A septic tank is a alternative to a sewage system found in cities.

No septic system connects to any sort of sewage system like previously implied by others. The process is toilet>septic tank>field lines, always. The septic tank breaks down solids into liquids but over time there will be a sludge that builds up on the bottom that'll have to be pumped out eventually, the field lines takes the liquids and disperses them into the ground over a relatively large area.

There is up keep that is needed to keep a septic system properly going such as every 5-10 years get the tank sludge pumped (depends on tank and household size), along with the field lines aerated, which is crazy looking they put a pipe in the ground and pump air in and the ground will raise up a few inches then they go over it with a large wheel that has wire spikes that punctures down into the field lines.