Today’s underground sewer infrastructure is the result of more than 100 years of investment. In the U.S., for example, the sewer system is now valued at an estimated $1 trillion. But that investment is under a chemical threat from sulfide-induced concrete corrosion, according to a research report. The problem is largely preventable by eliminating sulfate—the source of the sulfide—from water-treatment operations, the study’s authors say.
The sewer sulfide problem has been around since sewers were first built, according to Ilje Pikaar, Keshab R. Sharma, and Zhiguo Yuan of the University of Queensland, in Australia. To better understand the current impact of chemicals on sewer pipes, the researchers performed a two-year sampling campaign along with a water-treatment industry survey and conducted a computer model scenario analysis (Science 2014, DOI: 10.1126/science.1251418).
As Yuan and coworkers explain, anaerobic bacteria in biofilms and sediments reduce sulfate in sewage to hydrogen sulfide, which is bad enough because of its noxious odor and toxicity. But in sewers H2S is oxidized by aerobic bacteria on pipe surfaces exposed to air. The corrosive sulfuric acid formed eats away the calcium silicate-based concrete pipes at a rate of several millimeters per year.
Water and human and industrial waste naturally contain sulfate, the researchers note. But about half of the sulfate in wastewater comes from aluminum sulfate or ferric sulfate added as a coagulant during water treatment to help remove colloidal solids and natural organic matter. By switching to sulfate-free coagulants, such as polyaluminum chloride or ferric chloride, much of the sulfide problem could be solved, Yuan says.
Engineers estimate the average life span of sewers at 50 to 100 years, which leads to necessary annual investments of 1 to 2% of the replacement cost for repairs, note environmental engineers Wolfgang Rauch and Manfred Kleidorfer of the University of Innsbruck, in Austria, who wrote a commentary to accompany the report. “Any technological improvement that helps increase the service life of sewers by one year would save globally more than $1 billion per year,” Rauch and Kleidorfer say. Replacing sulfate in water treatment, they add, “is a simple source control measure that can at least counteract if not solve the sulfide problem.”