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Date of Award

5-1974

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Supervisor

Dr. J. D. Norman

Abstract

Canada and the United States by the "Great Lakes Water Quality Agreement" have designated that the discharge of phosphates in sewage to the Great Lakes will be controlled. In the majority of cases, chemical precipitation is the treatment method to be used. The selection of the chemical dosage needed is empirical and requires that extensive experimental studies be carried out at each wastewater plant.

The problem of estimating the chemical dosage is approached in this work on a more fundamental basis. The precipitation of phosphates with lanthanum is assumed to be representative of a chemical system at a metastable equilibrium. The dosage which results in the desired aqueous phosphate concentration at this state is estimated with an equilibrium model of the system. This model includes the reactions of major inorganic species found in wastewater.

Lanthanum is an efficient precipitating agent which causes a low residual phosphate concentration to result, even when added in quantities based upon the stoichiometric requirements. Also, lanthanum can be recovered by an economically viable process and recycled.

The solubility products of lanthanum, ortho-, pyro- and tripolyphosphate, lanthum hydroxide and carbonate and the stability constants of the hydroxide and polyphosphate ion complexes were measured. The experiments were carried out in distilled water at ion concentrations typical of the magnitude to be used in an actual treatment process. With this information, and solving the complex chemical equilibria problem for a set of over fifty separate reactions, it is shown that the extent of precipitation of ortho- and polyphosphates from a municipal wastewater can be accurately predicted. A computer program was written to predict the lanthanum dosage required for a specific residual of dissolved phosphates given the water quality of the wastewater expressed as the total aqueous concentrations of ortho- and polyphosphates, calcium, magnesium, carbonates, sodium, chloride, sulphates and pH.

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