Different ferric dosing strategies could result in different control mechanisms of sulfide and methane production in sediments of gravity sewers

Abstract

Ferric salt dosing is widely used to mitigate sulfide and methane emissions from sewers. In gravity sewers with sediments, responses of sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) residing in different zones to Fe3+ dosing strategies still remain unknown. In this study, we investigated the changes in behavior of SRB and MA in different depths of sewer sediment using laboratory-scale sewer sediment reactors with different Fe3+ dosing strategies (different instant dosages and frequencies). All Fe3+ dosing strategies examined efficiently suppressed sulfide concentration for a short time, but the control mechanisms were different. When a low-dosage, high-frequency Fe3+ dosing strategy was employed, Fe3+ could not penetrate into the sewer sediment, therefore, the abundances of SRB and MA in all zones of sewer sediment did not change substantially. As a result, the active sulfide-producing and methane-producing zones kept unchanged. Sulfide was controlled mainly via chemical sulfide oxidation and precipitation, and methane formation was not influenced. In contrast, when a high-dosage, low-frequency Fe3+ dosing strategy was used, the SRB activity in the upper layer of the sewer sediment was nearly fully suppressed according to the down moving zones of sulfide production (from 0-5 mm to 20–25 mm) and lower sulfate reduction, in which sulfate reduction decreased by 56% in the long-term trial. The generated sulfide was further removed via chemical sulfide oxidation and precipitation. This strategy also significantly suppressed MA activity (21% reduction in methane production). However, considering a long-term satisfactory sulfide control, a low operational cost and less sediments deposited in gravity sewers, a low-dosage, high-frequency Fe3+ dosing strategy would be a more cost-effective solution for sulfide control in gravity sewers with thin (<20 mm) or thick (>20 mm) sediments if methane mitigation does not need to be taken into account.