Returned activated sludge is sludge from the secondary
clarifier which is aged and stressed and returned to the aeration basin.
The microorganisms in this sludge are very efficient at digesting organic
matter in the aeration basin.
The sludge must be aged and stressed, or activated, before it is returned to the aeration basin. Each day's sludge (also known as floc) is pumped out of the clarifier and into a holding basin. There, it is added to the top of the previous day's sludge, as shown below. The sludge is allowed to age for ten days before it is pumped as seed to the aeration chamber.
The 10 days which sludge is aged is very important.
Aging stresses the sludge. Without any food, the microorganisms
in the sludge become very hungry. As a result, when they are added
to the aeration chamber, they are ready to feed again and are able to
reduce the B.O.D. in the water in a short period of time.
Stressing the microorganisms also prompts them to multiply
rapidly. This results in even more microorganisms to quickly eat
up the organic matter in the wastewater.
Packaged plants and extended aeration plants are both
able to remove B.O.D. in about two hours or less because of the aged
sludge used. The aged sludge provides optimum food removal and results
in a good quality effluent.