Craven County, North Carolina is located in the eastern part of the state near the Neuse River. The County installed a new water treatment plant using reverse osmosis. The corrosive permeate from this system (with low pH, low alkalinity, and hardness) requires remineralization prior to distribution. Such remineralization stabilizes and prevents corrosive water from being introduced to The County’s distribution system. Calcite contactors were developed by Tonka Water to address the needs of these membrane-based water treatment plants. For this project, Tonka Water provided a calcite contactor system including two contactors in horizontal vessels with isolated cell design. The entire calcite contactor system is controlled by Tonka Water through an integrated PLC system panel.

Once water has been treated through the reverse osmosis process it enters the horizontal vessels via the underdrain, flowing upwards through the vessel. As the low pH, low alkalinity, low hardness water flows upward, it comes into contact with
calcite (an engineered gradation of calcium carbonate). The water slowly dissolves the calcite particles, bringing the right amount of hardness and alkalinity into the water while restoring a neutral pH level to the effluent.

The isolated cell design of the horizontal vessel offers tremendous flexibility by allowing the operator to isolate individual cells while adjacent cells remain in service.

The raw water analysis summarized below reflects pH, hardness and alkalinity existing in the water. The treatment process has been highly effective in delivering quality water as indicated below.

Membrane PermeateCalcite Contactor Effluent
pH (pH Units)6.27
Hardness (mg/l as CaCO3)2.345-50
Alkalinity (mg/l as CaCO3)3.055-65