Elimination of COD, microorganisms and pharmaceuticals from sewage by trickling through sandy soil below leaking sewers

Abstract

To simulate the filtration and/or degradation of trickling sewage from leaky sewers through the non-water-saturated underground, sewage was trickled through sand of 0.4–2 mm from the Rhine valley in glass columns of 125 cm length. For the same sewage the chemical oxygen demand (COD) removal was almost independent of low or high trickling rates. The COD removal efficiency varied, however, from 67% to 79%, for sewage from rain and dry weather periods, respectively. The water content of the moist sand increased from initially 80 ml kg −1 with increasing sewage trickling rates to 108 ml kg −1 sand. It remained at 108 ml kg −1 at higher trickling rates higher than 600 ml d −1. Analyses of effluent of five consecutive 25-cm soil columns revealed that about 50% of the initial COD were filtrated off on top of the sand or degraded in the uppermost 25 cm at varying trickling rates. Another 6–12% of the COD were removed in the following 25–50 cm of sand, whereas almost no further COD removal was seen in the subsequent two or three 25-cm columns. The COD elimination during trickling of sewage through the segmented column (interrupted random flow) was slightly better than that in the non-segmented column. Total and faecal coliform bacteria decreased faster with increasing trickling depth than that of total aerobic or anaerobic bacteria. After a filter/degradation stretch of 125 cm elimination of all bacteria reached 96.2–99.9%. The sewage contained low concentrations of at least 10 different pharmaceuticals or X-ray media. During trickling of sewage through sand, elimination of these compounds by adsorption onto sand and/or biodegradation varied from a complete removal, e.g. Ibuprofen or Naproxen, to almost no removal for several X-ray contrast media. Some of the medicals were removed as effectively as during conventional sewage treatment.