Nitrogen and Sulfur Cycling During Wastewater Treatment

Abstract

Amid the challenges of climate change, aging infrastructure, and urbanization environmental engineers must develop resource efficient water and wastewater treatment. As the population in coastal communities continues to increase and effluent nitrogen regulations become more stringent, innovation in our wastewater treatment infrastructure can help promote resource efficient nitrogen removal. Sea level rise due to global climate change causes seawater intrusion to wastewater collection systems and increases sulfate concentrations in wastewater. When the wastewater collection system is anaerobic, sulfate is biologically converted to sulfide. Sulfide is an electron donor for denitrification, reducing the need for supplemental carbon addition for nitrogen removal. This dissertation presents advancements in our understanding of how sulfur can affect nitrogen cycling during wastewater treatment.

The effects of hydrogen sulfide on nitrogen cycling were evaluated in three wastewater treatment systems: two full-scale treatment processes that employ different redox environments, thereby supporting distinct microbial communities, and one lab-scale bioreactor. Studies using microbial communities from the full-scale treatment processes showed that nitrite oxidizing bacteria (NOB) were more sensitive to sulfide than ammonia oxidizing bacteria (AOB). Inhibiting nitrite oxidizing bacteria promotes resource efficient treatment because it can reduce the aeration demands of treatment and support nitrite-based denitrifying metabolisms. However, the extent of inhibition was distinct in the two treatment plants, demonstrating that the effect of sulfide is community specific.

Given the potential benefits of sulfide for both denitrification and for inhibiting NOB, the effect of sulfide was tested in a mixed-redox membrane aerated biofilm reactor (MABR). A MABR biofilm is counter-diffusional, meaning the electron donor and electron acceptor diffuse into the biofilm in opposite directions. Accordingly, sulfide is amended in the anoxic bulk liquid, which curtails aerobic oxidation and allows for sulfide oxidation using nitrite or nitrate that was formed in the inner regions of the biofilm as an electron acceptor. Incubation experiments with heavy nitrogen revealed that, consistent with the full-scale systems, sulfide could inhibit NOB but had no impact on the rates of ammonia oxidation. During routine reactor monitoring, inhibition of NOB was not apparent, most likely due to the rapid conversion of nitrite to ammonia. Higher effluent ammonia concentrations observed during operation were attributed to inhibition of AOB instead of nitrite reduction to ammonia. Biofilm modeling was used to elucidate dissimilatory nitrite or nitrate reduction to ammonia (DNRA). Simulation results show that DNRA with sulfide as the electron donor could increase effluent ammonium. The genetic potential for nitrite reduction to ammonia was found in a unique population of denitrifying anaerobic methane oxidizers. These organisms are beneficial in the treatment of effluents from mainstream anaerobic processes as they curtail an important greenhouse gas emission while denitrifying. On the other hand, results show that sulfide inhibits nitrous oxide reduction, leading to higher emissions of nitrous oxide, a greenhouse gas with a global warming potential 300 times higher than carbon dioxide. Overall, studies in the mixed-redox counter-diffusional biofilm enhanced our understanding of how sulfide affects microbial community interactions.

The results of this dissertation show that hydrogen sulfide could have beneficial impacts on nitrogen cycling in engineered systems. The effect of hydrogen sulfide is complex because microbial communities are adaptable and sulfide induces feedback effects which change overall microbial community interactions. Ultimately, this knowledge can spur the development of technologies that use hydrogen sulfide to develop resource efficient wastewater treatment technologies.