I. Description

Wastewater is collected in the building's plumbing system and conveyed outside by gravity to the buried septic tank. Within the septic tank, gravity causes the solids to sink to the bottom of the tank, and the oils/grease and scum float on the surface of the liquid in the tank. This is called primary treatment. Effluent from the septic tank is then conveyed either by gravity flow or pumping to either further treatment/pretreatment processes, or final treatment and disposal in a subsurface soil disposal system. Some anaerobic digestion of organic solids typically occurs in septic tanks, with the rate dependent on the waste stream and conditions in the tank.

Septic tanks typically have outlet piping structures ("tee" outlets) which allow wastewater to exit by gravity at a liquid elevation which should be several inches below the liquid surface, and well above the solids/sludge level in the tank so as to prevent solids and grease/scum from leaving the tank. Occasional inspection of the sludge levels in tanks, or regularly scheduled pumping intervals, are thus important for maintaining the proper functioning of septic tanks.

II. Common Modifications

Septic tanks are most commonly constructed of precast concrete, but may also be constructed of cast-in-place concrete, fiberglass, or polyethylene. If tanks are constructed of lightweight materials, such as fiberglass, it is important to "anchor" the tank in place with concrete or by other means if there is any indication of high groundwater which might cause flotation of the tank.

During the past five to ten years at least two different types of proprietary filters have been developed for use in septic tanks. These have become increasingly used to filter the effluent prior to discharging either to the disposal field, or further treatment processes. One type of filter uses a series of plastic trays, and another filters the effluent through a series of long tubes and screens as the effluent flows upward through the outlet pipe of the septic tank. These filters can provide for enhanced solids removal, with associated BOD reduction. Manufacturers recommend that the filters be cleaned once every two to three years.

III. Technology Status

Septic tanks are used by 25 to 30 percent of the residences in the United States. It is a well-developed technology.

IV. Applications

Septic tanks can be used by individual residences and establishments for providing primary treatment of wastewater prior to (1) further treatment/pretreatment, (2) final onsite subsurface treatment and disposal, or (3) conveyance to a centralized collection and treatment system for final treatment and disposal.

V. Limitations

If site conditions are such that primary treatment is not sufficient prior to final onsite subsurface disposal, then additional treatment will be necessary if the effluent is to be disposed of onsite.

VI. Typical Equipment/Number of Manufacturers

Septic tanks are supplied locally from several sources.

VII. Performance

Performance of septic tanks is a function of proper sizing, design, and installation, as well as use which is in keeping with the design assumptions. For typical residential wastewater, primary treatment by septic tanks can provide for approximately 40 to 60% removal of both BOD5 and TSS if the tank is sized for a detention time of about 3 to 4 days. Some organic nitrogen removal occurs through the solids removal process.

VIII. Residuals Generation

Numerous studies have been conducted to determine sludge accumulation rates for septic tanks under various conditions, and to develop recommendations for pumping (sludge/septage removal) intervals. Results and recommendations from these studies tend to vary widely. U.S. EPA literature generally recommends that septic tanks be pumped at least once every 2 to 5 years.

IX. Overall Reliability

Operating without mechanical equipment, and if properly constructed and installed, septic tanks can have service lives in excess of 20 years and be considered highly reliable.

X. Operation and Maintenance Requirements

As discussed above under "Residuals Generation", septic tanks should be pumped at an average frequency of 2 to 5 years, in order to prevent too much sludge and solids build-up.

XI. Potential Environmental Impacts

Table 6-1 from the EPA Design Manual for Onsite Wastewater Treatment and Disposal Systems is provided on the following page which shows typical concentrations of key parameters for effluent from residential septic tanks. A thorough site evaluation must be conducted in order to determine the appropriate level of pretreatment prior to final onsite subsurface disposal. Septic tanks can only provide for primary levels of treatment. Thus, some type of further treatment will need to be provided for if the soil and subsurface conditions at the site are such than ground or surface water may be adversely impacted by applying typical septic tank effluent.

XII. Energy Consumption

Septic tanks do not require power consumption. Wastewater drains by gravity from the house to the tank.

XIII. Costs

Costs for septic tanks depend upon a variety of factors including subsurface site conditions, location of and access to the site, and the type of tank. Costs of tanks, including installation, typically range from about $1.00 to $4.00 per gallon of tankage.

Current rates for pumping septic tanks are from about $150 to $200 for tanks less than 2,000 gallons. If a tank is pumped once every 3-1/2 years, the cost for maintaining the tank will be about $50 per year, with a pump and haul cost of $175.

Costs summary:

Capital: Tank (1,000 gallons) installed,	$2000
O&M: Pumped once every 3-1/2 yrs.,	$4.17/month
20-year NPW (not incl. design/permitting costs),	$2,498.54

XIV. Aesthetic Considerations

Septic tanks are buried below grade, and usually do not have significant visual impacts on the site.

XV. State and Local Acceptance

Septic tanks are the most frequently used pretreatment unit for the final onsite treatment and disposal of residential wastewater.

XVI. References

1. Anderson, J.L., R.E. Machmeir, and M.P. Gaffran, "Performance of Gravelless Seepage Trenches in Minnesota, Summer Meeting, American Society of Agricultural Engineers, June 1983.

2. Fritton, DD., W.E. Sharpe, A.R. Jarrett, C.A. Cole, and G.W. Peterson, "Restoration of Failing On-Lot Sewage Disposal Systems", U.S. EPA MERL, 1984. Summarized in: Sharpe, Cole Fritton, "Restoration of Failing On-Site Wastewater Disposal Systems Using Water Conservation, Journal of the Water Pollution Control Federation, Vol. 56, No. 7, pp 858-865.

3. U.S. EPA, "Onsite Wastewater Treatment and Disposal Systems Design Manual", EPA/625/l-80-0l2, October 1980.

4. Small Flows newsletter, Volume 9, No. 3, Summer 1995, National Small Flows Clearinghouse, West Virginia University, Morgantown, West Virginia.

5. National Small Flows Clearinghouse, West Virginia University, Morgantown, West Virginia; Publication WWPCGN28.

The above concentrations are for typical residential dwellings equipped with standard water-using fixtures and appliances (excluding garbage disposals), generating approximately 45 gpd. All values are for raw residential wastewater, and not effluent from septic tanks.

Coliform "concentrations" are presented in organisms per liter.