|Arsenic Removal From Drinking Water by Iron Removal, U.S. EPA Demonstration Project at City of Sandusky, MI, Six-Month Evaluation Report (EPA/600/R-07/147) December 2007
This report describes the activities and results of the first six months of the arsenic removal technology demonstration project at the City of Sandusky, Michigan, 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 system. The types of data collected include system operation, water quality, process residuals, and capital and O&M costs.
After engineering plan review and approval by the state, the AERALATER was installed and became operational on June 14, 2006. The fully automated packaged system consisted of a 12-foot-diameter aluminum detention tank atop a 12-foot-diameter, three-cell gravity sand filter plus ancillary equipment, including an air distribution grid, an air compressor pack, a blower, two chemical feed systems, a high service pump, sample taps, and associated instrumentation. The filter contained 226 cubic feet of sand and was designed for filtration rates up to 3 gallons per minute per square foot (gpm/ft2).
The source water had an average pH of 7.2 and contained fluctuating concentrations of arsenic and iron due, in part, to the use of up to four source water wells. Total arsenic concentrations ranged from 7.3 to 23.5 µg/L and averaged 10.9 µg/L. The predominant species was arsenic (III) with an average concentration of 7.8 µg/L. Total iron concentrations ranged from 236 to 3,214 µg/L and averaged 860 µg/L. Chlorine was used to oxidize arsenic (III) and iron (II) to form filterable arsenic (V)-laden particles within the detention tank. However, because of the presence of 0.3 milligrams per liter (mg/L) of ammonia (as nitrogen) in source water, breakpoint chlorination was not achieved with the 2.9 mg/L (as chlorine) of sodium hypochlorite applied. The formation of chloramines might have partially inhibited the oxidation of arsenic (III), leaving as much as 2.1 µg/L of arsenic (III) in the treated water. After gravity filtration, total arsenic concentrations ranged from 1.0 to 6.3 µg/L and averaged 2.3 µg/L, consisting of arsenic (III) and arsenic (V). The system operated at approximately 168 gallons per minute (gpm), producing approximately 29,406,000 gallons of water through December 14, 2006. The flow rate corresponded to a detention time of 67 minutes and a filtration rate of 1.5 gpm/ft2.
Comparison of the distribution system sampling results before and after system startup demonstrated a decrease in arsenic (7.4 to 3.0 µg/L) and iron (360 to 30 µg/L). Manganese and lead concentrations did not appear to be affected, but copper concentrations increased from 209 to 511 µg/L after system startup. Alkalinity and pH increased and decreased, respectively, at two locations. Uncertainties of water sources during baseline sampling and changes to the post-treatment chemicals might have affected the trends.
Filter tank backwash occurred automatically about three times per week based on a day and time setpoint. Approximately 6,000 galloons of wastewater was discharged to the sanitary sewer for each event, totaling 1.7 percent of the treated water volume during the first six months. On average, the backwash wastewater contained 109 mg/L of total suspended solids, 52 mg/L of iron, 0.9 mg/L of manganese, and 0.4 mg/L of arsenic, with the majority existing as particulates. Based on solids sampling, approximately 3 pounds of solids were discharged per event, including 2.45 pounds of iron, 0.05 pounds of manganese, and 0.02 pounds of arsenic.
The capital investment cost of $364,916 included $205,800 for equipment, $27,077 for site engineering, and $132,039 for installation, shakedown, and startup. Using the system’s rated capacity of 340 gpm (or 489,600 gallons per day [gpd]), the capital cost was $1,073 per gpm (or $0.75 per gpd). This calculation does not include the cost of the building to house the treatment system.
O&M costs, estimated at $0.24 per 1,000 gallons, included only the incremental cost for electricity and labor. There was no incremental chemical consumption cost because chlorination was previously performed on site.
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