Limestone contactors: Steady-state design relationships

Limestone contactors can mitigate corrosion in small water-supply systems that use dilute, acidic water. As water is transported through a packed bed of crushed limestone, CaCO₃ dissolves and the pH, calciumion concentration, and alkalinity increase. Operation of a contactor can be effectively modeled by considering the rate of dissolution and interfacial transport of calcium ions. The steady-state model developed and tested in this study relates the depth of limestone required in the contactor to the desired effluent water chemistry, influent water chemistry, limestone-particle size and shape, bed porosity, water temperature, and superficial velocity. The magnitude of the rate constant that describes the release of calcium ions from the calcite surface varies with the pH at the particle surface. When this pH is less than about 9.5, the rate constant for the surface reaction becomes large, and the rate of dissolution tends to be controlled solely by the transport of calcium ions away from the interface.