Catching a Killer
pH adjustments can increase the arsenic-removing capacity of adsorptive media
- By Peter Ritchey
- Jan 01, 2006
Arsenic (As) removal from drinking water by adsorptive media has been a proven technology for years. In the western and southwestern United States, however, much of the As-tainted groundwater has a pH value that is high enough to significantly reduce the operating capacity of adsorptive media, including the iron-, alumina-, and titanium-based media commercially available to municipalities. To lower the treatment costs for these higher-pH waters, pH adjustment has been successfully employed to increase the capacities of all of these media. A pH adjustment involves the use of an acid (an inorganic acid such as sulfuric or hydrochloric) or even gases such as carbon dioxide to depress the pH to a suitable treatment range of 6 to 7. While typically associated with larger flow rates from centralized water treatment plants, pH adjustment has also been adapted to lower flow rates from individual wells or even small community water supplies with flow rates of 50 gallons per minute (gpm) and lower. The pH adjustment systems are fully automated and require little operator maintenance or supervision.
On Jan. 22, 2001, the U.S. Environmental Protection Agency (EPA) adopted a new standard for arsenic in drinking water at 10 parts per billion (ppb) from its previous standard of 50 ppb. The revision will provide additional protection for 13 million Americans who are currently drinking water with arsenic levels greater than 10 ppb against cancer and other health problems, including cardiovascular disease and diabetes, as well as neurological effects.1 The previous standard of 50 ppb was set by the EPA in 1975 and based on a Public Health Service standard originally established in 1942. The new standard, requiring compliance by public water systems beginning Jan. 23, 2006, will finally elevate the federal standard to that of other developed countries. In 1993, the World Health Organization (WHO) set 10 ppb as the recommended limit for arsenic in drinking water while the 15-nation European Union, confirming the science behind the WHO limit, also adopted 10 ppb as its mandatory standard for arsenic in drinking water in 1998.2
Arsenic is a common, naturally occurring drinking water contaminant that originates from arsenic-containing rocks, minerals, and ores. It is transported to natural waters through weathering, erosion, and dissolution. While occurring in both the inorganic and organic forms, arsenic is predominantly inorganic in natural waters and is the most likely form of arsenic at concentrations that cause regulatory concern.3 Predominately found in groundwater as opposed to surface water, arsenic exists in drinking water in two primary valence forms: arsenate (As5+) and arsenite (As3+). The relative concentrations of each depend primarily on the geology of the surrounding rock formations and the water chemistry at each individual site. Within the two valence forms, arsenic exists in four different species for each valence state. Arsenite can exist as H3AsO3, H2AsO3-, HAsO32-, and AsO33- and arsenate as H3AsO4, H2AsO4-, HAsO42-, and AsO43-. The overall negative charges on the arsenic species is the inherent property that allows its removal from drinking water using adsorption, ion exchange, or co-precipitative processes.
Arsenic Adsorption on Media
The negative charge of arsenic species allows for its removal via adsorptive processes. The two predominant "sorption" processes that bind the arsenic ions to an adsorbent are ligand exchange and surface complexation, also known as inner-sphere complex formation. With the ligand exchange, the hydroxide groups on the surface of the adsorbent are replaced with the arsenic species as shown below:
With inner-sphere complex formation, the arsenic species are selectively bound to the oxide surface through the formation of inner-sphere complexes in which one or two of the arsenate's oxygen atoms bond to surface of the adsorbent as shown below:
The two attractive forces binding the arsenic to the adsorbent's surface during inner-sphere complex formation are coulombic bonding and a Lewis Acid-Base attraction.
Both adsorption processes are influenced by several fundamental properties of the untreated water -- most notably the pH of the water and the level of competing ligands, such as hydroxide (hence the pH dependence), silica, phosphate, vanadium, and selenium. With the concentration of arsenic in the parts-per-billion range, these competing ligands, often present in the groundwater at part-per-million levels, can significantly lower the arsenic capacity of any media. pH adjustment can therefore be used to suppress the presence of the ligand with the highest selectivity -- OH-. For most commercially available adsorptive media, the optimal pH is less than 7.5, and preferably a pH of 7 for optimal arsenic capacity.
Breakthrough column testing results (see Figure 1) completed on NSF-53 challenge water shows the dramatic impact of pH on both an activated alumina- and iron-based adsorptive media. At a contact time of 1.5 minutes, the capacity of the adsorptive media all show a dramatic increase in capacity with decreasing pH. For the iron-based media number 1, the ~7,500 BV capacity at pH 8.5 increases by 100 percent to 15,000 by decreasing the pH to 7.5 to 7.6. A similar decrease in pH increases the modified alumina capacity by more than 300 percent.
pH Effect On Adsorptive Media Arsenic Capacity
To translate the adsorptive media capacities into an operating cost, an all-inclusive (media, changeout, and disposal) costing of $100/cu. ft. and $170/cu. ft. for the activated alumina- and iron-based medias, respectively, for large commercial-scale systems was used. Additionally, large-scale systems operate at longer contact times (typically 3- to 4- minute EBCT) and a ratio factor between the laboratory and commercial EBCT (3 minutes / 90 seconds = 2) was used to determine the capacity at full-scale operating conditions. The operating costs can be effectively reduced from $1.52 to $0.47 per 1,000 gallons for the best performing iron-based media and from $2.67 to $0.35 per 1,000 gallons, respectively, based on ANSI NSF-53 water quality at an inlet arsenic V concentration of 50 ppb and a pH change from 8.5 to 6.5.
pH Adjustment -- Equipment and Controls
pH adjustment is a simple unit operation that can use a standard acid such as sulfuric or hydrochloric, or a more benign option -- carbon dioxide (CO2) gas. The acid is typically stored in plastic tanks and added or injected into the raw water line, typically prior to a static mixer. The static mixer provides turbulence to ensure that the acid is fully dispersed into the raw water. Next, a pH probe measures the pH of the adjusted water to ensure that the adsorption system will be operated at the appropriate pH.
pH Adjustment Costing
The costs for pH adjustment are primarily based on the acid consumption and are laid out in detail in the referenced EPA document.4 While the most accurate method for determining acid consumption is acid titration, a straightforward calculation can suffice when a raw water analysis is available and raw water samples are not. This method requires the pH, the total alkalinity (M as mg/L CaCO3), and/or the free CO2 (CO2 as mg/L). In the following example, sulfuric acid was used to decrease the pH of a sample of groundwater from 8.1 to 7. An acid cost of $0.10 per pound was used, which includes the acid and its storage and feed. In a trial-and-error iteration, a 25 mg/L reduction in the bicarbonate level to 125 mg/L as CaCO3 and its associated increase in free CO2 to 24 mg/L will yield a pH 7 water with 125 mg/L bicarbonate and 24 mg/L of free CO2.
As an example, a pH adjustment from pH 8.1 to 7.5, 7, or 6.5 requires a reduction in bicarbonate of 7, 25, and 65 mg/L as CaCO3, respectively. This reduction is achieved via the acid addition at an incremental increase in operating cost of less than $0.01 per 1,000 gallons, $0.02 per 1,000 gallons, and $0.05 per 1,000, respectively.
The cost of arsenic treatment can be dramatically reduced via this pH reduction. Adding in the loaded acid costs outlined above, an arsenic treatment cost ranging from $1.50 to $4.00/1,000 gallon can be substantially reduced to $0.40 to 0.70/1,000 gallon.
EPA is currently sponsoring two rounds of demonstration projects to develop more cost-effective technologies for arsenic removal, in addition to providing technical assistance and training to operators of small systems to reduce their compliance costs. The specific sites in the two rounds of demonstration projects were chosen to provide a cross-section of water qualities from several different states -- of the 12 Round-1 sites, nine are represented and of the 29 Round-2 sites, 17 are represented.
The projects were "bid" upon by solution providers who provided the capital and operating costs associated with their particular systems. This process, in effect, showcases the most cost-effective technologies and allows the bid selection panel to evaluate each technology on a site-by-site basis. Arsenic removal by adsorption was the dominant technology in both rounds of demonstration. In addition, pH adjustment was used in the bulk of these "small system" projects that had flow rates greater than that associated with a point-of-use application.
Water Quality Considerations
pH adjustment of water does have the potential for negative effects on water quality if not performed properly. The most noticeable effect could be the formation of "red" water, which is commonly associated with the dissolution of previously formed scaling on the inner surface of water pipe.
By calculating the Langelier Saturation Index (LSI) of both the raw water and pH-adjusted water, one can determine the potential for "red" water. The LSI is a simple prediction of the tendency of water to precipitate, dissolve, or be in equilibrium with calcium carbonate. Expressed as the difference between the actual pH and the saturation pH, the LSI can be positive (scale can form), negative (scale will dissolve), or zero (borderline scale potential).
From the example above for NSF-53 Challenge Water, the LSI for the pH 8.1 water is 0.234. Adjusting the pH of the water to 7.5 results in an LSI of -0.943. In this example, the water shifted from "borderline scale potential" to "scale-dissolving". The water could, therefore, potentially dissolve the existing deposits on the interior of any piping and fitting and could potentially lead to an unacceptable water quality at the tap.
While the addition of water stabilizers such as poly- or ortho- phosphates can remedy this problem, awareness of this potential will help minimize its impact on the end consumer.
pH adjustment is a fully implementable technology to substantially lower the cost of arsenic treatment. Operating costs for compliance with EPA's 2001 National Primary Drinking Water Regulation for Arsenic can be dramatically lowered for all adsorption-based media by using pH adjustment. pH adjustment allows all media to operate under conditions that boost their operating capacities and therefore, lower the total operating costs for arsenic removal. Based on the individual water quality, the lowest treatment cost technology can quickly switch from one media to another and municipalities and engineering firms should thoroughly evaluate these factors prior to recommending an arsenic treatment technology. The pH adjustment technology can be applied to large-, medium-, and small-scale treatment facilities with a minimum amount of effort from the operation and maintenance workforce.
- Fact Sheet: "Drinking Water Standard for Arsenic," EPA 815-F-00-015, January 2001.
- Council of the European Union, "Council Directive 98/83/EC of November 1998 on the quality of water intended for human consumption," Official Journal of the European Communities, May 12, 1998, pgs. L330/32-L330/52.
- Edwards, M., S. Patel, L. McNeil, H. Chen, M. Frey, A. D. Eaton, R. C. Antweiler, and H. E. Taylor, 1988. "Considerations in As Analysis and Speciation." J. AWWA (March): 103-113.
- Rubel Jr., Frederick, March 2003, Design Manual: Removal of Arsenic from Drinking Water by Adsorptive Media, EPA/600/R-03/019
This editorial originally appeared in the January/February 2006 issue of Water & Wastewater Products, Vol. 6, No. 1
This article originally appeared in the 01/01/2006 issue of Environmental Protection.