By Matthew Wirth and Kevin Osborn
They say a day is never a waste if we learn something new. In the world of arsenic remediation, learning something new is the norm, not the exception. With the severity of water contamination varying greatly throughout the arsenic (As) regions of the world, new challenges frequently arise and addressing these challenges takes knowledge and creativity. Whether it is particulates, iron, phosphate, silica, pH, chlorine or something else, the water is seldom just right for the technology in play. Working from accurate water analyses is the first step in eliminating arsenic. Having accurate water usage patterns and flowrates is the second step down the path to avoiding the bad and the ugly. This article looks at the various arsenic remedies and the conditions of operation that affect their application. In addition, it looks at the ugly things that can happen when bad planning supersedes good practices.
Arsenic business review
While the arsenic business is in its infancy on the POU, POE and small public water system (PWS) side of the industry, knowledge of arsenic as a poison goes back centuries. Arsenic (As) is a non-metal in the same chemical family as phosphorus (P) and other Group 15 non-metals in the Periodic Table of Elements. The properties of arsenic are relatively simple chemistry. Arsenic is a naturally occurring mineral found in groundwater throughout the US and the world. It predominantly exists in water as arsenate As+5 and arsenite As+3. Other names used in the industry for arsenate are arsenic five, As+5, AsO4–3 or pentavalent arsenic. Arsenite’s aliases are arsenic three, As+3, AsO3–3 or trivalent arsenic.
The good: the technologies
Specialty medias. There are a number of specialty media options available for arsenic removal. The common metals used in arsenic remediation are iron, titanium and zirconium. The most common are iron and titanium-based materials because arsenic readily binds to their oxides. Some manufacturers use granular material as a host carrier for hydroxide iron. Others synthetically manufacture iron hydroxide to create an adsorptive arsenic removal media. The industry refers to these materials as granular ferric oxide or granular ferric hydroxide (GFO/GFH, see Figure 1.) They are effective choices for As+3 and As+5 adsorption. They do require backwashing to avoid channeling and to clear fines from the bed. Titanium is a crystalline titanium oxide (TiO2) that requires the same backwashing equipment as the GFO/GFH applications. Again, it is an effective choice for arsenic removal. Zirconium binds arsenic too, and has applications where it works well—usually found in a cartridge platform. All these metals work to attract and tightly hold arsenic to them. Depending on the manufacturer, they have NSF 61 certification for material safety and pass US EPA’s Toxicity Characteristics Leaching Procedure (TCLP) tests for disposal. The equipment required for these systems are a simple backwashing filter control and an appropriately sized pressure vessel. In addition to granular and crystalline media, there are several manufacturers using a resin matrix for the backbone of their adsorption media. Hydroxide iron is either coated onto the bead or part of the bead itself. Hybrid resins can function in non-backwashing applications. Resin beads (see Figure 2) offer a more laminar flow pattern and are less susceptible to the creation of fines. Granular and/or crystals have a turbulent flow pattern (see Figure 3) that offers a more violent environment within the vessel.
Co-precip. Simple iron/oxidation filtration removes limited As+5 from water by what is referred to as coagulation/precipitation (co-precip). Through this process in iron-bearing water, ferric iron present in the water attracts As+5. The bonding of arsenate to iron creates ferric arsenite (FeAsO4), which precipitates as floc and is mechanically strained from the water through filtration. This process effectively reduces arsenic in the filter effluent if the iron-to-arsenic ratio is correct. Another use of co-precip filtration technology is to add ferric salts (ferric chloride) to the water to attract more As+5 and improve the removal percentage. Note: Studies show that the ratio of iron to arsenic is 20 ppb Fe to 1 ppb As+5 (or an even 50:1). This means that water with 50 ppb of As+5 requires 1,000 to 2,500 ppb (1 to 2.5 ppm) of iron to reduce As+5 levels to below 10 ppb (US EPA’s MCL.) The pH and phosphate levels in the water affect this ratio, and these conditions are described later in the article.
Ion exchange (IEx). Arsenate (As+5) will exchange with chloride in an anion exchange column; arsenite (As+3) will not. Total capacity of the particular anion exchange media is crucial when applying this technology as well as comprehensive water testing data to quantify interfering factors like nitrate, sulfate, silicate and alkalinity. When used in conjunction with an adsorptive media, it is effective in lowering pH (adsorption technologies have a longer life in a lower pH:, i.e., 6.5 to 7.0) and removal of the phosphate that competes with arsenic for adsorption sites.
Reverse osmosis. RO will effectively remove As+5; RO membranes are not as effective for the removal of As+3. There are a number of RO units available that are NSF certified for As+5 removal. Some units have postfilters for controlling As+3. They key is to recognize if the RO unit certification is correct for the intended use. NSF-certification challenge water used to test the system may not be as severe as the water at the intended installation site. Always ensure that a certified system will function properly on the installation source water. It must be known if the arsenic is in the As+5 or As+3 state, by doing an arsenic speciation test. Caution: As+3 will oxidize and become As+5 if it is exposed to air at atmospheric conditions or chemically oxidized. Ensure the speciation test takes this into consideration before sending water in for testing.
The bad: conditions affecting the arsenic technologies
Reverse osmosis. RO is non-selective; i.e., it does not target any one ion, thus making it unpredictable across a wide range of water severities for the removal of select ions. It has a waste stream of rejected water and this is not allowable under some regions’ discharge rules. Remember, an RO membrane is not an effective remediation tool for removing arsenite As+3. If the RO system has an As+3 polisher, scheduling the replacement of this polisher can be problematic if one does not know the effective throughput of the polisher.
IEx. Ion exchange systems must regenerate to work properly. If the system runs out of salt, the system stops working. This often disqualifies it as a choice for the sole treatment of arsenic. Without continued supervision, there is no guarantee that the system is functional. Additionally, there are mechanical operations (i.e., timers, injectors, meters, moving parts, resin attrition and fouling, etc.) to consider. There is plenty of support for redundancy for IEx applications. Any IEx system for level-one contaminants should include redundancy: either repetitive equal treatment or a polisher utilizing an adsorptive or alternate treatment.
Co-precip. Water chemistry in the source water can and does change. The test used in original sizing is a snapshot in time: a single moment. Whether using the existing iron present in a water supply to remove As+5 or adding ferric chloride to compensate for the lack of existing iron, it is possible for iron levels or arsenic levels in the source water to change or chemical feed pumps to fail. As a single treatment for arsenic, co-precip requires continuous testing and supervision to ensure that it is functioning. Both arsenic in the water and competing constituents, such as phosphate (PO4) and phosphorus (P), compete for free iron in the water. This ratio of iron (AsOH5 and PO4) is 20:1 (or an even 50:1). Example: 50 ppb of As+5 and 0.8 ppm (800 ppb) of phosphate is calculated as: 50 + 800 = 850; 850 x 20 = 17,000. 17,000 ppb of iron is 17 ppm. Very few waters containing arsenic have 17 ppm of iron present.
Specialty media. Adsorptive media have a finite life expectancy. They are affected by changes in water chemistry and quality of pretreatment. Without continual source water and effluent testing, it is difficult to know exactly when the system exhausts. Metering the effluent is only a secondary predictor of bed life. Only through testing can one know when the arsenic breaks through. Non-backwashing hybrid resins need clean water. They cannot remain functional if they plug with solids or microbiological matter. While the granular backwashing material can handle slightly higher solids content in the feedwater, they are not filters. As with any filter, backwash flowrates and pressure are critical to operation. If the filters plug or go without adequate backwash, they can channel or load with fines and solids. In addition, backwashing systems require operational controls and are subject to many of the same mechanical conditions as ion exchange. Backwashing systems require supervision and maintenance to remain functional.
The ugly: when things go terribly wrong
No matter which technology is in play, they are all subject to fluctuating conditions in the source water and, in some cases, the design of the equipment supporting the technology. Whether it is a change in the influent water, a prefiltration failure or a mechanical failure in membranes and/or controls, stuff happens. Following are some of the conditions to be aware of with arsenic remediation technologies.
IEx. If an anion resin is installed for arsenic remediation, whether as a pretreatment for adsorption to lower pH and control phosphate or a primary treatment system, it cannot be allowed to exhaust or run out of salt. Depending on whether the resin is a Type I or Type II, in the presence of more highly selective ions (i.e., sulfate) the arsenic concentration in the effluent can be higher than in the source water. Important: Always check with the resin manufacturer for the best choice of resin for the application.
Capacity estimates. These are estimates, not guarantees. Continual testing must be part of the ongoing maintenance of any adsorptive system. When predicting bed life, changes in water chemistry and/or inaccurate preliminary chemistry are common problems with adsorptive technologies. Because phosphate, silica and pH all affect bed life, a prediction of bed life without consideration of these items is folly. If original conditions change, the bed’s effective lifetime will change. Monitoring the water chemistry on both sides of treatment is a good practice with any treatment for harmful conditions.
Iron breakthrough. Media filters and co-precip iron filtration are both subject to iron breakthrough. If iron passes through a filter or releases from an iron-based media, arsenic can piggyback through the system on the iron as a ferric arsenate. Remember that 0.30-ppm iron content in the effluent is 300 parts per billion of iron—do the math. If metal-based media develop fines due to the violence of backwash or hydraulic shock, those fines may carry adsorbed arsenic. In addition, hybrid polystyrene resins do not perform well with high levels of chlorine or organic acids. They can break down and create excessive pressure drop and premature failure.
Arsenic speciation. While most specialty media manufacturers state their media removes both As+3 and As+5, their capacities for each species is not the same. As+5 is more readily removed by adsorption than As+3. If the ratio of As+3 to As+5 changes due to feedwater conditions and/or poor oxidation, the effective capacity of the media changes. It is possible to lose 75 percent or more of the media’s capacity if the arsenic changes from As+5 to As+3. RO membranes (as well as ion exchange media) do not effectively remove As+3. Changes in the ratio of As+3 to As+5 can render these technologies ineffective as protection from arsenic.
Monitoring. Lights, timers, TDS meters, water meters, etc. are all ways of indicating that a system requires maintenance. All are reliable and unreliable. An RO is non-selective. If a TDS monitor indicates 10-percent bleed through in the permeate, how does one know what is in the 10 percent? If a meter logs 100,000 gallons (378,500 liters), what is the accuracy and was the 100,000 gallons effectively treated? In arsenic remediation, the only true monitoring is water testing. Every system needs some safeguards in the way of secondary indicators, but the best way to know if everything is working is to test the water.
Conclusion
As water treatment professionals, it is unethical to abandon one’s customers when treatment is difficult. In the same way, it is not right for customers to attribute all responsibilities to the water treatment professional and require that they assume all the risk. Removing arsenic from water is a team effort. A professional tries to educate the customer about their issues and select the best technology for that particular water. The customer must acknowledge that they ‘own’ the problem and be proactive in monitoring the system for performance—or pay someone to do it. As with any project requiring a high level of supervision and risk, review the process with the customer. Identify their responsibilities and your responsibilities. Create a document stating that everyone understands the conditions of the sale, operation and maintenance. Document who is responsible for testing, monitoring and waste disposal. Have all the affected parties sign the document before bringing the system online. A professional protects both their business as well as the customer’s welfare.
About the authors
Matthew Wirth, Water Treatment Specialist–Industrial/Municipal Equipment for Layne Christensen, Inc., has 32 years of professional water industry experience and active training for several national organizations. Wirth is a graduate of Concordia University in St. Paul, MN with a BA Degree in organizational management and communications. He received his engineering training at the South Dakota School of Mines and Technology in Rapid City, SD. Wirth can be reached at [email protected] or cell (319) 333-4174.
Kevin Osborn is the Residential and Commercial Sales Manager for Layne Christensen’s POU/POE Division. He has been involved in operations and sales in the wastewater and drinking water industry for over 10 years and has a BA Degree in mechanical engineering from WPI in Worcester, MA. Osborn can be reached via email, [email protected] or phone (508) 397-1876.