Strategies to identify, understand & treat emerging contaminants in drinking water
Water purity is crucial to human health and safety, especially when it comes to drinking water. Advances in analytical chemistry have pushed the detection limits of chemicals lower and lower, such that chemicals in the water supply that previously were not detectable now are detectable. These detection limits allow for increased studies on chemicals’ effects on humans, which may result in new regulatory legislation. When new contaminants are discovered in the water supply or found in greater concentrations, the chemicals are considered “contaminants of emerging concern” or “emerging contaminants.” The risk of these chemicals is not well understood, so they have been highlighted as materials to study how they not only affect human health, but also the environment.
The U.S. EPA has the responsibility to research and understand water contamination, according to the Clean Water Act and the Safe Drinking Water Act. In some instances, EPA has the ability to enforce these regulations. Where the federal government has not acted, individual states may act and develop their own regulations.
EPA identifies contaminants that occur or may occur in drinking water with regular frequency and at levels that pose a threat to public health. Through its federal mandate, EPA created a contaminant candidate list (CCL), which identifies contaminants that may be regulated. To date, EPA created four CCLs with more than 100 contaminants, and the fifth list currently is being composed. Every five years, it must decide whether or not to regulate at least five contaminants. The agency continues to study the remaining contaminants. To this end, a regulation was created to monitor unregulated contaminants—up to 30 every five years.
When evaluating contaminants that may need regulation, there are two key questions that need to be addressed: Which contaminants are the most important to address because of immediate and future safety concerns? What technologies are available to treat potential contaminants?
The first question prioritizes the contaminants based on safety concerns, while the second question sets up potential regulations. Technologies must exist to treat the contaminants. If technologies do not exist or are not cost effective enough, new research must be completed and new products must be developed to treat the contaminants.
From a business perspective, the development of new technologies to treat contaminants comes down to cost and selectivity. There are many categories of emerging contaminants; these range from simple metals and metal ions, such as cobalt and strontium, to complex chemicals, such as pharmaceutical and personal care products (PPCU). Chemicals that are used in everyday life, such as pesticides, flame retardants and plasticizers, can make their way into water streams, contaminating the environment. Often technologies are not available to remove these contaminants; the technology is not practical, or it is too costly to be implemented appropriately in the water treatment markets.
Organic contaminants such as per- and polyfluoroalkyl substances (PFAS) are listed as emerging contaminants, but they have been recognized for a long time. More recently, studies addressing the health effects of PFAS have been published, leading to significant media attention and forthcoming regulations. With the possibility of regulation, businesses are investing in research and development to treat these harmful chemicals so that when regulations are introduced, the technologies will be available for the industry to use. With PFAS, there are technologies that can remove the chemicals, but more will be developed to bring options that are more cost effective and efficient.
A Deeper Dive
One way to better understand the challenges of emerging contaminants is to look at two examples that EPA is considering for regulation: strontium and nanomaterials.
Strontium is being considered a contaminant of concern for regulation according to CCL3. As of 2016, EPA delayed the final determination for strontium to consider additional data and decide whether there is a meaningful opportunity for health risk reduction by regulating strontium in drinking water. Strontium is a naturally occurring element utilized in several industries, including pyrotechnics, steel production and as a catalyst. The health concern arises out of the fact that strontium can replace calcium in bones, the effects of which are not well understood. There is approximately 8 ppm of strontium in seawater, and it is crucial to some sea life, such as corals. Strontium-88 (the natural, non-radioactive isotope) is found in well waters and high saline environments, specifically in the southwest U.S.
A nanomaterial is defined as a “material with at least one dimension smaller than 100 nanometers (nm).” Due to their small size, nanomaterials often exhibit unusual physical and chemical properties, making them of particular interest to industries. Carbon-based nanomaterials, such as carbon nanotubes and graphene already have been tested for applications in the electronics, medical, energy and automotive sectors. The yearly production of carbon-based nanomaterials is estimated to be hundreds of tons. Nanomaterials, particularly nanoparticles such as nano-gold and nano-silver, have been used as an anti-microbial agents for a long time. There are approximately 1,800 diverse, mass-consumer products containing nanomaterials already on the market. Unfortunately, increased use and production of these nanomaterials also leads to potential releases into the environment through multiple routes.
They are chemically stable and mostly non-biodegradable, making them difficult to treat and purify using conventional methods. Moreover, treatment of these nanomaterials is largely dependent on their structure, surface chemistry and particle size. Due to their small size, nanomaterials easily can infiltrate human tissue or escape into the natural environment in ways that larger particles cannot. The environmental exposure and potential adverse risk from nanomaterials in water remains ill-defined, making them an emerging contaminant. Nanomaterials found in drinking water may be regulated in the future, but currently no maximum contaminant level goals (MCLGs) or maximum contaminant levels (MCLs) have been established for these materials.
The good news is that technologies do exist to treat these contaminants. However, these technologies are not always practical and cost effective.
One method of treatment on the market today is reverse osmosis (RO) systems. RO technologies use specialty membranes that allow select ions and molecules to pass through, creating a permeate stream and a concentrate or “reject” stream. The permeate stream of an RO system essentially is pure water with some small ions present.
Where do the contaminants go? The RO selectively concentrates contaminants into a waste stream. Often it is this concentrate that can plug the membrane, causing it to foul and requiring membrane cleaning or replacement. This is an added expense for the consumer or municipality treating the water. Note that the contaminants are not only still in some water form, but are now concentrated significantly. While RO provides a robust and immediate treatment solution, the contaminants are not eliminated, and the waste still is difficult to handle.
Another treatment technology, coagulation, is a well-known method that can be used for the treatment of nanoparticles. Coagulation, combined with direct ultrafiltration and microfiltration, could be more effective in removing nanomaterials of different types with better efficacy.
For organic contaminants, another treatment option is advanced oxidation. There are many systems available to chemically convert organic constituents from pharmaceuticals, pesticides and personal care products to simple, non-toxic base chemicals. The approach in using advanced oxidation chemically transforms organic compounds to basic constituents such as carbon dioxide (CO2) and water (H2O). While this approach is one of the more appealing ones, the energy required to break chemical bonds such as these is high. Oxidation is another method that can be applied for the treatment of organic and carbon-based nanomaterials. However, carbon-based nanomaterials are difficult to degrade by common oxidation processes. Some recent studies claim hypochlorite, an environmentally friendly oxidizing agent, has the potential to degrade carbon nanomaterials such as carbon nanotubes completely from aqueous streams. The consumer would be unlikely to be able to afford a system to do this in their own home. Municipalities would need large systems and too much time to treat all of the water that flows through their systems.
There is no silver bullet to treat all types of contaminants, but combinations of technologies could significantly reduce risks to human health and the environment. Many companies, including Graver Technologies and Filtrex Technologies, are working on new sorbents (i.e., adsorbents and ion exchange materials) to selectively remove key contaminants. The use of adsorbents may be preferable to the other technologies mentioned for a few reasons.
First, the use of adsorbents removes the contaminants from a liquid phase and concentrates them in an easy to transport solid phase. Second, adsorbents may be easy to dispose of safely in a secured landfill or other disposal site. Sorbents can be modified to fit current equipment, reducing any capital expenses the other technologies discussed contain. Different sorbents may be combined to tackle a multitude of contaminants, and customized systems can be developed for different homes and municipalities.
Emerging contaminants are key guidelines for both water professionals and consumers alike. The lists by EPA and the resulting studies help professionals and the public determine the effective cleanliness of their water. While technologies exist to treat emerging contaminants, the practicality and cost of treatment may outweigh some of the risks. For new contaminants that become regulated, companies quickly will provide cost-effective treatments to generate clean water for consumers.