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Septic Drain Field Design and Maintenance

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Septic Drain Field Design and Maintenance t Manage es m B e n Septic Drain Field Design s t u P r r o a h c p t s i c o e h and Maintenance s P P Authors: Phillip R. Owens, USDA-ARS, Mississippi State, Mississippi; Protecting E. Moye Rutledge, University of Arkansas, Fayetteville, Arkansas Water Quality Defi nition: Typically, the drainfield in the area where the drain- Septic drain fields are consists of three to four field is located. Infiltration trenches that are 18 to 24 is inferred from the tex- Author’s email subsurface systems that re- inches deep with a perforat- ture or may be determined [email protected] cycle human wastewaters. Properly designed systems ed pipe in 12 to 18 inches of crudely by a percolation gravel covered by 6 inches test. A perched seasonal wa- Editing and Design: transmit and store the house- of soil. A properly designed ter table is determined by Forbes Walker hold effluent below the sur- drainfield should distrib- observing the color and uni- Wanda Russell face and allow soils and soil ute wastewater over a large formity of colors in the soil Gary Dagnan microorganisms to clean the enough area to allow it to in- and knowledge of the depth Anne Dalton wastewater before returning it to the hydrologic cycle. filtrate into the soil and be to the regional aquifer. A University of treated. A distribution box grey-colored soil or a soil Tennessee Extension Purpose: is often used to accomplish that has grey mottles (spots) The entire septic system equal distribution. indicates standing water serves as an onsite, house- The soil acts as a natu- during the rainy season. hold-sewage-treatment sys- ral filter and contains or- tem. The drainfield keeps ganisms that help treat Where This Practice human waste below ground, the wastewater. The nu- Applies and trients, such as phospho- away from human contact, Its Limitions: rus (P), in the wastewater Developed by SERA-17, which reduces the risk of Drainfields will function are adsorbed or chemically Minimizing Phosphorus nutrient and disease-caus- properly in a wide range of bonded to the soil particles, Losses from Agriculture ing pathogens from reaching soils in all climatic regions which limits mobility. The http://sera17.ext.vt.edu/ water bodies. within the U.S. Septic sys- pathogens in the wastewater tems are required in areas are typically destroyed by How Does where municipal wastewater the natural populations of treatment systems are not This Practice Work? soil microorganisms. available. A septic system is com- The septic drainfield There are limitations to posed of three parts: a sep- must be properly sized to these systems. Drainfields tic tank, drainfield and soil the soil conditions at each will not function hydrauli- (Fig. 1). location. Each soil should cally in soils with high clay The liquid and solid get a thorough examination or a perched seasonal wa- household waste flows into by a professional prior to in- ter table. The high-clay soils a septic tank, which is typi- stallation. There must be a will not allow the wastewa- cally belowground. The sep- uniform soil color (indicat- ter to infiltrate fast enough tic tank primarily functions ing well-drained conditions) as a separation chamber NRCS and an anaerobic digester. This project was funded in Solid material more dense part under an agreement than water sinks to the bot- with the USDA-NRCS. tom, and less dense mate- rial, such as fats and oils, float to the top. The liquid in the middle of the tank is termed grey water and exits the tank to flow to the sep- tic drainfield. Figure 1. Layout and components of a typical septic system. al guidelines apply. Prior to con- not build structures over the field structing a septic system and or cover the field with hard surfac- drainfield, a permit from the state es such as asphalt. department of health or depart- There is no evidence that sep- ment of environmental quality is tic tank additives improve the commonly required. Apart from performance of septic systems. the septic system material, such Additives are costly and may even as a septic tank, pipe and gravel, a be harmful to the system. licensed professional must evalu- ate the soil and provide a design. Gray soils indicate seasonal saturation References: Then, a licensed septic system in- with water. This soil would be Angoli, T. 2001. Summary of the unacceptable for a septic drainfield staller must be used. Status of Onsite Wastewater With a conventional (septic Treatment Systems in the to remain below the soil surface. tank, drainfield, trench) system, United States During 1998. On the contrary, soils with ex- the average cost of materials and In Proceedings of the tremely high sand content or shal- installation is $3200. When soils Ninth National Symposium low depth to bedrock do not reno- are limiting, alternative systems on Individual and Small vate the wastewater before reach- must be used. The most expensive Community Sewage Systems. ing the aquifer. The soils with a system is a recirculating sand fil- ASAE, St. Josephs, MI. perched seasonal water table near ter, which costs around $6300. the surface will fill with natural Siegrist, R.L. 2001. Advancing rainwater at some times during the Operation and Maintenance: the Science and Engineering year and will not have the capac- Proper drainfield function re- of Onsite Wastewater Systems. ity to accept additional wastewater. quires wastewater with a mini- In Proceedings of the This will cause the wastewater to mum amount of solid material; Ninth National Symposium surface, where it will run off into therefore, the septic tank should on Individual and Small streams or may come into direct be cleaned out every seven to 10 Community Sewage Systems. contact with humans. years. If the septic tank overfills ASAE, St. Josephs, MI. In some cases, alternative de- and solid materials begin exiting signs that involve pretreatment the tank with the wastewater, the USEPA. 2002. Onsite Wastewater of wastewater prior to reaching solids will cause a decrease in the Treatment Systems Manual. STP a drainfield may be acceptable if soil’s permeability, and the drain- EPA/625/R-0‰ 8. Washington, the soil does not provide proper field will fail by surfacing. DC. renovation. Limiting the flow of water from the household may also be For Further Information: Effectiveness: helpful. To lessen water flow, low- The National Small Flows With a properly designed sep- flush toilets and other water-con- Clearinghouse has free or low- tic system, the wastewater will re- serving measures are commonly cost information on septic system main below the soil surface, where used. In alternative systems, year- designs, functions and mainte- it will come in contact with suf- ly maintenance contracts are com- nance. They may be contacted at: ficient microorganisms and soil monly required by the state prior National Environmental complexing agents to be properly to approval. Services Center renovated and returned to the hy- Avoid driving or parking over West Virginia University drologic cycle. However, P remov- the septic drainfield. Heavy ve- P.O. Box 6064 al by adsorption/precipitation is hicles may crush the pipes within Morgantown, WV 26506-6064 highly dependent on the media ca- the drainfield. Also restrain from 1-800-624-8301 pacity. The total P concentration flushing non-degradable objects or http://www.nesc.wvu.edu/nsfc/ in wastewater averages 1-2 g/per- hazardous chemicals down the toi- son/day; however, the data analy- let. These items my clog your sys- The local Department of Health sis of wastewater through soils is tem or disrupt the microbiological or Department of Environmental limited and ranges from 0 percent treatment of the waste. Quality must be contacted for a to 100 percent removal. Trees or shrubs over the drain- permit prior to septic system in- field may cause problems in some stallation. These state agencies Cost of Establishing and septic systems. These plants have maintain a list of licensed pro- extensive root systems, which seek viders, as well as information on Putting the Practice in Place: out the nutrient-rich water, and The laws vary from state to proper drainfield function. thus may clog the system. Also, do state. However, the same gener- The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at 202-720-2600 (voice and TDD). To file a complaint of discrimination write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964 (voice or TDD). USDA is an equal opportunity provider and employer..
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