Aeration Explained

Why does aeration matter?

Wastewater refers to water that has been used and contaminated as a result of various processes including industrial, agricultural, municipal (sewage), and mining activities.

In wastewater treatment, aeration is the process of mixing oxygen into the wastewater. This is vital for biological treatment. Microorganisms in the wastewater require oxygen to break down pollutants. Without enough oxygen, these helpful microbes are ineffective, leading to poor treatment and potential environmental issues.

Aerators are engineered equipment that introduce oxygen into water as efficiently as possible. Different types of aerators are used depending on the size and design of the wastewater tank, pond, or basin. Each type comes with different pros and cons. Here is a short list of the main types.

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Surface Aerators

Surface aerators consist of an impeller or brush which agitates and churns the water surface. This creates a large surface area for oxygen to transfer from the air into the water. Surface aerators can either be mounted on a platform or floating on a floating structure or pontoon. They can operate in tanks, ponds or basins of any size, and typically require a water depth of 4 to 6 m.

Surface aerators offer several benefits. They need little civil work to install (especially for floating units). Also, aerator maintenance is simple because the tank, pond, or basin can remain full during servicing.

Within the surface aerator category, there are three main types of aerators:

• Low speed vertical shaft aerators (floating or platform mounted): an impeller churns the water surface and sprays water radially outwards at high velocity to maximise the oxygen mass transfer. Typically used in industrial or municipal wastewater treatment facilities. Low speed refers to the speed of the impeller tip, which is typically about 5 m/s.
• High speed vertical shaft aerators (floating): the impeller rotates at high speed, meaning that a gearbox is not necessary. High speed aerators are typically used in smaller scale water treatment applications as they are more compact, and lower priced due to no requirement for a gearbox. They have lower oxygen mass transfer efficiency than low speed aerators. High speed aerators can operate in shallower water depths than low speed aerators, but are also effective in large basins or ponds. High speed impellers can operated at tips speed of up to 18 m/s.
• Horizontal shaft brushed aerators (floating or platform mounted): a brush extends from a horizontal shaft and agitates the water surface. Horizontal shaft aerators are typically used in municipal waterworks.

Submerged Aerators

Self-aspirating submerged aerators have a submerged pump which sucks air from the surface to the bottom of the tank. Submerged aerators tend to be smaller than surface aerators, and can be quickly installed. They have the limitation of not operating well in deep basins. Submerged aerators can be floating or non-floating.

Diffused Aeration Bubble Systems

Diffused aeration systems use a network of pipes and diffusers to release fine bubbles of air into the water. The bubbles rise to the surface, transferring oxygen into the water as they ascend. Large blowers, housed in blower houses are typically used to supply air to the system. The floor of the tank is layered with diffusion disks which channel the air from the pipes into the water. Diffused aeration systems are typically used in large wastewater treatment plants, and can be more energy efficient than surface or submerged aerators. However, they require more maintenance due to the potential for diffuser fouling which severely decreases their efficiency.