Abstract

This report documents the activities performed and the results obtained from the first six months of the EPA arsenic removal technology demonstration project at the Oak Manor Municipal Utility District facility at Alvin, Texas. The main objective of the project is to evaluate the effectiveness of the Severn Trent Services Arsenic Package Unit (APU)-30S in removing arsenic to meet the maximum contaminant level of 10 micrograms per liter (μg/L). Additionally, this project evaluates 1) the reliability of the treatment system for use at small water facilities, 2) the required system operation and maintenance (O&M) and operator skill levels, and 3) the capital and O&M costs of the technology. The project also characterizes water in the distribution system and residuals generated by the treatment process. The types of data collected include system operation, water quality, process residuals, and capital and O&M costs.

After approval of a pilot-study exception request and engineering plans by the Texas Commission of Environmental Quality, the APU-30S system was installed and started up on April 25, 2006. The system consisted of two 63-inch-diameter and 86-inch-tall adsorption vessels configured in series with 53.6 cubic feet (ft³) of SORB 33™ in the lead vessel and 70.3 ft³ in the lag vessel, gas prechlorination equipment, sample taps, and associated instrumentation. At the design flow rate of 150 gallons per minute (gpm), the system had a hydraulic loading rate of 6.9 gallons per minute per square foot (gpm/ft²) and an empty bed contact time (EBCT) of 6.2 minutes. Based on the actual flow rate of only 134 gpm, the system operated at a hydraulic loading of 6.2 gpm/ft² and an EBCT of 6.9 minutes.

Source water supplied by two wells (Well 1 and 2) had a combined average concentration of 43.8 μg/L for total arsenic, with arsenic (III) as the predominating soluble species at 35.2 µg/L. Iron existed mostly in the particulate form, with concentrations ranging from 34.2 to 100 μg/L and averaging 60.5 μg/L. Total manganese concentrations averaged 54.4 μg/L, existing almost entirely in the soluble form. After prechlorination, arsenic (III) was effectively oxidized to arsenic (V), with concentrations averaging 0.6 and 27.1 μg/L, respectively. Somewhat unexpectedly, manganese (II) also was effectively oxidized, presumably, to manganese dioxide, leaving only 2.8 μg/L (or 6.5 percent) in the chlorinated water.

By the end of the first six months of system operation, after treating approximately 11,241,500 gallons (12,170 bed volumes [BV]) of water (1 BV = 124 ft³ of media in both the lead and lag vessels), arsenic concentration was 10.2 µg/L after the lead vessel and 1.4 µg/L following the lag vessel. Because the arsenic concentration following the lag vessel did not reach 10 µg/L, the media in the lead vessel was not changed out during the first six months of system operation.

Comparison of the distribution system sampling results before and after the system startup showed a considerable decrease in arsenic (38.2 to 2.0 μg/L), iron (115 to <25 μg/L), and manganese concentration (41.8 to 1.3 µg/L). Alkalinity, pH, lead, and copper did not appear to be affected.

Backwash was manually initiated by the operator when differential pressure across Vessel A reached 10 pounds per square inch, which occurred four times during the six-month period. About 6,058 gallons per vessel per event of wastewater was discharged to the roadside ditch during each backwash event. Approximately 14.9 pounds of solids were discharged from Vessel A, including 4.2 × 10^-5 pounds of arsenic, 0.9 pounds of iron, and 0.08 pounds of manganese. Approximately 2.9 pounds of solids were discharged from Vessel B, including 1.5 × 10^-4 pounds of arsenic, 0.2 pounds of iron, and 0.03 pounds of manganese. The reasons for the large amount of solids produced are being investigated and will be reported in the Final Performance Evaluation Report.

The capital investment for the system was $179,750, consisting of $124,103 for equipment, $14,000 for site engineering, and $41,647 for installation, shakedown, and startup. Using the system’s rated capacity of 150 gpm (or 216,000 gallons per day [gpd]), the capital cost was $1,198 per gpm (or $0.83 per gpd). This calculation does not include the cost of the building to house the treatment system.

O&M cost, estimated at $0.21 per 1,000 gallons, included only the incremental cost for labor. There was no incremental electricity cost or chemical consumption cost because gas chlorination was already performed prior to the demonstration study.

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Thomas Sorg

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