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.
The four stages of the water treatment process:
- Preliminary treatment
- Primary treatment
- Secondary treatment
- Tertiary treatment
This article describes the preliminary and primary treatments of wastewater. These treatments are split into four individual processes and comprise the first stage of wastewater treatment.
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.
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.
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, and grease and oil deposits rise to the top of the basin as scum. The sludge and scum are then pumped away from the wastewater to be treated separately.
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 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.