|Arsenic Removal From Drinking Water by Coagulation/Filtration, U.S. EPA Demonstration Project at Village of Pentwater, MI, Final Performance Evaluation Report (EPA/600/R-08/011) January 2008
This report describes the activities and results of the arsenic removal treatment technology demonstration project at the Village of Pentwater, Michigan, facility. The objectives of the project were to evaluate the:
The project also characterized the 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.
After review and approval of the engineering plan by the state, the FM-260-AS treatment system was installed and became operational on November 22, 2005. The system consisted of one 96–inch by 96–inch steel contact tank and two 60–inch by 96–inch steel pressure tanks configured in parallel. Each pressure tank was loaded with 40 cubic feet of Macrolite media to which filtration rates up to 9.3 gallons per minute per square foot (gpm/ft2) were applied The system used an existing chlorination system to oxidize arsenic (III) and iron (II) and the contact tank to improve the formation of arsenic (V)-laden iron particles prior to filtration. An iron addition system was installed midway through the study to improve arsenic removal.
On average, the system operated at approximately 350 gallons per minute (gpm) for 5.1 hours per day, producing 39,185,000 gallons of water through December 8, 2006. This average flow rate corresponded to a contact time of 6.8 minutes and a filtration rate of 8.9 gpm/ft2.
Several problems were encountered during the demonstration study, including programmable logic controller settings, backwash and service flow rates, media loss, influent pressure spikes, and chlorine addition. The actions taken to address these problems are detailed in the report.
Source water had an average pH value of 7.9 and contained 14.6 to 21.8 µg/L of total arsenic. The predominant arsenic species was arsenic (III) with an average concentration of 14.9 µg/L. Total iron concentrations ranged from 346 to 510 µg/L, which mostly existed in the soluble form. Chlorine was used to oxidize arsenic (III) and iron (II). Although breakpoint chlorination likely was achieved during most of the study period, chloramines might have been formed due to the occurrence of 0.3 milligrams per liter (mg/L) (as nitrogen) of ammonia in source water, causing incomplete arsenic (III) oxidation. As a result, as much as 1.6 µg/L of arsenic (III) was measured in the treated water. Total arsenic concentrations in the treated water ranged from 7.8 to 15.6 µg/L and averaged 9.9 µg/L. After months of system operations, provisions were made to add ferric chloride at an average dosage of 0.5 mg/L (as iron) to improve arsenic (V) removal. This pretreatment raised iron concentrations following the contact tank to 658 to 1,638 µg/L, thereby lowering the average arsenic concentration to 5.6 µg/L in the treated water.
The treatment system decreased arsenic levels in the distribution system from 16.5 to 7.5 µg/L. Iron and manganese levels also were reduced from 192 to less than 25 µg/L and from 23.8 to 13.7 µg/L, respectively. Alkalinity, pH, and lead levels did not appear to be affected.
Filters were backwashed automatically about 3 times per week triggered by 24–hour service time or 48–hour standby time. Approximately 749,800 gallons of wastewater, or 1.9 percent of the amount of water treated, was generated during the study. Without iron addition, the backwash wastewater contained 252 to 646 mg/L of total dissolved solids (TDS) and 24 to 166 mg/L of total suspended solids (TSS). With iron addition, TDS ranged from 354 to 498 mg/L and TSS from 160 to 282 mg/L with the majority existing as particulates. The backwash solids contained approximately 2.10 pounds of iron, 0.03 pounds of manganese, and 0.03 pounds of arsenic.
The capital investment of $334,573 included $224,994 for equipment, $30,929 for site engineering, and $78,650 for installation, shakedown, and startup. Using the system’s rated capacity of 400 gpm (576,000 gallons per day [gpd]), the capital cost was $836 per gpm ($0.58 per gpd). These calculations do not include the cost of the building to house the treatment system.
The O&M cost, estimated at $0.17 per 1,000 galloons, included only the incremental expenses for chemicals, electricity, and labor. Because chlorine addition already existed prior to the demonstration study, the incremental cost for chemical usage was for iron addition only.
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