WCW24

Have you ever looked at (or designed/installed) an isolation gate and thought “Glad I wont be around when this needs replacing…” or “I wonder how much life is left in this gate…”. Or have you ever been asked to shutdown half your plant process capacity (Bioreactors and Clarifiers)? This presentation covers the City of Saskatoon's innovative approach to replacing 70 water control slide gates in both of their Bioreactor trains. The original gates were made of Aluminum and installed in 1995. In the 27 years of service they had developed significant corrosion, pinholing and material loss. The condition raised concerns of imminent failure, thus required replacement. The Bioreactors were never designed for complete bypass (instead intending to just bypass a set of cells at any given time for normal maintenance using the control gates) and thus the shutdown to replace all the gates created an operational challenge. The options varied from accelerating the construction of a third train, to temporary secondary treatment systems, to modifying existing infrastructure. The City ultimately did a series of 1 day shutdowns to install coffer dams and three additional slide gates in the Bioreactor Effluent channels which then allowed for a modified treatment process. One bioreactor train was fully drained for gate replacement while the other was operated with a significantly increased MLSS and the Bio effluent flow was then split through the new gates to all six of the plant secondary clarifiers. The process was repeated for the other bioreactor the following summer (due to construction issues). This allowed the plant to maintain acceptable nutrient removal throughout the replacement process without excessive costs or accelerating future capital expansion.