Why is dechlorination sometimes needed after chlorine disinfection?

In the context of wastewater treatment, disinfection is the final step performed before treated effluent is discharged into surface water bodies to destroy remaining pathogens and protect public health.1

While chlorine is commonly used for this purpose, it is often necessary to perform dechlorination after the disinfection process. This is because residual chlorine remaining in the treated effluent can have adverse effects on aquatic life in the receiving water bodies. To mitigate these environmental impacts, an additional chemical is added to the effluent to neutralize the chlorine before it is released into the environment.1

As an alternative to chemical disinfection and the subsequent need for dechlorination, ultraviolet radiation is increasingly used because it can effectively disinfect wastewater without leaving any chemical residuals.1