By Matthew Wirth
In 2001, the U.S. Environmental Protection Agency (EPA) adopted a new standard for arsenic in drinking water of 0.01 milligrams per liter or 10 parts per billion (ppb), replacing the old standard of 50 ppb. When this happened, the industry was set for huge changes and an onslaught of application challenges.
Like a firecracker that fizzled out without exploding, this revision did not produce the expected bang. The public did not appear to be overly concerned. Why? Because when you can’t see it, smell it, or taste it, there doesn’t appear to be anything to fear and people are not alarmed—until something goes really wrong.
As water professionals, we know the seriousness of hazardous materials in water. We know that clear water does not mean safe water. Just because water looks good and is refreshing to drink doesn’t mean it is good for you. No matter the nonchalance about this Level I contaminant, we must remain vigilant about warning of the dangers of arsenic in the water. What has changed? Arsenic is still dangerous and a mystery to many. Let us revisit some facts, chemistry, and developments in applications.
The Basics of Arsenic
Where did arsenic come from, and why did the EPA change its maximum contaminant level (MCL)? Arsenic is a naturally occurring mineral in groundwater. It is found in water all over the world. Arsenic contamination in water is not just a man-made problem, but because arsenic also occurs naturally in water, people are often unaware of its presence.
This is not to say that there are no man-made pockets of arsenic contamination. According to “Groundwater Laced with Old Arsenic,” a Washington Post article by Curt Woodward, arsenic was a common pesticide used to control grasshopper swarms that plagued the Midwest in the 1930s. Arsenic is a byproduct of wood preservative and other industrial uses, but these sources of arsenic contamination are isolated and fairly rare. Most contamination occurs naturally.
The reason the public was unaware of an issue with this hazard was the inability to test very small levels of arsenic. For decades, the MCL set by the EPA was 50 ppb, the lowest possible test level.
But research showed that 50 ppb of arsenic is not a protective level. Once technology could test lower levels of arsenic, disturbing evidence emerged that ingesting arsenic as low as 0.17 ppb can result in arsenicosis. Arsenicosis has been defined by the World Health Organization working group as a “chronic health condition arising from prolonged ingestion (not less than 6 months) of arsenic above a safe dose, usually manifested by characteristic skin lesions, with or without involvement of internal organs.”
Ingesting water with just 1 ppb for an extended period can cause endocrine disruption. Endocrine disruptors such as arsenic are chemicals that interfere with the endocrine (or hormone) systems in animals, including humans. These disruptions can cause cancerous tumors, birth defects, and developmental disorders.
Treating Arsenic in Water
In water, arsenic is either pentavalent or trivalent. For this discussion, pentavalent (arsenate) is represented as As5. Arsenite, or trivalent arsenic, is As3. Most arsenic tests do not speciate between pentavalent and trivalent. This is not good.
To recommend a treatment solution for arsenic, one must know what state the arsenic is in. For most arsenic-reduction technologies, arsenic needs to be As5. As As3, arsenite becomes more difficult to remove. Ion exchange, reverse osmosis (RO), and most absorption technologies pass As5. In addition, applications using ferric chloride injection, or the iron naturally present in the water source, to attract and complex arsenic and iron into ferric arsenate require that the arsenic be pentavalent. Some of the hybrid iron-infused resin and exotic metal media remove both As5 and As3, but the system throughout is significantly changed by the ratio of As3 to As5. The presence of As3 reduces throughput, making bed life predictions difficult.
In Example 1, with 18 ppb of As5 and 5 ppb of As3, the throughput of one cubic foot of hybrid resin is 19,500 bed volumes. In Example 2, where all 23 ppb of arsenic is As5, the bed volumes are 73,000. The presence of As3 reduces the capacity of the media by 73 percent.
The levels of silica and phosphate and the pH all affect throughput. The stated bed volume capacity on a brochure is not the value used in building an arsenic solution. Because water chemistry changes, a best practice to ensure a predictable outcome is to include a final barrier, either a protective additional technology or installing a lead-lag system.
RO removes As5 but does not remove As3. All As3 present in water must be converted to As5 for an RO system to be effective. To get As3 to convert to As5, it needs to be oxidized. Chlorination and aeration are common methods of oxidizing arsenic from trivalent to pentavalent. NSF/ANSI 58 certifies RO systems for As5 reduction. Be aware that there are two certification levels for arsenic: 50 ppb of arsenate and 300 ppb.
Using RO for control of a Level I contaminant is a reasonable solution. Just remember that NSF/ANSI 58 is a removal certification; it does not state a throughput. In addition, RO is just part of a treatment train. If the oxidation component fails, the As3 will go right through the RO membrane. Depending on the levels of arsenic and the potential for harm in a failure, schedule preventive maintenance accordingly—and do not rely on most end users to be diligent about protecting themselves.
The lower the pH, the better media systems perform. Example 3 shows the effect of pH on throughput on the water from Example 2.
Phosphate is one of the ions that compete with arsenic for iron. This affects adsorption; it is a major issue when complexing arsenic to iron and filtering it from the water. While it varies with some waters, the standard ratio of iron to arsenic for conversion to ferric arsenic is 20:1, 20 ppb of iron to 1 ppb of arsenic. Remember, iron is found in parts per million (ppm) levels. So 2 ppm of iron is 2,000 ppb. Using this value, 50 ppb of arsenic needs 1,000 ppb of iron to be reduced using filtration. Just 1.5 ppm of phosphate will destroy the conversion factors for controlling arsenic with the iron naturally in the water. Knowing the competing ions is critical in systems where ferric chloride is added to the water to co-precipitate the iron and arsenic. Knowing if a residential system is losing capacity due to competing ions is critical, too. You must test the water for the ions at play and calculate their effect in reducing arsenic and throughput.
Silica appears in water in a dissolved and an ionized state. As the pH increases, the silica ionizes it and starts to adhere to the media. In effect, silica “glass” lines the media particles and blinds of the functional groups. Below pH 7, there is essentially no chance of precipitation because the silica exists in an unreactive, non-ionized form. As the pH exceeds 7.5, the silica starts to ionize more readily and become problematic. How much is a lot of silica? Less than 10 ppm of silica is not much; more than 50 ppm of silica is an issue. Silica and its effect on the chosen technology is relative to that application, so be sure to do the homework.
While the water industry has evolved and companies specializing in arsenic control have emerged, there are still people needing help from their local water dealer. The human brain is wired to believe “it won’t happen to me”—otherwise we’d all live in a constant state of fear.
When the trouble is a Level I contaminant in a person’s water, the water industry is the chosen provider of solutions. In taking on this responsibility, it must be extra diligent in providing the solution that fits the application, has a predictable challenge outcome, and has an identifiable life expectancy. Let knowledge, learning, and experience be your superpowers. If you’re going to fly to the rescue, be prepared to stay vigilant and for the long haul. There is no swooping in and flying away with Level I drinking water contaminants.
2. Endocrine Society. “Endocrine Disrupting Chemicals (EDCs).” Endocrine.org, Endocrine Society, December 8, 2022. https://www.endocrine.org/patient-engagement/endocrine-library/edcs
About the author
Matthew Wirth is a Water Professional with 42 years of experience, working at multiple levels in the water industry. The scope of his experience includes heavy industrial and commercial systems to public and private well applications – both customer direct and nationwide distribution. In addition to front line field support (including design, application and service troubleshooting), Wirth is an approved trainer for several industry organizations, state CEU programs and an author for trade periodicals. He holds a Water Conditioning Master license in the State of Minnesota, a BA Degree in organizational management and communication from Concordia University (St. Paul, MN) and received his engineering training at the South Dakota School of Mines and Technology in Rapid City, SD. Wirth is the General Manager of the Pargreen Sales Engineering-Water Division in Chicago, IL. He can be reached at (630) 433-7760.