| Abstract |
This report documents the activities performed and the results obtained for the arsenic removal treatment technology demonstration project at the Climax, Minnesota, site. The objectives of the project were to evaluate: (1) the effectiveness of Kineticos Macrolite pressure filtration process in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 mg/L; (2) the reliability of the treatment system; (3) the required system operation and maintenance (O&M) and operators skills; and 4) the capital and O&M costs of the technology. The project also characterized water in the distribution system and process residuals produced by the treatment system. The Macrolite FM-236-AS arsenic removal system consisted of two 42-in-diameter by 72-in-tall contact tanks (345 gal), and two 36-in-diameter by 72-in-tall filtration vessels (264 gal), each containing 14 ft3 of Macrolite media. The system also included two chemical addition assemblies, one each for prechlorination and supplemental iron addition. Prechlorination was used to oxidize As(III) to As(V) and form As(V)-laden iron solids prior to the Macrolite pressure filtration. The design flowrate was 140 gal/min (gpm), which yielded 5 min of contact time prior to pressure filtration and 10 gpm/ft2 of hydraulic loading rate to the filters. From August 11, 2004, through August 12, 2005, the system operated for a total of 2,086 hr at approximately 5.6 hr/day. Based on the totalizer to treatment readings, the system treated approximately 13,829,000 gal of water with an average daily water demand of 38,560 gal during this time period. The system operated in the service mode within the flow and pressure specifications. Operational issues related to the automated backwash process led to a number of backwash failures, but were later resolved. Total arsenic concentrations in source water ranged from 31.2 to 51.4 mg/L with As(III) being the predominating species at an average concentration of 35.8 mg/L. Iron in raw
water existed primarily in the soluble form with an average value of 485 mg/L. This amount of soluble iron corresponded to an iron:arsenic ratio of 13:1 given the average soluble iron and soluble arsenic levels in raw water. From August 11, 2004, to January 3, 2005, total arsenic levels in the treated water averaged 14.1 mg/L, indicating the need for supplemental iron addition to improve arsenic removal. Supplemental iron addition using ferric chloride was initiated on January 3, 2005, with an average iron dosage of approximately 0.85 mg/L (as Fe). Total arsenic levels in the treated water were reduced to 6.0 to 9.3 mg/L with no exceedances of arsenic above the 10-mg/L MCL. A slight increase in particulate iron was observed in the Macrolite filter effluent with concentrations increasing from <25 to 36.8 mg/L before iron addition to <25 to 104 mg/L after iron addition. However, filtration of arsenic-laden particles at a hydraulic loading rate of up to 10.7 gpm/ft2 (compared to 2 gpm/ft2 for conventional gravity filters) and a median filter run time of 11 hr did not appear to have caused significant penetration of particles through the Macrolite filters. The filters were set for backwash at 20 lb/in2 increase in differential pressure across the filters, 24 hr of run time, or 48 hr of standby time.
|
