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442-903

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*Overview of the Virginia Farm Assessment System

Table of Contents

Introduction
I. Septic Tank/Soil Absorption System
II. Quantity of Wastewater
III. Quality of Wastewater
IV. Collection of Wastewater
V. Treatment System
VI. Additional Treatment
VII. Disposal of Wastewater and Pumpage
VIII. Failing Septic Systems
Glossary
Contacts and References
Worksheet 3

 

Introduction

Household wastewater contains some contaminants that degrade water quality for such uses as drinking, stock watering, food preparation and cleaning. Potential contaminants in household wastewater include disease-causing bacteria, infectious viruses, household chemicals, and nutrients, such as nitrate. Viruses can infect the liver, causing hepatitis or infect the lining of the intestine, causing gastroenteritis (vomiting and diarrhea). If coliform organisms (a group of indicator bacteria) are found in well water, they show that the water is potentially dangerous for drinking and food preparation. Virtually all farmsteads use a septic system or similar on-site wastewater treatment system.

Groundwater and water supplies are least likely to be contaminated if appropriate management procedures are followed A properly installed and maintained system for treating and disposing of household wastewater will minimize the impact of that system on groundwater and surface water. The Virginia State Board of Health has published a document entitled "Sewage Handling and Disposal Regulations." Installation, permitting, servicing, and operation of private sewage systems are addressed in these regulations. All requests for a sewage disposal construction permit must be directed initially to the district or local health department. The district or local health department will evaluate soil, geological, and site conditions to make sure that installation of a sewage disposal system would not create any actual or potential health problem.

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I. Septic Tank/Soil Absorption System

The most common form of on-site wastewater treatment is a septic tank/soil absorption system. In this system, wastewater flows from the household sewer into an underground septic tank.
  • In the tank, waste components separate - the heavier solids (sludge) settle to the bottom and the grease and fatty solids (scum) float to the top.
  • Bacteria partially decompose and liquefy the solids.
  • The septic tank must have a minimum detention time of 48 hours. The minimum size for a septic tank is 750 gallons. The inlet and outlet structures of the tank function as baffles, preventing inlet and outlet plugging and rapid wastewater flow through the tank.
  • The liquid portion (effluent) flows through an outlet to a distribution box where the effluent is directed to individual subsurface trenches in the soil absorption field.
  • The absorption field is usually a series of trenches, placed on the contour, each containing a distribution pipe or tile line embedded in drain field gravel or rock.
  • The effluent drains out through holes in the pipe, then through the drain field gravel and into the soil.
  • The soil filters out remaining solids and pathogens (disease-producing micro-organisms). Excess water and dissolved substances slowly percolate down toward groundwater.

Figure 1 shows a typical household system for wastewater generation, collection, treatment and disposal. Note the lists of options below each part shown in the diagram. You may wish to circle the parts found in your system. The "leakage," "overflow," "infiltration" and "clearwater" components represent possible problems with the system. Unfortunately, these problems are often difficult to recognize. Overflow from systems may be noticed as wet spots, odors and some changes in vegetative cover. Water entry (infiltration and clear water) will be more difficult to detect, involving tracing where floor drains, roof drains, foundation drains and sumps are directing water that does not need treatment into the treatment system. Leakage from the collection and treatment system - as well as infiltration of water into the system through unsealed joints, access ports and cracks - can be very difficult to assess. The flow chart at the bottom of the box follows the flow of wastewaters and sludge through the treatment system.

house diagram
Figure 1. A Typical Household System for Wastewater Generation, Collection, Treatment and Disposal. (Source: National Farm*A*Syst Package)
WATER USECOLLECTIONPRETREATMENTADDITIONAL TREATMENTDISPOSALQuantity*, Quality*Leakage*, Clearwater, InfiltrationSeptic Tank*, Holding Tank, Aerobic SystemSand Filter, Nitrogen Removal, DisinfectionSoil Absorption*, In Ground, Bed, Trench, Mound, At Grade, Pump and Haul* = Elements Illustrated

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II. Quantity of Wastewater

Reducing the volume of wastewater entering the treatment system is important because less flow (volume) means improved treatment, longer system life and less chance of overflow. For septic tanks, less volume lowers costs by reducing the number of times the tank must be pumped.

The quantity of water used depends upon the number of people using the dwelling, how water is used, and maintenance of the water supply system. Average water use in rural households is estimated at 75 gallons per person per day. With low water use fixtures and individual awareness, the life of the subsurface soil absorption system can be lengthened. However, only permanent water saving plumbing services such as low flush toilets shall be considered in reducing the size of the absorption field.

Reducing the volume of water entering the system will also improve the treatment efficiency by increasing the time the waste is retained in the system, thus providing more time for settling, decomposition, aeration, and soil contact.

Consider the following ways to minimize water use:

  • Eliminate non-functional uses, such as flushing toilets to dispose of tissues or other wastes that should be handled as solid waste. Turn off water between uses and fix plumbing fixture leaks. Try to eliminate sources of infiltration into the system; for example, divert roof drains away from the soil absorption field.
  • Consider which actions use the most water. Toilet flushing usually ranks the highest. Low-flow models could decrease water use by more than half. In the United States, 35-40 percent of the population has plumbing codes that require a 1.5-gallon-or-less net flush volume of toilets in all new construction. Composting type toilets allow even greater reductions, but they can presentother waste disposal challenges.
  • Bathing and clothes washing are next in order of water use. For bathing, consider such reduction options as installing low-flow or controlled-flow showerheads and taking shorter showers. For clothes washing, use a suds saver and run full loads. Front loading washers use much less water. When running small loads, be sure to use the reduced water level setting.
  • Modern efficient plumbing fixtures, including 0.5 to 1.5 gallon/flush toilets, 0.5-2.0 gallons per minute (gpm) showerheads, faucets of 1.5 gpm or less, and front loading washing machines of 20 to 27 gallons per 10-12 pound dry load, can result in substantial reduction in residential water use and wastewater generation. These reductions have commonly amounted to between 30 and 70 percent of total in-house water use.
  • In hard water areas, the water softener may be a significant user of water. Proper adjustment and timing of the softener's regeneration mechanism can reduce excessive water use.
  • Keep in mind that your awareness of your family's water use and how each of you can reduce it is as important as the use of water conservation devices.

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III. Quality of Wastewater

Domestic wastewater usually contains relatively small amounts of contaminants - less than 0.2 percent - but their reduction can make a big difference in the efficiency of waste treatment.

Contaminants found in wastewater include:

  • Bacteria and viruses, some of which can cause disease in humans. Most microorganisms are large enough to be removed by settling, or through filtration in beds or soil. Many will die from the adverse conditions or aging in the system.
  • Suspended solids, or particles which are less dense (e.g. grease scum) than water, and can be removed by filtration. Most can be separated from liquid waste by allowing enough time in a relatively slow flowing tank. Filtration beds and absorption systems can be clogged by wastewater high in suspended solids.
  • Oxygen demand. The microorganisms that decompose organic wastes use oxygen. The amount of oxygen required to "stabilize" wastewater is typically measured as biochemical or chemical "oxygen demand." Wastes such as blood, milk residues, and garbage grindings have high oxygen demand. Aeration and digestion processes, in the presence of oxygen and organisms, can cause problems for soil absorption fields, groundwater, streams, and lakes by reducing levels of oxygen in the water.
  • Organic solvents from cleaning agents and fuels may not be degraded or removed through soil treatment and can contaminate groundwater and drinking water supplies.
  • Nutrients. Nitrogen from human wastes and phosphorus from machine dish washing detergents and some chemical water conditioners are the most notable. Nitrate-nitrogen is a common groundwater contaminant, and phosphorus can over-fertilize surface waters such as lakes and rivers.

Consider the following ways to improve wastewater quality:

  • Minimize use of the garbage disposal unit. Garbage disposals use a large volume of water and contribute a large load of suspended solids and organic matter to wastewater.
  • Do not put items down drains that may clog septic tanks (fats, grease, coffee grounds, paper towels, sanitary napkins, tampons, disposable diapers).
  • Do not put toxic substances in drains that might end up in the groundwater, such as solvents, degreasers, acids, oils, paints, disinfectants and pesticides.
  • Do not use chemicals to clean the system. They may interfere with the biological action in the tank, clog the drain field by flushing sludge and scum into the field or add toxic chemicals to groundwater.

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IV. Collection of Wastewater

Leaking pipes or treatment tanks (i.e., leakage losses) can allow wastewater to return to the groundwater without adequate treatment. Infiltration of clear water overloads the septic system and dilutes the wastes. Don't allow water that doesn't need treatment (basement floor drain sumps, foundation drains, infiltration of rain water, roof drainage) to add to your wastewater volume. Divert clear water, which doesn't require treatment, away from the house, and wastewater treatment system.

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V. Treatment System

Septic tanks retain most of the suspended solids (sludge and scum) from wastewater. In the tank, bacteria digest and compact the sludge. The partially treated water moves on to additional treatment or disposal (for example, in the soil absorption field).

Design and construction of septic tanks influence treatment effectiveness of sludge and scum removal. Multiple tanks or chambers in series can improve sludge and scum removal. Gas deflectors and filter screens or inclined-plate settling units help to minimize solids carryover. Tanks should be sized to accommodate at least 24 hours of wastewater flow, while still allowing for sludge and scum retention. Pumping the tank before it is more than one-third filled with scum and sludge improves functioning of the system. This is generally recommended every 3-5 years depending on amount of use. When the tank is pumped, you should also have the baffles checked and also check for tank leaks.

Aerobic (oxygen using) biological systems (packaged systems) provide more extensive treatment of wastewater than the typical anaerobic (no oxygen) septic units, while improving solids separation and reducing sludge volume. These systems are, however, more expensive to operate and maintain and are more subject to problems caused by changes in wastewater quality or environmental conditions.

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VI. Additional Treatment

Aerobic systems, described in the previous section, may be used for additional treatment of septic tank effluent, yielding a better quality effluent suitable for more disposal options.

Sand filters improve the quality of wastewater after septic tank pretreatment. Filters consist of 2 to 5 feet of sand (or other media) in a bed equipped with a distribution and collection system. Wastewater is applied by dosing, and it may be recirculated to improve treatment.

Constructed wetlands improve the quality of the wastewater by using plants to introduce air into the otherwise anaerobic liquid. Organisms associated with the plant roots are also beneficial in reducing levels of many contaminants. Wastewater treated in such systems is generally lower in bacteria, nitrogen, phosphorus, oxygen demand, suspended solids and organic matter.

Nitrogen removal can be achieved through denitrification (conversion of nitrate to nitrogen gas) or ion exchange. Denitrification requires anaerobic conditions in the presence of more decomposable organic matter for bacteria to reduce nitrate to nitrogen gas for removal from wastewater. Denitrification and ion exchange processes are not used extensively at this time, as they are quite expensive to install, operate and maintain.

Disinfection systems kill disease-causingmicroorganisms in wastewater and are used where discharge to surface water is permitted. Chlorine, iodine, ozone and ultraviolet light systems are available for treatment of good quality effluent, such as those from properly functioning aerobic units and sand filters.

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VII. Disposal of Wastewater and Pumpage

Subsurface treatment and disposal using soil absorption is the common practice for household wastewater after pretreatment in a septic tank or aerobic system. There are, however, sites where soil absorption systems are not acceptable because of high or low soil permeability, depth to bedrock or the saturated zone, or other factors. Deep, well-drained, well developed, medium-textured soils (such as silt loam and loam) are desirable soil absorption sites.

Holding tanks collect and store the entire wastewater flow. Disposal is generally done by a licensed contractor who spreads the waste on the land at an approved site or hauls it to a municipal waste treatment facility. Land application of wastewater provides an opportunity to recycle nutrients and to further reduce the contaminant content of wastewater in a safe manner. Disposal of pumpage from septic tanks and other treatment systems on-site should follow similar rules as for wastewater. Sludges are more concentrated than treated wastewater, so lower application rates are recommended.

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VIII. Failing Septic Systems

If you suspect your household wastewater treatment system is backing up or your distribution system is clogged, first contact your plumber or treatment system installer, who may have suggestions for extending the life of your system. Your county health department office should be contacted for permits to repair or replace your wastewater treatment system.
  • Do not use septic tank cleaners that contain degreasing solvents like TCE. They can contaminate groundwater.
  • Do not place more soil over a surfacing soil absorption field; this does not fix the system, and it will eventually surface again.
  • Do not pipe sewage to a road ditch, storm sewer, sink hole, drainage well, stream, or drain tile; this pollutes the water and creates a health hazard.
  • Do not wait for the system to fail before pumping the septic tank. Once a system fails, it is too late to pump the tank.

If existing septic systems are to be used by farm workers or the general public, e.g., retail sales operations, take into account the ability of the present system to handle the increased waste load. Even if the drain field is adequate, this consideration will require at least more frequent pumping of the septic tank.

A properly designed, constructed and maintained septic system can effectively treat wastewater for many years, but requires routine maintenance. For additional information on septic systems, contact your county Extension agent or local health department.

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Glossary

Approved: A site for land application of wastewater or tank pumpage that meets state standards of

Disposal site: the Virginia Department of Soil and Water Conservation.

Clear water infiltration: Entry of water that does not need treatment, such as rainfall or tile drainage into a system, through unsealed joints, access ports, and cracks.

Decomposition: Breaking down of organic wastes, such as sewage, by bacteria and other microorganisms.

Design capacity: Maximum volume of liquid that can be treated in a particular wastewater treatment system. For systems that include subsurface wastewater disposal and distribution, capacity is also based on the soil's ability to accept and treat sewage effluent. In filling out the worksheet, if you don't know the design capacity of your system, use 150 gallons per bedroom per day as an estimate.

Effluent: Liquid discharged from a septic tank or other treatment tank.

Holding tank: An approved watertight receptacle for the collection and storage of sewage.

Off-site disposal: Disposal of wastewater or sludge, as at a municipal treatment plant or approved disposal site.

Scum: Floatable solids, such as grease and fat.

Sludge: Settleable, partially decomposed solids resulting from biological, chemical or physical wastewater treatment.

Contacts and References

For additional information, consult the Virginia Farm*A*Syst directory. For technical assistance, you may contact your local Extension Office or local health department.

Acknowledgements

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Worksheet 3

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View a list of the Virginia Farmstead Assessment System publications

 

Reviewed by Brian Benham, Extension Specialist, Biological Systems Engineering


Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.

Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.

Publication Date

May 1, 2009