What likely causes higher fecal coliform numbers downstream of a wastewater treatment plant (WWTP)?

Higher fecal coliform numbers downstream of a wastewater treatment plant are typically associated with overflow of inflow and infiltration from the collection system. This situation occurs when excessive rainwater or groundwater enters the sewer system, which can overwhelm the capacity of the system, leading to combined sewer overflows or sanitary sewer overflows. During these events, untreated or partially treated sewage can be released directly into nearby water bodies, resulting in elevated levels of fecal coliform bacteria downstream.

In contrast, direct discharges from the plant are expected to be treated to meet regulatory standards, including limits on fecal coliform levels. Improper sludge handling generally affects the treatment process and can introduce contaminants in different ways, but it does not directly contribute to higher fecal coliform levels downstream in a straightforward manner. Increased rainfall contributes to the inflow and infiltration problem; however, the specific cause of the elevated fecal coliform is directly related to the system's capacity to handle excess water, thus making the overflow of inflow and infiltration the most relevant factor in this scenario.