The 10 million gallons of water stored in the Cool Circle are exposed to weather and pollution from the environment. Contaminated water leads to corrosion and blockages in the system. To combat this, a helophyte filter was installed. A helophyte filter is a filter that uses helophytes to purify wastewater to a quality that is harmless to the environment. Helophytes are plants that grow above water but grow with their roots in very wet soil and can transport oxygen to their roots by themselves.
Behind the Horst are two fields of gravel, sand and root cloth on which reed plants have been planted. The dirty water that flows in at the top of the filter is filtered through the gravel, where the roots of the reeds naturally extract dirt and other impurities. In the soil, the waste is converted into nutrients for the plants in the filter. The water leaving the helophyte filter is clean enough to be used again in the system.
