Experimental Study On Nitrate-based Combined Dosing Strategies For Sulfide Control In Sewer Systems

The odor nuisance caused by hydrogen sulfide in the sewer systems is one of the environmental problems leading to public complaints.Therefore,more and more attention is being paid to the control methods in the liquid phase.Among them,nitrate is widely used as an effective chemical for controlling sulfide.However,there are still some limitations in the nitrate dosing strategy.The quick recovery of the sulfide formation after nitrate depletion has been observed and results in serious odor problems.Besides,the organic carbon sources in sewage are consumed significantly by heterotrophic denitrification,which may cause an adverse effect on nitrogen removal in the wastewater treatment plants.Moreover,significant nitrous oxide（N₂O）is generated when dosing nitrate in sewers.Therefore,based on above problems,a novel and feasible control strategy for the removal of sulfide needs to be developed.In this study,sulfide is used as the research object.And the sewer reactors are operated to study the influence of general factors on sulfide control effects to provide guidance for the actual dosing strategy.Meanwhile,a novel combined dosing strategy of nitrate and sodium nitroprusside is evaluated,which reduced the consumption of nitrate and organic carbon in sewage.Additionally,the combined dosing strategy of nitrate and calcium peroxide for sulfide control is proposed.The practical application of the intermittent dosing strategy is discussed.The main conclusions are as follows:（1）With different concentrations of organic carbon,the rebound phenomenon of sulfide is different after dosing nitrate with the same nitrogen-sulfur ratio（N/S）.In high carbon-sulfur ratio（C/S）conditions,the rebound phenomenon of sulfide is more obvious.Most sulfate is reduced to sulfide rapidly through the dissimilatory sulfate reduction.However,in low carbon-sulfur ratio conditions,the sulfate is reduced to sulfide through assimilation sulfate reduction pathway.Additionally,a higher nitrogen-sulfur ratio（N/S）results in an increase in the consumption rate of organic carbon.The enrichment of heterotrophic denitrifying bacteria is observed with a higher concentration of organic carbon,resulting in a rapid consumption rate of nitrate and organic carbon source.（2）The combined addition of nitrate and sodium nitroprusside（SNP）can inhibit the formation of sulfide significantly.Nitrate and SNP are strongly synergistic.Compared with solely adding nitrate,70%of nitrate can be saved with the combination of nitrate and SNP dosing,and the carbon source loss is reduced by 80%.From the microbial community analysis,it is found that the combination of nitrate and SNP stimulated the activity of autotrophic nitrate-reducing-sulfur-oxidizing bacteria（NR-SOB）,especially for the Arcobacter and Sulfurimonas.The relative abundance of heterotrophic nitrate-reducing bacteria（h NRB）is decreased substantially.Therefore,most nitrate is consumed by autotrophic NR-SOB for sulfide control rather than h NRB for denitrification.Meanwhile,the combination of nitrate and SNP promotes the dissimilatory nitrate reduction process（DNRA）,which utilizes sulfide as an efficient electron donor for nitrate reduction.The economic evaluation results show that the chemical cost of combined dosing is about 35%lower than that of solely dosing nitrate to achieve 100%sulfide inhibition.（3）The simultaneous addition of nitrate and calcium peroxide（CaO₂）can also achieve a synergistic effect in controlling sulfide.The addition of nitrate and CaO₂stimulated the NR-SOB,in which Rhodobacteraceae is the predominant NR-SOB that participates in anoxic,nitrate-dependent sulfide oxidation.Additionally,the presence of oxygen in alkaline conditions generated by CaO₂ can facilitate the inhibition of SRB activities.Therefore,the intermittent addition of nitrate and CaO₂ is feasible in controlling sulfide.On the other hand,the combination of nitrate and CaO₂reduces the total global warming potential by nearly 90%.Thus,the proposed strategy will facilitate eliminating greenhouse gas when controlling sulfide.The economic evaluation results show that the chemical cost of the intermittent dosing strategy is around 84%lower than that of the continuous dosing nitrate strategy.Therefore,intermittent dosing nitrate combined with CaO₂ in sewers is an economical approach for sulfide removal in sewer systems.