Alpha Factors in Full-Scale Wastewater Aeration Systems

Surface active agents depress gas transfer at gas-liquid interfaces. They are present as measurable trace contaminants at all environmental and at most industrial gas-liquid interfaces. This paper presents the results of several studies, including lab and full scale studies that show the depression of gas transfer, usually quantified by the alpha factor, can be related to the Reynolds number and the level of wastewater treatment. Our results offer a tool for mass transfer prediction from flow regime and dynamic surface tension properties.

Aeration is an essential process in the majority of wastewater treatment processes and accounts for the largest fraction of plant energy costs (Reardon, 1995). The impact of contamination on aeration performance is usually quantified by the α factor (ratio of process water to clean water mass transfer coefficients, or KLapw/KLacw). Differences in α amongst aeration systems were noted in the 1930's (Kessener and Ribbius, 1935), but were generally forgotten until the energy crisis of the 1970’s increased the awareness for energy efficient technologies. Prior to the 1980s, many plants were designed with α of 0.8, which was considered as a “universal” α for all types of aeration systems. Lower flow regime gas-liquid interfaces (such as the ones produced by fine-pore diffusers) generally have lower α factors than higher flow regime interfaces (produced by coarse bubble diffusers or surface aerators) for similar conditions (Stenstrom and Gilbert, 1981). It has been shown that different aeration methods have different α, and for fine-pore diffusers the initial α decreases over time in operation due to fouling or scaling (Rosso and Stenstrom, 2005). Furthermore, for fine-bubble systems α is a function of process conditions such as the mean cell retention time (MCRT) or the airflow rate (Rosso et al., 2005).

There exists a debate over the reasons for different α factors for different aeration methods. Part of the difficulty is the reliance on different experimental methods to measure α factors, and the variations in the range of interfacial flow regimes. This research work summarizes previously observed phenomena, and explains the microscopic phenomena that cause different contamination effects at different flow regimes. The effects of contamination on gas-liquid interfaces due to surfactants were analyzed, compared, and explained for the cases of high and low interfacial velocities. These regimes correspond respectively to: 1. coarse-bubble and surface aerators: 2. fine-pore diffusers. The results of two laboratory studies using dynamic surface tension measurements and several other laboratory- and full-scale studies verify the conclusions. These results, along with our previously published work on the impact of MCRT and ageing or fouling on aeration efficiency, will be useful for aeration system design.

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