INFLUENT CALCIUM REMOVAL BY CRYSTALLIZATION REUSING ANAEROBIC EFFLUENT ALKALINITY

Waste waters from the waste paper industry and landfill leachates cont ain high calcium concentrations. Treatment of those waste waters in an anaerobic system will lead to extensive precipitation of calcium carb onates. Problems due to accumulation of calcium carbonate in anaerobic reactors can be avoided by applying a crystallization reactor to remo ve calcium from the influent. This solution, however, is expensive due to the need to dose alkalinity. In this paper, the possibility of reu sing the produced alkalinity by recirculating anaerobic effluent was i nvestigated. Calculations and experiments were carried out to investig ate to what extent calcium could be removed from the influent. Experim ents were carried out in a 4.5 1 UASB reactor and a 0.8 1 crystallizat ion reactor, seeded with sand. The influent contained up to 1800 mg.l( -1) calcium and the substrate was acetic acid. Removal of calcium from the influent could be successfully accomplished. Recirculation of the anaerobic effluent supplied sufficient bicarbonate alkalinity for cry stallization. Consequently, no extra chemicals (i.e. Na2CO3 and/or NaO H) were needed. A chemical equilibrium model, with adapted pseudo solu bility products, is a very useful tool to determine the proper recycle ratio of effluent for optimal performance. (C) 1997 IAWQ. Published b y Elsevier Science Ltd.