Effects of initial microbial density on disinfection efficiency and explanatory mechanisms

Abstract

Numerous water disinfection studies have reported deviations from the Chick-Watson Law, which was used to develop the CT tables provided by the USEPA’s SWTR. Some of the modifications of the Chick-Watson Law incorporate explicit dependence on initial microbial density. In this study a series of inactivation experiments were conducted with a gram-negative (E. coli) and a gram-positive bacterium (B. subtilis) and a protozoan (G. muris) under various growth stages to investigate cell density effects on inactivation. Cell density dependent inactivation was observed only during inactivation of continuous and batch cultures of E. coli at stationary growth phase and during inactivation of G. muris cysts. There was a statistically significant decrease in disinfection efficiency as the initial microbial density decreased. Inactivation of E. coli at exponential growth phase, vegetative cells of B. subtilis at exponential growth phase, and B. subtilis spores were independent of cell density. Statistically significant effects of growth phase and culture growth technique on inactivation efficiency were observed in E. coli experiments. Exponentially growing cells of E. coli were more sensitive to monochloramine than stationary phase cells. Batch cultures of E. coli were more resistant to monochloramine than continuous cultures of E. coli. Similarly, spores of B. subtilis were more resistant than the vegetative cells at exponential growth phase. The inactivation data of B. subtilis spores and G. muris cysts obtained in this study showed that B. subtilis spores can be used as a conservative surrogate to verify the removal efficiency of G. muris as they would overestimate CT requirement (over an order of magnitude) during the ozonation process. Comparison of survival data of B. subtilis spores from this study with survival data of C. parvum in the literature showed that B. subtilis spores were less resistant at higher ozone CT. However, the relation between survivals of C. parvum oocysts and B. subtilis spores can be expressed precisely using multiple linear regression. Using inactivation data of B. subtilis spores, an approximately 2-log reduction of C. parvum by ozonation was verified. B. subtilis spores could be used as a simple and inexpensive surrogate for on-site disinfection process performance.%%%%Ph.D., Environmental Engineering – Drexel University, 2003