Wastewater Treatment by Soil Absorption Systems

In the Federal Republic of Germany, the barely treated wastewater of about 7 million people is percolated into the soil. Groundwater quality has now become a major concern, and investigations into soil filtration as a means of eliminating pollution from household wastewater have been started. Experiments with six laboratory lysimeters (diameter = 0.4 m, height = 1.8 m) are described. Three lysimeters were filled with slightly silty sand, the other three with fine gravel sand. All lysimeters have been in operation for about two years. Initially they were loaded with mechanically treated wastewater from a municipal treatment plant and later with effluent from a septic tank. Hydraulic loading varied from 0.5 to 31 cm/d. Results showed good efficiency for fine gravel sand (aerobic conditions) in removal of BOD5 (98.0 to 99.2 %), COD (88.0 to 92.8 %) and Kjeldahl nitrogen (90.0 to 91.3 %). Elimination of phosphate ranged from 5.7 to 86.0 %. Slightly silty sand (anaerobic conditions) showed the best results in elimination of phosphate (86.0 to 99.3 %) and relatively good elimination rates were achieved for BOD5 (65.0 to 98.2 %) and COD (about 76 %). Elimination of Kjeldahl nitrogen ranged from 39.6 to 68.4 %. The results for the two soils can be summarized as follows: Fine gravel sand: high hydraulic loading capacity (aerobic conditions); high elimination rates of BOD5 and COD; high elimination rates of ammonium by nitrification; very low elimination of total phosphate. Slightly silty sand: low hydraulic loading capacity and, despite this, anaerobic conditions; high elimination rates of BOD5 and barely inferior elimination rates of COD in comparison to fine gravel sand; low elimination rates of ammonium; very high elimination rates of total phosphate.