The Importance of Side Water Depth in Sequencing Batch Reactor Design

When designing conventional secondary clarifiers, it is common to consider Initial Settling Velocity (ISV) a function of Mixed Liquor Suspended Solids (MLSS) concentration, using for example the Vesilind equation. A secondary clarifier is typically sized so that it would accommodate a given worst-case ISV at a given peak loading condition. However, some Sequencing Batch Reactor (SBR) design methods do not take this relationship into account, with the result that high MLSS concentrations are often used. In addition, SBR basin volumes are fixed by biological requirements. This implies that sufficient surface area must be provided so that the expected ISV can be accomodated. By contrast, some SBR design methods treat Side Water Depth (SWD) as an independent variable, ignoring the fact that for a given volume, SWD and surface area are inversely related. It is shown that high SWD has a negative impact on SBR tank capacity.