By Peter Chawaga
When Dr. Treavor Boyer, an associate professor of environmental engineering at Arizona State University, visited a small treatment plant, he saw a way to make things simpler.
The plant was using two typical processes for removing dissolved organic carbon (DOC) and hardness: anion exchange and lime softening. He thought it would make for a more streamlined treatment train if the plant augmented its process with a reactor that contains both anion exchange resin and cation exchange resin. That way, the combination could handle several pollutants in a single process.
A chief advantage of combined ion exchange over the use of both anion exchange and lime softening is the production of a single waste stream, instead of two. It also reduces the number of processes that an operator has to monitor and maintain.
“Innovations in treatment technology are important if they can allow water plants to perform a new function in a different way,” Boyer said. “For example, the combined ion exchange is envisioned as a simple process, especially for a small water system, that can remove multiple contaminants.”
While using fluidized beds has been shown to be an effective way of performing anion and cation exchange, little was known about combining the two processes, according to a paper by Boyer and fellow researcher Katrina A. Indarawis. They wondered if it was possible to predict the results of combined ion exchange.
Their study of the process found that combined ion exchange can reduce precipitation of sparingly soluble minerals like calcium carbonate and that sulfate and magnesium removal through the process is estimable, as is calcium removal if the initial concentration of calcium is low.
“The idea of combining anion exchange and cation exchange is to use both resins simultaneously in the same reactor or vessel,” Boyer said. “The anion exchange resin removes DOC, because the majority of DOC in natural water is negatively charged, and the cation exchange resin removes hardness, mostly divalent calcium ions.”
Boyer presented his research on the process in a recent U.S. EPA webinar, “Combined Ion Exchange for Removal of DOC and Hardness,” sharing its benefits with a wider industry audience.
“Combined ion exchange is still mostly at the research level,” he said. “It is being supported by two EPA grants with the goal of generating the necessary knowledge and understanding to lead to full-scale implementation.”
The two main barriers standing in the process’ way are the two that generally must be hurdled before new treatment technology can be implemented: understanding and approval. First, more must be understood about the chemistry of ion exchange and operating a dedicated reactor. Second, regulatory approval for the process must be obtained in any state or region where it would be implemented.
In order to implement the process if and when it is ready for wider adoption, a given treatment plant will need the necessary fixed bed or mixed tank reactor, vessel and feed tank for regeneration chemicals, and the right pumps. Beyond those requirements, Boyer sees it being an ideal fit for a particular kind of treatment plant.
“I think small water systems with source water that contains multiple contaminants, such as DOC and hardness, nitrate and hardness, and other combinations of inorganic anions and metal cations,” he said, when asked what type of treatment facility could benefit most from implementing the process.
A pilot plant study of the process is ongoing and will have results to share later this year. Students will present them during November’s AWWA Water Quality Technology Conference and the Florida Section AWWA Fall Conference in December.