Regular septic systems or On-Site Wastewater Disposal Systems (OSWDS) service almost 30% of the single family residences in the state of Virginia, and with more than 40,000 new permits applied for annually the potential for ground and surface water contamination from these systems is increasing (Stolt & Reneau, 1991). Septic systems are commonly referred to as subsurface soil absorption systems. "Subsurface soil absorption systems are sewage disposal systems which utilize the soil to further treat and dispose of effluent from a treatment works in a manner that does not result in point source discharge and does not create a nuisance, health hazard, or ground or surface water problem" (VDH, 1982).
Septic tank. Photo by E. Royer.
Devices which may be called septic tanks are known to have existed as far back as 2000 B.C. The first device like the septic tank as we know it today was patented in France in 1881 by M. Mouras. The device was called the Mouras Automatic Scavenger and described as a mysterious contrivance that transformed all excrement into a homogeneous solution. The device was believed to be self emptying and continuous in its working.
One-piece septic tank being lowered into position. Photo by W. Keeling.
OSWDS commonly consist of a septic tank, a distribution box, and drain or discharge lines, and the subsurface absorption or drainfield. The septic tank is usually made of concrete or less commonly fiberglass. The tank is designed to retain the solids from the waste that are flushed down the toilet and/or disposal. The tanks typically have baffles placed inside to force solids to settle on the bottom of the tank. Liquid and gaseous phase waste are allowed to pass through a septic tank and into an absorption field. The tank will usually have ports in the top in order that the tank can be inspected and the solids removed. The solids inside the tank are in an anaerobic environment and do undergo some decomposition by anaerobic microorganisms. However, it is advisable and usually necessary to pump the solids from the tank on a timely basis. This may be anywhere from 5 to 10 years depending on how heavily the system is used and what local and state regulations stipulate. This pumping would be accomplished by a septic tank pumping truck that would haul the solids plus what ever liquid waste where in the tank at the time of pumping to a conventional sewage treatment facility or a sewage lagoon.
Drainfield trench being excavated. Photo by W. Keeling
The distribution system frequently starts with the distribution box. This box is used to divert the effluent into the distribution lines. The distribution lines are typically corrugated and perforated plastic pipe. This pipe is surrounded by gravel to insure an aerobic environment and to help prevent clogging of the system by the soil. This provides an opportunity for the effluent to be renovated by aerobic microbial activity.
At the interface of the gravel and the soil a biological mat develops which helps filter out pathogens such as fecal coliforms. This mat consists of dead microbes and dissolved solids in the effluent. After the effluent passes through the mat the soil is utilized as a filtration and treatment system. Many biochemical reactions occur in the soil such as nitrification and denitrification. There is also chemical reactions that occur in the soil such as phosphorus adsorption and complexation reactions.
The major contaminants of concern from a septic system are pathogens and nitrate (NO3-) nitrogen. Pathogens such as fecal coliforms and coliphages are an obvious concern when dealing with ground and surface water because of the potential for the outbreak of water born diseases. Most outbreaks of waterborne decease can be traced to improperly functioning or improperly located OSWDS systems (Hagedorn et al., 1981). The EPA has set the limits for nitrate contamination in drinking water to 10 ml per liter.
OSWDS should be installed in locations that have adequate soil conditions. These include adequate depth to the water table or to bedrock and permeability. Wet soils and soils with too much slope gradient or too low hydraulic conductivities should be avoided. Landscape position is another factor to consider when deciding where an OSWDS should be placed. Soils located in flood plains and on side slopes are typically not suitable for OSWDS.
Drainfield that was installed in a wet soil. Photo by W. Keeling.
Septic tanks should be designed to remove close to 100 percent of solids from effluent. The tank design should provide:
Permeability is the quality of a soil that allows water and air to move through it. It is the rate at which a saturated soil transmits water. This rate is quantified by the saturated hydraulic conductivity and for all intensive purposes carries the units of inches per hour. Soil permeability's are in part used to describe a soil and its potential use. Permeability is the chief factor of the soil that you have in mind when designing irrigation systems, sizing seepage beds, designing terraces and other conservation practices, and most importantly when designing septic tank absorption fields. In order to classify the permeability of a soil measurements will be made of a saturated soil core to see the amount of water that passes through over a certain time period. See the table at the end of the text for the different permeability classes that soils can have. In order for a soil to be placed in a class a permeability test must be done. The test consists of a very simple apparatus shown in the figure below. In order to test for permeability a percolation test performed where a hole penetrating the soil to a depth of 24" is saturated for 24 hours and then the amount that the water level drops in an hour is recorded and this would tell you your class.
Illustration of percolation test. Drawing by G. Seyrlehner.
The permeability of a soil depends upon certain characteristics of that particular soil. These characteristics are texture, structure, total porosity, size and distribution of pores, and tortuosity (flow path of water) of the soil. As an example say we have a high shrink-swell clay (particle size of 0.002 mm) that has massive structure and a very low pore space percentage (20%-25%), this soil will most likely have a very extremely slow permeability class whereas a sand (0.20 mm) with loose structure and 50% pore space will be in a rapid permeability class. So now we partially have an idea of the factors comprising permeability.
Example Percolation classes. Drawing by G. Seyrlehner.
Once a soil has been classified it can be checked to see if it is suitable for a particular task. As an example when determining the use potential of a soil for a septic system, we would gather data on the soil in question and consult a soil survey interpretations and criteria table (compiled by the United States Department of Agriculture)to find the criteria required for our hypothetical use, lets say a septic system. From the table an ideal soil would be one that is in the moderately rapid class, however a class of moderate would also be acceptable.
The permeability of a soil is directly related to the hydraulic conductivity, K, of a soil. Hydraulic conductivity is a factor based upon the soil media and fluid within the media. In order to do any type of groundwater flow or remediation either the hydraulic conductivity or permeability (can be used to determine hydraulic conductivity) must be known. In conclusion, permeability along hydraulic conductivity are very important factors that must be considered when classifying and determining a soils suitability or when choosing a remediation method.
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Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
