What is the purpose of adding chlorine during the pre-chlorination phase?

The purpose of adding chlorine during the pre-chlorination phase is primarily to aid in odor control and the reduction of biochemical oxygen demand (BOD). When chlorine is introduced at this stage, it helps in the disinfection of the water and significantly reduces odors that arise from organic matter. This is particularly important as it prepares the water for subsequent treatment processes. Additionally, chlorine serves to oxidize organic compounds and pathogens present in the wastewater, ultimately enhancing the overall effectiveness of the treatment process by lowering the BOD levels before biological treatment stages.

The other options do not accurately reflect the primary goals of the pre-chlorination process. Promoting algae growth is counterproductive in wastewater treatment, as it can lead to further complications and imbalances. The removal of toxic substances is a more complex process that involves specific treatment methods and is not the main focus during pre-chlorination. Enhancing photosynthesis is irrelevant in this context, as the treatment of wastewater typically focuses on reducing rather than promoting biological processes associated with algae and other photosynthetic organisms.