The Impact of Depth on Secondary Clarifier Performance – An Industrial Perspective

Conventional wisdom for activated sludge secondary clarifier design is that deeper clarifiers will result in better effluent suspended solids (ESS) quality. Preferred design depths are in the 15 to 18 ft range. However in industrial applications value engineering is often performed to minimize wastewater treatment plant (WWTP) construction costs. Thus the question of how deep to build a new clarifier can arise. This paper will use two dimensional computational fluid dynamics (CFD) modeling to evaluate the impact of clarifier depth in industrial applications as compared to municipal designs. The industrial applications considered will be based on activated sludge systems at oil refineries. The paper will also use modeling to explore clarifier design modifications (e.g., Stamford baffles, inboard launders, larger flocculating centerwell) that can be used to offset shallower depths. The goal of the paper will be to inform industrial wastewater design engineers on how to select a clarifier depth that meets the ESS performance requirements in a cost-effective manner.