High concentrations of soluble calcium in industrial wastewater present problems due to the calcification of downstream processing. The current trend towards circuit closure and increased water re-use will escalate this problem. We investigated ureolytic microbial carbonate precipitation as a novel process for removing excess calcium from industrial effluents. Two laboratory-scale reactors, both with a hydraulic retention time of 8 h, were fed with 1.8 dm(3) of anaerobic effluent (about 11 mm Ca2+) from a paper recycling plant. Both reactors were inoculated with pre-cultivated calcareous sludge and the treatment reactor was additionally dosed with urea to a final concentration of S. 3 mm. Even though the anaerobic wastewater was saturated as such with respect to CaCO3, urea addition and hydrolysis was shown to be a pre-requisite for precipitation. Almost all (85-90% w/v) of the soluble calcium was precipitated as CaCO3 and removed through sedimentation in the treatment reactor. This bio-catalytic process presents an uncomplicated and efficient method for the removal of calcium from industrial wastewater. (C) 2003 Society of Chemical Industry.