By Matthew Wirth
There is more than one way to (blank)…add your own analogy. This is true about many things, including removing arsenic from water. Whether it is ion exchange, RO, adsorption or coagulation/filtration, each method has its limits and standard operating practices (SOP) unique to each technology. Let it be said right now, none are reliable options without scheduled testing and routine preventive maintenance (PM) service. Being acquainted with the rudimentary SOP related to each technology is important before investigating possible corrective actions related to arsenic issues.
Anion exchange
Anion exchange will remove arsenic five, As(V), from water. It does not remove arsenic three, As(III). For anion exchange to work as an option, the arsenic must be in its pentavalent state. If the As is not naturally in the As(V) state, it requires conversion to As(V) by oxidation. Though there are other options for oxidizing As(III) to As(V), the recognized best practice is chlorination. Chlorination is quite effective, but difficult to control on a smaller scale. Because ion exchange (IEx) resin is extremely susceptible to chlorine degradation, the SOP for a system using chlorine, in conjunction with IEx, includes dechlorination prior to the IEx columns. As with any ion exchange technology, anion resin is a mass transfer medium: anions in, with a similar mass of chloride ions out. This technology has an extremely limited throughput and requires frequent regeneration compared to other arsenic options. The SOP for regenerating anion resin in this application is NaCl (salt) @ 8-10lbs/ft3. Check with manufacturers’ specifications for capacity and brining instruction. There are instances where anion exchange works well as a sole treatment. If the competing anions are low in quantity, a strong-base anion resin can have several thousand gallons of capacity per regeneration. This is where a quality analysis helps establish the right technology. If the challenge water has arsenic, but is low in overall total solids, then consider IEx as the technology of choice.
The anion resin does not discriminate. To date, there are no advertised specific arsenicselect IEx resins on the market. The anionic constituents in the water source determine the system’s capacity (throughput). Determining the actual throughput of a specific resin requires a complete water analysis. Be aware that different types of Type I and Type II IEx resins have different selectivity. Common Type I and Type II anion resins prefer sulfate (SO2-4) to As(V). Allowing an anion column to exhaust (overrun capacity) in the presence of SO2-4 can dump or release arsenic from the resin column. The SOP for avoiding this condition is allowing a 20- to 25-percent safety factor in calculating capacity. In addition, the SOP for monitoring this type of technology looks for frequent, if not daily, system checks. In general, if the system does not regenerate, it does not work and can increase arsenic levels as the system exhausts due to SO2-4 loading.1 The SOP has an anion column downstream of a cation column (softener.) Calcium hardness will cause calcium sulfate deposits on the anion resin and reduce its performance. The anion column is commonly larger than the cation column because cation resin, as a rule, has a larger capacity than anion resin on the same water source. Both the cation and anion systems experience resin attrition, with the anion resin commonly suffering a high rate of loss over time. Anion resin is typically less robust than cation resin. Practice PM and audit resin attrition to ensure the system remains functional—and do not run out of salt! Anion exchange offers possibly the most economic option for whole-house arsenic removal, but it requires the highest level of monitoring due to its multiple incidents of failure. The SOP for IEx as an As(V) treatment has a redundant posttreatment (POU adsorption filter, RO, etc.) to protect against possible dumping and system overrun.
Reverse osmosis
Like anion exchange, RO is intended for As(V) removal, not As(III). A functioning RO system removes As(V), but not every POU RO has NSF certification for As(V). Check with the regional regulations to understand POU requirements and check that the system is NSF certified for As before installing the system for As(V) removal. Be aware that NSF certification tests to specific challenge water. As(V) challenge is either 50 ppb (ug/L) or 300 ppb. With liability being part of a level-one constituent removal program, do the due diligence to ensure the use of the correct system.
The SOP requires proper pretreatment upstream of an RO system. Membrane element fouling causes an increased pressure differential (ΔP) across the element. This excess pressure can damage the element’s glue lines, O-rings, etc. The common maximum recommended pressure differential across a 40-inch element is 10 to 15 psi.2 Excess pressure on the membrane element can telescope the element windings (see Figure 1) or split glue lines where the membrane perimeter is glued to envelope the permeate carrier sheet. O-rings break, bulge and leak under pressure stress (Figure 2). Damage to the element seals allows slippage of untreated water into finished water. Apply PM to pretreatment and monitor the ΔP (change in pressure) across the membrane and conductivity of the permeate to ensure system functionality.
Reverse osmosis offers an economic option for treating a single POU faucet. Keeping an entire household or institution drinking from the correct faucet may be the true challenge with a POU approach. POE treatment using RO becomes a more complex project. RO water is aggressive and requires special distribution materials or neutralizing agents to adjust the pH and increase the water’s conductivity. The SOP with either option using RO for As(V) removal includes a PM schedule and routine monitoring of the water’s conductivity to ensure the system is functioning properly. Best practices for preventive maintenance are supplied by a professional water company. Homeowners and janitors are not the first choice as PM contractors. They lack the training and organization structure needed to provide PM for a level-one constituent.
Adsorption
The engineered media used to target arsenic are adsorptive. The common As adsorption media commercially available use iron, titanium and zirconium as host metals for attracting arsenic. Adsorptive means these media collect arsenic on their surfaces. Note: Adsorption means onto, while absorption means into. Most of the media offered today remove both As(III) and As(V). Check the manufacturer’s specification. The SOP dictates that an arsenic speciation be part of planning if one intends to use an engineered media for removing both species of arsenic. These media have a greater capacity to remove As(V) than As(III). The levels of As(III) in the water will dictate the overall throughput of the media and this can be considerably less than if the As is converted to As(V) prior to adsorptive treatment. Note: The best practice for doing a speciation test is to collect the water onsite using a special speciation collection kit. These kits allow the collector to treat the samples onsite to ensure accurate arsenic speciation. Collecting samples and transporting them untreated to a lab can affect the results because As(III) oxidizes and converts to As(V) during transportation. Silica, pH and phosphate are common detractors of media throughput for adsorption. In the presence of pH 7.5, elevated silica shortens bed life. Basically, the silica gets sticky and blinds off the adsorption site on the media material. A lower pH water (< 7.0) provides a longer bed life and pH water > 8.0 greatly shortens bed life. Phosphate (PO4) is a competing ion with arsenic. Phosphorus (P) is located just above arsenic (As) on the Periodic Table of Elements. They are both in the same family of non-metals and adsorption technologies do not discriminate between the two elements. Therefore, if PO4 is present, especially in high quantities measuring in the ppm range, it drastically reduces the media throughput.
Where pH, silica and/or phosphate are issues, the SOP is to adjust their levels to manufacturers’ recommendations or use a different technology. There are various techniques and options for adjusting the levels of interfering conditions. Best practices with adsorption would have a complete analysis, listing the constituents to proper detection levels recommended by the media manufacturer. In addition, the arsenic should be speciated and the supplier asked to estimate bed life based on existing constituents and following a proposed pretreatment regiment.
Adsorption offers an option for treatment of POE systems and extended run-life if the conditions are favorable for the media. It offers an economic option for high-volume users with the ‘right water.’ Several of the options are regeneration-capable, allowing for multiple use at the original installation site. SOPs would not use regenerated media of an unknown origin at a different site. Most of the adsorption media are okay for landfill disposal. Some media offer little or no backwash requirements, making them excellent for users with waste discharge issue.
Coagulation/filtration
Arsenic loves iron. It will easily form ferric arsenate in the presence of ferric iron. This makes coagulation, in conjunction with filtration, an effective method for removing iron. The SOP compensates from 20:1 to 50:1 or higher iron for arsenic. This is because arsenic is not the only customer for the iron present in most waters. Remember PO4? It will use the iron in the same manner as As(III) and As(V). For example, if the water holds arsenic at 20 ppb and PO4 at 30 ppb, then the iron content required to attract a combined 50 ppb at 20:1 is 1,000 ppb or one part per million.
One of the first municipal arsenic treatment plants put into service was in Fallon, NV. With arsenic levels as high as 160 ppm, US EPA required Fallon to install treatment.3 The Fallon systems, along with other municipal systems, utilize iron-based coagulants. These systems work by adding coagulants, such as ferric chloride, to the water. Often, there is not enough natural iron present in the water to accommodate the levels of arsenic and other iron-using constituents. The arsenic adsorbs onto positively charged ferric hydroxide particles. In Fallon, microfiltration removes these particles from the water before it goes to distribution. Other systems will utilize backwashing multi-media or catalytic media horizontal or vertical filters. The SOP recommends full-time monitoring of large systems using coagulation and filtration to ensure the coagulant ration is correct and that the filters are functioning effectively. Coagulation with filtration is a very economical method for treating arsenic water. This assumes that waste discharge is not an issue and that qualified monitoring exists. Feeding coagulant correctly and keeping large filters operational is not a project for novices.
Conclusion
While there are successes using residential oxidizing iron filtration equipment for the removal of arsenic, these applications should include aggressive PM schedules. In addition, complete and comprehensive testing is just as important with coagulation/filtration as adsorption—or any of the other technologies. The best standards of practice do not rely on homeowners for monitoring complex systems. There is always risk involved when accepting responsibility for protecting others from possible harm. The forget rate among homeowners is high when it comes to monitoring water treatment systems, but their memories are quite good when it comes to laying blame. Be careful out there.
References
- Michaud, C.F. (2013), Personal discussions.
- https://knowledgecentral.gewater.com
- http://news.pall.com/article_display.cfm?article_id=4215
About the author
Matthew Wirth, Technical Advisor and Trainer for Pargreen Water Technologies (www.pargreen. com), is a second-generation water professional with over three decades in the industry. He received engineering training at the South Dakota School of Mines and Technology, Rapid City, SD and also earned a BA Degree in organizational management and communications from Concordia University, St. Paul, MN. In addition, Wirth holds a Water Conditioning Masters License in the State of Minnesota. A contributing author to WC&P, he is also a member of its Technical Review Committee. Wirth can be contacted via email, [email protected] or phone, (630) 443-7760.