Sewage Treatment Process

This article provides a detailed overview of the sewage treatment process, covering the three primary stages—mechanical treatment, biological (biochemical) treatment, and tertiary treatment—used to remove solids, organic matter, and nutrients from wastewater. Learn how these methods transform raw sewage into environmentally safe water through mechanical filtration, microbial degradation, and advanced disinfection technologies.

The sewage treatment process is a systematic and scientific approach to remove physical, chemical, and biological contaminants from wastewater before it is discharged into the environment or reused.
Sewage treatment typically includes three major stages:

1. Primary (Mechanical) Treatment – Removes large solid materials such as grit, oil, and suspended particles.
2. Secondary (Biological) Treatment – Uses microorganisms to degrade organic pollutants and convert them into stable sludge.
3. Tertiary (Advanced) Treatment – Polishes and disinfects the water, removing nutrients like nitrogen and phosphorus as well as pathogens.

Depending on local regulations, treatment goals, and influent quality, not all facilities employ all three stages. However, these core processes collectively ensure wastewater meets environmental discharge standards or can be safely reused.

The mechanical treatment stage—also called primary treatment—focuses on physically removing large particles and floating substances from sewage through screening, sedimentation, and flotation.

Key Equipment and Processes:

• Screens and Grids: Capture coarse solids such as rags, plastics, and stones.
• Grit Chambers: Settle out sand, gravel, and other heavy inorganic particles.
• Primary Sedimentation Tanks: Allow suspended solids to settle under gravity.

The goal is solid-liquid separation through physical means. This step typically removes 25% of Biochemical Oxygen Demand (BOD₅) and 50% of suspended solids (SS) from raw sewage.
Although simple, this step is essential to prevent clogging, reduce load on biological units, and enhance overall system performance.

In certain systems—especially those targeting biological nitrogen and phosphorus removal—aerated sedimentation tanks may be avoided to prevent premature degradation of organic matter needed for downstream biological processes. Careful design and monitoring of the primary sedimentation process help maintain the proper influent quality for subsequent treatment stages.

Biological treatment—also known as secondary treatment—is the heart of the sewage treatment process. It removes biodegradable organic matter and suspended solids through the activity of microorganisms such as bacteria, protozoa, and fungi.

Common Biological Treatment Methods:

• Activated Sludge Process (ASP)
• Oxidation Ditch
• A2/O (Anaerobic–Anoxic–Oxic) Process
• Stabilization Ponds
• Soil Filtration and Biofilm Reactors

Among these, the activated sludge process is the most widely used in municipal wastewater treatment plants worldwide.

Principle of Operation:

Microorganisms decompose organic pollutants, converting them into:

• Carbon dioxide (CO₂) – gaseous byproduct
• Water (H₂O) – liquid byproduct
• Biological sludge – solid microbial biomass rich in organic matter

The resulting sludge is then separated in secondary clarifiers, producing treated effluent suitable for discharge or further purification.

The efficiency of biological treatment depends on substrate quality and environmental conditions that affect microbial activity.

Microbes require adequate nutrients, primarily:

• Carbon sources (organic compounds)
• Nitrogen sources
• Phosphorus sources
• Trace elements (iron, zinc, manganese)
  Toxic substances like phenols, heavy metals (copper, cadmium, lead), and industrial chemicals can inhibit microbial activity and must be controlled.

• Temperature:
  Optimal range: 20–30°C.
  Below 10°C, microbial activity declines; above 35°C, enzymes denature and treatment efficiency drops.
• pH:
  Ideal range: 6.5–8.5.
  Highly acidic or alkaline conditions damage microbial flocs and reduce process stability.
• Dissolved Oxygen (DO):
  Essential for aerobic systems.
  Maintain ~2 mg/L in aeration tanks for optimal microbial respiration.
  Too low → sludge bulking; too high → unnecessary energy consumption.

Maintaining balanced DO, temperature, and pH ensures continuous and stable biological performance. In modern treatment plants, online monitoring systems for dissolved oxygen and other key indicators have become standard practice to support real-time control and optimization.

Tertiary treatment, or advanced treatment, is designed to further improve effluent quality after biological processing. Although still developing in some regions, it plays an essential role in producing reclaimed water for industrial, municipal, or environmental reuse.

Common Tertiary Treatment Methods:

• Nutrient Removal: Eliminates nitrogen and phosphorus through chemical precipitation or biological denitrification.
• Activated Carbon Adsorption: Removes trace organic compounds, color, and odor.
• Membrane Filtration (e.g., Reverse Osmosis, Ultrafiltration): Produces high-purity water for reuse.
• Disinfection: Kills pathogens using ozone, ultraviolet (UV) light, or chlorination.

Applications of Treated Water:

• Street cleaning and landscaping irrigation
• Industrial process water
• Firefighting systems
• Toilet flushing and non-potable municipal uses

Tertiary treatment not only protects ecosystems but also supports sustainable water resource management by promoting wastewater recycling.

The sludge generated from all treatment stages—primary, secondary, and tertiary—contains high levels of organic matter, heavy metals, and pathogens.
If not properly treated, it can rot, produce foul odors, and cause secondary pollution.

Key Sludge Treatment Processes:

• Thickening and Dewatering: Reduces sludge volume.
• Digestion (Aerobic or Anaerobic): Stabilizes organic matter and produces biogas.
• Drying and Composting: Converts sludge into reusable soil conditioner.
• Incineration or Landfilling: Final disposal methods when reuse is not viable.

Proper sludge management ensures the success of the entire wastewater treatment system by preventing recontamination of treated effluent.

The sewage treatment process is a cornerstone of modern environmental protection and public health.
Through mechanical, biological, and tertiary treatment, raw sewage is transformed into clean, safe, and reusable water. Each stage plays a crucial role—from removing solids and degrading organics to nutrient removal and final disinfection.

Moreover, efficient sludge management ensures complete pollution control and sustainable resource recovery.
As urbanization accelerates and environmental standards rise, integrated and intelligent wastewater treatment systems will continue to shape the future of sustainable water management.

The three stages are mechanical (primary), biological (secondary), and tertiary (advanced) treatment.

Biological treatment removes dissolved organic matter using microorganisms, converting pollutants into harmless end products like water and CO₂.

Tertiary treatment removes remaining nutrients and pathogens, ensuring the water is safe for reuse or discharge into sensitive ecosystems.

Sludge is thickened, digested, and dewatered to reduce volume and stabilize organic content, often reused as fertilizer or safely disposed of.

Yes. After tertiary treatment, reclaimed water can be safely used for irrigation, industrial cooling, landscaping, and other non-potable purposes.

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading:

• Characteristics and functions of municipal solid waste leachate
• Analysis of filtration technology in sewage treatment equipment
• What are the three major problems of sewage treatment?
• What are the treatment methods of domestic sewage treatment equipment?
• What is the dispersion technology of domestic sewage treatment equipment?