The Biology And Operation Of A Septic System Essay, Research Paper

Introduction to Septic SystemsThe 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 HorizonsWastewater 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 LayerThe 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 NitrogenForms 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 PhosphorusPhosphorus 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’sMechanisms 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 CompoundsToxic 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 septic system.

Role of BacteriaBacteria such as E. Coli may be trapped in the pore spaces between soil particles. This entrapment or filtration is an important mechanism for removal of enteric bacteria from effluent. The clogging mat which occurs at the interface between the leach field pipe and soil, serves to trap the bacteria before they enter the soil. Factors affecting the translocation of bacteria include bacterial numbers in the effluent, soil texture, soil wetness, loading rate, temperature, and bacterial type. Unsaturated flow beneath a drain field is important in ensuring slow travel, long residence time for bacteria, good aeration, increased opportunity for contact between effluent and soil particles, adsorption of bacteria to soil particles, and eventual die-off of bacteria.

* Wastewater contains a certain amount of organic material that bacteria in the environment will start decomposing, and using up oxygen in the surface waters. The lack of oxygen kills fish.

Removal of Viruses

Viruses are smaller than bacteria and have a behavior in the soil environment that is different from that of bacteria. Virus removal or inactivation in the soil may be accomplished by several mechanisms, including filtration, precipitation, adsorption, biological enzyme attack, and natural die-off. The small size of viruses, and their surface properties, deriving from a protein coat that may or may not have an electrical charge, causes removal of viruses to be controlled more by absorption to soil particles than by filtration. Many of the soil properties that affect adsorption of bacteria also affect adsorption of viruses. Cation exchange properties of soils, mineralogy, texture, pH , and temperature are just a few of the soil properties that influence the survival of a virus.

As with bacteria, unsaturated flow conditions in the soil beneath a septic system, resulting in good aeration, slow travel, long travel, long residence times, good effluent-contact, and opportunity for die-off, is very important in ensuring the cleanup of viruses in effluent.

Importance of the Unsaturated ZoneThe degree of saturation, or wetness, of the soil in influenced by several factors, including the depth to the wet season water table. The water table fluctuates as rains come and go, as rates of evaporation change with the seasons, and human activities (drainage, irrigations, storm water management etc.) all have an impact. One of the keys to a proper functioning of a septic system is ensuring the separation between the bottom of the drain field and the water table is large enough so that unsaturated conditions will be maintained even during wet seasons. Water travels more slowly through an unsaturated soil (i.e. a soil whose pores are not entirely filled with water) than it would travel through the same soil were it saturated. The slower the velocity of flow, the longer is the residence time of the effluent in the unsaturated zone and the greater the opportunity for cleanup of effluent. Good aeration is necessary to achieve decomposition of organic particles and compounds, biodegration of detergents, and die-off of bacteria and viruses.

Water table fluctuations are one of the major pitfalls of the percolation test, in which the rate of water in a standard-size hole is measured in order to estimate the ability of the soil to accept effluent. The percolation test has some value in estimating the ?perk? rate of soils at a site, establishing appropriate loading rates, and predicting system performance. The test might indicate a rapid perk rate during dry times, but these figures will change during the wet season when the watertable is just below the drainage field.

To design a septic system such that the unsaturated zone will exist, the depth to the wet season water table must be estimated at the site, by examining soil color patterns, features of the soil profile, landscape position, the vegetation growing on the land, and additional information on water table fluctuations in the soil survey reports for the area.

If a grain of sand was the size of a basketball, then a piece of silt would be the size of a marble and a particle of clay would be a pinpoint. Clay particles are so small, less than one 12 500th of an inch, that an electron microscope must be used to see them.

These tiny react well with contaminates found in waste water, but the problem is that they are shaped like tiny plates of flakes. When the cationic influence of sodium is present, these flakes tend to stick together like a peanut butter sandwich.

CLASSES OF FAILURE OF SEPTIC SYSTEMSThe homeowner tends to think that the septic system is working as long as the toilet works and there’s no smell in the yard or adjacent ditches. Shifts in our environmental awareness in recent years have led one realize, however, that there are other ways to define failure of septic systems. We might categorize these types of failures as follows:

1.Class I — Raw Sewage on the Bathroom Floor. This is the classic failure in which raw sewage is rejected by the disposal system.

2.Class II — Sewage in the Yard. In this class of failure the toilet and other facilities seem to function just fine, but untreated or poorly treated sewage is surfacing in the yard, in nearby ditches, in the neighbor’s yard, or elsewhere in the environment. It is probably going to be obvious to someone in the neighborhood that a failure has occurred.

3.Class III — Decline in Water Quality. In this case the household plumbing and drain field seem to be working perfectly. There is no smell in the neighborhood, and no excess wetness around the drain field. But a research team, using monitoring devices, groundwater sampling and tracers, observe that the system or systems are causing degradation of ground water and/or surface water.

4.Class IV — Long Term, Gradual, Environmental Degradation. Here there is little if any scientific evidence that waters are being degraded at a rate likely to be a problem to this or the next generation of residents. But computer modeling and/or long term monitoring indicates that very gradual environmental degradation will happen as a result of septic system practices at a particular home site, in a neighborhood, or in a region. This is the hardest type of “failure” to prove.

REASONS FOR FAILURE OF SEPTIC SYSTEMSA septic system failure, of whatever type, might have one or more of several causes.

Some of these causes might be:

High water table.

Slowly permeable subsoil (clay, cemented pan, etc.).

Inadequate setbacks from open water or wells

Organic material in mound fill

Fine-textured material in mound fill

Improper design and/or installation

Use of system beyond its designed hydraulic capacity

Improper disposal of solvents, grease, etc., in the septic tank

Failure to pump out

Excessive BOD, TSS, or other constituents in effluent

ReferencesBlair, Allen. The Septic System Owner?s manual. New York : Holiday House, 1999

Max, Alth. Wells and Septic Systems. New York : Tab Books, 1992.

Max, Alth. Constructing & Maintaining your well and septic system. Philadelphia : Blue Ridge Summit, 1984.

Warshall, Peter. Septic tank Practices. New York : Garden City. 1979.

http://www1.mhv.net/~dfriedman/septbook.htmAll Septic System Information website. Pertains to how the septic system works, drawings of the leach field and tank (http://www1.mhv.net/~dfriedman/septic/piclibp1.htm), links to corporate home pages, tips on maintaing the septic system. On line Articles

http://www.howstuffworks.com/sewer.htmHow Sewer a Septic Systems Work

http://www.aquadoc.com/Gayman_Soil_Failure.htmHow Sodium Contributes to Septic System and Soil Failure

http://www.cherryvalley.com/terragreen/page6.htmlSeptic Systems and the Environment

http://www.ianr.unl.edu/pubs/wastemgt/g514.htmSoils, Absorption Fields and Percolation Tests for Home Sewage Treatment