Transformation of organic matter in a gravity sewer

Abstract

Transformation of wastewater organic matter during 3 hours of transportation in an intercepting gravity sewer was measured. Dissolved and particulate fractions in terms of the specific organic pools: carbohydrate, protein, lipid, and volatile fatty acids were measured as well as dissolved organic carbon (DOC) and chemical oxygen demand (COD). The fate of dissolved organics in bulk water was the focus. Sampling was performed during dry weather periods, taking the residence time into account. The bulk‐water dissolved oxygen concentration was between 0.1 and 5 mg/L but relatively constant during each sampling period. Considerable removal of dissolved organics took place, with dissolved carbohydrate and acetate as the primary compounds removed. In COD units, the maximum measured value of the total changes in carbohydrate, protein, and acetate was 20 mg/L·h at 15°C. The DOC removal rates were slightly higher than what could be explained by the total changes of the dissolved specific pools. The removal of dissolved carbohydrate was dependent on concentration level and temperature. Described as a first‐order reaction, the removal rate (k) for dissolved carbohydrate was 0.58 h−1 and 0.27 h−1 at 15°C and 8°C, respectively. No net removal of particulate matter during transportation was found. There was evidence to conclude that growth of microorganisms took place because of a net removal of dissolved organics. From simple calculations, it was concluded that the possible removal of particles because of sedimentation during dry weather was in the same range as the growth. The observed change in wastewater composition during transportation in a gravity sewer affected the quality and the quantity of the organic matter and may be of importance for the biological nitrogen and phosphorus removal in wastewater treatment because a considerable part of the easy degradable fraction was removed.