Nanofiltration and reverse osmosis treatment reduce Total Dissolved Solids (TDS), incidentally lowering hardness and alkalinity in the process. If not treated further, this low alkalinity, low hardness permeate, can be extremely corrosive to downstream environments. Remineralization, the addition of hardness and alkalinity back into the water prior to distribution, can be a successful means of reducing this corrosion potential.
Remineralization options include chemical feed, lime and calcium carbonate. Chemical feed has a lower capital cost but can have higher operating expenses and create the potential for overfeeding. Using lime can be a messy and costly option. Calcium carbonate, also known as calcite or crushed limestone, provides easy operation with little need for process monitoring, costs 50% less than lime, and requires 50% less CO2 than lime to operate.
|Treatment begins as water enters the pressure vessel with water encountering the fixed bed of calcite. The water dissolves the calcite particles, neutralizing the effluent.
This crushed limestone, or calcite, is utilized as a fixed bed of particles in a pressure vessel – a calcite contactor. Calcite contactors have seen little use in the United States in the past but have recently gained popularity as a cost-effective and easy to use option for remineralization. As an industry leader, Tonka Water, a U.S. Water Brand, pioneered the development of quality contactors to address this need.
Recently, the community of Craven County, NC knew they needed to treat the corrosive permeate as part of a new reverse osmosis system. Tonka Water, a U.S. Water brand, provided a calcite contactor system in horizontal pressure vessels with an isolated cell design. Once water has been treated through a membrane system, 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. This process has been highly effective in providing remineralization of the permeate prior to distribution.
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