The concrete covers of the digester tanks were severely deteriorated with cracks allowing gases to escape and oxygen to enter the vessels. Numerous concrete “fins” (shown) from the original construction were difficult to coat and inspect.
Personnel frommultidisciplinary design firm Short Elliott Hendrickson Inc. (SEH), Champion Coatings, and Wapasha Construction, as well as local representatives from Sherwin-Williams Protective & Marine Coatings were all involved in restoring the two domestic anaerobic digesters for the City of Austin, Minn. The concrete structures were severely deteriorated with numerous cracks in their covers, allowing gases to escape. Rather than level and rebuild the structures con- structed in 1938 and 1959, the city was hopeful that high-performance protective coating and lining systems could extend the digesters’ lives. The winning bid team was confident this was possible, but it didn’t anticipate the numerous challenges it would encounter, including dif- ficult site conditions, installation issues, and failed validation testing. Urgent need for repairs The City of Austin operates several anaerobic digesters to manage the breakdown of biodegradable waste from various industries. The process requires an oxygen-free environment to operate at peak efficiency so microorganisms can proliferate and rapidly break down waste. With the two deteriorated digesters leaking gases out, they were also allowing oxygen to get in, making them increasingly inefficient. The city turned
Anaerobic digester restoration Challenging site conditions lead to repair trial and error for decades-old tank. By Greg Larson
Huddled inside a nearly 80-year-old anaerobic digester tank in the dead of a frigid Minnesota winter, contractors and engineers care- fully inspected every square inch of the freshly coated concrete ves- sel, looking urgently for a hidden leak. On top of the digester’s cover, crewmembers braved wind chill temperatures exceeding -20° F as they watched for signs of the leak following both smoke and soap tests. Eventually, the crew found an unfilled cold joint tucked in a hard-to- reach area. This discovery allowed them to finally stop the leak and get the digester back into service — just in time to begin restoring a second digester that was almost 60 years old.
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june 2018
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