The sniff test Charleston Water System volunteers use their noses to detect dirt-tasting algae

Miranda Torres, a Charleston Water Systems facility technician, was a sniff-tester Thursday. The samples were colored red to help prevent testers from using visual clues in their rankings.

HANAHAN — Sorry, no sipping at this tasting event. The participants delicately draw the snifter to their nose for a waft, then move to the next one.

And don’t bother about bouquet, finish, black cherry or plum.

It’s just water, and it smells like dirt.

Twice a week, a cadre of impromptu Charleston Water System volunteers edge their way into a lab at the Hanahan treatment plant to sniff for geosmin and methylisoborneol.

Their noses are your best friends these spring days, when warming waters have algae levels spiking in Bushy Park Reservoir, the source for much of the drinking water for 400,000 people in and around Charleston.

The organic chemicals in algae give the water that dirty smell and taste. They’re harmless, but they’re yuck.

The rule of thumb about geosmin and MIB is that customers can smell and taste them in the water when the amount reaches 15 parts per trillion. Yep, trillion. A part per trillion is the equivalent of an inch per 16 million miles.

As if that weren’t faint enough, the CWS experience has been that customer complaints start when it reaches 10 parts per trillion.

“We can tell people all day long that it’s not harmful. But when it smells like dirt they think it’s not clean,” said CWS chemist Lori Cowden.

So CWS tries to get the organics out using activated carbon, the stuff in your water filter.

The problem is, only so much carbon can be used, and you don’t want to use any more than you need. So you need to know how much algae is in the raw water and how much is left in the treated water.

Testing equipment costs $500,000 or so, plus training plus operating. Even sending off for the before-and-after tests costs $600 per pair, and takes at least a week. Meanwhile, as plant manager Jane Byrne points out, “You need that information now.”

Three years ago the light bulb lit up for Cowden: Why pay all that money for tests when the customer’s nose already can tell?

She devised a calibrated “taste and odor test” based on wine and food tastings.

One at a time, 10 “tasters” sniff out a series of vials with water from various stages in the treatment process. The sniffers sort the samples weakest to strongest, without knowing which one is which.

Two of the samples are controls with a known amount of the chemicals, so Cowden can gauge whose nose is sharp that day, and just how many parts per trillion the raw and treated water have.

Comparative lab-tested samples have proven her right.

A good sniffer, it turns out, can pick up the scent at 5 parts per trillion or less, and differentiate between the strengths of smells at that intensity.

“It’s kind of beyond comprehension,” said chemist Jason Thompson.

But the nose knows. Reach Bo Petersen at 937-5744, @bopete on Twitter or Bo Petersen Reporting on Facebook.

Science and environment reporter. Author of Washing Our Hands in the Clouds.