Waste water treatment: sedimentation to produce sewage sludge and effluent

The idea

After screening and grit removal, waste water is held in large sedimentation tanks so that suspended solids can settle. Solid organic material sinks to the bottom as sewage sludge, while the clearer liquid remaining above is the effluent. Sedimentation works because the solid particles are denser than water and settle under gravity when the water is allowed to move slowly. The sludge is drawn off from the bottom and the effluent from the top, so the two streams can be treated separately. Sludge may be digested anaerobically to produce biogas and a residue used as fertiliser, while effluent undergoes further aerobic biological treatment and disinfection before being released to a river or sea. Sedimentation is therefore a physical separation step that reduces the solid content of the water but does not by itself make the effluent safe to release.

What earns the marks

• Sedimentation allows suspended solid particles to settle under gravity in large tanks where the water moves slowly.
• The settled solid organic material is called sewage sludge and collects at the bottom of the tank.
• The clearer liquid remaining above the sludge is called effluent and is drawn off separately.
• Sedimentation is a physical separation process: it does not remove dissolved substances or kill microorganisms.
• Sludge and effluent are treated separately, for example sludge by anaerobic digestion and effluent by aerobic biological treatment and disinfection.
• Sedimentation reduces the solid content of the waste water and prepares the effluent for the next treatment stages.

Common mistakes and how to put them right

• Reversing the products: sludge is the settled solid at the bottom, while effluent is the liquid layer above it.

• Claiming sedimentation makes effluent safe to drink.

  Put it right: It only removes settleable solids, so dissolved pollutants and pathogens remain and need further treatment.

• Treating sedimentation as a chemical reaction.

  Put it right: It is a physical process in which denser particles settle under gravity.

Exam tips

• Name both products and state where each is collected: sludge at the bottom, effluent drawn from the top.
• Explain the separation in terms of density and gravity rather than saying the solids simply disappear.
• Link sedimentation to the stages around it: it follows screening and grit removal and is followed by further treatment of sludge and effluent.