Pós-tratamento com cal de efluentes de sistemas de tratamento de esgoto.

Abstract

In regions with long drought periods one way of guaranteeing the availability of water for industries is sewage treatment with the aim of effluent reuse. Depending on the kind of reuse in industry, it may be required to remove organic material and suspended solids in addition to biological treatment as well as nutrients and pathogens. In many cases also hardness removal may be necessary. One possibility is to treat biological effluent with lime, increasing pH and causing precipitation resulting in clarification of the pre treated effluent. At the same time the hardness and pathogenic bacteria may be removed. In this dissertation the technical feasibility to produce high quality water for industrial use by lime addition to biological effluents from an activated sludge system and a UASB reactor as evaluated. Comparative tests with public supply water were also carried out. Concerning the final effluent quality it was established that lime was an effective agent for the removal of nutrients and pathogens. Phosphate was removed as hydroxyl apatite and ammonia by stripping at high pH values. Colour was partially removed, but turbidity increased with increasing lime additions. Hardness removal was only partial and well below the efficiency that could be expected on the ground of thermodynamics. In all cases calcium carbonate precipitation was much smaller than expected, leaving a supersaturated effluent. The value determined for the apparent solubility constant was pKs' = 6,1, whereas the thermodynamic constant is 8,33. Thus the ionic product of CaCO$ was more than 100 times superior to the thermodynamic value. Magnesium removal with lime was also difficult: it was only possible to obtain partial removal for pH values higher than 11, so that the required lime dose was very large.