Nutrient and Pathogen Removal in a Passive On-Site Sewage Treatment and Disposal System (OSTDS) with a Recirculation Filtration Tank

Groundwater contamination due to the nutrients from a septic tank system is a vital concern in environmental health. Active OSTDS also know as on-site wastewater treatment is dependent on the use of pump to sustain the aerobic condition in the process and promote the nitrification which might not be sustainable in terms of energy saving. Experiments have been conducted over the past three years to find a cost-effective, passive on-site wastewater treatment process to improve nutrient removal. The inclusion of recirculation sand filter (RSF) might be a suitable intermediate process to trigger the proper nitrification/denitrification process between the septic tank and the drain field. This option may be adopted for these existing traditional septic tank systems as a retrofit project. Yet the science question as to how effectively the nitrification and denitrification can be fostered in an RSF remains unclear. Three types of medium: fine sand, course sand, and green sorption media (i.e. denoted as options 1, 2 & 3 in this study) were used to assess the performance of the RSF. In particular, phosphorus removal from the wastewater was one of the key concerns, and the capability of using limestone for phosphorus removal was also investigated and confirmed. It can be concluded that fine sand with limestone mixture had better performance for nutrient removal if clogging issue can be overcome by using a grinder pump when dosing. On average, the septic tank and RSF system with fine sand removal efficiencies were: 60.54% ammonium, 49.48% total Kjehldahl nitrogen (TKN), 42.57% total nitrogen (TN), 92.06% soluble reactive phosphorus (SRP) and 87.16% total phosphorus (TP). The E. Coli removal efficiency by RSF was 99.9% in all cases. However, the addition of green sorption media is not favored due to insufficient hydraulic residence time (HRT). If the size of the RSF can be increased, the use of green sorption media in combination with limestone might be beneficial.