The purpose of a Sewage Treatment Plant (STP) is to purify the sewage to a level that it can be safely disposed of in the environment without causing any harm to people or surroundings. Apart from that, the same treated wastewater can be reused and recycled too. There are scientific methods to check the quality of treated sewage, and the levels of BOD, COD, and other parameters in the treated water can give a fair indication of its quality.

There are different methods for testing the effectiveness of treated sewage. One way is by observing it and smelling it before filtration and disinfection. If the treated wastewater has an unpleasant smell, unusual color, or suspended matter, then it either needs further treatment or the current treatment is insufficient. When using treated sewage as toilet flushing water, it should not have any smell at all during flushing. As this is treated sewage, it may have a slight yellow tint, but this can be improved by chlorination to reduce the color.