|Arsenic Removal from Drinking Water by Iron Removal, U.S. EPA Demonstration Project at City of Sandusky, MI, Final Performance Evaluation Report (EPA/600/R-08/007) March 2008
This report describes the activities and results of the arsenic removal treatment technology demonstration at the City of Sandusky, Michigan, facility. The objectives of the project were to evaluate the:
The project also characterized water in the distribution system and residuals produced by the treatment system. The types of data collected included 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 of up to 2.5 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 11.4 µg/L. The predominant soluble species was arsenic (III) with an average concentration of 8.7 µg/L. Total iron concentrations ranged from 236 to 3,214 µg/L and averaged 896 µg/L. Chlorine was used to oxidize arsenic (III) and iron (II) to form filterable arsenic (V)-laden particles within the detention tank. However, due to the presence of 0.3 milligrams per liter (mg/L) of ammonia (as nitrogen) in source water, breakpoint chlorination was not achieved with an average of 2.5 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 3.2 µg/L of arsenic (III) in the treated water. After gravity filtration, total arsenic concentrations ranged from 0.4 to 9.8 µg/L and averaged 2.4 µg/L, consisting of soluble arsenic (III) and arsenic (V). Iron concentrations in the filter effluent were, in most cases, less than the method reporting limit of 25 µg/L; however, occasional elevated concentrations were measured in the range of 99 to 617 µg/L in the filter effluent. The system operated at approximately 163 gallons per minute (gpm), producing approximately 61,833,000 gallons of water through June 22, 2007. The flow rate corresponded to an average detention time of 69 minutes and an average filtration rate of 1.4 gpm/ft2, compared to the design values of 40 minutes and 2.5 gpm/ft2.
Comparison of the distribution system sampling results before and after system startup demonstrated a considerable decrease in arsenic (7.4 to 3.2 µg/L) and iron (360 to 35 µg/L). Manganese and lead concentrations did not appear to be affected, but copper concentrations increased from 209 to 473 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, based on a day and time setpoint. Approximately 6,000 gallons of wastewater were discharged to the sanitary sewer for each event, totaling 1.0 percent of the treated water volume when backwashing two times per week and 1.6 percent when backwashing three times per week. On average, the backwash wastewater contained 129 mg/L of total suspended solids, 0.5 mg/L of arsenic, 58 mg/L of iron, and 1.1 mg/L of manganese, with the majority existing as particulates. Approximately 3.5 pounds of solids were discharged per event, including 0.02 pounds of arsenic, 2.90 pounds of iron, and 0.06 pounds of manganese.
The capital investment of $364,916 included $205,800 for equipment, $27,077 for site engineering, and $132,039 for system 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 $0.50 per 1,000 gallons, included costs for chemicals, electricity, and labor.
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