WRRCSR No.12:28:84 Brackish Groundwater Desalting Tests Using Low Pressure Reverse Osmosis Membranes, Wai'anae, O'ahu, Hawai'i

Abstract

A desalting test using low pressure, reverse osmosis (RO) FT-30 and FT-50 membranes was conducted on Pump 10 well water at Oahu Sugar Company near Campbell Industrial Park on O'ahu. The brackish test water from the Waianae basalt aquifer had a chloride (Cl-) concentration of 450 mg/l and a total dissolved solids (TDS) concentration of 1250 mg/l. The applied pressure ranged from 40 to 75 psi with a daily incrementof 5 psi. The test operation duration was 239 hr. The FT-30 membrane demonstrated high rejection of Cl-, TDS, and silica (>90%), thus producing an excellent potable water. The permeate water production was slow, averaging 10.3 gpd or 4.6% of the feedwater. The production rate increased with pressure. The fouling rate was slow and longer runs could be made before cleaning became necessary. The FT-50 membrane yielded high permeate production (approximately four times that of FT-30), but was of poor (sub-potable) water quality with average rejection of Cl- at 17% and TDS at 25%. The fouling rate of the membrane was high, seriously reducing the permeate production in a short time. Cleaning with dilute phosphoric acid followed by Wisk and NaOH solution restored about 45% of the production rate. A large-scale RO module (element) should be tested for a longer period of time to determine sustainable performance of fouling rate. After reaching 15% reduction in the permeate production rate, various cleaning agents should be tested to restore the rate and to determine the type of fouling. By using a full-size commercial element, uncertainties of scale-up from small laboratory units can be eliminated. A longer operating period using the available pump pressure would yield more meaningful design data than the short, 2-wk test period using laboratory-size elements.