|Arsenic Removal From Drinking Water by Coagulation/Filtration, U.S. EPA Demonstration Project at the City of Okanogan, WA, Final Performance Evaluation Report (88 pp, 4.52 MB) (EPA/600/R-10/166) December 2010
This report documents the activities performed during and the results obtained from the arsenic removal treatment technology demonstration project at the City of Okanogan, Washington, facility. The objectives of the project were to evaluate the:
The project also characterized water in the distribution system and residuals generated by the treatment process. The types of data collected included system operation, water quality, process residuals, and capital and O&M costs.
After review and approval of the engineering plan by the State of Washington, the FH-13 Electromedia I treatment system was installed and became operational on August 14, 2008. The system consisted of two 4-foot by 8-foot carbon steel contact tanks, and one 7-foot by 9 1/3-foot horizontal carbon steel filter tank loaded with 174 cubic feet (ft3) of Electromedia I filter media, 33 ft3 of support media, and 43 ft3 of concrete. The filter tank was fitted with semi-elliptical ends and upper and lower manifold assemblies, providing a filtration area of 75 square feet (ft2). At a design flowrate of 750 gallons per minute (gpm), the hydraulic loading rate to the filter was 10 gpm per ft2. The system used two chemical addition assemblies, one each for prechlorination and supplemental iron addition. The chlorine addition system was installed to oxidize arsenic (III) and iron (II) and to form arsenic (V)-laden iron solids prior to the filtration tank. The iron addition system was installed to increase the removal of soluble arsenic (V) through adsorption and/or co precipitation with iron solids. The target chlorine and iron dosages were 0.7 milligrams per liger (mg/L) (as chlorine) and 0.9 mg/L (as iron), respectively.
A wastewater recycle system was incorporated into the treatment system to reclaim backwash wastewater and eliminate the need to discharge wastewater into the sanitary sewer. The recycle system consisted of a reclaim pump and a 22,500-gallon concrete reclaim tank equipped with high/low float switches.
From August 14, 2008, through August 14, 2009, the treatment system operated for an average of 13.6 hours per day, producing 139,435,000 gallons of water. This production rate corresponded to an average flowrate of 527 gpm, comparable to the 550-gpm extraction rate allowed for Well No. 4 by water rights. At 527 gpm, it yielded a contact time of 2.8 minutes in the two contact tanks and a filtration rate of 7.0 gpm per ft2.
Source water from Well No. 4 had an average pH value of 7.6 and contained 14.7 to 22.7 μg/L of total arsenic. The predominant arsenic species was arsenic (III) with an average concentration of 13.4 μg/L. Total iron concentrations ranged from less than 25 to 230 μg/L and averaged 78 μg/L, existing mostly in the soluble form (averaged at 49 μg/L). This amount of soluble iron corresponded to a soluble-iron-to-soluble-arsenic ratio of 2.7:1, indicating insufficient iron for arsenic removal. Ferric chloride was added to chlorinated water to achieve a target iron concentration of 0.9 mg/L (50 times the soluble arsenic concentration in source water) for more effective arsenic removal, presumably through adsorption and/or co-precipitation with iron solids.
Total arsenic concentrations after the pressure filter ranged from 2.9 to 14.9 μg/L and averaged 6.2 μg/L. Filter performance was maintained with backwash, which was triggered either by a preset run time of 8 hours or when the water level in the storage reservoir reached the “Stop” setpoint. Backwashing every 8 hours appeared to be adequate to maintain proper filter performance for arsenic and iron removal. The filter tank was backwashed 2.3 times per day, producing approximately 6,150 gallons of wastewater per time. A total of 4,667,850 gallons of wastewater was produced during the study, equivalent to 3.3 percent of the total amount of water treated. On average, the backwash wastewater contained 108 mg/L of total suspended solids, 462 μg/L of arsenic, 38.1 mg/L of iron, and 1,157 μg/L of manganese, with the majority existing as particulate. During each backwash, 2.5 kilograms of solids was produced, which included 10.6 g of arsenic, 882 grams (g) of iron, and 26.3 g of manganese.
Arsenic levels in distribution system water as sampled at DS3, a non-Lead and Copper Rule sampling location, were very close to those in treatment system effluent (i.e., 6.8 versus 6.2 μg/L, on average). Because the other two sampling locations (DS1 and DS2) selected for distribution water sampling were impacted by all four wells supplying Okanogan’s distribution system, the effect of the treatment system on the distribution water quality could not be evaluated directly. The average lead concentration within the distribution system was 1.5 μg/L with no samples exceeding the action level of 15 μg/L. The average copper concentration was 61.6 μg/L with no samples exceeding the 1,300 μg/L action level.
The capital investment for the system was $424,817, including $296,430 for equipment, $48,332 for site engineering, and $80,055 for installation, shakedown, and startup. Using the system’s rated capacity of 550 gpm (or 792,000 gallons per day [gpd]), the capital cost was $772 per gpm (or $0.54 per gpd). This unit cost does not include the cost of the building to house the treatment system and recycle system used to reclaim the backwash water. O&M cost, estimated at $0.18 per 1,000 gallons, included cost for chemicals usage, electricity consumption, and labor.
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