Liquid capacity of tank,
Liquid depth in feet
2 1/2 3
* Distance from bottom of outlet device to top of sludge, in inches.
Table 1-6. Allowable sludge accumulation.
(3) A long stick wrapped with rough, white toweling and lowered to the
bottom of the tank will show the depth of the sludge and the liquid depth of the tank.
The stick should be lowered behind the outlet device to avoid scum particles. After
several minutes, remove the stick carefully. The sludge line can be distinguished by
sludge particles clinging to the toweling.
(4) Cleaning is usually accomplished by pumping the contents of the tank
into a tank truck. Septic tanks should be washed or disinfected after pumping. A small
residual of sludge should be left in the tank for seeding purposes. The material
removed may be buried in an approved sanitary wastewater system. Spillage and
leakage during pumping and transporting should be avoided. When a large septic tank
is being cleaned, care should be taken not to enter the tank until it has been thoroughly
ventilated and gases have been removed to prevent explosion hazards or asphyxiation
of the workers. Anyone entering the tank should have one end of a rope tied around his
waist with the other end held above ground by another person. An atmosphere-
supplying or self-contained breathing apparatus should be used.
1-16. DOSING TANKS
When the quantity of wastewater exceeds the amount that can be disposed of in
about 500 linear feet of tile, a dosing tank should be used in conjunction with the septic
tank in order to obtain proper distribution of wastewater throughout the disposal area
and give the absorption bed a chance to rest or dry between dosings. The dosing tank
should be equipped with an automatic siphon that discharges the tank once every 3 or 4
hours. The tank should have a capacity equal to about 60 to 75 percent of the interior
capacity of the tile to be dosed at one time. Where the total length of tile lateral
exceeds 1,000 feet, the dosing tank should have 2 siphons dosing alternately and each
serving a half of the field. Figure 1-16 illustrates a septic tank with a dosing tank and