A new concept of waste iron recycling for the enhancement of the anammox process

Internal short-circuit revolution: Unveiling enhanced nitrogen removal efficiency in Mn-rich constructed wetlands

Electron acceptor limitation restricts ammonium removal in constructed wetlands (CWs). This study hypothesized that integrating carbon fiber filaments (CFs) within manganese-rich substrates (CF-CM) could enhance electron transfer and nitrogen transformation. Five lab-scale CWs were established. Results showed that CF-CM significantly improved NH4+-N and total nitrogen removal rates, reaching 8.1 and 13.1 gN/(m2·d), 3.1-fold and 5.4-fold higher than CF-CK, respectively. CFs facilitated internal short-circuiting, increasing anammox bacteria abundance and electron utilization of denitrifying bacteria. The qPCR gene chip testing revealed upregulated napA and narG, with nirS being the most abundant nitrogen transformation gene. And anammox genes (hzsA, hzsB, hzo) at 3.22 × 107 copies·g-1, 288 times higher than the control. This strategy enhanced regional electron transfer and regulated spatial electron supply-demand relationships, with the core enhancement being the enrichment of anammox bacteria, electroactive bacteria, and denitrifiers in CF-CM systems. One point of view is that integrating internal short-circuits in Mn-rich substrates CWs is an efficient treatment approach for nitrogen removal.

FeO might be more suitable than Fe2+ for the construction of anammox-dominated Fe-N coupling system: Based on 15N isotope tracing

Iron not only influences the activity of anammox bacteria (AnAOB) but also participates in complex Fe-N cycles. In this study, the advanced 15N isotope tracing method was set up to quantify the potential rates of full nitrogen metabolic pathways under different ferrous iron (Fe2+ and FeO) within two identical anammox granular reactors. The results indicated that both Fe2+ and FeO enhanced AnAOB activity. However, compared to Fe2+, which readily precipitates and oxidizes, the system supplemented with FeO exhibited higher Fe-N metabolic activity and greater metabolic diversity. This is attributed to the gradual release of Fe2+ from FeO, providing a sustainable and stable supply of Fe2+ for microorganisms. Furthermore, Subgroup_10 and Paludibaculum were identified as potential functional bacteria for feammox, while Denitratisoma, I-8 and Arenimonas were for NDFO. These results suggest that FeO addition is more beneficial for the construction of a Fe-N coupling system. Overall, this study enhances our understanding of how with exogenous iron can strengthen the anammox system, laying a theoretical foundation for the development of anammox-dominant Fe-N coupling systems.

Roles of waste iron scraps in anammox system treating sulfide-containing wastewater: Alleviating sulfide inhibition, promoting novel anammox bacteria enrichment, and enhancing nitrogen removal capacity

In this study, waste iron scraps (WIS) were exerted to alleviate sulfide inhibition on anammox bacteria and promote anammox nitrogen removal from sulfide-containing wastewater.Short-term batch experiments showed that WIS-addition led to the anammox bacteria activity increasing by 124.8 % at an initial sulfide concentration of 40 mgS/L. During the long-term experiments, the nitrogen removal rate (NRR) reached to 8.76 kg/(m3·d) in the WIS-added reactor, while the maximum NRR was only 3.77 ± 0.31 kg/(m3·d) in the non-WIS reactor. In contrast to anammox bacteria development in the non-WIS reactor, the relative abundance of Candidatus Kuenenia (1.4-3.7 %) declined significantly in the WIS-added reactor, but novel potential anammox bacteria Brocadiaceae_unclassified (60.1 %-78.6 %) were highly enriched. Overall, the experimental evidence suggested that WIS-addition not only mitigated the sulfide inhibition on anammox bacteria, but also promoted novel anammox bacteria proliferation. The findings of this work provide a promising solution for wide engineering applications of anammox treating sulfide-containing wastewater.

Roles of red mud-based biochar carriers in the recovery of anammox activity: characteristics and mechanisms

Anaerobic ammonium oxidation (anammox) sludge is easily deactivated in the process of treating ammonia-laden wastewater. To investigate an effective recovery method, red mud-based biochar carriers (RMBC) were prepared and added to a deactivated anammox reactor; the operation of this reactor had been interrupted for 6 months with starvation and low temperature. The deactivated sludge with added RMBC was recovered rapidly after 31 days, with the specific anammox activity rapidly increasing to 0.84 g N/(g VSS∙day), and the recovery efficiency of nitrogen removal rate increased by four times compared to the unadded control. The granulation degree and extracellular polymeric substances secretion of the anammox sludge with the added RMBC were significantly higher than that of the control group. In addition, a large number of spherical anammox bacteria were observed moored at the porous channels of RMBC, and the copy numbers of functional genes of anammox bacteria were approximately twice that of the control group. Hence, RMBC is a potential sludge activator, and it can provide a "house" to protect anammox bacteria, enhance the metabolic activity and the agglomerative growth of anammox bacteria, and synergistically achieve rapid recovery of deactivated anammox sludge.

Waste iron scraps promote anammox bacteria to resist inorganic carbon limitation

The anaerobic ammonium oxidation (anammox) process is adversely affected by the limitation of inorganic carbon (IC). In this research, a new technique was introduced to assist anammox biomass in counteracting the adverse effects of IC limitation by incorporating waste iron scraps (WIS), a cheap and easily accessible byproduct of lathe cutting. Results demonstrated that reducing the influent IC/TN ratio from 0.08-0.09 to 0.04 resulted in a 20 % decrease in the nitrogen removal rate (NRR) for the control reactor, with an average specific anammox activity (SAA) of 0.65 g N/g VSS/day. Nevertheless, the performance of the WIS-assisted anammox reactor remained robust despite the reduction in IC supply. In fact, the NRR and SAA of the WIS-assisted reactor exhibited substantial improvements, reaching approximately 1.86 kg/(m3·day) and 0.98 g N/g VSS/day, respectively. These values surpassed those achieved by the control reactor by approximately 39 % and 51 %, respectively. The microbial analysis confirmed that the WIS addition significantly stimulated the proliferation of anammox bacteria (dominated by Candidatus Kuenenia) under IC limitation. The anammox gene abundances in the WIS-assisted anammox reactor were 3-4 times higher than those in the control reactor. Functional genes prediction based on the KEGG database revealed that the addition of WIS significantly enhanced the relative abundances of genes associated with nitrogen metabolism, IC fixation, and central carbon metabolism. Together, the results suggested that WIS promoted carbon dioxide fixation of anammox species to resist IC limitation. This study provided a promising approach for effectively treating high ammonium-strength wastewater using anammox under IC limitation.

Efficient nitrogen removal and substrate usage in integrated fixed-film activated sludge-anammox system under seasonal temperature variation

To elucidate how integrated fixed-film activated sludge (IFAS) system favors nitrogen removal performance under seasonal temperature variations, two push-flow reactors were operated with and without carriers under the same operating conditions. The results show that the IFAS system had significant advantages in shock response and low temperature adaptation, with a nitrogen removal rate of 0.37-0.53 kg-N(m3·d)-1 at the temperature of 8-12 °C. Anammox bacteria on carriers were almost unaffected by temperature variation, and its nitrogen removal contribution rate stabilized at 55 % in the IFAS system. The Haldane model reveals that the specific anammox activity in the IFAS system was 28 % to 49 % higher than that in the control system at 13 °C. Candidatus_Jettenia, with the highest abundance of 45 %, was the dominant species in the IFAS system and preferred to attach to the carriers. This study provides a feasible scheme for the application of anammox process.

Chameleon-like Anammox Bacteria for Surface Color Change after Suffering Starvation

Bacteria are often exposed to long-term starvation during transportation and storage, during which a series of enzymes and metabolic pathways are activated to ensure survival. However, why the surface color of the bacteria changes during starvation is still not well-known. In this study, we found black anammox consortia suffering from long-term starvation contained 0.86 mmol gVSS-1 cytochrome c, which had no significant discrepancy compared with the red anammox consortia (P > 0.05), indicating cytochrome c was not the key issue for chromaticity change. Conversely, we found that under starvation conditions cysteine degradation is an important metabolic pathway for the blackening of the anammox consortia for H2S production. In particular, anammox bacteria contain large amounts of iron-rich nanoparticles, cytochrome c, and other iron-sulfur clusters that are converted to produce free iron. H2S combines with free iron in bacteria to form Fe-S compounds, which eventually exist stably as FeS2, mainly in the extracellular space. Interestingly, FeS2 could be oxidized by air aeration, which makes the consortia turn red again. The unique self-protection mechanism makes the whole consortia appear black, avoiding inhibition by high concentrations of H2S and achieving Fe storage. This study expands the understanding of the metabolites of anammox bacteria as well as the bacterial survival mechanism during starvation.