|Arsenic Removal From Drinking Water by Ion Exchange, U.S. EPA Demonstration Project at Fruitland, ID, Final Performance Evaluation Report (EPA/600/R-10/152) November 2010
This report documents the activities performed and the results obtained from this 32-month demonstration study, which evaluated a Kinetico ion exchange (IX) system to remove arsenic and nitrate from source water at the City of Fruitland in Idaho. The 250-gallon-perminute (gpm) IX system consisted of a bank of five sediment filters, two 48-inch-by-72-inch pressure vessels (configured in parallel), one 15-ton saturator, one 685-gallon day tank, and ancillary equipment. Each resin vessel contained 50 cubic feet of A300 E strong base anionic exchange resin manufactured by Purolite.
The 32-month demonstration study was divided into three major study periods: Study Period I extended from June 14, 2005, through July 25, 2006; Study Period II extended from July 25, 2006, through June 18, 2007; and Study Period III extended from June 18, 2007 to February 11, 2008. Study Period I evaluated performance of the IX system in a cocurrent regeneration mode. Due to leakage of both arsenic and nitrate after regeneration, attempts were made to switch the regeneration process from co- to counter-current mode in Study Period II. However, a series of mechanical failures was encountered while switching from co- to counter-current regeneration, causing the IX resin to foul. Therefore, Study Period III was devoted to resin cleaning, using a caustic/brine mixture before returning to regular but brief system performance evaluation.
Routine system performance evaluation was conducted in Study Period I, when the IX system operated in the co-current regeneration mode. During this period, the IX system operated for a total of 6,836 hours, averaging 17.4 hours per day. The system treated approximately 65,423,000 gallons of water with an average daily production of 166,895 gallons per day (gpd). The average flowrate was 157 gpm, which was 63 percent of the 250-gpm design flowrate. This average flowrate yielded an empty bed contact time of 4.8 minutes and a hydraulic loading rate of 6.2 gpm per square foot to each IX resin vessel.
Total arsenic concentrations in raw water ranged from 33.6 to 60.8 micrograms per liter (μg/L) and averaged 42.5 μg/L, which existed primarily as Arsenic(V). Nitrate concentrations ranged from 6.9 to 11.5 milligrams per liter (mg/L) (as N) and averaged 10.0 mg/L (as N). The water also contained, on average, 19.4 μg/L of uranium, 39.3 μg/L of vanadium, 59 mg/L of sulfate, 0.32 mg/L of phosphorus (as P), 57 mg/L of silica (as SiO2), and 387 mg/L of alkalinity (as CaCO3). After treatment, total arsenic and nitrate were reduced to below the respective maximum contaminant levels, except when the system was freshly regenerated or experiencing mechanical problems. Near complete removal of uranium, vanadium, and molybdenum by the IX system also was observed.
Sulfate, the most preferred anion by the IX resin, was removed from an average of 59 mg/L in raw water to less than 1 mg/L in the treated water for most sampling events, except when the system was experiencing mechanical problems. Raw water pH values ranged from 6.7 to 7.9. A significant reduction in pH in the treated water was observed immediately after resin regeneration, presumably due to the removal of bicarbonate ions by the freshly regenerated IX resin, as evidenced by the corresponding decrease in total alkalinity.
In addition to routine sampling, six run length and two regeneration (or elution) special studies were performed during Study Periods I and II. The purpose of the run length studies was to delineate arsenic and nitrate breakthrough behavior and determine the resin run length between two consecutive regeneration cycles. Based on the results of these special studies and routine sampling across the treatment train, the resin run length was upwardly adjusted from the initial factory setting of 214,000 gallons (or 286 bed volume [BV]) to 335,000 gallons (or 448 BV), then downwardly adjusted several times to 316,000 gallons (or 422 BV), 275,000 gallons (or 368 BV), 260,000 gallons (or 348 BV), and finally 220,000 gallons (or 294 BV) by the end of evaluation study. Effluent samples collected from the IX vessels indicated arsenic and nitrate leakage during the first 50,000 to 60,000 gallons (or 67 to 80 BV) of throughput.
The IX system was regenerated in a downflow co-current mode during Study Period I using brine at a target salt level of 10 pounds per cubic foot of resin. Triggered automatically by pre-set throughput in the programmable logic controller, the two IX vessels were regenerated sequentially, each cycling through the steps of brine draw, slow rinse, and fast rinse before returning to service. A total of 202 regeneration cycles took place during Period I, consuming approximately 271,640 pounds of salt. Depending on regeneration settings, average salt usage per regeneration cycle increased from 1,129 pounds to as high as 1,736 pounds and then decreased to 945 pounds, equivalent to a regeneration level of 11.3, 17.4, or 9.5 pounds per cubic foot. The regeneration settings were adjusted multiple times to reach 9.5 pounds per cubic foot regeneration level, which was within 5 percent of the target value of 10 pounds per cubic foot. Key settings included brine concentration, brine draw time, and brine draw flowrate. The system production efficiency was 98 percent considering the amount of treated water used for regeneration.
The purpose of the two regeneration (or elution) studies was to evaluate the effectiveness of the IX resin regeneration process and characterize the residuals produced. Although the majority of arsenic and nitrate on the resin was eluted during the brine draw and slow rinse steps, arsenic concentrations as high as 35 μg/L were still measured toward the end of the fast rinse step. Therefore, it was not surprising to detect over 10 μg/L of arsenic during subsequent service runs. Extending the fast rinse time from 6 to 15 minutes did not resolve the problem because the leakage was found to continue up to 52,000 gallons (or 70 BV) of throughput, or approximately 3 to 4 hours into service runs. The regeneration waste stream discharged to the sewer contained an average of 1.9 mg/L of arsenic and 0.31 grams per liter of nitrate, equivalent to a mass loading of 47 grams for arsenic and 7.9 kilograms for nitrate per regeneration cycle, based on the wastewater samples collected during nine regeneration events.
Attempts were made in Study Period II to convert the IX system from co- to countercurrent regeneration. The conversion, however, was unsuccessful due to various mechanical difficulties. Improper IX resin regeneration for an extended period during Study Period II resulted in resin fouling, which caused deteriorating resin performance. The fouled IX resin was cleaned with a 5 percent sodium hydroxide/10 percent brine mixture followed by regular co-current regeneration. Although the analytical data of IX resin samples showed some effectiveness, the system performance did not improve after the caustic/brine cleaning. The early leakage of arsenic and nitrate continued to exist after the system was reverted back to the co-current regeneration mode.
The capital investment cost was $286,388, which included $173,195 for equipment, $35,619 for site engineering, and $77,574 for installation. This capital cost was normalized to the system’s rated capacity of 250 gpm (360,000 gpd), which resulted in $1,146 per gpm ($0.80 per gpd). Funded separately by the City of Fruitland, the cost associated with the new building, sanitary sewer connection, and other discharge-related infrastructure was not included in the capital cost.
The operation and maintenance (O&M) cost for the IX system included the incremental cost associated with the salt supply, electricity consumption, and labor. Over the first year of system operation, the cost for salt supply was $0.49 per 1,000 gallons of water treated, which could be reduced to $0.35 per 1,000 gallons if a target salt usage rate of 3.16 pounds per 1,000 gallons was reached. The majority of the O&M cost was incurred by salt supply.
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