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What is the difference between primary and secondary treatment of wastewater?

Wastewater is a part of life - whether it is the water that is flowing down your bathroom sink, the liquids that are coming from your washing machine or the dirty water from your latest dishwashing cycle, wastewater is a big part of household or office waste. But, what happens to it after it makes it way out of sight?!

Household wastewater

Wastewater treatment is an incredibly important aspect of environmental safety - a lot of the changes that turn water into wastewater require varying levels of treatment before that water can be safely used again. As such, there are two major stages of wastewater treatment - primary and secondary.

Let’s start at the beginning though, for a little background knowledge on wastewater before we jump into the stages.

What is wastewater?

According to the Department of Environment and Science, wastewater is the used water from toilets, showers, baths, kitchen sinks, laundries and industrial processes. Around 99% of wastewater is water, and the other 1% is contaminating waste and solids. Wastewater can come from any of the following places : shops, homes, farms, factories, offices, fuel and transport depots, quarries, mines and vessels.

The term ‘domestic wastewater’ is the water that comes from kitchen sinks, showers, toilets, baths and laundries in your office or home. The term ‘industrial waste’ or ‘trade waste’ is water that comes from industrial and manufacturing scenarios, such as a food processing plant. This can also include water that has some alternative purposes, such as water to clean or cool machinery.

Industrial Wastewater

Finally, there is stormwater. Stormwater is a type of wastewater that runs off various areas, such as roofs, gardens, roads, paths or gutters in agricultural or urban regions. This water flows into stormwater drains, especially after it rains, and flows straight into our waterways untreated.

Did you know : Domestic households produce an average of 200–300L of wastewater per person every day!

What is wastewater treatment?

A large majority of the wastewater we all produce has been altered in a way that means unless it is treated, it can not be used again. The kinds of changes that change water to wastewater are :

  • Adding human waste
  • Adding grease, grime, oil or fat
  • Warming or cooling
  • Adding food waste or other organic matter
  • Adding poisons such as pesticides, some organic compounds, synthetic chemicals and heavy metals
Household Wastewater

A sewerage system sends wastewater to a sewage treatment plant so it can be treated and then released back into the environment - so, in other words, wastewater treatment.

What is primary treatment?

Primary wastewater treatment is the process of removing solid matter from wastewater. It is the first step in the wastewater treatment process, where wastewater flows into tanks and the solids settle on the bottom and the grease and scum rise to the top. The primary process is also called the ‘primary phase’ or the ‘septic process’. The lighter wastewater and matter leaves at around the mid-water level and then goes to the secondary pre-treatment chamber where anaerobic bacteria (otherwise known as bacteria that does not need oxygen to live) uses the nasties in the water as food and they begin to break them down.

Primary Treatment

What is secondary treatment?

The secondary wastewater treatment process uses aerobic microorganisms to breakdown and remove the remaining waste and other small particles. The waste and the microorganisms are both found in the sludge, and remove both the remaining solids and nutrients through bacterial composition. Secondary treatment utilises naturally occurring biological processes, meaning the wastewater oxygen level varies throughout and do not stay at one constant quantity. This results in microorganisms being either aerobic (require oxygen) or anaerobic (does not require oxygen), depending on the stage.

What is the difference between primary and secondary wastewater treatment?

There are quite a few differences between the primary and secondary treatment of wastewater, but the big one is the way each respective treatment is processed. Primary treatment works on sedimentation, where solids separate from the water through several different tanks. In contrast, secondary treatment uses aeration, biofiltration and the interaction of waste throughout its process.

Wastewater Treatment

There are some other differences between primary and secondary wastewater treatment, including:

  • Primary wastewater treatment generally utilises equipment to breakup larger particles (a physical process), whereas secondary treatment uses biological processes for extraction
  • The aim of secondary wastewater treatment is to remove smaller particles, whereas in primary treatment larger particles are to be removed
  • Primary wastewater treatment takes a lot less time than secondary treatment
  • Secondary treatment uses anaerobic or aerobic breakdown processes whereas primary wastewater treatment utilises filtration and sedimentation processes

Which wastewater treatment option is best?

This is an almost impossible question to answer, as everyone has different needs when it comes to wastewater treatment. Sure, a system that offers both primary and secondary wastewater treatment produces better quality water, however your home or business may not require that level of effluent or may not have the required space or resources for a system like that. It is best to contact your local wastewater professional and discuss your needs directly with them, as they will be able to guide you as to the most appropriate option for you.

Your local wastewater experts

If you have any questions about primary or secondary wastewater treatment, or feel it may be time to have a wastewater treatment plant installed in your home or business, give the friendly team at Express Wastewater a call on 1300 722 517 or complete an online job booking form today. We offer a free 30 minute wastewater consult with one of our experts to get your project off the ground, so get in contact now!

Resources :

Department of Environment and Science (Wastewater) :

Sydney Water Tour (Primary Treatment) :

Sydney Water Tour (Secondary Treatment) :

Organica :


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    Because we are not a manufacturer and not tied to one particular system we can always offer the best solution for your wastewater needs.


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    We will always look to repair your system rather than replace it if it does not need to be, which will generally save large amounts of money sometimes up to and over $10,000.