A septic system usually connected to the drainage pipes outside of a building consists of a holding tank and leaching drain field to dispose of domestic or commercial sewage or other liquid waste...
Everything from toilet or kitchen sink to laundry washing machine discharge into sewer line than exit the house , from there go into whats called the holding tank there everything rest and settle,...
It entirely depends on the system. There are expensive "mound systems" - or there are cheaper "gravel systems" - which consist of a buried septic tank to hold your household wastewater (everything...