Changing water treatment practices could prevent the costly concrete corrosion problem impacting sewers globally, a new study  in the 15 August issue of the journal Science reports.
In many countries, sewer networks contain pipes that are intended to last for a century or longer. But, time and pressures related to urban growth, including high concentrations of sewage that emit corrosive acid, can break down these pipes more quickly. Some pipes corrode and collapse in just ten or twenty years.
"Water utilities around the world are making strong efforts to reduce sewer corrosion, but this comes with a high price tag," said the study's lead author, Zhiguo Yuan, deputy director of the University of Queensland's Advanced Water Management Centre (AWMC).
As noted in a recent article  published on AAAS.org, it may cost as much as $3 $300 billion over the next twenty years to replace address corroding U.S. sewer pipes, which received a "D" grade in the 2013 Report Card for America's Infrastructure .
But, Yuan said that changing a commonly used water purifier could dramatically reduce sewer corrosion "at a much lower, and in many cases negligible, cost."
The main cause of sewer corrosion is the presence of a gas called hydrogen sulfide. Found in all types of sewage, hydrogen sulfide forms from sulfates found naturally in the raw source water supplied to water treatment plants and sulfates added to drinking water to help filter out suspended solids. The sulfate-treated water is used by homes and businesses, and then discharged into sewers as waste. In oxygen-poor environments, all the accumulated sulfates turn to sulfide, and ultimately to corrosive sulfuric acid.
To address concrete corrosion, some water sanitation facilities have tried to remove sulfide after it forms, but at great expense and with little success. "The methods for removing sulfide are suitable for controlling corrosion at hotspots," Yuan explained, "but, for network-wide corrosion control, they're cost-prohibitive."
Now, Yuan and other AWMC scientists provide an alternative — controlling sewer corrosion at the source by reducing added sulfate.
Yuan explained that this alternative hasn't been identified before because many cities may not have an integrated view of water supply and sanitation.
"Drinking water suppliers focus on delivering safe drinking water with minimum costs," Yuan said. Since the amounts of sulfate-based coagulants they add typically don't exceed drinking water guidelines, the suppliers have no incentive to avoid their use. "Similarly," he continued, "wastewater management utilities regard sulfide formation in sewers as unavoidable and therefore focus on sulfide removal after its formation."
Yuan and his team looked at the entire water management operation to develop a different approach. Based on an extensive water sampling campaign in suburban Australia near Queensland University, they show that the aluminum sulfate added as a coagulant during drinking water treatment is the primary source of sulfide in sewage — more so than sulfate from groundwater or waste. It contributes up to 50% of sulfate levels in Australian sewers, they say.
And, Australia's not alone in heavy used of sulfate-based coagulants. A comprehensive international literature survey by Yuan and colleagues revealed that aluminum sulfate is used to a similar extent globally.
The researchers took a close look at the impact of this heavy sulfate load on sewer corrosion with a quantitative model of simulated water flow in existing Australian sewer networks. When they replaced aluminum sulfate coagulants with sulfate-free compounds like ferric chloride, concrete corrosion in the model was dramatically reduced — by 35% after ten hours and 60% over longer durations.
"The proposed solution is very simple to implement," Yuan emphasized. "It simply requires a change of the coagulant. The change in chemicals should not require major changes to the dosing facilities that are already in place."