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CRYPTOSPORIDIUM LOG INACTIVATION CALCULATION METHODS
Citation:
Rakness, K. L., I. Najm, M Elovitz*, D. Rexing, AND S. Via. CRYPTOSPORIDIUM LOG INACTIVATION CALCULATION METHODS. Presented at IOA PAG Regional Conference, Windsor, ON, CANADA, September 12 - 15, 2004.
Impact/Purpose:
To inform the public.
Description:
Appendix O of the Surface Water Treatment Rule (SWTR) Guidance Manual introduces the CeffT10 (i.e., reaction zone outlet C value and T10 time) method for calculating ozone CT value and Giardia and virus log inactivation. The LT2ESWTR Pre-proposal Draft Regulatory Language for Stakeholder Review mentions the geometric mean method for calculating CT value and Cryptosporidium log inactivation. Chapter 11 and Appendix B of the draft Long Term 2 Enhanced Surface Water Treatment Rule Toolbox Guidance Manual (USEPA 2003) describes the Extended CSTR method for calculating Cryptosporidium log inactivation that is applicable for compartmentalized ozone contactors, such as over-under baffled basins. As an additional alternative, the Extended Integrated CT10 method, is a variation of the Extended CSTR and CeffT10 methods, and may be suitable for non-compartmentalized ozone contactors, such as pipelines or long-channels.
The draft Long Term 2 Enhanced Surface Water Treatment Rule contains Cryptosporidium log inactivation CT tables that were developed using conservative, 90% confidence limits. As such, high ozone CT value is required to achieve a specific log inactivation objective. Obtaining high ozone CT value requires elevated operating dose, which increases disinfection by-product formation such as bromate ion. Calculating CT value accurately will minimize ozone dose and disinfection by-product formation, and at the same time maintain disinfection protection through implementation of conservative log inactivation CT tables.
The CeffT10 log inactivation calculation method is most conservative resulting in maximum ozone dose. The geometric mean method is more correct, with its accuracy improved by using multiple ozone residual readings that are collected at strategic locations. The Extended CSTR and Extended Integrated CT10 methods are further improvements in the accuracy most accurate and are also easy to implement by using only three residual readings per contactor. Each calculation method will be described along with examples of optimized operating results at full-scale facilities.