Waste waters from the waste paper industry and landfill leachates contain high calcium concentrations. Treatment of those waste waters in an anaerobic system will lead to extensive precipitation of calcium carbonates. Problems due to accumulation of calcium carbonate in anaerobic reactors can be avoided by applying a crystallization reactor to remove calcium from the influent. This solution, however, is expensive due to the need to dose alkalinity. In this paper, the possibility of reusing the produced alkalinity by recirculating anaerobic effluent was investigated. Calculations and experiments were carried out to investigate to what extent calcium could be removed from the influent. Experiments were carried out in a 4.5 1 UASB reactor and a 0.8 1 crystallization 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 crystallization. Consequently, no extra chemicals (i.e. Na2CO3 and/or NaOH) were needed. A chemical equilibrium model, with adapted pseudo solubility products, is a very useful tool to determine the proper recycle ratio of effluent for optimal performance.

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