Calcium sulfate scale formation and control in nanofiltration of agricultural drainage water

Experimental studies with real drainage water samples and model solutions were conducted to evaluate scale formation and control by antiscalant treatment. Experiments were conducted in glassware and with a plate-and-frame nanofiltration (NF) membrane recirculation unit. Membrane experiments consisted of two types: permeate disposal to measure the effect of recovery increase, and feedwater recirculation at a specific concentration factor (CF) to establish the change of performance with time. A mathematical expression relating recovery to feedwater CF has been developed.; Incipient nuclei formation was observed in glassware experiments and related to ionic strength and concentration of calcium and sulfate ions. Membrane experiments showed that a continued surface reaction was the cause of gypsum scale once the corresponding crystallization CF was reached. On the other hand, calcium carbonate scale resulted from a continuous increase in concentration factor. There is a high probability that both scales result from precipitate particles formed in the concentration polarization layer and deposited on the membrane surface.; Specific glassware and membrane experiments showing the concentration polarization modulus (CPM) dependence on permeate flux compared very well with an equation based on semi-empirical mass transfer correlation. A proportionality factor, independent from ionic composition, represents the discrepancy between saturation level (CF_sat) predicted by a sophisticate gypsum solubility model and actual crystallization (CF _cryst) observed in glassware experiments. A reference aquatic humic acid significantly retarded scaling by tying up calcium ions that would have otherwise been used for the formation of incipient nuclei. Commercial polyacrylic acid (PAA) proved to be an effective antiscalant. The relationship between PAA dosage and CF of model solutions fitted a parabolic-type curve. It is hypothesized that PAA inhibits crystallization by coating the surface of nuclei.; Experimental findings were combined into a model, which predicts PAA dosage required for calcium sulfate scale control in nanofiltration of any given saline solution. Maximum product recovery was 60% with one type of drainage water and 80% with another. Operating pressures were 225 psi and 330 psi, respectively, thereby showing considerable improvement in performance, compared with pressures required by conventional reverse osmosis membranes used in earlier attempts at drainage water reclamation.