Sewage Treatment Plant: Design, Operation, and Benefits

A sewage treatment plant (STP) is an essential facility that transforms dirty wastewater into clean, reusable water. Whether it’s household sewage or industrial wastewater, STPs are designed to remove harmful pollutants and protect both public health and the environment. By using advanced technologies like anaerobic digestion and nutrient recovery, these plants play a vital role in water sustainability and combating climate change.

An STP is a specialized system designed to treat wastewater by removing organic, inorganic, and biological contaminants. It ensures that treated effluent can be safely discharged into the environment or reused for non-potable purposes like greywater recycling.

BOD (Biochemical Oxygen Demand) measures the oxygen required to break down organic matter. High BOD levels indicate excessive pollutants, which can affect nitrification and denitrification processes during treatment.

Imagine a world where untreated wastewater flows directly into rivers, lakes, and oceans. The consequences would be devastating—polluted water sources, damaged ecosystems, and widespread health issues. Sewage treatment plants act as a protective barrier, ensuring wastewater is treated and disposed of responsibly. They’re a critical part of modern infrastructure that we often take for granted.

The Sewage Treatment Plant Process

Preliminary Treatment

This step involves removing large debris using grit chambers and screens to protect downstream equipment.

Primary Treatment

In sedimentation tanks, heavy solids settle as sludge, while grease and oils float to the surface.

Secondary Treatment

Biological processes like activated sludge systems and trickling filters are employed to remove dissolved and suspended organic matter. Advanced methods like biological nutrient removal (BNR) focus on phosphorus removal and nitrogen reduction.

Tertiary Treatment

The final stage refines water quality with advanced filtration, UV or chlorine disinfection, and processes like sludge drying beds to manage solid waste.

Key Components of the Diagram

A typical sewage treatment plant diagram highlights the flow of water through each stage. Key components include:

  • Inlet Chamber: Where wastewater enters the plant.
  • Screening Unit: Removes large debris.
  • Sedimentation Tanks: Settles solids from the water.
  • Aeration Tanks: Promotes biological treatment.
  • Disinfection Units: Final purification stage.

Reading a sewage treatment plant diagram is like following a roadmap. Trace the flow of water step-by-step from the inlet to the discharge point. Look for symbols representing equipment and processes, such as tanks, filters, and pumps.

Environmental Impact

Sewage treatment plants prevent harmful pollutants from entering natural water bodies. They protect aquatic ecosystems, improve water quality, and reduce the risk of eutrophication—a condition where excess nutrients deplete oxygen levels in water.

Public Health Benefits

By treating sewage, these plants minimize the spread of waterborne diseases, ensuring safe water for drinking, bathing, and agricultural use. Clean water directly impacts public health and quality of life.

Even with advanced technology, STPs face challenges like:

  • High operational costs.
  • Sludge disposal problems.
  • Mechanical failures due to wear and tear.
  • Difficulty in handling industrial wastewater with high pollutant loads.

Operating a sewage treatment plant isn’t cheap. Regular maintenance of pumps, filters, and tanks is necessary to keep everything running smoothly. Energy consumption is another significant cost, especially during secondary and tertiary treatments.

What is a sewage treatment plant (STP)?

An STP is a facility designed to treat and clean wastewater (sewage) from residential, industrial, and commercial sources before releasing it into the environment or reusing it.

What is the principle of STP?

STPs work on the principle of separating solids, biological treatment to remove organic matter, and chemical/physical processes to remove harmful contaminants from sewage.

What is sewer treatment?

Sewer treatment refers to the process of removing contaminants from wastewater, including physical, biological, and chemical methods, to make it safe for discharge or reuse.

What is the function of an STP plant?

Remove physical, chemical, and biological impurities.
Treat wastewater for safe disposal or reuse.
Protect water bodies from pollution.

What are the 5 stages of sewage treatment?

Preliminary Treatment: Removes large debris like plastics and grit.
Primary Treatment: Settles solids and removes floating materials.
Secondary Treatment: Biological treatment to remove organic matter.
Tertiary Treatment: Advanced treatment to remove remaining impurities and pathogens.
Disinfection: Final stage to ensure treated water is safe for discharge or reuse.

What are the 4 types of sewage treatment?

Primary Treatment
Secondary Treatment
Tertiary Treatment
Sludge Treatment

What is the ASP process in STP?

MLSS (Mixed Liquor Suspended Solids) refers to the concentration of suspended solids, including microorganisms, in the aeration tank of an STP.

What is MBR in STP?

MBR (Membrane Bioreactor) combines biological treatment with membrane filtration to produce high-quality treated water.

How many types of STPs are there?

Conventional Activated Sludge Process
Extended Aeration
Sequential Batch Reactor (SBR)
Membrane Bioreactor (MBR)
Moving Bed Biofilm Reactor (MBBR)

What is a Type 2 sewage treatment plant?

Type 2 STP is a compact sewage treatment system that focuses on small-scale or decentralized treatment, often used for apartments or small communities.

Which STP system is best?

The best STP system depends on factors like size, capacity, and wastewater characteristics. MBR and SBR systems are highly efficient for residential and commercial use.

What is the difference between STP and ETP?

STP (Sewage Treatment Plant): Treats domestic sewage from homes and offices.
ETP (Effluent Treatment Plant): Treats industrial wastewater that contains chemical pollutants.

What is the construction of STP?

STP construction involves tanks for settling, aeration, filtration, and disinfection, along with pipelines, pumps, and electrical systems for operation.

What is the RAS pump in STP?

RAS (Return Activated Sludge) pumps are used to recirculate settled sludge from secondary clarifiers back into the aeration tank.

What chemicals are used in STP?

Chemicals like chlorine, alum, lime, and polymers are commonly used for disinfection and coagulation.

What is the operation of STP?

The operation includes:

Collecting sewage.
Treating it in various tanks.
Monitoring processes like aeration, filtration, and disinfection.
Ensuring proper sludge disposal.

How to calculate STP size?

STP size is calculated based on the expected daily sewage generation. For example, a standard calculation is 80% of water consumption.

What is the cost of a 100 KLD STP plant?

A 100 KLD (kiloliters per day) STP plant typically costs ₹25-40 lakhs, depending on the type of treatment process.

How is cost calculated in STP?

The cost is based on:

Capacity (e.g., KLD).
Type of technology (e.g., MBR, SBR).
Land and construction requirements.
Operating expenses.

What is the minimum area required for an STP?

The area required depends on the capacity. For small systems, 500-1000 square feet may suffice, while larger plants require several acres.

What is the STP project in Mumbai?

Mumbai’s STP projects aim to upgrade wastewater treatment facilities to handle increased sewage and reduce pollution in water bodies.

Which is the biggest STP plant in India?

The largest STP is the Okhla Wastewater Treatment Plant in Delhi, with a capacity of 564 million litres per day (MLD).

Which are the 7 STPs in Mumbai?

Mumbai has 7 major STPs located at Colaba, Worli, Bandra, Versova, Bhandup, Malad, and Ghatkopar.

Where does Mumbai sewage go?

Treated sewage in Mumbai is discharged into water bodies like the Arabian Sea after treatment at STPs.

Is STP mandatory for apartments in Mumbai?

Yes, Maharashtra regulations mandate STPs for apartment complexes with a built-up area exceeding a certain threshold (e.g., 20,000 square meters).

What is the full form of ETP?

ETP stands for Effluent Treatment Plant.

What is the full form of pH in STP?

pH stands for Potential of Hydrogen, which measures the acidity or alkalinity of water in the STP.

What is the slum sanitation project in Mumbai?

This project aims to provide improved sanitation facilities and STPs in Mumbai’s slum areas to enhance public health and hygiene.

What is STP in Swachh Bharat Mission?

Under the Swachh Bharat Mission, STPs play a critical role in managing sewage and ensuring environmental sustainability.