Sludge thickening and its methods

Thickening

Thickening (concentration) is the process of increasing the solids content of sludge by removing part of its water, so that the volume to be handled is reduced. For example, raising the solids from 2% to 5% reduces the volume to $2/5$ of the original, i.e. by 60%.

Purpose

• To reduce the volume of sludge, so that digesters, pumps, pipes, dewatering units and transport are smaller and cheaper.
• To increase the solids loading to digesters and so reduce the heat required for heating them.
• To reduce the chemical demand in conditioning.
• To remove supernatant and give a more uniform feed.

Methods of sludge thickening

1. Gravity thickening (most common for primary and mixed sludge).
2. Dissolved-air flotation (DAF) for waste activated sludge: air bubbles carry the light solids to the surface.
3. Centrifugal thickening (for waste activated sludge).
4. Gravity belt or drum thickeners.
5. Simple settling in the digester itself (supernatant withdrawal) and co-settling in the primary tank.

Gravity sludge thickener

 sludge in -> [feed well]   motor
              |    |        |
  supernatant |    v        | picket fence rake
  <-- weir ~~~~~~~~~~~~~~~~~~~~~~~~~~ water level
      |      \      thickening zone     /|  3-4 m
      |       \_ ___  ___  __/
      |    slope 1 in 6 to 1 in 4 floor
      |            \_/  sludge hopper
      +-------------> thickened sludge to digester

• A circular tank (like a sedimentation tank), 3-4 m side depth, with a floor slope of 1 in 6 to 1 in 4 and a deep central hopper.
• A very slowly rotating rake (tip speed of only a few metres per minute) with vertical pickets stirs the sludge gently, releases trapped water and gas, and moves the sludge to the hopper.
• Dilute sludge enters at the central feed well. The solids settle and compress, and clear supernatant overflows through peripheral weirs (and is returned to the plant inlet).
• The thickened sludge (5-10% solids for primary sludge) is drawn off from the hopper by pump.
• Design uses a solids loading of about 100-150 kg/m²/day for primary sludge and 20-40 kg/m²/day for activated sludge, and a hydraulic loading of 15-30 m³/m²/day.