The Biology and Operation of a Septic System Essay

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The Biology and Operation of a Septic System


Introduction to Septic Systems
The septic system is a grouping of components working together to decompose household
waste water. The septic tank holds the waste and breaks the organic compounds into a
solid, liquid or gas. The solid waste (called sludge) settles to the bottom of the tank
and must periodically be removed. The liquid waste is flushed to the distribution box and
is then allowed to drain into the ground through a series of perforated pipes (often
called lines or a leach field). Gases follow the same path as liquids and eventually rise
through the soil and enter the atmosphere. Tanks can be arranged in series for additional
waste treatment. Many older systems have no distribution mechanism.


The Septic Tank Layers or Horizons
Wastewater flows through a pipe into the septic tank. Baffles or "tees" at the inlet and
outlet of the tank slows the flow of water and prevents sewage from flowing directly
through the tank. Bacterial action within a septic tank helps to break down the solids in
the wastewater. The tank must be large enough, and the rate of flow small enough, to
ensure sufficient "residence time" of wastewater within the tank. The solids which cannot
be broken down sink to the bottom of the tank and accumulate as sludge.


Grease, foam, and lighter particles float to the surface and form a layer of scum. The
exit baffle holds back sludge and scum while allowing a partially digested wastewater to
flow out of the tank.


This picture depicts the three layers within the septic tank. Anything that floats rises
to the top and forms a layer known as the scum layer. Anything heavier than water sinks to
form the sludge layer. In the middle is a fairly clear water layer. This body of water
contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, but
it is largely free of solids.


Organic SubstancesThe concentration of natural and synthetic compounds in effluent are generally expressed in terms of:
Biological Oxygen Demand (BOD), the measure of how much oxygen is required to finish
digesting the organic material left in the effluent.

Chemical Oxygen Demand (COD), the measure of how much oxygen is required to finish the decomposion of chemicals.
Total Suspended Solid Content (TST)
Total phosphorus and nitrogen - measures the nutrients remaining in the water

A properly designed and maintained septic tank removes most of the organic
substances from raw wastewater. Additional removal of these materials from the septic
tank effluent occurs in the soil, where removal of organic?s is accomplished by filtration
(through the sand), decomposition, and the breakdown through the use of microbial.

The Formation of a Clogging Layer
The organic substances within wastewater plays an important part in the formation of a
biologically active clogging layer which forms between the leach field pipes and the soil
below thus, slowing down the rate at which effluent and its constituents flow into the
soil. Bacteria growing under conditions where effluent is plentiful store polysaccharides
as slime capsules, which cover the soil particles between the bottom of the disposal pipes
and the underlying soil, causing a reduction in pore diameter. In unmaintained situations,
the formation of a clogging layer can lead to hydraulic failure of the septic system.
Although the clogging layer has been found to be beneficial by filtering the solids? form
the effluent and allows for an unsaturated zone beneath the tank by slowing the entry of
effluent into the soil. The most effective way to control the formation of the layer is
through regular pumping of the tank. In extreme cases the clogging layer may have to be
removed by commercially spraying concentrated hydrogen peroxide into the system. ?This
form of chemical restoration was developed and patented (1977) by the Wisconsin Alumni
Research Foundation (WARF) and the process named POROX?

http://csbh.mhv.net/dfriedman/septic/septadds.htm

Role of Nitrogen
Forms of nitrogen found in the septic tank include ammonia, ammonium, organic nitrogen,
nitrate, and nitrite. The types of nitrogen compounds and their concentrations are
important functions of the treatment of effluent in the septic tank.


Only a small part perhaps 10% (Wells and Septic Systems, pg. 102), of the total nitrogen
in raw wastewater is removed through the extraction of sludge that accumulates at the
bottom of the tank. Present in the soil are several mechanics which break down nitrogen
through denitrification, absorption, plant uptake and volatilization (turning into a gas).


Some nitrogen in effluent may be removed by one or more of these mechanics before the
effluent reaches groundwater. But half or more of the nitrogen is likely to travel with
effluent to the groundwater


Nitrification the conversion of ammonium-nitrogen into nitrate form, occurs in the first
foot or so of soil below the drain field, assuming that the water table is not present and
the soil is unsaturated. Nitrate is very soluble and does not interact with soil
components allowing it to travel through the soil practically untouched. Unless conditions
for denitrification (conversion of nitrate to nitrogen gas) exist, nitrate will not
undergo further transformation in the ground water. Therefore, dilution is the best hope
of reducing concentrations of nitrite from septic systems in ground water.

*Waste water leaching into surface waters contains nitrogen and phosphates that, being
fertilizers, encourage the growth of algae. Excessive algae growth can block the sun and
foul the water.


Role of Phosphorus
Phosphorus in septic tank effluent originates from two main sources: detergents containing
phosphates, and human excreta. Anaerobic digestion in septic tanks converts most of the
phosphorus into soluble orthophosphates.


In contrast too then non reactive nitrate, most phosphate will react vigorously with the
soil. Phosphate ions in the waste water are removed from the soil by several mechanisms
including absorption, precipitation, plant uptake, and biological breakdown.


However phosphorus transport through the soil, to the water table is more likely to occur in:
1. Coarse-textured soils
2. Soils with low organic matter
3. Soils which have a shallow depth to the water table and/or bedrock

This can become a problem when wells and surface water become contaminated. Although over
time phosphate removal will occur in the water table through, precipitation, absorption
and dilution.


Detergent Surfactant's
Mechanisms for removal of detergent surfactants from effluent in the soil include
biodegration and absorption to soil particles. Absorption is influenced by several soil
properties, including soil texture, mineralogy, organic matter content, soil chemistry,
soil pH, and the formation of a clogging mat. Absorption of detergent surfactants allows
more time for biodegradation through microorganisms to occur. Therefore, it is even more
important to purchase biodegradable soaps.


Toxic Organic Compounds
Toxic non biodegradable organic compounds, such as chlorinated hydrocarbons,
trichloroehtylene (TEC), and methyl chloroform (MC) has been found in septic tank cleaners
and additives. TEC and MC have a greater density than water, allowing them to sink to the
bottom of the tank, and are not readily biodegradable in this environment. These and other
toxic materials such as pesticides, solvents, and compounds containing heavy metals have a
high potential for contaminating the soils and groundwater, and should not be put into the
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