Sulfide Generation in Force Mains and Its Control Using Nitrate Dosing

Sulfide generation rates in the force mains of four pump stations in Edmonton, Alberta, Canada, were evaluated in the field, and a continuous nitrate dosing was applied in the pump wet well of the Big Lake pump station to investigate its effectiveness on sulfide control. The sulfide production rates at these force mains were estimated to be $0.08–0.15\mathrm{g}/{\mathrm{m}}^2\mathrm{h}$ under 20°C, which could be modeled by using the $1/2$-order biofilm kinetics of dissolved chemical oxygen demand (DCOD). The modeled rate constant $a$ was obtained at around 0.006 using the field monitoring data. It was found that $20\mathrm{mg}/\mathrm{L}$ sulfide was formed in the force main of the Big Lake pump station, which resulted in 400–500 parts per million (ppm) ${\mathrm{H}}_2\mathrm{S}$ gas in the discharge manhole. It was found that a nitrate dosing concentration of $85\mathrm{mg}/\mathrm{L}$ could completely suppress the sulfide generation in the force main. A constant dosing rate resulted in nitrate overdosage at midnight and underdosage in peak hours. Injecting nitrate in the wet well led to a large portion of nitrate consumption where a heterotrophic denitrification process happened. The appropriate dosing location for this dosing strategy is at the beginning of force main instead of the wet well. The corresponding dosing rate is a flow-paced dosing rate instead of a constant dosing rate. A more cost-effective dosing strategy is to add the nitrate at downstream section of the force main where sulfide generated upstream could be removed in the presence of nitrate when passing downstream. This dosing strategy could significantly reduce the nitrate demand.