Date of Award

6-2003

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Science

First Advisor

Dr. Munjed Maraqa

Second Advisor

Dr. Abdul Majid Khaja

Third Advisor

Ronald Lawrence Drost

Abstract

The presence of Legionella pneumophila in cooling tower waters is a serious health hazard. If the contaminated mist from the cooling tower is inhaled, Legionnaires' disease could be contracted. Of the 2.4 million cases of pneumonia that occur each year in the United States, an estimated 8,000 to 18,000 are actually cases of Legionnaires' disease. Several factors have been suggested in the literature that enhance the high bacterial count in cooling towers including: temperature, pH, water stagnation, and presence of algae and protozoa.

On few occasions, water samples from the cooling tower at Dubai Hospital tested positive for Legionella pneumophila despite the routine application of chlorine. With additional doses of chlorine or bromine, Legionella pneumophila was not detected for a certain period of time after the shock treatment, but tested positive afterwards. The inner concrete surfaces of the cooling tower at Dubai Hospital were left without a smooth finish. Therefore, it is possible that the interior rough surfaces of the cooling tower enhance the development of Legionella due to water stagnation within the exposed large pores. Rough surfaces may also serve as a good environment for algal growth. No previous experimental work has been conducted to investigate the effect of lining cooling towers interior walls on the growth of Legionella. As such was the main objective of this study. Both the cooling tower at Dubai Hospital and a built prototype were used to determine Legionella count during the cold and the hot season of 2001. Results obtained in this study revealed that the growth of Legionella in the cooling tower and the prototype is linear under both lined and unlined conditions during the study duration. It was further found that lining reduces the number of Legionella by about 37-68% but does not eliminate its presence in the cooling tower. In the prototype, the percent reduction in Legionella count due to lining appears to decrease with the time until it retains a constant value of approximately 30%. It was further found that, under similar conditions and sampling periods, the number of Legionella in the cooling tower were higher than those in the prototype despite the use of biocides in the cooling tower. This could be due to addition of make-up water to the cooling tower that may contain organic matter and some minerals which enhance the growth of Legionella. It was further demonstrated that there is an enhanced growth of Legionella during the hot season as compared to the cold season, but the increase was generally less than double with a 13.5°C change in temperature. A strong relationship (R2 = 0.86) was found between Legionella count determined using swab sampling and that using water sampling.

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