Design and Performance of Chlorine Contact Tanks

Most existing chlorine contact tanks suffer from serious drawbacks of dead spaces, short circuiting, and solids accumulations that subsequently putrefy and exert undue chlorine demand. Hydraulic model studies of chlorine contact tanks indicate that air agitated, baffled contact units have better flow characteristics than just baffled units or air agitated contact tanks without baffles. Air agitation does not result in a loss of total chlorine residuals, and it improves the bactericidal efficiency of chlorination. Also air agitation eliminates the problems caused by solids accumulation in the contact units. Split chlorination does not appear to be an advantageous modification in chlorination practice. INTRODUCTION Continuous disinfection of waste water effluents likely to contain fecal coliform bacteria has become mandatory in the state of Illinois. The pertinent rule of the Illinois Pollution Control Board regarding effluent standards stipulates that "no effluent shall exceed 400 fecal coliforms per 100 ml after July 31, 1972 . . . ." This rule, in effect, requires year-round disinfection practices, even in winter months. In Illinois, as in most states, the principal method used for the disinfection of treated effluents is chlorination. Since chlorination practices in waste water treatment entail a significant amount of capital outlay and recurring expenditure, it is important that the process be effective and efficient. Any improvement in the efficiency over that obtainable with the current chlorination practice should result in better economy. The design of chlorine contact tanks on the basis of the concept of plug flow assumes that the fluid entering the contact chamber is evenly distributed over the entire cross section of the basin and moves in parallel stream lines with a constant and uniform velocity to the outlet. It is tacitly assumed that each particle of fluid entering the basin remains in it for a period called the 'theoretical detention time.' However, in actual practice, particles of fluid entering at the same time are found to have unequal times of passage, and a significant portion of the fluid leaves the tank with a residence time considerably less than the theoretical detention time. Consequently, to maintain the effectiveness of chlorine contact tanks for the disinfection of waste water effluents, the chlorine dosage must be increased resulting in significant increases in operational costs. In addition to being beset with the problems of dead spaces and short circuiting, contact tanks also suffer from the problems of solids accumulations that putrefy and exert undue chlorine demand. These accumulated solids must be removed periodically, which involves varying degrees of human effort and results in nuisance conditions and interrupted operations. These deficiencies in turn reduce the bacterial removal efficiencies of the contact units.