Advanced purification of sewage-works effluent using a combined system of lime softening and flotation

Further results are given of pilot-plant studies on the removal of resistant organic matter from secondary sewage effluents by coagulation and flotation (see also Wat. Pollut. Abstr., 1967, 40, Abstr. No. 1323). Full treatment comprised coagulation with excess lime, flotation with micro-bubble aeration achieved by centrifugal pump action, recarbonation with carbon dioxide, contact stabilization on ferric hydroxide to remove slow-settling calcium carbonate, sand filtration, break-point chlorination to remove ammoniacal-nitrogen, and filtration through activated carbon; this process produced a final effluent equal in quality to drinking water. The advantages of using excess lime instead of aluminium sulphate as coagulant include its sterilizing action, low cost, and complete removal of phosphates; the aeration process used also appears to have economic advantages over conventional dissolved-air flotation. Some studies were also carried out on final treatment of the effluent by the Desal ion-exchange process to reduce the dissolved mineral salts that tend to build up in reclaimed water. probable costs of a 1 m.g.d. demonstration plant using this process are estimated.