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Making Water Treatment Plants Sustainable

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Each year millions of tons of sludge laden with iron -the byproduct of drinking water treatment- are transported and deposited into landfills. While this practice is mandated by the U.S. Environmental Protection Agency and is the accepted industry standard for safe sludge disposal, it is expensive. In fact, sludge disposal costs the water-treatment industry millions of dollars per year.

Meanwhile, half way around the world, an environmental health disaster has been unfolding over the past two decades: some tens of millions of people in Bangladesh and West Bengal, India, are routinely exposed to drinking water contaminated with high levels of arsenic. Thousands have died, and estimates suggest some one-hundred million more people are at risk of developing arsenic poisoning.

A team of students from Lehigh University addressed these two seemingly distant issues as part of the U.S. Environmental Protection Agency's P3 (People, Prosperity, and the Planet) sustainability design competition.

At its simplest, water treatment is a filtration process. The liquids and the solids must be separated to ensure that the water meets health and safety standards. This happens in large part due to gravity, with the heavy solids sinking to the bottom of the tank. However, there are still microscopic particles suspended in the water. A coagulant, which is a chemical additive, combines the small particles into larger ones that are heavy enough to fall to the bottom, collecting as sludge.

Many different chemicals can act as coagulants in water treatment, but the most prevalent are iron and aluminum. These metals are quite effective at combining particles but utilizing these coagulants entails costs at two points. The water treatment facility must first purchase the chemicals and then pay for the disposal of the metal-laden sludge in landfills.

The P3 team used the Donnan Membrane Process (DMP) to efficiently recover the coagulant, iron in this case, from the sludge. DMP operates by exploiting an electrochemical gradient across an ion exchange membrane (between sludge and acid solutions) to effectively recover iron from the sludge.

DMP worked for the team, recovering 70 percent of the iron within 24 hours, thereby reducing the amount of sludge that would wind up in a landfill. This accomplishment is notable because the process allows for high amounts of coagulant recovery over short periods of time with minimal operating costs. Needed capital investments to implement DMP would be recovered through lower raw coagulant requirements and disposal costs.

Although the first part of the team's technology worked, the second stage, exploring if the recovered iron could be embedded as nanoparticles into polymeric ion exchange resins towards ultimate removal of arsenic from drinking water, did not operate as planned. While the process did work, it required additional inputs, making the process less attractive than standard, existing practices. However, the team was able to commercialize a similar resin, known as HAIX, which is now available through two companies, SolmeteX and Purolite. HAIX is currently being used to remove arsenic from drinking water at several facilities in West Bengal, India.

The potential benefits of switching from disposal of the sludge to treatment using DMP are vast: millions less tons of iron-laden sludge produced annually, continued access to clean drinking water at a lower cost, and even lower municipal taxes.

The team from Lehigh is now using a computer simulation and a pilot-scale prototype to demonstrate the effectiveness of the process both in the laboratory and at water treatment plants.

After the team gathers and organizes their findings, they will share detailed process reports with two water treatment plants, the Allentown Water Treatment Plant in Allentown, PA, and the Baxter Water Treatment Plant in Philadelphia, PA. The Civil and Environmental Engineering Department at the university has worked with these plants for years on water treatment and coagulant recovery issues.

In the near future, the directors of the water treatment plants will be invited to see a demonstration of the operational simplicity and economic potential provided by a DMP reactor. If officials approve the method, they could adopt similar processes in their own plants.

Last April, one of the team members, Lee Blaney spoke in New York City at the 13th United Nations Commission on Sustainable Development. Mr. Blaney was invited to give a presentation after finishing first in the Citizen Science writing contest for a report about this water treatment project. Citizen Science, organized by SustainUS, is a science and technology program which mobilizes young people to advance scientific approaches to sustainable development. SustainUS is a nonprofit organization that promotes sustainable development. Mr. Blaney is now serving as a member of the technical review board of the Citizen Science program.

Published results and advertisements of the abilities of a full-scale Donnan Membrane Process reactor will help spread the word and encourage industry to take advantage of this sustainable practice for water treatment plants.

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