Arsenic Removal from Drinking Water by Iron Removal
USEPA Demonstration Project at Climax, MN
Six-Month Evaluation Report (58 pp, 3.8 MB) March 2006
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This report documents the activities performed and the results obtained from the first six months of the arsenic removal treatment technology demonstration project at the Climax, MN site. The objectives of the project are to evaluate (1) the effectiveness of Kinetico's Macrolite® pressure filtration process in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 µg/L, (2) the reliability of the treatment system, (3) the simplicity of required system operation and maintenance (O&M) and operator's skills, and (4) the cost-effectiveness of the technology. The project also is characterizing water in the distribution system and process residuals produced by the treatment system.
The Macrolite® FM-236-AS arsenic removal system at the Climax, MN site consisted of two 42-inch diameter by 72-inch-tall contact tanks (345 gal) and two 36-inch-diameter by 72-inch-tall pressure vessels (264 gal), each containing 14 ft3 of Macrolite® media. The design flow rate was 140 gal per minute (gpm), which yielded 5 min of contact time prior to pressure filtration. From August 11, 2004 through February 28, 2005, the system operated for a total of 1,075 hrs at approximately 5.3 hrs per day. Based on the totalizer to treatment readings, the system treated approximately 6,758,000 gal of water with an average daily water demand of 34,850 gal during this time period. The system operated in the service mode within the flow and pressure specifications provided by the vendor. During this time period, however, operational issues were noted with the automated backwash process that led to relatively frequent backwash failure conditions as discussed in this report.
Total arsenic concentrations in the source water ranged from 32.1 to 51.4 µg/L with As(III) being the predominating species at an average concentration of 35.5 µg/L. Prechlorination was used to oxidize As(III) to As(V) and promote the precipitation of iron solids prior to the Macrolite® pressure filtration. From August 11, 2004 to January 3, 2005, the total arsenic levels in the treated water ranged from 9.7 to 19 µg/L and averaged 14.1 µg/L, indicating that the natural iron content of the water was not high enough for sufficient arsenic removal to below 10 µg/L. The natural soluble iron concentrations in the raw water varied from 342 to 520 µg/L and averaged 455 µg/L. This corresponds to an iron:arsenic ratio of 12:1 given the average soluble iron and soluble arsenic levels in the source water. Supplemental iron addition using ferric chloride was subsequently initiated on January 3, 2005 in order to provide sufficient iron for effective arsenic removal. After iron addition at a target dose of 0.5 mg/L, total arsenic levels in the treated water averaged 6.0 µg/L. However, a slight increase in iron leakage from the Macrolite® filters was noted with total iron levels (existing solely as particulates) in the treated water ranging from <25 to 122 µg/L.
During this time period, the rate of backwash water production ranged from 2.2% to 2.4% of the treated water production. Prior to the iron addition, soluble arsenic concentrations in the backwash water ranged from 12.3 to 21.6 µg/L and soluble iron concentrations ranged from <25 to 39.9 µg/L. After iron addition, soluble arsenic concentrations decreased and ranged from 6.4 to 9.2 µg/L, while soluble iron concentrations increased and ranged from 27.3 to 148 µg/L.
Comparison of the distribution system sampling results before and after the system operation showed a decrease in the arsenic levels at all three sampling locations. Arsenic concentrations in the baseline samples ranged from 21.8 to 52.3 µg/L. Since the treatment system startup, arsenic levels in the distribution samples decreased from 11.3 to 17.0 µg/L before iron addition to 5.9 to 11.8 µg/L after iron addition. Neither lead nor copper concentrations at the sample sites appeared to have been affected by the operation of the system.
The capital investment cost was $249,081, which included $137,970 for equipment, $39,344 for engineering, and $71,767 for installation. Using the system's rated capacity of 140 gpm (201,600 gallons per day [gpd]), the capital cost was $1,779 per gpm ($1.24 per gpd) and equipment-only cost was $986 per gpm ($0.68 per gpd). These calculations did not include the cost of a building addition to house the treatment system.
O&M costs for the Macrolite® FM-236-AS system included only incremental costs associated with the chemical supply, electricity, and labor. O&M costs were estimated in this report at $0.27/1,000 gal and will be refined at the end of the one-year evaluation period.