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Abstract

 

Arsenic Removal from Drinking Water by Coagulation/Filtration U.S. EPA Demonstration Project at Town of Felton, DE Final Performance Evaluation Report (EPA/600/R-10/039) April 2010

This report documents the activities performed during and the results obtained from the arsenic removal treatment technology demonstration project at the Town of Felton, DE. The objectives of the project were to evaluate: (1) the effectiveness of Kinetico's FM-348-AS coagulation/filtration (C/F) system using Macrolite® media in removing arsenic to meet the maximum contaminant level (MCL) of 10 µg/L, (2) the reliability of the C/F system for use at small water facilities, (3) the required system operation and maintenance (O&M) and operator skill levels, and (4) the capital and O&M cost of the technology. The project also characterized water in the distribution system and residuals generated by the treatment process. The types of data collected included system operation, water quality, process residuals, and capital and O&M cost.

After review and approval of the engineering plan by the state, the C/F system was installed and became operational on September 14, 2006. The system consisted of two 48-in × 72-in fiber reinforced plastic (FRP) contact tanks and three 48-in × 72-in FRP filtration vessels configured in parallel. Each filtration vessel was loaded with 25 ft3 of M2 Macrolite® media for a design filtration rate of 10 gpm/ft2. The system also used two chemical addition assemblies, one each for prechlorination and iron addition. An existing prechlorination system was used to oxidize As(III) and soluble iron (Fe[II]); an iron addition system was installed to inject ferric chloride (FeCl3) to form arsenic-laden particles prior to Macrolite® pressure filtration. A recycle system was incorporated into the treatment system to reclaim backwash wastewater and eliminate the need to discharge wastewater into a sanitary sewer. The recycle system consisted of a pump controller, two booster pumps, and a 16-ft × 6-ft × 10-ft concrete recycle tank equipped with four float switches.

From September 14, 2006, through November 3, 2007, the treatment system operated at 263 gal/min (gpm) for 6.5 hr/day, on average, producing 43,446,110 gal of water. This average flowrate corresponded to a contact time of 4.3 min through the two contact tanks and a filtration rate of 7.0 gpm/ft2. The recycle system operated for 29.4% of the time when the treatment system was in operation during the demonstration study.

Source water had an average pH value of 8.3 and contained 27.2 to 43.3 µg/L of total arsenic. The predominant arsenic species was As(III) with an average concentration of 29.1 µg/L. Total iron concentrations ranged from <25 to 62.5 µg/L and averaged 26.1 µg/L, existing mostly in the soluble form. This amount of soluble iron was not adequate for arsenic removal; therefore, ferric chloride was added to achieve an iron concentration of 1.2 to 2.0 mg/L to effectively remove arsenic to below the MCL.

Following prechlorination, arsenic existed mostly as particulate arsenic, which was removed by the pressure filters to levels below 7.4 µg/L (on average). Throughout the performance evaluation study, total arsenic concentrations in system effluent exceeded the arsenic MCL on 14 sampling occasions, which were due to either insufficient iron addition or particulate breakthrough from the filters. Shortening run lengths from 17.0 to 9.1 hr (by lowering the differential pressure [Δp] trigger from 25 to 18 lb/in2 [psi]) appeared to be useful for decreasing particulate breakthrough from the pressure filters.

Each filter was backwashed automatically approximately 5 time/week with the backwashing process triggered by either high Δp, standby time, or run time. High Δp triggered approximately 94% of backwashes. Backwash durations averaged 6.7 min, generating approximately 724 gal of wastewater per vessel during each backwash. A total of 673,450 gal of wastewater was produced during the performance evaluation study, equivalent to 1.6% of the total amount of water treated. The backwash wastewater contained, on average, 336 mg/L of total suspended solids (TSS), 1,229 µg/L of arsenic, 107 mg/L of iron, and 551 µg/L of manganese, with the majority existing as particulates. As such, approximately 920 g of solids were discharged from each filtration vessel during each backwash event, including 3.4 g of arsenic, 293 g of iron, and 1.5 g of manganese.

Comparison of the distribution system water sampling results before and after system startup demonstrated a considerable decrease in arsenic concentration (i.e., 34.4 to 8.5 µg/L, on average). Arsenic levels in the distribution system were slightly higher than those in treatment system effluent, indicating resuspension and/or redissolution of arsenic in the distribution system. Copper concentrations decreased from an average baseline concentration of 85.6 to 44.0 µg/L after system startup. Manganese and lead concentrations decreased slightly from 1.7 to 0.5 µg/L and 2.4 to 1.6 µg/L, respectively. Iron concentrations increased slightly from 26.9 to 38.1 µg/L. pH and alkalinity levels did not appear to be affected.

The capital investment for the system was $334,297, including $201,292 for equipment, $44,520 for site engineering, and $88,485 for installation, shakedown, and startup. Using the system's rated capacity of 375 gpm (or 540,000 gal/day [gpd]), the capital cost was $891/gpm (or $0.62/gpd). This unit cost does not include the cost of the building to house the treatment system or the cost of the recycle system used for reclaiming the backwash wastewater. O&M cost, estimated at $0.30/1,000 gal, included the cost for chemical usage, electricity consumption, and labor.

Contact

Thomas Sorg

See Also

Arsenic Research

Arsenic Research Publications

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