sewage i believe is generally referred to in sections of primary, secondary and tertiary
Primary refers to the removal and/or sedimentation of solids
seives catch solid material such as plastics. chemicals may be added to assist sedimentation
Secondary refers to the chemical breakdown. This is where bacteria (both aerobic - with oxygen and anaerobic - without oxygen) "eat" or breakdown the biological material. basically they "digest" the nitrates and phosphates.
These nitrates and phosphates are converted by the bacteria into other materials which is termed "activated sludge".
This activated sludge is removed/sedimented out as a part of the secondary phase of treatment.
This process can be done in tanks (where the foecal-water is aerated for periods, then not aerated) or by pouring the water over the surface of jagged rocks where the bacteria digest it. Both methods are viable, and both require sedimentation of activated sludge.
Tertiary treatment is not always undertaken in all circumstances. It refers to the sterilisation and/or release of water.
This can be accomplished by the use of
UV Light - breaks down cell structure
OR
Chlorine - used in bleach and an excellent 'bug' killer
OR
Ozone gas - basically it is poisonous at ground level
Some water is simply released into the oceans untreated.
The detail is not really required; they just want you to know a flow chart i believe. that means a small piccie and arrows between
Primary - remove solids
----->
Secondary - bacteria break down material
- activated sludge removed
----->
Tertiary - Sterilisation (UV, Cl or Ozone)
Release
pee.dot.ess
i am not perfect and definitely not always right
i hope it has helped