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Filtration
How It Works
Filtration SystemParticles in surface water are too small to be trapped in a filter, but they are processed to stick together to make larger particles. This process is called coagulation and flocculation. Chemicals, such as polymers, are added prior to the filters to change the stability of the individual particles.

First Stage
Flocculation, which occurs in the first stage of the filters, causes the particles to impact each other so they become attached. The first stage also acts as a roughing filter removing the larger particles. Water then flows down through a bed of granular media (anthracite and sand) trapping the smaller particles that are larger than the filter pores. Particles smaller than the pores become attached to the granular media of the filter by absorption.

Back-Wash
The filters are back-washed to clean the captured material from the filter media. This is accomplished by forcing air and treated water up through the media. The waste flows to the backwash water recovery tank where approximately 75% of the water is recovered and returned to the treatment facility with the raw water, while the solids settle out and are pumped to the sanitary sewer.

Chemical Treatment
Chemical additives are used in treating Rainy River water. Multiple application points for the chemicals are incorporated into the treatment scheme for operating flexibility and for the changing characteristics of the water supply. The chemical feed rates are set by automatic controls with manual overrides.

For More Information
Contact the Water Plant at (218) 283-2990 for more information.