Which of the following is a type of secondary treatment system at municipal wastewater treatment facilities consisting of a bed of stones through which sewage is passed?

The trickling filter is a specific type of secondary treatment system used in municipal wastewater treatment facilities. This system consists of a bed of stones, or other media, that provides a surface for biofilm to grow. As sewage is passed over the stones, microorganisms in the biofilm consume organic matter and nutrients in the wastewater, effectively treating the sewage.

The design of the trickling filter allows for a large surface area for the bacteria to thrive, enhancing the biological treatment process. The wastewater trickles over the stones, allowing gravity to aid in the flow and ensuring that the biofilm can efficiently break down contaminants as it comes into contact with the effluent. This method helps to reduce the biochemical oxygen demand (BOD) and other pollutants in the wastewater before it is further treated or released into the environment.

Other options, while related to the treatment of wastewater, do not specifically match the description of passing sewage through a bed of stones. For instance, biological contactors and activated sludge systems utilize different processes for biological treatment and do not rely on a bed of stones as the primary mechanism for treating sewage. Suspended-growth systems focus on microorganisms that are suspended in the water rather than attached to a fixed surface like in trickling filters. Thus, the defining characteristics