Article | Primary treatment of wastewater

Wastewater is often moved to a wastewater treatment plant. At the plant, it undergoes a series of treatment processes. These processes essentially return clean and safe water back into the environment.

Wastewater treatment plants commonly combine various processes to remove pollutants and treat industrial wastewater. Wastewater treatment projects and services cover physical, chemical and biological processes to perform the treatments to the highest efficiency.

If you are experiencing any of the problems listed above, NCH offers a range of solutions to tackle all of your wastewater needs.


  • Preliminary treatment
  • Primary treatment
  • Secondary treatment
  • Tertiary treatment

Preliminary treatment is an important stage in wastewater treatment because this is where the majority of the waste removal happens – solid separation happens at this stage. These treatments are split into four individual processes.

Primary wastewater treatments usually and most commonly involve the removal of solids, while secondary wastewater treatments involve bacterial decomposition. And lastly, tertiary wastewater treatment services involve extra filtration.


Screening is the preliminary process when treating wastewater. During this stage, large solid materials are separated from the water by means of metal screens.

The wastewater passes through metal screens to filter out large objects from the wastewater (such as metal or plastic objects, tissue paper, organic matter and other waste that can be found in wastewater from industrial and household sewage). The wastewater will then pass through a second screen with smaller openings to capture any smaller objects.

Grit removal

During this stage, grit, sand, stones and other fine particles are removed from the wastewater. The process takes place in a grit chamber or grit channel, where the speed of the water flow is slowed, allowing the particles the time to settle to the bottom of the chamber.

There are three common types of grit removal chambers:

  • Horizontal – this design allows the water to flow horizontally into the chamber at a controlled speed to encourage grit particles to settle.
  • Aerated – water flows in a spiralling motion in the chamber. Water flowing at the correct velocity will help the grit particles to settle.
  • Vortex – water flows through the chamber in a ‘whirlpool’ fashion. Again, the velocity is controlled to encourage the settlement of the grit particles.

Primary sedimentation

In the primary sedimentation stage, wastewater flows into pre-settling basins where the water is left for a period of time.

Any remaining organic material or solid particles sink to the bottom of the basin to form sludge build-up, and grease and oil deposits rise to the top of the basin as scum. The sludge and scum build-up are then pumped away from the wastewater to be treated separately.

Primary wastewater treatment is also referred to as primary sedimentation. The primary treatment utilises a sanitation technology which removes solids and organic material, usually suspended or floating (scum). This water treatment plant project helps in reducing the suspended solids by following treatment procedures.


Pumping takes place in larger wastewater treatment plants. Centrifugal pumps are the most commonly used pumps due to their ease of operation. However, there are other types of pumps during the treatment of wastewater, such as:

  • Progressive cavity pumps
  • Positive displacement pumps
  • Trash pumps
  • Submersible pumps

Centrifugal wastewater pumps are primarily used to pump wastewater upwards and return sludge build-up in a constant rotation whilst the pump station is in operation. Once the primary treatment processes are completed, the water is pumped into tanks to prepare for the secondary treatment stage.

Solid separation stage at preliminary treatment usually allows for water to remain suspended or solids collected may rest for an extended period of time. During this period, Odor control may become an issue as water and solids are still decomposing at this point.

NCH Asia offers a range of solutions to aid in the wastewater treatment processes, including odor control and FOG, BOD and TSS removal. Speak to one of our industry specialists for more information.

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