Waste water treatment: aerobic biological treatment of effluent.

The idea

After screening and sedimentation, the liquid effluent still contains dissolved and suspended organic matter. Aerobic biological treatment uses microorganisms that respire aerobically to break down this organic material. Air is bubbled through the effluent, or the liquid is trickled over surfaces, so oxygen dissolves and aerobic bacteria can respire. The bacteria digest organic matter, converting it into carbon dioxide, water and other harmless products, and the treated effluent is clean enough to be released into rivers or the sea. The process is assessed by describing the oxygen supply, the role of microorganisms and the reduction in organic content.

What earns the marks

• Effluent is the liquid remaining after screening and sedimentation, and it still contains organic matter that must be removed.
• Aerobic microorganisms break down the organic matter in the effluent by aerobic respiration.
• Oxygen must be supplied, for example by bubbling air through the effluent or trickling it over a bed, so the microorganisms can respire.
• The organic matter is converted into carbon dioxide, water and other less harmful substances, reducing the effluent's polluting effect.
• The treated effluent can be discharged safely into rivers or the sea because its organic content has been greatly reduced.
• The process relies on microorganisms, so conditions such as oxygen supply and temperature must be controlled for effective treatment.

Common mistakes and how to put them right

• Saying the effluent is treated without oxygen.

  Put it right by stating that air or oxygen is deliberately supplied for aerobic respiration.

• Confusing effluent with sludge.

  Put it right: Effluent is the liquid, while sludge is the settled solid treated separately by anaerobic digestion.

• Believing the microorganisms are removed by the treatment.

  Put it right: Explain that they carry out the breakdown, and the treated liquid is then discharged.

Exam tips

• Name the gas supplied as oxygen or air and link it explicitly to aerobic respiration by microorganisms.
• Describe one way oxygen is supplied, such as bubbling air through the effluent, to show understanding of the process.
• State that organic matter is broken down into carbon dioxide and water, and that this makes the effluent safe to release.