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Septic System Filter Fields: Stress-Period Loading as a Design Factor

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E.M. Rutledge,
Professor and Research Specialist, respectively,
Department of Agronomy,
Fayetteville, AR 72701

M. A. Gross
Associate Professor,
Department Civil Engineering,
Fayetteville, AR 72701

P. R. Owens
Professor and Research Specialist, respectively,
Department of Agronomy,
Fayetteville, AR 72701

University of Arkansas

The objective of this report is to outline a morphological technique for designing septic tank filter fields and to report on the performance of fields designed by this approach. Filter fields, like any structure must be designed to withstand some maximum stress or limiting factor. The limitations on hydraulic loading due to crust formation at the grave-soil interface is widely recognized and included in most design approaches. Monitoring of filter fields in soils with naturally occurring perched seasonal water tables indicated that during the wetter parts of most years the effluent rose in the trench an din the soil adjacent to the trench. In this situation, the climatic load (infiltrated precipitation minus evaportranspiration) plus the effluent load exceeded a horizon’s ability to transmit. Under these conditions, the filter field’s ability to withstand a hydraulic stress period depends on its ability to store effluent, both in the trench and in the soil. Soils with naturally occurring perched water tables store free-water during hydraulic stress, filter field, whichever is most limiting.

Using redoximorphic features the perched seasonal water tables were divided into three duration groups; brief, moderate and long. Our data indicated we should store the effluent from the house for 18 days above a perched seasonal water table of moderate duration. We assumed 6 days above a brief and 36 days above perched seasonal water table of long duration. For soils that do not readily exhibit redoximorphic features, clay content is used as a backup. We developed guides to estimate hydraulic conductivity (utilizing texture, structure and mineralogy) which is needed to estimate the effluent storage volume. Thus, by estimating the depth from the soil surface and duration of the perched seasonal water table, the hydraulic conductivity, and the daily effluent load, a filter field can be designed and sized.

The perched seasonal water tables are deep enough in some soils that they do not influence the loading rate because the crust is the limiting factor. In other soils the seasonal water tables are so near the surface that adequate storage can not be provided. This approach is for parallel distribution, not serial, since storage is much greater with parallel distribution. It also assumes that the distribution system can raise the effluent to the soil surface.

The Arkansas Department of Health records indicate 190 system designed on the storage-crust approach were installed in the last few years. Information to date indicates all are functioning well. It is noteworthy that Arkansas estimates 150 gal/day of effluent output from each bedroom. This is likely an over estimate, especially for the 3 bedroom and larger houses. Thus, it can be concluded that the information to date indicates the storage-crust design approach works well with some over designing.