Field study of performance of rectangular chlorine contact tanks

Abstract

The purpose of this study was to make a field evaluation of the efficiency of three types of rectangular chlorine contact tanks. Efficiency of the tanks was evaluated in terms of the type of initial mixing of the chlorine and fluid, location and configuration of the inlet and outlet structures, the influence of baffles and the effect of sludge deposits. Two unbaffled tanks, two end-around baffled tanks and one over-and-under baffled tank were sampled. The sampling procedure consisted of measuring free and total chlorine residuals throughout each tank by the Amperometric Titration method. Each tank was divided into a grid system, and samples were taken at various depths across the width and length of the tank. List results showed that the baffled tanks provided less short circuiting, especially the over-and-under baffled units, as compared to the unbaffled tank. Premixing of the chlorine and fluid upstream from the tank produced a good distribution of chlorine at the influent of the tank. The need for sludge removal was shown by the high chlorine demand of sludge deposits in the baffled tanks. A second investigation was conducted to make a comparison between the Amperometric titration and Orthotolidine methods of measuring total chlorine residuals. Results showed that the Amperometric titration method measured much higher residuals. An Amperometric titration residual of 0.1 to 0.98 mg/l was required to produce any Orthotolidine reading.