Mass balances, activated sludge plants
Fig4.3
Activated sludge floes magnified 200 times. The floes include micro-organisms,
organic and inorganic particles. They are glued together by polymers produced by the
bacteria. Dense regular floes with protozoa indicate that the biomass is healthy. In the
event of inhibition or other mistreatment, the protozoa will die simultaneously with
the disaggregation of the floes. Filamentous bacteria (not visible in this picture) re duce
the settling and concentration ability of the floes.
(Photo: Consorcio des Aquas, Bilbao)
The concentration of sludge is increased while passing through the aeration tank. A
sludge mass balance around the aeration tank itself may be used to calculate the
effluent concentration, Xs,2.2 (see Fig 4.2).
or
Xs,z.z= Xs,2.1 + (Cz.t- Cz.v · Yobs
(4.6)
Example 4.3 shows how to calculate the effluent concentration from the aeration
tank. The amount of the sludge concentration in the aeration tank itself, Xs,2..
depends on the hydraulics. If the tank has an ideal mix
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