The older-style graveled systems contain a layer of gravel in the drainfield. While some treatment of waste occurs in the septic tanks, most of the treatment occurs as wastewater discharged from the tank enters the drainfield and is filtered through the gravel and the soil below. Over time, bacteria and other organisms in the soil consume any organic material in the wastewater. These organisms multiply and form a layer called a bio-mat that sits on the soil layer. When the drain field is in balance, parasites and other organisms keep the bio-mat from becoming too thick to allow passage of wastewater to the soil.
If the drain field is not in balance there may be an overload of the drain field, which can occur when the water table rises above the drain pipe and stops the drain field from discharging water. At this point baths and toilets will start to back up.
Drainfield overload can occur when there is overuse of water in the house, such as guests in the house for long periods of time, if faucets or toilets are left running for prolonged periods, if an unusually heavy amount of laundry is done over a short period of time, or if the drain field soil is saturated by heavy rains. If the system is overwhelmed by overuse of water, the drain field may fail to function properly and may become damaged to the point of needing to be replaced.