As the layer of sludge builds up higher and higher on the bottom of the septic tank, it must be removed occasionally, depending on how rapidly the buildup occurs. If the sludge isn’t removed, it will in time reach the height of the outlet line. When this happens, the sludge will pass into the absorption system, where it will clog the drain tile. The entire absorption system will then have to be dug up and cleaned out.
Another reason for cleaning the septic tank at periodic intervals is that the greater the volume of sludge in the tank, the smaller the volume of liquid. As a result, incoming sewage will spend less time within the tank, too little time for it to be completely decomposed. As a result, the effluent will enter the dispersal system only partially decomposed.
To prevent this excessive buildup of sludge, a septic tank should be inspected at least twice a year, during the spring and autumn, say. The depth of the sludge is checked by removing the cover from the top of the tank, lowering a rod into the tank, and noting how far it goes before it touches the layer of sludge. A more accurate method (since the sludge may be too loose for anyone to gauge easily what its height is) is to lower into the tank a length of pipe connected to the inlet of a pump. The pump is started up and the pipe is carefully lowered into the tank, to avoid stirring up the sludge. At some point, as the pipe is being continually lowered, the liquid being discharged by the pump will turn black. This indicates that the top of the sludge layer has been reached. The length of the pipe within the tank is then measured, and this distance is subtracted from the overall depth of the tank. The difference is the depth of the sludge.
If the sludge layer is more than 12 in. deep, it should be removed. How long it takes for this amount of sludge to collect will depend on the capacity of the tank, the size of the house, the living habits of its occupants, and whether they occupy the house continuously all year round. It may, therefore, take less than a year, or it may take several years for 12 in. of sludge to accumulate. To be on the safe side, many local regulatory authorities require that a septic tank be inspected annually.
The sludge must be disposed of. It has no fertilizing value, it smells too bad to be used for landfill or anything else, and it is also a health hazard. The only thing one can do is burn or bury it. The sludge can be run into an isolated trench that has been dug for this purpose. The liquid portion will slowly seep into the soil. The solid matter that remains can be covered over, or it can be allowed to dry; it is then burned. If the tank has been cleaned out by a firm that specializes in this work, they will cart the sludge away to some facility approved by the local health authorities, probably a nearby municipal sewage treatment plant.