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What Is Primary Wastewater Treatment and How Does It Work?

By: Tom Frankel
Post Date: November 21st 2022

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What Is Primary Wastewater Treatment and How Does It Work?

Primary wastewater treatment is the first phase in the water treatment process and an essential step in turning wastewater into clean water that can safely return to a natural source. It removes solid materials that could damage equipment during the secondary wastewater treatment phase and prepares water for further filtration. Primary wastewater treatment uses gravity and physical processes to remove materials that can float or settle in the water.

Wastewater treatment is incredibly important for the environment, human health and community water supplies. Proper water treatment processes prevent industries from releasing overwhelming amounts of pollutants into the environment. These processes ensure that water released back into the environment is safe for animals, plants, habitats and humans.


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The Importance of Wastewater Treatment

Wastewater treatment is an essential part of protecting human health, preserving the environment and alleviating water scarcity. Contaminants in untreated wastewater threaten human health and can cause severe illness. Treating wastewater removes harmful sewage discharge from water, making it safe to release back into the environment for future use.

Water treatment helps to preserve the environment by reducing contaminants that can kill animals and plants or damage their habitats. Primary wastewater treatment eliminates approximately 60% of suspended solids from used water, and secondary treatment removes over 90%.

Water treatment also alleviates water scarcity. Treating water for reuse helps to preserve and recycle water supplies, ensuring communities have sufficient water to sustain living conditions. The following industries and processes produce wastewater that requires treatment:

What Is Primary Wastewater Treatment?

What Is Primary Wastewater Treatment?

Primary wastewater treatment is the first of three treatment phases that follow pretreatment. The wastewater treatment process consists of three phases — primary treatment, secondary treatment and tertiary treatment. Primary treatment removes material that will settle with gravity or float.

During the primary stage, wastewater sits in primary clarifiers to allow impurities to settle to the bottom or float to the top. Clarifiers may also contain mechanical scraping equipment that removes solid materials from the wastewater and moves it to sludge treatment equipment. Primary treatment also skims out any grease and oil that the pretreatment process did not remove. If the plant produces glycerol and soaps, it will add alkali substances to the skimmed oil and grease to begin the saponification process.

What Is Wastewater Pretreatment?

Like primary treatment, pretreatment uses physical processes such as settling and filtration to remove solids from wastewater. However, pretreatment removes larger solids before wastewater enters the primary phase.

Water passes through bar screens — long, narrow, closely-spaced metal bars — as it flows into treatment basins and tanks. These screens block large pieces of debris that could block or clog pumps and pipes. The screening process collects large debris such as:

Modern plants mechanically clean screens and swiftly dispose of debris by burying it on plant grounds. Some treatment plants also remove grease and oil during the pretreatment phase, while others remove those substances later in the treatment process.

How Does Primary Wastewater Treatment Work?

Primary wastewater treatment removes solid materials from wastewater using physical processes and gravity. The primary wastewater treatment process consists of the following physical methods:


The comminution process shreds and grinds debris that passes through the screens during pretreatment. The wastewater treatment plant will remove this shredded debris during the flotation or sedimentation processes.

Grit Removal

The grit removal process removes smaller solid materials. Grit chambers slow the flow of materials such as eggshells, coffee grounds and sand. These narrow, long tanks help grit settle out of the wastewater, preventing it from causing wear and tear on plant equipment such as pumps.

The grit removal stage is crucial in cities that utilize combined sewer systems. These systems collect a significant amount of sand, gravel and silt that washes off of the land or streets during storms. Water treatment plants pump the grit out of water tanks and move it to landfills.


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:

  1. As wastewater flows through sedimentation tanks, the solid materials gradually settle at the bottom.
  2. Mechanical scrapers then rotate slowly to move the solids — also referred to as primary or raw sludge — along the bottom of the tank.
  3. The scrapers also collect any materials that float on the water’s surface.
  4. A hopper collects the sludge and pumps it out for removal.

Sedimentation tanks typically provide two hours for solids to sink.

Why Is Primary Wastewater Treatment Important?

The primary treatment stage is crucial to the water treatment process. Removing larger debris and pollutants that could damage equipment safeguards the remainder of the plant’s treatment processes. This first stage is essential to keep wastewater treatment moving smoothly.

Additionally, removing the total suspended solids (TSS) and reducing the biochemical oxygen demand (BOD) in wastewater prepares it for the bacterial processes in the secondary treatment phase.

Contact SSI Aeration, Inc. for All Your Primary Treatment Needs

Contact SSI Aeration, Inc. for All Your Primary Treatment Needs

Primary wastewater treatment is a crucial part of the water treatment process. It removes large debris, suspended solids and floating solids from wastewater so the water can enter secondary treatment. The primary phase ensures water is free of larger solid materials that could harm equipment during the secondary phase, which removes even more solids from the water before disinfecting it.

SSI Aeration, INC. offers water treatment solutions for all phases of the wastewater treatment process. SSI Aeration designs and manufactures equipment for sewage treatment plants and industrial wastewater treatment plants. Their innovative treatment solutions provide long-lasting, energy-efficient options for all types of wastewater treatment plants. SSI also offers expert installation services and has installed systems in some of the world’s largest cities.

Different industries require different wastewater treatment equipment and processes. With SSI Aeration’s innovative equipment and professional installation services, your wastewater treatment plant can operate with the highest efficiency possible. The SSI Aeration team is trained and certified to handle design, manufacturing, retrofitting, installation and part replacement to ensure the highest system performance possible. Contact SSI Aeration to learn more about wastewater treatment solutions.