What percentage of settleable solids should primary treatment ideally remove from raw wastewater?

The ideal percentage of settleable solids removed during the primary treatment of raw wastewater is typically in the range of 90-95%. Primary treatment focuses on the physical processes of sedimentation and flotation to separate solids from the liquid wastewater. This is achieved through the primary clarifier, where heavier solids settle to the bottom, forming a sludge layer, while lighter materials, such as grease and oils, float to the surface.

Achieving a removal rate of 90-95% is critical because it significantly reduces the load of solids and organic matter entering subsequent treatment processes, leading to more efficient biological treatment and helping to meet regulatory discharge standards. This high level of removal also helps in reducing odors and improving the overall quality of effluent.

While the removal of settleable solids can exceed 90-95% in some advanced treatment systems, achieving complete removal (100%) is not practical or feasible, as certain solids remain suspended or dissolved in the liquid phase. Therefore, the target percentage reflects the ideal operational efficiency expected in traditional primary treatment processes.