A septic tank is a key component of the septic system, a small scale sewage treatment system common in areas with no connection to main sewage pipes provided by local governments or private corporations. (Other components, typically mandated and/or restricted by local governments, optionally include pumps, alarms, sand filters, and clarified liquid effluent disposal means such as a septic drain field, ponds, natural stone fiber filter plants or peat moss beds.) Septic systems are a type of On-Site Sewage Facility (OSSF). In North America approximately 25% of the population relies on septic tanks; this can include suburbs and small towns as well as rural areas (Indianapolis is an example of a large city where many of the city's neighborhoods are still on separate septic systems). In Europe they are generally limited to rural areas only.
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| A Septic Tank |
The term "septic" refers to the anaerobic bacterial environment that develops in the tank and which decomposes or mineralizes the waste discharged into the tank. Septic tanks can be coupled with other on-site wastewater treatment units such as biofilters or aerobic systems involving artificial forced aeration.
Periodic preventive maintenance is required to remove the irreducible solids which settle and gradually fill the tank, reducing its efficiency. In most jurisdictions this maintenance is required by law, yet often not enforced. Those who ignore the requirement will eventually be faced with extremely costly repairs when solids escape the tank and destroy the clarified liquid effluent disposal means. A properly maintained system, on the other hand, can last for decades and possibly a lifetime.
Description
A septic tank generally consists of a tank (or sometimes more than one tank) of between 4000 - 7500 litres (1,000 and 2,000 gallons) in size connected to an inlet wastewater pipe at one end and a septic drain field at the other. These pipe connections are generally made via a T pipe which allows liquid entry and exit without disturbing any crust on the surface. Today the design of the tank usually incorporates two chambers (each of which is equipped with a manhole cover) which are separated by means of a dividing wall which has openings located about midway between the floor and roof of the tank.
Wastewater enters the first chamber of the tank, allowing solids to settle and scum to float. The settled solids are anaerobically digested, reducing the volume of solids. The liquid component flows through the dividing wall into the second chamber where further settlement takes place, with the excess liquid then draining in a relatively clear condition from the outlet into the leach field, also referred to as a drain field or seepage field, depending upon locality. Septic tank lift pump alarm system located in a house.
The remaining impurities are trapped and eliminated in the soil, with the excess water eliminated through percolation into the soil (eventually returning to the groundwater), through evaporation, and by uptake through the root system of plants andeventual transpiration. A piping network, often laid in a stone filled trench, distributes the wastewater throughout the field with multiple drainage holes in the network. The size of the leach field is proportional to the volume of wastewater and inversely proportional to the porosity of the drainage field. The entire septic system can operate by gravity alone, or where topographic considerations require, with inclusion of a lift pump. Certain septic tank designs include siphons or other methods of increasing the volume and velocity of outflow to the drainage field. This helps to load all portions of the drainage pipe more evenly and extends the drainage field life by preventing premature clogging.
An IMHOFF TANK is a two-stage septic system where the sludge is digested in a separate tank. This avoids mixing digested sludge with incoming sewage. Also, some septic tank designs have a second stage where the effluent from the anaerobic first stage is aerated before it drains into the seepage field.
Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank, or else the septic tank fills up and undecomposed wastewater discharges directly to the drainage field. Not only is this bad for the environment, but if the sludge overflows the septic tank into the leach field, it may clog the leach field piping or decrease the soil porosity itself, requiring expensive repairs.
How often the septic tank has to be emptied depends on the volume of the tank relative to the input of solids, the amount of indigestible solids and the ambient temperature (as anaerobic digestion occurs more efficiently at higher temperatures). The required frequency varies greatly depending on jurisdiction, usage, and system characteristics. Some health authorities require tanks to be emptied at prescribed intervals, while others leave it up to the determination of the inspector. Some systems require pumping every few years or sooner, while others may be able to go 10–20 years between pumpings. Contrary to what many believe, there is no "rule of thumb" for how often tanks should be emptied. An older system with an undersized tank that is being used by a large family will require much more frequent pumping than a new system used by only a few people. Anaerobic decomposition is rapidly re-started when the tank re-fills.
A properly designed and normally operating septic system is odour free and, besides periodic inspection and pumping of the septic tank, should last for decades with no maintenance. A well designed and maintained concrete, fibreglass or plastic tank should last about 50 years.
Wastewater enters the first chamber of the tank, allowing solids to settle and scum to float. The settled solids are anaerobically digested, reducing the volume of solids. The liquid component flows through the dividing wall into the second chamber where further settlement takes place, with the excess liquid then draining in a relatively clear condition from the outlet into the leach field, also referred to as a drain field or seepage field, depending upon locality. Septic tank lift pump alarm system located in a house.
The remaining impurities are trapped and eliminated in the soil, with the excess water eliminated through percolation into the soil (eventually returning to the groundwater), through evaporation, and by uptake through the root system of plants andeventual transpiration. A piping network, often laid in a stone filled trench, distributes the wastewater throughout the field with multiple drainage holes in the network. The size of the leach field is proportional to the volume of wastewater and inversely proportional to the porosity of the drainage field. The entire septic system can operate by gravity alone, or where topographic considerations require, with inclusion of a lift pump. Certain septic tank designs include siphons or other methods of increasing the volume and velocity of outflow to the drainage field. This helps to load all portions of the drainage pipe more evenly and extends the drainage field life by preventing premature clogging.
An IMHOFF TANK is a two-stage septic system where the sludge is digested in a separate tank. This avoids mixing digested sludge with incoming sewage. Also, some septic tank designs have a second stage where the effluent from the anaerobic first stage is aerated before it drains into the seepage field.
Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank, or else the septic tank fills up and undecomposed wastewater discharges directly to the drainage field. Not only is this bad for the environment, but if the sludge overflows the septic tank into the leach field, it may clog the leach field piping or decrease the soil porosity itself, requiring expensive repairs. How often the septic tank has to be emptied depends on the volume of the tank relative to the input of solids, the amount of indigestible solids and the ambient temperature (as anaerobic digestion occurs more efficiently at higher temperatures). The required frequency varies greatly depending on jurisdiction, usage, and system characteristics. Some health authorities require tanks to be emptied at prescribed intervals, while others leave it up to the determination of the inspector. Some systems require pumping every few years or sooner, while others may be able to go 10–20 years between pumpings. Contrary to what many believe, there is no "rule of thumb" for how often tanks should be emptied. An older system with an undersized tank that is being used by a large family will require much more frequent pumping than a new system used by only a few people. Anaerobic decomposition is rapidly re-started when the tank re-fills.
A properly designed and normally operating septic system is odour free and, besides periodic inspection and pumping of the septic tank, should last for decades with no maintenance. A well designed and maintained concrete, fibreglass or plastic tank should last about 50 years.
Potential Problems
- Excessive dumping of cooking oils and grease can cause the inlet drains to block. Oils and grease are often difficult to degrade and can cause odour problems and difficulties with the periodic emptying.
- Flushing non-biodegradable items such as cigarette butts and hygiene products such as sanitary towels and cotton buds will rapidly fill or clog a septic tank; these materials should not be disposed of in this way.
- The use of garbage disposer for disposal of waste food can cause a rapid overload of the system and early failure.
- Certain chemicals may damage the working of a septic tank, especially pesticides, herbicides, materials with high concentrations of bleach or caustic soda (lye) or any other inorganic materials such as paints or solvents.
- Roots from trees and shrubbery growing above the tank or the drainfield may clog and/or rupture them.
- Playgrounds and storage buildings may cause damage to a tank and the drainage field. In addition, covering the drainage field with an impervious surface, such as a driveway or parking area will seriously affect its efficiency and possibly damage the tank and absorption system.
- Unsupervised septic tanks may cause serious injury or death to children playing nearby.
- Excessive water entering the system will overload it and cause it to fail. Checking for plumbing leaks and practicing water conservation will help the system's operation.
- Over time biofilms develop on the pipes of the drainage field which can lead to blockage. Such a failure can be referred to as "Biomat failure".
- Septic tanks by themselves are ineffective at removing nitrogen compounds that can potentially cause algal blooms in receiving waters; this can be remedied by using a nitrogen-reducing technology, or by simply ensuring that the leach field is properly sited to prevent direct entry of effluent into bodies of water.
Historically at least, not all varieties of toilet paper were suitable for disposal in a septic tank as they did not deteriorate sufficiently (or, at least at some points in history, some toilet paper was specifically marked as suitable for use in septic systems and some was not).
Environmental Issues
Some pollutants, especially sulfates, under the anaerobic conditions of septic tanks, are reduced to hydrogen sulfide, a pungent and toxic gas. Likewise, methane, a potent greenhouse gas is another by-product. Nitrates and organic nitrogen compounds are reduced to ammonia. Because of the anaerobic conditions, fermentation processes take place, which ultimately generate carbon dioxide and methane.
The fermentation processes cause the contents of a septic tank to be anaerobic with a low redox potential, which keeps phosphate in a soluble and thus mobilized form. Because phosphate can be the limiting nutrient for plant growth in many ecosystems, the discharge from a septic tank into the environment can trigger prolific plant growth including algal blooms which can also include blooms of potentially toxic cyanobacteria.
Soil capacity to retain phosphorus is large compared with the load through a normal residential septic tank. An exception occurs when septic drain fields are located in sandy or coarser soils on property adjoining a water body. Because of limited particle surface area, these soils can become saturated with phosphate. Phosphate will progress beyond the treatment area, posing a threat of eutrophication to surface waters.
In areas with high population density, groundwater pollution levels often exceed acceptable limits. Some small towns are facing the costs of building very expensive centralized wastewater treatment systems because of this problem, owing to the high cost of extended collection systems.
To slow development, building moratoriums and limits on the subdivision of property are often imposed. Ensuring existing septic tanks are functioning properly can also be helpful for a limited time, but becomes less effective as a primary remediation strategy as population density increases.
Trees in the vicinity of a concrete septic tank have the potential to penetrate the tank as the system ages and the concrete begins to develop cracks and small leaks. Tree roots can cause serious flow problems due to plugging and blockage of drain pipes, but the trees themselves tend to grow extremely vigorously due to the continuous influx of nutrients into the septic system.
How it Works?
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| X-Sec of a Septic Tank |
In this picture, you can see three layers. 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.
Wastewater comes into the septic tank from the sewer pipes in the house, as shown here:
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| Sewage Mechanism |
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| Top View |
A septic tank naturally produces gases (caused by bacteria breaking down the organic material in the wastewater), and these gases don't smell good. Sinks therefore have loops of pipe called P-traps that hold water in the lower loop and block the gases from flowing back into the house. The gases flow up a vent pipe instead -- if you look at the roof of any house, you will see one or more vent pipes poking through.
As new water enters the tank, it displaces the water that's already there. This water flows out of the septic tank and into a drain field. A drain field is made of perforated pipes buried in trenches filled with gravel.
A typical drain field pipe is 4 inches (10 centimeters) in diameter and is buried in a trench that is 4 to 6 feet (about 1.5 m) deep and 2 feet (0.6 m) wide. The gravel fills the bottom 2 to 3 feet of the trench and dirt covers the gravel, like this:
The water is slowly absorbed and filtered by the ground in the drain field. The size of the drain field is determined by how well the ground absorbs water. In places where the ground is hard clay that absorbs water very slowly, the drain field has to be much bigger. A septic system is normally powered by nothing but gravity. Water flows down from the house to the tank, and down from the tank to the drain field. It is a completely passive system.
You may have heard the expression, "The grass is always greener over the septic tank." Actually, it's the drain field, and the grass really is greener -- it takes advantage of the moisture and nutrients in the drain field.
Maintenance of a Septic Tank
Help is on hand in the form of OE42 pre-packed synergistic bacteria. This product will cure or help alleviate most septic tank problems by restoring the natural balance found in healthy tanks.
It is safe to use and contains naturally occurring bacteria, ready to quickly get to work on the fats, paper, food waste and other contents of the tank. Unlike other products which claim to improve your septic tank performance, OE42 does not contain harsh chemicals which can attack sewers nor does it contain harmful solvents which can easily make matters worse.
Don't pour grease, coffee grounds and similar materials down your kitchen drain. Never Ever pour paints, cleaners, solvents or any caustics down any drain. Some people toss cigarette ends, sanitary products, wads of hair and paper and other solid materials into the toilet - this is a serious mistake! Don't use your toilet and septic system as a dust bin... You will be adding to the sludge build up and overloading the bacteria in the septic tank!
If you have a kitchen sink waste disposal, you should inspect your septic tank and pump out the solids about twice as often as you normally would. Therefore, try to limit the amount of food waste you put through the disposal. Even without a sink waste disposal, have your septic system inspected occasionally and the solids pumped out.
There are beneficial bacterial products such as OE42, which homeowners (as well as businesses) use to restore and optimize the natural biology of their wastewater system.
OE42 does not emulsify or liquefy oil, grease and organics, but eliminates it through natural biological digestion.
Many discharge problems can be completely eliminated by constructing a reed bed to treat the water from the septic tank. Odour-free and in keeping with the environment, these are often the ideal way to clean up wastewater.
In review, you can prolong the life of your home septic system by:
(1) Conserving water
(2) Reducing the amount of bleach poured down the toilet. If possible, switch to oxygen based bleach.
(3) Avoiding the installation of a waste disposal system
(4) Having your septic tank inspected every few years
(5) Using OE42 to maintain and optimize natural biological activity
(6) Not pouring grease, coffee grounds, medicines, solvents and other chemicals down your household drains.
About the Author
Mohsin Hanif Awan He is B.Sc in Civil Engineering from "University of Engineering and Technology Peshawar,Pakistan"