On-site treatment of septic tank effluent by using a soil adsorption system

A septic tank and soil absorption system (SAS) is a simple means of treating domestic wastewater using the filtration, sedimentation, chemical absorption, and biological characteristics of soil. Data obtained from countries, such as the United States and Australia, where on-site wastewater treatment is common, suggests that SAS have become a main application for on-site treatment. In this research, the performance of SAS was investigated through an outdoor pilot study and two laboratory tank studies. The treatment capacity of the outdoor pilot plant was 1.5 m3/day, and used alternating anaerobic and aerobic units. The objective of the pilot study was to verify the occurrence of nitrification and denitrification in the system. In the tank study, the first setup utilized capillary and siphonage mechanisms to change wastewater distribution within the filter bed. The effect of wastewater distribution on hydraulic loading, pollutant removal, and system reliability were then investigated. In the second setup, three different media (sand, gravel, and soil) were used to treat typical domestic wastewater to assess their removal of nitrogen and phosphorus-rich pollutants (including ammonia, nitrate, and TP). Last, three different hydraulic loadings were examined to investigate the maximum possible treatment loading. The pilot scale SAS experimental results show that through a batch system of anaerobic and aerobic units, the SAS was effective in removing nitrogen by nitrification and denitrification. In the pilot study, ammonia decreased by 76.0%, nitrate increased by 91.8%, and the sum of ammonia and nitrate decreased by 51.3%. Results from the water distribution test show that a minimum grain size of 2mm should be used for the filter bed medium when capillary layers are used to distribute water. The hydraulic loading can be up to 50 L/m2day for a well-constructed capillary and siphonage trench. Results from the test with different filter media show that sand is the best of the three media tested in removing nitrogen and phosphorus. For typical nitrogen and phosphorus concentration of domestic wastewater, the influent concentration had little effect on the rate of reduction when sand and gravel were used. For hydraulic loading, under the maximum loading chosen in this experiment, ammonia nitrogen and phosphorus removal of greater than 90% still could be achieved for sand and soil. We therefore believe that hydraulic loading in SAS can be further increased to reduce the treatment site area. © 2007 ASCE.