Activated sludge and ASP treatment process

Activated sludge is the biologically active, brown, flocculent mass of bacteria, protozoa, fungi and other micro-organisms, together with organic solids, that is produced when sewage is aerated for some hours. It adsorbs and oxidises organic matter and settles readily; a portion is returned to the process (hence "activated").

Treatment process (conventional ASP)

 Raw      +---------+  +-----+  +----------+  +----------+
 sewage ->| Screen  |->| PST |->| Aeration |->|Secondary |-> Effluent
          | + grit  |  |     |  | tank+air |  |clarifier |
          +---------+  +--+--+  +-----^----+  +----+-----+
                          |           | return     |
                    primary sludge    | sludge(RAS)|
                          |           +------------+
                          |                  | excess sludge
                          +--> thickener -> digester -> drying bed

1. Preliminary and primary treatment: screens remove floating matter, the grit chamber removes grit and the primary sedimentation tank (PST) removes settleable solids and about 30-35% of the BOD.
2. Aeration tank: the settled sewage is mixed with return activated sludge. Compressed air through diffusers (or surface aerators) supplies oxygen and keeps the flocs in suspension for 4-8 h (MLSS 2000-4000 mg/l). Bacteria oxidise the organic matter and build new cells.
3. Secondary clarifier: the mixed liquor flows in, the flocs settle in 2-3 h and the clear effluent (BOD 20-30 mg/l) overflows.
4. Return sludge: 25-50% of the settled sludge is pumped back to the aeration tank to maintain the MLSS.
5. Excess sludge: the surplus is wasted to thickening and digestion, then to drying beds, because it would otherwise raise the MLSS.
6. The BOD removal is 85-95%, and the effluent may be disinfected before discharge.

Modifications: tapered aeration, step aeration, contact stabilisation, extended aeration, oxidation ditch and sequencing batch reactor.