Septic tanks are a component of septic systems, which are used for partial sewage treatment and disposal when it's impossible to connect to main sewage pipes.
Partially treated sludge accumulates in the tanks over time. Because of this, they need to be emptied or pumped periodically, which is one of player's jobs or activities, since they are in possession of a Gifu, a septic vacuum truck.
Currently there are five septic tanks that the player can empty and get paid for doing so.
The player can actually fall into the septic tank and drown, respawning at the house afterwards if permadeath is turned off. Be careful not to walk over or into the septic tank while working around it.