Nutrient removal from septic effluents as affected by soil thickness and texture

This study evaluated the effectiveness of soils with different textures and thickness to treat BOD, N and P eluted from household septic effluent. The assessments were accomplished by leaching undisturbed soil monoliths of 30, 45 and 60 cm thickness and 25 cm 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 household septic system. Effluent concentrations were monitored daily over a 15 d period for biochemical oxygen demand (BOD), total-N, NH₄–N, NO₃–N and total-P concentrations. The results of the study indicate an alarming frequency of failure to comply with EPA criteria for BOD, total-N and NH₄–N concentrations when using a 30 cm vertical separation distance between the bottom of the drain field and a limiting soil interface. The treatment performance was particularly poor in coarse-textured soils, apparently due to insufficient reactive surface area. Although biomat development over time is expected to improve the treatment for some of these parameters, the high influent levels of BOD pose great concerns for surface and groundwater contamination during the early stages of operation. Fine-textured soils generally provided better treatment efficiency and more consistent compliance with EPA standards for BOD, total-N, NH₄–N and total-P, as well as greater nitrification/denitrification potential. Treatment efficiency and compliance usually improved with increasing soil depth, with the 60 cm thickness providing the most consistent performance and compliance with MDL requirements. Considering that increasing soil thickness requirements 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.