Hydrogen sulfide (H2S) generation in sewer systems presents a significant cost to public due to metal and concrete corrosion and destruction of the asset and is responsible for obnoxious odour and potential toxicity.
Several methods are adopted to control the corrosion. Most popular methods are dosing chemicals either to bind the H2S or to adjust pH to higher values to convert H2S to a form that will prevent the emission to air. Effectiveness and cost of these methods vary from researcher to researcher. Mainly due to the poor access system, fundamentals behind the effectiveness are not clear. Fundamentals are not properly understood because simulating the sewer system, especially partial flow sewer, is challenging due to exposure to atmosphere and a high flow velocity and hence there was no single laboratory model that was available to simulate conditions.
The main objective of this research is:
• To develop a laboratory testing system capable of generating H2S in similar conditions to a real sewer system
• To evaluate various control measures of sulfate reduction in the sewer system
• To understand the mechanism of H2S generation and control
• Formulate a more cost-effective H2S control method
• Development of the model
To achieve the main objectives, a combination of two traditionally used methods –pH control using lime and ferrous chloride addition- were tested. Synthetic wastewater was fed into the reactor. Produced hydrogen sulfide concentrations were measured using the colorimetric method. Reduction of H2S in the gas phase was monitored with the concentration of dissolved oxygen in the liquid phase. The comparison of the existing hydrogen sulfide control methods with the tested combined method of pH adjustment and ferrous chloride dosing exhibited important benefits of the new method. This method was found to be more efficient and cost effective. Due to sludge reduction and chemical reduction, the treatment process (sludge treatment process) will be much easier in the downstream.
The addition of ferrous chloride at higher pH reduced the gas phase H2S concentration even further as did the presence of oxygen. The discrepancy between actual H2S gas phase measurements and the theoretical gas phase values was identified. Silver nitrate titration using a silver electrode was used to identify the final products of the reactions. To further identify the sulphur species, a model using stoichiometric coefficients was developed and tested under various experimental conditions. It was noted, that SO42-, S2O32- , So and disulfide were the major species formed in the sewer with the variation of DO level and H2S level in the liquid phase. Formation of disulfide is advantageous, as it requires low DO level which may already exist in gravity sewers. In addition, the disulfide can not be converted back to gaseous form or processed by microbes into H2S.
The findings of this work were used for the design of the proposed experimental and theoretical work. The data produced in the experimental work was used to develop a process model of the combined oxidation of H2S and reduction of its concentration in the sewer headspace. Such a model can be used to optimize H2S in a gravity sewer system, achieving the required gas concentration reduction at a minimum cost
