Co-precipitation Technology

The City of Corcoran in the San Joaquin Valley in Southern California historically had only needed to chlorinate its groundwater supply prior to distribution. With a reduced arsenic MCL standard, the City found all of its nine groundwater wells out of compliance. The City explored various treatment possibilities. Aft er conducting cost studies and running pilot tests, Quad Knopf, the city’s engineering fi rm, designed a state-of-the-art 18 MGD treatment plant based on blending the raw water into a single stream and removing arsenic using co-precipitation technology.

Process Responsibility and Guarantee

One of the most important treatment objectives for the city was to have a supplier take on process responsibility and guarantee a maximum effl uent arsenic level of 5ppb for 98% of the treatment period. The city and engineers selected Tonka Equipment Company to provide a cost eff ective treatment plant to meet the requirements of the City.

Treatment and Design

The city’s multiple wells are combined at the head of the plant in a raw water blend tank. This blending reduces the arsenic levels from the most contaminated wells. Aft er chemical addition, the water is pumped by multiple feed pumps through a static mixer and through the fi lters into a 2 million gallon ground storage tank. Each of the fi ve (5) 12’ diameter x 40’ OAL horizontal pressure fi lters is comprised of two cells sharing a common underdrain. Prior to entering the fi lters, sodium hypochlorite, ferric chloride, and a small dose of polymer are added. The chlorine serves to oxidize any natural iron and change the valence of the arsenic from As+3 to As+5. This arsenate co- recipitates with the iron from the ferric chloride addition, forming a fi lterable solid. The water is evenly split between each fi lter cell. Within the fi lter cells, the water passes through the dual-media fi lter bed consisting of anthracite and Greensand Plus™.

Optimizing Performance

Water conservation was another system objective. The fi lter backwash water is sent to a backwash reclaim tank and, aft er a sett ling period of up to 4 hours, the water is recycled back to the raw water blending tank.

The system is controlled by Tonka’s PLC-based automatic control panel which has been customized to automatically backwash the fi lters by loss of head, time, or operator initiation. The duration of the backwash and other functions is selectable and adjustable by the operator through a color touch screen interface to optimize operating results.

The treatment process since start up has been highly eff ective in delivering quality water to the population of Corcoran.