Wastewater flow in gravity sewers produces concurrent airflow in the headspace, which results in the transport of gaseous components such as hydrogen sulfide from one location to another within the sewer pipe. The movement of air in a sewer pipe and the transfer and transport of hydrogen sulfide, a key compound of interest in sewers, is illustrated in Figure below.
The air transport is important mainly due to the followings:
- the extent of transport determines spatial variation of gaseous phase hydrogen sulfide concentration, which contributes to the corrosion in sewer pipes; and
- the extent of air exchange between the ambient atmosphere and the sewer headspace, which affects the release of odorous gases from openings along a sewer network.
The release of odorous gases through the manhole openings and lateral pipes is the direct consequence of pressure buildup in the sewer headspace due to the resistance to airflow. Accurate knowledge on the gas velocity in sewer headspace is necessary to estimate the variation of gas phase H2S concentration along the length of sewer and the rate of emission at the available openings.