Save our sewers
An additive used to clean drinking water causes concrete pipework to crumble, scientists have found. James Mitchell Crow reports.
What was the greatest medical advance of the modern age? Not antibiotics, or any other drugs, according to the British Medical Journal. In a recent vote its readers declared clean drinking water and proper sewage disposal the greatest advances. And yet most people only think about sewers when they stop working properly.
A report published this week in Science says the world’s sewer networks are ageing. Failing pipes in the US alone cost $US14 billion per year – most of that due to corroded concrete pipes.
Now water management researcher Zhiguo Yuan and his team at the University of Queensland have discovered that an ingredient most water utilities add to clean up drinking water is the major culprit.
Sulphate in waste water damages concrete pipes. Bacteria feed on it turning it first into the foul-smelling gas hydrogen sulphide, and then into sulphuric acid, which eats into the pipes.
When sewerage systems were first installed in Australia they included regularly spaced air vents to release the hydrogen sulphide into the atmosphere, says Tung Nguyen, a recently retired chemical engineer from Sydney Water. “That design was very good at preventing corrosion, but not very good because of complaints about the odour,” he says. As urban populations grew people ended up living close to vent sites. “So many utilities closed up the vents.”
Since then, years of research have gone into looking for alternative ways to slow the corrosion. “Most of the research, including my work, has focused on removing sulphate,” says Yuan. “I don’t know anybody who has asked the question, where does this sulphate come from?”
Sulphate is present in the water we drink from the tap. But by analysing the sulphate content of water at the point that it is sourced from the environment, out of the tap, and in the sewer, Yuan has shown that more than half the sulphate in southeast Queensland’s waste water comes from the aluminium sulphate added as a coagulant to remove solid particles and organic matter when drinking water is purified. He then compared available data on natural sulphate concentrations in water around the world, concluding that aluminium sulphate is the main source of sulphate in waste water in most places.
There’s a simple solution says Yuan – use a sulphate-free coagulant, such as iron chloride. Some utility companies, including Sydney Water, already use iron chloride because they have a cheap local supply. The iron is a waste product from nearby steelworks.
“I think there is the potential many other utilities will switch to iron chloride, based on Yuan’s work,” Nguyen says.