Network Modelling of the Formation and Fate of Hydrogen Sulfide and Methane in Sewer Systems

Hydrogen sulfide is produced by sulfate reducing bacteria, which are mainly associated with the biofilms covering the surfaces in rising mains. Sulfide control strategies commonly used such as chemical dosage are costly for long-term management. The effect of physical and hydraulic conditions of sewers on sulfide formation has been investigated in recent years. One of the key parameters in modelling the formation of hydrogen sulfide is the pipe area- volume ratio (A/V), as this indicates the relative contribution between biofilm and wastewater processes. The A/V is naturally related to the pipe diameter. A high A/V is associated with small pipe diameter, and would lead to a high contribution from the biofilms and hence the potential for high hydrogen sulfide formation. However, it would also decrease the residence time of the wastewater, which would tend to decrease the amount of hydrogen sulfide formed. Based on the results of in-sewer process modelling, this study quantifies the importance of the pipe diameter and pumping strategy for optimal design for rising mains, to minimise hydrogen sulfide production, either to improve the life time for the downstream sewer structures or to minimise the potential chemical dosing needed in the rising mains. The model results from this study show optimal diameter options for both existing rising mains in terms of minimum hydrogen sulfide formation. The sensitivity analysis on model parameter based on the case studies also indicates the most uncertain parameters and COD fractions.
Methane is a problem acknowledged in some sewer networks around the world and is particularly of concern in China where sewer explosions can occur. Septic tanks are integrated parts of many Chinese sewer systems and methane is believed to be produced not only in sewer pipes, but also in septic tanks. Work has been done to look at how the anaerobic digestion model can be applied in combinations with sewer processes models to describe such a system. Model simulation results on methane formation in rising mains is similar to reported literature. The field measurements in the Chinese sewer system demonstrated high gas phase hydrogen sulfide and methane concentrations both in gravity systems and rising mains due to the surcharging sewer conditions. Future work has been proposed according to the local problems by discussing the benefits of applying sewer process and anaerobic digestion models based on this study.