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INVESTIGATIONS INTO BIOFOULING PHENOMENA IN FINE PORE AERATION DEVICES
Citation:
Jansen, W., J. W. Costerton, AND H. Melcer. INVESTIGATIONS INTO BIOFOULING PHENOMENA IN FINE PORE AERATION DEVICES. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-94/107 (NTIS 94-200953), 1994.
Impact/Purpose:
Information.
Description:
Microbiologically-based procedures were used to describe biofouling phenomena on fine pore aeration devices and to determine whether biofilm characteristics could be related to diffuser process performance parameters. Fine pore diffusers were obtained from five municipal wastewater treatment plants in Wisconsin, one in Michigan, and one in California. Biofilm thickness and carbohydrate content were measured as were the film's ability to retain air bubbles. Scanning electron microscopy (SEM) procedures were used to examine biofilm structure. Diffusers were also tooted for standard oxygen transfer efficiency (SOTE), bubble release vacuum (BRV), and dynamic wet pressure (DWP). Subsequently, one of the plants in Wisconsin was revisited where the three most useful of the biofilm characterization techniques were deployed to determine any relationship between biofilm thickness and content and operational parameters such as SOTE. The effect of four different cleaning procedures on diffuser characteristics was also measured. Parallel laboratory-scale investigation simulated microbially induced fouling of small-scale diffusers and the effect of bleach and acid cleaning. Microbial fouling of the diffusers occurred irregularly at all seven sites. Biofilm thickness and its distribution over a diffuser appeared to vary randomly. Highly structured biofilm were observed by SEN and are thought to interfere with the passage of air bubble, such that the size of the bubble released from a fouled diffuser surface is probably not the same as that from a cleaned and unfouled diffuser surface. Two types of biofoulant were observed. One appeared to reduce SOTE with minimal impact on diffuser headloss as measured by DWP. The second appeared to increase DWP with minimal impact on SOTE. In general, measurements of biofilm thickness and carbohydrate content do not appear to be directly related to measurements of SOTE, DWP, and BRV. However, the microbiological and process characteristics are complementary and support the trends and observations made, particularly when supported by foulant properties such as mass per unit area, volatility, and acid soluble content. Taken collectively, the data contribute to an improved understanding of the plant diffuser operation. After cleaning in the laboratory, the diffuser SOTEs were restored to values ranging from 16.3 to 20.9% at 1 cfm/diffuser. Inadequate data were collected to demonstrate statistically significant differences, but the data show little difference in SOTE improvement between the cleaning processes used. Values of BRV, coefficient of variability, DWP, and BRV/DWP were restored to clean water equivalent values. The laboratory investigations showed that the progressive development of a bacterial biofilm on the surface of a fine bubble diffuser can increase the size of bubbles released from that fouled surface. % bleach was found to be more effective in removing the biofilm than 14% HCL on both artificially induced and naturally occurring biofilms, and a combination of bleach followed by acid returned the stone to its original condition. Bleach cleaning from the air side was effective in removing the diffuser biofilm in the laboratory-scale model system. Project summary may be ordered as EPA/600/S-94/001).