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 Abstract

  Arsenic and Antimony Removal from Drinking Water by Point-of-Entry Reverse Osmosis Coupled with Dual Plumbing Distribution U.S. EPA Demonstration Project at Carmel Elementary School in Carmel, ME Final Performance Evaluation Report (EPA/600/R-11/026) March 2011

This report documents the activities performed for and the results obtained from the arsenic and antimony removal treatment technology demonstration project at the Carmel Elementary School (CES) in Carmel, ME. An innovative approach of employing point-of-entry (POE) reverse osmosis (RO) coupled with dual plumbing was demonstrated at CES as a low cost alternative to achieve compliance with arsenic and antimony maximum contaminant levels (MCLs) compared with conventional RO treatment. The objectives of the project were to evaluate the performance of the RO/dual plumbing system in meeting the new arsenic MCL of 10 µg/L and the antimony MCL of 6 µg/L, the reliability of the treatment system, the required system operation and maintenance (O&M) and operator skill levels, and the capital and O&M cost of the technology. Additionally, the project characterized the water quality of the distribution system and process residuals produced by the RO system.

The original treatment system selected for demonstration at CES was a Watts Premier 9,600-gal/day (gpd) RO treatment system, which would require a significant building modification/expansion to house the new system and construction of a larger septic/leach field to receive residual water from the RO system. To reduce the financial burden on the school, a joint decision was made by United States Environmental Protection Agency (EPA), Maine Drinking Water Program (MDWP), CES, and Battelle to use a smaller RO system coupled with a dual plumbing distribution system so that only a portion of raw water would be treated and consumed as potable water while the untreated water was available for non-potable use. The only modification required was re-plumbing of the existing distribution system to convert it into a duplex system with separate potable and non-potable lines. The potable line supplied RO-treated water to the kitchen sinks and dishwasher (both cold and hot water), water fountains in school buildings, and cold water facets in restrooms. Based on a water demand study conducted upon completing the plumbing modification, it was determined that the potable water demand could be met by a Crane Environmental EPRO-1,200 treatment system. A similar but smaller unit, EPRO-600 system, had been used by EPA/Battelle for a pilot study conducted at CES in 2006 and was effective at removing arsenic and antimony to levels well below their respective MCLs.

The Crane Environmental EPRO-1,200 RO treatment system consisted of an RO unit, a calcite filter for pH adjustment, two 300-gal atmospheric storage tanks, a re-pressurization system, and a post-chlorination system. Major components of the RO unit included a 5-µm sediment filter, a ½-horsepower (hp) booster pump, and two 2.5-in × 40-in thin-film composite RO membrane modules. The RO permeate water passed through the calcite filter to raise its pH level to near neutral, then was stored in two 300-gal atmospheric storage tanks. The water from the storage tank was re-pressurized by a 1-hp booster pump before entering the potable distribution line.

Operation of the EPRO-1,200 RO treatment system began on February 4, 2009, but logging of operational data did not begin until April 16, 2009. The types of data collected included system operation, water quality (both across the treatment train and in the distribution system), process residuals, and capital and O&M cost. Through the performance evaluation study period from April 16 through December 15, 2009, the system operated for approximately 1,474 hr, processing approximately 180,700 gal of water. With an average recovery rate of 40%, the system generated 71,100 gal of permeate and 109,600 gal of reject water. Daily system run times averaged 11.7 hr/day when the school was in session and 1.9 hr/day when the school was out of session.

Arsenic concentrations in source water ranged from 13.6 to 22.6 µg/L and averaged 18.2 µg/L. Soluble As(V) was the predominating species, with concentrations ranging from 14.3 to 18.7 µg/L and averaging 16.7 µg/L. Antimony concentration in source water ranged from 8.6 to 13.2 µg/L and averaged 10.8 µg/L, with the majority present in the soluble form. Total arsenic concentrations in permeate water averaged 0.1 µg/L. Total antimony concentrations in permeate water were below the MDL of 0.1 µg/L. Based on the average arsenic and antimony concentrations in raw and permeate water, the RO system had achieved 99% removal efficiency for both analytes. The RO system had achieved an average of 97% rejection for total dissolved solids (TDS), slightly below the specified 98% rejection.

pH values measured in source water averaged 7.9 and decreased to an average of 6.9 after the RO unit. Alkalinity concentrations were reduced from an average of 206 mg/L (as CaCO3) in source water to an average of 5.6 mg/L (as CaCO3) in permeate water, causing the decrease in pH. After pH adjustment via the calcite filter, pH values and alkalinity concentrations were raised, on average, to 7.4 and 16.6 mg/L (as CaCO3), respectively.

The RO process concentrated the contaminants into the reject water, which was discharged to the existing septic system. During the performance evaluation study, approximately 109,570 gal of reject water was generated. The reject water contained, on average, 31.9 µg/L of arsenic, 17.7 µg/L of antimony, 410 mg/L of TDS, 340 mg/L (as CaCO3) of alkalinity, 352 mg/L (as CaCO3) of total hardness, 18.1 mg/L of silica (as SiO2), and 17.9 mg/L of sulfate. Mass balance calculations showed that the RO process (permeate and reject water) had recovered 107% of arsenic and 100% of antimony from raw water.

Distribution system “first draw” samples were collected from a cold water tap in the kitchen on a monthly basis to determine if the RO treatment had any impacts on the distribution water quality. pH values of the distribution “first draw” samples ranged from 6.8 to 9.2, and averaged 8.4. Alkalinity concentrations ranged from 10.1 to 58.3 mg/L, and averaged 24.6 mg/L. Arsenic and antimony concentrations in the distribution “first draw” samples were both in the sub-parts per billion (ppb) levels (except for one time at 2.7 µg/L of arsenic), similar to those in the treatment effluent. Lead and copper concentrations were well below the respective action levels. Therefore, the RO treatment system did not have any adverse effects on the water quality in the distribution system.

Operational problems encountered during the demonstration study included a bearing failure on the RO motor and pump assembly. The problem was corrected promptly by the vendor and has not re-occurred. The replacement parts were covered under warranty; however, the cost to diagnose the problem and install the replacement parts was not.

The capital investment for the system was $20,452, including $8,600 for the dual plumbing and $11,942 for the EPRO-1,200 RO system. With the system’s rated capacity of 1,200 gpd, the normalized capital cost was $17.12 per gpd of design capacity.

The O&M cost included the cost incurred by system repairs, electricity consumption, and labor to operate the system. The cost to diagnose and install a faulty RO motor and pump assembly was $321. Annual electricity consumption was estimated to be 5,078 kWh and cost $376. Routine labor activities consumed 10 min per day, which translated into $666/yr. The total annual O&M cost was estimated to be $1,404, or $12.89/1,000 gal of permeate water produced.

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Thomas Sorg

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