This disclosure relates to physical selection, deselection or outselection for smaller, less dense, sheared or compressed particles in sludge, wherein the first deselection step occurs at the reactor or at a clarification step, by separately deselecting for such particles and then a second deselection step occurs in an external selector. This double deselection promotes the more efficient removal of slow settling particles, while simultaneously allowing for maintenance of multiple solids residence times for fast and slow growing organisms. The deselection in a clarifier occurs typically at the periphery of the tank or at the surface of a blanket using a positive or negative pressure device. Structures such as slotted or perforated plates, pipes or manifolds can be used to assist in such deselection. Baffles can also be used for such deselection.