When you flush the toilet, drain the sink after washing the dinner dishes or rinse the shampoo out of your hair, dirty water flows through pipes and out of your home. Those pipes connect to a sewer or a septic tank, depending on where you live.

Septic systems are also called on-site systems because the sewage and wastewater are treated right there at the home site. They are common in rural areas where houses are too far apart to justify the costs of sewers.

Wastewater gets flushed into a holding tank. Solid waste settles at the bottom and liquid rises to the top. Bacteria breaks down some of the solids into liquid and gas. Septic owners must have their tanks regularly pumped out and serviced.

The liquid flows out to an underground area called a drain field. Gravity pulls the water through sand, rock and soil underneath the drain field, which filters it. In a properly functioning septic system, water filters through enough substrate and soil that it is safe and clean by the time it reaches a water supply, such as a drinking well, river or lake.

A sewer system is a network of large underground pipes that collect waste and wastewater from many homes. Wastewater flows through larger and larger pipes until it arrives at a treatment facility. There, sewage is pumped into a large pool where the solid waste settles to the bottom. Bacteria begins to break down this waste.

The wastewater gets pumped into other pools where chemicals are added to it. Again, bacteria breaks down some of the waste. Eventually, this partially cleaned water gets sprayed over fields that act like giant drain fields. Crops are usually planted on the fields, to absorb some of the remaining waste, which works like a fertilizer.

Managers must monitor the amount of wastewater applied to the field and the amount of waste absorbed by the growing crop. The idea is that by the time it reaches another body of water — the aquifer or a nearby river, for example — pollutants should have been filtered out by plants, soils and rocks.

— Lily Raff