Air flow model development and application in a complex combined sewer system.

A steady-state air flow model was developed and applied in a complex combined sewer system in the city of Edmonton, Alberta, Canada. The model solves the continuity at each junction and the momentum equation for the links coupled with dropshaft and other manholes. The dropshaft pressure gradient is computed using the dropshaft equation and air flow rate through manhole pickholes is computed considering the opening as an orifice. A leakage factor is used as a calibration parameter to represent the area through which air can leak from the manholes into the neighborhood. The model uses an iterative solution algorithm with a forward sweep for the continuity and backward sweep for the momentum equation. An underrelaxation is applied to update pressure in each iteration for model stability. The model was calibrated and validated by using the measured air flow rate and manhole pressure in the sewer network, with good results. An analysis of the air flow characteristics shows that a significant amount of air is brought into the system due to a large headspace in the upstream trunk but over 70% of this air is released into the neighborhood due to reduced headspace in the downstream trunk.

Statistical analysis of sewer odour based on 10-year complaint data.

The City of Edmonton has been suffering from sewer odour problem for many years. Ten years of odour complaints data from 2008 to 2017 were statistically analyzed to identify major factors that relate to the odour problem. Spatial and temporal distributions of odour complaints in the city were first presented. Then relationships between the complaints and physical attributes of the sewer systems were analyzed by introducing a parameter of risk index. It was found that the snowmelt and storm events could possibly reduce odour complaints. Old sewer pipes and large drop structures are statistically more linked and thus significantly contribute to the complaints. The risk index relationship for three pipe materials is clay pipe > concrete pipe > PVC pipe. Combined sewers are more problematic in terms of odour complaints than sanitary sewers. And no clear correlation has been found between the changes of sewer pipe slope or angle and the complaints.

Case study of H2S release and transport in a trunk sewer with drops.

Field work was performed to investigate the release of hydrogen sulphide (H2S) and its transport in the sewer trunk with drops in the Bonnie Doon area in Edmonton, Alberta, Canada, in order to develop a proper odor control strategy. The liquid sulfide concentration in the upstream trunk was low (less than 1.0 mg/L), and no H2S gas was detected in the head space under this low concentration. However, high H2S gas concentration was detected in the middle reach of the trunk due to the stripping effect of the three drops (2.7 m, 5.2 m and 2.0 m) along the trunk. The released H2S at drops was then transported in the sewer system and emitted at various locations and caused odor concerns. These drops played an important role in H2S release, and the overall H2S mass transfer coefficient at drops was much higher than that in normal gravity sewers. The overall oxygen and H2S mass transfer coefficient (KLa) was estimated to be around 200 h-1 and 300 h-1 at the first two drops, respectively. Field sampling of biofilm indicates that Desulfomicrobium was identified as the sulfate-reducing bacteria (SRB) responsible for sulfide generation in sewer wall biofilm and Thiobacillus was the only predominant member in manhole wall biofilm contributing to sewer manhole corrosion.

Case study of sulfide generation and emission in sanitary sewer with drop structures and pump station.

Field work was carried out to identify sewer odor hotspots and corresponding causes in a sanitary sewer trunk with drop structures and pump station in Steinhauer area, Edmonton, Canada. Relatively high concentrations of H2S were detected at the beginning and the end of the trunk with odor complaints. At the beginning of the trunk, sulfide emission was mainly caused by the increased stripping effect of the drop structures. The pump operation at the end of the trunk causes the long retention time of the sewage and the subsequent sulfide generation. The sulfide generation was modeled and calibrated using field measurements. The model was applied to assess mitigation strategies, and optimized pump operation was found to be able to reduce sulfide generation in the study trunk sewer.