ly. For most households, once every three years is adequate to prevent the accumulated sludge from being carried over into the drain field. Until you have an idea of the sludge accumula- tion rate in your tank, you should check the sludge level each year. This procedure is as follows: (a) Nail a 3-in. square block to the end of a pole and poke the block through the floating scum. Carefully move the pole up and down to feel the resistance as you move the block up against the bottom surface of the scum (Figure 4.4). Mark that place on the pole which is level with the top of the septic tank. Then feel around for the bottom of the outlet baffle or tee and mark that level on the pole. If the two marks are 3 in. or less apart, the tank needs to be pumped out. ONSITE WASTEWATER DISPOSAL Figure 4.5 Measuring the sludge depth. (b) Next, wrap an old towel around the bottom of the pole and fasten it with tape or string. Push the towel down to the bottom of the tank and twirl it around several times (Figure 4.5). Mark the pole at the level of the top of the septic tank. After a minute or so, pull it out and measure the distance between the top of the sludge layer (the top of the black material on the towel) and the mark for the bottom of the tank. Subtract from this distance the earlier distance you measured between the bottom of the outlet baffle or tee and the top of the tank. If the resulting distance is 12 in. or less, your tank needs to be pumped out. Septic tank pumpers are advertised in the yellow pages of the telephone directory. Most of them are willing to do this inspection for you-for - a fee. Be sure that the floating scum layer is removed, and that after your septic tank has been pumped out, a small amount of liquid is left in the tank to act as seed to start the regrowth of the necessary bacteria. It is unnecessary to clean the sides of the tank or to add yeast, powdered bacteria, or other additives to get your tank started up again. Never disinfect the tank. And, if the tank was installed in a high ground wate