| (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.
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