|Arsenic Removal From Drinking Water by Iron Removal, U.S. EPA Demonstration Project at Vintage on the Ponds in Delavan, WI, Six-Month Evaluation Report (PDF) (68 pp, 1.63 KB) (EPA/600/R-07/083) September 2007
This report describes the activities and results of the first six months of the arsenic removal treatment technology demonstration project at Vintage on the Ponds at Delavan, WI. The objectives of the project are to evaluate the:
The project is also characterizing the water in the distribution system and residuals produced by the treatment system.
Source water at Vintage on the Ponds contained 14.3 to 29.0 µg/L of total arsenic, with arsenic (III) being the predominating species at an average concentration of 16.7 µg/L. The source water also contained 1,165 to 2,478 µg/L of total iron present, mostly in the soluble form. The ratio of soluble iron to soluble arsenic concentrations was 78:1, indicating sufficient iron present in the source water for effective arsenic removal.
A Macrolite PM2162D6 system was installed to remove arsenic via iron removal from source water. The system consisted of one 21-inch by 62-inch contact tank and two 21-inch by 62-inch pressure vessels, each containing 5 cubic feet of Macrolite filter media. The treatment process included chlorine addition to oxidize arsenic (III) to arsenic (V) and iron (II) to iron (III), adsorption and/or coprecipitation of arsenic (V) onto/with iron (III) solids, and filtration of arsenic (V)-laden iron solids with the Macrolite media. The design flow rate was 45 gallons per minute (gpm) based on the well capacity, which yielded 1.8 minutes of contact time prior to filtration and 9.4 gallons per minute per square foot (gpm/ft2) of hydraulic loading to the filters. Because the actual treatment flow rates fluctuated with the water demand from the distribution system and never exceeded 20 gpm, the minimum contact time and the maximum hydraulic loading rate would be 4.1 minutes and 4.2 gpm/ft2, respectively.
From July 12, 2005, through January 17, 2006, the well operated for a total of 446 hours at 2.4 hours per day (hr/day) (on average). The treatment system processed approximately 1,031,200 gallons of water with an average daily demand of 5,485 gallons during this time period.
Due to the presence of approximately 3.0 milligrams per liter (mg/L) (as nitrogen) of ammonia in source water, chloramines were formed upon chlorine addition. The breakpoint chlorination was not performed because of the unrealistically high chlorine dosage (up to 23 mg/L [as chlorine]) that would be required to completely oxidize ammonia and chloramines formed during chlorination and because ammonia could be easily removed by the preexisting softener located downstream from the Macrolite pressure filters before water entered the distribution system.
For the first several months of operation, little or no chlorine residuals were detected in the treated water due to repeated operational problems with the chlorine feed system, including failures of the feed pump and the chlorine injector, pipe leaks due to incompatibility of plumbing materials with a 12.5 percent sodium hypochlorite solution, and difficulties associated with chlorine residual and chorine dosage measurements. After the working condition of the chlorine feed system was restored in late October 2005, chlorine dosing rates varied from 2.1 to 4.1 mg/L (as chlorine), although less than 1 mg/L (as chlorine) of chlorine residuals (chloramines) were being targeted in order to minimize adverse impact on the resins in the downstream softener. The erratic chlorine residual data might have been caused by the on-demand system operation, which had made it difficult to adjust the dosing rates.
The working condition of the chlorine addition system had direct impacts on the effectiveness of treatment. Among the six arsenic speciation sampling events that took place, there were two events when chlorine was not injected properly so that iron (II) and arsenic (III) were not oxidized or only partially oxidized, resulting in elevated soluble iron and arsenic (III) levels after treatment. For the other four events, when the chlorine system was in good working condition, iron (II) and arsenic (III) were mostly oxidized and total iron and arsenic were removed to less than 25 and 10 µg/L, respectively, after filtration. During this reporting period, total arsenic concentrations exceeded the MCL of 10 µg/L in 7 out of the 25 sampling events, mostly due to poor chlorine addition.
For the four speciation events meeting the treatment goals, arsenic (III) concentrations after the contact tank were reduced to 5.0, 5.8, 4.1 and 9.7 µg/L, respectively, and averaged 6.2 µg/L. This average arsenic (III) concentration corresponded to a 63 percent conversion rate, based on the average 16.7 µg/L of arsenic (III) in raw water. Arsenic (III) concentrations after filtration were 5.8, 5.9, 1.5, and 3.9 µg/L, respectively, and averaged 4.3 µg/L, suggesting that additional arsenic (III) oxidation (11 percent) might have occurred in the filters.
The conversion of arsenic (III) to arsenic (V) after the contact tank, however, was not as complete as that observed at many other sites where little or no ammonia was present in raw water, suggesting that the presence of ammonia in the Vintage’s raw water might have impacted the effectiveness of arsenic (III) oxidation. Although monochloramine was reported as an ineffective oxidant for arsenic (III) by other researchers, the observation at the Vintage suggested that when chlorine was added to the water, a fraction of the chlorine reacted with arsenic (III) before it was completely quenched by ammonia to form monochloramine.
Similarly, lower total and soluble iron concentrations were observed after the filtration vessels than after the contact tank (29 and less than 25 µg/L versus 1,363 and 520 µg/L [on average]). As expected, elevated total arsenic concentrations were associated directly with elevated total iron concentrations in the treated water after both filtration vessels.
Total manganese concentrations averaged 19.4 µg/L in source water, existing primarily in the soluble form as manganese (II). Manganese remained in the soluble form in the treated water at levels ranging from 17.0 to 20.2 µg/L, indicating insignificant oxidation of manganese by the addition of chlorine.
During the six-month period, the Macrolite system was backwashed approximately 60 times using treated water; each time generated approximately 720 gallons of wastewater. It processed 7,900 to 26,900 gallons of water between two consecutive backwash cycles; thus, the productivity of the filters was 91 to 97 percent. Backwash wastewater was sampled three times, including two with grab samples and one with composite samples. The composite samples were taken from a side stream of the backwash effluent, which presumably was more representative of the overall wastewater quality. The analyses of the composite samples showed 121 and 46 µg/L of total arsenic, 13,543 and 4,486 µg/L of total iron, and 26 and 22 µg/L of total manganese in the samples collected from Vessels A and B, respectively. The total suspended solids levels in the backwash water were uncharacteristically low at 5 and 12 mg/L, most likely due to insufficient mixing of solids/water before sample collection.
Comparison of the distribution system sampling results before and after the system operation showed a decrease in the arsenic, iron, and manganese levels at all three sampling locations. Although total arsenic levels in the distribution system were slightly higher (from 3.1 to 23.3 µg/L), they mirrored the total arsenic levels in the treated water (from 2.6 to 18.0 µg/L). Neither lead nor copper concentrations at the sample sites appeared to have been affected by the operation of the system.
The capital investment cost of $60,500 included $19,790 for equipment, $20,580 for engineering, and $20,130 for installation. Using the system’s rated capacity of 45 gpm (64,800 gallons per day [gpd]), the capital cost was $1,344 per gpm ($0.93 per gpd).
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