Implications of Downstream Nitrate Dosage in anaerobic sewers to control sulfide and methane emissions.

Nitrate (NO₃⁻) is commonly dosed in sewer systems to reduce sulfide (H₂S) and methane (CH₄) produced in anaerobic rising main pipes. However, anoxic conditions along the whole rising pipes are difficult and costly to maintain since nitrate is added at the upstream sections of the sewer. In this study we tested the effects of the Downstream Nitrate Dosage strategy (DND) in anaerobic pipes in a specially designed laboratory-scale systems that mimics a real rising main. Effectiveness of the strategy was assessed on H₂S and CH₄ abatement on the effluent of the lab sewer system. A combination of process (Normal Functioning monitoring and batch tests) and molecular (by 454-pyrosequencing) methods were used to investigate the impacts and microbial activities related to the nitrate addition. Results showed a complete abatement of H₂S generated, with a fraction transformed to elemental sulfur (S⁰). Methane discharged was reduced to 50% while nitrate was added, due to the CH₄ oxidation in the anoxic conditions established at the end of the pipe. Both sulfidogenic and methanogenic activities resumed upon cessation of NO₃⁻ dosage. An increase of microorganisms of the genera Simplicispira, Comamonas, Azonexus and Thauera was detected during nitrate addition. Regarding anoxic methane oxidation, only one Operational Taxonomic Unit (OTU) was identified, which is likely related with this metabolism. Obtained results are relevant for the optimal management of nitrate dosage strategies in sewer systems.