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Abstract

 

Arsenic Removal from Drinking Water by Iron Removal U.S. EPA Demonstration Project at Sabin, MN Final Performance Evaluation Report (EPA/600/R-10/033) April 2010

This report documents the activities performed and the results obtained from January 30, 2006 to April 29, 2007 at the U.S. Environmental Protection Agency (EPA) Arsenic Removal Technology Demonstration site in Sabin, MN. The main objective of the project was to evaluate the effectiveness of Kinetico's FM-248-AS arsenic removal system using Macrolite® media in removing arsenic from raw drinking water to meet the arsenic maximum contaminant level (MCL) of 10 µg/L. Additionally, this project evaluated: 1) the reliability of the treatment system for use at a small water facility; 2) the required system operation and maintenance (O&M) and operator skill levels; and 3) the cost-effectiveness 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 operational data, water quality data (both across the treatment train and in the distribution system), process residual data, and capital and O&M cost data.

After engineering plan review and approval by the state, the treatment system was installed and became operational on January 19, 2006. System inspection and operator training were performed on January 30 and 31, 2006, and the system performance evaluation began officially on January 30, 2006. The system consisted of two 63-in × 86-in fiberglass reinforced plastic (FRP) contact tanks and two 48-in × 72-in FRP pressure tanks, all configured in parallel. Each pressure tank contained 25 ft3 of Macrolite® media, a spherical, low density, chemically inert ceramic media designed for filtration rates up to 10 gal/min (gpm)/ft2 at 125 gpm. The system used prechlorination to oxidize soluble As(III) and Fe(II) and the contact tank to promote the formation of As(V)-laden iron particles prior to entering the pressure filters. Later in the study, this prechlorination step also was used to increase oxidation, precipitation, and subsequent removal of manganese. The system operated at approximately 231 gpm, producing 14,884,800 gal of water through April 29, 2007. This represents an average finished water production of 32,858 gal/day (gpd). The average flowrate corresponds to a contact time of 7.4 min and a filtration rate of 9.2 gpm/ft2. A number of issues related to the control of backwash frequency and duration were experienced and are discussed in the report.

Source water had an average pH of 7.3 and an average of total arsenic of 41.8 µg/L. The soluble fraction consisted of both As(V) and As(III) with concentrations varying from <0.1 to 41.7 and from 3.8 to 35.6 µg/L, respectively. Soluble As(III) concentrations exhibited a decreasing trend and soluble As(V) concentrations exhibited an increasing trend. Total iron concentrations ranged from 1,005 to 2,757 µg/L and averaged 1,350 µg/L, which existed primarily in the soluble form. Average soluble iron and soluble arsenic concentrations corresponded to a ratio of 29:1, which was sufficient for arsenic removal via iron removal. Manganese levels ranged at 153 to 449 µg/L and averaged 341 µg/L, which existed entirely in the soluble form. The source water also contained 0.2 mg/L of ammonia (as N), 30.4 µg/L of phosphorus (as P), 29.9 mg/L of silica (as SiO2), and 1.7 mg/L of total organic carbon (as C).

With sufficient chlorine addition, soluble As(III) was effectively oxidized to soluble As(V), which was then adsorbed onto or co-precipitated with iron solids, formed during prechlorination, to become particulate arsenic. Total arsenic concentrations in the treated water were significantly reduced with concentrations averaging at 6.6 µg/L at Tanks A and B sampling locations and 8.3 µg/L at the combined effluent sampling location. Three exceedances (above the arsenic MCL) experienced were attributed to particulate arsenic and iron breakthrough. A special study was conducted in November 2006 to investigate the filter run length; a maximum of 12 hr was recommended to minimize particulate iron and arsenic breakthrough.

Prechlorination did not effectively oxidize Mn(II). Low rates of manganese removal across the treatment system and accumulation within the distribution system were issues. Manganese concentrations averaged 114 and 75 µg/L within the distribution system before and after system startup, respectively. In June 2006, the facility operator received complaints from a few customers concerning periodic slugs of dark solids from their taps, which might have been related to iron and/or manganese solids accumulating within the distribution system. It was hypothesized that the increased chlorine dose from post-chlorination and contact time within the distribution system resulted in manganese oxidation and subsequent attachment to pipe walls or iron deposits (tubercules), which are characteristic of older distribution systems.

Chlorine dosages for prechlorination were subsequently increased to enhance soluble manganese oxidation and particulate manganese removal by the pressure filters. Conversion from soluble Mn(II) to manganese solids increased from an average of 38.6% to 71.8% as chlorine residuals increased from an average value of 0.6 to 1.0 mg/L (as Cl2). Manganese solids removal rates increased correspondingly to as high as 92%. Because of additional manganese solids loading, the maximum filter run length was reduced to 5 hr, which was just below the system median run length of 6 hr based on a 48-hr standby trigger.

Arsenic concentrations in the distribution system water samples were reduced from a pre-startup average of 27 µg/L to a post-startup average of 8.7 µg/L (excluding two outliers in the first quarter of operation). In general, total arsenic concentrations in the distribution system water were slightly higher than those in the treatment system effluent. Iron concentrations decreased significantly and averaged 1,211 µg/L and 157 µg/L before and after system startup, respectively. Lead and copper concentrations were reduced slightly since system startup. Alkalinity and pH did not appear to be significantly affected.

Filter tank backwash occurred automatically 1 to 4 times/tank/week, which was triggered primarily by the 48-hr standby time setpoint, due to low operational time of the treatment system. Approximately 521,250 gal of wastewater were generated during the performance evaluation study, which represents approximately 3.5% of the total amount of water treated. Under normal operating conditions, 1,924 gal of wastewater and 3.6 lb of solids were generated per backwash cycle (for two tanks). The solids generated included 0.6 lb of elemental iron, 0.03 lb of elemental manganese, and 0.01 lb of elemental arsenic.

The capital investment for the system was $287,159, consisting of $160,875 for equipment, $49,164 for site engineering, and $77,120 for system installation, shakedown, and startup. Using the system's rated capacity of 250 gpm (or 360,000 gpd), the capital cost was $1,149/gpm or $0.80/gpd. This calculation does not include the cost of the building to house the treatment system.

The estimated O&M costs included chemical supply, labor and electricity consumption. O&M costs were estimated at $0.43/1,000 gal.

Contact

Thomas Sorg

See Also

Arsenic Research

Arsenic Research Publications

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