tails address only the setting of the tank. Construction The septic tank should be located far enough from buildings SO as not to cause a problem with the foundation should it leak (or more accurately, when it leaks), and far enough from wells and streams SO they won't cause a pollution problem. State or local codes will give you the specific setback requirements. If there is no code to follow, Table 4.3 shows reasonable setback distances. The bottom of the excavation which will hold the septic tank should be level, SO that the flow patterns assumed in the tank design will occur, and flat, to minimize stresses on the bottom slab. The bottom of the hole should be at a level which allows the drainpipe from the house to the septic tank inlet to drop at a grade of not less than 0.25 in. per ft. Try to get the proper depths for both the house drainpipe trench and the septic tank hole without having to backfill, WASTE PROCESS Table 4.3 Commonly Required Liquid Waste System Setback Distances Setback Distance (ft) Septic Tank and Protected Item Enclosed Systems Drain Fields Buildings Domestic wells and suction linesa Public wells and suction linesa Water lines under pressure Water courses Lakes aSuction lines exist between the water source and the water pump. for both the house drain and the septic tank should rest on undis- turbed earth. If backfilling is necessary, use sand to minimize poten- tial settling. Freezing of the liquid in a septic tank is not generally a problem because of its salt content, the routine input of warm water, the heat generated by bacterial action, and movement toward the outlet. After the tank is placed in the hole and the inlet and outlet are connected, repair any damage to the watertight coating. Then back- fill around the tank, compacting the earth around the sides, inlet, and outlet as you work. If the top of the tank is to be more than a couple of feet under- ground, you may wish to place risers over the manhole openings SO that the riser covers lie just below the ground surface (Figure 4