Phosphate Precipitation with On-Site Alkali Generation

Abstract

Commercially available membrane electrolysis equipment can be used to produce substantial quantities of caustic soda from salt (NaCl) with chlorine gas being produced as a by-product. This caustic soda may be employed to raise the pH of the final effluent in order to achieve high levels of phosphate precipitation or removal in the form of crystallization. The pH may then be lowered, prior to final discharge, by re-carbonation utilising combusted biogas, which is produced on-site. At the Kempton Park wastewater treatment works, which achieves biological phosphate removal by means of the Phostrip process, laboratory scale tests proved the commercial viability of this process. It was found that a dosage of 150 mg/l of caustic soda was needed to reach a redox potential of −250 mV, at which point the phosphate content of the stripper overflow was reduced from 30 mg/l to under 1 mg/l. The savings effected in the cost of chlorinating the final effluent, entirely offsets the cost involved. A number of other possible process configurations are proposed, with treatment costs varying from 0,8 to 5,3c/kl, which compares favourably with biological or other methods of chemical treatment, which typically range from 3 to 6 c/kl (costs given in SA cents = 0,35 US cents).