Enhanced shortcut nitrogen removal and metagenomic analysis of functional microbial communities in a double sludge system treating ammonium-rich wastewater

Metagenomic insights into the mechanism for the rapid enrichment and high stability of Candidatus Brocadia facilitated by Fe(Ⅲ)

The rapid enrichment of anammox bacteria and its fragile resistance to adverse environment are the critical problems facing of anammox processes. As an abundant component in anammox bacteria, iron has been proved to promote the activity and growth of anammox bacteria in the mature anammox systems, but the functional and metabolic profiles in Fe(III) enhanced emerging anammox systems have not been evaluated. Results indicated that the relative abundance of functional genes involved in oxidative phosphorylation, nitrogen metabolism, cofactors synthesis, and extracellular polymers synthesis pathways was significantly promoted in the system added with 5 mg/L Fe(III) (R5). These enhanced pathways were crucial to energy generation, nitrogen removal, cell activity and proliferation, and microbial self-defense, thereby accelerating the enrichment of anammox bacteria Ca. Brocadia and facilitating their resistance to adverse environments. Microbial community analysis showed that the proportion of Ca. Brocadia in R5 also increased to 64.42 %. Hence, R5 could adapt rapidly to the increased nitrogen loading rate and increase the nitrogen removal rate by 108 % compared to the system without Fe(III) addition. However, the addition of 10 and 20 mg/L Fe(III) showed inhibitory effects on the growth and activity of anammox bacteria, which exhibited the lower relative abundance of Ca. Brocadia and unstable or even collapsed nitrogen removal performance. This study not only clarified the concentration range of Fe(III) that promoted and inhibited the enrichment of anammox bacteria, but also deepened our understanding of the functional and metabolic mechanisms underlying enhanced enrichment of anammox bacteria by Fe(III), providing a potential strategy to hasten the start-up of anammox from conventional activated sludge.

Application of a Partial Nitrogen Lab-Scale Sequencing Batch Reactor for the Treatment of Organic Wastewater and Its N2O Production Pathways, and the Microbial Mechanism

Partial nitrification (PN) is a widely used wastewater treatment process. Here a lab-scale sequencing batch reactor for PN (PN-SBR) was constructed and run with artificial organic wastewater for 225 days. Results showed that the SBR reached a stable PN state after 174 days of operation and >98% of NH4+-N was removed and >60% was converted to NO2−-N with low effluent NO3−-N content. In a PN-SBR cycle at stage IV, the release of N2O was accompanied by the production of hydroxylamine, occurring mainly in the conversion from anaerobic to aerobic phases, and the amount of N2O produced was about 6.3% of the total nitrogen. The N2O isotopic signature results suggested that hydroxylamine oxidation was the main pathway for N2O production. Illumina MiSeq sequencing results showed that Proteobacteria and Bacteroidetes were the dominant phyla throughout the operation period. Many heterotrophic nitrifiers were significantly enriched, leading to ammonia removal and nitrite accumulation, including Acidovorax, Paracoccus, Propionibacteriaceae_unclassified, Shinella, Comamonas and Brevundimonas. Representative strains were isolated from the reactor and they were capable of efficiently producing nitrite from ammonia. These results provide a guide for the direct running of PN reactors for treating organic wastewater and help to understand the microbial processes and N2O release pathways and the microbial mechanism of partial nitrification.

Recent advances in nitrous oxide production and mitigation in wastewater treatment

Nitrous oxide (N₂O) emitted from wastewater treatment plants has caused widespread concern. Over the past decade, people have made tremendous efforts to discover the microorganisms responsible for N₂O production, elucidate metabolic pathways, establish production models and formulate mitigation strategies. The ultimate goal of all these efforts is to shed new light on how N₂O is produced and how to reduce it, and one of the best ways is to find key opportunities by integrating the information obtained. This review article critically evaluates the knowledge gained in the field within a decade, especially in N₂O production microbiology, biochemistry, models and mitigation strategies, with a focus on denitrification. Previous research has greatly deepened the understanding of the N₂O generation mechanism, but further efforts are still needed due to the lack of standardized methodology for establishing N₂O mitigation strategies in full-scale systems. One of the challenges seems to be to convert the denitrification process from a net N₂O source into an effective sink, which is recommended as a key opportunity to reduce N₂O production in this review.