Positive effects of magnetic Fe3O4@polyaniline on aerobic granular sludge: Aerobic granulation, granule stability and pollutants removal performance

Promoting mechanism of nitrogen removal by Fe3O4 magnetic particles during the start-up phase in sequencing batch reactor

In this paper, a magnetic sequencing batch reactor (SBR) was constructed, and the influence rule of magnetic particle dosing performance of denitrification was investigated. The diversity, structure, and potential functions of the microbial community were comprehensively explored. The results showed that the particle size and the dosage of Fe3O4 magnetic particles were the main parameters affecting the sedimentation performance of activated sludge. The start-up phase of the SBR reactor with Fe3O4 magnetic particles was 5 days less than the control. Moreover, total nitrogen removal efficiency during the start-up phase was improved, with the maximum value reaching 91.93%, surpassing the control by 9.7% with the Fe3O4 dosage of 1.2 g L-1. In addition, the activated sludge concentration and dehydrogenase activity were improved, compared to the control. High-throughput sequencing showed that the denitrifying bacterium Saccharimonadales dominated the reactor and was enriched by magnetic particles. According to predicted functions, the abundance of genes for denitrification increased with the addition of magnetic particles, suggesting the capacity of nitrogen removal was enhanced in the microbial community. Overall, the Fe3O4 magnetic particles provide great potential for enhanced wastewater nitrogen removal.

Enhanced aerobic granular sludge with micro-electric field for sulfamethoxazole degradation: Efficiency, mechanism, and microbial community

In this study, an aerobic granular sludge electrochemical system (AGES) was established by applying the micro-electric field to an aerobic granular sludge (AGS) reactor for the degradation of sulfamethoxazole (SMZ). Under the stimulation of the micro-electric field, the granulation of sludge was improved and the degradation rate of SMZ was enhanced. The features of granular sludge were characterized by scanning electron microscopy and X-ray diffraction. The optimal degradation rate of SMZ (88%) was obtained at the voltage of 3 V and the effective electrode area of 800 mm2. The results of kinetics analyses revealed that the degradation of SMZ by AGES can be fitted with the second-order kinetic equation, showing a degradation rate constant (k) of 0.001 L mol-1·min-1. The degradation products of SMZ in the AGES system were detected by LC-MS and their possible degradation routes were elucidated. The micro-electric field in the AGES system played a selective role in microbes' enrichment and growth, changing the diversity of the microbial community. Pseudomonas, Tolumonas, and Acidovorax were the dominant bacteria in the AGES system, which is accountable for the abatement of SMZ and nutrients. This work provides a green means for improving AGS and paves the way for applying the AGS process to real-world wastewater treatment.

Role of Nano-Fe3O4 for enhancing nitrate removal in microbial electrolytic cells: Characterizations and microbial patterns of cathodic biofilm

Conductive magnetite nanoparticle (Nano-Fe3O4) can facilitate numerous biological reduction reactions as an outstanding electron mediator for electron transfer. The positive role of Nano-Fe3O4 for nitrate removal has gradually gained attention recent years, however, it has not been clarified for the persistence of the promoting effect under different concentrations addition. Performance of nitrogen removal and characteristics of cathodic biofilm were evaluated in this study after Nano-Fe3O4 addition with gradient concentration of 100∼500 mg L-1 in microbial electrolytic cells (MEC). Our study illustrated that the optimal concentration was 200 mg L-1 as the removal rate of nitrate increased by 24.76% and the removal rate of total dissolved nitrogen by 29.72%. At the optimal concentration, Nano-Fe3O4 increased cathodic biofilm DNA concentration by 61.04%, enhanced electron transport system activity, enriched iron redox bacteria, denitrifying bacteria and genes, as well as increased extracellular polymeric substances (EPS) amount, especially the protein content of soluble-EPS. However, promoting effect on nitrate removal was not visible in high concentration (500 mg L-1) addition, its electron transport system activity and EPS content were even declined. XPS results indicated that high concentration of Nano-Fe3O4 may reduce the availability of electrons to cathodic biofilm by competing for electrons, which inhibit nitrate removal.

Rapid, stable, and highly-efficient development of salt-tolerant aerobic granular sludge by inoculating magnetite-assisted mycelial pellets

Long cultivation time hinders the industrial applications of aerobic granular sludge (AGS) in treatment of hypersaline wastewater. Mycelial pellets (MPs) have been used to efficiently strengthen the flocculent sludge aggregation and accelerate the formation of AGS. However, the MPs-based AGS was easily crushed or fragmented into several small pieces/granules that brought the uncertainty and extended the transition process to form mature AGS. In this study, magnetite was used to strengthen MPs (halotolerant fungus Cladosporium tenuissimum NCSL-XY8), and co-culture and adsorption type of magnetite-assisted mycelial pellets (CMMPs and AMMPs) were prepared and used for acceleration of salt-tolerant aerobic granular sludge (SAGS) cultivation under 3% salinity conditions. Compared to inoculating MPs, the inoculation of either CMMPs or AMMPs could stably transition to mature SAGS without evident fragmentation, which obviously increased the certainty and stability of SAGS formation. Also, highly-efficient simultaneous nitrogen and carbon removal (∼98% TOC and ∼80% TN removal) could be reached in 8 days. Typically, the granules maintained perfect characteristics (D50 > 1300 μm, D10 > 350 μm, SVI30 < 45 mL/g, and SVI30/SVI5 = 1.0) during the whole cultivation/transition processes (Day 0-55) by using the inoculum of CMMPs. ITS rDNA sequencing revealed the inoculated fungus Cladosporium tenuissimum played key roles in the formation of SAGS. All the phenomena indicated the rapid, stable, and highly-efficient start-up of SAGS could be successfully realized by inoculating CMMPs.

Synergistic analysis of performance, microbial community, and metabolism in aerobic granular sludge under polyacrylonitrile microplastics stress

Aerobic granular sludge (AGS) has proved to be a promising biotechnology for microplastics wastewater treatment. However, polyacrylonitrile microplastics (PAN MPs), the most widely used plastic in textile materials, have not been investigated. Therefore, the effect of the neglected PAN MPs on AGS at different concentrations (1, 10, and 100 mg/L) was evaluated. The results indicated that PAN MPs with 1 and 10 mg/L concentrations had no obvious effect on granular stability and nutrient removal performance, but greatly promoted the secretion of EPS. Remarkably, the granule structure was severely damaged under 100 mg/L PAN MPs. Moreover, microbial community analysis showed that phylum Proteobacteria played a dominant role in resistance to PAN MPs. Metabolic analysis further revealed that genes related to denitrification pathway (nasA, nirK, nirS and norB) and membrane transport were significantly inhibited under PAN MPs stress. This study may provide additional information on the treatment of microplastics wastewater using AGS.

Ball-milled magnetic sludge biochar enables fast aerobic granulation in anoxic/oxic process for the treatment of coal chemical wastewater

Coal chemical wastewater (CCW) containing toxic and hazardous matters requires to be treated prior to discharge. Promoting the in-situ formation of magnetic aerobic granular sludge (mAGS) in continuous flow reactor process has a great potential for CCW remediation. However, long granulation time and low stability limit the application of AGS technology. In this study, Fe₃O₄/sludge biochar (Fe₃O₄/SC) with biochar matrix derived from coal chemical sludge were applied to facilitate the aerobic granulation in two-stage continuous flow reactors, containing separated anoxic and oxic reaction units (abbreviated as A/O process). The performance of A/O process was evaluated at various hydraulic retention times (HRTs) (42 h, 27 h, and 15 h). Magnetic Fe₃O₄/SC with porous structures, high specific surface area (BET = 96.69 m²/g), and abundant functional groups was successfully prepared by ball-milled method. Adding magnetic Fe₃O₄/SC to A/O process could promote aerobic granulation (85 days) and the removal of chemical oxygen demand (COD), ammonia nitrogen (NH₄⁺-N)_, and total nitrogen (TN) from CCW at all tested HRTs. Since the formed mAGS had high biomass, good settling ability, and high electrochemical activities, mAGS-based A/O process had high tolerance to the decrease of HRT from 42 h to 15 h for CCW treatment. The optimized HRT for A/O process was 27 h, at which Fe₃O₄/SC addition can result in the increase of COD, NH₄⁺-N and TN removal efficiencies by 2.5 %, 4.7 % and 10.5 %, respectively. Based on 16S rRNA genes sequencing, the relative abundances of genus Nitrosomonas, Hyphomicrobium/Hydrogenophaga and Gaiella in mAGS accounting for nitrification, denitrification as well as COD removal were increased during aerobic granulation. Overall, this study proved that adding Fe₃O₄/SC to A/O process was effective for facilitating aerobic granulation and CCW treatment.

Mechanisms of persistence and impact of ordinary heterotrophic organisms in aerobic granular sludge

The stability of granules, contaminant removal and microbial structure of an aerobic granular sludge (AGS) process were investigated with a focus on ordinary heterotrophic organisms (OHOs). Long-term stable granules and high removals of COD (97 %), NH₄⁺ (98 %), P (85 %) and total N (77 %) were achieved. Sequencing analyses identified 6.6 % of phosphorus-accumulating organisms in the sludge, concordant with the observed bio-P removal capacity. However, OHOs were the most abundant bacteria in the sludge (70-93 %) without resulting in unstable aggregates. Under current dogmas of microbial competition in activated sludge, it seemed contradictory that OHOs could persist in the long term in the AGS where COD was depleted beginning in the anaerobic phase. Microbial analyses showed that OHOs could survive in granules by micropredation, proteolysis, fermentation and EPS consumption. Heterotrophic-nitrification/ aerobic-denitrification was an active pathway in the AGS. These findings contribute to a better understanding of microbial competition in AGS and its stability.