A correlation of continuous mixing and the disinfection of domestic sewage

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Virginia Polytechnic Institute

When domestic sewage is disinfected it is necessary to destroy the pathogenic organisms before discharge to the environment of mankind. Some of the more resistant pathogens may survive the disinfection process and escape to the environment. To minimize this possibility it is advisable to study the unit process know as disinfection and to determine if the disinfectant is adequately mixed to minimize the number of pathogens surviving.

Eliassen et al (2) studied disinfection of sewage subjected to initial mixing. He reported the disinfection in systems subjected to slow initial mixing and systems subjected to rapid initial mixing were approximately equal.

The method devised to study the mixing was the determination of the disinfection rate in continuously mixed systems disinfected with iodine. The mixing intensity was evaluated and the rate of disinfection determined by plotting the percent of microorganisms remaining against time m on semi-logarithmic paper (3). The slope of the data plotted was approximately -2.0 when the percent remaining was plotted against time 1/3 for all experiments where the mean temporal velocity gradient (1) was greater than 11 per second and the power to volume ratio (4) was greater than 0.29 foot pounds per second per 1000 gallons.

From these results and interpretation of Eliassen's studies (2), the initial mixing time required was estimated as one minute and the approximate quantity of mixing required was estimated as 1.3 tank turnovers corresponding to a product of mean temporal velocity gradient and time of 660 and a power to volume ratio for one minute of 21.6 foot pounds per 1000 gallons.

The design and operation of plant scale disinfection processes could be evaluated considering the minimum mixing defined by this thesis, such evaluation might indicate the need for additional mixing other than the turbulence induced by the baffling presently used in the disinfection processes.