Function of soil-based on-site wastewater treatment systems - Biological and chemical treatment capacity

Sammanfattning: On-site wastewater treatment systems are among the main Swedish anthropogenic sources of nutrients causing euthropication of the Baltic Sea. Among on-site systems in Sweden almost half have septic tank treatment followed by a soil-based system, in which the wastewater is treated through soil filtration. In this study a soil based technique for on-site wastewater treatment is studied where wastewater is filtered through a sand filter. Composite samples of influent and effluent at two sand filters in the area of Stockholm are sampled to determine their chemical and biological function and to compare their treatment capacity to requirements. Parameters within the scope of the study are tot-P, NH4-N, DOC, pH, turbidity and dissolved oxygen. Biological function was considered to be good in both systems as nitrification was high and the effluent had sufficient levels of dissolved oxygen suggesting aerobic conditions. Prevailing aerobic conditions in the sand filters would also indicate good reduction of organic substances which is the case for DOC as it was reduced by above 85 % for one site and almost 70 % for the other site. The overall high reduction of organic micropollutants in the systems, reported in another study, also suggests that biological function when it comes to reduction of organic substances is good. On the other hand, chemical function, with respect to reduction of phosphorus, was not sufficient as none of the systems fulfilled the requirements from HaV for normal or high protection level. In the systems tot-P was reduced by 42 and 54 % respectively. A drawback with the method approach used in the study is that the obtained reduction results only can represent the actual situation if variations in incoming and outgoing flow, variations in influent concentrations and magnitude of dilution of effluent compared to daily wastewater load are small. As these are unknown in this case it adds uncertainty to the results.