What is sewage?

Sewage is water (and any other waste) that enters the sewerage network.

It is produced by homes and businesses from: showers, baths, kitchens, toilets and industrial processes.

Sewage is 99% water, the remainder is organic and inorganic matter.  

Sewage doesn’t just disappear when you flush the toilet or drain the sink. It must be treated.

That's where we come in! 

Watch: the sewage treatment process

The treatment process

Stage 1
Inlet works

Here, raw sewage is screened to remove sand, grit and other foreign materials that could damage mechanical equipment and impact treatment efficiency. The removed material is disposed of at a secure landfill. 

Stage 2
Balance tanks

The remaining sewage is held in balance tanks that regulate the amount of sewage being treated. This ensures optimum efficiency of treatment.  


Stage 3

Phosphorus and nitrogen are organic pollutants that can impact waterways and the environment. Inside the bioreactor, micro-organisms 'eat' these pollutants and reduce their volumes. The bioreactor has three zones to keep these micro-organisms active and healthy:

  • anaerobic (no dissolved oxygen and no nitrates)
  • anoxic (no dissolved oxygen by nitrates present) and
  • aerobic (dissolved oxygen supplied in order to convert ammonia into nitrates). 


Stage 4
Odour control
Odours from the sewage treatment process are treated using activated carbon, which absorbs odorous gas on its surface. Odourous gases can also be chemically treated using sodium hypochlorite and caustic soda to break down odorous compounds.
Stage 5

The treated effluent settles inside the clarifiers allowing the solid material (activated sludge) and the liquid (clarified effluent) to separate. The liquid is sent for further treatment. The activated sludge contains active micro-organisms and unwanted organic matter. Some of the activated sludge is returned to the bioreactor to maintain the micro-organism population, while the remainder is dried out to be used under strict guidelines as biosolids.  

Stage 6

Clarified effluent is filtered to remove any remaining solid particles.  

Stage 7

The final treatment process disinfects the treated effluent to destroy any pathogens. The Murrumba Downs and Noosa Sewage Treatment Plants use ultra-violet disinfection to replicate the sun's UV radiation and destroy micro-organisms. Other treatment plants use chlorine disinfection. 

Stage 8

The final treated effluent is now ready to be discharged to the environment - normally to a river or other waterway. This effluent can also be recycled for industrial, agricultural or irrigation purposes.  


Unitywater Sewage Treatment Plant, Murrumba Downs

 Balance tanks


Unitywater Sewage Treatment Plant, Murrumba Downs

Odour control

Unitywater Sewage Treatment Plant, Clarifier, Murrumba Downs


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