Engineering aspects of calcium carbonate and magnesium hydroxide precipitation in waste water reclamation

Doctoral Thesis


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University of Cape Town

This thesis attempts to resolve some of the major problems associated with lime treatment in waste water reclamation. The contribution to knowledge is briefly outlined below. One of the major problems associated with lime treatment is the instability of lime-treated effluent, which may result in serious calcium carbonate scale formation problems. In the thesis this instability is attributed to two fundamental causes, (1) Incomplete precipitation, i.e. a kinetic problem. (2) The unintentional absorption of carbon dioxide from the air by the highly alkaline lime-treated effluent, i.e. a contamination problem. Calcium carbonate and magnesium hydroxide precipitation are time dependent. As a consequence of this time dependency unstable effluent may be produced under reaction conditions commonly encountered in practice. An exhaustive study identified the major factors affecting the precipitation kinetics. Reaction system conditions required for producing a stable effluent are, (1) Lime slurry and sludge, in that sequence, must be thoroughly mixed with the waste water, preferably by means of in-line static mixers, before discharge to a completely stirred tank reactor. (2) A completely stirred tank reactor with a minimum mean residence time of two minutes must be provided for the dissolution and precipitation reactions to go as near to completion as possible. (3) The reactor contents must have a sludge concentration of the order of 10 000 mg l⁻¹.

Includes bibliography.