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.
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.
Every day, cities, factories, and industries produce millions of gallons of wastewater. If that water is released untreated into rivers, lakes, or oceans, it poisons drinking water sources, wipes out fish and other aquatic life, and spreads diseases like cholera and typhoid. Nature cannot clean water fast enough to keep up with what modern society produces; that is why treatment plants exist.
Secondary treatment is especially important because it handles what the first stage misses. Primary treatment uses physical settling and screening to remove large solids, but it captures only about 40–60% of suspended solids and does little to remove dissolved organic matter. Secondary treatment uses bacteria and other microorganisms to break down what remains, removing 85–95% of biochemical oxygen demand (BOD) and total suspended solids (TSS), bringing the water to a level safe for discharge or further treatment
In the United States, the EPA requires publicly owned treatment works to meet a 30-day average of no more than 30 mg/L BOD and 30 mg/L TSS. The EU, Australia, and most other major markets have similar rules in place. Facilities that fall short face fines and the risk of causing lasting harm to waterways and communities around them.
For industries generating large volumes of process wastewater, including mining, food and beverage manufacturing, pharmaceuticals, and petrochemicals, understanding and investing in effective secondary treatment is not optional. It is a baseline operational requirement.
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.
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.
Not all secondary treatment systems deliver the same results. Before choosing a method, it’s worth knowing what each one can realistically achieve and what regulations require.
The table below shows how effluent quality improves at each stage of treatment, from primary through standard secondary to advanced MBR systems.
| Parameter | After Primary Treatment | After Secondary Treatment | After MBR Treatment |
| BOD removal | 25–40% | 85–95% | >98% |
| TSS removal | 40–60% | 85–95% | >99% |
| Effluent BOD | 120–200 mg/L | 20–30 mg/L | <5 mg/L |
| Effluent TSS | 60–140 mg/L | 15–30 mg/L | <1 mg/L |
| Pathogens removed | Minimal | Moderate | High |
Under 40 CFR Part 133, the EPA requires publicly owned treatment works to hit a 30-day average of no more than 30 mg/L BOD and 30 mg/L TSS, with a minimum 85% removal rate for both. Membrane bioreactors regularly beat these numbers by a wide margin, producing water clean enough for reuse.
For facilities in water treatment and industrial manufacturing that need consistent, high-quality effluent, understanding these benchmarks is essential when evaluating treatment options.
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.
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.
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.
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.
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 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.
Incoming wastewater is mixed with a recycled population of microorganisms in an aerated tank. The bacteria break down dissolved organic matter while oxygen is continuously fed in. Solids then settle in a secondary clarifier; some sludge is returned to the tank to keep bacteria active, and the rest is removed as waste.
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.
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.
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.
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.
The right system depends on your specific conditions. Here are the six key factors to consider:
Secondary wastewater treatment is the biological core of the treatment process, the stage responsible for removing dissolved organic matter and biological contamination that physical methods cannot handle. The range of available technologies, from activated sludge and trickling filters through to modern membrane bioreactors, means engineers and plant operators can match system performance to their regulatory requirements and site conditions.
Water scarcity is growing, and discharge regulations are tightening across most markets. The industry is moving toward treatment systems that produce water fit for reuse, not just water clean enough to discharge. Membrane bioreactors and advanced membrane filtration products are central to that shift.
Knowing how the full water treatment process fits together, and where secondary treatment sits within it, is the starting point for building systems that protect both the environment and the operation behind it. For water treatment and industrial manufacturing applications, Brother Filtration supplies filtration systems, membrane products, filter housings, and cartridge filters built for wastewater environments.
Explore our full product range or contact us to discuss your secondary treatment filtration needs.
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