Environmental Engineering FOURTH EDITION

Wastewater Treatment 185

Table 9-3. Loadings and Efficiencies of Activated Sludge Systems

Loading: F/M Aeration period BOD removal

Process (lb BOD/day/lb MLSS) (h) efficiency (%)

Extended aeration 0.05-0.2 30 95

Conventional 0.2-0.5 6 90

High rate 1-2 4 85

EXAMPLE 9.4. The BOD5 of the liquid from the primary clarifier is 120 mg/L at a flow

rate of 0.05mgd. The dimensions of the aeration tank are 20 x 10 x 20 ft3 and the

MLSS = 2000 mg/L. Calculate the F/M ratio:

lb MLSS = (20 x 10 x 20) ft3 (2oyg) - (5) (?$El) (E)

x (&) = 229 lb

F 50 lb BOD/day

• 
  
  
  
  • 
    
    
    
    • 
      

      = 0.22
      M 229

      
      

      lb MLSS

      
      
    
    
    
    
  
  
  
  

When the microorganisms begin to metabolize the food, a great deal of oxygen is

required. Accordingly, the DO in the aeration tank decreases suddenly after the point

at which waste is introduced. The resulting low DO concentration at the influent end of

the aeration tank may be detrimental to the microbial population. In order to sustain the

microorganisms, the activated sludge process may incorporate a variation like tapered

aeration or step aeration (see Fig. 9-13). In tapered aeration, air is blasted into the

tank where needed, while in step aeration, waste is introduced at several locations in

the tank, thus spreading out this initial oxygen demand.

Another process modification is contact stabilizution, or biosorption, in which the

sorption and bacterial growth phases are separated by a settling tank (see Fig. 9-14).

Contact stabilization provides for growth at high solids concentrations, thus saving tank

space. An activated sludge plant can often be converted into a contact stabilization plant

when tank volume limits treatment efficiency.

The two principal means of introducing sufficient oxygen into the aeration tank are

by bubbling compressed air through porous diffusers or by beating air in mechanically.

These are shown in Figs. 9-15 and 9-16.

The success of the activated sludge system also depends on the separation of the

microorganisms in the final clarifier. When the microorganisms do not settle out as

anticipated, the sludge is said to be a bulking sludge. Bulking is often characterized

by a biomass composed almost totally of filamentous organisms that form a kind of