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Septic Systems
What are they and how do they work?

If you are like most people, you might not know much about your septic system this will help you understand what a septic system is and how it works.

The Septic system itself is made up of two parts:

1. The Septic Tank (see diagram below)
- Newer tanks are usually constructed of cement, or polyurethane, but older systems can be made from anything from wood to metal.

- All household waste (i.e. toilet / kitchen / laundry / bathing) is carried out to your septic system usually by a 3 or 4 pipe called the main line.

- All the organic solids (i.e. waste / food / paper ) will float to the surface and form a layer commonly referred to as The Crust or Scum.
*The bacteria culture in the septic tank will break down these solids into gasses and liquids.

- Inorganic solids (i.e. plastic / dirt / other inert materials) and the by-products of the bacteria culture digestion all will settle to the bottom of the tank and is called Sludge.

- Only relatively clear water should exist between the crust and sludge this is called Leachate. This clear water is the only material that should be allowed into the leach area.

- Both the crust and sludge materials should not be allowed to flow into the leach area at any time. If this is allowed to occur, it can clog soil pores in the leach area and cause septic systems to fail.

- The three factors that can cause solids to travel into the leach area are missing outlet tee, bacterial deficiency, and failure to maintenance pump your septic tank on a regular basis as per Current County Recommendations.

A Typical Septic Tank

The diagram above is of a standard cement tank with a normal water level, two cement lids, a baffle that is cement and intact, and inlet and outlet Tees intact. Most septics will resemble this model in design.

How do they work?

A septic system is a highly efficient, self-contained, underground wastewater treatment system. Because septic systems treat and dispose of household wastewater onsite, they are often more economical than centralized sewer systems in rural areas where lot sizes are larger and houses are spaced widely apart.

Septic systems are also simple in design, which make them generally less expensive to install and maintain. And by using natural processes to treat the wastewater onsite, usually in a homeowner's backyard, septic systems don't require the installation of miles of sewer lines, making them less disruptive to the environment.

A septic system consists of two main parts a septic tank and a drainfield. The septic tank is a watertight box, usually made of concrete with an inlet and outlet pipe. Wastewater flows from the home to the septic tank through the sewer pipe. The septic tank treats the wastewater naturally by holding it in the tank long enough for solids and liquids to separate. The wastewater forms three layers inside the tank. Solids lighter than water (such as paper, fecal matter, greases, and oils) float to the top forming a layer of scum. Solids heavier than water settle at the bottom of the tank forming a layer of sludge. This leaves a middle layer of partially clarified wastewater.

The layers of sludge and scum remain in the septic tank where bacteria found naturally in the wastewater work to break the solids down. The sludge and scum that cannot be broken down are retained in the tank until the tank is pumped.

The layer of clarified liquid flows from the septic tank to the drainfield or to a distribution device, which helps to uniformly distribute the wastewater in the drainfield. A standard drainfield (also known as a leachfield, disposal field, or a soil absorption system) is a series of trenches lined with gravel and buried one to three feet below the ground surface. Perforated pipes or drain tiles run through the trenches to distribute the wastewater. The drainfield treats the wastewater by allowing it to slowly trickle from the pipes out into the gravel and down through the soil. The gravel and soil act as biological filters.

More Detailed information:

How many people would buy an expensive car and never change the oil? How many would never remove the ashes from a wood stove or fireplace? When the car breaks down or the stove is so full of ashes that no more wood can be put in, we don't blame the car or the stove. But when a septic system fails for lack of proper maintenance, the blame is often incorrectly placed on that "no-good" septic tank.

A septic tank is the essential first part of an onsite sewage treatment system; and it's both very simple and very complex. Raw sewage flows into the tank from the house sewer. The solids separate from the liquid and stay in the tank. The liquid flowing out is called septic tank effluent. A septic tank is watertight so that when 5 gallons of sewage enters from the house, 5 gallons of effluent must flow out of the tank. Bacteria that do not use oxygen (from the air) grow in the tank. These bacteria are called anaerobic and the by-products of their activity are methane and hydrogen sulfide gas, plus other substances having an odor. Hence, the word "septic" has been applied to this tank. The septic tank might also be described as a settling tank where the sewage solids are stored while the bacteria decompose them and reduce their volume. [The volume is never reduced to zero, so a residue remains.] It is this residue that must be cleaned out of the tank when the volume becomes too great.

Sewage flows to the septic tank through the house sewer. This pipe must have the proper slope; not too steep so that the liquids run away from the solids and not too flat so that the solids settle out in the sewer pipe. A grade of from one to two inches in eight feet is used. This is a slope of one to two percent. A one percent slope is a one-foot drop in a hundred feet of pipe.

The house sewer pipe should be smooth on the inside so that sewage won't catch and start a blockage. Toilet paper might hang up on a rough spot at a pipe joint causing a continuing problem of sewer pipe plugging every so often. If the homeowner notices that the toilet isn't flushing as fast as it used to, or the floor drain is backing up when the clothes washer discharges, the problem may be a partially plugged house sewer pipe. There are also many other causes but this is one place to start looking.

The house sewer is the inlet pipe to the septic tank. The bottom (invert) of this inlet pipe should be two to three inches higher than the invert of the outlet pipe of the septic tank. As the sewage reaches the tank it drops into the liquid in the tank with a downward flow direction. This drop tends to move the sewage into the depth of the tank using a sanitary tee. The purpose of this sanitary tee is to prevent the floating solids, called the scum layer, from building up and plugging the end of the sewer pipe.

The bottom of the sanitary tee should extend below the surface of the liquid to the middle of the liquid depth. If the tee is not deep enough, the floating scum layer may tend to plug it. If the tee extends too deep, the downward flow may cause agitation in the tank and result in more solids carried out with the effluent.

There are three distinct zones or layers in a septic tank. At the top is the floating scum layer which collects wastes such as fecal matter, toilet paper, soap or detergent scum, cooking fats, cigarette filters, and any other material that floats. Most of the material in the scum layer does decompose under the bacterial action in a septic tank. At the bottom of the tank is the sludge layer which consists of decomposing and partially decomposed solids which sink to the bottom of the tank. The decomposition process continually goes on in the sludge layer. Some solids can't make up their mind whether they will sink or float and may remain in the clear zone between the scum and sludge layers until such time as they are carried out through the outlet baffle and pipe. These items have the same density as water so will tend to neither sink or float. Inorganic items such as plastic film, condoms, etc., can cause serious plugging problems in a septic tank and should never be flushed down the toilet.

The center zone is called the clear zone which is liquid that contains suspended solids and bacteria. It is important that the tank have a deep clear zone. As the scum and sludge layers become thicker, the depth of the clear zone decreases. Then the speed at which liquid flows through the tank speeds up and begins to carry some of the solids out of the tank. It is these solids that begin to plug the pores in the soil. The tank needs cleaning whenever the bottom of the scum layer gets too close to the bottom of the outlet device or the top of the sludge layer gets too close.

The sewage flowing into the tank determines the nature of the bacteria in the tank. Some families of bacteria do better with one type of sewage, others do better with different sewage. No two septic tanks operate exactly the same. The nature of the raw sewage is different because of the eating and living habits of the family. The amount of water use is different so that the dilution of sewage is different. Different home cleaning products are used, as are varying amounts of other household chemicals and cleaners. Tank temperatures differ depending on type of water use, depth that the tank is buried, tank insulation, etc. All of these factors affect the bacterial population and the intensity of bacterial action. Bacteria are always present in sewage, however. If there is food and the proper temperature, they will grow and multiply. Septic tank bacterial action starts by itself and continues as long as solids are deposited in the tank.

A sanitary tee definitely is needed on every septic tank. The bottom of this device should extend into the liquid a distance equal to 40 percent of the liquid depth. This is the location determined by a number of tests to provide for the clearest effluent to leave the septic tank. If there is no outlet device or if it falls off or is removed, then the scum layer will flow out of the tank and into the soil treatment area, plugging the soil pores. This is why it is a good idea to have access to the outlet side of the septic tank to inspect the quality of the septic tank effluent.

Septic tank effluent is usually cloudy and contains suspended solids and pathogens (disease-causing bacteria and viruses). No, it is not good enough to drink, regardless of what you may have heard at one time. A half cup of effluent will likely contain a million or more bacteria and as many viruses. Suspended solids have a concentration of 45 to 65 mg/l (milligrams per liter). These are the solids that won't settle out, and cause the cloudy color. Since only about half of the oxygen-demanding material in the raw sewage has been removed in the septic tank, the effluent must have further bacterial action for treatment. This treatment takes place in the soil.

Risers or lid extensions can be installed over the inlet and outlet lids. Their basic purpose is to provide access for removing obstructions and determining the amount of sludge in the bottom of the tank or the amount of scum floating on top of the tank, and pumping the septic tank.

Water softener discharges are often blamed for the malfunction of the septic tank. A slight saline solution is good for bacterial growth. The agar culture used to grow bacteria in the laboratory is saline. However, too much salt will be detrimental to bacterial growth. A properly functioning water softener will not add much sodium chloride to the recharge waste water. As the softener gets older and particularly if there is iron in the water, the efficiency may drop unless the resin beads are kept clean. Resin beads that are iron-fouled cannot take on the same amount of salt and the unused salt goes into the waste water. As the beads become more and more fouled, the frequency between softener recharges is more often in order to get soft water, and more and more of the salt is wasted. This means that more salt gets into the septic tank and may be too strong for the bacteria to grow.

Another problem is the extra water that the water softener recharge adds to the septic system. A system that is just big enough to take the daily sewage flows may experience back-ups when a softener is added. This extra water cannot be handled by the system which is now too small. The softener gets blamed for the sewage system failure. Remember that each septic system has a maximum daily capacity. When more liquid flows in than the system can handle, back-ups or surfacing will occur.

Septic tanks should be constructed of good quality materials, which are not subject to excessive decomposition or deterioration. The tank material must be strong enough to withstand earth pressures and not collapse when the tank is cleaned and pumped. The septic tank, tank cover, and manhole extensions must be water tight to prevent leakage or infiltration of ground water.

Remember that the septic tank is a settling tank which collects and stores sewage solids. When the storage gets full, the tank must be cleaned and pumped.