Abstract
Fine bubble diffusers are utilised within the activated sludge process to fulfil two requirements; they supply dissolved oxygen to satisfy the respiratory demands of the microbial population and maintain the mixed liquor suspended solids in suspension. The rate at which oxygen is transferred into solution by an aeration device, and the amount of energy used during the process will effect the efficiency and hence the cost of treatment. There often exists a difference in wastewater oxygenation capacity compared to that of potable water under the same conditions. This reduction is due to the presence of mechanisms that inhibit the oxygen transfer process. This inhibition of oxygen transfer within wastewater is characterised by the alpha factor. There are a number of variables, be they physical, chemical or biological, that are believed to contribute towards the alpha factor. A comprehensive literature review of these factors is provided. Three distinct areas that influence the performance of fine bubble diffusers in a wastewater environment are examined. A photographic approach was used to investigate the influence of surfactants upon the bubble formation process with respect to fine bubble diffusers. Anionic surfactants were found to have a significant impact upon at the point of bubble formation, reducing the major and minor mean bubble diameters and producing a more uniform bubble size. In the presence of anionic surfactants, the generic type of diffuser material was found to determine the level of impact of the surfactant upon the bubble formation process with ceramic diffusers being the most adversely affected. The influence of the physical variables of depth of immersion, airflow rate, and generic type of diffuser material upon the oxygen transfer performance of fine bubble diffusers were investigated in a 6m deep aeration test rig. Comparisons of results in clean water and clean water with anionic surfactants were undertaken in order to simulate the effects of the alpha factor. In the presence of anionic surfactants, ceramic diffusers were found to be the most adversely affected in terms of oxygen transfer performance when compared to that of plastic and membrane diffusers. The alpha factor was found to be insensitive to increases in airflow rate per diffuser for the range 0.5 - 4m3/hr per diffuser. With respect to the influence of the depth of diffuser submergence upon the alpha factor, it was found that once the rising bubble plume had become fully developed at a depth of 1.75m, the alpha factor was found to be constant up to the maximum depth investigated of 5.75m. The causes and consequences of diffuser fouling are discussed. Investigations into the rate of fouling and the impact upon fine bubble diffuser performance are presented. Dynamic Wet Pressure (DWP) and Standard Oxygen Transfer Efficiency (SOTE) data are presented from several full-scale operational plants and two experimental rigs. The fouling of fine bubble diffusers was found to increase the DWP whilst decrease the SOTE. The degree of fouling was found to be related to the position of the diffuser within the treatment process where the greater the treatment received, the less the degree of fouling. The major inorganic foulant was identified as Calcium. The generic type of diffuser material was found to significantly affect the changes in the DWP during diffuser operation, with plastic diffusers being most adversely affected. The implication for practice of the research findings are discussed and it is recommended that the operational cost implications of the choice of specific diffusers be considered along with the capital costs when making future procurement decisions.