(a) Solids Recycling and Monitoring.(1) A return sludge system must operate as designed in all flow conditions.(2) A monitoring and control system must provide a means to control return and waste sludge flows from each clarifier, to control return sludge flows into each aeration basin, to meter return sludge flows, and to measure waste sludge flows.(b) Solids Wasting. The solids management system must be able to store and process the waste activated sludge under all flow conditions.(c) Return Activated Sludge (RAS) Pump Design.(1) A centrifugal sludge pump must have a positive suction head, unless the pump is self-priming.(2) An airlift pump must comply with requirements of §217.162 of this title (relating to Air Lift Pump Design).(3) An RAS system must have sufficient pumping units to maintain the maximum design return pumping rate with the largest single pumping unit out of service.(d) Waste Activated Sludge Pump Design. A waste activated sludge pumping system requires at least two pumping units and must be sized to prevent excessive solids accumulation in the clarifiers.(e) Sludge Piping System.(1) The design of a sludge piping system must accommodate cleaning and flushing.(2) The design of a sludge piping system must accommodate a minimum velocity of 2.0 feet per second at the maximum wasting rate to prevent solids from settling. The velocity in a sludge piping system must prevent scouring.(3) A sludge pipe must have a minimum diameter of 4.0 inches.