Secondary sewage treatment in a sewage treatment plant is mainly a ______________ process.
The correct option is A
Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage. It includes physical, chemical, and biological processes to remove these contaminants and produce environmentally safe, treated wastewater (or treated effluent). Secondary treatment is a biological treatment process of wastewater (or sewage) to remove the dissolved and suspended organic compounds by using aerobic microbes to degrade the organic content in the water.