There are three main stages of the wastewater treatment process, aptly known as primary, secondary and tertiary water treatment. In some applications, more advanced treatment is required, known as quaternary water treatment. This stage deals with part per million to part per billion levels of contamination and often involves oxidation or fine filtration processes. Each of these stages tackles different pollutants, with water becoming cleaner as it moves through the phases.
What are the three stages of sewage treatment
Primary treatment (stage 1)
The first stage is called primary treatment, where wastewater is held in large tanks. Gravity helps heavier solids settle at the bottom, and chemicals can be added to remove more solids. This process creates sewage sludge, while the treated water moves to the next stage for further treatment.
Secondary treatment (stage 2)
In the second stage, soluble organic matter and smaller suspended solids are removed. Various methods like bioreactors, filter beds, and activated sludge are used. A membrane bioreactor is a modern and efficient technology that eliminates the need for additional clarifiers. Water from this stage can be released into the environment without harming aquatic life.
- Bioreactors
- Filter beds
- Aerated ponds
- Activated sludge
- Rotating biological contactors
Removal of organic contaminants is best achieved using a membrane bio reactor, where a biological process takes place in the reactor where microorganisms consume the organic matter for energy. The microorganisms replicate to create solids. The solids are then filtered by the membrane to produce effluent.
The key benefit to this technology is it eliminates the need for a secondary clarifier or filtration system for solids. The biological process in the reactor and the physical separation of solids takes place efficiently in one system.
Other methods of secondary treatment include filter beds, aerated ponds, biofilters, activated sludge and rotating biological contactors. However, membrane bioreactors are the most efficient technology and the most modern. Water that has gone through secondary treatment can be released into the environment without damaging aquatic life and ecosystems.
Tertiary treatment (stage 3)
The third stage, tertiary treatment, focuses on disinfecting water to high standards. This step is crucial for producing water according to specifications and treating wastewater for public water systems. Methods include UV disinfection, which requires no chemicals, and chemical disinfection using substances like chlorine or iodine. UV disinfection is often preferred for its effectiveness without altering the water’s properties.
Tertiary treatment methods include:
- UV disinfection
- Chemical disinfection
Four types of wastewater treatments
Sewage Treatment Plants (STP)
This treatment method involves the following steps: infiltration, grit removal, primary sedimentation tank, secondary sedimentation tank, and secondary clarification. This process is used to treat domestic wastewater and commercial waste.
Effluent Treatment Plants (ETP)
This method uses the same process as STPs but uses more advanced technology resulting in the effluent having a lower BOD (biological oxygen demand) level.
Common and Combined Effluent Treatment Plants (CETP)
This type of wastewater treatment is for small businesses that are required to process their wastewater before releasing it into the water bodies but can’t afford to do so. CEPTs allow small industrial businesses to pool their wastewater with other businesses like them and divide the cost of treating wastewater to save their resources and the environment.
Activated Sludge Plants (ASP)
This involves creating a highly concentrated sludge with activated sludge, which is mixed with air and septic tanks to cause oxidation and breakdown of organic matters.
