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Primary Treatment of Sewage: How Does It Work and Why Is It Important?

Primary treatment of sewage is the first of the three distinct steps involved in traditional sewage treatment plants.

In today's modern world, effective sewage treatment is essential for maintaining public health and environmental sustainability.

Here’s a quick look at sewage primary treatment process functions and main features.

Primary Treatment of Sewage in a Sewage Treatment Plant Aerial View
Primary Treatment of Sewage in a Sewage Treatment Plant




Primary Treatment of Sewage Introduction

The primary treatment of sewage involves the removal of a portion of the suspended solids and organic matter from sewage.

Primary sewage treatment is the first of three treatment phases that follow pre-treatment. In fact, the sewage treatment process consists of three phases: primary treatment, secondary treatment and tertiary treatment. Primary treatment main function is to remove materials from waste water that will settle with gravity or float on the surface.

How Does Primary Treatment of Sewage Work?

Primary treatment of sewage consists of allowing sewage to pass slowly through a basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface and are skimmed off.

Primary treatment of sewage involves sedimentation of solid waste within the water. This is done after filtering out larger contaminants within the water. Wastewater is passed through several tanks and filters that separate water from contaminants. The resulting sludge from the primary clarifier or sedimentation tank is then fed into a digester, in which further processing takes place.

Primary Treatment of Sewage in a Sewage Treatment Plant Drawing
Primary Treatment of Sewage in a Sewage Treatment Plant Drawing

Primary sewage treatment uses gravity and physical processes to remove materials that can float or settle in the water. These processes are performed by primary sedimentation tanks or primary clarifiers, as shown in the picture below.

Primary treatment sedimentation tanks can be expected to remove 50-70% of the suspended solids and 25-40% of the biological oxygen demand (BOD) from sewage.

Below we can see an example of DAF equipment which is used in the primary treatment stage of sewage:

Main Objectives of Sewage Primary Treatment

In a sequence of operations, sewage is treated primarily through physical, chemical, and biological processes in domestic sewage treatment and has the following main objectives.

  • The main objective of primary treatment is to remove floating materials or “scum” that float on the surface. Scums can be made up of organic matter, oils, grease, and other lighter solids.

  • The primary treatment is usually a preliminary stage to further sewage treatments.

  • Primary treatment of sewage has been demonstrated by research to improve the quality and settling characteristics of sewages that are otherwise difficult to treat.

  • Sewage primary treatment is beneficial for reducing the BOD (Biological Oxygen Demand) of sewage.

  • Removal of settleable solids is the most crucial objective of this process.

The primary effluent treatment is a must have in any treatment system, both in domestic and industrial applications.

What are the Processes of Primary Sewage Treatment?

Sewage Primary Treatment is the first step in the water treatment process meant for removing suspended solids (TSS), oil and grease, colour, and odour. The key components in this step are screens, grit chamber, flow equalization tank, and clarifier.

The primary sewage treatment process consists of the following physical methods:


In primary treatment the sewage water passes through a screening process to remove all large objects like cans, rags, sticks, plastic packets, etc.

The solids are collected and later disposed in a landfill, or incinerated. Bar screens or mesh screens of varying sizes may be used to optimize solids removal. If gross solids are not removed, they become entrained in pipes and moving parts of the treatment plant, and can cause substantial damage and inefficiency in the process.

Screening Equipment Used During Sewage Primary Treatment
Screening Equipment Used During Sewage Primary Treatment


In primary treatment of sewage, comminution is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, and vibrating.

The sewage treatment plant will remove these shredded debris during the flotation or sedimentation processes in their primary clarifiers.

Grit Removal

Grit consists of sand, gravel, rocks, and other heavy materials. Preliminary treatment may include a sand or grit removal channel or chamber, where the velocity of the incoming sewage is reduced to allow the settlement of grit.

Grit removal is necessary to:

(1) reduce formation of deposits in primary sedimentation tanks, aeration tanks, anaerobic digesters, pipes, channels, etc.

(2) reduce the frequency of tank cleaning caused by excessive accumulation of grit;

(3) protect moving mechanical equipment from abrasion and accompanying abnormal wear.

Fat And Grease Removal

In some larger plants, fat and grease are removed during primary treatment by passing the sewage through a small tank where skimmers collect the fat floating on the surface.

Air blowers in the base of the tank may also be used to help recover the fat as a froth. Many plants, however, use primary clarifiers with mechanical surface skimmers for fat and grease removal.

Oil and Fat Removal Phase Drawing in the Primary Treatment of Sewage
Oil and Fat Removal Phase in the Primary Treatment of Sewage


Sedimentation tanks remove any suspended solid materials that make it past the screens and grit removal chambers. These circular tanks, also known as primary clarifiers, allow time for suspended solids to settle via gravity; as sewage flows through sedimentation tanks, the solid materials gradually settle at the bottom.

The Importance of Primary Treatment of Sewage

Primary sewage treatment is the first phase in the treatment process and an essential step in turning sewage into clean water that can safely return to a natural source or be reused.

Primary treatment of waste water removes larger debris and pollutants that could damage the equipment used in the following stages and the remainder of the plant’s treatment processes. This first stage is essential to keep sewage treatment working effectively.

As mentioned earlier, primary sewage treatment involves screening, sedimentation and primary clarifiers that remove solid materials that could damage equipment during the secondary sewage treatment phase and prepares water for further filtration.

Primary VS Secondary Sewage Treatment

Primary and secondary treatment of sewage are two different stages of sewage treatment and are necessary steps to produce freshwater.

Primary treatment of sewage main purpose is to remove the solids and prepare the waste water for the second step of the treatment system. Instead, secondary treatment produces water free of solids, and with a greatly reduced concentration of contaminants.

In municipal sewage, secondary treatment usually uses biological processes such as biofiltration, aeration and oxidation. In industrial effluents treatment applications, the preferred treatment methods may vary.

The initial and primary water treatment process removes large matter from sewage while the secondary treatment will remove smaller particles already dissolved or suspended. Sedimentation and filtration are the processes involved in the primary treatment method while biological breakdown occurs through aerobic or anaerobic units in secondary processes.

Another difference between these processes is how much time they take to complete. The primary treatment takes a shorter period to finish, but the secondary takes much longer as organic microbes consume the waste.

Primary, Secondary, Tertiary Treatment of Industrial Sewage

Primary, secondary, and tertiary sewage treatment are three distinct stages in the process of wastewater treatment. Each stage focuses on different methods and processes to purify and remove contaminants from sewage before it is discharged back into the environment.

Primary Treatment

Primary treatment is the initial stage of sewage treatment. It primarily involves physical processes to remove larger solids and suspended matter from wastewater.

The key methods used in primary treatment include screening, where large debris like plastics and rags are removed, and sedimentation, where gravity is utilized to allow the settling of solids. Primary treatment helps protect downstream treatment processes, prevents clogging of pipes and equipment, and reduces the organic load in sewage.

Secondary Treatment

Secondary treatment follows primary treatment and is a biological treatment process. It focuses on the removal of dissolved and colloidal organic matter, as well as the reduction of nutrients such as nitrogen and phosphorus.

The most common method used in secondary treatment is the activated sludge process, where microorganisms break down organic matter in the presence of oxygen. This process produces a biological floc, which settles in sedimentation tanks, forming sludge. The treated water, known as effluent, undergoes disinfection before it is discharged into natural water bodies.

Tertiary Treatment

Tertiary treatment is the final stage of sewage treatment and involves advanced treatment processes to achieve a higher level of purification. It is primarily employed to remove remaining contaminants and to improve the quality of the effluent.

Tertiary treatment methods include filtration through sand or activated carbon filters, disinfection using chemicals or ultraviolet light, and nutrient removal through processes like biological nitrogen removal or chemical phosphorus precipitation. The objective of tertiary treatment is to produce high-quality effluent that meets specific water quality standards for its designated reuse or safe discharge.

In summary, primary treatment focuses on physical processes to remove larger solids, while secondary treatment utilizes biological processes to break down organic matter. Tertiary treatment employs advanced methods to further purify the effluent, achieving a higher level of water quality.

Together, these three stages of sewage treatment work in harmony to effectively treat wastewater and protect the environment.


Primary treatment serves as a crucial initial step in the sewage treatment process. By effectively removing large solids and suspended matter, it lays the foundation for further treatment stages and prevents potential damage to equipment.

The significance of primary treatment lies in its ability to protect public health and the environment by minimizing pollution and promoting sustainable wastewater management.



Q1. Can primary treatment completely purify sewage?

No, primary treatment alone cannot fully purify sewage. It focuses on the removal of larger particles and suspended matter, but additional treatment processes are required to achieve higher levels of purification.

Q2. How long does primary treatment take?

The duration of primary treatment can vary depending on the flow rate and the size of the treatment plant. On average, it takes several hours for sewage to undergo primary treatment.

Q3. What happens to the collected sludge during primary treatment?

The collected sludge undergoes further treatment, which may include processes like digestion, dewatering, or composting, to reduce its volume and stabilize its composition.

Q4. Can primary treatment eliminate pathogens from sewage?

Primary treatment alone is not sufficient to eliminate pathogens from sewage. It mainly focuses on physical processes and does not effectively remove dissolved or microscopic contaminants. Advanced treatment methods, such as disinfection or tertiary treatment, are necessary to ensure pathogen removal.

Q5. What are the energy requirements for primary treatment?

Compared to secondary and tertiary treatment methods, primary treatment has lower energy requirements. It is a relatively simple process that relies on gravity and basic mechanical components, resulting in reduced energy consumption.

Q6. Why is sewage primary treatment important?

Sewage treatment plays a vital role in removing harmful contaminants from sewage before it is released back into the environment. Primary treatment serves as the initial stage of this complex process, focusing on the removal of large solid particles and suspended matter.


What Are The Best Solutions for Sewage Primary Treatment?

The best equipment for sewage primary treatment depends on various factors such as the volume and quality of the wastewater to be treated, the available space, and the budget.

Also, it is important to consider factors such as the type and quality of materials used in construction, the efficiency of the unit in removing pollutants, and the maintenance requirements before choosing a treatment unit for your sewage treatment needs.

Ultimately, the best system for sewage primary treatment will depend on the specific needs and requirements of the wastewater treatment plant.

YASA ET helps companies all around the world to find the best solution to their water and sewage problems. Get in touch with our team for more information:


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