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A baffle is a vertical section or wall within the tank that reduces the velocity of, and directs the waste through, the tank and prevents solids from flowing into the outlet pipe and leaving the tank. Baffles are also installed to assist in the further breakdown of the waste by creating multiple compartments that allow the effluent to sit and separate longer, providing the bacteriological farm more time to operate at greater efficiency. The sizing of the baffles is important. If the baffle or tee is not deep enough, the floating buoyant waste may enter and plug it. If the baffle or tee extends too deep, the downward flow of incoming waste may cause excessive agitation within the tank and result in solids being carried out with the effluent. If sludge or buoyant waste is forced into the distribution box and into the leaching field, the soil will quickly become clogged or saturated. If this happens, the liquid will no longer percolate (absorb) into the soil. Broken baffles in the septic tank that allow sludge or scum to escape can cause this condition. Failure to perform periodic maintenance by having the tank pumped can also lead to a situation where the sludge and buoyant waste overwhelm the baffles.
When septic tanks have to be buried more than 18 inches below finished grade, a riser or access tube/tunnel is installed on top of the tank body above the access cover, and is extended up to a point that is less than 18 inches from the ground surface. The riser(s) needs to be completely sealed to the tank to prevent ground water from entering the riser cavity, which may cause a flooding of the tank and the whole system.
The sludge layer consists of the heavier waste solids that separate and settle to the bottom. The sludge layer is where the decomposition process continues by means of bacteriological interaction. These bacteria live and grow without the presence of air in what is called an anaerobic bacteriological farm. (When oxygen is introduced, this is called an aerobic bacteriological farm.) The average efficiency of an anaerobic bacteriological farm is approximately 60 to 70 percent. If a second tank is installed, as is becoming more commonplace in several parts of the country, then the efficiency will be increased. Although decomposition is a continual process, the breakdown is not complete, which can eventually result in waste residue build-up if not pumped out on a regular basis (every 2-4 years). This residue can build up to the bottom of the inlet or outlet tee and block flow into and/or out of the tank.
In order to make repairs or perform regular maintenance or cleaning/pumping of the tank, access must be provided. There are usually two or three access hatches located at the top of the septic tank-one located over the inlet and outlet tees, and sometimes one larger one located at the center of the tank.
The septic tank is a watertight structure that is the main collection point for human waste by-products. It is at this tank that the solid waste is separated from the liquid waste and the biological digestion of the waste matter takes place, referred to as part of the closed portion of the treatment system. Until about 1965, most septic tanks were constructed of steel plates, which were welded together. After 1965, the primary construction materials were fiberglass or cast concrete. Septic tanks are sized according to the amount of liquid waste they must process. A typical sized tank is 1000 gallons for a one, two, or three-bedroom house. For each bedroom after three, add 250 gallons to the size of the tank. If a garbage grinder/disposal is present at the kitchen sink, it counts as an additional bedroom.
All septic tanks have an opening for the waste to enter the tank and another one for the waste to exit the tank. The exit is called the outlet. Inside the tank, there will be either a PVC or metal tee fitting, consisting of a short section of horizontal piping leading into a slightly longer vertical section of piping that is open on both the top and the bottom. The top of the vertical section must extend above the level of the scum mat, and the bottom of the vertical section must extend below the bottom level of the scum mat. The outlet tee is usually several inches below the level of the inlet tee.
The liquid effluent is made up of the remaining liquids and semi-buoyant waste particles after the sludge and buoyant waste have separated. A normally operating septic tank maintains a constant effluent level at the height of the bottom of the outlet tee opening. Consequently, when new waste enters the tank, the liquid effluent level rises and the effluent is forced out of the tank through the outlet into the distribution box and into the absorption area for dispersment and continued treatment. Septic tank effluent is usually cloudy and contains suspended solids and pathogens, including disease-causing bacteria and viruses. This condition requires more bacterial action for treatment than can occur in the tank alone.
Any single trench in an absorption area is called a lateral line. Where property conditions permit, it is best to keep the laterals the same length, with the number of laterals being determined by the needs of the house. The trenches are approximately 2 to 3 feet wide, 3 to 4 feet deep, and about 9 feet apart. Some modern systems may have shallower trenches. The trench should be sloped enough so that the liquid effluent flows fast enough to allow for even dispersal. A grade of one to two inches within eight feet is what is commonly used. If the slope is too flat, the effluent may stall close to the distribution box and cause premature saturation. If the slope is too steep, the effluent may run to the end of the lateral line and, again, cause premature saturation.
Leach line piping is usually made of perforated piping, approximately four inches in diameter. The piping is laid in a bed of gravel at the bottom of a series of interconnected trenches. The gravel is then covered by a layer of backfilled soil that covers the trench system. Some older septic systems actually used a series of segmented, non-perforated tubes set in a bed of gravel, with the tubes set between ½ inch and 1 inch apart. These tubes were sometimes constructed of a thickened roll of asphalt-saturated fibrous material (similar to building felt paper), or they were made of an orange ceramic tile material, similar to clay flue tiles. Both of these materials have been found to deteriorate and collapse.
All septic tanks have an opening for the waste to enter the tank and another one for the waste to exit the tank. The entrance is called the inlet. Inside the tank there will be either a PVC or metal, tee-shaped fitting, consisting of a short section of horizontal piping leading into a slightly longer, vertical section of piping that is open on both the top and the bottom. The bottom of the tee is usually below the level of the bottom of the outlet and is inside the liquid effluent layer. This is to prevent any buoyant waste from backing up into the house side of the drain lines and into the house plumbing.
Graywater waste is defined as the wastewater produced from baths and showers, clothes washers, lavatories, laundry systems, perhaps the effluent from a sump pump, the foundation footing drains, and sometimes roof runoff. This waste usually contains significant amounts of soaps from personal hygiene, dishwashing and laundry, as well as greases and oils from cooking. If a dishwasher or garbage disposal is used, there will also be food solids present. Because the bacteriological farm has greater difficulty breaking down undigested food material, a separate collection tank aids in the efficiency of the main waste system. Also, soaps and greases can prematurely saturate and rapidly clog the main absorption area if they manage to reach it. Graywater does not contain human waste products and does not need to be digested like human waste. The disposal requirements for this type of water are less stringent than those for human waste.
When the disposal of blackwater waste and graywater waste is separate, there will be a separate tank installed for the graywater waste. This tank is usually made of the same materials as the main septic tank, however, it can also be made of other materials such as metal, plastic, fiberglass and concrete. This tank is usually smaller in size than the primary septic tank. The interior components of the graywater septic tank are similar to those of the larger septic tank, with inlet and outlet tees, baffles, access covers and tank connections. The graywater tank may be connected to the primary septic tank and absorption area, or it may discharge into its own absorption area.
In some specialized cases, there will be a separate septic tank for graywater waste. These systems are usually for the kitchen and laundry plumbing, and sometimes the bathroom sinks.The graywater outlet will usually be constructed of 2 or 3-inch drain pipe. Like the blackwater outlet, the point where the drain line exits the house may serve as an indication of where the graywater tank is located, but not always.
There are usually two ways that graywater waste can be treated and disposed; one way is through its own absorption area and another is to tie into the primary waste system. When the graywater discharge is tied into the primary house waste system, the outlet line should be tied into the primary drain line of the main waste system. This connection can be located between the foundation exit and the main septic tank itself
The gravel bed is crushed or screened stone that comes from river sand or is quarried. The open porosity of the gravel allows rapid dispersal of the liquid effluent. The gravel bed is laid at the bottom of the lateral line trench, with the actual lateral plumbing line sandwiched within the gravel.
For ease of maintenance and inspection, most properties with private on-site waste disposal systems will have an additional cleanout pipe and access installed to aid in removing waste from the septic tank without having to actually excavate the main tank access. The cleanout is usually made of 4-inch diameter PVC or cast iron piping with a screw-on cover. The cover is usually flush with the finish grade or extends up high enough to be noticeable so as not to be run over with yard equipment. If there is sewage backup in the home, it can be a possible sign of a failing system. However, backup can also be the result of a blockage somewhere between the house and the septic tank.
When the effluent leaves the septic tank, it is sent to the distribution box (D-box). The D-box usually has a single inlet (from the tank) and several outlets that lead to the individual lines in the absorption area. The D-box will usually have one or more separate baffles that reduces the velocity of liquid moving through the D-box to allow more even displacement of the liquid effluent into the separate lateral lines.
The buoyant waste is made up of greases and soaps from the graywater waste. When a septic tank is opened, this is usually the first thing that is seen floating on top. If periodic maintenance is not performed (i.e. pumping the tank clean every 2-4 years), this waste can build up to the point of going above the top of the inlet and outlet tees and clogging the inlet into the tank, as well as possibly clogging the soils in the absorption area.
The point at which the drain/waste pipes for the bathroom plumbing connect and leave the house structure is the blackwater outlet. In typical on-site waste disposal systems, there is usually only a single primary septic tank that all of the plumbing drain lines empty into. This includes toilet, as well as sink, shower, and tub waste. In most cases, when looking for the blackwater drain outlet, you will find a drain line of approximately 4-4 ½ inches in diameter exiting the foundation wall, to which all of the other drain lines lead into.
Backfill is clean soil, free of debris or large organic material, that is used to fill the lateral trench lines after the gravel and piping is installed. The backfill needs to be finish-graded so that surface water does not pond above the lines.
The absorption area is a system of subsurface piping, known as lateral lines, that forms a series of water-carrying channels into which the effluent is discharged for direct treatment and absorption into the soil. A leaching system placed in unsuitable soil, a system that is too small for the house it serves, or an improperly constructed system may lead to early failure. The natural soil conditions will need to be able to dissipate and disperse the discharged effluent without becoming oversaturated. It should also provide enough capacity to store effluent during periods of unusually heavy use or when rainfall or subsurface flooding reduces the ability of the system to disperse and distribute the liquid. The most readily, noticeable sign that an absorption area has failed is “breakout.” When this occurs, you will notice ponding water laying on the soils directly above the individual lateral lines. Another possible sign of failure is a smell of sewage outside the house. If this smell is more noticeable after a lot of water has been put into the system-multiple showers or several loads of laundry, for example-this may be an indication that the leaching field is failing. The smell may also be accompanied by a “spongy” feeling in some areas of the leaching field.