Elimination of Organic Compounds of Sewage or Sewage Fractions in Sandy Soil below Leaking Sewers

Abstract

AbstractTo determine the purification effect of a sandy underground for pollutants from leaky sewers, raw sewage, clarified sewage and the resuspended solids fraction of sewage were trickled through sand columns of 1.25 m length.On average 79 % of the chemical oxygen demand (COD) of raw sewage from dry weather seasons and 62 % of the COD of raw sewage from wet seasons were eliminated during trickling through the sand. During trickling of the soluble sewage fraction 61 % of the COD were eliminated, whereas the COD of the hydrolysed portion of the particulate fraction was eliminated to an extent of 85 %. The non‐hydrolysed residues (26 % of the COD of sewage) remained as a discrete sludge layer on top and in the first 5 cm of the sand, not causing colmation during one year.The mean hydraulic residence time (HRT) of sewage in the sand columns was determined by spiking with potassium chloride and conductivity measurement. It was 24 h initially and after a trickling time of more than 8 months 32 h for the soluble fraction of sewage or 35 h for raw sewage. The increasing HRT of the soluble sewage fraction in the sand columns with trickling time indicated the formation of a water‐retaining biofilm on the sand particles in the columns, which was more pronounced when a greater portion of degradable COD was supplied with the raw sewage as compared to clarified sewage. The HRT of all columns was drastically shorter at flow rates of more than 6.4 L d—1, presumably by preferential, non‐restricted wastewater flow through large pores.