Changes of Chemical and Biological Parameters in Soil Caused by Trickling Sewage

AbstractThe effect of leaking sewers on soil and groundwater was simulated by trickling sewage through 125 cm sand columns in the laboratory. The filtering effect of sludge and soil was tested and a detailed analysis of the soil and column effluent was performed after 3 months and 18 months. The trickling rates were highly variable. Sewage components that sedimented on top of the sand or were swept into the upper sand layers reduced the trickling rates to almost zero after a few days. However, biogas production by the anaerobic bacterial consortium in the sludge and upper soil layers periodically led to a significant increase of the trickling rates for a while, after the gas bubbles escaped and destroyed the dense sludge structure that restricted trickling with increasing trickling time. The COD elimination averaged 71.7…74 %. The total nitrogen content of the sewage was reduced after trickling through the sand column by 34 %. The column effluent contained mainly Norg and ammonia. Since the sand columns were not explicitly kept anaerobic, some oxygen was apparently available and little nitrite/nitrate was generated. As judged from the COD, TOC, or the total organic solids, the organic matter on the sand grains decreased rapidly and exponentially from top to bottom. This was in accordance with a decrease of the population density that was determined after resolution of the biofilm from the sand at different depth. More culturable bacteria per gram sand at every depth were found in the column that was run for 18 months than in the column that was run for only 3 months. This may be an indication for a less developed biofilm after 3 months in comparison to a more developed, thicker biofilm after 18 months. After trickling of sewage for 3 months the biofilm on the sand grains contained more aerobic bacteria, the biofilm that had developed after 18 months contained a higher portion of anaerobic bacteria.