The Complete Guide to Sewage Treatment: Four Essential Stages

by Anna

Sewage treatment is an indispensable process for maintaining public health and preserving the environment. It involves the careful and systematic removal of contaminants from wastewater before its release back into the environment. This process comprises four fundamental stages, each with its own set of methods and technologies. In this comprehensive guide, we will delve into each of these four stages, shedding light on their significance and the technologies employed.

Stage 1: Preliminary Treatment

The first stage in sewage treatment is the preliminary treatment. It serves as a vital preparatory phase for the subsequent processes. The primary objectives of this stage are to remove large objects and grit, as well as to equalize the flow of influent wastewater. Here are some key aspects of preliminary treatment:


Screening: Large objects such as sticks, leaves, plastics, and debris must be removed from the incoming wastewater to prevent clogs and damage to equipment. This is typically achieved using bar screens or rotating screens.


Grit Removal: Grit, which includes sand, gravel, and other heavy particles, can cause damage to downstream equipment and hinder the treatment process. Grit removal is accomplished through settling tanks or cyclone separators.


Flow Equalization: The flow of wastewater can fluctuate significantly throughout the day. Flow equalization tanks are used to balance the flow, ensuring that subsequent treatment stages receive a consistent and manageable load.


Stage 2: Primary Treatment

Following preliminary treatment, the wastewater moves into the primary treatment stage. Primary treatment is a physical separation process that targets the removal of settleable organic and inorganic solids. The primary clarifiers, also known as sedimentation tanks, play a crucial role in this phase. Key aspects of primary treatment include:

Sedimentation Tanks: In these large tanks, the wastewater is allowed to sit undisturbed, facilitating the settling of solids. Heavier particles settle to the bottom, forming sludge, while lighter materials float to the top, creating a scum layer. This process effectively removes up to 30-40% of the contaminants in the wastewater.

Sludge Management: The collected sludge at the bottom of the sedimentation tanks is pumped out and further treated in sludge treatment processes, such as anaerobic digestion or dewatering, to reduce its volume and minimize its environmental impact.

Stage 3: Secondary Treatment

Secondary treatment is a biological process designed to further purify the wastewater by breaking down dissolved and suspended organic matter. This stage is critical for significantly reducing the levels of pollutants in the effluent. There are two primary methods for secondary treatment:

Activated Sludge Process: In this method, microorganisms (activated sludge) are introduced to the wastewater in aeration tanks. These microorganisms metabolize organic matter, transforming it into biomass and carbon dioxide. The treated water then moves to secondary clarifiers, where the activated sludge is separated from the clarified water before being returned to the aeration tanks or sent for further treatment.

Trickling Filters: In trickling filters, wastewater is distributed over a bed of media (typically rocks or plastic) covered with a biofilm of microorganisms. As the wastewater trickles over the media, the microorganisms break down the organic matter. The treated water is then collected, and excess biomass is periodically removed from the media.

Stage 4: Tertiary Treatment

Tertiary treatment is the final phase of sewage treatment and focuses on the removal of remaining impurities to meet specific water quality standards. While not always required, tertiary treatment is essential when higher water quality standards must be achieved. Key components of tertiary treatment include:

Filtration: Filtration involves the passage of treated wastewater through sand or other media to remove fine particles, residual organic matter, and any remaining microorganisms. Filtration can be accomplished through rapid sand filters, multimedia filters, or membrane filtration systems.

Disinfection: Disinfection is employed to kill or inactivate any remaining pathogenic microorganisms in the treated wastewater. Chlorination, ultraviolet (UV) disinfection, and ozonation are common methods used for this purpose.

Nutrient Removal: In some cases, wastewater may contain excess nutrients, such as nitrogen and phosphorus, which can contribute to water pollution. Nutrient removal processes, such as biological nutrient removal (BNR) or chemical precipitation, are employed to reduce these contaminants to acceptable levels.


Sewage treatment is a multi-stage process that plays a crucial role in safeguarding public health and protecting the environment. The four primary stages—preliminary treatment, primary treatment, secondary treatment, and tertiary treatment—work in harmony to remove contaminants and pollutants from wastewater. Each stage serves a specific purpose, from removing large debris to breaking down organic matter and ensuring the treated effluent meets stringent water quality standards.

Understanding these four stages is essential for municipalities, environmental agencies, and wastewater treatment professionals tasked with managing and maintaining sewage treatment plants. As environmental regulations become more stringent and water quality standards rise, the importance of efficient and effective sewage treatment processes continues to grow.

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