Dosing of Septic Tank Effluent
to Improve System Performance
By Don R. May, P.E.
Fluid Dynamic Siphons, Inc.
1405 E. Olive Court Unit B
Fort Collins, CO 80524
Periodic dosing of septic tank effluent into a soil absorption field by septic tank siphon
or pump eliminates many of the problems associated with conventional gravity flow systems.
Septic tank-soil absorption systems continue to be an important method of sewage disposal for single family homes, clusters of homes and small communities2
throughout the United States and the world. In fact, about a quarter of the population in the industrialized countries use this method either as a long term or a short term solution to effluent disposal17
. The excessive costs of conventional wastewater facilities, the limitations on funding for local sewer extension projects, and the increased demand for rural building sites are forcing engineers and planers to seriously consider the septic tank and other on-site systems as permanent rather than interim solutions to wastewater treatment and disposal problems in unsewered areas. 17, 12
Historically many septic tank-soil absorption systems have been unreliable. Failures occur because of poor siting, design and construction. Because of this engineers are often reluctant to use this method. Recent research in site evaluation, design and construction techniques has helped to identify the major problem areas and has led to improved performance of septic tank-soil absorption systems.
This paper focuses on a major design problem; effluent distribution in absorption fields 17, 12, 15
. Three distribution methods are presented and compared.
Effluent Distribution in Absorption Fields
Three generic methods of distribution have been identified13
. The simplest and most common method is gravity or trickle flow. With this method, wastewater is allowed to flow by gravity into the absorption field as it is discharged from the septic tank. Each incremental inflow to the septic tank requires an equal outflow into the absorption field. Distribution is usually localized to a few areas within the absorption field resulting in an overloading of the infiltrative surface in these areas. This can lead to groundwater contamination in coarse granular soils due to insufficient treatment9, 10
, or rapid clogging in fine textured soild2, 5
. Many gravity flow systems also experience a crusting phenomenon at the interface of the gravel-filled seepage bed and the underlying soil6
. The effect of the crust is to greatly reduce the infiltration rate into the soil. This may result in surface seepage of unpurified septic tank effluent3
The second and third methods of effluent distribution, alleviate many of the problems associated with the gravity flow system. They use the septic tank to store effluent for periodic discharge into the soil absorption field by a siphon or pump. This process is called dosing and can be achieved by using either a pressurized or a nonpressurized system. The dosing interval is controlled by the liquid level within the tank.
Nonpressurized dosing (commonly referred to simply as dosing) has been evaluated by many intvestigators7, 14
and results indicated that:
- Effluent is distributed over a larger portion of the absorption area12.
- The rest period between doses allows the infiltrative surface to drain12.
- The exposure of the soil-seepage bed interface to air between doses, results in a reduction of crust resistance and build-up.4, 3, 11, 14
- Soil clogging is not as severe as with the gravity flow method. 1, 5, 15, 16
- Localized overloading still occurs.7
The third method, provides uniform application of sewage effluent by using pressurized dosing, and has, the advantages of (1) through (4) and yet is solves the overloading problem of (5) by applying effluent uniformly over the entire absorption area at a rate below the saturated hydraulic conductivity of the soil. This insures adequate treatment by the soil at all times and seems to reduce clogging15
. However achieving uniform application is difficult and can be costly and therefore is recommended only where the other methods are not acceptable12
Figure 1. Effluent distribution performance
for three different systems and the ideal
condition. Adapted from Converse (reference 5).
These distribution systems have been experimentally compared in the laboratory 7, 14
to determine their operating characteristics, and hence advantages and disadvantages. Converse tested the gravity flow, dosing and pressurized dosing systems under similar operating conditions. He found that the dosing and pressurized dosing methods greatly improved the performance of a conventional trickle flow system and would result in dramatic increase in the life of the system7
. Figure 1 shows the comparative performance of the three systems. The ideal performance is represented by a straight line, uniform distribution of sewage effluent along the entire length of the absorption field.
In a similar study Popkin and Bendixen14
experimentally compared the gravity flow and dosing methods of distribution. They found that a vastly improved design and operation of soil absorption systems could be obtained through the use of periodic dosing. In their experiments "relative wetted area" is inversely proportional to absorption efficiency and the effluent loading rate is expressed as "hydraulic load". In figure 2 the 35 doses/week line represents gravity flow or a near continual flow loading rate. The lower wetted area - higher absorption efficiency is attained with a once a week dosing interval.
Periodic dosing of sewage effluent from a septic tank into a soil absorption field by siphon or pump results in improved effluent distribution throughout the field. This eliminates many of the problems associated with the conventional gravity flow systems such as, localized overloading and soil clogging. Implementation of these design principals along with improved site evaluation and construction techniques will make the septic tank-absorption field system an efficient and cost effective solution to many sewage disposal problems.
Figure 2. The effect of different dosing intervals
on absorption efficiency (relative wetted area).
35 doses per week represents gravity flow.
From reference 14.
- Bendixen, T.W., Berk, M., Sheehy, J.P., Uribel, S.R., "Studies on Household Sewage Disposal Systems" Part II Federal Security Agency, Public Health Service, Environmental Health Center, Cincinnati, Ohio, 1950.
- Bouma, J., Converse, J.C., Carlsen, J., Baker, F.G., "Soil Absorption of Septic Tank Effluent in Moderately Permeable Fine Silty Soils", Transactions, American Society of Agricultural Engineers, Vol. 18, 1975, p. 1094-1100.
- Bouma, J., Denning, J.L., "Field Measurement of Unsaturated Hydraulic Coductivity By Infiltration Through Gypsum Crusts", Soil Science Society of American Proceedings, Vol 36, Sept.-Oct. 1972, No. 5, p. 846-847.
- Bouma J., Hillel, D.I., Hole, F.D., Amerman, C.R., "Field Measurement of Unsaturated Hydraulic Coductivity By Infiltration Through Artificial Crusts", Soil Science Society of American Proceedings, Vol 35, March-April 1971, No. 2, p. 362-364.
- Bouma, J., Converse, J.C., Magdoff, F.R., "Dosing and Resting to Improve Soil Absorption Beds", Transactions of the American Society of Agricultural Engineers, Vol. 17 No. 2, March-April 1974, p. 295-298.
- Bouma, J., Ziebell, W.A., Walker, W.G., Olcott, P.G., McCoy, E., & Hole, F.D., "Soil Absorption of Septic Tank Effluent", Information Circular No. 20, Geol. and Nat. Hist. Serv., univ. Ext., Univ. of Wisconsin 1972, p. 235.
- Converse, J.C., "Distribution of Domestic Waste Effluent in Soil Absorption Beds", Transactions of the American Society of Agricultural Engineers, Vol. 17, 1974, p. 295-298.
- Fey, Robert T., "Cost Minded Community Chooses Small Diameter Gravity System", Water & Sewage Works, June 1978, pp. 58-61.
- Green, K.M., "Sand Filtration for Virus Purification of Septic Tank Effluent", thesis presented to the University of Wisconsin at Madison, Wisconsin, 1976 in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
- McCoy, E., Ziebell, W.A., "Ythe Effects of Effluents on Groundwater: Bacteriological Aspects", Proceedings, Second National Conference on Individual Onsite Wastewater Systems, National Sanitation Foundation, Ann Arbor, Michigan, 1975.
- McGachey, P.H., Krone, R.B., "Soil Mantle as a Mantle as Wastewater Treatment System", SERL Report No. 67-11, University of California, Berkeley, 1967, p. 200.
- Otis, Richard J., "Pressure Distribution Design for Septic Tank Systems", Journal of Environmental Engineering Division, American Society of Civil Engineers, Vol. 108, No. EE1, Feb. 1982, p. 123-140.
- Otis, R.J., Converse, J.C., Carlile, B.L., Witty, J.E., "Effluent Distribution", Home Sewage Treatment, American Society of Agricultural Engineers, Publication 5-77, St. Joseph, Michigan, 1977.
- Popkin, Robald A., Bendixen, Thomas W., "Improved Subsurface Disposal", Journal Water Pollution Control Federation, August 1968, p. 1499-1514.
- University of Wisconsin, "Management of Small Waste Flows", U.S. Environmental Protection Agency Publication, EPA-600/2-78-173, Cincinnati, Ohio, 1978.
- Winneberger, J.H., Francis, L., Klein, S.A., McGauhey, P.A., "Biological Aspects of Septic Tank Percolation Systems", Final Report Sanitary Engineering Research Laboratory, University of California, Berkeley, California, 1960.
- Wiuff, Rasmus, "Distribution of Wastewater in Drain Field Pipes", Journal of the Environmental Engineering Division, American Society of Civil Engineers, Vol. 107, No. EE5, Oct. 1981, p. 1009-1024
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