A Comprehensive Guide to Sewage Treatment: Main Steps

by Anna

Sewage treatment is a vital process that ensures the safe disposal of wastewater from homes, industries, and communities. It plays a critical role in protecting the environment and public health by removing contaminants and pollutants from sewage before it is released into water bodies or reused. This comprehensive article delves into the main steps of sewage treatment, highlighting the importance of each phase and its impact on our environment. Additionally, we’ll address some frequently asked questions (FAQs) related to sewage treatment.


Main Steps in Sewage Treatment

Sewage treatment involves a series of physical, chemical, and biological processes designed to remove contaminants from wastewater. The process typically consists of several key steps, each contributing to the overall purification of the sewage.


1. Screening

The first step in sewage treatment is screening. Wastewater entering treatment facilities contains a variety of solid materials, including leaves, sticks, plastics, and other debris. Screening involves passing the wastewater through a series of screens or bars with different-sized openings. These screens trap and remove larger particles and debris, preventing them from entering the treatment process. This step not only protects downstream equipment but also ensures the effectiveness of subsequent treatment processes.

2. Primary Treatment

After screening, the wastewater undergoes primary treatment. This step involves the removal of settleable organic and inorganic solids from the sewage. The wastewater flows into a sedimentation tank or primary clarifier, where it is allowed to settle for a period of time. During this process, gravity causes heavier particles to sink to the bottom of the tank, forming a layer of sludge, while lighter materials float to the surface, creating a scum layer.

The clarified water, which is now substantially free of large solids, is removed from the middle of the tank and directed to the next treatment stage. The sludge collected at the bottom is pumped or scraped out for further processing in the secondary treatment phase.

3. Secondary Treatment

Secondary treatment is a biological process that focuses on the removal of dissolved and suspended organic matter from the wastewater. This step relies on microorganisms to break down the organic materials present in the sewage. There are two primary methods for secondary treatment:

A. Activated Sludge Process:

In this method, wastewater is mixed with a culture of microorganisms (activated sludge) in aeration tanks. Oxygen is supplied to the tanks to support the growth and activity of these microorganisms, which consume and biologically convert organic pollutants into harmless substances. After aeration, the wastewater and activated sludge mixture is allowed to settle, and the clarified water is separated and discharged. Some of the activated sludge is returned to the aeration tank to maintain the microbial population.

B. Trickling Filters:

Trickling filters are another biological treatment method. Wastewater is distributed over a bed of rocks or plastic media, creating a surface for the attachment of microorganisms. As the sewage trickles over the media, these microorganisms consume organic matter, purifying the water. Periodically, the media is cleaned to remove excess biomass.

4. Tertiary Treatment (Optional)

Tertiary treatment is an additional step that further polishes the effluent to meet specific water quality standards or for reuse purposes. This step is not always required but may be necessary in cases where stringent water quality standards must be met. Tertiary treatment can include various processes such as filtration, chemical coagulation and flocculation, and advanced oxidation. It helps to remove remaining impurities, including nutrients like phosphorus and nitrogen, and ensures the water is safe for discharge into sensitive environments or for reuse in non-potable applications like irrigation.

5. Disinfection

Disinfection is a critical step in sewage treatment to eliminate harmful pathogens and bacteria from the effluent. Common disinfection methods include:

A. Chlorination: Chlorine gas or chlorine compounds are added to the wastewater to kill or inactivate harmful microorganisms. Chlorine is effective in disinfection but must be carefully monitored to ensure it does not create harmful byproducts or residuals.

B. UV (Ultraviolet) Disinfection: UV light is used to disinfect wastewater by disrupting the DNA of microorganisms, rendering them unable to reproduce. UV disinfection is an environmentally friendly method that does not introduce chemicals into the water.

C. Ozonation: Ozone gas is introduced into the wastewater to destroy pathogens and organic contaminants. Ozonation is effective in disinfection and also helps to remove color, taste, and odor from the water.

6. Sludge Treatment

Throughout the sewage treatment process, various forms of sludge are generated, including primary sludge and excess activated sludge. These sludges undergo further treatment to reduce their volume, stabilize their composition, and make them suitable for disposal or beneficial use. Sludge treatment processes may include thickening, dewatering, anaerobic digestion, and in some cases, incineration or landfill disposal.

FAQs about Sewage Treatment

1. Why is sewage treatment important?

Sewage treatment is essential for several reasons:

It prevents the release of untreated sewage, which can contain harmful bacteria, viruses, and pollutants, into the environment, protecting public health and aquatic ecosystems.

It reduces the risk of waterborne diseases and contamination of drinking water sources.

It allows for the safe disposal of wastewater from homes, industries, and municipalities.

It promotes water conservation and the potential for wastewater reuse, helping to address water scarcity issues.

2. How does sewage treatment benefit the environment?

Sewage treatment benefits the environment in several ways:

It removes harmful contaminants and pollutants from wastewater, preventing their entry into natural water bodies and protecting aquatic ecosystems.

It reduces the oxygen demand of discharged effluent, preventing oxygen depletion in receiving waters, which can harm aquatic life.

It helps to minimize the release of nutrients like nitrogen and phosphorus, which can lead to harmful algal blooms and eutrophication in water bodies.

3. Is sewage treatment the same as wastewater treatment?

Sewage treatment specifically refers to the treatment of domestic wastewater or sewage, which comes from homes and residential areas. Wastewater treatment, on the other hand, is a broader term that encompasses the treatment of all types of wastewater, including industrial wastewater, agricultural runoff, and stormwater, in addition to sewage.

4. What happens to the treated sewage water after the treatment process?

After sewage treatment, the treated water, known as effluent, is typically discharged into a receiving water body, such as a river, lake, or ocean, in compliance with regulatory standards. In some cases, the treated water may be further treated to meet stringent water quality requirements or reused for non-potable purposes like irrigation or industrial processes.

5. Are there any environmental concerns associated with sewage treatment?

While sewage treatment is critical for environmental protection, there are some environmental concerns associated with the process, including:

The energy consumption required for treatment processes, especially in energy-intensive methods like aeration.

The potential for the release of greenhouse gases from sludge treatment processes.

The need for careful management of residual sludge to prevent groundwater contamination and soil pollution.

To mitigate these concerns, ongoing research and advancements in treatment technologies focus on improving energy efficiency and reducing the environmental footprint of sewage treatment plants.


Sewage treatment is a complex and crucial process that ensures the safe disposal of wastewater while safeguarding public health and the environment. The main steps in sewage treatment, including screening, primary and secondary treatment, tertiary treatment (if necessary), disinfection, and sludge treatment, work together to remove contaminants and pollutants, making wastewater safe for discharge or reuse.

As populations grow and environmental concerns increase, sewage treatment plays an ever-more-important role in managing our water resources responsibly. By understanding the steps and processes involved, we can appreciate the significance of sewage treatment and its positive impact on our communities and ecosystems.


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