Rational Basis for Septic Tank System Designa

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Abstract
About 17 million families in the U. S. A. are served by septic tank and subsurface seepage systems. Rational design of these wastewater disposal systems includes three aspects. First the system must be hydraulically sound. This means that the flow regime and the storage and water carrying capacity of the receiving soil should be measured before design. A soil with a coefficient of permeability of less than 10‐4 ft/min (5 × 10‐5 cm/sec) suggests, for example, that the hydraulic capacity of the system governs the size of the subsurface leaching field. Seasonally high water tables or impervious strata may retard the flow and reduce the quantity of wastewater that can be carried away from the subsurface disposal area. In this case an elevated bed can be designed to increase the potential hydraulic gradient.The second consideration concerns the biological mat in leaching fields. Leaching fields can be designed with higher loadings in soils having a greater coefficient of permeability than 10‐4 ft/min (5 × 10‐5 cm/sec) if increased pretreatment is used. A mathematical relationship was developed for reducing the size of leaching fields for effluents with a BOD5 plus suspended solids less than 250 mg/1. Leaching fields were projected to operate indefinitely. Long‐term loading rates for different soil permeabilities were plotted on a graph which can be used for sizing fields.The third design consideration concerns preservation of the ground‐water quality. The travel of phosphate and nitrogen has been studied by others and by the authors. Concrete sand and silt with an unsaturated thickness of 1.5 feet (38.1 cm) still removed after 2 years of operation 30% of septic tank effluent phosphate and nitrogen. The effluent from the test soils had a concentration of about 13 mg/1 phosphate and 15 mg/1 nitrogen mostly in the NO‐3— N form.

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