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 EPA/600/R-08/141

 

Arsenic Removal from Drinking Water by Adsorptive Media U.S. EPA Demonstration Project at Queen Anne’s County, Maryland
December 2008

This report documents the activities performed and the results obtained from the arsenic removal treatment technology demonstration project at the community of Prospect Bay at Grasonville in Queen Anne’s County, MD. The main objective of the project was 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 g/L. Additionally, this project evaluated 1) the reliability of the treatment system (Arsenic Package Unit [APU]-300) 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 cost of the technology. 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 (both across the treatment train and in the distribution system), process residuals, and capital and O&M cost.

The STS system consisted of two 63-in-diameter, 86-in-tall fiberglass reinforced plastic (FRP) vessels in parallel configuration, each containing approximately 80 ft3 of SORB 33™ 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 300 gal/min (gpm) (150 gpm to each vessel), corresponding to a design empty bed contact time (EBCT) of about 4.0 min per vessel and a hydraulic loading rate of 6.9 gpm/ft2. Actual flowrates through the system averaged 207 gpm, corresponding to an EBCT of 5.6 min in Vessel A and 6.0 min in Vessel B.

Upon review and approval of the engineering plan by the State, the APU-300 treatment system was installed and became operational on June 30, 2004. From June 30, 2004, through April 2, 2007, the APU-300 system operated an average of 6.2 hr/day for a total operating time of 5,890 hr. The system treated approximately 71,533,000 gal of water, or 59,800 bed volumes (BV), which was approximately 52% of the vendor-estimated working capacity for the SORB 33™ media. Several problems were encountered during the performance evaluation study, including the need to implement prechlorination, shortened run times between backwashes, and equipment malfunctions. The corrective actions taken to address the problems are detailed in the report.

Total arsenic concentrations in raw water ranged from 16.0 to 25.8 g/L with soluble As(III) being the predominating species, averaging 18.9 g/L. After treating only 7,400 BV of water, the arsenic concentration in the treated water exceeded the target concentration of 10 g/L. To improve arsenic removal by the media, prechlorination was implemented in early November 2004. (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 effective As(III) oxidation. Since then, arsenic removal improved significantly, with its concentrations in the treated water decreasing from over 10 to 0.9 g/L within two weeks following the switch to prechlorination. Total arsenic concentrations in the treated water remained at levels less than 10 g/L (averaged at 2.1 g/L) during the remainder of the performance evaluation study.

The APU-300 system was designed and programmed with an automatic backwash feature that would trigger backwash by either a differential pressure (p) setting of 10 pounds per square inch (psi) or a timer. However, backwash of the system was initiated manually by the system operator because there was no onsite disposal facility to receive the backwash wastewater, which had to be discharged into a tanker truck and transported to the Stevensville Wastewater Treatment Plant (WWTP) for disposal.
When post-chlorination was performed during the first four months of system operation, the adsorption vessels were not backwashed because there were little or no changes in pressure across both vessels. The pressure readings began to rise once the switch to prechlorination had been implemented. The run times between backwashes shortened significantly from the initial 2,359 BV to less than 800 BV (or 2 month/backwash to less than 2 week/backwash). Media attrition appears to be the main reason for the shortened run times and more frequent backwash.

During the 33-month performance evaluation study, the adsorption system was backwashed a total of 50 times, generating 229,646 gal of wastewater. The backwash wastewater contained 100 to 430 mg/L of total dissolved solids (TDS) and 12 to 130 mg/L of total suspended solids (TSS). The backwash solids generated by each backwash cycle contained approximately 1.3 lb of iron, 0.003 lb of manganese, and 0.02 lb of arsenic.

Results of the distribution system sampling showed a distinct effect of the treatment system on arsenic concentrations in the treated water. The treatment system decreased arsenic levels in the distribution system from an average of 19.0 to 8.4 μg/L prior to the switch to prechlorination and to 3.6 μg/L after the switch to prechlorination. The results from the distribution system sampling mirrored those seen from the treatment system sampling, with As concentrations dropped once the system was put into service, rose gradually as As(III) began to break through during the first four months of system operation, 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 system.

The capital investment cost of $211,000 included $129,500 for equipment, $36,700 for site engineering, and $44,800 for installation. Using the system’s rated capacity of 300 gpm (or 432,000 gal per day [gpd]), the unit capital cost was $703/gpm (or $0.49/gpd). This calculation does not include the cost of the building to house the treatment system. O&M cost, estimated at $0.31/1,000 gal, included only the incremental cost for electricity, replacement parts, and labor. The estimated media changeout cost was $27,728, which would represent the majority of the O&M cost. Media changeout did not occur during the performance evaluation period.

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Thomas Sorg


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