The use of ultra fine bubble aeration in the activated sludge process
Barter, P.1 and Novellino, M.2, 1Hydro International, UK, 2 Parkson Corporation, UK
(free)
The Cost of Aeration
• Aeration consumes 54-97% of total energy depending on configuration (Young and Koopman, 1991)
• Energy costs: 15-30% of large & 30-40% of small WWTPs’ O&M budgets (Kennedy et al., 1997)
• Increasing the efficiency of the activated sludge system will significantly reduce the operational cost
Options for increased efficiency
To increase efficiency we have to increase oxygen transfer, this can be done by:
Increasing contact area – If the contact area is larger there is more opportunity for the oxygen to
transfer to the wastewater.
For submerged system this normally relates to having smaller bubble
• Smaller the bubbles – Larger Area / Volume ratio – Higher Oxygen Transfer
o 1mm bubble have 3 time the more surface area than 3 mm bubbles
Increasing contact time – If you can increase how long the oxygen and wastewater are in contact you
give the oxygen more time to transfer from the air to the water.
For submerged systems a smaller bubble has low buoyancy, therefore rises through the wastewater
at a slower rate
• Buoyancy is a function of the volume and density difference of fluid displaced therefore
small bubbles have much lower buoyancy
• Therefore a 1mm bubble has 4 time less buoyancy force than a 2mm bubble and 9 time less
than a 3 mm bubble
o This decrease in buoyancy force means that the bubble has a slower ascent through
the wastewater
o The increased ascent time translates to increase contact time for oxygen transfer
Decrease the resistance to transfer – There are many constituents of wastewater that restrict the
transfer of oxygen from the air to the wastewater, these are loosely collected together and applied
as the alpha and beta factors for oxygen transfer converting the Actual Oxygen Requirement (AOR)
into a Standard Oxygen Requirement (SOR).
