· 2020. 4. 30. · The septic tank is a buried, watertight container typically made of concrete, fiberglass, or polyethylene. It holds the wastewater long enough to allow solids

EADE Organization vision in designing of the septic tanks Submitted by: Sati M. Al-Rawi, MSc

EADE Environmental Consultant [email protected]

18 Feb, 2018 Preface Septic tanks are still a common method of treatment and disposal of domestic wastewater in many parts of the world. Their removal efficiency of potential pollutants depends on many parameters. In Mosul city and due to the absence of the sewerage system, dwellings are provided with individual septic tanks. An investigation of these tanks showed a good percent removal of pollutants. Some constructional defects and faulty location of some arrangements had affected tank performances. Soil type necessitated rapid desludging or effluent overflow which might affect the environment. An oversizing of the tanks might cause some constructional danger. Besides, it added an overburden cost to the owner. Septic System Components Septic systems are wastewater treatment systems that collect, treat, and dispose of wastewater generated by homes or businesses. The wastewater is treated on site, rather than collected and transported to a centralized community wastewater treatment plant. There are several variations of the basic septic system design in use today. While many systems are individually designed or adapted for a specific site, most work using the same basic principles. A septic system consists of two main parts; a septic tank and a soil absorption system (SAS), also known as a drain field, leach field, or disposal field. The entire system is connected by pipes, and a sewer pipe connects the home or business to the septic system. The Septic Tank The main function of the septic tank is to collect household wastewater, which includes water from the toilet, commonly referred to as black water, and water from the bathtub, showers, sinks, and laundry, which is known collectively as gray water. However, some states include kitchen sink waste as black water. The septic tank treats the wastewater naturally by holding it in the tank long enough for solids and liquids to separate. Treatment begins when the household wastewater flows from the home to the septic tank through the sewer pipe. A baffle (an internal flap) or tee (a T shaped pipe) at the inlet slows the flow of wastewater going into the tank and directs it downward toward the middle of the tank. The wastewater is then retained for a day or more in the tank to allow the solids in the wastewater to separate from the liquids. Inside the tank, solids lighter than water—such as

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greases, oils, and sometimes, other solid materials like toilet paper—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. An outlet baffle in the septic tank is positioned to allow only the partially treated liquid waste in the middle layer to flow out of the tank for further treatment. The layers of scum and sludge remain in the septic tank where bacteria found naturally in the wastewater work to break the solids down. This process takes place anaerobically (without the presence of oxygen), and gases produced from the decaying solids are vented back through the sewer line and released, usually through a plumbing vent located on the roof of the house. The sludge and scum that cannot be broken down are retained in the tank until the tank is eventually pumped. After the wastewater is allowed to settle and separate in the septic tank, the partially treated liquid from the middle layer flows through the outlet baffle or tee to the SAS. Septic tanks are usually made of precast concrete, fiberglass, or plastic, bricks or block masonry and come in a variety of shapes and sizes. In order for septic tanks to work properly, they must be watertight and resistant to corrosion. For this reason, metal tanks are not recommended. Most septic tanks are single-compartment tanks. Some people prefer tanks with two or more compartments because they feel settling ability may be enhanced. Multi-compartment tanks use the same process to treat the wastewater as single-compartment tanks. Some states recommend or require two or more compartments for septic tanks that hold 1,000 gallons or more, or two or more septic tanks used in series, one after the other, to provide additional treatment. Septic Systems Soil Absorption System In a conventional septic system the wastewater flows by gravity from the septic tank to the SAS or to a distribution device, which helps to uniformly distribute the wastewater flow in the drain field. The soil absorption field provides the final step in the wastewater treatment process. The size of a SAS is usually based upon the size of the house and percolation rate of the soil. A standard field is a series of trenches or a bed lined with gravel or coarse sand and buried one to three feet below the ground surface. Perforated pipes or drain tiles run through the trenches to distribute the wastewater. The drain field 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 in a drain field act as biological filters. As the wastewater percolates (moves through the soil) to the groundwater below, the filtration process and organisms in the soil work together to remove toxics, bacteria, viruses, and other pollutants from the wastewater. Soil particles, particularly clay, chemically attract and hold sewage nutrients, metals, and disease carrying organisms. This process can effectively treat the wastewater to an acceptable level that will not contaminate the groundwater. Therefore, it is very important that adequate separation exists between the bottom of the trench/bed and a limiting layer, such as groundwater or bedrock. Certain toxics, such as paints, paint thinners, pesticides, waste oils, and other hazardous chemicals, cannot be treated by the drain field and should never be disposed of through a septic system. Some of the chemicals also kill the bacteria found in the septic tank, temporarily disrupting the natural treatment process that occurs in the septic tank. A thorough site evaluation should be conducted at the beginning of the planning stage. Septic system failures are often caused by poorly conducted evaluations or results of evaluations that did not occur at the beginning of the planning process. Site Evaluation Is Essential In a typical site evaluation, a sanitarian, engineer, or other wastewater professional examines the soils, landscape features, and past surveys of the potential site. He or she makes special note of the location of nearby wells, other septic systems, the slope of the land, depth to the groundwater source and to impermeable layers (such as bedrock), natural drainage patterns, and the boundaries of the lot. An important feature of the site evaluation is a thorough study of the soil. Marking the position of the absorption field, the sanitarian digs an observation pit to examine the soil layers for texture, structure, and color patterns that will give clues about the soil’s permeability and potential for seasonal water saturation. Toxic gases, including methane and hydrogen sulfide, are produced by the natural treatment processes in septic tanks. These gases can kill in minutes Extreme care should be taken when inspecting your tank, even when just looking in. Never enter a septic tank or try to inspect the tank alone. Most communities have licensed septic contractors who can inspect your system periodically. For guidelines on how to safely and properly inspect your system, call your local permitting authority. The size of a septic tank is usually determined by the number of bedrooms (not bathrooms) in a home. One way to estimate the size of a septic tank necessary for an average household


would be to multiply 150 gallons per bedroom per day, and then multiply this number by two to allow for two days retention time. 1 How Septic Systems Work Septic systems are wastewater treatment systems that collect, treat, and dispose of wastewater generated by homes or businesses. The wastewater is treated onsite, rather than collected and transported to a centralized community Wastewater treatment plant. If properly designed, installed, and maintained, a septic tank system can effectively treat household wastewater for up to 20 years or more. There are several variations of the basic septic system design in use today. While many systems are individually designed or adapted for a specific site, most work using the same basic principles. A septic system consists of two main parts—a septic tank and a drain field, also known as a leach field, disposal field, or a soil absorption system. The entire system is connected by pipes, and a sewer pipe connects the home or business to the septic system. The Septic Tank The main function of the septic tank is to collect household wastewater, which wastewater to separate from the liquids. Inside the tank, solids lighter than water—such as greases, oils, and, sometimes, other solid materials like toilet paper—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. An outlet baffle in the septic tank is positioned to allow only the partially treated liquid waste in the middle layer to flow out of the tank for further treatment. The layers of scum and sludge remain in the septic tank where bacteria found naturally in the wastewater work to break the solids down. This process takes place anaerobically, or without the presence of oxygen, and gases produced from the decaying solids are vented back through the sewer line and released, usually through a plumbing vent located on the roof of the house. The sludge and scum that cannot be includes water from the toilet, referred to as black water, and water from the bathtub, showers, sinks, and laundry, which is known collectively as gray water. The septic tank treats the wastewater naturally by holding it in the tank long enough for solids and liquids to separate.



Components A typical septic system has four main components: A pipe from the home, a septic tank, a drain field, and the soil. Microbes in the soil digest or remove most contaminants from wastewater before it eventually reaches groundwater. Pipe from the home All of your household wastewater exits your home through a pipe to the septic tank. Septic tank The septic tank is a buried, watertight container typically made of concrete, fiberglass, or polyethylene. It holds the wastewater long enough to allow solids to settle out (forming sludge) and oil and grease to float to the surface (as scum). It also allows partial decomposition of the solid materials. Compartments and a T-shaped outlet in the septic tank prevent the sludge and scum from leaving the tank and traveling into the drain field area. Screens are also recommended to keep solids from entering the drain field. Newer tanks generally have risers with lids at the ground surface to allow easy location, inspection, and pumping of the tank. Drain field The wastewater exits the septic tank and is discharged into the drain field for further treatment by the soil. The partially treated wastewater is pushed along into the drain field for further treatment every time new wastewater enters the tank. If the drain field is overloaded with too much liquid, it will flood, causing sewage to flow to the ground surface or create backups in plumbing fixtures and prevent treatment of all wastewater. A reserve drain field, required by many states, is an area on your property suitable for a new drain field system if your current drain field fails. Treat this area with the same care as your septic system. Soil Septic tank wastewater flows to the drain field, where it percolates into the soil, which provides final treatment by removing harmful bacteria, viruses, and nutrients. Suitable soil is necessary for successful wastewater treatment.


Design The design of a septic tank aims at: • producing a tank in which wastewater is contained for long enough for the maximum removal of suspended solids; • preventing suspended solids from being discharged with the effluent; • providing sufficient space for the sludge and scum to accumulate between desludging; • ensuring that no blockages are likely to occur; and • provide adequate ventilation for gases. Tank volume There are many different formulae in use for calculating the size of a septic tank. They produce widely differing results. The reason for the difference is not completely clear but it can be partly attributed to the broad range of environments in which septic tanks are expected to work. For example, will a septic tank in some countries, where the climate is cool and water consumption high, function in the same way as one in other countries where the temperature is much hotter and water consumption per capita is generally lower? There are also variations in desludging frequencies. This may also be because many of the formulae in use were developed many years ago and there is little reliable evidence to support their accuracy. The method presented here was developed at UK some years ago by synthesizing the results produced by other formulae then being used.


There are three divided spaces within a septic tank: A. Space for the retention of the volume of clear liquid (settlement zone). B. Storage space for sludge and scum. C. Ventilation space. Each is designed separately and the results combined to give the total volume.

Tank components Except for small domestic units, the structural components of septic tanks should be designed professionally. The floor, cover and possibly the walls may need to be reinforced, requiring the skills of a structural engineer. Base In small tanks, the base is usually made of unreinforced concrete about 100 to150 mm thick. . This is thick enough to withstand the uplift pressure when the tank is empty and act as a foundation for the walls. In larger tanks the floor will have to be reinforced to carry the imposed loads and


prevent cracking. Walls Walls are made of bricks, blocks, stone or concrete. It is conventional to make the walls watertight, but in most cases the effluent is going to be disposed of underground so there should be no problem with allowing some of it to escape through the walls of the tank. Check that there is no potential for pollution of nearby water sources or the water table is not so high as to cause groundwater to flow into the tank rather than effluent out. If there is a potential for groundwater pollution and the tank effluent is being discharged to a sewer for disposal elsewhere then the tank should be made watertight. The space between the walls and the sides of the excavation should be filled with a granular material such as fine gravel. Cover The tank cover is usually made of concrete. Access holes with covers should be provided over the inlet, outlet and dividing walls to allow for desludging and maintenance. In large tanks the cover may be made of a series of removable slabs that are light enough so that each can be removed manually. Inlet and outlet The construction of the inlet and outlet is critical to the performance of the tank. The wastewater must enter and leave the tank with the minimum of disturbance (to optimize settlement of the solids) and be easy to maintain. For smaller units (serving one or two families) a ‘tee’ piece design is the most appropriate. In larger units the outlet tee piece should be replaced by a weir and scum board plate. The scum board is to prevent the floating solids from being washed into the outlet pipe. Do not use a weir on the inlet, as it will become fouled with large solids. Dividing wall The dividing wall must have a facility for allowing the effluent to pass from one compartment to another. This can be achieved either by installing tee pieces through the wall at water level or leaving holes (slots) in the wall. In either case, the openings should be designed to minimize turbulence by ensuring the average velocity through them does not exceed 0.1m/s. The openings must be in the middle of the settlement zone, not in the sludge or scum zones. Ventilation


The decomposition of the organic wastes produces gases and a route must be provided for their escape. The simplest option is to allow the gases to escape along the inlet wastewater pipe. This can only be used if the pipe system is fitted with a ventilation pipe at its highest point. The other option is to fit a screened ventilation pipe in the roof of the tank. The gases given off by septic tanks have a strong smell so they should be vented above normal head height.

Operation Preventing problems To avoid blockages and make emptying easier, only toilet paper and faeces should be flushed into a septic tank. Large amounts of chemicals (including bleach) should not be disposed of into the tank as this will kill the bacteria that are digesting the sludge. Sludge removal Sludge should be removed at regular intervals. This should be carried out from the surface as the tank itself will lack enough oxygen for people to breathe. Even though a septic tank can be left for several years without desludging, a regular programme will prevent it from neglect. If the sludge is more than a third of the tank volume, then the tank needs emptying.



Some sludge should be left in the tank after removal to ensure the right bacteria are present to digest faeces.


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· 2020. 4. 30. · The septic tank is a buried, watertight container typically made of concrete, fiberglass, or polyethylene. It holds the wastewater long enough to allow solids