A dynamic sewer ventilation model for sewer pipes

Abstract

Sewage collection and transport in an urban drainage system is often planned, designed and operated based on hydraulic and process modelling. A number of different sewer models are increasingly used to quantify odorous chemicals released into sewer as well as the ambient environment, which could provide information of likely “lifespan” of sewer pipes as well as support the management of odour and corrosion in sewer.Most of the sewer models, however, focus on water-phase hydraulic modelling only and neglect air transportation process in gravity sewers. Several experiments have been conducted for accurate and rapid air flow estimation in sewer headspace, in order to improve the understanding of the impacts of pressure difference, wastewater drag and concrete wall friction on the air movement in sewer headspace. Empirical models, steady-state models and dynamic models have been used for air flow prediction under various sewer conditions. However, some of these models are complex and demand high computational resources. In order to understand the air movement process in sewers and accurately predict air flow in a sewer pipe, a process-based Integrated Dynamic Sewer Ventilation Model (IDSVM), consisting of a dynamic hydraulic model and a dynamic sewer ventilation model, has been developed. The model is able to predict the dynamic variation of water velocity and air flow with the change in sewer hydraulics and physical properties. Results from simulation process showed good reliability of the model.The first part of the study focused on air flow in closed sewer pipe, in which an attempt to demonstrate the effect of changes in pipe diameters, pipe slope and sewer flow on air velocity and air to water velocity ratio has been made. Based on the simulation results, both the increase in flow rate and the increase in pipe slope are found to result in increased air velocity. On the other hand, the increase in pipe diameter is found to result in decreased air velocity. Similar observation was also made for the air to water velocity ratio except that a pipe with larger slope is found to result in reduced air to water velocity ratio. A sensitivity analysis was carried out to quantify the sensitivity of the model in terms of air velocity and air/water velocity ratio towards the changes in inputs of the model.Application of the model to a simple sewer network has also been demonstrated. The model is able to predict the inflow or outflow of air at the junctions (manholes). It has also been found that the air eduction/induction at manholes results from air flow difference between upstream and downstream link at nodes.Simple empirical models based on a large number of simulations for varying sewer conditions have been found to provide reasonably good results in terms of both air velocity and air to water velocity ratio. The simplified empirical model, which is based on the full hydraulic and ventilation model, can be used in sewer models to predict the air flow rate based on sewer properties and the water velocity.