A Case for Low Return Sludge Flow Rates

A common problem encountered by traditional activated sludge systems involves failure
to develop biomass that separates efficiently from the liquid, leaving behind a clear
effluent that is low in BOD5 and suspended solids. Another problem is the bleed-through
of ammonia due to low detention time in the aeration tank. Oftentimes, failure may be
attributed to high return sludge flow rates (RSF) that affect not only clarifier hydraulics,
but also the growth of bacteria in the system. In order to promote efficient separation and
nitrification, system conditions should be maintained that favor the growth of flocforming
bacteria and nitrifiers over nuisance microorganisms that may include filaments.
Favorable conditions are encouraged by a regime of higher detention time and feast and
famine experienced by the bacteria in the system. By viewing system operation through
this lens, the following paper proposes that many activated sludge treatment systems can
achieve significant operational improvement through reduction in RSF. This paper
further provides a method for minimizing RSF and presents examples of successful
application of this method.