[en] The purpose of this study was threefold: to investigate the relationship between settleability and the physical properties of activated sludge, to correlate settleability and key operational parameters such as chemical oxygen demand (COD) concentration at the aeration basin inlet and to determine potential operational improvements to the Oakville Oil Refinery Wastewater Treatment Plant. Nine separate sets of stirred sludge-settling tests were conducted over a six-month period to measure settleability. The stirred sludge volume index (SSVI), the zone settling constants, and the Zheng-Bagley compression constants were determined. State point analyses were conducted to determine the maximum solids flux of the clarifiers. The concentrations of Al, Mn, Mg, Fe, Ca, Na, and K were determined to identify potential correlations to settling characteristics. Extracellular polymeric substances were extracted from the sludge and analyzed for the same reason. The performance of the clarifiers was highly variable during the period examined. As the refinery process flows and effluent streams changed, influent COD increased, from an average of 180mg/L to 600mg/L. Concurrently the settleability of the sludge decreased from excellent (SSVI = 68mL/g) to very poor (150mL/g), perhaps as the result of under aeration. State point analyses indicated that the clarifiers are under-sized for process upsets of this magnitude. (author)