Activated Sludge
The sludge from the final clarifiers is mostly returned to the aeration tanks.
Biomass is created in the aeration tanks and therefore some sludge must be
wasted to keep the system from clogging up. The Waste Activated Sludge (WAS)
is sent to a concentrator. In the concentrator a flocculant is added and the
sludge is dewatered to get the WAS to about 3 to 4 percent solids. The
sludge is then pumped into a second sludge holding tank (recall that there
was one holding tank for just primary sludge). The sludge level in the primary
tank is controlled to keep a constant 3 foot blanket. The secondary sludge
level is controlled by a timing system. Sludge in either tank must pass
through an in line grinder before going on for further dewatering.
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