Sewage discharged by the residents is treated to the required standards so as to make it fit for disposal into a water body or for irrigation purposes to the required standards. Sewage Treatment Plants (STP) are set up for this purpose.
Sewage from the habitated areas generally flows by gravity up to the STPs through sewers laid in the area. These are internal sewers and also trunk sewers, for carrying discharge from larger areas. Minimum size of internal sewers is 250mm dia to allow for self cleaning velocity. Bigger sizes up to 2500mm dia are provided for carrying sewage from catchment areas up to STPs. These pipes are generally of R.C.C., collar jointed or socket and spigot jointed with flexible joints. In between, for day to day maintenance of the sewerage system, manholes are provided.
On account of topography and ground conditions of soil, when sewers cannot be laid very deep, intermediate pumping stations are provided to pump sewage up to STPs. Pumping mains known as rising mains can be of C.I., D.I. MS or PSC pipes depending upon hydraulic considerations.
Sewage Treatment Plants are designed to provide primary treatment, secondary treatment and tertiary treatment as per the requirements of the utilization of treated effluent. These are of conventional type, biological plants with Aeration system and also with extended Aeration system. Economic considerations as well as availability of land has also to be kept into view, as the same is vital.