Purpose of Aeration | Activated Sludge Process | Wastewater Treatment Plant

 Purpose of Aeration

Aeration serves two purposes: first, it dissolves oxygen in the wastewater in the aeration tank, and second, it mixes the suspended solids from the mixed liquor in the aeration tank with the incoming wastewater.

Aeration Methods

There are two types of aeration in wastewater treatment plants: mechanical and diffuse. Each method has its own air delivery system as well as a mixing system for the combined liquor and wastewater.

• Mechanical: Devices are used to either splash water into the air or mix air into the wastewater.
• Diffused: compresses air and disperses it to provide oxygen to and mix wastewater

Mechanical Aeration Systems

The majority of mechanical aeration equipment operates in a similar manner.

Equipment Is Designed To:

• Agitate the water surface in the aeration tank to produce spray and waves, which increases oxygen transfer from the atmosphere into the wastewater.
• To improve oxygen transfer into wastewater, splash water into the air or entrain (mix) air into the water.
• In the aeration tank, combine the incoming wastewater, mixed liquor suspended solids, and air bubbles to improve the aeration process.

Simple mechanical aerators spray water into the air or mix air with wastewater to allow oxygen to enter. Mechanical aerators are used for a variety of purposes, including:

• For power, use an electric motor.
• A fixed-location design (platform-mounted) or a floating design is possible.
• Use a submerged or partially submerged splashing impeller.

Equipment Can Consist of:

• Paddlewheels or rotating brushes that are horizontally oriented, or
• Vertically oriented turbines or mixers

Submerged Turbine Aerators

Are hybrid mechanical aerators that are commonly used:

• Impellers that are submerged.
• Air that has been diffused and dispersed beneath the aerator.
• To induce a more effective flow pattern for transferring oxygen to wastewater, use a draught tube.

Diffused Aeration Systems

The most common type of aeration system used in the activated sludge process is diffused aeration. These systems work by supplying pressurized air to a network of distribution devices (diffusers) that are submerged in an aeration tank.

• Blowers are the machines that supply pressurized air.
• The pressurized air is transported from the blowers to the diffusers via air piping.
• Diffusers are the parts of the aeration tank that distribute air.

Diffused aeration systems introduce oxygen into wastewater using diffusers, blowers, transfer piping, and other equipment. The diffusers are designed to continuously release bubbles into the wastewater. The air contained within the bubbles dissolves into the wastewater. As the contact time with the wastewater and the surface area of the bubbles increase, so does the amount of air transferred. As a result, increasing the submergence depth of the diffusers and decreasing the size of the air bubbles will increase the rate at which air is dissolved into the wastewater for a given air volume.

Diffusers placed in the aeration tank can improve the contact time between bubbles and wastewater. This is accomplished by inducing a rolling pattern that keeps the bubbles submerged for a longer period of time. In order to adequately mix the suspended solids in the mixed liquor with the incoming wastewater and bubbles, the induced rolling pattern is also required. When this occurs, the solids do not settle and intimate contact is maintained, allowing the microorganism-initiated decomposition processes to proceed effectively.