Returned Activated Sludge

Microorganisms are an essential part of the wastewater treatment process. They are added to the aeration chamber and allowed to digest organic matter in the water.  Then they are removed in the clarifier.  Some of the microorganisms from the clarifier are added back to the aeration basin to start the process over again.  The use of microorganisms to remove dissolved organic materials from the water is a form of secondary treatment of the wastewater.

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.