Abstract
Saprolite, weathered bedrock, is being used to dispose of domestic sewage through septic system drainfields, but the thickness of saprolite needed to remove biological contaminants is unknown for most saprolites. This study developed and tested a simple method for estimating the thickness of saprolite needed below septic drainlines to filter E. coli from wastewater using estimates of the volume of pores that are smaller than the length of the coliform (≤10 μm). Particle size distribution (texture) and water retention data were obtained for 12 different saprolites from the Piedmont and Mountain regions of North Carolina (N.C.). Saprolite textures ranged from clay loam to coarse sand. The volume of pores with diameters ≤10 μm were determined by water retention measurements for each saprolite. The data were used in an equation to estimate the saprolite thickness needed to filter E. coli. The estimated saprolite thicknesses ranged from 36 cm in the clay loam to 113 cm for the coarse sand. The average thickness across all samples was 58 cm. Saprolite thickness estimates increased as silt percentage decreased and as sand percentage and in situ saturated hydraulic conductivity increased. Silt percentage may be most useful for estimating appropriate saprolite thicknesses in the field.
Highlights
While pore size distribution was used to estimate the saprolite thicknesses needed to filter E. coli from wastewater, such measurements are time-consuming and expensive to conduct
We developed and used an equation to estimate the thickness of saprolite needed to situ measured Ksat and thickness of saprolite needed for removing E. coli
The equation was derived from results of prior experiments with saprolite columns, which used simulated wastewater having an E. coli concentration of 1 × 105 CFU/100 mL
Summary
On-site wastewater management systems (OSWMS), commonly referred to as septic systems, are used to treat and dispose of sewage in areas not served by a municipal sewer system. 20% of the households in the US, and one-half of those in North Carolina (N.C.), use these systems to manage their domestic sewage on-site [1]. A conventional OSWMS consists of a septic tank and a drainfield [2]. The septic tank provides primary treatment to the sewage by allowing solids to settle and go through anaerobic digestion. The liquid from the septic tank (referred to as wastewater) containing dissolved and suspended organic materials, microbial organisms (e.g., Escherichia coli (E. coli)), and chemical pollutants are dispersed into the soil through a series of trenches in the drainfield [3]. In the unsaturated and aerated environment below the trenches, some pathogenic bacteria are removed through physical filtration, and the anaerobic bacteria typically die off in the aerobic soil environment [4,5]
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