Wastewater treatment levels

Treating wastewater is about removing or breaking down what people have added to the water that leaves their home or business.

We use different processes to remove impurities from wastewater at our treatment plants. The type of treatment needed depends on:

  • the location of the plant
  • where the treated water will be discharged or re-used
  • the nature of the plant’s catchment area, including wastewater quality
  • when the plant was built.

What are the treatment levels?

Aerial view of North Head Wastewater Treatment Plant

Our North Head plant is on the coast near Manly.

Primary treatment includes screens, sedimentation and grit removal.

Primary treatment methods include:

  • filtering wastewater through fine screens to remove items such as paper, cotton tips and plastic
  • removing sand and grit that has fallen to the bottom of aerated grit tanks
  • removing solids that have settled to the bottom of sedimentation tanks
  • removing oil and grease that floats to the top of tanks using scrapers.

The main purpose of secondary treatment is to remove nutrients.

Biological reactor system

The biological reactor creates different environments for microorganisms to treat pollutants in wastewater. There are five key stages:

Fermentation tank

Solids from the sedimentation tanks are broken down to produce a better carbon supply for microorganisms in the anoxic and aerobic zones. This makes it easier to remove phosphorus. 

Anaerobic zone

Water from primary treatment is pumped into the anaerobic zones. Microorganisms absorb carbon into their cells and release phosphates.

Anoxic zone

No oxygen is available for microorganisms. They use carbon in the organic matter as a food source, converting nitrates to nitrogen gas which is released to the atmosphere.

Aeration zone

The air works with microorganisms in the tank to further break down the wastewater. The microorganisms also take up phosphorus from the wastewater. This results in phosphorus-rich solids that are used to make biosolids.

Secondary clarifiers

Remaining solids are settled in a tank. The settled solids can be returned to the anaerobic zone and the clear wastewater maybe sent on for tertiary treatment.

Intermittently decanted aerated lagoons (IDAL) 

An alternative secondary process is the intermittently decanted aerated lagoons (IDAL).  

Settled wastewater is pumped from the primary distribution structure to the IDAL anaerobic zone. Iron-rich spent pickle liquor is added to help remove phosphorus.

In the IDALs, wastewater goes through three stages in the one tank - aeration, settling and decanting.


 In this stage, air is pumped into the IDAL through diffusers. The air works with microorganisms in the tank to break down:

  • ammonia into nitrates and water (nitrification)
  • organic matter, reducing the BOD.


In this stage, air is no longer pumped into the tank and the water is still.

No longer supplied with oxygen, the microorganisms use carbon in the organic matter as a food source, converting nitrates to nitrogen gas. This is then released to the atmosphere.

The solid particles settle to the bottom. Some go to a thickening tank before being treated for biosolids production. The rest of the solids are returned to the IDAL to provide microorganisms for incoming wastewater.


After settling, the clear wastewater flows over weirs from the top of the lagoon into an equalising basin. This basin controls the flow to the tertiary treatment process.


Wastewater contains nutrient-rich solids. We treat these solids so they can be re-used as biosolids to improve soil for agriculture and gardens. 

There are four key steps in the treatment process:

  1. Solids are collected from the primary and secondary treatment tanks.
  2. Settled solids may go into digestion tanks for further break down of organic matter.
  3. Solids go through a range of different processes to remove water, for example using centrifuges. The matter left behind is known as biosolids.
  4. Biosolids are ready for re-use in agriculture, forestry, land rehabilitation and landscaping.

Tertiary treatment includes filtering, disinfecting and preparing wastewater for recycling.

Filtering and disinfecting

Treated wastewater from a biological reactor and an intermittently decanted aerated lagoon (IDAL) are combined.

We add alum to help remove additional phosphorus particles and group remaining solids together for easy removal in the filters.

The treated wastewater then flows to sand filters. The wastewater sinks down through these filters where the sand traps particles.

Filtered water then flows to a chlorine contact tank for disinfection. After the water is disinfected, we remove any remaining chlorine before discharging the treated wastewater. Alternatively, we may use ultraviolet lamps for disinfection.

Preparing wastewater for recycling

Imag eof aerial view of Wollongong Water Recycling Plant

Our Wollongong plant supplies recycled water to industry.

Treated wastewater from biological reactors can be passed through deep sand filters where the sand traps any remaining particles. Then clear wastewater goes to a water recycling water plant where it is filtered through fine membranes to remove very small particles.

The water is pumped at high pressure through reverse osmosis membranes. These are the finest level of filtration and remove molecules including bacteria, viruses and parasites.

The recycled water may also be treated with chlorine before it enters the recycled water distribution pipes.