Aeration is the heart of every activated sludge system, as well as the most energy-intensive process. When designing or modifying a wastewater treatment facility, the aeration system design will have lasting impacts on effectiveness, capital and operating costs.
What Is The Alpha Factor?
The alpha factor accounts for impurities in the water and their effect on oxygen transfer.
To understand alpha, it’s helpful to understand some basics about oxygen transfer. A theory of oxygen transfer describes a thin film on each side of the gas/liquid interface. There is a resistance to the transfer of solutes across the two films.
KLa is the overall mass transfer coefficient where “KL is the liquid film mass transfer coefficient, and “a” is the surface area of the interface.
The alpha factor is the ratio of dirty-to-clean water mass transfer of oxygen:
The alpha factor is the most important consideration when sizing an aeration system in terms of capital costs and energy. It has a major impact on both the size of the aeration system and the power required to operate it.
Issues Affecting The Alpha Factor
Type of Aeration
The surface area of the liquid/gas interface is part of the mass transfer coefficient. Small diameter air bubbles provide more interface surface area. Therefore, in theory, aeration systems that produce smaller air bubbles will have a higher alpha value.
Fine bubble diffusers are well known for their high clean water oxygen transfer efficiency. Fine bubble diffusion works by pumping the air through a grid of diffusers that cover the bottom of the tank. The diffusers have tiny holes, letting air escape and rise to the surface. These systems are used most frequently at municipal plants but are not uncommon in industrial plants. However, in industrial applications which have higher biomass concentrations, the oxygen transfer efficiency can drop quickly due to reduced alpha factors and biofouling of the diffusers. The diffusers are also prone to alpha depression due to the presence of surface active agents that impede diffusion. The design alpha factor is typically in the range of 0.4-0.5. These aeration systems must be closely monitored, and the diffusers should be cleaned or replaced regularly to maintain efficiency
Jet aeration systems have a lower clean water transfer efficiency than fine bubble diffusers but provide high alpha values, especially in industrial applications with high concentrations of organic pollutants and higher biomass levels. Jet aerators transfer oxygen by simultaneously introducing large volumes of mixed liquor suspended solids and air through a series of jet nozzles. Intense mixing and a high degree of turbulence rapidly dissolves the air into a fine dispersion of gas and liquid. The hydrodynamic conditions within the jet and fine bubble cloud produce continuous surface renewal at the gas/liquid interface. This results in higher alpha factors. Alpha factors for jet aeration systems for industrial plants range from 0.8 to 0.9, about twice that of fine bubble diffusion.
Jet aeration systems have other advantages over fine bubble diffusion. Because the jets have large diameter openings of an inch or two, there is no problem with plugging or fouling. In addition, the equipment is robust. The major components are constructed of fiberglass or stainless steel with a 20-year life expectancy.
Wastewater Characteristics
Contaminants in the wastewater always affect the alpha factor. Soaps, detergents, and other surfactants lower the surface tension of the liquid. This may reduce the air bubble diameter, increasing the bubble surface area, though this can be offset completely by the reduction in oxygen diffusivity. Dissolved salts may keep the alpha high by inhibiting the coalescing effects of the bubble plume. Emulsified oil and grease can depress the alpha factor. Strong industrial wastes may lower the alpha factor for diffused aeration systems such that consideration should be given to aeration systems with higher alpha values to compensate.
Wastewater Additives
Adding chemicals to wastewater to resolve problems or enhance the process may greatly affect the alpha factor.
For example, foaming is a common problem at many industrial facilities. Many anti-foams and defoaming chemicals change the surface tension of the liquid. These chemicals can enlarge the air bubbles and depress oxygen transfer. Even at doses as low as 3 ppm, they can lower the alpha factor by 50 percent. Replacing these chemicals with others that aren’t as aggressive can prevent the alpha factor from dropping, thus getting the aeration plant back to optimum performance and energy usage.
Polymer used for solids separation may have a similar effect if it ends up in the aeration basin in excessive concentrations. Excess polymer makes the mixed liquor more viscous, creating a barrier to diffusion. The result may be an alpha factor approaching zero and an inability to transfer oxygen.
How To Ace The Alpha Factor
The goal of acing the alpha factor is to save capital costs, use less energy, and improve sustainability.
When designing an aeration system, complete a thorough preliminary evaluation. A good understanding of the specific wastewater characteristics is imperative. Depending on the project, evaluation may include computer modeling, field study, and pilot testing.
Get technical help from someone with lots of experience in a similar industry. Work with someone who has successfully overcome some of these challenges. Manufacturers and their local representatives have a great depth of knowledge about aeration system design and which aeration technology best fits the application. They’ve helped solve problems that affect the alpha factor, saving industry both capital and operating costs. Often, they are willing and able to perform pilot testing or at least consult with the engineer to offer examples of similar projects where they have had success. Take advantage of that knowledge and experience.
With a thorough evaluation and a trusted technical partner, you can ace the alpha factor.
