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Views: 222 Author: Carie Publish Time: 2025-04-26 Origin: Site
Content Menu
● Introduction to Sewage Treatment
● What is Primary Sewage Treatment?
● Key Processes in Primary Treatment
>> Screening
>> Comminution
>> Grit Removal
● What Does Primary Sewage Treatment Remove?
>> Limitations of Primary Treatment
● Advanced Technologies in Primary Treatment
>> Enhanced Primary Treatment (EPT)
>> Dissolved Air Flotation (DAF)
>> Mechanical Skimmers and Scrapers
● Environmental Impact of Primary Treatment
● Visual Guide: Primary Sewage Treatment Process
● FAQ
>> 1. What is the main purpose of primary sewage treatment?
>> 2. How effective is primary treatment at removing pollutants?
>> 3. What happens to the sludge collected during primary treatment?
>> 4. Is primary treatment enough to make wastewater safe for the environment?
>> 5. What are the main components of a primary sewage treatment system?
● Citation
Primary sewage treatment is a crucial first step in making wastewater safer for the environment and public health. This article provides a comprehensive overview of what primary sewage treatment removes, how the process works, and why it is essential. Throughout the article, you'll find diagrams, process illustrations, and educational videos to enhance your understanding.
Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage, to produce an effluent suitable for discharge into the environment or for reuse. The treatment process is typically divided into three main stages:
- Primary Treatment: Physical removal of solids and floating materials.
- Secondary Treatment: Biological treatment to degrade dissolved and suspended organic matter.
- Tertiary Treatment: Advanced processes to further improve effluent quality, including nutrient removal and disinfection.
The importance of sewage treatment cannot be overstated. Untreated sewage can pollute water bodies, harm aquatic life, and pose serious health risks to humans. Primary treatment serves as the first line of defense in reducing pollution loads and protecting downstream treatment processes.
Primary sewage treatment is the initial phase in the wastewater treatment process. Its main goal is to remove materials that will either float or readily settle out by gravity. This step is essential because it reduces the load on subsequent treatment stages and prevents damage to equipment.
Key Objectives:
- Remove suspended solids
- Reduce organic load (measured as Biochemical Oxygen Demand, BOD)
- Prepare wastewater for further biological treatment
Primary treatment is largely a physical process - it does not rely on biological reactions or chemical additives. Instead, it uses mechanical and gravitational forces to separate solids and oils from the wastewater.
Screening is the first step, where large objects such as rags, sticks, cans, and plastics are removed using bar or mesh screens. These screens prevent damage to pumps and other equipment downstream.
The screens are regularly cleaned to remove trapped debris, which is then disposed of in landfills or incinerated.
Comminution involves shredding or grinding any remaining large solids that pass through the screens into smaller pieces, making them easier to remove in later stages.
This process prevents clogging and facilitates smoother flow through the treatment plant.
Grit chambers slow down the flow of sewage, allowing heavy inorganic materials like sand, gravel, and eggshells to settle out. Grit can cause abrasion and damage to pumps and other equipment if not removed.
Grit is collected and often disposed of in landfills or used in construction.
Sedimentation tanks, also called primary clarifiers, allow suspended solids to settle to the bottom as sludge, while oils and lighter materials rise to the surface to be skimmed off.
The sludge collected at the bottom is pumped to sludge treatment facilities, while the floating scum layer is skimmed and disposed of separately.
In some plants, fats and oils are removed by skimming devices or air blowers that collect floating materials for separate disposal. These substances can clog pipes and interfere with biological treatment if not removed early.
Primary sewage treatment primarily removes the following:
- Suspended Solids: About 50–70% of total suspended solids (TSS) are removed. These solids include organic and inorganic materials that are suspended in the wastewater.
- Organic Matter: About 25–40% of the biological oxygen demand (BOD), which is a measure of organic material, is reduced. Organic matter is what bacteria consume, so reducing it helps prevent oxygen depletion in natural waters.
- Grit and Heavy Inorganics: Sand, gravel, and other heavy particles are settled out in grit chambers.
- Oils, Fats, and Grease: These float to the surface and are skimmed off to prevent clogging and interference with treatment.
- Large Debris: Rags, plastics, and other large objects are physically screened out at the start.
Note: Primary treatment does not remove dissolved pollutants, pathogens, or nutrients such as nitrogen and phosphorus. These are addressed in secondary and tertiary treatment stages.
| Removed Component | Removal Efficiency (Typical) |
|---|---|
| Total Suspended Solids (TSS) | 50–70% |
| Biochemical Oxygen Demand (BOD) | 25–40% |
| Grit, Sand, Gravel | Nearly 100% (if present) |
| Oils, Fats, Grease | High (varies by system) |
- Does not remove dissolved organic or inorganic substances: Many pollutants remain dissolved in the water after primary treatment.
- Does not eliminate pathogens: Disease-causing microorganisms require disinfection or biological treatment.
- Does not remove nutrients (nitrogen, phosphorus): Excess nutrients can cause eutrophication in receiving waters.
- Effluent still requires further treatment: Secondary and tertiary treatments are necessary to meet environmental discharge standards.
Despite these limitations, primary treatment is essential for reducing the load on downstream processes and preventing mechanical damage.
While traditional primary treatment relies on gravity and mechanical separation, advanced technologies have been developed to enhance removal efficiency:
Enhanced primary treatment uses coagulation and flocculation chemicals to increase the settling of suspended solids and organic matter. This can improve removal efficiencies to:
- 60–80% for suspended solids
- 40–60% for BOD
EPT is often used when secondary treatment capacity is limited or to meet stricter discharge regulations.
DAF systems inject fine air bubbles into the wastewater, which attach to suspended particles and float them to the surface for removal. This method is effective for removing fats, oils, and greases that do not settle easily.
Modern primary clarifiers are equipped with automated skimmers and scrapers that continuously remove floating scum and settled sludge, improving operational efficiency.
Primary sewage treatment plays a significant role in reducing environmental pollution:
- Reduces solids and organic matter: This helps prevent oxygen depletion in rivers and lakes, protecting aquatic life.
- Removes grit and debris: Prevents damage to downstream equipment and reduces sedimentation in natural water bodies.
- Prevents clogging: Removal of fats and oils reduces blockages in sewer systems and treatment plants.
However, because primary treatment does not remove nutrients or pathogens, untreated or partially treated effluent can still cause environmental harm if discharged directly. Therefore, it is critical to follow primary treatment with secondary and tertiary processes.
Primary sewage treatment is a vital step in the wastewater treatment process, focusing on the removal of suspended solids, organic matter, grit, and floating materials through physical means such as screening, sedimentation, and skimming. While it significantly reduces the pollutant load and protects downstream equipment, primary treatment alone does not make wastewater safe for discharge. It serves as a foundation for more advanced secondary and tertiary treatments, which are necessary to remove dissolved contaminants, pathogens, and nutrients.
Advancements in technology, such as enhanced primary treatment and dissolved air flotation, have improved the efficiency of primary treatment, helping wastewater treatment plants meet stricter environmental standards. Understanding what primary treatment removes-and what it does not-is essential for appreciating the complexity of wastewater management and the importance of integrated treatment systems.
The main purpose of primary sewage treatment is to physically remove suspended solids, organic matter, grit, and floating materials from wastewater, thereby reducing the pollutant load before biological and chemical treatments.
Primary treatment typically removes 50–70% of suspended solids and 25–40% of BOD. It is less effective at removing dissolved substances, pathogens, or nutrients, which require further treatment.
The sludge (settled solids) collected at the bottom of sedimentation tanks is usually pumped to sludge digesters for further treatment, stabilization, and eventual disposal or reuse.
No. While primary treatment removes a significant portion of solids and some organic matter, many harmful substances remain. Secondary and tertiary treatments are essential to further purify the water and meet environmental standards.
The main components include:
- Screens: For removing large debris
- Comminutors: For shredding solids
- Grit Chambers: For settling heavy inorganic particles
- Sedimentation Tanks (Primary Clarifiers): For settling suspended solids and skimming off floating materials
[1] https://www.yasa.ltd/post/primary-treatment-of-wastewater-how-does-it-work
[2] https://www.ssiaeration.com/what-is-primary-wastewater-treatment/
[3] https://www.britannica.com/technology/wastewater-treatment/Primary-treatment
[4] https://www.thameswater.co.uk/about-us/community/education/the-sewage-treatment-process
[5] https://www.yasa.ltd/post/primary-treatment-of-sewage-how-does-it-work-and-why-is-it-important
[6] https://www.dsd.gov.hk/EN/Sewerage/Sewage_Treatment_Facilities/Type_of_Sewage_Treatment_Facilities/index.html
[7] https://en.wikipedia.org/wiki/Sewage_treatment
[8] https://www.khanacademy.org/science/ap-college-environmental-science/x0b0e430a38ebd23f:aquatic-and-terrestrial-pollution/x0b0e430a38ebd23f:waste-disposal-reduction-and-treatment/v/ap-es-sewage-treatment
