1. (a) Solids Recycling and Monitoring.

   1. (1) A return sludge system must operate satisfactorily in all flow conditions.
   2. (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.
2. (b) Solids Wasting. A design must be adequate to store and process the waste activated sludge under all flow conditions.

3. (c) Return Activated Sludge (RAS) Pump Design.

   1. (1) A centrifugal sludge pump must have a positive suction head, unless the pump is self-priming.
   2. (2) An airlift pump must allow for cleaning without removal from a basin.
   3. (3) A RAS system must have sufficient pumping units to maintain the maximum design return pumping rate with the largest single pumping unit out of service.
4. (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.

5. (e) Sludge Piping System.

   1. (1) The design of a sludge piping system must accommodate cleaning and flushing.
   2. (2) A sludge piping system must be sized for a minimum velocity of 3.0 feet per second at the maximum waste or return rate to prevent solids from settling and must prevent scouring at anticipated normal operating conditions.
   3. (3) A sludge pipe must have a minimum diameter of 4.0 inches.

Source Note:The provisions of this §217.158 adopted to be effective August 28, 2008, 33 TexReg 6843.