(1) Tank proportions. For tanks of a given capacity, shallow tanks function
as well as deep. However, it is recommended that the liquid depth range between 30
and 60 inches and that the minimum plan dimension be at least 2 feet.
(2) Storage above liquid level. Capacity is required above the liquid level to
provide for that portion of the scum that floats above the liquid. To allow for this scum
storage, the distance from the liquid level to the top of the tank should be approximately
20 percent of the liquid depth (21 percent of the diameter for horizontal cylindrical
(3) Inlet. The inlet invert should enter the tank at least 3 inches above the
liquid level to allow for momentary rises in level during discharges into the tank. A
vented inlet tee or baffle should be provided to divert the incoming wastewater
downward. It should penetrate at least 6 inches below the liquid level, but in no case
should the penetration be greater than that allowed for the outlet device.
(4) Outlet. The outlet device (or baffle) should extend to a distance below
the liquid level equal to 40 percent of the liquid depth (35 percent for horizontal
cylindrical tanks) to provide a balance between scum and sludge storage.
(1) Septic tanks should be cleaned before too much sludge or scum is
allowed to accumulate. If either the sludge or the scum approaches too closely to the
bottom of the outlet device, particles will be scoured into the disposal field and will clog
the system. Eventually, when this happens, liquid may break through to the ground
surface and wastewater may back up in the plumbing fixtures. When a disposal field is
clogged in this manner, it is not only necessary to clean the tank, but it also may be
necessary to construct a new disposal field. The tank capacities, as given before, will
ensure a reasonable period of good operation before cleaning becomes necessary.
There are wide differences in the rate that sludge and scum will accumulate from one
tank to the next. Tanks should be inspected at least once a year and cleaned when
necessary. The only way to determine definitely when a tank needs to be cleaned is to
measure the depth of the scum and sludge. The tank should be cleaned if:
(a) The bottom of the scum mat is within approximately 3 inches of the
bottom of the outlet device, or
Sludge comes within the limits specified in Table 1-6.
(2) Scum can be measured with a stick to which a weighted flap has been
hinged or with any device that can be used to feel out the bottom of the scum mat. The
stick is forced through the mat, the hinged flap falls into a horizontal position, and the
stick is raised until the resistance from the bottom of the scum is felt. With the same
tool, the distance to the bottom of the outlet device can be found.