Renewable energy powered membrane technology: Salt and inorganic contaminant removal by nanofiltration/reverse osmosis

The objective of this study was to evaluate the effects of fluctuating energy and pH on retention of dissolved contaminants from real Australian groundwaters using a solar (photovoltaic) powered ultrafiltration–nanofiltration/reverse osmosis (UF–NF/RO) system. Four NF/RO membranes (BW30, ESPA4, NF90, and TFC-S) were used. Energy fluctuations affected pressure and flow. Solar irradiance levels impacted retention of fluoride, magnesium, nitrate, potassium, and sodium where convection/diffusion dominated retention. Retention of calcium, strontium, and uranium was very high and independent of solar irradiance, which was attributed to a combination of size and charge exclusion and for some solutes sorption and precipitation. Groundwater characteristics affected retention and the solutes were categorized into two groups according to retention as a function of pH: (1) pH-independent retention (arsenic, calcium, chloride, nitrate, potassium, selenium, sodium, strontium, and sulfate) and (2) pH-dependent retention (copper, magnesium, manganese, molybdenum, nickel, uranium, vanadium, and zinc). The retention of Group 1 solutes was typically high and attributed to steric effects. Group 2 solutes had dominant, insoluble species under certain conditions which led to deposition on the membrane surface (and thus varying apparent retention). The renewable energy membrane system removed a large number of groundwater solutes reliably over a range of real energy and pH conditions.