The key difference between primary and secondary sewage treatment is the degree of treatment and the type of processes involved.
Primary sewage treatment involves the physical removal of large solids and organic matter from wastewater through processes such as screening, sedimentation, and flotation. The goal of primary treatment is to reduce the overall volume of wastewater and remove large, easily settleable particles such as grit, rocks, sand, and organic matter.
Secondary sewage treatment, on the other hand, involves biological processes that break down and remove dissolved and suspended organic matter, nutrients, and pathogens in wastewater. This is typically achieved through processes such as activated sludge treatment, trickling filters, or rotating biological contactors. Secondary treatment can significantly reduce the amount of organic material and pathogens in wastewater, making it suitable for discharge into natural waterways or for reuse.
In summary, primary treatment is a basic form of sewage treatment that mainly involves physical processes, while secondary treatment is a more advanced form of treatment that involves biological processes to further purify the wastewater.