ASP working, advantages and disadvantages

Activated sludge process (ASP)

The activated sludge process is an aerobic, suspended-growth biological treatment in which settled sewage is mixed and aerated with a flocculent mass of micro-organisms called activated sludge. The organisms oxidise organic matter, and the mixed liquor is then settled; a part of the settled sludge is returned to the aeration tank to maintain the biomass.

Working

1. Primary treatment (screens, grit chamber, primary sedimentation) removes about 30-35% of the BOD.
2. Aeration tank: the settled sewage is mixed with return activated sludge (RAS). Air (diffused or mechanical) supplies oxygen and keeps the flocs in suspension for 4-8 hours. Bacteria and protozoa in the floc adsorb and oxidise organic matter to CO₂ and water and form new cells.
3. Secondary sedimentation tank: the flocs settle and the clear effluent (BOD 20-30 mg/l) overflows.
4. Sludge return: 25-50% of the settled sludge is recycled to the aeration tank to keep MLSS at about 2000-4000 mg/l. The excess (waste) sludge is sent to thickening and digestion.

 Raw    +--------+ +-----+ +---------+ +---------+   Effluent
 sewage |Screen/ | |PST  | |Aeration | |Secondary|-->
 ------>|Grit    |>|     |>|tank+air |>|settling |
        +--------+ +--+--+ +----^----+ +----+----+
                      |        | RAS       |
                      |        +-----------+
              primary sludge     waste sludge -> thickener
                                 -> digester

Advantages

• High BOD removal efficiency (85-95%) and a good quality effluent.
• Needs less land than a trickling filter or oxidation pond.
• No fly or odour nuisance if properly run; no head loss problem.
• Flexible: loading can be adjusted (conventional, extended aeration, step aeration, etc.).
• Capable of nitrification with long sludge age.

Disadvantages

• High capital and operating costs, especially for power for aeration.
• Needs skilled and continuous operation and control (F/M, MLSS, DO, sludge return).
• Sensitive to shock loads and toxic substances.
• Large quantity of sludge is produced; it needs treatment and disposal.
• Sludge bulking (high SVI) and foaming problems may occur.