The following picture shows how a typical septic systems works. Wastewater leaves the house through the plumbing and enters the septic tank. The septic tank settles the solid materials to the bottom and the oils and greases float to the top. The partly treated wastewater in the middle of the tank exits the tank through piping and enters the drainfield. As the wastewater spreads through the drainfield, it is absorbed into the soil where microbes and chemical reactions treat the wastewater. Studies have shown that at least 84 percent of the wastewater winds up back in our groundwater after treatment.