|Arsenic Removal from Drinking Water by Iron Removal U.S. EPA Demonstration Project at Northeastern Elementary School in Fountain City, IN Final Performance Evaluation Report (EPA/600/R-11/025) March 2011
This report documents the activities performed and the results obtained from the arsenic removal treatment technology demonstration project at Northeastern Elementary School in Fountain City, IN. The main objective of the project was to evaluate the effectiveness of US Water Systems’ iron removal (IR) system in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 µg/L. Additionally, this project evaluated (1) the reliability of the treatment system, (2) the required system operation and maintenance (O&M) and operator skill levels, and (3) the capital and O&M cost of the technology. The project also characterized the water in the distribution system and process 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 cost.
The system consisted of four 36-in × 72-in composite poly-glass vessels configured in parallel. Each vessel contained 17.7 ft3 of G2® media consisting of a granular, calcined diatomite substrate coated with ferric hydroxide developed by ADI. The treatment system was designed for a peak flowrate of 60 gal/min (gpm) (15 gpm per vessel), which corresponds to a hydraulic loading rate of 2.1 gpm/ft2. Over the performance evaluation period, the actual average flowrates were 11.3, 11.3, 11.4, and 13.1 gpm for Vessels A, B, C, and D, respectively, based on readings from the flow meter/totalizer installed on each vessel. The average hydraulic loading rates for Vessels A, B, C, and D were 1.6, 1.6, 1.6, and 1.8 gpm/ft2, respectively.
The pre-existing chlorination system was replaced with a new Stenner Model 85MPHP5 peristaltic pump, a 30-gal chemical feed tank, an injector, and 2-in inline mixer. Sodium hypochlorite (NaOCl) was injected prior to the filtration vessels to oxidize As(III) to As(V) and form arsenic-laden iron solids, which were then filtered by G2® media. The chlorination system also was used to maintain a target combined chlorine residual of 1.0 mg/L (as Cl2) in the distribution system for disinfection.
From September 22, 2008, through the end of the performance evaluation study on October 29, 2009, the treatment system operated for a total of 349.1 hr, treating approximately 941,500 gal of water. The average daily operation time was 1.4 hr/day when the school was in session and 0.3 hr/day when the school was out of session. The average system flowrate was 47.1 gpm.
Total arsenic concentrations in raw water ranged from 24.0 to 39.3 µg/L and averaged 29.4 µg/L. Soluble As(III) was the predominating arsenic species with concentrations ranging from 10.8 to 23.9 µg/L and averaging 17.7 µg/L. Total iron concentrations in raw water averaged 1,865 µg/L, while soluble iron concentrations averaged 1,058 µg/L, which was over 52 times the average soluble arsenic concentration (20.2 µg/L) in raw water. Therefore, supplemental iron addition was not necessary for arsenic removal. Following chlorination, over 85% of arsenic existed as particulate arsenic (23.8 µg/L [on average]), which was removed by the pressure filters to an average concentration of 3.6 µg/L. The system also reduced total iron concentrations to 99 µg/L (on average), while total manganese concentrations remained relatively unchanged.
During the performance evaluation period, the vessels were backwashed eight times. Backwash might be triggered manually or automatically with either a time, a throughput, or a pressure differential (Δp) as a setpoint. Throughput was chosen as the setpoint to initiate backwash. To give the operator better control over when backwash would occur, the throughput was set to 90,000 gal. Each backwash cycle lasted 26 min, including 14 min for counter-current backwash, 5 min for co-current slow rinse, and 7 min co-current fast rinse. Each vessel generated 1,137 gal of wastewater (on average), or 4,548 gal per event. Assuming an average of 677 mg/L of total suspended solids (TSS) in 1,137 gal of wastewater produced by backwashing one vessel, 2,914 g of solids would be discharged to the sewer. The solids were composed of 2.2, 396, and 1.6 g of arsenic, iron, and manganese, respectively.
Comparison of the distribution system sampling results before and after the system startup showed a significant decrease in arsenic concentration (i.e., from 17.0 to 5.2 µg/L [on average]). Arsenic concentrations in the distribution system were slightly higher than those in the system effluent. Iron was significantly reduced in the distribution system, while manganese remained relatively unchanged. Copper levels in the distribution system increased after the system was put into service, but their concentrations were always below their respective action levels.
The capital investment cost for the system was $128,118, including $103,118 for equipment, $7,500 for site engineering, and $17,500 for installation. Using the system’s rated capacity of 60 gpm (86,400 gal/day [gpd]), the normalized capital cost was $2,135/gpm ($1.48/gpd). The O&M cost was $2.26/1,000 gal of water treated and only included the cost associated with labor.