Ranking of antiscalant performance for gypsum scale suppression in the presence of residual aluminum

An approach to ranking antiscalant effectiveness for gypsum scale suppression using combined bulk crystallization and membrane scaling diagnostic tests was explored with a focus on the adverse impact of residual aluminum in dissolved (Al3+) and as colloidal species on gypsum scale suppression. Five commercial antiscalants were ranked based on the crystallization induction time as determined in a well-mixed crystallization vessel with a back-light scattering turbidity meter to monitor gypsum formation. The presence of aluminum, even at trace levels (up to 100 µg/L), significantly reduced the crystallization induction time, thereby reducing antiscalant effectiveness. For a given antiscalant dose, the crystallization induction time decreased with increasing total aluminum concentration according to a log-linear relationship. Although the above qualitative behavior was similar for the tested commercial anti-scalants, the adverse impact of aluminum on antiscalants effectiveness differed markedly among the different antiscalants. Diagnostic scaling tests in a plate-and-frame RO module demonstrated antiscalant performance ranking of the same order as that obtained based on bulk crystallization induction time measurements.

The present study showed that both bulk crystallization along with diagnostic membrane scaling tests can be used for rapid ranking and selection of antiscalants. An additional outcome is a quick method for the determination of the optimal antiscalant dose. Studies with source water containing Al3+ demonstrated a reduction in antiscalant effectiveness for suppression of gypsum crystallization. Ranking of five different commercial antiscalants for gypsum scale suppression revealed significant performance differences among the antiscalants. It was shown via both crystallization induction time and membrane scaling tests that the adverse impact of Al3+ can be overcome by increasing antiscalant dose, although this would increase the RO process chemical treatment cost. Membrane scaling experiments confirmed that ranking of antiscalant effectiveness and impact of Al3+, based on flux decline and surface scale coverage, correlated with ranking obtained from the bulk solution crystallization test. It is suggested that the above approach can be valuable in identifying effective candidate antiscalants, thereby assisting in the design and optimization of field pilot membrane studies. Acknolwedgements This work was funded in part by the Metropolitan Water District of Southern California and the California Department of Water Resources.