Biological Systems Engineering


Date of this Version



WASTE MANAGEMENT D-1, Home Waste Systems Issued June 1979, 15,000


Copyright 1979 U.S. Department of Agriculture


Once a septic tank and absorption field are installed, you can do several things to prolong their life, thus protecting an investment which may cost $2500 or more. Here are several tips you can follow.

Often overlooked or neglected is the fact that septic tanks should be inspected at least once annually. In every properly functioning septic tank, sludge accumulates in the bottom. This sludge is composed of solid materials and must be removed periodically. If the sludge is not removed, the accumulated solids will build up in the septic tank and will begin to wash out into the absorption field. Allowing solids from the septic tank to wash out into the absorption field will eventually clog it to the point where a new field will be needed.

Most authorities agree that for a typical three-bedroom home, a 1 ,000-gallon (3,800 I) septic tank will need to have the solids removed every three to five years. Smaller tanks must be pumped more often. Septic tank additives that "clean" the tank are available, but these are generally not recommended. Some additives may cause the solids to be flushed from the septic tank into the absorption field, causing clogging problems. Other compounds may produce a septic tank effluent which will destroy soil structure and cause premature failure of the soil absorption system.

To determine if your septic tank needs pumping, the thickness of the sludge can be measured as illustrated in Figure 1. To measure the depth of the sludge, wrap a long stick with a piece of rough, white toweling and tie it securely. Lower the stick through the inlet tee (to avoid the scum) to the bottom of the tank. Wait about 30 seconds and remove the stick slowly and carefully. Black particles will cling to the towel indicating the depth of the sludge. The sludge should be removed if its depth is equal to one third or more of the liquid depth.

Occasionally, a floating scum layer may develop in septic tanks. This scum layer can also cause clogging and should be checked annually. The scum layer thickness can be measured with a stick and hinged flap device (Figure 1). Push the stick through the scum until the hinged flap falls into the horizontal position. Raise the stick until you feel the bottom of the layer. Mark the stick to indicate the depth of the scum layer. Now use the same procedure to locate the lower end of the submerged inlet pipe. If the bottom side of the scum layer is within three inches (7.6 em) of the lower end of the submerged inlet, the septic tank should be pumped.

Most communities have contractors who pump septic tanks. It may cost $50 or more, but it is necessary for maintaining the life of the absorption field. The contractors pump the contents into a tank truck and dispose of it at an approved treatment site or by proper land application. Be sure the workman who cleans your tank mixes the liquid, sludge and scum before pumping so that all of the material can be removed, not just the liquid. It is not recommended to wash, scrub or disinfect the septic tank when pumping. Similarly, it is not necessary to leave solids in the septic to "start" it again. Normally, as the septic tank fills, the natural processes begin. Products to " seed" the system with desirable bacteria are available, but they are also not necessary.