Effects of nitrate dosing on methanogenic activity in a sulfide-producing sewer biofilm reactor

Nitrate dosing is widely used by water industry to control hydrogen sulfide production in sewers. This study assessed the impact of nitrate addition on methane generation by sewer biofilms using a lab-scale rising main sewer reactor. It was found that methanogenesis could coexist with denitrification and sulfate reduction in sewers dosed with nitrate. However, methane production was substantially reduced by nitrate addition. Methanogenic rates remained below 10% of its baseline level, with 30 mg-N/L of nitrate dosing for each pump event. By calculating the substrate penetration depth in biofilms, it is suggested that methanogenesis may persist in deeper biofilms due to the limited penetration of nitrate and sulfate, and better penetration of soluble organic substrates. The control of methane and sulfide production was found to be determined by the nitrate penetration depth in biofilms and nitrate presence time in sewers, respectively. The presence of nitrous oxide after nitrate addition was transient, with a negligible discharge of nitrous oxide from the sewer reactor due to its further reduction by denitrifiers after nitrate depletion.