After the septic tank, effluent water is discharged into the drainfield for further treatment by bacteria in the soil and filtering from the soil itself. The water moves along into the drainfield every time more sewage enters the tank. The effluent water is moved by gravity or by a pump depending on the design. If the drainfield receives too much water it will flood, causing sewage to pool on the ground surface or backup into the house plumbing fixtures. Overloading also prevents proper treatment of all the wastewater.