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Primary Treatment of Wastewater: How Does It Work?

Updated: May 24, 2023

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

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


Article Outline


Primary Treatment of Wastewater Introduction

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

Primary wastewater treatment is the first of three treatment phases that follow pre-treatment. In fact, the wastewater 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 Wastewater Work?

Primary treatment of wastewater 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 wastewater involves sedimentation of solid waste within the water. This is done after filtering out larger contaminants within the water. Waste water 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 wastewater 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 50-65% of the suspended solids and 30-40% of the biological oxygen demand (BOD) from sewage.

Main Objectives of Wastewater Primary Treatment

In a sequence of operations, wastewater is treated primarily through physical, chemical, and biological processes in domestic sewage treatment.

  • 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 wastewater treatments.

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

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

  • 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 Wastewater Treatment?

Wastewater 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 wastewater 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.


In primary treatment of wastewater, 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 wastewater 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.

Primary Sedimentation

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

The Importance of Primary Treatment

Primary wastewater treatment is the first phase in the treatment process and an essential step in turning wastewater 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 wastewater 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 wastewater treatment phase and prepares water for further filtration.

Primary VS Secondary Wastewater Treatment

Primary and secondary treatment of wastewater 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 wastewater 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 and Wastewater

Due to the characteristics of industrial wastewater, the use of primary, secondary, and tertiary treatment is often necessary.

In these applications, the primary treatment uses similar processes, such as sedimentation, to reduce the solids in the industrial effluent.

Instead, secondary treatment involves the use of membrane filtration, evaporators, crystallizers, and electrocoagulation technology which are more suitable than biological processes. These methods are the most common in wastewater treatment inside of factories.

In some cases, factories need to reuse the treated wastewater in their productions processes, thus a tertiary treatment of the liquid is often necessary.

The combination of primary, secondary, and tertiary treatment is used for the most advanced Zero Liquid Discharge systems.

Design of Primary Treatment for Wastewater and Industrial Effluents

The best design for a primary treatment system for industrial wastewaters and sewage often involves various stages and technologies. The correct equipment needs to be selected and carefully manufactured to reach the optimal results.

For the right treatment system, you need the right expertise.

For more information about YASA ET wastewater treatment systems kindly get in touch at:


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