Types of Secondary Treatment Systems

Wastewater can be treated in a variety of ways, with septic tanks and home sewage treatment plants offering some of the most common arrangements across Australia. They also offer different levels of treatment, which many people are unaware of.

Wastewater treatment can have up to three stages - primary, secondary and tertiary. We are discussing the first two stages here, but head to our What are the stages of sewage treatment? information page to learn more about the third stage.

What is a secondary treatment system?

The secondary wastewater treatment process utilises aerobic microorganisms to both breakdown and remove the remaining waste and other small particles. The waste and the microorganisms are both located within 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 does not stay at one constant quantity. This results in microorganisms being either aerobic (need oxygen) or anaerobic (do not need oxygen), depending on the stage.

What is the difference between primary and secondary treatment systems?

There are a range of differences between the primary and secondary treatment of wastewater, but the major one is the way each treatment is processed. Primary treatment focuses 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.

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

Primary wastewater treatment generally utilises a physical process by using equipment to break up larger particles, whereas secondary treatment uses biological processes for extraction.
The aim of primary wastewater treatment is to remove larger particles, whereas in secondary treatment smaller particles are to be removed.
Secondary treatment uses anaerobic or aerobic breakdown processes whereas primary wastewater treatment utilises filtration and sedimentation processes.

Septic Tanks

Although septic tanks do not generally have a secondary treatment system, a sand filter can be used.

In simple terms, a sand filter is basically a large sandpit that sits under a grassed area. This sandpit has concrete pits at each end, which can be seen at ground level. The difference between the sand filter and the septic tank is the conditions - a septic tank works in anaerobic conditions (no oxygen) whereas a sand filter works in aerobic conditions (oxygen present).

*Image courtesy of Manningham City Council*

The effluent is transported into the sand, which acts as a highly effective and naturally absorptive material. This in turn can treat effluent to a very high standard which can then be used in irrigation systems.

Find out how a septic tank system works here.

Home Sewage Treatment Plants

Alternatively, home sewage treatment plants (also known as HSTPs or AWTSs) have an inbuilt secondary treatment system. The secondary treatment chamber(s), known as the aeration chamber(s), takes partially treated wastewater and distributes air through it. This injects gaseous oxygen into the water, becoming dissolved oxygen, which then allows aerobic bacteria to get involved.

This process is much quicker than the primary phase, as this kind of bacteria can multiply very quickly when a food source, such as that contained in the wastewater, is combined with lots of oxygen and time. The bacteria both eat the food and each other (this is part of nature, as harmful as it may sound), which then results in countless naturally-occurring bacteria cleaning the water contained within the chamber. The outcome is a very clear liquid which contains considerably less contaminants and nutrients.

Find out how a HSTP works here.

Is primary or secondary treatment best?

This is a difficult question to answer, as everyone has different needs when it comes to wastewater treatment. A system that offers both primary and secondary wastewater treatment definitely offers better quality water, however, your circumstances may not require that level of effluent or you 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!

Would you like to learn more about septic tanks and HSTPs? Take a look at the below: