Effect of residual commercial antiscalants on gypsum scaling and membrane wetting during direct contact membrane distillation

Membrane distillation (MD) is considered as a promising technology for reverse osmosis (RO) brine treatment, but application of MD is significantly hampered by gypsum scaling and subsequent membrane wetting. As antiscalants are widely used in RO desalination, it is of interest to understand effects of residual antiscalants on membrane scaling during RO brine treatment using MD. In this work, MDC220, PTP-0100 and SHMP were adopted to study effects of residual antiscalants on water vapor flux, distillate conductivity and scale growth using a direct contact membrane distillation (DCMD) device. Feed temperature and antiscalant dose were investigated in ranges of 40-60 degrees C and 0-10 mg L-1, respectively. Moreover, morphological characterization of scale layers in the presence of antiscalants were performed using a field emission scanning electron microscopy. The results show that all the antiscalants neither decreased feed surface tension nor worsened distillate quality. The presence of antiscalants effectively retarded flux decline and distillate conductivity increase; SHMP and MDC220 outperformed PTP-0100 in restricting gypsum scaling. The performance of antiscalants was temperature dependent, and increased feed temperature adversely affected scaling inhibition performance. The dose of antiscalants played a significant role in gypsum scaling restriction and the inhibition performance was superb at a MDC220 dose not less than 5 mg L-1.