Effects of using a specially designed sludge draw‐off pipe for circular secondary clarifiers to mitigate underflow short‐circuiting

Abstract Short‐circuiting in secondary clarifiers is a well‐known problem that can occur through up‐flow or underflow routes. The underflow short‐circuiting is not as visible as up‐flow short‐circuiting but can affect clarifier performance. The energy‐dissipating inlet (EDI) is a type of inlet structure that is used in secondary clarifiers to dissipate the energy of larger influent volumes, allowing clarifiers to operate at higher treatment capacities. The underflow short‐circuiting is encountered particularly in clarifiers equipped with EDIs. As influent volume increases, conventional draw‐off pipes cannot handle high sludge capacities, deforming the sludge blanket and leading to lower concentration of solids being withdrawn. Retrofitting the design of draw‐off pipes is an effective way to mitigate underflow short‐circuiting and enhance treatment performance. In this study, a snail‐shaped sludge draw‐off pipe was designed and tested in two types of EDIs using computational fluid dynamics tools, showing a 20% increase in withdrawn sludge concentration and mitigating underflow short‐circuiting potential. The optimal retrofit option was identified as equipping the clarifier with a snail‐shaped draw‐off pipe and an innovative EDI, known as multilayer EDI column, which would save almost half of the energy and operational costs of the biological processes while meeting discharge limits.