Though oxygen is already present in the water molecule (H2O), aquatic life cannot utilize it. Instead, they require dissolved oxygen which enters the water bodies from the atmosphere. This dissolved oxygen is also used by microorganisms living in it, which helps clean the natural water streams.
Though microorganisms can treat water, they take a long time to degrade pollutants. As a result, it develops intense odors and toxic chemical reactions as it sits. However, the treatment process can be sped along by adding oxygen.
Similarly, wastewater from municipalities and industries can also be treated by adding oxygen to stimulate aerobic degradation of organic matter in biological treatment. The use of oxygen in the treatment of wastewater is beneficial for removing pollutants without using toxic chemicals and harming the environment.
In wastewater treatment plants, optimum levels of dissolved oxygen are achieved by the aeration process.
What is the aeration process, and how does it work?
Aeration is the process of infusing air into wastewater, and it enables the growth of microorganisms that help degrade pollutants.
Air bubbles enter wastewater through an aeration system, they supplement aerobic bacteria for biodegradation, and apart from that, air bubbles bind with suspended solids and take them up on the surface. Then, the impurities from the surface are mechanically skimmed off.
For some time, the use of pure oxygen in wastewater treatment has prevailed because of the following benefits,
- Increased oxygen mass transfer
- Compact installations as it requires low tank volume
- Reduced power requirements
- Decreased sludge production
- Minimization of bulking and foaming problems
- Increases the capacity of wastewater treatment
- Eliminates foul odor
- Lowers emission rates
- Avoid stripping large amounts of nitrogen
- Eliminate low dissolved oxygen issues
- It is cost-efficient
Generally, air or oxygen is added during secondary treatment through aerators in wastewater treatment plants.
Role of oxygen in secondary wastewater treatment
After primary treatment, wastewater is passed forward for secondary treatment, where contaminants are broken down through biological processes.
Usually, secondary treatment involves fixed-film systems or activated sludge plants, which use aeration to enhance the biodegradation of organic matter.
- In fixed-film systems, bacteria are allowed to grow on a media, upon which wastewater is passed, and microbes consume organic material. Here, oxygen is added to stimulate the activity of bacteria.
- Activated sludge treatment, also known as a suspended growth system, consists of an aeration tank where wastewater is introduced. Here, the air and bacterial sludge are incorporated into water and allowed to settle for several hours so that bacteria can break down organic matter into harmless by-products.
To get efficient results from the processes mentioned above, it is essential to supply enough air and its even distribution across wastewater.
When is more oxygen needed in wastewater treatment plants?
- If the facility does not have sufficient airflow at the secondary or biological facility
- If the operator is trying to get more out of the existing system
- During the time of summer or high wastewater temperature
- Substantial changes in flow rates
Importance of Oxygen Injection for Wastewater Treatment
- It directly affects the quality of treated wastewater
- Oxygen is vital for the survival of aerobic bacteria in wastewater
- Even distribution and optimum supply of air ensure effective wastewater treatment
- It reduces the time required by bacteria to degrade contaminants
How does aeration improve the treatment process?
- It reduces the pungent odor caused by stagnant water
- It improves sludge settling
- It minimizes volatile organic compounds from wastewater
- It improves the ammonia treatment
- It enhances the system operation
- It helps meet the central wastewater discharge limits
- Sufficient dissolved air in the system maintains the health of the wastewater treatment plant
- It is a cost-efficient treatment method
- It saves the environment because it does not use chemicals in the treatment process
It does not do to simply inject air into wastewater; it should be added strategically to get effective results. Moreover, the amount of air and the way it is introduced makes a huge difference. Often, temperature, pressure, salinity, and flow rate affect the amount of dissolved oxygen in wastewater and simultaneously affects the treatment procedure. Hence, the amount of oxygen should be enough to supplement all bacteria, and it should be evenly distributed across the water.