Chlorination Of Wastewater
The disinfection process is the most important phase of wastewater treatment. Proper disinfection ensures removal of pathogens from wastewater before it is discharged to the environment. The most widely used disinfectant is chlorine. The use of chlorine in municipal wastewater treatment is commonly practiced in many countries.
Please note that there is a discharge limitation on chlorine residuals in the effluent as low as zero (no detectable amount).

Application Points
Up-Sewer Chlorination
Up-sewer chlorination is the addition of chlorine to the wastewater before the wastewater reaches the treatment plant. The function of up-sewer chlorination is to control odors and septicity, effectively decreasing the load imposed upon the wastewater treatment plant.

The addition of chlorine to wastewater at the entrance of the wet well, ahead of settling tanks and before the addition of other chemicals, is called pre-chlorination. Pre-chlorination is primarily applied to prevent odor problems associated with the breakout of hydrogen sulfide (H2S).
Pre-chlorination also helps to reduce plant load, (BOD) and aids in the settling of solids.

Pre-chlorination can also be used to treat specialized industrial wastes (e.g. metal bearing wastewater) before discharge to wastewater collection systems.

In-Plant Chlorination
Chlorine can be applied between primary settling units to control filter ponding, psychoda flies and to aid filter operation. Chlorine can also be added to return activated sludge, to control bulking and to activated sludge prior to the return of the sludge to primary settling units, to assist sedimentation.
The load on primary or secondary treatment processes, imposed by recycled supernatant liquor, can be reduced by chlorine addition to supernatant liquor from digesters. Chlorine will aid odor control from supernatant liquor, digested sludge, and sludge drying beds.

The primary purpose of post chlorination is disinfection. Chlorine is applied prior to or at the inlet of the chlorine contact chamber. Rapid initial mixing of chlorine solution and wastewater through an efficient diffusion device is essential for effective disinfection.

For proper operation and disinfection, chlorine dosages may need to be applied at a value which will exhibit a residual in excess of limits in effect at the discharge of the wastewater treatment plant. Reduction of chlorine dosage may not be possible while providing adequate treatment. At this point, dechlorination, using sulfur dioxide or another reducing agent will need to be evaluated.