Final answer:
A foul odor from a trickling filter suggests a problem with aeration, leading to buildup of gases such as hydrogen sulfide, which emits a rotten egg smell.
Step-by-step explanation:
A foul odor coming from a trickling filter in a wastewater treatment system suggests that there may be an issue with the aeration process. Typically in the water treatment cycle, after wastewater goes through a grit chamber to remove larger particles like sand or dirt, it is then aerated. Aeration involves bubbling air through the water to prevent smaller particles from settling and to release dissolved gases such as hydrogen sulfide, which has a rotten egg smell. A sharp smell coming from the water, like that around swimming pools, is often due to chlorine used in disinfection. If proper aeration is not maintained, gases like hydrogen sulfide can build up, leading to foul odors from the trickling filter.