Ontario Operator-in-Training (OIT) Practice Exam

In what stage of wastewater treatment is disinfection typically applied?

1. Before primary treatment

2. After secondary clarification

3. During the initial screening

4. After chemical precipitation

The correct answer is: After secondary clarification

Disinfection is commonly applied after secondary clarification in the wastewater treatment process. After wastewater undergoes primary treatment, which removes large solids, and secondary treatment, where biological processes reduce organic matter and nutrients, the water is clarified to separate remaining suspended solids. At this phase, disinfection is crucial as it targets pathogenic microorganisms that could still be present in the treated water. Applying disinfection at this stage ensures that the effluent meets health and environmental standards before it's released back into the environment or used for purposes such as irrigation or even potable reuse, depending on the treatment quality. The common disinfection methods include chlorination, ultraviolet (UV) light, and ozonation, which effectively kill or inactivate harmful pathogens, enhancing the overall safety of the treated wastewater. This is why secondary clarification serves as the appropriate context for applying disinfection—it's the last step before effluent release, solidifying the treatment's effectiveness.