Arsenic Removal from Drinking Water by Iron Removal and Adsorptive Media
U.S. EPA Demonstration Project at Stewart, MN
Six-Month Evaluation Repor
(75 pp, 2299 Kb)
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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 Stewart, MN facility. The main objective of the project is to evaluate the effectiveness of Siemens' Type II AERALATER® system for iron removal and AdEdge Technologies' Arsenic Package Unit (APU)-300 system for subsequent arsenic removal. The effectiveness is evaluated based on the system's ability to remove arsenic to below the new arsenic maximum contaminant level (MCL) of 10 µg/L. Further, this project also 1) evaluates the reliability of the treatment system for use at small water facilities, 2) determines the required system operation and maintenance (O&M) and operator skill levels, 3) characterizes process residuals generated by the treatment process, and 4) determines the capital and O&M cost of the technology. The types of data collected include system operation, water quality (both across the treatment train and in the distribution system), process residuals, and capital and O&M cost.
The 250-gal/min (gpm) treatment system consists of an AERALATER® pretreatment unit and an APU-300 arsenic removal unit. Used for iron removal, the 11-ft diameter x 26-ft carbon steel AERALATER® package unit is composed of an aeration tower, a detention tank, and a four-cell gravity filter in one stacked circular configuration. The effluent from the gravity filter is subsequently polished with AD-33 media, an iron-based adsorptive media developed by Bayer AG for arsenic removal. The APU-300 system consists of two skid-mounted 63-in x 86-in fiberglass vessels configured in parallel. Each vessel contains 64 ft3 of pelletized AD-33 media supported by gravel underbedding.
The treatment system began routine operation on January 18, 2006. Through the period from January 30 to August 1, 2006, the system treated approximately 10,039,000 gal of water with an average run time of 4.9 hr/day. The average daily demand was 54,822 gal with the peak daily demand of 126,779 gal occurring on July 12, 2006. Water to the treatment system was supplied by two wells (i.e., Wells No. 3 and 4) each operating at an average flowrate of 194 and 184 gpm, respectively, on an alternating basis. These reduced flowrates resulted in longer contact times (i.e., 44 to 46 min versus the design value of 34 min) within the AERALATER® detention tank and lower hydraulic loading rates (i.e., 1.9 to 2.0 gpm/ft2 versus the design value of 2.6 gpm/ft2) to the gravity filter. The corresponding flowrates measured through the APU-300 system also resulted in longer empty bed contact time (EBCT) (i.e., 4.6 to 6.8 min compared to the design value of 3.8 min) in each vessel. No significant operational or mechanical issues were experienced during the six-month study period.
The source water contained 35.5 to 56.4 µg/L of total arsenic, with As(III) at an average concentration of 34.9 µg/L as the predominant species. With NaMnO4 addition prior to aeration (based on February 2, 2006 data), most As(III) was oxidized to As(V), which, along with the pre-existing As(V), was partially adsorbed onto and co-precipitated with iron solids also formed during this preoxidation step, resulting in 57% As(V) removal. The arsenic-laden iron solids were effectively removed by the gravity filter, achieving approximately 60% total arsenic and 100% total iron removal. The untreated arsenic was present mostly as As(V) at 17.2 µg/L, which was subsequently removed by the AD-33 media during the polishing step. The higher-than-expected amount of As(V) in the gravity filter effluent was thought to have been caused by the relatively high levels of pH, competing anions (such as phosphorous and silica), and total organic carbon in source water.
NaMnO4 addition was inadvertently discontinued after one week of operation due to problems with the chemical feed pump. Total arsenic removal was 34% and the iron removal rate 100% across the gravity filter. The oxidation of Fe(II) was accomplished through aeration. It was also observed that the oxidation of As(III) to As(V) was occurring at a rate of over 95% across the gravity filter due to natural biological processes with only 1.2 µg/L of As(III) in the filter effluent. The As(V) concentration averaged 24.5 µg/L after the gravity filter. Nitrification was also observed to within the gravity filter, but was not related to the microbially-mediated As(III) oxidation as noted in this report.
In both cases, the levels of As(V) remained above 10 µg/L in the gravity filter effluent, which required further polishing in the APU-300 unit. Through 10,900 bed volume (BV), the effluent arsenic concentration averaged 3.1 µg/L in the APU-300 effluent.
Comparison of the distribution system sampling results before and after system startup showed a significant decrease in arsenic concentration from an average of 31.2 to 5.5 µg/L. However, the average arsenic concentration in the distribution system at 5.5 µg/L was higher than the average arsenic concentration of 0.9 µg/L following the AD-33 adsorption vessels. Iron and manganese also were significantly reduced in the distribution system.
AERALATER® backwash was manually initiated by the operator on a weekly basis. The APU-300 system was backwashed manually on two occasions during the six-month study period. Approximately 168,900 gal of wastewater, or 1.7% of the quantity of the treated water, was generated during the first six months from the AERALATER®. The AERALATER® backwash water contained, on average, 108 mg/L of total suspended solids (TSS), 46 mg/L of iron, 415 µg/L of arsenic, and 68 µg/L of manganase with the majority existing as particulate. The average amount of solids discharged per backwash cycle was approximately 6.1 lb, which was composed of 2.6 lb of elemental iron, 0.004 lb of elemental manganese, and 0.02 lb of elemental arsenic. In addition, 13,472 gal of wastewater were generated by the APU-300 unit or 0.1% of the quantity of treated water.
The capital investment for the system was $367,838, consisting of $273,873 for equipment, $16,520 for site engineering, and $77,445 for installation, shakedown, and startup. Using the system's rated capacity of 250 gpm or 360,000 gal/day (gpd), the capital cost was $1,471 per gpm of design capacity ($1.02/gpd). This calculation did not include the cost of the building to house the treatment system. The O&M cost consisted primarily of the media replacement cost, which was estimated by the vendor at $41,370 to change out the AD-33 media. The O&M cost is presented as a function of potential media run length and will be refined in the Final Evaluation Report once the actual bed volumes to breakthrough become available.
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