What is the typical removal efficiency for BOD5 in primary treatment?

In primary treatment of wastewater, the typical removal efficiency for Biochemical Oxygen Demand over a 5-day period (BOD5) generally falls within the range of 30-35%. This phase of treatment primarily focuses on the physical processes, such as sedimentation and flotation, to remove solids and settleable materials from the wastewater.

While some biological activity may occur, the main purpose is not to significantly reduce BOD but rather to reduce the volume and concentration of suspended solids and some of the organic matter. The removal efficiency can vary depending on factors such as the type of treatment system, the characteristics of the influent wastewater, and operational conditions, but a typical removal rate of 30-35% for BOD5 is considered standard in primary treatment facilities.

Higher removal efficiencies, such as those seen in secondary treatment processes, are achieved through biological treatment methods designed specifically for the breakdown of dissolved organic matter, which typically yields much higher percentages of BOD reduction.