What is the Difference Between Primary and Secondary Wastewater Treatment?

By Stefano Tommasone Ph.D.Reviewed by Lexie CornerJan 27 2025

As the global population expands and industrial activities grow, significant volumes of wastewater are produced, often filled with various contaminants.

Image Credit: Bilanol/Shutterstock.com

These pollutants come from various sources, including industrial processes such as petrochemical production, pharmaceuticals, and metal manufacturing, as well as domestic sewage, agricultural runoff, and hospital waste.¹

Globally, over 380 billion cubic meters of wastewater are generated annually—a figure expected to rise by 51 % by 2050. This growing volume exacerbates water pollution, accelerates eutrophication, and threatens aquatic biodiversity.

Treating wastewater is essential to protecting public health and maintaining environmental stability. Primary and secondary treatment methods use targeted techniques to remove contaminants, preserve water resources, and reduce the ecological impact of discharge.

Primary Wastewater Treatment

Primary wastewater treatment is designed to remove suspended solids, debris, fats, and oils through physical methods such as sedimentation and dissolved air flotation. These processes reduce the pollutant load sent to secondary treatment, helping protect downstream systems from potential damage.

Large objects such as plastics, rags, and other debris are removed mechanically using screens to prevent clogging. Sedimentation tanks allow suspended solids to settle to the bottom, creating a sludge layer, while lighter substances (like oils and grease) float to the surface and are removed through skimming. However, this process is insufficient for removing dissolved organic materials, pathogens, or nutrients, necessitating further treatment.

This initial stage is a practical solution for managing large volumes of wastewater and is widely used in industrial facilities. For example, in the cement industry, where wastewater often has high pH levels and turbidity, treatment involves neutralization followed by sedimentation.² In steel production, adsorbents like activated carbon or metallurgical slag are commonly used to remove heavy metals and other pollutants efficiently.³

Companies like Hach support the optimization of primary wastewater treatment with tools like advanced monitoring systems for sedimentation tanks and real-time data analytics. A typical primary clarifier can remove about 70 % of solids and 45 % of the biochemical oxygen demand (BOD) from screened wastewater. These technologies help facilities improve efficiency and maintain compliance with regulatory standards.

Secondary Wastewater Treatment

Secondary wastewater treatment removes pollutants like organic matter, nitrogen, and phosphorus that remain after primary treatment. This process uses biochemical reactions to degrade dissolved organic matter, reducing biochemical oxygen demand (BOD) and improving water quality.

A key method in secondary treatment is the use of activated sludge systems. In these systems, aeration tanks introduce air to encourage microbial growth, enabling microorganisms to consume organic pollutants.

Another approach involves biofilm-based technologies, such as the moving bed biofilm reactor (MBBR) developed by SSI, where microorganisms attach to media surfaces and break down organic compounds, further reducing BOD.

Secondary treatment is a standard component of municipal wastewater management and is also widely applied in settings with high organic loads. For example, hospital wastewater often contains heavy metals, bacteria, viruses, and pharmaceuticals. A combination of activated sludge processes and membrane bioreactors is often used to address these complex contaminants effectively.⁴

Companies like Veolia support these efforts by providing advanced technologies and operational expertise. Their solutions integrate membrane bioreactors for high-efficiency filtration with biological treatments to remove organic pollutants, simplifying wastewater management and enhancing overall treatment performance.

Key Differences Between Primary and Secondary Treatment

Primary and secondary wastewater treatments serve distinct but complementary purposes in ensuring effective water purification. Primary treatment removes physical debris, while secondary treatment addresses dissolved organic matter and pathogens to improve water quality.

Primary treatment relies on physical processes, such as sedimentation, to remove floating and settleable materials like scum and sludge. In some systems, such as septic tanks, limited anaerobic biological activity occurs, partially digesting organic matter. This stage eliminates around 50–70 % of suspended solids and organic pollutants, preparing the water for further treatment.

Secondary treatment, on the other hand, uses biological processes to target dissolved contaminants. Aeration and filtration systems encourage microbial activity to break down organic matter, significantly reducing BOD and TSS. Disinfection methods, such as chlorine or ultraviolet light, may also be incorporated to eliminate harmful microorganisms.⁵ This stage achieves a higher removal rate, eliminating up to 85–95 % of organic pollutants.

As water demand rises and pollution becomes more complex, the need for robust wastewater treatment systems grows. Emerging pollutants present new challenges, highlighting the importance of developing advanced technologies and improving existing methods to ensure effective and sustainable water purification.

To learn more about advanced water filtration methods, visit:

• Advanced Nanocatalysts in Wastewater Treatment
• Graphene Membranes: Applications in Modern Water Purification
• Utilizing Green Technology for Wastewater Treatment

References and Further Reading

1. Kato, S., Kansha, Y. (2024). Comprehensive review of industrial wastewater treatment techniques. Environ Sci Pollut Res Int. DOI.10.1007/s11356-024-34584-0. https://pubmed.ncbi.nlm.nih.gov/39107648/
2. Zhu, X., Yang, J., Huang, Q. Liu, T. (2022). A Review on Pollution Treatment in Cement Industrial Areas: From Prevention Techniques to Python-Based Monitoring and Controlling Models. Processes. Available: https://www.mdpi.com/2227-9717/10/12/2682
3. Manchisi, J., Matinde, E., Rowson, NA., Simmons, MJH., Simate, GS., Ndlovu, S. Mwewa, B. (2020). Ironmaking and Steelmaking Slags as Sustainable Adsorbents for Industrial Effluents and Wastewater Treatment: A Critical Review of Properties, Performance, Challenges and Opportunities. Sustainability. https://www.mdpi.com/2071-1050/12/5/2118
4. Liu, A., Zhao, Y., Cai, Y., Kang, P., Huang, Y., Li, M. Yang, A. (2023). Towards Effective, Sustainable Solution for Hospital Wastewater Treatment to Cope with the Post-Pandemic Era. Int J Environ Res Public Health. DOI: 10.3390/ijerph20042854. https://pmc.ncbi.nlm.nih.gov/articles/PMC9957062/
5. Kehrein, P., Van Loosdrecht, M., Osseweijer, P., Garfí, M., Dewulf, J.  Posada, J. (2020). A critical review of resource recovery from municipal wastewater treatment plants – market supply potentials, technologies and bottlenecks. Environmental Science: Water Research & Technology, 6, 877-910.10.1039/C9EW00905A. http://dx.doi.org/10.1039/C9EW00905A

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