If a secondary clarifier has floatable debris in the effluent, what is the most likely cause?

When floatable debris is observed in the effluent of a secondary clarifier, it often suggests that the system is experiencing hydraulic overload. Hydraulic overload occurs when the flow of influent into the clarifier exceeds its designed capacity, which can hinder the effective separation of solids from the liquid. This excess flow can prevent solids from settling properly, leading to the carryover of floatable materials in the treated effluent. Thus, the presence of these materials indicates that the clarifier may not be able to adequately manage the incoming flow rates, resulting in poor performance.

In contrast, if the clarifier were undersized or had insufficient detention time, it could also contribute to issues with settling and solids removal. However, these factors are typically more about capacity and retention rather than directly causing floatation issues. A cleaner-than-expected influent, option C, would not logically lead to excess floatable matter in the effluent, as cleaner influent usually results in easier treatment and potentially better settling conditions. Therefore, the most direct and relevant cause of floatable debris in effluent is the hydraulic overload scenario.