Sewage treatment generally involves three stages called primary, secondary and tertiary treatment. Primary treatment consists of temporarily holding the sewage, separation of oil, grease, floating matter and grit etc. Secondary treatment removes suspended and dissolved biological matter. It also includes separation of micro-organisms from the treated water prior to tertiary treatment.Tertiary treatment involves fine filtration and disinfection for final reuse or discharge.
Biological treatment comprises an aeration tank with external air supply. Surface aeration and diffused aeration are used to provide required oxygen. Suspended growth systems where biomass is mixed in sewage and recycled back to aeration. Fixed film or attached growth systems use media where biomass grows and sewage passes over its surface. Attached growth system requires lesser foot print than suspended growth system and able to cope with varying loads of influent.
Recycling and reuse of maximum sewage on site will help in reducing the fresh water use for various non potable uses like landscaping, WC flushing,external cleaning, vehicle washing,construction and for utilities like cooling towers.