What is sewage? In which way can this be harmful?

Explanation

Domestic Sewage. It is domestic waste that is passed down into sewer system. Domestic sewage has wastes from toilets, kitchens and washings. The important components are : -
1. Suspended Solids, (a) Grit made of sand, silt and some clay.
(b) Large sized fibrous matter that floats over the surface as scum.
2. Colloidal Particles. They include clay, faecal matter, fine fibres of paper and cloth.
3. Pathogenic contaminants. A number of pathogens, their spores, cysts or eggs, coliforms and Enterococci contaminate sewage.
4. Dissolved Material. They are inorganic nutrients (e.g, nitrate, phosphates, ammonia, sodium, calcium), toxic metal ions and organic compounds.
Effects of Sewage discharge on rivers. The following effects occur on river water if untreated sewage is allowed to flow into it.
1. Quality of Water. Water becomes unfit for bathing, domestic or industrial use as it becomes coloured, turbid, odorous with a lot of particulate matter suspended or floating on water.
2. B.O.D. (Biochemical Oxygen Demand). Decomposer organisms will become active and increase in number. They require lot of oxygen. Therefore, BOD, biological or biochemical oxygen demand increases. Most of the dissoved oxygen is consumed by the decomposers. Quantity of dissolved oxygen (=D.O.) falls. Fish and other clean water organisms are killed. With the degradation of organic matter, the BOD falls. This happens downstream. With this the fish and clean water organisms are found to occur here.