Direction: The sludge from the aeration tank of the activated sludge process (ASP) has solids content (by weight) of 2%. This sludge is put in a sludge thickener, where sludge volume is reduced to half. Assume that the amount of solids in the supernatant from the thickener is negligible, the specific gravity of sludge solids is 2.2 and the density of water is 1000 kg/m³.

What is the density of the sludge removed from the aeration tank?

 990 kg/m³
 1000 kg/m³
 1011 kg/m³
 1022 kg/m³
Correct Option: C
Density =  
sludge water 
=  
V_{sludge} + V_{water} 
=  
+  
GP_{w}  P_{w} 
= 1011.03 kg/m³.