The Sewage Treatment Process

The Sewage Treatment Process

The collection, treatment and disposal of sewage is an integral part of the water cycle in our city.
This brochure explains the process undertaken in our sewage treatment plants. It follows the journey from
collecting sewage from homes and businesses, through to releasing recycled water to our environment.

Sewage treatment process

1. Pre-treatment
Sewage enters the treatment plant and is screened. Rotating
drums remove the inorganic waste such as plastic and other
larger objects from the sewage. The inorganic waste is taken
to a waste facility.

2. Grit removal
Grit removal eliminates inorganics and solids to protect
downstream process equipment from abrasion and blockage.
Grit is often made up of sand and stones that pass through
the sewers to the pre-treatment zone.

3. Aeration tanks (bioreactors)
Bioreactors are large tanks containing
many micro-organisms that make up
what's known as activated sludge. The
bioreactor provides a range of conditions
necessary for a variety of bacteria to grow
and work to remove the contaminants
(e.g. nitrogen and phosphorus).

4. Clarifier
This area allows the activated sludge
floc particles to settle in a conicalshaped
tank leaving clear liquid on the
surface that is decanted as effluent.
The settled sludge in the bottom of the
clarifier is returned to the aeration tanks
to mix with the incoming sewage and
ensure an adequate biological population (FM ratio) to remove nutrients and pollutants.

This is also where the treatment plant stream divides to
Output A: Recycled water and Output B: Biosolids.

Output A: Recycled water

5a Chlorine contact tank
The effluent travels from the secondary clarifier in pipes to
the chlorine contact tank. The effluent is contained here
for 30 minutes ‘contact time' with chlorine to kill bacteria
and pathogens.
6a Storage lagoon
This is the storage area for the final product recycled water,
which is pumped out on an outgoing tide or supplied to
recycled water customers for uses including golf course and
sporting field irrigation or dust suppression on construction
and development sites.

Output B: Biosolids used for fertiliser

5b Belt press
Rollers in the belt press squeeze water out of the sludge toform a ‘solid sludge cake'.

6b Sludge drying beds
Rollers in the belt press squeeze water out of the sludge to
form a ‘solid sludge cake'.
6b Sludge drying beds

Further information