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Will Fine Bubble Aeration Systems 
Work for You?

     The following article presents an overview of a very complex subject: fine bubble aeration systems. Appreciation is extended to the following individuals who contributed to this article: Mary Michaud, Teresa Libby, Kevin Flanders, Steve Cassidy, Kurt Marston, Anne Tischbein, Lisa Napolitano, Guy Vaillancourt, Hugh Tozer, James Morris, Steve Woodard, Greg Cataldo, George BWm, Charlene Powell, Wiff Peterson, Albert Presgraves, Doug McKeown, Tracy Planinsek, Steve Fogg, Adam Yanutis, Henri Vincent, and Jim Fitch; and Chris Branch, City of Lewiston, Maine; Russell Ferrelli, Bert LeBlanc, and Jim Lavacchia, Town of Weston, Massachusetts

     Activated sludge aeration systems consume from 50 percent to 90 percent of the energy used at most wastewater treatment plants, according to the U.S. EPA. A significant component of operation and maintenance costs, aeration systems have an equal effect on plant performance and capacity.

     Many wastewater treatment facilities worldwide operate inefficient and expensive aeration systems. As operators look to optimize their plants and streamline costs, many turn to fine bubble technology. Compared to many older systems, fine bubble aeration can reduce the power required to transfer oxygen (and its associated costs) by up to 50 percent.

     Would your wastewater treatment plant benefit by replacing an older aeration system such as coarse bubble diffusion with fine bubble technology? It's well worth investigating. It is important to understand some critical terms, such as oxygen transfer efficiency and alpha, before beginning your evaluation. The oxygen transfer efficiency of an aeration system is the ratio of the amount of oxygen that actually dissolves into the water to the total amount of oxygen pumped into the water. Only the dissolved oxygen is available for treatment, and any portion of oxygen that does not dissolve is a waste of energy and, therefore, money.

   For clean water aeration, the oxygen transfer efficiency provided by fine bubble diffusers usually exceeds that available from coarse bubble systems. In wastewater, this may not always be the case. The design engineer and equipment vendor use a factor called alpha to adjust the clean water transfer efficiency to take into account the affects of wastewater characteristics on transfer efficiency. Alpha is the ratio of oxygen transfer in a generic wastewater to oxygen transfer in clean water. The clean water transfer efficiency multiplied by alpha yields the wastewater transfer efficiency. When evaluating your system, keep in mind that the use of generic or assumed values can pose problems. Your wastewater may not behave like the assumed generic wastewater.

     Many factors affect oxygen transfer efficiencies, alpha values, and ultimately the performance of one aerator versus another. The following are some issues to consider and understand when evaluating aeration systems.

The bubble size affects the oxygen transfer efficiency. Smaller bubbles have more surface area per unit volume. This provides more area through which oxygen can diffuse and thereby increase overall transfer efficiency. Visualize a basketball filled with ping pong balls. Both take up the same volume, but the ping pong balls have a much greater cumulative surface area. Also, since fine bubbles provide larger total surface area, they create more friction and rise slower than coarse bubbles. The combination of more transfer area and a greater contact time enhances transfer efficiency.

Surface active agents (surfactants), such as detergents, lower alpha and the oxygen transfer efficiency. By altering the surface tension, they often cause fine bubbles to coalesce into fewer, larger bubbles. In addition, the thin film of detergent molecules between the air bubble and the wastewater can act as a barrier, increasing resistance to oxygen transfer.

Fouling decreases transfer efficiency. Most fine bubble systems being installed today use a diffuser made of a flexible synthetic membrane with many small slits or a ceramic disk with very fine holes. Both provide attractive sites for microbial growth and precipitation of inorganic compounds. This type of fouling can cause bubbles to coalesce, thereby decreasing the overall transfer efficiency.

Finally, aeration basin and aerator layout geometry can dramatically alter oxygen transfer efficiency. Standard clean water transfer efficiency tests are usually based on full floor aerator coverage. This creates a nearly ideal situation for oxygen transfer. A turbulent counter-current flow regime is established with a volume of water being dragged upward with the rising bubbles, being opposed by an equal flow of water traveling downward. In large basins, such as lagoons, full floor coverage is not normally needed to meet the biological aeration demands. As a result of incomplete coverage, large scale currents can form a spiral roll in the basins. The air bubbles flow with the water, which significantly reduces the transfer efficiency. Compared with the full floor configuration, many more air bubbles short-circuit their way to the top of the aeration basin.

     Although it may be true that fine bubble systems can provide an appreciable savings in many situations, those who purchase and operate these systems without verification do so at the peril of their own bottom line cost. How can you avoid this pitfall? First, test the system proposed with your wastewater. Do not trust a vendor who does not recommend oxygen transfer testing. Second, make sure that oxygen transfer testing mimics the actual conditions, including the configuration of your basin. The cost of aeration system testing can be high and may not be affordable for many plants. Before testing, the potential cost savings of switching to fine bubble aeration should be evaluated. Literature, vender, and EPA values for oxygen transfer efficiently and alpha can be used. Find plants with similar wastewater to refine from their experiences the values and contact plants to learn from their experience. If the investment in fine bubble diffusion appears favorable, consider testing before replacing the system.

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