Technologies and Costs for Removal of Arsenic From Drinking Water

solids (1,500 mg/L). San Ysidro is a small community with limited financial resources, and centraltreatment was not a viable treatment option. San Ysidro applied to the DWRD for a cooperativeagreement to evaluate POU RO treatment for the entire village. The project was funded in August1995, and seventy-three units were initially purchased and installed in homes, restaurants, gas stationsand municipal buildings.During the study, observed arsenic concentrations of 0.068 mg/L to 0.02 mg/L wereconsistently reduced to less than the detectable limit (0.005 mg/L) by the POU RO units. Othercontaminants were also effectively removed, including manganese (80 percent), iron (85 percent), andTDS (95 percent). Based on the manufacturer’s literature, it appeared that the units were operatingat an approximate recovery rate of 25 percent, i.e., for every 100 gallons of influent, 25 gallons oftreated water are produced.The water supply for San Ysidro is chlorinated at the wellhead. As a result, a carbon prefilterwas installed with each unit to remove residual chlorine and particulates to prevent membranefouling. A carbon post-filter was installed for polishing treated water. Since the conclusion of thestudy, the village has assumed ownership of the units and is now responsible for their maintenance.5.4 REVERSE OSMOSISReverse osmosis (RO) is a separation process that utilizes a membrane system to rejectcompounds based on molecular properties. Water molecules pass through the membrane, but mostcontaminants, including arsenic, are rejected by the membranes. While a portion of the feed waterpasses through the membrane, the rest is discharged with the rejected contaminants in a concentratedstream. Membrane performance can be adversely affected by the presence of turbidity, iron,manganese, scale-producing compounds, and other contaminants. A detailed discussion of the ROremoval mechanism is presented in Chapter 2.POU RO systems can be operated at both high (approximately 200 psig) and low (40 - 60 psig)pressures. High pressure RO devices typically operate at a product-to-reject water ratio of 1 to 3(Fox, 1989), and require a booster pump to achieve the desired operating pressure. Low pressure ROdevices are less efficient and operate with a product-to-reject water ratio of about 1 to 10 (Fox,1989). This can be a significant deterrent to RO treatment in dry regions or regions with frequentwater shortages.5-6