|Arsenic Removal From Drinking Water by Coagulation/Filtration. U.S. EPA Demonstration Project at Town of Arnaudville, LA, Final Performance Evaluation Report (EPA/600/R-11/060) May 2011
This report documents the activities performed during and the results obtained from the arsenic removal treatment technology demonstration project at the United Water Systems’ facility in Arnaudville, Louisianan. The objectives of the project were to evaluate the:
The project also characterized water in the distribution system and residuals generated by the treatment process. The types of data collected included system operation, water quality, process residuals, and capital and O&M costs.
Upon approval of the engineering plan by the Louisiana Department of Health and Hospitals (LADHH), the treatment system was installed and became operational on June 23, 2006. The system consisted of a 5,000-gallon contact tank (converted from a pre-existing aeralater) and two 84-inch by 96-inch steel pressure tanks configured in parallel. Each pressure tank was loaded with 75 cubic feet of Macrolite media, a spherical, low density, and chemically inert ceramic media, to which filtration rates up to 10 gallons per minute (gpm) per square foot (at a design flowrate of 770 gpm) might be applied. The actual flowrate was 335 gpm on average.
Due to the presence of ammonia (1.9 milligrams per liter (mg/L) [as nitrogen]) and total organic carbon (TOC) (1.3 mg/L) in source water, potassium permanganate was selected as the oxidant to oxidize arsenic(III) (24.4 μg/L [on average]) and iron (II) (1,906 μg/L [on average]). After arsenic-laden iron solids had been removed by the pressure filters, the treated water was softened, with 30 percent bypass, and chlorinated before entering the distribution system.
Source water was supplied by two 10-inch production wells, Wells No. 1 and No. 2, at 350 and 375 gpm, respectively. Quality of well water from both wells was similar, containing 24.1 to 43.0 μg/L of arsenic (existing mostly as soluble arsenic[III]), 1,477 to greater than 3,000 μg/L of iron (existing almost entirely in the soluble form), and 96.2 to 196 μg/L of manganese (also existing almost entirely in the soluble form).
Because the aeralater was used not only as a contact tank, but also for aeration (although unintentionally), a number of operational and performance issues occurred during the performance evaluation study. After approximately five months into system operation, extensive biofouling became evident, causing the filters to be backwashed up to eight times per day (from one to two times per day after system startup). Aeration in the aeralater, with an average dissolved oxygen (DO) level of 5.5 mg/L, apparently had caused biological activities, including nitrification, to occur. To curb continuing biological activities in the filters, several actions were taken, including performing a hydrochloric acid/caustic wash of the filter media, replacing potassium permanganate with gas chlorine, and turning off the blower of the aeralater. Due to the presence of elevated soluble arsenic(V) in the filter influent/effluent, a system modification application package was prepared and submitted to LADHH for supplemental iron addition. While the benefit of supplemental iron usage was inconclusive, extra solids loading to the filters caused them to be backwashed more frequently (from one to two times per day to two to three times per day). Iron addition was discontinued after 19 months.
Although the ratio of soluble iron to soluble arsenic in source water was over 65 (on average)—a value higher than the rule-of-thumb value of 20—elevated soluble arsenic(V) (close to or over 10 μg/L) continued to be measured through the most of the four-year study period. Factors affecting removal of soluble arsenic(V) in the filter influent might include elevated phosphorus levels (648 μg/L [on average]), elevated silica levels (42.5 mg/L as silicon dioxide [on average]), and elevated DO levels due to aeration in the aeralater. Aeration continued even after shutdown of the blower (DO levels reduced from 5.5 to 2.4–3.4 mg/L) and removal of aluminum trays (DO levels further reduced to 2.4 mg/L). Aeration discontinued only after the aeralater had been bypassed (DO levels further reduced to 0.5 mg/L). The presence of oxygen might have caused some soluble iron to precipitate (even though potassium permanganate or chlorine was added ahead of the aeralater), rendering it ineffective to remove soluble arsenic(V) via adsorption and/or co-precipitation. A series of jar tests was conducted onsite to examine the authenticity of this postulation.
Results of distribution system water sampling before and after system startup indicated that the water quality in the distribution system was comparable to that of the pressure filter effluent. Thus, the treatment system appeared not to have beneficial effects on arsenic, iron, and manganese concentrations. Arsenic concentrations remained essentially unchanged from baseline levels; iron and manganese concentrations actually increased slightly. Alkalinity, pH, and lead concentrations also increased slightly. Copper concentrations increased rather significantly from the average baseline level of 108 μg/L to 267 μg/L.
Analyses of backwash wastewater samples indicated that approximately 4.9 pounds of solids (including 0.01 pounds of arsenic, 1.8 pounds of iron, and 0.08 pounds of manganese) would be discharged, assuming that 87.8 mg/L of total suspended solids and 6,752 gallons of wastewater would be generated during each backwash event.
The capital investment for the treatment system was $427,407, including $281,048 for equipment; $50,770 for site engineering; and $95,589 for installation, shakedown, and startup. Using the system’s rated capacity of 770 gpm (or 1,108,800 gallons per day [gpd]), the capital cost was $555 per gpm (or $0.38 per gpd). This calculation did not include the cost of the building to house the treatment system. O&M cost, estimated at $0.07 per 1,000 gallons, included only the incremental cost for chemicals, electricity, and labor. Since chlorine addition already existed prior to the demonstration study, the incremental cost for chemical usage was for iron addition only.
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