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Role of saturated and unsaturated zones in soil disposal of septic tank effluent

University of British Columbia

The guidelines in British Columbia for the use of the septic tank-soil absorption system (ST-SAS) are very specific in regard to the separation distances between the ground surface and the groundwater table (minimum 1.2 metres), and between the tile field and perimeter drains or ditches (minimum 3.0 metres). A pilot scale experiment utilizing simulated sections of a septic tile field, with zones of saturated and unsaturated soil, was used to evaluate the scientific basis for these guidelines. Septic tank effluent was applied to depths of unsaturated soil, which varied from 0.91 metres (3 feet) to 0.00 metres, at the inlet end of a saturated zone. Samples were taken at distances of 0.61 metres (2 feet), 1.67 metres (5.5 feet), and 2.74 metres (9 feet) in the saturated zone. Measurements were made of total and fecal conforms, chemical oxygen demand, ammonia, nitrate and orthophosphate. Continuous operation of the sections produced effluent with total coliforms generally less than 400 coliforms/100 mL, and fecal coliforms generally less than 200 coliforms/100 mL. Varying degrees of nitrification occurred in the unsaturated zones resulting in relatively high concentrations of nitrate in some of the sections. The removal of orthophosphate was greater than 90 percent in all of the sections,and the removal of nitrogen varied from 25 to 90 percent. Reductions in measured influent parameters were substantial in all of the sections. This supports the 1.2 metre separation distance in the guidelines, and suggests that 3.0 metre horizontal separation may be conservative in some cases. Of concern were the high nitrate values observed in some of the sections, which may require consideration of nitrification potential in some soils.

Text

2010-07-10

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http://hdl.handle.net/2429/26301

Civil Engineering

University of British Columbia

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