|Arsenic Removal from Drinking Water by Iron Removal U.S. EPA Demonstration Project at Vintage on the Ponds in Delavan, WI Final|
This report documents the activities performed and the results obtained for the arsenic removal treatment technology demonstration project at Vintage on the Ponds in Delavan, WI. The objectives of the project were to evaluate 1) the effectiveness of a Kinetico Macrolite® pressure filtration system in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 μg/L, 2) the reliability of the treatment system; 3) the required system operation and maintenance (O&M) and operator skill levels; and 4) the capital and O&M cost of the technology. The project also characterized water in the distribution system and process residuals produced by the treatment system.
The Macrolite® pressure filtration system removed arsenic via iron removal from source water. The system consisted of one 21-in × 62-in contact tank and two 21-in × 62-in pressure vessels, each containing 4.8 ft3 of Macrolite® filter media, a spherical, low-density ceramic media manufactured by Kinetico for high-flow filtration. The treatment process included chlorine addition to oxidize As(III) to As(V) and Fe(II) to Fe(III), adsorption and/or coprecipitation of As(V) onto/with iron solids, filtration of As(V)-laden particles with the Macrolite® media, and softening (preexisting). The design flowrate was 45 gal/min (gpm) based on the well capacity, which yielded 1.8 min of contact time prior to filtration and 9.4 gpm/ft2 of hydraulic loading to the filters. Because the actual flowrates 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 min and 4.2 gpm/ft2, respectively. From July 12, 2005, through September 3, 2006, the well operated for a total of 1,072 hr at 2.6 hr/day (on average). The treatment system processed approximately 2,500,200 gal of water with an average daily demand of 5,981 gal during the study period.
Source water at Vintage on the Ponds contained 14.3 to 29.0 μg/L of total arsenic with As(III) as the predominating species at an average concentration of 16.3 μg/L. Source water also contained 997 to 2,478 μg/L of total iron present mostly in the soluble form. The average soluble iron concentration was 80 times the average soluble arsenic concentration and thus was sufficient for effective arsenic removal via iron removal.
Due to the presence of approximately 2.9 mg/L of ammonia (as N) in source water, chloramines were formed upon chlorination. Breakpoint chlorination was not performed because it would require a unrealistically high chlorine dosage (i.e., up to 22 mg/L [as Cl2]) to obtain free chlorine and because ammonia could be easily removed by the preexisting softener units located downstream from the pressure filters.
For the first three months of system operation, little or no chlorine residual was detected in the treated water due to repeated operational problems with the chlorine feed system. After the working condition of the chlorine feed system was established in late October 2005, both chlorine dosing rates (based on chlorine tank level measurements) and total chlorine residuals (measured in the system effluent) varied widely from 1.3 to 5.9 mg/L and from <0.1 to 4.7 mg/L (as Cl2), respectively. These values were much higher than the 1-mg/L target level recommended for the downstream softener units. The erratic chlorine residuals observed might have been caused, in part, by the on-demand system operation, which made it difficult to adjust the dosing rates.
The working condition of the chlorine addition system had direct effects on the effectiveness of the treatment system. Of the 14 arsenic speciation sampling events that took place, there were two where the chlorine injection system did not work properly. Under the circumstances, soluble Fe(II) and As(III) were either not oxidized or only partially oxidized, resulting in elevated soluble iron and soluble As(III) levels after Macrolite® filtration. For the other 12 events where the chlorine addition system was in good working order, soluble As(III) concentrations were reduced to 4.6 μg/L after the contact tank and then to 2.9 μg/L after the pressure filters. Meanwhile, particulate arsenic concentrations increased to 10.8 μg/L after the contact tank and then decreased to 1.2 μg/L after the pressure filters (except for one sampling event where particulate arsenic breakthrough was observed due to a system backwash failure). As expected, total arsenic concentrations increased with total iron concentrations in the filter effluent. Soluble iron levels were reduced to an average of 39 μg/L after the pressure filters.
Due to the presence of chloramines, incomplete As(III) and Fe(II) oxidation was observed, with as much as 4.6 and 429 μg/L (on average) of As(III) and Fe(II), respectively, measured after the contract tank. Additional contact time in the pressure filters appeared to have enhanced oxidation of As(III) and Fe(II), reducing their concentrations to 2.9 and 39 μg/L (on average), respectively, in the filter effluent.
Total manganese concentrations averaged 19.2 μg/L in source water, existing primarily in the soluble form as Mn(II). Manganese remained in the soluble form in the treated water at levels ranging from 16.1 to 20.8 μg/L, indicating insignificant oxidation of manganese by chloramines. Soluble Mn(II) was almost completely removed by the downstream softener units.
During the performance evaluation study, the pressure filters were backwashed 102 times using chlorinated water from the contact tank. Each backwash generated approximately 360 gal of wastewater. Backwash wastewater was sampled nine times, including two grab samples and seven 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 11.7 to 322 μg/L of total arsenic, 0.27 to 37.1 mg/L of total iron, and 16.5 to 32.9 μg/L of total manganese. Total suspended solids (TSS) levels in the backwash wastewater were uncharacteristically low at 13.2 mg/L (on average), most likely due to insufficient mixing of solids/water mixtures before sampling.
Comparison of the distribution system water sampling results before and after system startup showed a decrease in arsenic, iron, and manganese levels at all three sampling locations. Total arsenic levels in the distribution system ranged from 3.1 to 23.3 μg/L, which, although slightly higher, mirrored the total arsenic levels in filter effluent. Neither lead nor copper concentrations appeared to have been affected by the operation of the system.
The capital investment cost was $60,500, which included $19,790 for equipment, $20,580 for engineering, and $20,130 for installation. Using the system’s rated capacity of 45 gal/min (gpm) (64,800 gal/day [gpd]), the capital cost was $1,344/gpm ($0.93/gpd).
The O&M cost for the system included only incremental cost associated with the chemical supply, electricity consumption, and labor. The O&M cost was estimated at $0.26/1,000 gal.
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