What are the three stages of sewage treatment and How does each stage work?
The Sewage Treatment Process essentially includes three stages. The three stages can be divided into primary, secondary, and Tertiary. In each step, water is purified to the next level to access clean water for humans and the environment.
There may be a situation where only one or two stages are required, but that depends upon the intent of using the water and what kind of environment it is discharged into. Let us understand the three stages of sewage water and how they are treated.
Stage 1: Primary Treatment (Solid Removal)
This stage essentially includes the process of sedimentation. The water is held in the large sedimentary or rainwater tanks where the settleable solids are removed. Since the sedimentation tanks work on the principle of gravity, the solids settle at the bottom, and the lighter solids float in the tanks.
Also, some chemicals can be added as coagulants to refine it more and remove the solids. Have you wondered what the sedimentation process does? Sedimentation helps in creating a sewage sludge as a by-product at the bottom of tanks. This by-product is further treated and used as fertilizer. It may also find its use in anaerobic digestion or as a liming material.
Anyhow, let's move forward to stage 2 of secondary treatment. After the sludge settles at the bottom, the water is then released for its secondary treatment.
This stage involves the removal of soluble organic solid matter. It also deals with the treatment of smaller suspended particles. In this process, waste is broken down by aerobic bacteria and incorporated into the wastewater system. There are various methods under the secondary treatment. These are as follows:
1. Use of Bioreactors
2. Use of Filter Beds
3. Use of Aerated Ponds
4. Activated Sludge
5. Use of rotating biological contactors
Stage 3: Tertiary Treatment (Extra Filtration)
Tertiary treatment is also known as polishing and disinfecting the water with the highest standards. This stage is critical to producing the water to a particular specification such as technical water, mineral water etc. It is also used to treat the water in public systems.
Tertiary treatment methods include the two following ways:
1. UV disinfection
2. Chemical disinfection
It is important to note that UV disinfection does not require any chemicals and, therefore, can be used in the place of chemicals. Essentially, It does not affect the Ph, appearance, taste or smell of water, but it destroys all the microorganisms.