Basic aerator selection

Key points

• Provide as much information to Zentec as possible and we will try help
• Useful information includes
  • basin dimensions,
  • water quality,
  • estimated aerator size in power (for example 30 kW),
  • number of aerators required
  • site location

Why Aerators Matter: The Basics

In wastewater treatment, aeration is the process of adding oxygen into the wastewater which 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.

Surface aerators work by agitating the water surface, creating a large surface area for oxygen to transfer from the air into the water.

Aerators types

There are numerous aerator types with different pros and cons depending on the waste water plant’s needs. Here is a short list of them:

• Surface aerators
  • High speed vertical shaft floating
  • Low speed vertical shaft platform mounted or floating
  • Horizontal shaft brushed aerators
• Submerged aerators
  • Self-aspirating
  • can be floating or non floating
• Diffused aeration bubble systems

Key Considerations for Your Selection

Whether you are designing the basin and sizing the equipment from scratch or purchasing replacement equipment, there are some key points to consider. If you can provide some of the information given below, Zentec can provide recommendations

Know Your Pond or Basin

This is probably the most critical piece of information you can gather. You’ll need to know:

• Size (length, width, depth) of the basin or pond: This dictates the scale of the aeration needed. The type of aerator that can selected depends on the liquid depth. High speed aerators tend to be for ponds with depths of bout 1.5 m.
  
• Shape of the basin: Irregular shapes might require multiple smaller aerators or specific placement.
• Current water levels and variations: Aerators need to be correctly submerged to function optimally. Fixed mounted aerators need to accommodate varying liquid levels.
• Existing infrastructure: Available power supply - Typically 400 V, three-phase
• Supporting structure: Type of available supporting structure. This will limit the type and size of aerator. If there is no structure, then floating aerators need to be considered.

Gathering drawings, photos and basic sketches of the area can be incredibly helpful for suppliers.

Estimating the aerator size

Here we provide two ways of estimating the size of aerator. The first is based on the size of the basin and the second is based on basic waste water data if this is available. It should just be used as a guide, but will give some idea of what to expect.

If you are supplying to an existing plant, the best is to look at the existing aerator unit nameplates. Site documentation should also be able to help.

Estimating aerator size based on basin/pond size

When the aerator oxygen requirements are known, the design is based on this. If it is unknown, the below can be used as a guide:

• Pond: Depth < 1.5 m. High speed aerators min 4-k w/m3. More may be required depending on water quality
• Basin: Depth between 3 and 6 m. Low speed aerators, typically 40 w/m3

The difference in required power per cubic meter of water between ponds and basins arises from the quantity of oxygen required by the process. High speed aerators, are less efficient than low speed aerators meaning that they transfer less oxygen to the water for the same size motor.

Depending on the basin size size, the power calculated above can be be split across multiple aerators.

Estimating based oxygen requirements

When a water sample has been taken, the key values to consider are biological oxygen demand (BOD), chemical oxygen demand (COD) and Oxygen for Nitrification.

If you have this available, include it in your query to us and we can estimate the required oxygen transfer rate.

To use these values, we must know the retention time of the water in the basin or pond. These values should be given in grams per cubic meter or similar units.

The total oxygen required is about 4.6x the ammonia-nitrogen and 1.5x BOD.

Divide this this by the number of hours the water spends in the basin to get the oxygen required in kilograms of oxygen per hour (kg/hr). This will help greatly in estimating the required aerator size.

Additional considerations

Estimating power and oxygen requirements is just the first part of a aerator selection. Below is a list of some additional details to consider.

• Lead time: Aerators are not off-the-shelf items and you can expect a lead time of 6-8 weeks on smaller units and up to 24 weeks on larger units. This needs to be considered in project planning

• Materials of construction and corrosion protection of wetted parts

• Extras - These include motor and gearbox special requirements, any site specifications, aerator accessories like draft tubes, pontoons, mooring cables, electrical cables

• Delivery, installation and commission. Zentec can assist here on request