What is a Secondary treatment

Secondary treatment is the core biological stage of the wastewater treatment process. It follows primary or mechanical treatment, which removes large solids and floating debris through screening and sedimentation. While the primary stage deals mainly with physical impurities, the secondary treatment focuses on the removal of dissolved and suspended organic matter that cannot be settled out mechanically. This is achieved through the controlled use of microorganisms that naturally consume organic pollutants as food, converting them into biomass, carbon dioxide and water.

The process plays a vital role in protecting rivers, lakes and coastal waters from pollution, reducing the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of effluent to levels that meet environmental discharge standards. It is the biological heart of any wastewater treatment plant and a cornerstone of sustainable sanitation systems.

The purpose of secondary treatment

The main objective of secondary treatment is to biologically degrade organic matter that remains in the wastewater after primary clarification. This organic matter, often referred to as biochemical oxygen demand, would otherwise consume oxygen from receiving waters and harm aquatic life.

Through aeration and microbial activity, the organic load is stabilised. Microorganisms metabolise the organic compounds, using them as an energy source, and in doing so, they reproduce and form biological flocs or biofilms that can be separated from the treated water in a subsequent clarification stage.

This step significantly improves water quality by:

  • Reducing suspended solids and turbidity.

  • Lowering concentrations of dissolved organic carbon and nutrients.

  • Eliminating odour and improving clarity of the effluent.

The success of secondary treatment depends on maintaining optimal conditions for microbial activity, including adequate oxygen supply, temperature, and pH balance.

Biological principles behind secondary treatment

Secondary treatment relies on the natural ability of microorganisms, primarily bacteria, protozoa and fungi, to break down organic matter. In an oxygen-rich environment, these microorganisms convert complex organic substances into simpler inorganic compounds through aerobic respiration.

The biological reactions can be summarised as follows:

  1. Organic matter (carbon-based compounds) from wastewater serves as a food source for microorganisms.

  2. Oxygen is supplied to support aerobic metabolism.

  3. The microbes oxidise organic carbon to carbon dioxide and water, releasing energy for growth.

  4. New microbial cells form and accumulate as sludge.

The sludge produced in this stage contains both active biomass and inert material. A portion of it is recycled to maintain the microbial population, while the excess sludge is removed and sent for further processing, such as digestion or dewatering.

Main types of secondary treatment systems

A wide variety of biological treatment systems exist, but they can generally be divided into two categories: suspended growth systems and attached growth systems. Each has its own characteristics, advantages and operational requirements.

Suspended growth systems

In these systems, microorganisms are freely suspended in the wastewater as flocs. The most common and widely used process is the activated sludge system.

The activated sludge process involves aerating wastewater in a large tank where microorganisms metabolise organic matter. Air or pure oxygen is introduced through diffusers or mechanical aerators to maintain aerobic conditions. The mixed liquor (a combination of wastewater and microbial biomass) is then transferred to a secondary clarifier, where solids settle and clarified effluent is discharged.

A portion of the settled sludge, known as return activated sludge (RAS), is pumped back to the aeration tank to maintain a stable microbial population. The remaining portion, called waste activated sludge (WAS), is removed for further treatment.

Variations of the activated sludge system include:

  • Extended aeration, which provides longer retention times for complete stabilisation of organic matter.

  • Sequencing batch reactors (SBR), which carry out aeration and clarification in a single tank through timed cycles.

  • Oxidation ditches, where wastewater circulates continuously through a looped channel with surface aerators.

Attached growth systems

In attached growth systems, also known as fixed film systems, microorganisms grow on the surface of a solid medium such as rocks, plastic media or ceramic plates. Wastewater flows over or through the medium, allowing organic matter to be absorbed and degraded by the biofilm.

Examples of attached growth systems include:

  • Trickling filters, where wastewater is distributed over a bed of media and trickles downward while bacteria on the surface metabolise the organic load.

  • Rotating biological contactors (RBC), where large discs partially submerged in wastewater rotate slowly, alternately exposing the biofilm to air and water.

Attached growth systems are known for their simplicity, robustness and low operational costs, although they may require larger land areas compared to suspended systems.

Factors affecting the efficiency of secondary treatment

The performance of a secondary treatment process depends on several interrelated factors. Maintaining the correct balance of these parameters is essential for stable operation and high-quality effluent.

  1. Oxygen supply: Sufficient aeration is required to sustain aerobic microorganisms. Inadequate oxygen leads to incomplete oxidation and possible odour formation.

  2. Temperature: Most biological treatment systems operate optimally between 15°C and 35°C. Lower temperatures slow down microbial activity, while high temperatures can inhibit growth.

  3. pH and alkalinity: The pH should generally remain between 6.5 and 8.5 to support biological activity. Buffering capacity is needed to counteract acid formation during oxidation.

  4. Nutrient balance: Microorganisms require nitrogen and phosphorus for cell growth. A typical ratio of carbon:nitrogen:phosphorus (C:N:P) of 100:5:1 is desirable.

  5. Hydraulic retention time: This determines how long wastewater remains in the treatment tank, influencing the degree of biological degradation.

  6. Sludge age and concentration: The amount of biomass and its age (mean cell residence time) affect both treatment efficiency and sludge production.

Careful monitoring of these parameters ensures that the system operates within optimal limits, avoiding upsets or process failures.

Secondary clarifiers and sludge management

After biological oxidation, the mixed liquor must be separated into treated effluent and biological solids. This is achieved in the secondary clarifier, where gravity allows the flocs or biofilm particles to settle. The clear supernatant is discharged for tertiary treatment or directly to the environment if it meets regulatory standards.

The settled biomass is partially recycled and partially wasted. The waste sludge is typically thickened and stabilised before disposal. Proper sludge management is essential to prevent accumulation in the system and to maintain treatment performance.

Excess sludge can be further treated by anaerobic digestion, producing biogas that can be used for energy recovery, thereby improving the sustainability of the treatment plant.

Advanced secondary treatment and nutrient removal

In many modern treatment plants, secondary treatment is combined with nutrient removal processes to reduce nitrogen and phosphorus in the effluent. These nutrients, if discharged in high concentrations, can cause eutrophication in receiving waters, leading to algal blooms and oxygen depletion.

Biological nutrient removal (BNR) processes integrate aerobic, anoxic and anaerobic zones to enable simultaneous oxidation of organic matter, nitrification, denitrification and phosphorus uptake by specialised bacteria. The result is an effluent with significantly reduced nutrient levels, meeting stricter environmental standards.

Comparison of secondary and primary treatment

While primary treatment relies on physical separation, secondary treatment is biological in nature. Primary processes typically remove 50 to 60 percent of suspended solids and 30 to 40 percent of BOD, whereas secondary treatment can achieve reductions of over 90 percent for both parameters.

This dramatic improvement is why secondary treatment is considered the most critical stage in producing environmentally safe effluent. It bridges the gap between simple sedimentation and advanced tertiary purification.

Maintenance and operational challenges

Operating a biological system requires continuous monitoring and adjustment. Some of the most common challenges include:

  • Foaming or bulking sludge caused by filamentous bacteria.

  • Oxygen deficiencies leading to incomplete degradation.

  • Shock loads from industrial discharges that disrupt microbial populations.

  • Temperature variations or toxic substances that inhibit biological activity.

Operators must respond promptly to any signs of process imbalance, using routine laboratory tests such as BOD, COD, mixed liquor suspended solids (MLSS) and microscopic examination of biomass.

The role of secondary treatment in environmental protection

Secondary treatment plays an indispensable role in safeguarding water bodies and human health. By reducing the organic and microbial load of effluent, it prevents the depletion of dissolved oxygen in rivers and estuaries, protecting aquatic life and maintaining ecological balance. It also ensures compliance with discharge permits and contributes to sustainable urban sanitation systems.

In the United Kingdom and across Europe, secondary treatment is a legal requirement for most municipal wastewater discharges under the Urban Waste Water Treatment Regulations. Its widespread implementation has led to a significant improvement in water quality over recent decades.

Future developments in secondary treatment

The field of biological wastewater treatment continues to evolve. Modern trends include energy-efficient aeration systems, membrane bioreactors (MBRs) that combine biological treatment with ultrafiltration, and digital monitoring systems that optimise performance in real time. There is also growing interest in using microbial communities for resource recovery, such as producing biogas, bioplastics or fertiliser from waste sludge.

The future of secondary treatment lies in integrating these technologies into smart, resilient and low-carbon treatment plants that support the circular economy.

Conclusion

Secondary treatment is the essential biological engine of wastewater purification. It transforms dissolved organic pollution into stable biomass through natural microbial processes, achieving high levels of water quality improvement. By combining biological science with engineering, secondary treatment ensures that wastewater can be safely returned to the environment without harming ecosystems.

Its importance extends far beyond its technical function; it represents a balance between human development and environmental responsibility. As treatment technologies continue to advance, the principles of secondary treatment will remain central to the design and operation of sustainable wastewater systems worldwide.