Effect of soil depth and texture on fecal bacteria removal from septic effluents

Abstract

This study evaluated the effectiveness of soils with different texture and depth to treat fecal bacteria eluted from a house-hold septic effluent. The assessments were accomplished by leaching undisturbed soil monoliths of 30, 45, and 60cm thickness and 25cm in diameter, representing the four different textural groups and hydraulic loadings recommended by the Kentucky Health Department, with domestic wastewater effluent collected regularly from a house-hold septic system. Eluent concentrations were monitored daily over a 15 day period for fecal coliform and fecal streptococci concentrations. The results of the study indicate an alarming frequency of failure to comply with United States Environmental Protection Agency (USEPA) criteria for depth to groundwater, when using a 30 cm vertical separation distance between the bottom of the drain-field and a limiting soil interface. The treatment performance was especially poor in coarse-textured soils. Although biomat development over time is expected to improve treatment, the high influent levels of fecal bacteria pose great concerns for surface and groundwater contamination. Fine-textured soils generally provided better treatment efficiency and more consistent compliance with EPA standards. Treatment efficiency and compliance usually improved with increasing soil depth, with the 60cm thickness providing the most consistent performance and compliance with maximum discharge limit (MDL) requirements. The findings of this study document a general inadequacy of the 30cm vertical separation distance to provide effective treatment of septic effluents in Kentucky soils, particularly in coarse-textured soils. Considering that increasing the soil depth thickness may be impractical in many marginal soils, complementary or alternative treatment technologies should be adopted to improve treatment efficiency and prevent further deterioration of the quality of water resources.

Spatial Distribution of Fecal Indicator Bacteria in Groundwater beneath Two Large On-Site Wastewater Treatment Systems

“…Therefore, the vadose zone was on average over 1.6 m thicker beneath Field 2 relative to Field 1 at JWS. Prior studies have shown that bacteria treatment typically improves as the unsaturated zone thickness increases [7,8,12,14,15,26]. Therefore, there was likely more bacterial filtration beneath Field 2, relative to Field 1.…”

“…OWS installed in soils with higher percentages of fine-textured materials such as silt and clay, typically are more efficient at reducing microbial concentrations than OWS installed in sandy soils, due to the increased reactive surface area and residence time [7,14,15]. Research has also suggested that higher wastewater loading rates may lead to decreased treatment [8,[16][17][18] and that OWS that utilize low pressure pipe (LPP) distribution can more efficiently treat wastewater constituents such as bacteria relative to gravity flow effluent distribution systems [16,19,20].…”

Bacteria Transport in a Soil-Based Wastewater Treatment System under Simulated Operational and Climate Change Conditions

“…These results are consistent with other studies in which the removal efficiency of fecal bacteria in the STA was investigated in relation to soil texture and depth (e.g., Karathanasis et al, 2006; Amador et al, 2009). The positive correlation between bacteria removal and depth points to mechanical filtration (straining) processes that are most pronounced in finer textured soils (Keswick and Gerba, 1980; Powelson et al, 1990). Finer textured soils have smaller pores and lower hydraulic conductivity, which promotes the mechanical filtration of bacteria and attachment.…”

Groundwater Quality

“…A first order reliability methods algorithm (FORM) was used to model the fate and transport of benzene in groundwater (Kim et al, 2006b). Removal of nutrients from septic tank effluents was found to be related to soil texture, with coarse grained soils having the poorest removal (Karathanasis et al, 2006a). A minimum thickness of 60 cm was found to provide consistent compliance with EPA standards for BOD, ammonia, and total-phosphorous removal efficiency.…”