Conventional Activated Sludge (CAS)

Conventional Activated Sludge (CAS) system commonly include an aeration tank, which is used for biological degradation, and a secondary clarifier (sedimentation tank), where the sludge in separated from the treated wastewater (refer to process flow diagram).
The first step of a CAS system is the aeration tank, where the wastewater is mixed with air to activate micro-organisms. While digesting the wastewater, the organisms collide with each other, forming larger particles called flocs, which have a larger capacity to degrade the biological components of the wastewater.


The aeration basin is followed by a secondary clarifier or settling tank. During this step, micro-organisms with their adsorbed organic material settle.
Water from the clarifier is transported to installations for disinfection and final discharge or to other tertiary treatment units for further purification.
The surplus micro-organisms can easily be channeled to any of our sludge treatment solutions where energy can be recovered from the bio solids. This additional step closes the energy cycle of the wastewater treatment plant allowing it to run independently of fossil fuel.
Another part of the micro-organisms is fed back into the aeration tank in order to keep the load of micro-organisms at a sufficient level for the biological degrading processes to continue.

Advantages of Conventional Activated Sludge (CAS):

• Low installation cost (Cost effective)
• Good quality effluent
• Small land area requirement
• Loss of head is small
• Freedom from fly and odor nuisance high degree of treatment
• Easily maintained mechanical work
• Self-sustaining system


Activated Sludge Principles
• Wastewater is aerated in a tank 
• Bacteria are encouraged to grow by providing 
• Oxygen 
• Food (BOD)
• Nutrients 
• Correct temperature 
• Time 
• As bacteria consume BOD, they grow and multiply 
• Treated wastewater flows into secondary clarifier 
• Bacterial cells settle, removed from clarifier as sludge 
• Part of sludge is recycled back to activated sludge tank, to maintain bacteria population 
• Remainder of sludge is wasted


Table : Basic design parameters for conventional activated sludge process



Basic considerations for design of a conventional activated sludge (CAS) 

BOD5 and COD removal efficiency in conventional activated sludge process:


Comparison of SBR, CAS and MBR processes:

Kinetics of Microbial Growth

• Biochemical reaction

• Biomass concentration. 
• The concentration of biomass, X (mg/L), increases as a function of time due to conversion of food to biomass:

Where m is the specific growth rate constant (d-1). This represents the mass of cells produced/mass of cells per unit of time.

Effect of substrate concentration on growth rate constant

Monod Kinetics
• Growth rate 
o Growth rate constant, m , is a function of the substrate concentration, S. 
o Two constants are used to describe the growth rate 
o m m (mg/L) is the maximum growth rate constant (the rate at which the susbtrate concentration is not limiting) 
o Ks is the half-saturation constant (mg/L) (i.e., concentration of S when m = m m/2 

• Biomass production

• Where kd represents the endogenous decay rate (d-1) (i.e., microorganism death rate).
– Substituting the growth rate constant:

• Substrate utilization

Where Y is the yield factor (mg of biomass produced/mg of food consumed)
• Y range:
– Aerobic: 0.4 – 0.8 mg/mg

• Food to microorganism ratio (F/M) 
• Represents the daily mass of food supplied to the microbial biomass, X, in the mixed liquor suspended solids, MLSS 
• Units are Kg BOD5/Kg MLSS/day


• Since the hydraulic retention time, q = V/Qo, then

Typical range of F/M ratio in activated sludge units


Design parameters for activated sludge processes


Operational characteristics of activated sludge processes


Activated Sludge Design Equations

Mass balance of food substrate

Overall equations