High Efficiency Diffusers vs. Surface Aerators

Technical Bulletin 162: High Efficiency Diffusers versus Surface Aerators

EDI has engineered systems that are well suited for almost any aeration mixing application. With multiple diffuser platforms available, it gives design engineers or users choices for coarse bubble, fine bubble of different types, and several different configurations for the diffuser systems. With this flexibility and multiple configurations of diffuser systems available it has become obvious that the process efficiency gains and the energy efficiency of diffusers are causing diffused air technology to supplant and replace surface aeration components.

Many people want a comparison of the surface aerators versus diffusers. There is a wide spectrum of diffuser choices and a wide difference of performance of the splash type surface aerators versus induced draft aerators. With all of the variations, it is clear that EDI Advanced Technology Diffuser Systems offer huge process and economic advantages over surface aeration systems. The process benefits and control capabilities of diffused aeration are particularly attractive. The ability to save energy of 30% to 70% over surface aerators depending on the type of surface aerator used is a big benefit of fine bubble diffusers. Coarse bubble diffusers by comparison with surface aeration are still attractive from process standpoint, but the energy consumption of a coarse bubble diffuser system is virtually identical to the splashing type surface aerators while saving about 35% energy versus the induced draft aerators!

Key elements in the comparison of diffusers versus surface aerators are to get the performance on a common basis. Items that are important to evaluate are as follows:

  • Surface aerators typically have limited standby capacity. It is important when comparing surface aerators and diffusers to have a similar standby capacity. Diffusers will typically have 50% or 100% standby capacity of the blower systems. Similar standby capacities or spare capacity should be considered with surface aerators as well.
  • With the diffusers, the cost of a blower building may be included for the diffusers. There are two important issues to remember:
    • Most of the blower systems can also be installed outside with little if any protection. The cost of a blower building is optional and not a direct cost comparison for the diffuser system versus surface aerators.
    • If a blower building or shelter is employed, it is critical to recognize that cost of an office for the plant operator, housing for other components in the treatment plant, storage space for spare parts, etc which are typically employed inside a blower building and operations building are really not associated with diffusers, but are the convenience for the total plant operation. Those costs should not be assigned to diffusers in a comparison of diffusers versus surface aerators.
  • Electrical switch gear items need to be compared for total systems as well. Blowers will typically have less electrical components as all starters and electrical controls are in one location. The cost for the electrical components that are distributed around the treatment plant or lagoon are part of the surface aerator system cost.

It should also be noted that on the electrical controls, when there is a 50% to 100% standby of blowers provided there should be an accounting for a similar electrical switch for capability on the surface aerators.

In general, mechanical aerators have been considered as the lowest first cost of most systems. In reality when total system cost are considered on a similar design criteria and similar design basis, diffusers and surface  aerators can be very similar in capital cost.

Once the capital cost issue has been addressed, the operating savings from diffused aeration systems will typically pay for complete abandonment of new surface aeration systems and converting to fine bubble diffusers with less than 3 year pay back. When comparing surface aerators with diffusers for new construction, comparing them on an equal scope of supply basis will show that the diffusers are very close to the same first cost and if the first year’s operation cost are incorporated, the installed cost of diffusers are less! Beyond that point, savings of energy is huge and is contributing to the owner or to the operating entity.

For convenience, a quick checklist of operating and performance characteristics of high efficiency diffusers versus surface aerators is below for reference and use.




Superior Mixing

  • Full depth mixing any depth
  • Full basin utilization with large number of energy points and full basin distribution
  • Act as hydraulic baffles to eliminate short circuiting
  • Limited mixing below 12’ SWD
  • Limited energy locations – poor distribution
  • Limited energy points channelize flow and loss basin utilization

Major Energy Savings

  • Approximately 40% energy savings vs splash draft type
  • Approximately 70% savings vs induced draft type
  • Low demand charge
  • High energy cost
  • High demand charge

Maximum Process Flexibility

  • Turn down or throttle entire system
  • Throttle individual air laterals
  • Generally 50% to 100% spare blower
  • Loss of air when units turned off
  • No throttling
  • No spare capacity

Reduced Electrical

  • Lower Hp and smaller station
  • Only one to three blowers for most systems for reduced switch gear
  • No electrical cables in basin
  • Large horsepower load
  • Large number of starters
  • Cables to each unit hazard and costly
Suitable for cold weather operation Freezing in may require removal
Reduced foaming Generates foam and/or nuisance aerosols
Reduced maintenance with no moving parts in aeration basin Major maintenance of mechanical items in aeration basins
Can be installed in earthen basins with no floor protection required Floor protection or erosion control required
No protection required of basin liners Liners vulnerable to aerator vortex
System life 10 – 20 years 10 years approximately

Capital Cost

  • Deep basins – very economical
  • Shallow basins – competitive
  • Present worth analysis of 5 years or more – Diffusers always win
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