A Rude Surprise That Turned Out Surprisingly Well

by Angus W. Stocking, L.S.

A routine five-year inspection of a 36” reinforced concrete pipe (RCP) sewer interceptor in Rockland County (NY) Sewer District No. 1 revealed a rude surprise for Engineer IV Martin Dolphin P.E.—CCTV showed that the top half of the interceptor, which had been in good shape just five years previously, was almost completely rotted out by microbiologically-induced corrosion (MIC). Even the reinforcing rebar was gone in 20-30 foot stretches of the 750 lf sewer and the only factor preventing complete collapse appeared to be solid root masses that held soil in place.

It was a shock, but not completely unexpected. Rockland has been implementing an aggressive I&I reduction plan for several years that has reduced excess flows to their sewer plant by almost 50%. This is a good thing of course, but reduced flows in this sewer may have allowed greater accumulations of organic matter, fostering growth of Thiobacillus bacteria and the consequent production of hydrogen sulfide and concrete-destroying sulphuric acid. Ironically, in this critical sewer, Rockland Sewer District No. 1 may have been a victim of their own success. “It is now taking more time for waste to get to the plant, and that may be causing more of this type of corrosion,” Dolphin says. “In the past we were more used to seeing cracks and other deformations affect our network, but now we are seeing more corrosion caused by hydrogen sulfide accumulation.”

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