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A charcoal scrub for drinking water

Most of us don’t think much about water.

It’s just the twist of a handle away when we want to take a shower or make that first pot of coffee in the morning. But behind the scenes at the local water plant, water treatment professionals are hard at work to make sure that water is safe to use, and especially safe to drink.

They use all kinds of technologies to do their work, but one of their most common tools is activated carbon (sometimes called activated charcoal). When it’s mixed into the water, it absorbs organic contamination and compounds that create taste or smell problems in tap water supplies. In drinking water drawn from Lake Erie, one of its jobs is to remove toxins created by harmful algal blooms, such as microcystins and saxitoxins.

The difficult part for treatment plant staff is knowing exactly how much activated carbon to use, because its effectiveness depends on many different factors, from toxin levels and the type of activated carbon they have on hand to whether the raw water has plants or lots of sediment in it. So they prefer to be safe, possibly using more activated carbon than necessary and raising treatment costs by hundreds of dollars each day for as long as an algal bloom is present near their water intake.

Professor John Lenhart, CEGEDr. John Lenhart, an associate professor in the Department of Civil, Environmental and Geodetic Engineering, and his research team are now working on guidelines that address some of these issues, using both powdered and granular activated carbon made from wood, coconut shells, bituminous coal and a coal blend. Powdered products are used on an as-needed basis, while granular activated carbon is built into the overall treatment process when a particular plant tends to have chronic issues to address.

How much microcystin toxin granular activated carbon can adsorb depends on what the carbon is made from, and whether the water is clear (blue bars) or contains organic matter like decomposing plants (orange bars). Results from test solutions that model the water typically going through an Ohio treatment plant show that organic matter reduces the amount of adsorbed microcystin and needs to be addressed during water treatment. Adsorption capacity is defined as milligrams (mg) of microcystin removed per gram (g) of activated carbon added.The research is funded by the Ohio Department of Higher Education’s Harmful Algal Bloom Research Initiative (HABRI), a statewide response to the threat of harmful algal blooms that arose out of the 2014 Toledo drinking water crisis, where elevated levels of algal toxins in Lake Erie shut down water supplies for half a million people in northwest Ohio. Lenhart recently received funding for a third project focusing on saxitoxin removal (completing in 2020), adding to the team’s work on microcystin toxins (completed in 2016) and granular vs. powdered activated carbon products (completing in mid-2018).

Highlights from the previous projects suggest that wood-based powdered activated charcoal is the best choice for removing microcystins, especially because it interacts with the toxin and any organic matter in the water more quickly than coal-based activated carbon. Coconut-based powdered activated carbon, a common choice in water pitcher filters, was a poor option for removing microcystin. Granular activated carbon followed similar trends during initial laboratory tests.

The scientists also saw that natural organic matter like decomposing plant material can decrease the amount of toxin removed (see graph). They are currently running experiments at a larger scale, in columns similar to those used in treatment plants.

“Our partners include water treatment plants in Wilmington and Barberton that face issues related to harmful algal blooms and saxitoxins, and that will directly benefit from the results of this research,” said Lenhart. “Our continued goal is safe drinking water for communities and money savings for water plants that will be able to apply activated carbon in a more targeted fashion.”

by Christina DierkesOhio Sea Grant College Program