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Arsenic Removal from Drinking Water by Coagulation/Filtration
U.S. EPA Demonstration Project at Village of Pentwater, MI
Six-Month Evaluation Report
(57 pp, 1356 Kb)
June 2007

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This report documents the activities performed during and the results obtained from the first six months of the EPA arsenic removal technology demonstration project at the Village of Pentwater, MI facility. The main objective of the project is to evaluate the effectiveness of Kinetico's FM-260-AS treatment system using Macrolite® media in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 g/L. Additionally, this project evaluates the reliability of the treatment system for use at small water facilities, the required system operation and maintenance (O&M) and operator skill levels, and the cost-effectiveness of the technology. The project also characterizes water in the distribution system and residuals generated 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-260-AS treatment system was installed and became operational on November 22, 2005. The system consisted of one 96-in-diameter, 96-in-tall steel contact tank and two 60-in-diameter, 96-in-tall steel pressure tanks configured in parallel. Each pressure tank contained 40 ft3 of Macrolite® media, which is a spherical, low density, chemically inert ceramic media designed for filtration rates up to 10 gpm/ft2. The system used an existing chlorination system to oxidize As(III) and Fe(II) and the contact tank to improve the formation of As(V)-laden particles prior to entering the pressure filters. The system operated at approximately 353 gal/min (gpm) for 3.2 hr/day, producing 12,714,000 gal of water through May 22, 2006. The flowrate corresponded to a contact time of 6.8 min and a filtration rate of 9 gpm/ft2. A number of issues related to backwash operation were experienced and are being addressed by the vendor. The resolution of these issues will be further discussed in the Final Performance Evaluation Report.

The source water had an average pH of 8.0 and contained 15.3 to 21.8 g/L of total arsenic. The predominant species was As(III) with an average concentration of 17.2 g/L. Total iron concentrations ranged from 346 to 510 g/L, which existed primarily in the soluble form with an average concentration of 367 g/L. Raw water soluble iron and soluble arsenic concentrations corresponded to a ratio of 21:1. Total arsenic concentrations in treated water ranged from 7.8 to 15.6 g/L and averaged 10.0 g/L. To further reduce arsenic concentrations in treated water, provisions were made to enable supplemental iron addition. This condition will be initiated after backwash issues are resolved by the vendor and evaluated in the Final Performance Evaluation Report.

Comparison of the distribution system sampling results before and after the system startup demonstrated a considerable decrease in arsenic (16.5 to 8.8 g/L), iron (192 to <25 g/L), manganese (23.8 to 11.5 g/L), and copper (131 to 70.4 g/L) concentrations. Alkalinity, pH, and lead concentrations did not appear to be affected.

Filter tank backwash occurred automatically about 3 time/tank/week triggered by 24-hr service time, 48-hr standby time, or 22-psi differential pressure setpoints, whichever occurred first. Due to low operational time of the treatment system, the majority of backwash cycles was initiated by the standby time setpoint. Approximately 403,900 gal of wastewater, or 3.2% of the amount of water treated, was generated during the first six months. Under normal operating conditions, the backwash wastewater contained 24 to 106 mg/L of total suspended solids (TSS), 1.5 to 29.5 mg/L of iron, 66 to 1,206 g/L of manganese, and 30 to 610 g/L of arsenic, with the majority exisiting as particulates. The highest amount of solids discharged per backwash cycle was approximately 0.96 lb, including 0.235 lb of iron, 0.009 lb of manganese, and 0.005 lb of arsenic.

The capital investment for the system was $334,573 consisting of $224,994 for equipment, $30,929 for site engineering, and $78,650 for installation, shakedown, and startup. Using the system's rated capacity of 400 gpm (or 576,000 gal/day [gpd]), the capital cost was $836/gpm (or $0.58/gpd). This calculation does not include the cost of the building to house the treatment system.

O&M cost, estimated at $0.22/1,000 gal, included only the incremental cost for electricity and labor. Since chlorination already existed prior to the demonstration study, the incremental cost for chemical usage will only be incurred for iron addition once initiated. The associated costs for iron addition will be discussed in the Final Performance Evaluation Report.

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Thomas Sorg

Office of Research & Development | National Risk Management Research Laboratory

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