Central Softening by Crystallization in a Fluidized‐Bed Process

Abstract

Water softening installations at Amsterdam (the Netherlands) Water Supply's two main drinking water treatment plants reduce hardness from 250 to 150 mg as CaCO 3 /L. In this softening process, pH is raised by adding caustic soda to produce calcium carbonate, which crystallizes on seeding grains in a fluidized bed within a cylindrical reactor. The process, which is not temperature sensitive, allows intensive mixing of water and caustic soda without causing turbidity, and sodium concentrations are not raised above regulatory guidelines. The only by‐product of the process, which is stable and makes few demands in terms of operation and maintenance, is recyclable seed grains layered with calcite. Additionally, increased pH lowers lead and copper solubility by factors of 2 and 5, respectively, and the process can be used to remove heavy metals and phosphates and for denitrification. Total investment and operations cost is equivalent to $4 per household per year, and total savings on household detergents, soaps, energy, and maintenance are equivalent to $25–$40 per household per year.