As your system is used, solid materials settle to the bottom of the tank, forming a sludge layer. Grease and lightweight materials float to the surface of the septic tank as scum. You can avoid costly repairs by having your septic tank inspected on a regular basis. When the solids accumulation is greater than 40 percent, have your septic tank pumped. Normally, properly designed tanks have enough space for up to three to five years' safe accumulation of sludge, depending on the size of tank and number of users per household.