|Arsenic Removal From Drinking Water by Iron Removal, U.S. EPA Demonstration Project at Climax, MN, Final Performance Evaluation Report (EPA/600/R-06/152) December 2006
This report describes the activities and results of Kinetico’s Macrolite arsenic removal treatment technology demonstration project at Climax, Minnesota. The objectives of the project were to evaluate the:
The project also characterized the water in the distribution system and residuals produced by the treatment system process.
The Macrolite FM-236-AS arsenic removal system consisted of two 42-inch-diameter by 72-inch-tall contact tanks (345 gallons) and two 36-inch-diameter by 72-inch-tall filtration vessels (264 gallons). Each contained 14 cubic feet of Macrolite media. The system also included two chemical addition assemblies, one each for prechlorination and supplemental iron addition.
Prechlorination was used to oxidize arsenic (III) to arsenic (V) and form arsenic (V)-laden iron solids prior to the Macrolite pressure filtration. The design flow rate was 140 gallons per minute (gpm), which yielded 5 minutes of contact time prior to pressure filtration and 10 gallons per minute per square foot (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 hours at approximately 5.6 hours per day. Based on the totalizer-to-treatment readings, the system treated approximately 13,829,000 gallons of water, with an average daily water demand of 38,560 gallons 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, which were later resolved.
Total arsenic concentrations in source water ranged from 31.2 to 51.4 µg/L, with arsenic (III) being the predominating species at an average concentration of 35.8 µg/L. Iron in raw water existed primarily in the soluble form with an average value of 485 µg/L. This amount of soluble iron corresponded to a 13:1 ratio of iron to arsenic, 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 µg/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 milligrams per liter (mg/L) (as iron). Total arsenic levels in the treated water were reduced to 6.0–9.3 µg/L with no exceedances of arsenic above the 10-µg/L MCL. A slight increase in particulate iron was observed in the Macrolite filter effluent, with concentrations increasing from less than 25–36.8 µg/L before iron addition to less than 25–104 µg/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 hours did not appear to have caused significant penetration of particles through the Macrolite filters. The filters were set for backwash at a 20-pounds-per-square-inch increase in differential pressure across the filters, with 24 hours of run time or 48 hours of standby time.
After adjustments were made to the backwash control settings, the rate of backwash water generation was reduced to approximately 1.6 percent of the amount of treated water produced. The backwash water contained relatively low levels of soluble arsenic (8.7 µg/L on average) and soluble iron (86.4 µg/L on average). Total arsenic levels ranged from 1,420 to 1,850 µg/L and total iron levels ranged from 74.2 to 97.6 mg/L. The iron levels in the solids ranged from 2.46 × 105 to 3.12 × 105 micrograms per gallon (µg/g) and the arsenic levels ranged from 3,830 to 4,540 µg/g.
Given an average total suspended solid loading of 233 mg/L and 1,000 gallons per backwash event, approximately 1.9 pounds of solids were generated per backwash event. The backwash solids passed the Toxicity Characteristic Leaching Procedure (TCLP) test for all analytes, with only barium showing detectable concentrations ranging from 0.189 to 0.231 mg/L. The TCLP regulatory limit set by EPA is 5 mg/L for arsenic and 100 mg/L for barium. As such, the backwash solids were nonhazardous.
After iron addition, arsenic levels in the distribution system water samples averaged 10.3 µg/L; this was higher than the average arsenic level in the treated water, which was 7.4 µg/L. The higher arsenic levels in the distribution system are an indication of potential solubilization, destabilization, or desorption of arsenic-laden particles and scales in the distribution system.
Total iron levels in the distribution system at an average of 74.7 µg/L were also higher in the distribution system, compared to the average value of 41.8 µg/L in the treated water.
Manganese levels were generally lower in the distribution system samples at 33.8 µg/L, compared to 83.4 µg/L in the treated water.
Lead levels in the distribution system were not affected by the treatment system.
Copper concentrations appeared to have increased with concentrations ranging from 53 to 1,027 µg/L after system startup, but the treatment system did not appear to affect the pH, temperature, or hardness of the water in the distribution system.
The capital investment of $270,530 included $159,419 for equipment, $39,344 for engineering, and $71,767 for installation. The equipment cost can vary based on the level of preassembly, automation, and instrumentation included on the system. Using the system’s rated capacity of 140 gpm (201,600 gallons per day [gpd]), the capital cost was $1,932 per gpm ($1.34 per gpd). These calculations did not include the cost of a building addition to house the treatment system. The total capital cost of $270,530 was converted to a unit cost of $0.35 per 1,000 gallons, using a capital recovery factor of 0.09439 based on a 7 percent interest rate and a 20-year return period. These calculations assumed that the system operated 24 hours a day, 7 days a week, at the system design flow rate of 140 gpm. The system operated only 5.6 hours per day and produced 13,829,000 gallons of water during the study period. At this reduced usage rate, the total unit cost was increased to $1.85 per 1,000 gallons.
The O&M cost for the system included only incremental expenses associated with the chemical supply, electricity consumption, and labor. The total O&M cost was estimated at $0.29 per 1,000 gallons. Based on the actual water usage rate and capital and O&M costs, the total cost for arsenic removal incurred during the one-year demonstration study period was estimated at $2.14 per 1,000 gallons.
Thomas Sorg (513) 569-7370
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