If BOD removal rates in a primary clarifier drop to 15%, what is the most likely cause?

When the BOD (Biochemical Oxygen Demand) removal rates in a primary clarifier drop to 15%, the most likely cause is an issue related to hydraulic loading, specifically too much flow leading to a short detention time.

In the primary clarifier, wastewater is meant to settle solids and remove some organic matter primarily through sedimentation. Each clarifier has an optimal flow rate that allows for sufficient time for solids to settle. If the flow into the clarifier exceeds its designed capacity, this can lead to shorter detention times. When detention time is reduced, there is not enough time for the biological processes to effectively remove BOD, resulting in poor treatment performance.

Insufficient oxygen levels is primarily a concern for biological treatment processes, such as in activated sludge systems, rather than in primary clarifiers. Excessive algae growth could potentially interfere with settling; however, this is less directly related to a sudden drop in BOD removal rates in this context. Likewise, inadequate polymer dosage typically affects the flocculation and dewatering processes rather than the primary removal capabilities of a clarifier. Thus, an overload of flow causing short detention time is the most fitting explanation for the drastic reduction in BOD removal efficiency.