In which type of treatment is disinfection always the last step?

Disinfection is critical in wastewater treatment as it serves to eliminate pathogenic microorganisms before the treated effluent is released into the environment or returned to the water supply. In tertiary treatment, which is often considered the final stage of treatment processes, disinfection becomes essential after the wastewater has undergone previous stages, such as primary and secondary treatment.

During primary treatment, larger solids are removed through screening and settling, while secondary treatment typically focuses on biological processes to degrade organic matter. After these processes, the effluent still contains various contaminants, including pathogens. Tertiary treatment addresses these remaining contaminants and typically incorporates physical, chemical, or biological methods to further improve water quality. The final disinfection step helps ensure that any harmful microorganisms are killed or inactivated, making the effluent safe for discharge or reuse.

Advanced treatment can also refer to technologies used to further treat wastewater beyond the conventional methods, but in this context, it includes disinfection as a critical final step to ensure water safety. Thus, identifying that disinfection is the last step emphasizes its importance in tertiary treatment, making this the correct choice.