Your septic system is designed to receive, treat, and dispose of waste from your plumbing system. Water is separated from the sludge and scum (solids and fats, oils, and grease) and then moved into a drain field. Here, it is filtered by bacteria and finally reentered into the groundwater.
As for the sludge and scum, they must be periodically manually pumped and should never enter the drain field with the water.