Renovation of Septic Tank Effluent in Sand-Clay Mixtures

Sand required by Pennsylvania code for use in elevated sand mounds is not specified by sand mineralogy but does require 5–15% clay by weight. Since this required sandy material is expensive and research elsewhere would suggest that a specific clay percentage is not required, the objective of this research was to test the validity of the clay content requirements and the effect of sand mineralogy on septic tank effluent renovation. Limestone, sandstone, and shale sands were used with clay added to give 0, 3, 6, and 12% clay. Septic tank effluent was applied at the rate of 1.5 cm/day to the surface of each sand-clay mixture for 23 weeks. The effluent was analyzed at regular intervals before and after flowing through 60 cm of the sand for the following 16 chemical and biological variables: pH, chemical oxygen demand (COD), fecal coliforms, fecal streptococci, total Kjeldahl N, NH₄-N, NO₂-N, NO₃-N, (NO₃-NO₂)-N, PO₄-P, total P, Cl, Ca, Na, K, and specific conductivity. All the sand-clay combinations (including the 0% clay samples) removed over 99% of the fecal coliforms and streptococci. The removal of P and COD and the transformation of N were highly efficient in the limestone and shale sands regardless of clay content. Renovation in the sandstone sand, which is a relatively inert material, was increased by the addition of small amounts of clay. It was concluded that sand type influences the renovation of effluent and is more important than the amount of clay. The results of this study indicate that limestone is the most desirable sand type, although long-term dissolution may be a problem, followed by sandstone with a small percentage of clay, and then by shale. Although shale was a relatively efficient renovator of effluent over the time period studied, it is anticipated that the susceptibility of shale to weathering would lead to early clogging.