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What is a Sewage Treatment Plant & How does it work?

Membracon > Water Processing Blog  > What is a Sewage Treatment Plant & How does it work?
What is a Sewage Treatment Plant & How does it work?

What is a Sewage Treatment Plant & How does it work?

It might surprise you to know that we all drink and bathe in recycled water. The water we flush down the toilet is turned into potable water, and it is the job of sewage treatment plants to make this water fit for human consumption or release into rivers and oceans.


Wastewater from sinks, baths, washing machines, toilets and other appliances has to go somewhere. After traversing miles of sewer network, it ends up in sewage treatment plants whose job is to treat and discharge it.


Sewage treatment plants collect, treat, and discharge wastewater, providing a service essential to environmental and public health.


Without adequate treatment, sewage will leach into the environment and contaminate ecosystems. For example, sewage contains bacteria and chemicals that break down using oxygen in the water. In doing so, they use oxygen that fish and aquatic life needs to survive, so it needs treatment to preserve the ecosystem. 


Returning sewage/wastewater to a specified quality for safe discharge is the most crucial role of the sewage treatment plant.

How does a sewage treatment plant work?

Sewage treatment plants run wastewater through multiple treatment stages. After preliminary filtration, there are three main stages of wastewater treatment (primary, secondary, and tertiary), with the third stage reserved for polishing.

Preliminary treatment

Sewage enters the plant network, pushed through various screens to remove large solids and waste, with grit removed by flow attenuation. The purpose of this stage is to filter the sewage of debris, sand, grit, and large particles.


Primary stage

The primary stage involves separating solids from liquids. The sewage is pumped into sedimentation tanks, where gravity forces solids to the bottom of the tank. The water is then released, leaving behind a sludge/slurry.


The sludge is a by-product of primary treatment and can sometimes be repurposed as a fertiliser, but it requires treatment such as de-watering to stabilise it. Incineration is the most likely destination for heavily contaminated sludge. 

Secondary treatment

Secondary treatment is the biological treatment stage that breaks down organic contaminants in wastewater.


The two most frequently used processes are activated sludge (aerated ponds) and filter beds (sewage tricked over aggregate), where ‘good’ bacteria in the sludge/aggregate break down the pathogens in the wastewater.


After secondary treatment, wastewater can sometimes be released, providing there’s a low risk to human and animal life and the environment.


Tertiary treatment

Wastewater is considered clean after secondary treatment, but tertiary treatment returns it to an even higher quality for release in protected waters.


The type of tertiary treatment depends on the wastewater. For example, suppose we should release wastewater into bathing or shellfish waters. In that case, it requires disinfection, and nutrients in the water, like phosphorous, must also be removed.

Types of tertiary treatment include:


  • Microfiltration (where water passes through tiny holes at high pressure).
  • Ion exchange (where ions in the water are exchanged for other ions).
  • Activated carbon adsorption (which removes organics).
  • Disinfection (where UV light or chemicals kill organic pathogens leftover).

Sewage treatment plants treat sewage from public sewers, producing clean, odourless water we can drink and use to bathe. All the water we consume from the tap is recycled, with drinkable water undergoing tertiary treatment stages.


Today’s sewage treatment plants are highly automated, improving plant performance and reducing the risk of human error. Sensor technology, networks and automatic controls keep plants ticking without human intervention.


If you found this article helpful, read our piece on water recycling and how water recycling systems work.

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