Arsenic Removal from Drinking Water by Iron Removal
EPA Demonstration Project at Big Sauk Lake Mobile Home Park in Sauk Centre, MN
Six-Month Evaluation Report
(PDF, 1505 Kb, 68 pp) June 2007
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This report documents the activities performed and the results obtained from the first six months of the arsenic removal treatment technology demonstration project at the Big Sauk Lake Mobile Home Park (BSLMHP) in Sauk Centre, MN. The objectives of the project are to evaluate the effectiveness of Kinetico's Macrolite® pressure filtration process in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 µg/L, the reliability of the treatment system, the required system operation and maintenance (O&M) and operator skill levels, and the capital and O&M cost of the technology. The project also is characterizing water in the distribution system and process residuals produced by the treatment system.
The Macrolite® CP-213f arsenic removal system at BSLMHP consisted of two 36-in diameter by 57-in tall contact tanks (205 gal each) and four 13-in diameter by 54-in tall pressure tanks (two for each duplex unit), all configured in parallel. Each pressure tank contained 20 in (or 1.5 ft3) of Macrolite® filter media. The maximum design flowrate was 20 gpm, which yielded at least 20 min of contact time prior to pressure filtration and at least 5.4 gpm/ft2 of hydraulic loading to the Macrolite® filters. Because the system operated in an on-demand configuration, the actual flowrates ranged from 1 to 15 gpm, corresponding to 27 to 410 min of contact time and 0.3 to 4.1 gpm/ft2 of hydraulic loading. From July 13, 2005, through January 17, 2006, the system operated for a total of 617 hr at approximately 3.4 hr/day. Based on the totalizer readings, the system treated approximately 863,470 gal of water with an average daily demand of 4,617 gal during this time period.
Total arsenic concentrations in source water ranged from 20.6 to 36.6 µg/L with As(III) being the predominating species at an average concentration of 23.0 µg/L. Potassium permanganate (KMnO4) was used to oxidize As(III) and Fe(II) prior to Macrolite® pressure filtration. KMnO4 was selected as the oxidant because of the presence of elevated total organic carbon (TOC) levels (at 3.2 to 4.8 mg/L) in source water and high formation potential of disinfection byproducts with the use of chlorine.
After the contact tanks, As(III) concentrations were reduced to an average value of 1.9 µg/L, suggesting effective oxidation of As(III) to As(V) with KMnO4. Meanwhile, arsenic was present primarily in the particulate form at an average value of 22.9 µg/L, presumably, by being bound to iron particles. During the first six months from July 13, 2005, through January 17, 2006, total arsenic levels in the treated water were reduced to 2.9 to 17.7 µg/L (averaged 7.6 µg/L). Out of 24 sampling occasions, arsenic concentrations exceeded the 10-µg/L MCL for a total of eight times, with all but one due to particulate breakthrough. Two samples exceeded total arsenic concentrations of 10 µg/L due to low KMnO4 dosage, resulting in incomplete oxidation of As(III) and Fe(II). In order to address particulate arsenic breakthrough, the backwash frequency was increased from every 2,743 gal to every 1,714 gal of throughput during this time period.
Total iron concentrations averaged 2,760 µg/L in source water, which is above the secondary MCL of 300 µg/L. With an average soluble iron to soluble arsenic ratio of 100:1, there was sufficient natural iron present in source water for effective arsenic removal. After the contact tanks, iron was present primarily in the particulate form, suggesting effective oxidation even in the presence of elevated TOC levels. Total iron concentrations in the treated water ranged from <25 to 1,067 µg/L and averaged 259 µg/L. An increase in particulate iron correlated with an increase in particulate arsenic, indicating iron breakthrough from the Macrolite® filters. Total manganese concentrations averaged 144 µg/L in source water, existing primarily in the soluble form as Mn(II) at 132 µg/L. After the addition of 2.6 to 3.8 mg/L of KMnO4 and after the contact tanks, manganese was present primarily in the soluble form based on the use of 0.45-µm disc filters, with levels ranging from 337 to 946 µg/L before November 15, 2005. The high levels of TOC in source water appeared to have inhibited the formation of filterable manganese solids. Based on the results of a series of jar tests, the KMnO4 dosage applied to the treatment system was increased to 5.6 mg/L. The increased KMnO4 dosage enabled most manganese to precipitate after the contact tanks, leaving only 108 to 166 µg/L measured as soluble manganese. Further adjustments will be made to the KMnO4 dosing in the next six-month period to further lower the soluble manganese levels.
During this time period, the backwash water production rates ranged from 2.8 to 7.2%. The control disc on top of each set of duplex units was changed out twice to increase the backwash frequency in order to address the particulate arsenic, iron, and manganese breakthrough. The backwash frequency was increased from the initial field setting of every 2,743 gal to every 1,714 gal. If needed, further adjustments will be made in the next six-month study period. After November 15, 2006, when the modified backwash procedure was implemented, total arsenic concentrations in the backwash water ranged from 114 to 417 µg/L; total iron concentrations ranged from 14,069 to 77,641 µg/L; and total manganese concentrations ranged from 1,595 to 16,178 µg/L. Using 130 gal of backwash water produced, this equates to approximately 0.17 lb of solids, including 4.4 x 10-4 lb of arsenic, 0.08 lb of iron, and 0.01 lb manganese, generated per backwash event.
Comparison of the distribution system sampling results before and after system startup showed a significant decrease in arsenic and iron levels and a significant increase in manganese levels at all three sampling locations. The distribution water sampling results essentially mirrored the treatment results of the Macrolite® filters. Neither lead nor copper concentrations at the sample sites appear to have been affected by the operation of the system.
The capital investment cost was $63,547, which included $22,422 for equipment, $20,227 for engineering, and $20,898 for installation. Using the system's rated capacity of 20 gal/min (gpm) (28,800 gal/day [gpd]), the capital cost was $3,177/gpm ($2.21/gpd).
The O&M cost for the Macrolite® CP-213f system included only incremental cost associated with the chemical supply, electricity consumption, and labor. The O&M cost was estimated in this report at $0.43/1,000 gal and will be refined at the end of the one-year evaluation period.
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