Domestic Wastewater Treatment Process Explained

Waste Water Treatment Plant

Let’s face it, we all use water every day for various purposes, such as cooking, cleaning, and personal hygiene. But have you ever wondered what happens to the water after we’ve used it? Read on as we take a deep dive into the domestic wastewater treatment process, explaining how it works and why it’s so important.

What is Domestic Wastewater?

Domestic wastewater, also known as sewage, is the water that comes from households, which includes water from sinks, showers, toilets, and washing machines. This wastewater contains a mix of organic and inorganic substances, such as food particles, oils, chemicals, and human waste.

Why is Wastewater Treatment Important?

Wastewater treatment is crucial for protecting public health and the environment. Untreated wastewater can contain harmful bacteria, viruses, and parasites that can cause diseases if it contaminates drinking water or enters our natural water systems. Moreover, wastewater can carry pollutants that can harm aquatic life and ecosystems.

The Domestic Wastewater Treatment Process

The domestic wastewater treatment process typically involves several stages, each designed to remove specific types of contaminants. Let’s explore each stage in detail.

How Do Wastewater Treatment Plants Work?

Preliminary Treatment

In the preliminary treatment stage, wastewater undergoes initial screening to remove large objects, such as rags, plastics, and other debris. This is usually achieved using screens or grates that filter out these large items, preventing them from clogging the treatment equipment downstream.

Primary Treatment

Next up is the primary treatment, which involves separating solid particles from the wastewater. This is typically done using a sedimentation tank or clarifier, where the water slows down, allowing heavier solids to settle at the bottom. These settled solids, known as primary sludge, are later removed for further treatment.

Secondary Treatment

The secondary treatment stage aims to remove dissolved and suspended organic matter from the wastewater. There are several methods used in this stage, such as:

  • Activated Sludge Process – In the activated sludge process, wastewater is mixed with microorganisms in an aeration tank. These microorganisms consume the organic matter, converting it into carbon dioxide, water, and new biomass. The treated water then moves to a clarifier, where the biomass settles and is removed.
  • Trickling Filters – Trickling filters consist of large beds filled with porous media, such as rocks, gravel, or plastic. Wastewater is sprayed over the media, allowing microorganisms to grow on the surface and break down the organic matter as the water trickles down.
  • Rotating Biological Contactors – Rotating biological contactors (RBCs) feature large, circular discs covered in a thin layer of microorganisms. As the discs rotate, they come into contact with the wastewater, allowing the microorganisms to consume the organic matter.

Tertiary Treatment

Tertiary treatment, also known as advanced treatment, is an additional step taken to remove any remaining contaminants not eliminated during the primary and secondary stages. These may include nutrients like nitrogen and phosphorus, which can contribute to algal blooms and harm aquatic ecosystems. Tertiary treatment methods can involve filtration, membrane processes, or chemical treatments.


Once the wastewater has undergone tertiary treatment, it is disinfected to kill any remaining harmful microorganisms. Disinfection is typically achieved using methods such as chlorination, ultraviolet (UV) radiation, or ozonation. After disinfection, the treated water is released into rivers, lakes, or the ocean, or it may be used for irrigation or other non-potable purposes.

Sludge Treatment

Throughout the wastewater treatment process, sludge is generated from the various settling stages. Sludge treatment is crucial to reduce its volume and remove harmful substances. This process typically involves thickening, digestion, dewatering, and disposal or reuse. Digested sludge can be used as fertiliser, soil conditioner, or even as a source of energy through anaerobic digestion.

Wastewater Reuse

Treated wastewater can be reused for various purposes, such as irrigation, industrial processes, or even as a source of potable water. Reusing wastewater helps conserve water resources, reduce the demand for freshwater, and alleviate the pressure on our environment.

In Summary

In conclusion, the domestic wastewater treatment process is essential for safeguarding public health and protecting our environment. By employing various treatment stages, including preliminary, primary, secondary, tertiary, and disinfection, we can ensure that the wastewater we generate is properly treated before being released back into the environment or reused for other purposes.


Q. What is the difference between greywater and blackwater?

A. Greywater is wastewater from sinks, showers, and washing machines, while blackwater comes from toilets and contains human waste. Greywater can be more easily treated and reused compared to blackwater, which requires more advanced treatment processes to remove harmful contaminants.

Q. Can treated wastewater be used for drinking purposes?

A. Yes, treated wastewater can be used as a source of drinking water, but it requires additional treatment processes, such as reverse osmosis, advanced oxidation, or UV disinfection, to ensure it meets the required drinking water standards.

Q. What is the role of microorganisms in wastewater treatment?

A. Microorganisms play a crucial role in the secondary treatment stage of wastewater treatment, as they consume organic matter present in the wastewater, converting it into less harmful substances. Different types of microorganisms are involved in various processes, such as the activated sludge process, trickling filters, and rotating biological contactors.

Q. How can I reduce the amount of wastewater I generate at home?

A. You can reduce the amount of wastewater generated at home by adopting water-saving practices, such as using low-flow fixtures, fixing leaks, only running full loads in washing machines and dishwashers, and collecting rainwater for irrigation.

Q. What happens to the solids removed during the wastewater treatment process?

A. The solids removed during wastewater treatment, known as sludge, undergo further treatment to reduce their volume and remove harmful substances. Treated sludge can then be used as fertiliser, soil conditioner, or even as a source of energy through anaerobic digestion.