Effect of irradiation on microbial indicators during sewage treatment

A study was carried out to assess the changes in the population densities of certain microbial groups of hygienic significance throughout the successive steps of treatment in Virginia Key Wastewater Plant in Miami, Florida. Samples were taken from raw sewage, oxygenated tanks, settling tanks and chlorinated effluent. Total count, total coliform, Streptococcus faecalis, Aeromonas hydrophila and coliphage increased or remained unchanged after oxygenation but they sharply decreased in settling and chlorination tanks were more than 99% reduction in counts initially present was observed. A. hydrophila and coliphage were still detected in the final effluent. With respect to the last stages of treatment till the production of dewatered sludge, all tested groups of microorganisms attained their highest values in the effluent of concentration tanks. They decreased throughout the steps of treatment till the dewatered sludge to attain 106-105 cfu/g for total counts and total coliform as well as 104 cfu/g for A. hydrophila and S. faecalis and 103 pfu/g for coliphage. An experiment was conducted to study the efficiency of gamma radiation in reducing the microbial load in the chlorinated effluent and dewatered sludge. In the chlorinated effluent, total counts, S. faecalis and coliphage were eliminated with 2 kGy while 1 kGy was sufficient to eradicate total coliform and A. Hydrophila. Regarding the dewatered sludge, a dose of 6 kGy was quite sufficient to decrease total counts from 6x105 to few cells/g. Whereas, total coliform, S. Faecalis and A. hydrophila could not be detected after irradiation with 2,4 and 1 kGy respectively. Coliphage was eliminated after irradiation with 6 kGy. Another experiment was carried out to compare the effect of gamma radiation with that of electron beam on microbial groups in chlorinated effluent. At any given dose of radiation, gamma rays proved to be of more lethal than electron beam for all types of organisms