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Arsenic Removal from Drinking Water by Point of Use Reverse Osmosis EPA Demonstration Project at Sunset Ranch Development in Homedale, ID: Six-Month Evaluation Report (52 pp, 984 KB) October 2006
The treatment system at Sunset Ranch Development in Homedale, ID, consisted of one Point of Use (POU) reverse osmosis (RO) unit each at nine participating residences to remove arsenic, nitrate, and uranium from source water. Softening of source water was performed as pretreatment to meet the feed water quality requirements for the RO units. Six Point of Entry (POE) softeners and nine POU RO units were provided by Kinetico. Each POU RO unit consisted of a 20-µm pre-filter, a RO module with a 1.7-in-diameter by 11-in-tall, thin film composite, semi-permeable membrane element, a 3-gal storage tank, and a MACguard post-filter. The RO units were capable of producing up to 35.5 gal/day (gpd) of permeate water and had a feed water to permeate water ratio of 2.7 to 1, a 37% recovery rating. The RO units automatically shut down the production after 500 gal of permeate water have been processed and resume operation only after the replacement of pre- and post-filters.
The POU RO units began regular operation on July 15, 2005. The types of data collected included volume of permeate water produced, quality of feed, permeate, and reject water, required system operation and maintenance (O&M), and capital and O&M cost. Through the period of July 15, 2005, to January 17, 2006, one residence used 242 gal of water from the RO tap; another residence used 500 gal of water and the pre- and post-filters had to be replaced before the unit resumed operation. The rest of seven units were not tracked for water usage, but had not reached the 500-gal level.
Arsenic speciation results indicated that As(V) was the predominant species in raw water, ranging from 49.5 to 56.4 µg/L. Only a trace amount of As(III) existed, ranging from 1.7 to 2.9 and averaging 2.2 µg/L. As expected, the softeners did not remove any arsenic, but reduced the water hardness from 232-247 mg/L (as CaCO3) to an average of 0.5 mg/L (as CaCO3). Total arsenic concentrations in the permeate water were less than 0.1 µg/L for all samples except for two at 5.1 and 8.7 µg/L. Based on the average arsenic concentrations in the feed and permeate water, the RO units achieved higher than 99% removal efficiency for arsenic.
Nitrate was consistently removed by the RO units from an average of 10.1 mg/L (as N) in raw water to an average of 0.8 mg/L (as N) in the permeate water, representing a 94% reduction. Uranium was removed from 24.3 to 31.0 µg/L in raw water to below 0.1 µg/L in the permeate water. In addition, the RO units achieved 100% removal for iron, 99% for vanadium, 96% for silica, and 97% for total dissolved solids (TDS). pH values also were reduced to 6.5-6.7, due to the reduction of alkalinity by the RO units.
Regeneration brine waste from the softener and reject water from the RO units were discharged to the septic tank at each residence. The RO reject water contained 56 to 92 µg/L of arsenic, 11 to 16 mg/L (as N) of nitrate, 27 to 42 µg/L of uranium, and 892 to 1,060 mg/L of TDS. The mass balance across the RO unit was calculated for total arsenic and nitrate for each sampling event. During the first six months, the mass balance ranged from 66% to 114% for total arsenic and from 83% to 99% for nitrate.
Operational problems encountered during the reporting period included water pulsing from the faucet, incorrect outlet elbow installation, water quality monitor malfunction, and a loose wire on the TDS monitor indicator light. These problems were corrected promptly by the vendor and have not re-occurred; any cost incurred was covered under warranty.
The capital investment for this project was $31,877.50, including $21,732.50 for equipment and $10,145 for installation. Each water softener cost $2,395, including $1,585 for equipment and $810 for installation. Each RO unit cost $1,220, including $1,025 for equipment and $195 for installation. O&M cost per household during a six-month period was near $144 or $24/month, which included salt usage and RO filter replacement. Neither electricity nor labor cost was incurred because the water softener and the RO unit did not consume electricity and did not require a treatment-level certified operator.