Waste water treatment: anaerobic digestion of sewage sludge

The idea

Sewage treatment produces sludge, the settled solid material removed from waste water. Anaerobic digestion treats this sludge in a sealed tank where microorganisms break it down in the absence of oxygen. The process reduces the volume and mass of sludge, kills many harmful microorganisms and produces biogas, mainly methane and carbon dioxide. The biogas can be burned to generate electricity or heat, making the treatment self-supporting. The remaining digested sludge is rich in nutrients and can be used as fertiliser on farmland, provided it meets safety standards.

What earns the marks

• Sludge is the solid material that settles out during waste water treatment and must be treated before disposal.
• Anaerobic digestion occurs in a sealed tank, so oxygen is absent and anaerobic microorganisms decompose the sludge.
• The process produces biogas, which contains methane and can be burned as a fuel for heating or generating electricity.
• Digestion reduces the volume and mass of the sludge, lowering transport and disposal costs.
• Harmful microorganisms in the sludge are largely destroyed, reducing the risk of disease when the digested sludge is used.
• The digested sludge retains nutrients and can be applied to land as fertiliser, subject to appropriate controls.

Common mistakes and how to put them right

• Thinking that anaerobic digestion uses oxygen.

  Put it right by stressing that the tank is sealed and oxygen is excluded, so anaerobic respiration occurs.

• Confusing biogas with natural gas.

  Put it right: Biogas is a mixture, mainly methane and carbon dioxide, not pure methane.

• Assuming all sludge is simply buried.

  Put it right: Explain that digestion reduces and sanitises it, and the residue can be used as fertiliser.

Exam tips

• State clearly that the digester is sealed or oxygen-free, because this is the key condition for anaerobic digestion.
• Name biogas and give methane as its useful combustible component, then link burning it to generating electricity or heat.
• Give two benefits of digestion, such as reduced sludge volume and destruction of pathogens, to cover the treatment and disposal points.