A comparative study of RO membrane scale inhibitors in wastewater reclamation: Antiscalants versus pH adjustment

Scaling is the most common challenge that reverse osmosis (RO) membranes encounter in potable reuse applications due to the high product recovery. Usage of antiscalants or pH adjustment can suppress scale formation on membranes without sacrificing overall recovery. A series of experiments was conducted to evaluate the effectiveness of two different antiscalants (phosphonate-based and acrylic acid-based) and pH adjustment for the inhibition of scaling on polyamide RO membranes during the separation of microfiltration (MF) treated secondary effluent wastewater. Product recovery was targeted at 50%, increasing the residual concentration of scale forming ions by a factor of 2 in the feed. Substantial scale formation, dominated by calcium carbonate, was observed during the separation of the feed. In the presence of antiscalants, calcium carbonate scaling was successfully abated, while iron based scalants did not significantly decrease. Phosphonate-based antiscalants were found to be more effective in inhibiting calcium carbonate scaling as compared to acrylic acid-based antiscalants. As such, RO membranes treated with phosphonate-based antiscalants demonstrated lower permeate flux decline and less calcium scale deposition on membrane surfaces under the tested conditions. The addition of a strong acid to control scaling is a less expensive alternative to antiscalant addition. pH adjustment using sulfuric acid was effective for the inhibition of calcium carbonate scaling. However, acid addition resulted in corrosion of piping leading to considerable leaching of iron and copper into the feed. The presence of iron and copper adversely increased the scaling potential of the membranes.