What is a Tertiary treatment

Tertiary treatment, often referred to as advanced treatment or polishing, represents the final and most sophisticated stage of wastewater treatment. It follows secondary (biological) treatment and is designed to remove any remaining suspended solids, nutrients, pathogens and trace contaminants that earlier stages cannot fully eliminate. The goal of tertiary treatment is to produce effluent of such high quality that it can be safely discharged into sensitive natural environments or even reused for industrial, agricultural or domestic purposes.

This stage of treatment has become increasingly important in modern wastewater management, particularly as environmental standards tighten and water scarcity drives the need for water recycling. Tertiary processes use advanced physical, chemical and biological methods to achieve a level of purification that goes well beyond the capabilities of conventional treatment.

The role of tertiary treatment in the wastewater process

Primary treatment removes large solids through sedimentation and screening, while secondary treatment uses biological processes to degrade organic matter. However, even after these stages, the treated water still contains fine particles, dissolved nutrients like nitrogen and phosphorus, microorganisms and traces of chemicals such as detergents, heavy metals and pharmaceuticals.

Tertiary treatment addresses these remaining contaminants. Depending on the desired effluent quality, it may include filtration, chemical precipitation, membrane separation, or disinfection methods such as ultraviolet (UV) irradiation and chlorination. Each technique is carefully selected based on local regulations, discharge conditions and intended reuse applications.

In essence, tertiary treatment acts as a polishing step, refining the quality of wastewater to meet specific standards for discharge or reuse. It transforms treated effluent from a basic environmental safeguard into a valuable water resource.

Main objectives of tertiary treatment

The key objectives of tertiary treatment are to:

• Remove residual suspended and colloidal solids that cause turbidity.

• Eliminate nutrients, especially nitrogen and phosphorus, which contribute to eutrophication of natural waters.

• Destroy or deactivate pathogenic microorganisms to ensure sanitary safety.

• Reduce concentrations of dissolved inorganic salts, heavy metals and emerging contaminants such as pharmaceuticals and microplastics.

• Improve colour, taste and odour of the final effluent when reuse is intended.

Each of these goals requires specific treatment technologies, often used in combination to achieve optimal results.

Common tertiary treatment processes

Tertiary treatment encompasses a wide range of technologies, from traditional sand filtration to advanced membrane and oxidation systems. While each plant is designed according to local needs and regulatory limits, the following processes represent the most widely adopted approaches.

Filtration

Filtration is one of the most common forms of tertiary treatment. It physically removes fine suspended solids and colloidal particles that remain after secondary clarification. Several types of filters are used:

• Rapid gravity filters, which pass water through layers of sand or anthracite to trap suspended particles.

• Pressure filters, similar in principle but enclosed in pressurised vessels, suitable for compact installations.

• Disk or cloth filters, which use rotating discs or fabric media to achieve very fine filtration with minimal space requirements.

Filtration not only improves clarity but also reduces microbial load by removing organisms attached to suspended solids. Regular backwashing or automated cleaning is essential to maintain filter efficiency.

Disinfection

Disinfection ensures the destruction of pathogenic microorganisms, including bacteria, viruses and protozoa. This step is crucial before effluent is discharged to surface waters or reused. The most common disinfection methods include:

• Ultraviolet (UV) irradiation, where high-intensity UV light penetrates microbial cells and damages their DNA, preventing reproduction. UV systems are highly effective, chemical-free and environmentally friendly.

• Chlorination, in which chlorine gas or sodium hypochlorite is added to the effluent. While effective, chlorination can produce harmful by-products such as trihalomethanes, so careful control is required.

• Ozonation, which uses ozone gas to oxidise and destroy microorganisms. It is a powerful and fast-acting method but requires complex equipment and energy input.

Modern treatment plants increasingly prefer UV and ozone disinfection due to their efficiency and lower environmental impact compared to traditional chlorination.

Nutrient removal

Nutrients such as nitrogen and phosphorus can cause serious environmental damage when discharged in excess, leading to algal blooms and oxygen depletion in rivers, lakes and estuaries. Tertiary nutrient removal processes are therefore essential in many facilities.

• Nitrogen removal is typically achieved through a combination of nitrification and denitrification. Ammonia is first oxidised to nitrate by aerobic bacteria, and then nitrate is converted to nitrogen gas by anoxic bacteria, which is released harmlessly into the atmosphere.

• Phosphorus removal can be carried out chemically or biologically. Chemical precipitation involves adding metal salts such as aluminium sulphate (alum) or ferric chloride, which bind to phosphorus and form insoluble compounds that can be settled and removed. Biological phosphorus removal relies on specific bacteria that absorb phosphorus during alternating aerobic and anaerobic conditions.

Membrane filtration technologies

Membrane-based systems have revolutionised tertiary treatment in recent decades. They provide exceptionally fine filtration, capable of removing not only suspended solids but also viruses and dissolved organic molecules. Common membrane technologies include:

• Microfiltration (MF) and ultrafiltration (UF), which remove particles, bacteria and some macromolecules using semi-permeable membranes with precise pore sizes.

• Nanofiltration (NF) and reverse osmosis (RO), which can remove dissolved salts, heavy metals, nitrates and even trace organic contaminants.

These systems are particularly valuable for producing high-quality reclaimed water suitable for industrial or agricultural reuse. However, they require significant energy and maintenance to prevent membrane fouling.

Advanced oxidation processes

Advanced oxidation processes (AOPs) are used when the removal of persistent organic compounds or micropollutants is required. They rely on highly reactive hydroxyl radicals generated through combinations of UV light, ozone and hydrogen peroxide. These radicals can oxidise a wide range of contaminants, including pharmaceuticals, pesticides and industrial chemicals, which conventional biological treatment cannot degrade.

Although AOPs are energy-intensive, they are increasingly being implemented to meet stringent environmental and health regulations.

Factors influencing tertiary treatment selection

Choosing the right tertiary treatment method depends on several interrelated factors:

• Regulatory standards for effluent discharge or reuse, which define allowable concentrations of solids, nutrients and pathogens.

• Intended use of treated water. For example, irrigation reuse requires lower disinfection levels than drinking water production.

• Operational costs and energy efficiency, as advanced systems can be expensive to install and maintain.

• Local environmental sensitivity, including proximity to protected habitats or watercourses.

• Space availability, since filtration and membrane systems vary in their footprint requirements.

A combination of these factors determines the optimal configuration for each facility. Many plants adopt modular designs, allowing additional stages to be added as standards evolve or new reuse opportunities arise.

Tertiary treatment and water reuse

One of the most significant applications of tertiary treatment is in water reuse. By achieving near-drinking-water quality, treated effluent can be used for irrigation, industrial cooling, toilet flushing, landscape maintenance or groundwater recharge. In some regions, especially those facing water scarcity, tertiary-treated water is even used for indirect potable reuse after further treatment and monitoring.

Water reuse reduces the demand on freshwater sources and contributes to sustainable water management. The United Kingdom, like many countries, is increasingly exploring these options to cope with population growth and climate variability.

Environmental and public health benefits

Tertiary treatment provides multiple environmental and health benefits. It prevents nutrient enrichment of water bodies, protects aquatic ecosystems, and reduces the spread of waterborne diseases. The production of high-quality effluent also enables safe and sustainable resource recovery.

By reducing the concentration of contaminants such as microplastics, heavy metals and pharmaceuticals, tertiary treatment helps address emerging pollution concerns that have gained international attention. It also supports the circular economy model by turning wastewater into a reusable and valuable resource.

Challenges and limitations

Despite its advantages, tertiary treatment involves technical and economic challenges. Advanced systems require high energy input, careful maintenance and skilled operation. Membrane fouling, chemical usage and disposal of concentrated waste streams must be managed responsibly.

There are also issues related to monitoring and ensuring consistent water quality, especially when reuse is intended for sensitive applications. Research continues into more energy-efficient and environmentally friendly processes, including low-pressure membranes, natural-based systems and hybrid configurations.

Future trends in tertiary treatment

Technological innovation continues to shape the future of tertiary treatment. Developments in nanotechnology, biofiltration, and photocatalysis are opening new possibilities for removing emerging contaminants more effectively. The integration of artificial intelligence and digital monitoring systems is improving real-time control, enabling treatment plants to operate with greater efficiency and adaptability.

Sustainable design is another major focus, with many facilities incorporating renewable energy sources and resource recovery units. The trend toward decentralised treatment systems, which allow localised water reuse, is also gaining momentum, particularly in new urban developments and industrial estates.

Conclusion

Tertiary treatment represents the pinnacle of wastewater purification, transforming effluent into a clean, reusable resource. It removes the final traces of solids, nutrients and microorganisms, ensuring that discharged water meets the highest environmental standards. Through processes such as filtration, nutrient removal, membrane separation and disinfection, tertiary treatment protects ecosystems, conserves water resources and promotes sustainable development.

As global demand for clean water continues to rise, tertiary treatment will play an increasingly central role in the future of sanitation and environmental protection. By combining innovation, engineering expertise and environmental stewardship, it ensures that the water leaving treatment plants not only meets legal standards but also supports a more resilient and sustainable world.