exposed to wastewater that contains pathogenic microorganisms. 

Analysis of critical health risks
A new project aims to map out and identify the health risks when visiting recreational areas after a flood and at the same time investigate how to mitigate the health risks. 
In a previous project, DHI conducted a study of health risks from cloudburst water mixed with wastewater, including the length of time required before the risk of exposure to pathogenic bacteria is reduced to a level where all health risks are acceptable. 

In the previous study, two experimental grass areas were flooded. One area with clean water, the other area with a mixture of clean water and wastewater. The concentrations of pathogens in the grass and their decay over time were measured. Then hydraulic models, estimating the impact of wastewater, were combined with the actual measurements and used for a microbial risk assessment to estimate the probability of diarrhoeal infection.

The results showed that there is a significant health risk after a urban flood, and that it may take up to 14 days after heavy rainfall before the level of pathogens is so low that the organisms no longer can be detected. 

DHI will now test the experimental findings in practise in cooperation with the Municipality of Copenhagen, HOFOR, Aalborg Kloak A/S, VandCenter SydNovafos as well as the National Food Institut and Center for Diagnostik DTU. The project will investigate how high the risks are in selected parks and how to manage unacceptable risks by changing cloudburst plans, restricting access to the parks or providing information about the risks to citizens.
The project is funded by the Foundation for Development of Technology in the Danish Water Sector and will be completed during 2020.