we use water for lots of different things in the house hold: flushing the toilet, washing up, having a bath
agricultural systems alison produce a lot of waste water including nutrient run off from fields and slurry from animal farms
sewage from domestic or ag sources have to be treated to get rid of any organic matter or harmful microbes before it can be put back in fresh waters otherwise it would make them polluted
swage treatment step 1
screening
removing any large bits of materials (twigs & plastic bags)
swage treatment step 2
sedimentation
heavier suspended solids sink to the bottom to produce sludge while the lighter effluent floats on the top