The study explored the autotrophic and heterotrophic denitrification in batch anaerobic reactors with nitrite in toxic conditions in the first stage. After that, autotrophic and heterotrophic interactions were evaluated in a innovative structured bed reactor.
Improper disposal of wastewater with carbon, ammonia, and sulfate compounds, like leachate and vinasse, poses threats to the environment and public health. Treating these pollutants is complex, requiring an understanding of mechanisms and biological interactions, particularly in high nitrogen conditions. This knowledge is crucial for advancing wastewater treatment. The project aims to explore the feasibility of simultaneously removing carbon, nitrogen, and sulfur. The approach involves utilizing an innovative structured bed reactor configuration. In the initial phase (STEP 1), we evaluated the impact of acetate as an external carbon source in various stoichiometric proportions on myxotrophic denitrification through batch assays. The conditions under investigation ranged from scenarios with no acetate addition to a nitrite/acetate ratio of 10.28, with a nitrite concentration approaching to 500 mg.L-1.  In STAGE 2, a structured bed and recirculation reactor (SBRR) was operated in a single stage with 3 compartments (anaerobic/anoxic/aerobic) using synthetic wastewater, changing the ammonia nitrogen concentration from 150 to 250 mg.L-1 (Phases 1 and 2, respectively), sulfate addition (Phase 3), normal to intermittent aeration (Phase 4), aeration cycle and aeration flow rate (Phase 5) and carbon/nitrogen ratio (Phase 6).. This stage involved variations in normal and intermittent aeration, aeration cycle, aeration flow rate, and carbon/nitrogen ratio. The assessment of the batch reactors in STEP 1 revealed that acetate in proportions exceeding the stoichiometric values favored the heterotrophic denitrification pathway. Conversely, proportions below the stoichiometric values led to a more active myxotrophic denitrification. Despite the potential toxic effect of nitrite at the concentrations studied, all reactors demonstrated adaptability and efficient nitrite removal, with removal efficiencies ranging from 73% to 99%. In STAGE 2, intermittent aeration was employed with a cycle consisting of 2.5 hours of non-aeration followed by 0.5 hours of aeration. The aeration flow was maintained at 9 mL/min, and the COD/nitrogen ratio was set at 4 during Phase 6. This operational strategy led to the attainment of the highest average nitrification efficiency at 67% and an impressive total nitrogen removal rate of 62%. Throughout the various phases studied, the reactor consistently demonstrated its capability to effectively reduce ammoniacal nitrogen loads in rates of: 32.4 mgN.L.-1d-1 (Phase 1), 25.4 mgN.L.-1d-1 (Phase 2), 21.7 mgN.L.-1d-1 (Phase 3), 33.7 mgN.L.-1d-1 (Phase 4), 59.9 mgN.L.-1d-1 (Phase 5), and 55.0 mgN.L.-1d-1 (Phase 6). Intermittent aeration during Phases 4 to 6 promoted simultaneous nitrification and denitrification, enhancing overall system performance. Compartment 1 played a pivotal role in carbon, nitrogen, and sulfur compound removal, significantly contributing to total nitrogen removal across all phases. Microbial analysis showed nitrifying bacteria prevailing in compartments 2 and 3, with denitrifying bacteria were more abundant in Compartment 1. Elevated free ammonia in Compartment 1, due to increased pH, limited nitrification in Compartments 2 and 3, affecting overall system performance. The study highlighted intricate interactions among nitrifiers, denitrifiers, anammox, and sulfate-reducing bacteria, emphasizing the complex microbial community. The obtained results represent significant advancements in the field, showcasing the promising potential of this reactor configuration for wastewater treatment.