Activated sludge: properties, mechanism

Activated sludge is the flocculent, brown, microbial mass formed in aerated sewage; it consists of bacteria, protozoa, fungi, rotifers and organic and inorganic solids, and is capable of adsorbing and oxidising organic matter in sewage. A part of it is returned to the aeration tank to keep the process active.

Properties of activated sludge

• Colour and odour: healthy sludge is brown to chocolate and has an earthy odour; black colour and foul smell show septic (oxygen-deficient) conditions.
• Floc form: gelatinous, sticky floc, 50-200 μm in size, which settles well.
• Concentration: MLSS 2000-4000 mg/l in the tank; return sludge 8000-12000 mg/l (about 0.5-1% solids).
• Sludge volume index (SVI): $SVI=\dfrac{\text{volume of settled sludge (ml/l)}\times1000}{MLSS\ (\text{mg/l})}$; good sludge has SVI 50-150 ml/g; above 150 is bulking sludge.
• Specific gravity: slightly above 1 (about 1.005-1.02), so it settles slowly but compacts.
• Moisture: about 99% water in the aeration tank and 98-99% in the secondary underflow.
• Biological activity: mostly aerobic bacteria (Zoogloea ramigera, Pseudomonas, Flavobacterium), with ciliated protozoa which graze free bacteria and make the effluent clear.
• Volatile fraction: MLVSS is 70-80% of MLSS.

Biochemical mechanism

  Organic matter + O2 + nutrients
        |        (bacterial floc)
        v
  1. Adsorption on floc  ---- (first 15-30 min)
  2. Oxidation  -> CO2 + H2O + energy
  3. Synthesis  -> new cells (more sludge)
  4. Endogenous respiration -> cells + O2 -> CO2 + H2O + NH3
  5. Flocculation -> settling in clarifier -> RAS / waste

1. Adsorption (biosorption): colloidal and suspended organic matter is quickly adsorbed on the floc surface. Soluble matter is taken up through cell walls with the help of exo-enzymes.
2. Oxidation: $\text{organics}+O_2\rightarrow CO_2+H_2O+\text{energy}$ (about one-third to one-half of the BOD removed).
3. Synthesis: $\text{organics}+O_2+NH_3\rightarrow C_5H_7NO_2\ (\text{new cells})$ with energy from step 2.
4. Endogenous respiration: $C_5H_7NO_2+5O_2\rightarrow5CO_2+2H_2O+NH_3+\text{energy}$, reducing the sludge quantity.
5. Nitrification (at long sludge age): $NH_4^+\rightarrow NO_2^-\rightarrow NO_3^-$.
6. Flocculation and settling: the bacteria secrete a polysaccharide slime that binds the cells into flocs, which settle in the clarifier. Settled sludge is returned to the aeration tank and the excess is wasted.