North Water Treatment Plant

North Water Treatment Plant

The NWTP has a permitted capacity of 6.67 mgd.  Raw surface water, which has a total hardness of 400 mg/L is pumped from the Source Water Facility.  Water hardness (calcium and magnesium ions) is the soap- or detergent-consuming power of water, which causes difficulties in doing laundry, washing dishes and cause a coating to form in hot water heaters.  Therefore, the City's raw surface water is treated to reduce the water's hardness through the lime softening process.  

Raw surface water enters the facility and is mixed with hydrated lime along with alum (aluminum sulfate) in a vessel known as a "lime reactor".  This process causes the bicarbonate in the water to precipitate and settle out, thus reducing the level of hardness leaving the NWTP (120-140 mg/L).  The overflow (or effluent) is treated with sodium hydpochlorite and ammonia to form chloramines.  The effluent from the lime reactor goes to a low-pressure hollow-fiber membrane system for polishing.

Although the NWTP treats approximately 75% of the City’s potable water, only 33% enters the distribution system from the NWTP.  Typically, 3 to 3.5 mgd of the finished lime-softened water is pumped to the South Water Treatment Plant (SWTP), where it is blended with the SWTP’s finished water and sent out into the water distribution system from the SWTP (SWTP treats brackish ground water by reverse osmosis membranes).