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Removal from Drinking Water by Adsorptive Media
U.S. EPA Demonstration Project at Bow, NH Six-Month Evaluation Report (60 pp, 3.2 MB) April 2006
This report documents the activities performed during and the results obtained from the first six months of the arsenic removal treatment technology demonstration project at the White Rock Water Company (WRWC) public water system, a small residential drinking water facility in Bow, NH. The objectives of the project are to evaluate the effectiveness of the ADI Group, Inc. (ADI) G2 media in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 µg/L, the reliability of the treatment system, the required system operation and maintenance (O&M) and operator's skills, and the capital and O&M costs of the technology. The project also characterizes the water in the distribution system and process residuals produced by the treatment system.
The arsenic adsorption system consisted of two vertical, 72-inch-diameter and 72-inch-sidewall-height stainless steel vessels configured in series. The adsorption vessels were originally designed to operate in parallel for the Holiday Acres Mobile Home Park in Allenstown, NH with a flowrate of 70 gallons per minute (gpm) (35 gpm per vessel). Due to the switch to the site in Bow with a total flowrate of about 40 gpm, the flowrate was reduced by 43%; therefore, the system was reconfigured to operate in series. At 40 gpm, each vessel provided an empty bed contact time (EBCT) of 16 min (or 32 min total contact time) and a hydraulic loading rate of 1.4-gpm/ft2. The 16-min EBCT was 60% longer than that normally recommended by the vendor and the 1.4-gpm/ft2 hydraulic loading rate was about 50% lower than that normally applied to the G2 media.
The G2 media is a granular, calcined diatomite substrate coated with ferric hydroxide. Because of its inherently high pH values from the manufacturing process, the G2 media was conditioned on-site with sulfuric acid before the system was put into service. To increase the media adsorption capacity, the raw water was adjusted to a target value of 6.8, and later 6.4, using a 93% sulfuric acid solution. The treated water was adjusted for pH again to a target value of 7.5 using a 25% caustic solution before entering the distribution system. In-line pH probes were used to monitor the pH values of the feed water and treated water but the rates of acid and caustic addition were controlled via manual adjustments to the pump stroke length. The relative feed rates were then flow-paced with a water meter located on the discharge line following the treatment system.
The arsenic adsorption system became operational on October 13, 2004. Through April 24, 2005, the system operated for 1,741 hr, treating approximately 3,858,000 gal of water or 6,067 bed volumes (BVs). Total As concentrations in the raw water averaged 49.3 µg/L, present almost entirely as As(V). After the lead vessel, greater than 30 µg/L of total As was unexpectedly detected in samples collected just after startup on October 13 and about one week later on October 19, 2004. After about 380 BVs of throughput, total As concentrations decreased to 12.6 to 15.6 µg/L before beginning a steady increase to 26.3 µg/L at about 2,600 BVs by April 12, 2005. Total As concentrations after the lag vessel also were high during the first two weeks of system operation, with 16.7 to 21.8 µg/L of arsenic measured on October 13 and October 19, 2004, respectively. Afterwards, the concentrations dropped to 1.7 µg/L after about 2,500 BVs and then increased steadily to 5.8 µg/L after about 5,700 BVs by April 12, 2005. ADI attributed the elevated arsenic concentrations just after the system startup to the leaching of arsenic from the G2 media prepared with FeCl3 containing arsenic and manganese as impurities. While this might explain the elevated arsenic levels observed in the treated water during the first two weeks of system operation, it does not explain why the arsenic concentrations remained high (i.e., 12.6 µg/L or greater) following the lead vessel throughout the first six months of operation.
Increases in both manganese and silica were observed in the treated water following the adsorption vessels, indicating leaching of these constituents from the media. After about 3,000 BVs, manganese concentrations decreased to levels similar to those in the raw water. The leaching of silica from both vessels leveled off after about 2,000 BVs, but continued throughout the remainder of the study period with an increase in concentrations ranging from 1.6 to 6.2 mg/L.
The system was backwashed only twice during this period because of low pressure losses (i.e., 1-2 pounds per square inch [psi]) across the adsorption vessels. Analysis of the backwash water indicated that soluble As concentrations were either similar to or lower than the levels measured in the source water. Since finished water was used for backwash, some arsenic might have been desorbed from the media during backwashing. Future backwash samples will include collection and analysis of total suspended solids (TSS) and total As, Fe, and Mn.
Comparison of the distribution system sampling results before and after the installation of the ADI G2 media system showed a decrease in arsenic concentration (from 36.9 - 52.3 µg/L to 3.9 - 12.4 µg/L) at all three EPA Lead and Copper Rule (LCR) sampling locations. Manganese concentrations increased to as high as 16.0 µg/L in the distribution system during the first three months of system operation, apparently due to leaching of manganese from the G2 media, as mentioned above. Following a drop in pH of the treated water in December 2004, the lead concentration in the January 12, 2005 sample increased to 9.9 µg/L at one sampling location and copper levels increased across all three sampling locations, with the most noticeable increase exceeding the action level of 1.3 mg/L at one location. During the subsequent monthly sampling events, the pH values were better controlled; however, the lead and copper levels continued to be higher than those observed before the pH drop in January.
The most significant operational issue observed was related to the addition of acid and caustic necessary to maintain the desired pH ranges of the feed water to the treatment system and the finish water to the storage tank and distribution system. Confounding the proper addition of acid and caustic were continuing discrepancies observed in pH readings from the inline pH probes versus a WTW field meter used to measure pH at sampling locations across the treatment train. In fact, an inadvertent lowering of the caustic addition in late December resulted in the pH drop observed in the distribution system samples collected on January 12, 2005, and the corresponding increase in lead and copper levels in the distribution system as described above.
The capital investment cost of $154,700 includes $102,600 for equipment, $12,500 for site engineering, and $39,600 for installation. Using the system's actual capacity of 40 gpm (57,600 gal per day [gpd]), the capital cost was $3,868/gpm ($2.68/gpd) and equipment-only cost was $2,565/gpm ($1.78/gpd). These calculations did not include the cost of the building construction.
O&M costs included only incremental costs associated with the adsorption system, such as media replacement and disposal, chemical supply, electricity, and labor. Incremental costs for electricity were negligible. Although media replacement and disposal did not take place during the first six months of operation, the cost to change out the lead vessel was estimated to be $9,396 based on information provided by the vendor and a local subcontractor. This cost was used to estimate the media replacement cost per 1,000 gal of water treated as a function of the projected media run length to the 10-µg/L arsenic breakthrough.