Masters Theses

Date of Award

5-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

R. Bruce Robinson

Committee Members

Joseph J. Perona, Gregory Reed

Abstract

Fluidized bed crystallization (or pelletization) is an attractive alternative to conventional water softening processes. Water softening pelletizers (WSPs) precipitate hardness onto seed sand particles producing hard spheres (consisting primarily of calcium carbonate as calcite) that continue to grow until wasted. The product pellets are easily removed from the reaction vessel and drained free of water. Pelletizers offer operational simplicity, low residual water content (thus, reduced sludge volume), and ease of waste handling and disposal. The low capital costs and operational simplicity are especially desirable for small water treatment plants. The major disadvantage of WSPs is that relatively high effluent turbidity levels are common. The carry over of tiny floe particles reduces run times of subsequent filtration operations.

The scope of this investigation is to characterize WSPs from an engineering perspective in order to quantify their performance and explore the potential for new or improved design applications. Three different WSPs located in Florida have been sampled at various heights within the operating reactors and analyzed for hardness, suspended solids, alkalinity, pH, turbidity, and temperature. The distribution of particle (pellet) sizes are determined. Mixing regimes and other operational factors are also evaluated. The data is compared to predictive models.

This study finds that the sampling method used is adequate for chemical characterization of operating WSPs, calcium depletion is very rapid and a large portion of the pellet bed is unutilized in the crystallization process, and high supersaturation levels near the raw water and lime feed inlets (caused both by fluid maldistribution and by large initial reactant concentrations at this level) promotes microfloc production and contributes most of the solids carryover. It is suggested that uniform flow and chemical addition should be investigated as a means to reduce local supersaturations and thus reduce microfloc production and subsequent suspended solids carryover.

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