Non-Stormwater Fact Sheet: Septic Systems

View an Annotated Septic System Bibliography



Septic systems provide a means of treating household waste for those areas that do not have access to public sewer or where sewering is not feasible. In the state of Maryland, over eighty percent of the land developed in the last decade has been outside the sewer and water "envelope" (MOP, 1991). Currently it is estimated that twenty-five percent of the population of the United States rely on onsite wastewater systems to treat and dispose of their household waste. Of that number, about ninety-five percent of the disposal systems are septic tank systems.

Managing septic systems requires regular maintenance, proper installation and siting, and the detection and correction of existing failing systems. A failing septic system is considered to be one that discharges effluent with pollutant concentrations exceeding established water quality standards. Failure rates for septic systems typically range between one and five percent each year (De Walle,1981) but can be much higher in some regions (Schueler, 2000) (for more information see "Microbes in Urban Watersheds: Concentrations, Sources and Pathways," Article 17 in The Practice of Watershed Protection). Failure of on-site disposal systems can be due to a number of causes including unsuitable soil conditions, improper design and installation, or inadequate maintenance practices. Improperly functioning septic systems are recognized as a significant contributor of pollutants (especially nitrogen) and microbiological pathogens and dispense more than one trillion gallons of waste each year to subsurface and surface waters (NSFC, 1995). Identifying and eliminating these failing septic systems will help control contamination of ground and surface water supplies from untreated wastewater discharges.


Conventional septic systems are used throughout the United States and are the wastewater treatment method mostly commonly selected for those areas without public sewer systems and treatment plants. In areas without sewer systems, there are a number of factors that should be examined to determine if conventional septic systems are the right treatment choice. The first is the size of the lot where the system is installed. Conventional septic systems have a relatively large lot size requirement to allow for even effluent distribution across the drainfield. A second factor is the soil type within a region, which influences the ability of the soil to purify effluent and allow the effluent to percolate. Other conditions which can affect septic system applicability include: separation distance from the water table and bedrock, topography, flooding frequency, density of development, and distance to streams or shorelines.

Siting and Design Considerations

The best way to prevent septic system failure is to ensure that a new system is sited and sized properly and to employ appropriate treatment technology. Septic systems should be located to ensure a horizontal distance between surface waters and vertical separation to groundwater. Setback requirements are determined by each state or region regarding the vertical and horizontal distances that soil absorption field must be located from building foundations, property boundaries, water supply wells, and other surface waters. The distances between septic system components and man-made and natural water supplies will vary according to local site factors such as soil percolation rate, grain size, and depth to water table. The most effective siting distances for efficient on-site wastewater disposal are determined by doing individual site assessments prior to installation. For more information see "Dealing With Septic Systems Impacts," Article 123 in the Practice of Watershed Protection.

The proper sizing of a system is necessary to avoid hydraulic overloading. Overloading a system can cause the system to back up or can force waste through the septic tank before it receives adequate treatment (Perkins, 1989). Overloading can result in anaerobic conditions in the drainfield and might not give solids time to settle out before being pushed through the system.

In some cases, modifications to septic systems may be necessary in order to ensure proper treatment of wastewater discharges. The size of the septic drainfield must be enlarged in cases were soil permeability is low, steep slopes are present, or where increases in daily sewage flow is expected. Limiting factors such as inadequate lot size, limited separation distances, and the presence of problem pollutants such as nitrogen may require the use of alternative on-site disposal systems such as mound or recirculating sand filters. Selecting the right system to handle site specific problems often decreases the likelihood of septic failure. Systems can be designed to control pollutants such as nitrogen and phosphorus (Denitrification Systems or Aquaculture System ) or as retrofits for conventional systems that were inadequately sited or sized (Alternating Bed System, Mound System, Pressure Distribution (Low Pressure Pipe) System, Sand Filter System, or Constructed Wetlands).

Proper siting and postconstruction inspection will work to prevent new systems from failing, but planning for existing systems is needed as well. A septic system management program of scheduled pumpouts and regular maintenance is the best way to reduce the possibility of failure for currently operating systems. A number of agencies have taken on the responsibility for managing septic systems and Table 1 provides some examples of programs and how they seek to control system failures.

Table 1. Examples of Septic System Management Programs
(from USEPA 1993; CWP 1995)

Georgetown Divide Public Utilities (CA)

Approximately 10% of agency's resources are allocated to septic system management
Provides comprehensive site evaluation, septic system design, and makes inspections during construction
Conducts scheduled post-construction inspections
Homeowners pay $12.50 per month for services

Stinson Beach County Water District (CA)

Monitors septic system operation to identify failures
Detects contamination of groundwater, streams, and sensitive aquatic systems from septic systems
Homeowners pay $12.90 per month, plus cost of construction or repair

Puget Sound Water Quality Authority (WA)

Member jurisdictions have established revolving loan funds to provide low interest loans for repair of failing septic systems

Chesterfield County (VA)

Private pumpers submit form to county, and county maintains database tracking pumpout
Every five years county sends residents notification for pumpout requirement
County contracts to have pumpout performed if owner does not comply and can fine or back charge to owner.


Field screening, which can pinpoint areas where more detailed on-site inspection surveys are warranted, should be used in these programs designed to address failing septic systems. There are several good references available discussing field screening techniques for identifying sources of contamination (Lalor and Pitt, 1999; Center for Watershed Protection, 1999). However, there is not much information available dealing with specific techniques for identifying existing individual septic systems that might be failing.

Two field screening techniques that have been used with success at identifying possible locations of failing septic systems are the brightener test and color infrared (CIR) aerial photography. The first involves the use of specific phosphorus-based elements found in many laundry products-often called brighteners- as an indicator of the presence of failing on-site wastewater systems. The second technique uses color infrared (CIR) aerial photography to characterize the performance of septic systems. This method has been found to be a quick and cost-effective method for assessing the potential impacts of failing systems and uses variations in vegetative growth or stress patterns over septic system fieldlines to identify those systems which may potentially be malfunctioning. Then a more detailed on-site visual and physical inspection will confirm whether the system has truly failed and the extent of the repairs needed. These inspections may be carried out by county health departments or other authorized personnel.

Once a septic system has been identified as failing, procedures must be in place to replace that system. The cost to replace a septic system typically ranges between $3,000 and $7,000 per unit (NSFC, 1999) but costs vary significantly depending on site conditions and geographic location. Various methods have been used to finance septic system replacement, including money from state revolving funds or from local utilities through user fees.


Septic systems can have numerous impacts on the quality of ground and surface water supplies. Improperly located or failing systems can discharge inadequately treated sewage which may pond on the ground and runoff into surface waters, and inappropriate vertical distances from groundwater can result in contamination of water supply wells. The wastewater and sewage that may be discharged from failing on-site systems will contain bacteria and viruses that present problems for the health of both humans and aquatic organisms. In addition, excess nitrogen and phosphorus can cause algal blooms that reduce the level of available oxygen in the water and prevent sunlight from reaching desirable submerged aquatic vegetation.

There are also economic impacts associated with failing or overtaxed systems. Beach and shellfish bed closures affect tourism and the vitality of local businesses that rely on fishing and seafood.

The lack of proven field methods for identifying malfunctioning systems other than individual on-site inspection is another current limitation. These individual on-site inspection is very labor-intensive and requires access to private property to pinpoint the exact location of the failing system. Property owners may be reluctant to provide this access and an ordinance mandating inspection authority may be required. In addition, the replacement of failing systems may be limited due to economic situation of septic owners, who due to financial hardship may not have the funding to pay for replacement of their system.

Perhaps the biggest limitation to correcting the impacts of failing septic systems is the lack of techniques for detecting individual failed systems. While visual inspections and dye testing can locate a malfunctioning system, they require access to private property and demand staff time. A number of communities have dealt with access issues by using an ordinance requiring inspection at time of property transfer to pinpoint systems requiring repairs. A key point in dealing with failing septic systems is the need for a stronger emphasis on developing screening techniques for local governments to use to detect and correct improperly operating systems.

Maintenance Considerations

Periodic maintenance of on-site systems is necessary to ensure their proper functioning. Since many homeowners do not employ these routine maintenance practices, it may be necessary for agencies to establish programs to track pumpout and maintenance requirements. Table 1 gives some examples of programs that include maintenance tracking as part of their plan.


The effectiveness of septic systems at removing pollutants from wastewater varies depending on the type of system used and the conditions at the site. The fact is even a properly operating septic system can release more than 10 pounds of nitrogen per year to the groundwater for each person using it (Matuszeski, 1997). Table 2 gives an overview of the average effectiveness for seven types of on-site systems for removing total suspended solids (TSS), biological oxygen demand (BOD), total nitrogen (TN), and total phosphorus (TP). As can be seen, even properly operating conventional septic systems have relatively low nutrient removal capability, and can be a cause of eutrophication in lakes and coastal areas. Communities may elect to require new septic systems to use more advanced treatment technologies to address concerns regarding pollutant loads from improperly functioning systems.

Table 2. Average Effectiveness of On-site Disposal Systems (total system reductions) (Source: USEPA, 1993)
Onsite Wastewater Disposal Practice TSS (%) BOD (%) TN




Pathogens (Logos)
Conventional System 72 45 28 57 3.5
Mound System NA NA 44 NA NA
Anaerobic Upflow Filter 44 62 59 NA NA
Intermittent Sand Filter 92 92 55 80 3.2
Recirculating Sand Filter 90 92 64 80 2.9
Water Separation System 60 42 83 30 3.0
Constructed Wetlands 80 81 90 NA 4.0


The costs associated with detecting and correcting septic system failures are subject to a number of factors including availability of trained personnel, cost of materials, and the level of follow-up required to fix the system problems. Mason County Washington Department of Health Services has conducted on-site sewage inspections for a number of years and has found that dye tests, while reasonably affordable, were too costly to conduct on a regular basis. The estimated cost for each dye test survey conducted was $290 dollars, and the cost for each visual inspection was $95 (Glasoe and Tompkins, 1996). Most of the causes of system failure were found to be relatively easy and inexpensive to repair, and the cost to oversee the repairs was estimated to be $285.

There are also significant cost differences between the various technologies available for on-site wastewater treatment. Table 3 gives both capital and maintenance costs for seven different on-site disposal systems. The installation cost for alternative systems may be higher due to variables like requirements for additional system equipment and the cost of permit approval for the system. Differences in maintenance costs may possibly be due to factors such as increased demand for replacement of treatment media and the lack of available personnel with training in maintenance of alternative systems.

Table 3. Cost of On-site Disposal Systems (Source: USEPA 1993)
Onsite Wastewater Disposal Practice Capital Cost($/House) Maintenance ($/Year)
Conventional System 4,500 70
Mound System 8,300 180
Anaerobic Upflow Filter 5,500 NA
Intermittent Sand Filter 5,400 275
Recirculating Sand Filter 3,900 145
Water Separation System 8,000 300
Constructed Wetlands 710 25


Center for Watershed Protection. 1999. "Resources for Detecting Bacterial Sources." Watershed Protection Techniques, Volume 3. Number 1, April, 1999.

Center for Watershed Protection. 2000. "Dealing With Septic System Impacts," Article 123 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, MD.

De Walle, F.B. 1981. "Failure Analysis of Large Septic Tank Systems." Journal of Environmental Engineering. American Society of Civil Engineers.

Glasoe, S. and M. Tompkins. 1996. "Sanitary Surveys in Mason County. Puget Sound Water Quality Authority." Puget Sound Notes Number 39, June 1996.

Lalor, M. and R. Pitt. 2000. "Use of Tracers to Identify Sources of Contamination in Dry Weather Flow," Article 125 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, MD.

Maryland Office of Planning. 1991. Maryland's Land: 1973-1990, A Changing Resource. Maryland Office of Planning, Baltimore, MD.

National Small Flows Clearinghouse (NSFC). Summer 1995. Pipeline. Vol.6, No. 3.

Perkins, Richard. 1989. Onsite Wastewater Disposal. Lewis Publishers, Inc., Chelsea, MI.

Sagona, Frank. 1988. Color Infrared Aerial Surveys of Septic Systems in the Tennessee Valley Region. Tennessee Valley Authority, Water Quality Branch, Chattanooga, TN.

Sagona, Frank. 1986. "Monitoring and Planning for Onsite Wastewater Disposal Along TVA Reservoirs." Lake and Reservoir Management: Volume II. North American Lake Management Society, Madison, WI.

Schueler, T. 2000. "Microbes in Urban Watersheds: Concentrations, Sources and Pathways." Article 17 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, MD.

Texas Water Resource Institute. 1997. Brazos River Authority Uses "Bright" Idea to Search for Failing On-Site Wastewater Systems. Texas Water Resources Institute, Texas A&M University, College Station, Texas.

U.S. EPA. 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. USEPA, Office of Water, Washington, DC.