In the septic tank, heavier substances settle on the bottom and remaining liquid gets pumped over a drainage field. Soil bacteria decompose biodegradable material. Found in most rural areas. In urban areas, the volume of sewage is simply too great for the area they occupy. Sewage gets sent to treatment plants.
Sewage is carried by pipes and culverts.
Some have combined systems because it is cheaper to run one line than two. Problem is that during heavy rains, huge deluges of sewage reaches the treatment plant that exceed capacity and must be diverted to rivers, lakes or oceans. Primary Treatment Mechanical process to screen sewage. Suspended solids in settling tank sludge removed. A lot of organic material remains that encourages algae growth, which chokes out sunlight and eventually oxygen from bodies of water.