This report documents the activities performed and the results obtained from the first six months of the arsenic removal treatment technology demonstration project at the community of Prospect Bay at Grasonville in Queen Anne's County, MD. The objectives of the project were to evaluate the effectiveness of Severn Trent Services (STS) SORB 33™ media in removing arsenic to meet the new arsenic maximum contaminant level (MCL) of 10 µg/L. Additionally, this project evaluates the reliability of the treatment system (Arsenic Package Unit [APU]-300), the simplicity of required system operation and maintenance (O&M) and operator's skills, and the cost-effectiveness of the technology. The project also characterizes the water in the distribution system and process residuals produced by the treatment process.
The STS system consisted of two 63-inch-diameter, 86-inch-tall fiberglass reinforced plastic (FRP) vessels in parallel configuration, each containing approximately 80 ft3 of SORB 33T media. The media is an iron-based adsorptive media developed by Bayer AG and packaged under the name SORB 33™ by STS. The system was designed for a flowrate of 320 gallons per minute (gpm) (160 gpm to each vessel), corresponding to a design empty bed contact time (EBCT) of about 3.8 minutes per vessel and a hydraulic loading to each vessel of 7.4 gpm/ft2.
The treatment system began regular operation on June 30, 2004. The types of data collected included system operation, water quality (both across the treatment train and in the distribution system), process residuals, and capital and O&M costs. Through the period June 30 through December 30, 2004, the APU-300 system operated an average of 6.3 hours per day for a total operating time of 1,082 hours. The system treated approximately 14,856,000 gallons of water, or 12,400 bed volumes (BV), which was approximately 11% of the vendor-estimated working capacity for the SORB 33™ media. Total arsenic concentrations in raw water ranged from 18.3 to 25.8 µg/L with As(III) being the predominating species, averaging 18.7 µg/L. By the end of September 2004, the arsenic concentration in the treated water exceeded the target concentration of 10 µg/L after approximately 7,400 BVs of water treated. To improve arsenic removal by the media, prechlorination was implemented in early November. (Prior to this, chlorine was added at the end of the treatment train.) Arsenic in samples collected following prechlorination existed primarily as As(V) and particulate As, indicating the effectiveness of chlorination in oxidizing As(III) to As(V). In the week following the switch to prechlorination, arsenic in the treated water existed primarily as As(III) (i.e., 10.4 out of 12 µg/L). Within two weeks of switching to prechlorination, total arsenic concentrations in the treated water reduced to 0.9 µg/L.
Because there was no on-site disposal facility for the backwash water and because there was little change in the differential pressure during the first four months of operation, the system was backwashed only once during the first six months of operation. The backwash was initiated manually and the backwash water was discharged into a tanker truck and transported to the Stevensville Wastewater Treatment Plant (WWTP) for disposal. Each vessel was backwashed separately at a flowrate of 200 gpm for a period of 20 to 25 minutes, generating approximately 9,500 gallons of backwash water. Soluble arsenic concentrations in the backwash water were 5.4 and 3.4 µg/L from Vessels A and B, respectively, which was significantly lower than those in the raw water that was used for backwash, indicating some arsenic removal by the media during backwash.
Results of the distribution system sampling showed a distinct effect of the treatment system on the arsenic concentrations in the treated water. The results mirrored those seen from the treatment system sampling, as As concentrations dropped once the system was put into service, rose gradually during the first four months of operation as As(III) began to break through, and then went down again once the switch to prechlorination was made. The APU-300 did not appear to have an effect on the Pb or Cu levels in the distribution.
The capital investment cost of $211,000 included $129,500 for equipment, $4,907 for site engineering, and $19,580 for installation. Using the system's rated capacity of 320 gpm (460,800 gallons per day [gpd]), the capital cost was $659/gpm ($0.46/gpd) and equipment-only cost was $405/gpm ($0.28/gpd). These calculations did not include the cost of the building construction.
O&M costs included only incremental costs associated with the adsorption system, such as media replacement and disposal, chemical supply, electricity, and labor. Although media replacement and disposal did not take place during the first six months of operation, the vendor estimated $26,800 to change out both vessels. This cost was used to estimate the media replacement cost per 1,000 gallons of water treated as a function of the projected media run length to the 10 µg/L arsenic breakthrough.