KWRRI Research Reports


The air surrounding three activated sludge tanks was sampled over a two year period for the emission of bacterial aerosols under a variety of climatic conditions and at varying distances upwind and downwind of the aerated tanks. All plants emitted species of enteric bacteria which are significant as index organisms and as frank pathogens. The emission pattern of these bacteria were influenced by distance from the plant and wind direction. Within the parameters of a plant, defined arbitrarily in this study by sampling sites less than 150 m upwind and less than 900 m downwind, distance from the source was the only reliable predictor of emissions, and no statistical significance was found in the differences between upwind and downwind samples at the same distances from the plant. Multiple regression analysis revealed no consistent influences of any environmental factor on emission rate, but relative humidity, wind speed, air temperature, and ozone levels showed some contribution on the bacterial count, while light intensity appeared to have little influence. The deposition and retention of enteric bacteria on foliage plants near aerated basins was used as an alternate sampling method, and it emphasized the potential hazard of these aerosols. This method confirmed the inability to predict the emission rate by climatic factors, but wind speed contributed directly to the counts, and there was a pronounced difference in the average counts of upwind and downwind samples.

The LD50 in mice was the same for aerosolized Klebsiella pneumoniae as for a strain of Klebsiella pneumoniae recovered from the sputum of a patient with pneumonia. Escherichia, Enterobacter and Klebsiella were recovered from the respiratory tract of mice forced to inhale air at a sewage treatment plant when the respiratory organs were assayed immediately after exposure, but not when the assay followed a prolonged period of observation, during which there was also an absence of clinical symptoms. This investigation also included a pilot study of the numbers of viable cells in the colony forming units on plates exposed in an Andersen Sampler, and CFU was found to be an unreliable index of viable cell counts. The study concludes that bacterial aerosols are a hazard for residents living near package plants, and recommends adopting alternate methods of sewage treatment that would remove the emissions of numerous package plants from the environment of densely populated regions.

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The work on which this report is based was supported in part by funds provided by the Office of Water Research and Technology, United States Department of the Interior, as authorized under the Water Resources Research Act of 1964.