What Is Secondary Treatment Of Sewage

by Anna

Effective wastewater treatment is a critical aspect of modern sanitation and environmental protection. As populations grow and urban areas expand, the need for efficient sewage treatment becomes paramount to safeguard public health and preserve the environment. Secondary treatment of sewage plays a crucial role in this process, representing a key step in transforming raw sewage into treated effluent that can be safely discharged into natural water bodies. This article delves into the significance of secondary treatment and explores the various processes involved.


Primary Treatment Overview:

Before delving into secondary treatment, it is essential to understand the primary treatment of sewage. Primary treatment involves the removal of large debris, grit, and solid particles from raw sewage. This is typically done through physical processes such as screening, sedimentation, and grit removal. While primary treatment reduces the organic load of wastewater, it does not adequately address dissolved and colloidal pollutants. This is where secondary treatment comes into play.


The Need for Secondary Treatment:

Secondary treatment is a biological process designed to further purify the wastewater that has undergone primary treatment. The primary objective is to remove the dissolved and colloidal organic matter that remains in the sewage. These organic substances, if left untreated, can contribute to water pollution, deplete oxygen levels in receiving water bodies, and harm aquatic ecosystems. Secondary treatment is indispensable in meeting stringent water quality standards and ensuring the safe discharge of treated effluent.

Biological Processes in Secondary Treatment:

Secondary treatment primarily employs biological processes to break down and remove organic contaminants from sewage. The two main types of biological treatment processes are activated sludge and trickling filter systems.

Activated Sludge Process:

The activated sludge process is a widely used biological treatment method that involves the use of microorganisms to break down organic matter in sewage. In this process, sewage is mixed with a culture of microorganisms, known as activated sludge, in aeration tanks. These microorganisms utilize the organic pollutants as a food source, converting them into harmless byproducts like carbon dioxide and water. The treated effluent is then separated from the activated sludge through sedimentation, and the sludge is either recycled or subjected to further treatment.

Trickling Filter System:

Trickling filters are another common form of secondary treatment. In this process, sewage is distributed over a bed of rocks or synthetic media, allowing a thin film of microbial slime (biofilm) to develop. Microorganisms within this biofilm metabolize and digest organic pollutants as the sewage trickles over it. The treated effluent is collected and further processed, while excess microbial growth is periodically removed to maintain the system’s efficiency.

Advantages of Biological Treatment:

Biological treatment methods, whether through activated sludge or trickling filters, offer several advantages. Firstly, they are highly effective in breaking down organic pollutants, providing a reliable means of improving water quality. Additionally, these processes are cost-effective compared to some chemical treatment methods. Moreover, they can adapt to varying loads and are suitable for treating a wide range of wastewater types.

Challenges and Innovations:

While secondary treatment processes have proven effective over the years, challenges persist, such as the need for large treatment facilities and the generation of sludge that requires proper disposal. Researchers and engineers continually seek innovative solutions to enhance the efficiency and sustainability of secondary treatment. Advanced technologies, such as membrane bioreactors and sequencing batch reactors, aim to address these challenges by improving treatment performance and reducing the environmental impact.

Membrane bioreactors integrate membrane filtration technology with biological treatment, resulting in higher-quality effluent and a more compact footprint. Sequencing batch reactors offer flexibility in operation and better nutrient removal capabilities. These advancements contribute to the ongoing evolution of sewage treatment processes, aligning with the global push for more sustainable and environmentally friendly solutions.


In conclusion, secondary treatment of sewage is a pivotal step in the wastewater treatment process, playing a crucial role in ensuring the safety of water bodies and protecting public health. By employing biological processes like activated sludge and trickling filters, secondary treatment effectively removes dissolved and colloidal organic pollutants from sewage, meeting stringent water quality standards. Ongoing innovations in treatment technologies further enhance the efficiency and sustainability of these processes, emphasizing the importance of continued research and development in the field of wastewater treatment. As urbanization continues and environmental concerns grow, the significance of secondary treatment in safeguarding our water resources remains more vital than ever.


You may also like

Copyright © 2023