Backpressure rises alone can result in blower power increases of 10-20%, but it is not uncommon to measure oxygen transfer efficiency losses of 30% or more after some years of service. The combined effects of both changes can increase blower power costs by more than 50% from the ‘as new’ condition.
Changes in oxygen transfer efficiency are best measured on-site using ‘Off-Gas’ techniques. These involve capturing gases released from the tank surface and passing these through an oxygen analyser.
Off-gases are typically collected from a hood floating on the tank surface and moved from position to position by a portable davit assembly. Simultaneous readings of air flow, water temperature and dissolved oxygen concentration are taken.
Atmospheric air contains 20.9% oxygen by volume. The gases collected from under the hood are usually 16-19% oxygen, depending on the efficiency of oxygen transfer, and hence how much oxygen has been absorbed from the bubbles rising to the tank surface.
Post-test calculations enable actual oxygen transfer rates to be determined over a range of air flows, and usually for a number of different measurement stations. These, in turn, allow effective Alpha values to be determined if clean water oxygen transfer efficiencies are also known.
Use of off-gas testing enables the operator to track changes in real oxygen transfer efficiencies as diffusers age and foul, and they can help the operator gauge the benefits of periodic cleaning operations.