Remedying acidification and deterioration of aerobic post-treatment of digested effluent by using zero-valent iron

Organic carbon source excites extracellular polymeric substances to boost Fe0-mediated autotrophic denitrification in mixotrophic system

Fe⁰-mediated autotrophic denitrification (ADN) can be suppressed by iron oxide coverage resulting from Fe⁰ corrosion. The mixotrophic denitrification (MDN) coupling Fe⁰-mediated ADN with heterotrophic denitrification (HDN) can circumvent the weakening of Fe⁰-mediated ADN over operation time. But the interaction between HDN and Fe⁰-mediated ADN for nitrogen removal of secondary effluent with deficient bioavailable organics remains unclear. When the influent COD/NO₃^--N ratio increased from 0.0 to 1.8-2.1, the TN removal efficiency was promoted significantly. The increased carbon source did not inhibit ADN, but promoted ADN and HDN synchronously. The formation of extracellular polymeric substances (EPS) was also facilitated concomitantly. Protein (PN) and humic acid (HA) in EPS increased significantly, which capable of accelerating electron transfer of denitrification. Due to that the electron transfer of HDN occurs intracellularly, the EPS with the capacity of accelerating electron transfer had a negligible influence on HDN. But for Fe⁰-mediated ADN, the increased EPS as well as corresponding PN and HA facilitated TN and NO₃^--N removal significantly, while accelerated the electron release originating from Fe⁰ corrosion. The bioorganic-Fe complexes were generated on Fe⁰ surface after used, meaning that the soluble EPS and soluble microbial products (SMP) participated in the electron transfer of Fe⁰-mediated ADN. The coexistence of HDN and ADN denitrifiers demonstrated the synchronous enhancement of HDN and ADN by the external carbon source. From the perspective of EPS and related SMP, the insight of enhancing Fe⁰-mediated ADN by external carbon source is beneficial to implement high-efficiency MDN for organics-deficient secondary wastewater.

Nitrogen removal in improved subsurface wastewater infiltration system: Mechanism, microbial indicators and the limitation of phosphorus

To enhance the nitrogen removal capacity, scrap iron filings and Si-Al porous clay mineral material (PCMW) was used to improve a subsurface wastewater infiltration system (SWIS). The results showed TN and NH₄⁺-N removal efficiencies of improved SWIS were 20.72% and 5.49% higher than those of the control SWIS, respectively. Based on the response of the removal performance, microbial community and function analysis of 16s rRNA amplicon sequencing results, the amending soil matrix substantially enriched the nitrogen removal bacteria (Rhizobiales_Incertae_Sedis and Gemmatimonadaceae), and significantly improved the activities of key enzymes (Hao, NasAB, NarGHI, NirK, NorBC, NirA and NirBD), particularly at co-occurrence zone of nitrification and denitrification (70-130 cm depth). The amending soil matrix not only extended the growth space of microbes, but also provided additional electrons and carbon sources for denitrifying bacteria by regulating the structure and function of the microbial community. In addition, amending soil matrix could enhance phosphate metabolism genes and phosphate solubilizing microbes in the denitrification zone by increasing the phosphorus source, thus strengthening nitrogen metabolism. Nitrospiraceae, Rhizobiales_Incertae_Sedis and Gemmatimonadaceae related to nitrogen removal and Bacillaceae with phosphate-solubilizing ability could be used as microbial indicators of nitrogen removal in SWISs. The reciprocal action of environmental on microbial characteristics exhibited microbial functional were related to DO, Fe²⁺, TOC, TP, TN, NH₄⁺-N and NO₃^--N. Those could be used as physicochemical and biological indicators for application and monitoring of SWIS. In conclusion, this study provided a low-cost and efficient enhancement approach for the application of SWIS in decentralized domestic sewage treatment, and furnished theoretical support for subsequent applications.

Co-treatment of landfill leachate with urban wastewater by chemical, physical and biological processes: Fenton oxidation preserves autochthonous bacterial community in the activated sludge process

The impact of Fenton oxidation (FO) and Air stripping (AS) pre-treatments on the bacterial community of a biological activated sludge (B-AS) process for the co-treatment of mature landfill leachate (MLL) and urban wastewater (UWW) was assessed. In this work high-throughput sequencing was used to identify changes in the composition of the bacterial communities when exposed to different landfill leachate's pre-treatments. The combination of FO and AS to increase biodegradability (BOD₅/COD) and reduce ammonia concentration (NH₃) respectively, allowed to successfully operate the B-AS and effectively treat MLL. In particular, BOD₅/COD resulted to be the key factor for bacterial community shifting. The microbiological community of the B-AS, mainly composed by the phylum Bacteroidota (Saprospiraceae, PHOS-HE51, Chitinophagaceae) after FO pre-treatment, shifted to Pseudomonadota (Caulobacteraceae and Hyphomicrobiaceae) when FO was not used. At the same time a drastic reduction in BOD₅ removal was observed (90%-58%). On the other hand, high NH₃ concentration affected the abundance of the family Saprospiraceae, known to play a key role in the degradation of complex organic compounds in B-AS. The results obtained suggest that a suitable combination of pre-treatments can reduce the negative effect of MLL on the B-AS process, reducing the pressure on autochthonous bacteria and therefore the acclimatization time of the biological process.

Enhancement and mechanisms of iron-assisted anammox process

Anaerobic ammonium oxidation (anammox) is a sustainable biological nitrogen removal technology that has limited large-scale applications owing to the low cell yield and high sensitivity of anammox bacteria (AnAOB). Fortunately, iron-assisted anammox, being a highly practical method could be an effective solution. This review focused on the iron-assisted anammox process, especially on its performance and mechanisms. In this review, the effects of iron in three different forms (ionic iron, zero-valent iron and iron-containing minerals) on the performance of the anammox process were systematically reviewed and summarized, and the strengthening effects of Fe (II) seem to be more prominent. Moreover, the detailed mechanisms of iron-assisted anammox in previous researches were discussed from macro to micro perspectives. Additionally, applicable iron-assisted methods and unified strengthening mechanisms for improving the stability of nitrogen removal and shortening the start-up time of the system in anammox processes were suggested to explore in future studies. This review was intended to provide helpful information for scientific research and engineering applications of iron-assisted anammox.

Promotion of nitrogen removal in a zero-valent iron-mediated nitrogen removal system operated in co-substrate mode

In this study, the performance and mechanism of nitrogen removal were investigated in a zero-valent iron-mediated nitrogen removal system operated in co-substrate mode with sodium acetate as the organic carbon source. The results showed that the additional organic matter had the capacity to promote NH4+-N and total inorganic nitrogen (TIN) removal with efficiencies of 91.09% and 84.10%, and increases of 60.06% and 75.32% compared with the control group, respectively. The organic matter also stimulated the production of extracellular polymer substances that reduced the passivation and toxicity of iron to microorganisms. The ammonia oxidation activity was 2.5 times higher than that in the control group, and the anammonia oxidation activity and denitrification activity were substantially higher than in the control group with TIN removal efficiencies of 1.02 and 1.19 mgN/(gVSS·d), respectively. In addition, the organic matter increased the enrichment of the heterotrophic denitrification bacterium Diaphorobacter and facultative iron salt-based bacterium Dechloromonas.

Autotrophic Fe-Driven Biological Nitrogen Removal Technologies for Sustainable Wastewater Treatment

Fe-driven biological nitrogen removal (FeBNR) has become one of the main technologies in water pollution remediation due to its economy, safety and mild reaction conditions. This paper systematically summarizes abiotic and biotic reactions in the Fe and N cycles, including nitrate/nitrite-dependent anaerobic Fe(II) oxidation (NDAFO) and anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox). The biodiversity of iron-oxidizing microorganisms for nitrate/nitrite reduction and iron-reducing microorganisms for ammonium oxidation are reviewed. The effects of environmental factors, e.g., pH, redox potential, Fe species, extracellular electron shuttles and natural organic matter, on the FeBNR reaction rate are analyzed. Current application advances in natural and artificial wastewater treatment are introduced with some typical experimental and application cases. Autotrophic FeBNR can treat low-C/N wastewater and greatly benefit the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.

Removal of bisphenol A by waste zero-valent iron regulating microbial community in sequencing batch biofilm reactor

The removal of bisphenol A (BPA) by waste zero-valent iron (ZVI) regulating microbial community in sequencing batch biofilm reactor (SBBR) was investigated. Compared with SBBR_-BPA, the acclimation time of microorganisms in the presence of waste ZVI and BPA (SBBR_-ZVI+BPA) decreased from 56 d to 49 d. During stable operation period, BPA was removed completely at 150th min and 100th min in the SBBR_-BPA and SBBR_-ZVI+BPA, respectively. The optimal initial pH and BPA concentration in the SBBRs were respectively 8.0 and 10 mg/L. The composition and content analysis of extracellular polymeric substances (EPS) using fluorescence spectrometer showed that the yield of EPS was enhanced by the addition of ZVI. The analysis of microbial community structure in the SBBRs using Illumina Miseq sequencing method indicated that the indexes of ACE, Chao1 and Shannon were higher and Simpson index was lower in the SBBR_-ZVI+BPA. Moreover, the abundance of BPA biodegradation strains was increased in the presence of ZVI. This study provided a promising method with low cost of effectively removing BPA from wastewater.

A state-of-the-art review of quinoline degradation and technical bottlenecks

Quinoline is a critical raw material for the dye, metallurgy, pharmaceutical, rubber, and agrochemical industries, and its use poses a serious threat to human health and the ecological environment. Quinoline has carcinogenic, teratogenic and mutagenic effects on the human body through food accumulation. However, due to the steric hindrance of its bicyclic fused structure and its long photooxidation half-life, quinoline is too difficult to decompose naturally. To date, numerous technologies have been used to degrade quinoline, whereas only a few have been reviewed. Therefore, this paper is focused on offering a comprehensive overview of the state of quinoline degradation in an effort to improve its degradation efficiency and fully utilize the carbon and nitrogen within quinoline without causing any damage to the environment. Accordingly, the strains, research progress and mechanisms of various methods for degrading quinoline are explored and elucidated in detail, especially quinoline biodegradation and the combination of these technologies for efficient removal. The state-of-the-art processes and new findings of our team on the biofortification of quinoline degradation are also presented. Finally, research bottlenecks and gaps for future research were identified along with the prospects and resource utilization of quinoline. These discussions facilitate the realization of the zero discharge of quinoline.

Construction of autotrophic nitrogen removal system based on zero-valent iron (ZVI): performance and mechanism

In this study, the performance and mechanism of nitrogen removal in sequencing batch reactors (SBRs) with and without zero-valent iron (ZVI) was investigated. The results showed that ZVI had a capacity to promote NH₄⁺-N conversion, NO₂^--N accumulation and total inorganic nitrogen (TIN) removal, with the TIN removal rate being increased by 29.45%. The ZVI also had a significant impact on microbial community structure by means of high-throughput pyrosequencing, increasing the enrichment of Anammox (anaerobic ammonium oxidation) bacteria Candidatus Brocadia and Feammox (anaerobic ferric ammonium oxidation) bacteria Ignavibacterium. With ZVI addition, the main pathway of nitrogen removal was changed from nitrification-heterotrophic denitrification to Anammox and Feammox.

Enhanced phycocyanin and DON removal by the synergism of H2O2 and micro-sized ZVI: Optimization, performance, and mechanisms

Micro-sized zero-valent iron (mZVI) has proven effective for phycocyanin (PC) removal, but efficiency needs to be enhanced. Here, hydrogen peroxide (H₂O₂) was used to enhance PC removal by mZVI and the corresponding mechanisms are discussed. The results showed that H₂O₂ could effectively enhance PC removal by mZVI and the PC removal efficiency increased from 37.8% to 80.6% with 1.5 g/L mZVI in 60 min reaction time. The trends of dissolved organic nitrogen (DON) removal were consistent with PC removal. Low pH value, high mZVI dosage, and a suitable amount of H₂O₂ were conducive to PC removal. The SEM-mapping indicated that PC removal was not primarily by adsorption. Similarly, no obvious change was observed in PC molecular structure based on fluorescence spectroscopy and SDS-PAGE analyses. However, the PC removal mechanism could be inferred from the variation of iron concentration in the process. The coagulation of dissolved iron ions dissolved from mZVI was the main removal pathway. The OH oxidation only accounted for 20% of PC removal. PC removal led to the reduction of disinfection by-products with similar efficiency. The combination of mZVI and H₂O₂ is a promising strategy for the simultaneous removal of PC and DON in drinking water treatments.

Removal efficiency and mechanism of phycocyanin in water by zero-valent iron

The efficiencies and mechanism of phycocyanin removal from water by zero-valent iron (ZVI) were studied. The trend for dissolved organic nitrogen removal was similar to phycocyanin and the removal efficiency was high at ∼81% and 95%, respectively, in 90 min. The experimental results showed that the phycocyanin removal efficiency was higher at pH < 6, with an almost complete removal. However, only 68% was removed at pH 9. Within 30 min, the removal efficiency of phycocyanin for 1-4 tested cycles was reduced from 55.8% to 15.2%. Scanning electron microscopy and energy dispersive spectroscopy, Fourier transform infrared spectroscopy analysis and X-ray photoelectron spectroscopy were used to analyze the mechanisms of phycocyanin removal, which indicated that a small amount of phycocyanin was immobilized on the ZVI surface by adsorption. In addition, the main removal pathway was coagulation by dissolved iron ions. The Fe oxide formed in situ from ZVI had a higher removal efficiency than that in FeCl₃, which can play improved roles in charge neutralization. The production of disinfection byproducts also decreased because of the decrease of precursors.