During the treatment of sewage water, the chemical process is carried out in which step of water treatment?

Treatment of sewage water is carried out in three stages: primary treatment is a physical process in which filtration and sedimentation are carried out to remove floating debris and girt. Secondary treatment is a biological treatment in which aerobic heterotrophic microbes are used, which consumes the major part of organic matter present in the effluent. ​Tertiary treatment: Physiochemical process in which chlorine gas , zirconium, ozone gas, perchlorate salts, UV rays or reverse osmosis etc. are used to remove DDT, pesticides, pathogens and remove turbidity present in wastewater.