A standard septic system contains two major components: a septic
tank and an absorption field (either a drainfield or a seepage pit).
Most systems in the Virgin Islands use a seepage pit. These parts of
a septic system are all connected by pipes called conveyance lines
(see Figure 1). The septic tank is usually made of concrete,
fiberglass, or plastic so that it is resistant to corrosion and decay.
A septic system is a wastewater treatment system that uses the soil
to treat the small amounts of wastewater generated by individual
homes. A septic system is typically used in suburban and rural
areas where public sewage treatment plants are impractical. There
are many different types of septic systems (or on-site sewage
disposal systems, OSDS) in use today. While all septic systems are
designed according to each site's conditions, most systems are based
on the same principles.
Your septic system treats your household wastewater by temporarily
holding it in the septic tank where heavy solids and lighter scum (oils and
grease) are allowed to separate from the wastewater. The solids collect on
the bottom of the tank as sludge, and the scum floats to the top of the
wastewater (see Figure 2). This separation process is known as primary
treatment. Septic tanks are typically designed to hold 1,000 gallons of
liquid, but the size of the tank depends on the number of bedrooms in the
home. The sludge stored in the septic tank is decomposed by bacteria and
later removed, along with the lighter scum, by a professional septic tank
pumper.
After the partially-treated wastewater leaves the tank, it flows into either a
seepage pit or a distribution box, which separates the flow evenly into a
network of drainfield trenches. A seepage pit is simply a deep pit
excavated downhill from the septic tank, from which the wastewater
spreads or seeps into the surrounding soil. In a drainfield, holes at the
bottom of each line allow the wastewater to drain into gravel trenches for
temporary storage. The effluent then slowly seeps into the soil.
Many of the harmful bacteria present in the effluent
are filtered out. Some nutrients are also retained in the soil, and
dissolved and solid organic matter are broken down
biologically. This treatment of wastewater by the soil is called secondary treatment.
A properly designed septic system will have a septic tank with enough space to accumulate sludge for up to three years.
The septic tank should be pumped when the sludge fills the bottom-third of the tank. When the level of sludge fills too
much space in the holding tank, the wastewater has less time to settle properly and solid particles will begin to flow into
the absorption field. This will cause the absorption field to clog up and ultimately fail, so that a new drainage field will
need to be installed.
Septic systems must be maintained regularly to stay working properly.
The accumulated solids in the bottom of the septic tank should be pumped out every two to five years,
or when the sludge fills the bottom third of the tank. How
often you need to pump your septic tank depends on three factors. First is the size
or capacity of your tank. If more
people are living in the home than when the tank was installed, the capacity could be too small. You would then have
to pump your system more frequently (see Table I).
The number of people in the home also relates to the second factor, the flow of wastewater. The more people in the
household, the more water will flow through the system. The third factor that determines how often you have to pump
your septic tank is the volume of solids in the wastewater. If you have a garbage disposal you will have to pump out
your system more frequently. A garbage disposal can increase the amount of solids in your septic tank by up to 50%.
Keep a record of pumping, inspections and other maintenance of your septic system, so you'll know when to pump it
next.
Other ways to care for your septic system include:
• Conserve water. Repair dripping faucets and leaking toilets, run washing machines and dishwashers only when full,
avoid long showers, and use water-saving features in faucets, shower heads and toilets.
• Divert water from cistern overflows, driveways, and hillsides away from the septic system. Flooding of the
absorption field with excessive water will prevent the soil from naturally filtering the wastewater.
• Use non-caustic products for unclogging drains, such as boiling water, baking soda and/or vinegar, or a drain snake to
free up clogs.
• Use mild detergents or baking soda, borax, vinegar or rubbing alcohol rather than the stronger and potentially
system-damaging chlorine and commercial household cleaners.
• Do not park or drive over any part of your septic system. Driving over the septic system can break the concrete walls
of the tank or compact the soil in the drainage field. The area over the septic system should be left undisturbed with
only grass or very shallow-rooted plants, like bananas, as cover. Roots from nearby trees or shrubs can clog and damage
drain lines, and break cement walls.
• Do not cover the absorption field with a hard surface such as concrete or asphalt. Grass or shallow-rooted shrubs are
the best cover for the absorption field. The grass will not only prevent erosion, but will help remove excess water.
• Do not use commercial septic tank additives. Studies have shown that these products usually do not help, and some
may harm your system in the long run.
• Never use your septic system as a trash can! Do not dump non-degradable
items down your toilet or drains. Also, don't poison your septic system and
ground and surface water by pouring harmful chemicals down the drain. They can
kill the beneficial bacterial that treat your wastewater. Keep these materials out of
your septic system.
Small amounts of paints, varnishes, thinners, waste oil, photographic solutions,
pesticides, and other organic chemicals can destroy the biological digestion that
takes place within your septic system. Grease and fats can solidify in your
plumbing and septic system, contributing to blockages. Plastics, cat box litter,
cigarette filters, sanitary napkins, paper towels and facial tissues also should not be
disposed of in your septic system. These items quickly fill your holding tank with
solids, decrease its efficiency, and will require you to pump out the septic tank
more frequently.
Tank Size (gal)
Household Size (Number of People)
1
2
3
4
5
6
500
5.8
2.6
1.3
1.0
0.7
0.4
750
9.1
4.2
2.6
1.8
1.3
1.0
900
11.0
5.2
3.3
2.3
1.7
1.3
1000
12.4
5.9
3.7
2.6
2.0
1.3
1250
15.6
7.5
4.8
3.4
2.6
2.0
1500
18.9
9.1
5.9
4.2
3.3
2.6
1750
22.1
10.7
6.9
5.0
3.9
3.1
2000
25.4
12.4
8.0
5.9
4.5
3.7
2250
28.6
14.0
9.1
6.7
5.2
4.2
2500
31.9
15.6
10.2
7.5
5.9
4.8
grease, disposable diapers, plastics, etc.
POISONS:
gasoline, oil, paint, paint thinner, pesticides, coolant, etc.
Neglect or abuse of your septic system can cause it to fail. Failing
septic systems can:
• Cause a serious health threat to your family and neighbors;
• Degrade the environment, especially ground water and coastal waters;
• Reduce the value of your property;
• Be very expensive to repair; and
• Put thousands of water supply users at risk if you live in an
area adjacent to water pipes or cisterns.
Be alert to these warning signs of a failing septic system:
• Sewage surfacing over the drainfield (especially after storms);
• Sewage back-ups in the house;
• Lush, green growth over the drainfield;
• Slow-draining toilets or drains; or
• Sewage odors.
For more information on septic systems or to report a failing septic system, contact:
UVI Cooperative Extension Service RR02, Box 10,000 UVI Water Resources Research Institute DPNR Coastal Zone Management Program DPNR Division of Environmental Protection Department of Health, Division of Environmental Health Charles Harwood Complex, Rm 331 USDA Natural Resources Conservation Service
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