Diagnostic analysis of RO desalting treated wastewater

Diagnostic analysis of reverse osmosis membranes that were fed with Western treatment plant (WTP) recycled water was investigated by both thermodynamic calculations and laboratory experiments in order to predict the feasibility of RO desalting for WTP. The thermodynamic calculations suggested that RO recoveries of 80–85% were feasible with careful control of feed water pH and the use of chemical additives such as antiscalants and chelating agents, it also predicted the major minerals of concern to be silica, calcium fluoride, calcium carbonate, and calcium phosphate. Following the thermodynamic simulations, diagnostic laboratory experiments were undertaken. The experiments showed that the major contributor to scale formation was indeed calcium phosphate and possibly another calcium based compound, which was strongly suspected to be calcium carbonate. Based on previously published literature that indicated anti-scalants did not substantially decrease the scaling effect of calcium phosphate and laboratory tests that indicated controlling the pH to 6.4 in the feed water dramatically reduced scaling formation, it was suggested that the feed water could be controlled by pH adjustments only. Inter-stage pH correction was suggested as an optional technique to enhance the overall water recovery to above 95%.