According to the U.S. EPA, what is one of the most important factors for establishing effluent limitations for secondary treatment?

The significance of the flow of the receiving stream in establishing effluent limitations for secondary treatment stems from its direct effect on the dilution and dispersion of the treated wastewater as it enters a natural water body. The flow rate of the receiving stream affects the assimilative capacity of the water body, which is the ability of the stream to receive wastewater without harming the aquatic ecosystem.

When setting effluent limitations, regulators consider the flow of the receiving stream to ensure that the concentration of pollutants from the treated wastewater does not exceed levels that could adversely affect water quality and aquatic life. Higher flow rates can result in greater dilution, potentially allowing for higher permissible pollutant concentrations compared to a low-flow scenario where the pollutants could concentrate and lead to environmental harm.

In contrast, while factors like temperature, concentration of suspended solids, and type of treatment technology used are relevant to the efficacy and efficiency of treatment processes, they do not account for the receiving environment's capacity to mitigate discharges. Therefore, the flow of the receiving stream is a fundamental consideration in determining appropriate and effective effluent limitations.