Using Advanced Sewer Ventilation Modeling to Size Vapor Phase Controls

Learning Objectives 1.Summarize the developments that have led to creation of sewer ventilation models. 2.Explain the steps required to build and calibrate a sewer ventilation model. 3.Show an application for using the ventilation model to size an odour control unit. Introduction Sizing air extraction for vapor phase odour control is a difficult task often leading to large safety factors and oversized equipment. Fan testing can be used to determine flow rates more accurately; however, it is a costly exercise that often only captures a limited range of wastewater flow conditions. Sewer ventilation modeling has evolved to accurately predict air extraction rates and optimum locations for installing odour control. This paper discusses the history of sewer ventilation modelling and presents case studies demonstrating applications of ventilation modeling for odour control in sewers. Development of Ventilation Modelling In 2007 the Water Environment Research Foundation undertook a review of all published literature on sewer odours and corrosion. Sewer ventilation was found to be the subject in most need of further research (Apgar et al, 2007). Field tests using tracer gas were used to measure air flows in large sewers leading to a proposed conservation of momentum approach to natural ventilation with the air/water interface modelled as flat plate with a drag coefficient. However, the drag coefficient value was unknown. (Witherspoon, 2009) In 2011 the Australian research Council initiated a project to determine the interface drag coefficient. This work culminated in a model for calculating air pressure and flow in a linear sewer network (Ward, 2011), subsequently, flat plate drag coefficients were measured in laboratory-scale studies at the University of Aalborg (Bentzen,2016) for use in the conservation of momentum method. In 2020 a software tool was developed capable of solving air pressure and ventilation in large, branched sewer networks, including representation of air extraction fans, and then overlay the ventilation results in sewer process models (Ward, 2022). Work is continuing to better represent how drop structures impact ventilation rates. The ability to overlay this ventilation model with the sewer process model allows for more accurate estimation of headspace H2S concentration and the required airflow rates to maintain sewer headspaces under negative pressure. Two case studies are presented where this methodology is applied. In one, the client was able to make changes to the sewer network prior to completion of design, and in the second, odour control systems were sized without the need for costly fan testing. Case Studies Region of Vaughan New Trunk Sewer Construction: The region of Vaughan, part of the Greater Toronto Metropolitan Area (GTA) is constructing a new major truck sewer, Jane St Trunk, and twinning portions of existing trunk sewers, Keele Langstaff. The trunk sewers will provide sufficient capacity to service the projected growth in Northeast Vaughan to the year 2051. Sewer process models and overlaid ventilation models were developed for 2024 (completion of the Keele-Langstaff segments) and 2028 (completion of the Jane St segments) growth scenarios. These time frames represent the lowest flow scenarios, i.e. highest sewer detention time, which are the conditions most susceptible to sulphide generation and potential odour release. Sampling took place to allow calibration of the models. A comparison measured versus modelled data will be detailed and as well as assumptions that were made during calibration. The major finding of the study was that the planned dropped structures along the Jane St Trunk caused sulphides to be stripped from the water into the gas phase, leading to elevated levels of H2S, see Figure 1. The ventilation model predicted the trunk would operate at a slight negative pressure due to the drag forces and closed nature of the sewer (minimal laterals and openings to atmosphere) with in gassing (negative pressures) at each maintenance hole/drop structure, creating a scenario in which the odours would not escape to the ambient environment. However, the region has had issues with corrosion at drop structures and the H2S levels were high enough for the region to request a redesign of the Jane St Trunk to mitigate H2S release and corrosion. The sewer design work is currently underway. Following its completion, the sewer process modelling will be recompleted. Adelaide Trunk System Odour Investigations: 18 of the 21 odour control facilities in the Adelaide Trunk system have been turned off to avoid odour complaints, resulting in a lack of ventilation along the system. The lack of ventilation has been identified as a major risk to a significant asset with severe corrosion consequences. Sampling took place to calibrate sewer process models and an overlaid ventilation model. The calibration of the ventilation model provided numerous challenges due to the large headspace in the main trunks resulting in slight pressures during sampling, uncertainty of the state of repair of large curtains pervious installed in the network to assist with zoned ventilation control and a siphon located close to the edge of the modelled domain. The zone of influence from the current fans was found to be minimal and reinstatement of decommissioned fans was not considered a suitable solution to the systemic odour and corrosion issues in the Adelaide Trunk system. Recommendations from the modelling will be discussed including recommended chemical dosing, lining sections of the trunk sewer and installing a large odour control unit in the vicinity of a new development, see Figure 2. Conclusions The development of a ventilation model which can be overlaid on a sewer process model is a value tool for accessing the generation and release of odours from the collection system and sizing odour control fans.