Optimization of Chlorine Disinfection Efficiency

In order to show that savings in chlorine can be achieved by improved process design, parallel chlorination studies were conducted in an optimized chlorination pilot plant and each full-scale system at six different sewage treatment plants. Concurrent flow-through fish bioassays were also conducted at each location. At all test sites the optimized pilot plant used significantly lower chlorine dosages and residuals to achieve the same degree of disinfection, and its effluent was markedly less toxic than the full-scale effluents. The reasons for the good pilot plant results were variously good initial mixing, better chlorine control, plug flow contact, and better operation. Nitrified effluents required relatively high chlorine dosages for adequate disinfection. In most cases the disinfection efficiency could be expressed by the product of chlorine residual and contact time. The degree of bacterial kill obtained during initial mixing was a function of chlorine residual. Sulfur dioxide removed all chlorine-induced toxicity from the effluents.