Limits of RO recovery imposed by calcium phosphate precipitation

The presence of phosphate ions causes a difficulty confronting RO purification of secondary treated wastewater and limits the water recovery. These ions can readily lead to membrane blockage by precipitation of sparingly soluble calcium phosphate salts. Currently, it is far from clear if calcium phosphate scale deposition can be reliably inhibited by dosage of antiscalants. Major efforts were devoted to a systematic evaluation of the effectiveness of currently available calcium phosphate antiscalants. The inhibitory capability of the tested antiscalants was assessed using a continuous-flow laboratory system, equipped with a tubular RO membrane. Feed solution of controlled composition, dosed with an antiscalant, was continuously passed through the membrane. Both concentrate and permeate recycled to the feed vessel. Antiscalant effectiveness was evaluated from the rate of membrane permeability decay. Five antiscalants were tested under various solution supersaturation conditions and antiscalant concentrations. All antiscalants proved to be ineffective over most solution compositions tested. Results of this study delineate the restricted range of conditions under which currently available antiscalant are likely to provide an acceptable calcium phosphate scale inhibition.