Sludge treatment need; digester sketch

Necessity of sludge treatment

Sludge is the semi-solid residue (about 95-99% water) removed from screens, sedimentation tanks, filters and activated sludge units. Raw sludge is a highly putrescible mixture of organic matter, pathogens and heavy metals, so it must be treated before disposal.

• Prevent odour, flies and insect nuisance from putrescible sludge.
• Destroy pathogens and protect public health.
• Reduce the volume and make handling and disposal easier.
• Stabilise the organic matter and recover biogas.

Sludge digester (anaerobic, floating-cover type)

        gas dome / floating cover
      gas out <-  ____|____
   sludge in ->  |  gas    |<- supernatant draw-off
   ~~~~~~~~~~~~~~|~~~~~~~~~|~~~~~~~~~ liquid level
   (heated)      | scum    |
   [heating coil] supernatant
                 | digesting sludge |
                 \  digested sludge /
                  \_______________/ slope 1:1 hopper
                     digested sludge out

Construction: a closed circular tank (diameter 6-30 m, depth 6-12 m), with a conical hopper bottom for sludge withdrawal, a fixed or floating gas-collecting cover (to avoid air entering), inlet for raw sludge, draw-off pipes at different levels for supernatant, a pipe for digested sludge, a gas outlet, a heating coil or heat exchanger to keep 35°C, and a mixer or recirculation pump (in high-rate type).

Working (three stages):

1. Acid fermentation: acid-forming bacteria break down the complex organic matter into volatile acids; pH falls to 5-6.
2. Acid regression: the acids are partly broken down, and the pH rises slowly.
3. Alkaline (methane) fermentation: methane bacteria convert the acids to CH₄ and CO₂ at pH 6.8-7.4. The gas (65% CH₄) collects under the cover and is used for heating.

Raw sludge is fed regularly; after 30-60 days (conventional rate), the stabilised sludge is withdrawn from the bottom to drying beds, the supernatant is drawn off and returned to the plant inlet.