What Are the 3 Types of Wastewater Treatment and How Do They Work?

1. Primary Wastewater Treatment – Removing the Obvious

The first stage in any STP plant is the Primary Wastewater treatment, which focuses on removing large, visible, and settleable solids from the incoming sewage.

How It Works:

At this stage, wastewater flows through bar screens and grit chambers to trap debris such as plastics, sand, and silt. It then moves into a primary sedimentation tank where heavier particles settle at the bottom, forming sludge, while grease and oils float to the top and are skimmed off.

This phase is purely physical and doesn’t eliminate dissolved organic matter, but it’s essential for preventing blockages and preparing the sewage for further treatment.

Across India, this process is widely implemented in sewage facilities for both residential and industrial setups, forming the backbone of most municipal STPs.

2. Secondary Wastewater Treatment – The Biological Clean-up

Next comes the secondary wastewater treatment, which is where the real purification begins. This stage relies on biological processes to break down the organic pollutants present in sewage.

How It Works:

Wastewater from the primary stage enters aeration tanks, where oxygen is added to promote the growth of aerobic bacteria. These microbes feed on the organic waste, converting it into less harmful substances. The resulting mixture then flows into a secondary clarifier where the biological sludge settles and is separated.

This method is commonly known as the activated sludge process and is a standard in any modern sewage treatment plant design, especially for housing societies and commercial projects.

Secondary treatment drastically reduces biological oxygen demand (BOD) and removes pathogens—ensuring that the STP plant meets national discharge norms.

3. Tertiary Wastewater Treatment – Making Water Reusable

The final stage, tertiary treatment, is all about polishing the treated water to a quality level where it can be reused or safely discharged into the environment.

How It Works:

This advanced treatment involves several processes such as sand filtration, membrane filtration (like RO or UF), disinfection using chlorine or UV light, and sometimes chemical dosing to remove nutrients like nitrogen and phosphorus.

Many waste water treatment plants in India now include tertiary treatment, especially in regions facing water shortages. The reclaimed water is reused for flushing, gardening, and even cooling towers in industries.

Packaged STPs are often equipped with these systems in high-rise apartments and IT parks, turning wastewater into a valuable resource instead of waste.