Most septic systems depend on the gravity to transport the liquid from home to the leaching bottom through the tank. Septic tanks are usually located one to two meters below the ground surface to keep the sewage away from people and pets, but not much deeper, because a significant amount of water flows through evaporation or is inspired by the grass growing above it. Wastewater leaves the house and enters a septic tank, usually located 20 to 50 meters from the house, and begins the treatment process. If the septic tank is located above the house, a sanding pump is installed in the basement or cellar of the house to liquefy solids. According to the Environmental Protection Agency, the discharge of toxic substances into a well can lead to the death of organisms in the septic tank, which digests and processes household waste. Every two years, particulate matter in the pit must be pumped through the wastewater treatment system. In such situations, barrow systems are built where the projected fill material is placed in the natural soil and the leaching bottom extends to the top of the barrow. If a pump is needed between the tank and the field, it is located in an underground well accessible through a manhole in the grass. However, sometimes the slope of the field has to be higher than the house. If sewage seeps into the field, the soil absorbs the remaining bacteria and particles, so that when the water reaches deeper in the aquifer it is clean enough to drink. In the plant, the waste is treated and separated into sufficiently clean water to be discharged into the river and solids, called waste.