Softening by fluidized bed crystallizers

Fluidized bed crystallizers (FBCs), also called pelletizers or pellet reactors, are an attractive alternative to conventional water softening. FBCs produce a coarse, spherically grained, easily dewatered, calcitic sand as the waste product, rather than a gelatinous sludge. Two operating full-scale municipal FBCs used for softening in southern Florida were sampled and characterized. The study concludes that: (1) Calcium removal is very rapid within the reactor, leaving a large portion of the bed unused at any one time; (2) the calcium removal rate can be described by the Reddy-Nancollas crystallization model; (3) high supersaturation levels due to poor mixing at the point of chemical mixing promotes microfloc production and solids carryover in the effluent; (4) classification of the pellets within the fluidized bed does not occur; (5) the sampling techniques appear to be adequate for characterizing FBC reactors; and (6) operation of FBCs would be aided by the installation of a pressure gauge on the FBC influent to monitor bed growth.