Septic Systems
If you are buying a home with a septic tank, we recommend having
it inspected by a professional septic contractor.

Our standard home inspection does not include this type of
specialized, intrusive inspection. To properly inspect the system,
the contractor will need to dig holes to access the underground
parts of the system. This will include inspecting the tank, as well
as the leach field.

It makes good sense to have the tank pumped at the time of this
inspection. A professional septic contractor can perform both the
inspection and pump the tank, killing two birds with one stone
assuring that you begin with an empty tank and a system that has
been inspected. Often, you can negotiate with the seller to have
them pay for the pumping.

The State of Florida requires that all septic inspections be
performed by a Licensed Septic Contractor.

FUNCTION OF THE SEPTIC TANK

While relatively simple in construction and operation, the septic
tank provides a number of important functions through a complex
interaction of physical and biological processes. The essential
functions of the septic tank are to: receive all wastewater from the
house separate solids from the wastewater flow cause reduction
and decomposition of accumulated solids provide storage for the
separated solids (sludge and scum) pass the clarified wastewater
(effluent) out to the drain field for final treatment and disposal.


Primary Treatment

As stated, the main function of the septic tank is to remove solids
from the wastewater and provide a clarified effluent for disposal to
the drain field. The septic tank provides a relatively quiescent
body of water where the wastewater is retained long enough to let
the solids separate by both settling and flotation. This process is
often called primary treatment and results in three products: scum,
sludge, and effluent.

Scum: Substances lighter than water (oil, grease, fats) float to the
top, where they form a scum layer. This scum layer floats on top of
the water surface in the tank. Aerobic bacteria work at digesting
floating solids.

Sludge: The "sinkable" solids (soil, grit, bones, unconsumed food
particles) settle to the bottom of the tank and form a sludge layer.
The sludge is denser than water and fluid in nature, so it forms a
flat layer along the tank bottom. Underwater anaerobic bacteria
consume organic materials in the sludge, giving off gases in the
process and then, as they die off, become part of the sludge.

Effluent: Effluent is the clarified wastewater left over after the
scum has floated to the top and the sludge has settled to the
bottom. It is the clarified liquid between scum and sludge. It flows
through the septic tank outlet into the drain field.


HOW LONG LIQUIDS MUST REMAIN IN TANK

Effective volume: The floating scum layer on top and the sludge
layer on the bottom take up a certain amount of the total volume in
the tank. The effective volume is the liquid volume in the clear
space between the scum and sludge layers. This is where the
active solids separation occurs as the wastewater sits in the tank.

Retention time: In order for adequate separation of solids to occur,
the wastewater needs to sit long enough in the quiescent
conditions of the tank. The time the water spends in the tank, on
its way from inlet to outlet, is known as the retention time. The
retention time is a function of the effective volume and the daily
household wastewater flow rate:

Retention Time (days) = Effective Volume (gallons)/Flow Rate
(gallons per day) A common design rule is for a tank to provide a
minimum retention time of at least 24 hours, during which one-half
to two-thirds of the tank volume is taken up by sludge and scum
storage. Note that this is a minimum retention time, under
conditions with a lot of accumulated solids in the tank. Under
ordinary conditions (i.e., with routine maintenance pumping) a tank
should be able to provide two to three days of retention time.

As sludge and scum accumulate and take up more volume in the
tank, the effective volume is gradually reduced, which results in a
reduced retention time. If this process continues unchecked-if the
accumulated solids are not cleaned out (pumped) often
enough-wastewater will not spend enough time in the tank for
adequate separation of solids, and solids may flow out of the tank
with the effluent into the drain field. This can result in clogged
pipes and gravel in the drain field, one of the most common
causes of septic system failure.


SOLIDS STORAGE

In order to avoid frequent removal of accumulated solids, the
septic tank is (hopefully) designed with ample volume so that
sludge and scum can be stored in the tank for an extended period
of time. A general design rule is that one-half to two-thirds of the
tank volume is reserved for sludge and scum accumulation. A
properly designed and used septic system should have the
capacity to store solids for about five years or more. However, the
rate of solids accumulation varies greatly from one household to
another, and actual storage time can only be determined by
routine septic tank inspections.


ANAEROBIC DECOMPOSITION

While fresh solids are continually added to the scum and sludge
layers, anaerobic bacteria (bacteria that live without oxygen)
consume the organic material in the solids. The by-products of this
decomposition are soluble compounds, which are carried away in
the liquid effluent, and various gases, which are vented out of the
tank via the inlet pipe that ties into the house plumbing air vent
system.
Anaerobic decomposition results in a slow reduction of the volume
of accumulated solids in the septic tank. This occurs primarily in
the sludge layer but also, to a lesser degree, in the scum layer.
The volume of the sludge layer is also reduced by compaction of
the older, underlying sludge. While a certain amount of volume
reduction occurs over time, sludge and scum layers gradually
build up in the tank and eventually must be pumped out.


FLOW INTO AND OUT OF THE TANK

The inlet and outlet ports of the tank are generally equipped with
devices such as baffles, concrete tees, or in more recent years,
sanitary tees (T-shaped pipes with one short and one long leg).

Inlets

The inlet device dissipates the energy of the incoming flow and
deflects it downwards. The vertical leg of the tee extends below
the liquid surface well into the clear space below the scum layer.
This prevents disturbance of the floating scum layer and reduces
disruptive turbulence caused by incoming flows. The inlet device
also is supposed to prevent short-circuiting of flows across the
water surface directly to the outlet.
The upper leg of the inlet should extend well above the liquid
surface in order to prevent floating scum from backing up into, and
possibly plugging, the main inlet pipe. The open top of the inlet tee
allows venting of gases out of the tank through the inlet pipe and
fresh air vents of the household plumbing.

Outlets

The outlet device is designed to retain the scum layer within the
tank. A sanitary tee can be used with the lower leg extending
below the scum layer. The elevation of the outlet port should be 2
to 3 inches below the elevation of the inlet port. This prevents
backwater and stranding of solids in the main inlet pipe during
momentary rises in the tank liquid level caused by surges of
incoming wastewater.

Gases are produced by the natural digestion of sludge at the
bottom of the tank, and particles of sludge can be carried upward
by these rising gases. Some tanks have a gas deflection baffle,
which prevents gas bubbles (to which solid particles often adhere)
from leaving the tank by deflecting them away from the outlet and
preventing them from entering the drain field.


THE EFFLUENT FILTER

In newer systems, there is often an effluent filter: one of the
significant improvements in septic tank design in decades. They
range from 4 to 18 inches in diameter. As we have described, the
most serious problem with septic systems is the migration of
solids, grease, or oil into the drain field, and the filter is effective in
preventing this. A filter restricts and limits passage of suspended
solids into the effluent. Solids in a filtered system's effluent
discharge are significantly less than those produced in a
non-screened system.


FLOW BUFFERING

The septic tank also provides a buffering of flows between the
house and the drain field. Large surges from the household, such
as toilet flushing or washing machine drainage, are dampened by
the septic tank so that the flows leaving the tank and entering the
drain field are at substantially lower flow rates and extend over a
longer period of time than the incoming surges.


It makes good sense to have the tank pumped at the time of this
inspection. A professional septic contractor can perform both the
inspection and pump the tank, killing two birds with one stone and
assuring that you begin with an empty tank and a system that has
been inspected. Often, you can negotiate with the seller to have
them pay for the pumping.