Secondary wastewater treatment is the second stage in the sewage or wastewater treatment process. Its main purpose is to remove the remaining suspended and dissolved organic matter that cannot be removed in the primary treatment.

The most commonly used processes in secondary wastewater treatment are the activated sludge process, trickling filter, anaerobic treatment anoxic treatment, and membrane bioreactor.

Secondary wastewater

In this blog, we will provide an overview of secondary wastewater treatment and how it works. We will also discuss the various processes involved in secondary wastewater treatment, including biological, physical, and chemical processes.

Why is wastewater treatment important?

Water treatment facilities are primarily a natural process that accelerates the purification of water. Natural processes are already overloaded by human water and wastewater discharges. Wastewater that is not treated will harm the environment’s ecology and have an impact on people’s ability to produce and live.

Precisely because wastewater treatment is directly linked to our environment and the health of humans and animals. Then not only will our water sources be destroyed, but also natural habitats will be affected, and we will even be susceptible to unknown serious diseases.

wastewater treatment important

The treatment method of the general sewage treatment plant is to treat and purify the polluted water produced by production activities or the water flowing into the sewer and then discharging it back into the environment. Primary treatment generally only removes about 60% of suspended solids in wastewater, while secondary treatment can remove more than 90% of suspended solids.

What is secondary wastewater treatment?

Wastewater treatment is divided into primary treatment, secondary treatment, and even tertiary treatment. As long as the process technology is good and the treatment method is appropriate, the reuse rate of wastewater after treatment will increase. Secondary treatment is one of them that is quite significant.

Secondary wastewater treatment is a process of biological purification of wastewater, involving the digestion of organic matter by microorganisms, which belongs to biological treatment. It also means that the oxygen level in the wastewater is constantly changing and will not be maintained at a constant amount.

Water plant

In secondary treatment, aerobic microorganisms are used to break down and remove waste and other small particles. Both waste and microorganisms are present in the sludge, and the removal of solids and nutrients by a combination of bacteria requires biofiltration, aerobic, and anaerobic processes.

What is the difference between primary and secondary treatment?

For the treatment of wastewater, there are great differences between primary treatment and secondary treatment. The most significant difference lies in the differences in their respective wastewater treatment methods. Primary treatment, that is, physical treatment, is to remove floating and settleable substances in wastewater through sedimentation. Secondary treatment involves chemical treatment that removes suspended solids and biodegradable organic matter through aeration and filtration processes.


Primary Wastewater Treatment:

Step 1: Primary wastewater treatment is the first step in wastewater treatment.

Step 2: The primary purpose of its treatment is to remove sediment and some floating particles in the wastewater.

Step 3: Its treatment method is mainly a sedimentation and filtration process.

Step 4: The processing takes less time.

Secondary Wastewater Treatment:

Step 1: Secondary wastewater treatment is the second step in wastewater treatment.

Step 2: Its treatment involves the digestion of organic matter by microorganisms, and the biological oxygen demand in wastewater is reduced.

Step 3: The treatment methods are biological filtration and aerobic and anaerobic processes.

Step 4: The processing takes a long time.

The above are the two basic stages of wastewater treatment, namely the primary treatment stage and the secondary treatment stage. The preliminary stage is to allow the solids to settle and remove them from the wastewater. The secondary stage is to use biological processes to further purify the wastewater. Sometimes these two stages are also combined to operate.

How does secondary wastewater treatment work?

In the secondary wastewater treatment stage, the bacterial process is used to remove impurities in the water. This process is applied after the wastewater has undergone pretreatment and primary treatment, that is, after most of the suspended solids have been removed from the water. The removal rate of secondary treatment for the remaining suspended solids can be as high as 90%.


The aerobic biological process is mainly used in the treatment of urban sewage. During aerobic wastewater treatment, organic pollutants are converted into carbon dioxide, water, and other microorganisms. A key point here is that the method requires the microorganisms to have metabolic processes that require oxygen.

Trickling Filter method

During the trickling filter process, physical and chemical interactions take place, with bacteria and some other biological material adsorbing on the granular media passing upwards. This process helps break down contaminants and also provides organic matter for bacterial growth media, which is critical for secondary treatment.


Biofilm reactor

Biofilm reactors use media filters to facilitate aerobic digestion and are a common media filtration system. Systems like this work by using thousands of small pieces of plastic media on which biofilms naturally form as bacteria attach to them.

Activated sludge

Activated sludge treatment is a relatively common aerobic process and a biological process of suspended growth. The process uses aeration and flocculation, allowing the floc solids to form a sludge layer that settles to the bottom during the aeration process.

Anaerobic treatment

Anaerobic processes take place in the absence of oxygen and mainly convert organic pollutants into biofuel gases. The process produces by-products such as biogas (water vapor, carbon dioxide, and methane) that are used as fuel for the plant.

Biogas usage

Anoxic treatment

Anoxic treatment is the treatment of wastewater by microorganisms that involve metabolic processes that do not require oxygen. This means that the process takes place in the absence of free molecular oxygen, or in the presence of some oxygen (in the form of sulfate, or nitrate). It is mainly used in factories to treat wastewater with high nitrogen content.

What types of secondary treatment systems are available?

The secondary treatment currently used in the wastewater industry mainly involves aerobic treatment systems and biological filtration systems. The aerobic treatment system refers that injecting air into the wastewater in order to generate healthy aerobic microorganisms and use these microorganisms to decompose the organic pollutants in the wastewater.

Aerobic treatment systems generally have three compartments, the first compartment is used to precipitate solid pollutants, the second compartment is used to inject air using compressors or other equipment, and the last compartment removes additional solid particles.

wastewater treatment facility

Biofiltration systems consist of many modules containing porous filter media such as peat, foam, sand, or synthetic textiles. The system provides a large surface area for aerobic organisms to attach and grow. As contaminants drip from the media, they are removed by microbial activity and filtration.


Water is one of the most precious natural resources. However, the waste and consumption of water are unavoidable in people’s production and life. Therefore, the water treatment system plays a vital role, and wastewater treatment is also the top priority for the water treatment system.

When wastewater treatment systems are installed or misoperated, a host of serious problems can arise. The secondary wastewater treatment system plays a leading role in wastewater treatment. We can make good use of various secondary treatment technologies and filtration systems to optimize wastewater treatment.

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