Disinfection step that kills remaining pathogens, often using chlorine, ozone, or UV light.

• A. Primary sewage treatment
• B. Bar screens
• C. Secondary sewage treatment
• D. Final chemical treatment

Answer: (D)

Disinfection is the final stage of wastewater treatment, designed to kill any pathogens that survived the earlier steps so the water is safe to discharge or reuse. After solids are removed in primary treatment and organic matter is reduced in secondary treatment, this last step uses methods like chlorine, ozone, or UV light to inactivate microorganisms. Chlorine provides a lasting disinfectant residual, ozone is a strong oxidant that rapidly inactivates microbes, and UV light damages microbial DNA to prevent replication. This step specifically targets pathogens, whereas primary treatment focuses on removing large solids and secondary treatment focuses on reducing organic load, so the other steps do not achieve the necessary microbial kill in the final effluent. Bar screens are just a preliminary barrier to remove large debris and do not disinfect.