Which of the following is typically used at a residential sewage treatment system with a surface discharge?

In a residential sewage treatment system with a surface discharge, both sodium hypochlorite and calcium hypochlorite are commonly used as disinfecting agents. These compounds serve effectively in reducing pathogens in wastewater prior to the discharge into the environment, ensuring that the effluent meets safety and health standards.

Sodium hypochlorite, a liquid form of chlorine, is frequently utilized in various disinfection processes due to its efficacy and ease of use. It is generally favored in residential applications because of its relatively low concentration and ability to be applied directly as a solution.

Calcium hypochlorite, on the other hand, is a solid form that can be used for disinfection and is known for its stability and effectiveness as well. While it requires a bit more handling and preparation to use since it is in powder form, it remains a reliable choice for wastewater treatment.

Both compounds achieve similar results in disinfecting sewage treatment effluents, thus either sodium hypochlorite or calcium hypochlorite could be employed in a system designed for surface discharge. This is why selecting option D, which includes either of the two substances as acceptable disinfectants, is justified.