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 Abstract

  Arsenic Removal From Drinking Water by Point-of-Use Reverse Osmosis (POU RO), U.S. EPA Demonstration Project at Sunset Ranch Development in Homedale, ID, Final Performance Evaluation Report (58 pp, 1.17 MB) (EPA/600/R-07/082) August 2007

This report documents the activities and results of the arsenic removal technology demonstration project at the Sunset Ranch Development in Homedale, Idaho. The objectives of the project were to evaluate the:

  • Effectiveness of a point-of-use (POU) reverse osmosis (RO) technology in removing arsenic, nitrate, and uranium to meet the respective maximum contaminant levels (MCLs) of 10 micrograms per liter (µg/L), 10 milligrams per liter (mg/L) (as nitrogen), and 30 µg/L
  • Reliability of the treatment units
  • Simplicity of the required system operation and maintenance (O&M) and operator skill level
  • Capital and O&M costs of the technology

The project also characterized process residuals, i.e., reject water, produced by the RO units. The types of data collected included volume of permeate water produced; quality of feed, permeate, and reject water; required system O&M; and capital and O&M costs.

The treatment system at Sunset Ranch Development consisted of one POU RO unit at each of 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 (three homes had existing POE softeners).

Each POU RO unit consisted of a 20-micrometer (µm) pre-filter, an RO module with a 1.7-inch × 11-inch thin film composite, semi-permeable membrane element, a 3-gallon storage tank, and a MACguard post-filter. The RO units were capable of producing up to 35.5 gallons per day (gpd) of permeate water and had a feed water to permeate water ratio of 2.7 to 1, a 37 percent recovery rating. The RO units automatically shut down production after 500 gallons 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. Through January 17, 2006, one residence used 481 gallons of water from the RO tap; another residence used 500 gallons of water and the pre- and post-filters had to be replaced before the unit resumed operation. The remaining seven units were not tracked for water usage, but had not reached the 500-gallon mark.

Source water at the Sunset Ranch Development contained elevated levels of arsenic, nitrate, and uranium. Arsenic speciation results indicated that arsenic (V) was the predominant species in raw water, ranging from 49.5 to 64.8 µg/L and averaging 56.3 µg/L. Only a trace amount of arsenic (III) existed, ranging from 0.5 to 2.7 and averaging 1.5 µg/L. Nitrate concentrations averaged 10.2 mg/L (as nitrogen), just over the 10-mg/L (as nitrogen) MCL. Uranium concentrations ranged from 23.4 to 31.0 µg/L, very close to the 30-µg/L MCL.

As expected, the softeners did not remove any arsenic, but reduced the water hardness from 216–251 mg/L (as calcium carbonate) to an average of 1.7 mg/L (as calcium carbonate). Total arsenic concentrations in the permeate water were less than 0.1 µg/L for all samples except four at 8.7, 5.1, 1.2, and 1.2 µg/L. Based on the average arsenic concentrations in the feed and permeate water, the RO units achieved higher than 99 percent removal efficiency for arsenic.

Nitrate was consistently removed by the RO units from an average of 10.2 mg/L (as nitrogen) in raw water to an average of 1.0 mg/L (as nitrogen) in the permeate water, representing a 90 percent reduction. Uranium was removed from 23.4–31.0 µg/L in raw water to below 0.1 µg/L in the permeate water. In addition, the RO units achieved 100 percent removal for iron, 99 percent for vanadium, 96 percent for silica, and 96 percent for total dissolved solids (TDS). Because of the reduction of alkalinity by the RO units, pH values were reduced to between 6.4 and 6.9.

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 55.9 to 92.3 µg/L of arsenic, 8.3 to 19.2 mg/L (as nitrogen) of nitrate, 23.0 to 42.3 µg/L of uranium, and 740 to 1,080 mg/L of TDS. The mass balance across the RO unit was calculated for total arsenic and nitrate for each sampling event. During the entire study period, the mass balance data, in terms of the mass recovered in the permeate and reject water against the mass in the raw water, ranged from 63 to 114 percent and averaged 83 percent for total arsenic, and from 66 to 100 percent and averaged 89 percent for nitrate.

Operational problems included water pulsing from the faucet, incorrect outlet elbow installation, water quality monitor malfunction, a loose wire on the TDS monitor indicator light, and reduced flow from the RO tap. These problems were corrected promptly by the vendor; any cost incurred was covered under warranty.

The capital investment cost of $31,877.50 included $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. If the cost of materials and vendor travel was included, the total cost for each household system was nearly $4,000, which is equivalent to an annualized cost of $570 based on a 10-year life and a 7 percent interest rate.

O&M costs per household were nearly $202 annually or $17 per 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 certified operator.

Contact

Thomas Sorg


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