Disinfection by chlorination: is it a viable technology to reduce the quantity of bacteria in sewage?
The chlorination of sewage by the addition of chlorine or chlorine compounds is required to achieve the following purposes:-
(a) To disinfect sewage, especially where the effluent is to be discharged into a body of water to be used for bathing, recreation or water supply.
(b) To control the odors due to hydrogen sulfide (H2S) by either preventing its formation or reducing or neutralizing the amount after it has been produced.
(C) To reduce the BOD, 10 to 35 per cent by oxidizing the organic matter and killing the microorganisms necessary for decomposition.
(d) To kill the filter (psychoda) flies .
(e) To reduce ponding in the case of trickling filters.
(f) To form floc by coagulation in combination with other chemicals.
The action of the various forms of chlorine in disinfecting sewage and the methods of application of chlorine is mostly similar to those used for disinfecting water. However, the amount of chlorine required to reduce bacteria in sewage is much larger because, a great amount of organic matter in sewage tends to neutralize the chlorine.
Applied Chlorine Dose
Type of effluent – Settled sewage (fresh) = Dose in mg/l – 10;
Type of effluent – Settled sewage (septic) = Dose in mg/l – 20;
Type of effluent – Imhoff-tank effluent = Dose in mg/l – 10;
Type of effluent – Trickling filter effluent = Dose in mg/l – 3 to 7;
Type of effluent – Intermediate sand filter effluent = Dose in mg/l – 2;
Type of effluent – Activated sludge effluent = Dose in mg/l – 5;
Recommended doses for disinfection by chlorination are given in the table above. Chlorine residual is 0.15 mg/l with a contact period of 15 – 30 minutes. It is, however, more usual to regulate the dosages by reduction in coliform organisms, instead of obtaining the chlorine residuals. Thus, a 99.9 per cent reduction in coliforms may be considered as having eliminated the less resistant bacteria. But, it should be understood that chlorination is not effective in killing all types of bacteria in sewage, as for instance, certain spores and bacteria protected in the organic matter are not penetrated by chlorine and thus escape the disinfecting effect during chlorination of sewage.