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

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cannot handle high sludge capacities, deforming the sludge blanket and leading to lower concentration of solids being withdrawn.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pipe was designed and tested in two types of EDIs using computational fluid dynamics tools, showing a 20% increase in withdrawn sludge concentration and mitigating underflow short-circuiting potential.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.