No abstract

Many chemical, physical and biological transformations of wastewater in sewer systems may take place and cause significant changes in the composition during transportation. This paper focuses on the methods for measuring changes in wastewater composition (organic matter, dissolved oxygen, nitrogen and sulfur compounds), characteristics for changes in wastewater composition and empirical equations for oxygen uptake rates, sulfide production rates and BOD removal rates in both pressure mains and gravity sewers. Simple and more complicated biofilm models are evaluated for use in sewer systems, but so far no suitable advanced model for sewer biofilms exists, due to a lack of quantitative information about the transformations in such high-loaded multispecies biofilms.

An empirical model for predicting sulfide production in pressure mains (Hvitved-Jacobsen et al., 1988) was evaluated and modified based on results obtained from two intercepting pressure mains located in the Northern part of Jutland, Denmark. Mass balances in pipe influent and effluent were made for volatile fatty acids, VFA (formate, acetate, propionate and butyrate), dissolved COD, DOC and sulfide and biofilm surface rate for sulfide and organic matter were calculated. Relatively high sulfide formation rates were observed at low temperatures (5-12°C). The sulfide production rate strongly depended on wastewater quality in terms of VFA and dissolved carbohydrate concentration. Based on these two sets of observations - wastewater quality and temperature - the original empirical model was modified.

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.

The main objective was to study the anaerobic transformation of organic matter and sulfide production during wastewater transportation. Emphasis was on the transformation of easily biodegradable organic matter in terms of volatile fatty acids (VFA). Samples from two intercepting pressure mains located in the Northern part of Jutland, Denmark, were taken. The concentration of VFA in the wastewater varied considerably during day and night, being typically between 5 and 50 g/m3. A net production as well as a net removal of VFA was seen during transportation in the pressure mains probably depending on the quality and quantity of the organic matter. Typically 85% of the VFA was acetate and 10% propionate; the remainder was formate, n-butyrate and iso-butyrate. Observed sulfide production rates from the sewer biofilms were typically 0.05 and 0.005 g S/m2 h at 12 and 4 °C, respectively. A high sulfide production rate corresponded with a high VFA concentration. TOC removal was observed in the pressure mains.