|Arsenic Removal From Drinking Water by Iron Removal, U.S. EPA Demonstration Project at Sabin, MN, Six-Month Evaluation Report (EPA/600/R-08/005) January 2008
This report describes the activities and the results of the first six months of the arsenic removal technology demonstration project at the Sabin, Minnesota, facility. The objectives of the project are to evaluate the:
The project is also characterizing the water in the distribution system and residuals produced by the treatment process. The types of data collected include system operation, water quality (both across the treatment train and in the distribution system), process residuals, and capital and O&M costs.
After engineering plan review and approval by the state, the FM-248-AS treatment system was installed and became operational on January 19, 2006. The system consisted of two 63-inch-diameter, 86-inch-tall fiberglass reinforced plastic (FRP) contact tanks and two 48-inch-diameter, 72-inch-tall FRP pressure tanks, all configured in parallel. Each pressure tank contained 25 cubic feet of Macrolite media, which is a spherical, low-density, chemically inert ceramic media designed for filtration rates up to 10 gallons per minute per square foot (gpm/ft2). The system used prechlorination to oxidize arsenic (III) and iron (II) and the contact tank to improve the formation of arsenic (V)-laden particles prior to entering the pressure filters. The system operated at approximately 238 gallons per minute (gpm) for 3 hours per day (on average), producing 6,650,000 gallons of water through July 30, 2006. The average flow rate corresponded to a contact time of 7.1 minutes and a filtration rate of 9.5 gpm/ft2. A number of issues related to the control of the frequency and duration of backwash operation were experienced, as discussed in the report.
The source water had an average pH of 7.4 and total arsenic concentrations ranged from 32.8 to 49.8 µg/L, with the soluble fraction consisting of arsenic (V) at 23.9 µg/L and arsenic (III) at 13.2 µg/L. Concentrations of both arsenic (V) and arsenic (III) varied considerably during the course of this six-month study period; arsenic (III) concentrations exhibited a decreasing trend and arsenic (V) concentrations exhibited an increasing trend, especially during the first month. Total iron concentrations ranged from 1,203 to 1,936 µg/L and existed primarily in the soluble form with an average concentration of 1,135 µg/L. Raw water-soluble iron and soluble arsenic concentrations corresponded to a ratio of 31:1. Total arsenic concentrations in treated water averaged 6.3 µg/L and ranged from 3.5 to 10.6 µg/L. Because of total arsenic breakthrough at 10.6 µg/L on July 26, 2006, a run length study will be conducted during the next six-month period.
Comparison of the distribution system sampling results before and after the second quarter of operations demonstrated a considerable decrease in arsenic (27.4 to 7.1 µg/L), iron (1,211 to 75 µg/L), and manganese (114 to 60 µg/L). Additional decreases were observed in manganese concentrations within the distribution system when compared to the concentrations in the filter effluent (203 and 217 [on average] following Tanks A and B) to those in the distribution system (60 µg/L [on average] in the second quarter of system operation). Copper (179 to 127 µg/L) and lead concentrations (4.2 to 1.3 µg/L) also decreased. Alkalinity and pH did not appear to be significantly affected.
Filter tank backwash occurred automatically about three times per tank per week and was triggered primarily by the 48-hour standby time setpoint that was due to low operational time of the treatment system (3 hours per day). Approximately 161,550 gallons of wastewater, or 2.4 percent of the amount of water treated, was generated during the first six months. Under normal operating conditions, the backwash wastewater contained 116 to 550 milligrams per liter (mg/L) of total suspended solids, 29.8 to 176.8 mg/L of iron, 2.0 to 8.6 mg/L of manganese, and 6.1 to 27.6 µg/L of arsenic, with the majority existing as particulates. The average amount of solids discharged per backwash cycle was approximately 5.2 pounds, including 1.6 pounds of elemental iron, 0.09 pounds of elemental manganese, and 0.01 pounds of elemental arsenic.
The capital investment cost of $287,159 included $160,875 for equipment, $49,164 for site engineering, and $77,120 for system installation, shakedown, and startup. Using the system’s rated capacity of 250 gpm (or 360,000 gallons per day [gpd]), the capital cost was $1,149 per gpm ($0.80 per gpd). This calculation does not include the cost of the building to house the treatment system.
O&M costs, estimated at $0.69 per 1,000 gallons, included only chemical supply and labor and will be refined at the end of the one-year evaluation period.
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