Thursday, April 3, 2014

Concrete-dissolving bacteria are destroying our sewers

Sarah Zhang in Gizmodo: Underground in places nobody likes to look, bacteria are doing terrible things to our sewage pipes. The concrete pipes that carry our waste are literally dissolving away, forcing engineers into a messy, expensive battle against tiny microbes.

“The veins of our cities are in serious trouble, and they’re in serious trouble because of corrosion, and this corrosion has been unanticipated and it’s accelerating,” said Mark Hernandez at a symposium on the microbiology of the built environment in Washington DC yesterday. Hernandez is a civil engineer, but he’s meeting with microbiologists because this problem is bacterial. Essentially, it’s an infection of the nation’s sewage system.

....One set of microbes emits hydrogen sulfide, the gas that is also responsible for raw sewage’s unpleasant smell. This gas fills the empty space between the top of the pipe and the water flow. Another set of microbes living in this headspace turns hydrogen sulfide to sulfuric acid, which eats away at concrete, leaving behind gypsum, the powdery stuff you find in drywall.

“Essentially what we’re ending up with is wet drywall,” said Hernandez. This is one reason the American Society of Civil Engineers has gave our wastewater infrastructure a D grade. The current solution is to put plastic liners into the concrete pipes, a process that is almost as expensive as digging them up entirely. But if the problem is microbial, perhaps the solution is, too?

...The solution they settled on was shotcrete — a type of concrete that can be sprayed onto the inside of pipes — embedded with small amounts of charcoal and bacteria-killing metals like chrome. As the acid ate through this concrete, metal would be released. Based on Hernandez’s tests, this concrete did hold up better against sewage microbes — suggesting a promising solution to corrosion....

Concrete sewer pipes, shot by TUBS, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 Unported2.5 Generic2.0 Generic and 1.0 Generic license

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