Slanted Soil Treatment System (SSTS) was previously designed for onsite greywater treatment. In this study, several configurations were tested with granitic gravel in order to improve its efficiency. The lowest average removal efficiencies for fecal coliforms, enterococci and E. coli were 1.78, 2.15 and 2.21 log u. respectively and originated from case 3 (length 3 m, width 20 cm, grain size in second box 1-4 mm). The highest removal efficiencies originated from case 1 (length 5 m, width 20 cm, grain size in second box 1-2 mm) with values of 2.66, 2.56 and 2.51 log u. for fecal coliforms, enterococci and E. coli respectively. The average removal of suspended solids varied from 62% (case 3) to 92% (case 1). The comparison of the performances highlighted that the removal of indicator bacteria was more affected by the variations in the characteristics of the SSTS than that of organic matter. Based on these results, a SSTS with a length of 5 m, a width of 30 cm and a grain size of 1-4 mm (to avoid early clogging) is suggested in order to enhance the removal of indicator bacteria. However, for organic matter, further studies are necessary to improve the removal efficiency.

Granit, Greywater, Indicator bacteria, Slanted soil system