Simplex method of aeration

The simplex aerator is a mechanical surface aeration device used in the activated sludge process (a mechanical, non-diffused system).

Purpose

To supply oxygen to the mixed liquor in the aeration tank and to keep the activated sludge flocs in suspension and well mixed with the sewage, without compressed-air diffusers.

Working

   motor + gear
      |
   +--+--+            ~~~~~~~~~~~~~~ spray at surface
   | shaft|      draft tube/cone  ^ sewage lifted
   +--+--+ ___________|__|_______|_
  ~~~~|~~~~~~|  cone  /  \   /|~~~~~~  water level
      |      | impeller   draft tube
      |______|_____|______
        aeration tank (liquor drawn up from the bottom)

• A vertical hollow draft tube (uptake tube) is fixed at the centre of the tank, with a cone-shaped impeller at its top, driven by a motor through a gear box.
• The impeller draws the mixed liquor up through the tube from the bottom and throws it out over the surface as a thin sheet or spray, so that it picks up oxygen from the air.
• This sets up a continuous circulation in the tank, so the whole of the mixed liquor is mixed and aerated and the flocs stay in suspension.
• Sewage and return sludge flow into the tank, and the mixed liquor goes to the secondary settling tank.

Design considerations

• Oxygen transfer is about 1.2-2 kg O₂ per kWh; select the aerator for the oxygen requirement of the BOD load.
• The tank is usually square (or circular), with a side water depth of 3-4.5 m, and is sized so that one aerator can keep its whole plan area mixed and aerated.
• The power per unit volume must keep the MLSS in suspension (about 10-20 W/m³ for mixing).
• The tank is designed from the F/M ratio (0.2-0.4), MLSS (2000-4000 mg/l) and detention time (6-8 h).
• Provide standby units, access for maintenance and a splash guard against spray and noise.
• Wind and temperature affect efficiency; a sludge return line and a scum removal arrangement are needed.