Advances in Arsenic Removal

Rebecca Wilhelm: What is your research currently focusing on?
Frederick Vance:
Current research is focused on improving the physical properties of the adsorbent media. Feedback from customers has indicated that qualitative issues like ease of handling and use can be as important in their purchasing decision as quantitative issues like the cost of water treated.

Our current research is focused on further improving the physical properties by narrowing the particle size distribution of the beads as well as improving the sphericity. Ultimately, we believe we could eliminate the initial backwash and perhaps improve somewhat on the efficiency of the media bed at the same time.

Wilhelm: What challenges have you faced in your research on the development and application of media for the selective removal of arsenic?
Vance:
The biggest challenge has been finding the balance between competing drivers or customer desires.

For example, we knew early on in our research that we could best take advantage of the fast kinetics of our media by making very small particles. However, using smaller particles creates problems with pressure drop, so we ended up with a compromise, which represents good kinetics with good handling properties, but not the ideal for either attribute alone. The same can be said for looking at physical strength versus capacity of the media. It is relatively easy to make a very strong media, but making it strong while still adsorbing a high amount of arsenic is what the customer needs.

End users don’t want to pay more for their water just because the arsenic has been removed, so the goal is always to keep costs at a minimum. This means that any performance improvements incorporated have to be justified by the cost at which they are delivered—this is why you now see that iron- and titanium-based media are being used preferentially over alumina. Even though the alumina is cheaper, the added performance of the other media more than justifies the added costs.

Wilhelm: Is there currently a “best practice” for arsenic removal?
Vance:
There is not one best practice, but several, depending on the water chemistry and size of the system

.For very large systems, coagulation filtration using ferric chloride is often the best treatment option, and can cost as little as a few cents or less per thousand gallons.

For medium and small systems, the best treatment technology is most often selective adsorption. Adsorbia media is used in these systems to treat water at a cost of a few cents to a dollar per thousand gallons, depending on the water chemistry.

There can also be circumstances which would change your treatment choice. For example, if there is high iron present, the best solution may be oxidative iron removal, which may take the arsenic down to safe levels. Similarly, if there are multiple contaminants present, a broad spectrum treatment approach like reverse osmosis membranes may make more sense

At the end of the day, “cost-effective” is always relative to what consumers want to pay. Any of the treatment options give utilities the ability to deliver safe, high-quality water at a fraction of the cost of bottled water, for example.

Wilhelm: What will be the focus for future research regarding arsenic removal from drinking water?
Vance:
We will see increased attention paid to the entire life cycle of arsenic management. While current selective adsorbents can pass standard tests to be classified as non-hazardous waste for disposal, some researchers are already raising questions about the ultimate fate of the arsenic in landfills, where high pH and reducing conditions can be found.

At the same time, the desire to lower operating costs has spurred the development of regeneration approaches for dealing with arsenic.

In either of these cases, the ultimate question is where should we be putting the arsenic once we’ve taken it out of our drinking water? The selective adsorbent media provide a convenient means for disposing of the arsenic as a solid, and with the level of arsenic found in U.S. drinking waters, can be managed safely and effectively. However, with the higher levels of arsenic found in isolated pockets and in other parts of the globe, the issue can be further complicated since the high levels mean you need to worry about acute rather than chronic effects.

Waste handling is always a regional issue, which is exacerbated when you have a natural contaminant like arsenic affecting so many of the globe’s poorest people. This brings affordability back to the center of attention, since an ideal solution here in the U.S. may simply be cost-prohibitive somewhere else.

Fredrick W. Vance is global application development specialist at Dow Water & Process Solutions. Vance can be reached at 989.636.0390 or by e-mail at fwvance@dow.com.

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