After screening is completed and grit has been removed, sewage still contains organic and inorganic matter along with other suspended solids. These solids are minute particles that can be removed from sewage by treatment in a sedimentation tank . When the speed of the flow of sewage through one of these tanks is reduced, the suspended solids will gradually sink to the bottom, where they form a mass of solids called raw primary sludge . Sludge is usually removed from tanks by pumping, after which it may be further treated for use as a fertilizer, or disposed of through incineration if necessary. To complete primary treatment, effluent from the sedimentation tank is usually disinfected with chlorine before being discharged into receiving waters.
Chlorine is fed into the water to kill pathogenic bacteria, and to reduce unpleasant odors.
Over the years, primary treatment alone has been unable to meet many communities' demands for higher water quality. To meet higher requirements, cities and industries often treat to a secondary treatment level, and in some cases, also use advanced treatment to remove nutrients and other contaminants. The secondary stage of treatment removes about 85 percent of the organic matter in sewage by making use of the bacteria in it. The two principal techniques used in secondary treatment are trickling filters and the activated sludge process . After effluent leaves the sedimentation tank in the primary stage of treatment, it flows or is pumped to a facility using one or the other of these processes.