You pump out your septic tank to get rid of the solid sludge that accumulates at the bottom of the tank and the grease that accumulates as scum floating near the top. Both sludge and grease can foul the leaching field. How often to pump out depends on your tank size, family size, and how much of your wastewater is solid waste.
For example: suppose your tank is 1,500 gallons, you’ve got four people in the family, and your solid waste is the average amount. Then you should pump your tank every 4 years. See Guidelines in Further Reading below. On the other hand, suppose there’s only one person using a 1,500-gallon tank. Then pumping it every 19 years is fine. The number of people dumping wastewater down the drain makes a big difference in how often the tank needs pumping.