Evaluation of hydrogen sulfide concentration and control in a sewer system
This study focuses on monitoring hydrogen sulfide (dissolved and atmospheric) generation and wastewater volumetric flow in a 13.4 mile sewer line of the City of San Antonio, Texas. Results are used to evaluate daily and seasonal trends of atmospheric and dissolved sulfide, and to better apply sulfide control practices to prevent odor and corrosion. The work was performed in conjunction with The San Antonio Water System (SAWS). SAWS is currently dosing ferrous sulfate (FeSO4) to control odor and corrosion in their sewers. As part of this study, the evaluation of a cost-effective dosing strategy with FeSO4 was performed to better control the sulfide contents in wastewater.
Sulfide concentrations, flow and physical properties of the wastewater were monitored at two points (upstream site and downstream site) along the sewer line to provide enough data to analyze variations and identify diurnal trends and possible seasonal differences. In addition, dosing studies were performed in the laboratory to find out the required ratio of ferrous sulfate for acceptable sulfide removal. These laboratory measurements were compared with chemical equilibrium model simulations.
Flow was higher at the upstream site than the downstream site due to flow splitting 8.7 miles downstream. Sulfide concentrations were found higher at the downstream site, especially in the summer months. Laboratory dosing studies indicate a 1.25 mole ratio requirement, to reduce sulfide by 93%. The dosing results were compared with the chemical equilibrium software finding a similar trend in sulfide precipitation at pH of 7.06. However, sulfide removal gets more difficult as pH decreases due to solubility constraints.
Sulfide requirements were calculated based on total flow downstream and the respective sulfide concentrations, excluding the values below 2 mg/L where corrosion has been reported to be minimal. Over a typical daily diurnal cycle, necessary dosing rates to maintain sulfide concentrations below 2 mg/L varied between 0 and 9,077 gpd with a daily average rate of 3,565 gpd. If instead of dosing at the maximum required rate, dosing were matched over the diurnal cycle, chemical savings would amount to 5,512 gpd while achieving sulfide control. The approximate cost of the ferrous sulfate solution dosed by SAWS is $0.52 per gallon and this amount of chemical savings translates into roughly $2,866 per day. Actual dosing cost for the hypothetical average day will be $1,854 per day. These cost savings can easily recoup the required instrumentation costs to achieve this diurnal dose matching.