Predicting and measurement of pH of seawater reverse osmosis concentrates

The pH of seawater reverse osmosis plants (SWRO) is the most influential parameter in determining the degree of supersaturation of CaCO3 in the concentrate stream. For this, the results of pH measurements of the concentrate of a seawater reverse osmosis pilot plant were compared with pH calculations based on the CO2―HCO3−―CO32− system equilibrium equations. Results were compared with two commercial software programs from membrane suppliers and also the software package Phreeqc.Results suggest that the real concentrate pH is lower than that of the feed and that none of the used programs was able to predict correctly real pH values. In addition, the effect of incorporating the acidity constant calculated for NaCl medium or seawater medium showed a great influence on the concentrate pH determination. The HCO3− and CO32− equilibrium equation using acidity constants developed for seawater medium was the only method able to predict correctly the concentrate pH.The outcome of this study indicated that the saturation level of the concentrate was lower than previously anticipated. This was confirmed by shutting down the acid and the antiscalants dosing without any signs of scaling over a period of 12 months.

► This research expedites the saturation of CaCO3 in seawater reverse osmosis systems.
► pH calculations were compared to the concentrate pH measurements in the SWRO plant.
► None of that applied methods was able to predict the measured pH accurately.
► The saturation level of the concentrate was much lower than previously anticipated.
► Acid and antiscalant doses were stopped for 6 months without experiencing scaling.