Effect of ferric iron and nitrate on hydrogen sulfide control in lab-scale reactors

Four bioreactors imitating force mains of pump stations were operated over 8 months to evaluate sulfide generation rate in force mains. Ferric iron and nitrate were added into two of the four bioreactors to investigate their effects on sulfide control. The half order kinetics regarding dissolved COD was capable of predicting the sulfide formation in the bioreactors and the rate constant was around 0.005–0.010 g S g COD−0.5 m−0.5 h−1. Sulfide controlled by ferric iron was only via chemical oxidation and precipitation and total sulfate reducing bacteria (SRB) population did not change. The heterotrophic nitrate reducing bacteria (hNRB), Thauera, outcompeted SRB for organic matter as electron donor as the dominant bacteria when nitrate was added at the beginning of the pump cycle. The sulfide generation was controlled by hNRB activities. The amount of nitrate required for sulfide control depended on heterotrophic denitrification rate which can be well described by half order kinetics with regard to DCOD concentration. A modified empirical equation was proposed and the modelled empirical rate constant was around 0.05 g N g COD−0.5 m−0.5 h−1. A cost-effective nitrate dosing strategy is proposed to add the nitrate at the end of the pump cycle instead of at the beginning of the pump cycle which can save up to 75% nitrate dosage. The co-existence of sulfide and nitrate stimulated the development of nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB), Sulfurovum, which was responsible for sulfide removal in the presence of nitrate. Both ferric iron and nitrate did not have the long-lasting inhibitory/toxic effect on sulfate reduction.