Microbial community and N removal of aerobic granular sludge at high COD and N loading rates

Organic acids released by submerged macrophytes with damaged leaves alter the denitrification microbial community in rhizosphere

Submerged macrophytes have important impacts on the denitrification and anaerobic ammonia-oxidizing (anammox) processes. Leaf damage in these plants probably changes the rhizosphere environment, affecting organic acid release and denitrifying bacteria. However, there is a lack of comprehensive understanding of the specific changes. This study investigated these changes in the rhizosphere of Potamogeton crispus with four degrees of leaf excision. When 0 %, 30 %, 50 % and 70 % of leaves were excised, the concentrations of total organic acid were 31.45, 32.67, 38.26, and 35.16 mg/L, respectively. The abundances of nirS-type denitrifying bacteria were 2.10 × 1010, 1.59 × 1010, 2.54 × 1010, and 4.67 × 1010 copies/g dry sediment, respectively. The abundances of anammox bacteria were 7.58 × 109, 4.59 × 109, 3.81 × 109, and 3.90 × 109 copies/g dry sediment, respectively. The concentration of total organic acids and the abundance of two denitrification microorganisms in the rhizosphere zone were higher than those in the root zone and non-rhizosphere zone. With increasing leaf damage, the number of OTUs in the Pseudomonas genus of nirS-type denitrifying bacteria first increased and then decreased, while that of the Thauera genus was relatively stable. The overall increase in the OTU number of anammox bacteria indicated that leaf damage promotes root exudates release, thereby leading to an increase in their diversity. The co-occurrence network revealed that the two denitrification microorganisms had about 60.52 % positive connections in rhizosphere while 64.73 % negative connections in non-rhizosphere. The abundance and community composition of both denitrification microorganisms were positively correlated with the concentrations of various substances such as oxalic acid, succinic acid, total organic acids and NO2--N. These findings demonstrate that submerged plant damage has significantly impacts on the structure of denitrification microbial community in the rhizosphere, which may alter the nitrogen cycling process in the deposit sediment. SYNOPSIS: This study reveals leaf damage of macrophyte changed the rhizosphere denitrification microbial community, which is helpful to further understand the process of nitrogen cycle in water.

Nitrogen metabolism network in the biotreatment combination of coking wastewater: Take the OHO process as a case

As steel production increases, large volumes of highly toxic and nitrogen-rich coking wastewater (CWW) are produced, prompting the development of a novel oxic-hydrolytic-oxic (OHO) biological treatment combination designed for highly efficient removal of nitrogen-contained contaminants. However, previous studies have not comprehensively explored the CWW biotreatment from the perspective of nitrogen metabolism functional genes and pathways. Based on the investigation of taking the full-scale OHO biotreatment combination as a case, it was found that the O1 and O2 bioreactors remove nitrogen through the ammonia assimilation accounting for 33.87% of the total nitrogen (TN) removal rate, while the H bioreactor removes nitrogen through the simultaneous nitrification-denitrification accounting for 61.11% of the TN removal rate. The major ammonia assimilation taxa include Thauera, Immundisolibacter and Thiobacillus; the major nitrifying taxa include Nitrospira and Nitrosomonas; and the major denitrifying taxa include Thiobacillus, Lautropia and Mesorhizobium. Additionally, the H bioreactor exhibits the potential to be optimized for simultaneous nitrification-denitrification coupled with anaerobic ammonium oxidation (Anammox). These understandings will guide the optimization of engineering design and operational practices, contributing to more effective and sustainable wastewater treatment strategies.

Characteristics and performance of algal-bacterial granular sludge in photo-sequencing batch reactors under various substrate loading rates

The algae-bacterial granular sludge (ABGS) technology has garnered significant attention due to its remarkable attributes of low carbon emissions. To investigate the performance of the ABGS system under various substrate loading rates, the parallel photo-sequencing batch reactors (P1 and P2) were set up. The results indicated that chlorophyll-a content and extracellular polymeric substance content were measured at 10.7 ± 0.3 mg/L and 61.4 ± 0.7 mg/g SS in P1 under relatively low substrate loading rate (0.9 kg COD/m3/d and 0.09 kg N/m3/d). Moreover, kinetic study revealed that the maximal specific P uptake rate for P1 reached 0.21 mg P/g SS/h under light conditions, and it achieved 0.078 mg P/g SS/h under dark conditions, highlighting the significant role on phosphorus removal played by algae in the ABGS system. The microbial analysis and scanning electron microscopy confirmed that filamentous algae predominantly colonize the surface in P1, whereas spherical bacteria dominate the surface of granular sludge in P2. Additionally, a diverse array of microorganisms including bacteria, algae, and metazoa such as Rotifers and Nematodes were observed in both systems, providing evidence for the establishment of a symbiotic system. This study not only confirmed the ability of ABGS for efficient N and P removal under different substrate loading conditions but also highlighted its potential to enhance the ecological diversity of the reaction system.

Efficient nitrogen removal from ammonia rich wastewater using aerobic granular sludge (AGS) reactor: Selection and enrichment of effective microbial community

Anaerobically digested sludge supernatant, characterized by its high ammonia and low biodegradable chemical oxygen demand (COD) content, has raised concerns when returned to mainstream treatment lines due to potential impacts on effluent quality. Addressing this, an aerobic granular sludge (AGS) reactor adopted nitritation/denitritation with external COD addition was utilized and achieved a considerable nitrogen treatment capacity of 4.2 kg N/m3/d, reaching over 90% removal efficiencies for both ammonia and total inorganic nitrogen. This study applied progressively increased nitrogen loading to select for a microbial community that exhibited high nitrogen oxidation and reduction rates, demonstrating peak rates of 0.5 g N/g VSS/d and 3 g N/g VSS/d, respectively. The enrichment of highly efficient microbial community was achieved along with the increased biomass density peaked at 17 g/L MLVSS, with the system retaining small-sized granular sludge at 0.5 mm. The primary ammonia oxidizing bacteria was Nitrosomonas, while Thauera was the dominated denitrifiers. Quantitative polymerase chain reaction analyses reinforced the enhanced nitrogen removal capacity based on the progressively increased abundance of nitrogen cycling functional genes. The high nitrogen treatment capacity, synergistic attributes of high specific microbial activities and the substantial biomass retention, suggest the AGS's efficacy and capacity in ammonia rich wastewater treatment.

Mixotrophic denitrification process driven by lime sulfur and butanediol: Denitrification performance and metagenomic analysis

Heterotrophic sulfur-based autotrophic denitrification is a promising biological denitrification technology for low COD/TN (C/N) wastewater due to its high efficiency and low cost. Compared to the conventional autotrophic denitrification process driven by elemental sulfur, the presence of polysulfide in the system can promote high-speed nitrogen removal. However, autotrophic denitrification mediated by polysulfide has not been reported. This study investigated the denitrification performance and microbial metabolic mechanism of heterotrophic denitrification, sulfur-based autotrophic denitrification, and mixotrophic denitrification using lime sulfur and butanediol as electron donors. When the influent C/N was 1, the total nitrogen removal efficiency of the mixotrophic denitrification process was 1.67 and 1.14 times higher than that of the heterotrophic and sulfur-based autotrophic denitrification processes, respectively. Microbial community alpha diversity and principal component analysis indicated different electron donors lead to different evolutionary directions in microbial communities. Metagenomic analysis showed the enriched denitrifying bacteria (Thauera, Pseudomonas, and Pseudoxanthomonas), dissimilatory nitrate reduction to ammonia bacteria (Hydrogenophaga), and sulfur oxidizing bacteria (Thiobacillus) can stably support nitrate reduction. Analysis of metabolic pathways revealed that complete denitrification, dissimilatory nitrate reduction to ammonia, and sulfur disproportionation are the main pathways of the N and S cycle. This study demonstrates the feasibility of a mixotrophic denitrification process driven by a combination of lime sulfur and butanediol as a cost-effective solution for treating nitrogen pollution in low C/N wastewater and elucidates the N and S metabolic pathways involved.

Polydopamine and calcium functionalized fiber carrier for enhancing microbial attachment and Cr(VI) resistance

The formation of biofilm determines the performance and stability of biofilm system. Increasing the hydrophilicity of the carrier surface could efficiently accelerate the attachment and growth of microorganisms. Here, the surface of polypropylene (PP) fiber carrier was modified with polydopamine (PDA) and calcium (Ca(II)) to enhance microbial attachment and toxicity resistance. The results of surface characteristic confirmed the self-polymerization of PDA and the chelation mechanism of Ca(II). Subsequently, the biofilm formation experiments were conducted in sequencing batch biofilm reactors using both normal and chromium-containing wastewater. The biofilm on the surface of the modified carrier exhibited better nitrogen removal and Cr(VI) reduction ability. The biomass of the modified carrier was significantly increased, and the maximum microbial attachment amounts in normal wastewater and chrome-containing wastewater were 1153.34 and 511.78 mg/g carrier, respectively. Furthermore, the confocal laser scanning microscope (CLSM) indicated that the modified carrier coated with PDA and Ca(II) were both biocompatible, and the cell activity was significantly increased. 16S rRNA sequencing results showed that the modified carrier efficiently enriched both denitrification bacteria (Thauera and Flavobacterium) and chrome-reducing bacteria (Simplicispira and Arenimonas) to improve system stability and Cr(VI) resistance. Microbial phenotype prediction based on BugBase analysis further verified the enrichment effect of modified carriers on microorganisms responsible for biofilm formation and oxidative stress resistance. Overall, this work proposed a novel functional carrier that could provide references for advancing the application of biofilm systems in wastewater 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.

Pilot-scale study of step-feed anaerobic coupled four-stage micro-oxygen gradient aeration process for treating digested swine wastewater with low carbon/nitrogen ratios

This study developed an innovative step-feed anaerobic coupled four-stage micro-oxygen gradient aeration process to treat digested swine wastewater. An anaerobic zone was used for prepositive denitrification; four micro-oxygen reactors (zones O1-O4) were used for simultaneous partial nitrification and denitrification through low-dissolved oxygen gradient control, step-feed, and swine wastewater-digested swine wastewater distribution. The nitrogen-removal efficiency was satisfactory (93 ± 3 %; effluent total nitrogen, 53 ± 19 mg/L). Mass balance coupled with quantitative polymerase chain reaction analysis revealed that simultaneous partial nitrification and denitrification was achieved in four micro-oxygen zones. Zones O1 were the major denitrification zones for nitrogen removal; nitrification was primary happened in zones O2 and O3. Correlation analysis confirmed that low-dissolved oxygen gradient control was the key to achieving efficient nitrogen removal. This study provides a low oxygen energy consumption method to treat digested swine wastewater with a low carbon/nitrogen ratio (<3).

Responses of various carbon to nitrogen ratios to microbial communities, kinetics, and nitrogen metabolic pathways in aerobic granular sludge reactor

The role of different ammonia concentrations (mg N/L) (of 100 (carbon to nitrogen ratio (C/N) = 12; Stage I), 200 (C/N = 6; Stage II), 400 (C/N = 3; Stage III) and 200 (C/N = 6; Stage IV)) in nitrogen metabolic pathways, microbial community, and specific microbial activity were investigated in an aerobic granular sludge reactor. Heterotrophic ammonia oxidizing bacteria (HAOB) showed higher ammonia oxidation rates (AORs) than autotrophic ammonia oxidizing bacteria (AAOB) at higher C/N conditions (Stages I and II). Paracoccus was the dominant HAOB. AAOB, with only 0.2-0.3 % in relative abundance, showed 2.7-fold higher AORs than HAOB at elevated ammonia and free ammonia (FA) concentrations with C/N at 3. Nitrosomonas and a genus in Nitrosomondaceae family were the major AAOB. This study proposed that FA inhibition on heterotrophic bacteria might be the mechanism that contributes to the development of the autotrophic ammonia oxidation pathway and enrichment of AAOB.

Ecological restoration performance enhanced by nano zero valent iron treatment in constructed wetlands under perfluorooctanoic acid stress

Perfluorooctanoic acid (PFOA) of widespread use can enter constructed wetlands (CWs) via migration, and inevitably causes negative impacts on removal efficiencies of conventional pollutants due to its ecotoxicity. However, little attention has been paid to strengthen performance of CWs under PFOA stress. In this study, influences of nano zero valent iron (nZVI), which has been demonstrated to improve nutrients removal, were explored after exemplifying threats of PFOA to operation performance in CWs. The results revealed that 1 mg/L PFOA suppressed the nitrification capacity and phosphorus removal, and nZVI distinctly improved the removal efficiency of ammonia and total phosphorus in CWs compared to PFOA exposure group without nZVI, with the maximum increases of 3.65 % and 16.76 %. Furthermore, nZVI significantly stimulated dehydrogenase (390.64 % and 884.54 %) and urease (118.15 % and 246.92 %) activities during 0-30 d and 30-60 d in comparison to PFOA group. On the other hand, nitrifying enzymes were also promoted, in which ammonia monooxygenase increased by 30.90 % during 0-30 d, and nitrite oxidoreductase was raised by 117.91 % and 232.10 % in two stages. Besides, the content of extracellular polymeric substances (EPS) under nZVI treatment was 72.98 % higher than PFOA group. Analyses of Illumina Miseq sequencing further certified that nZVI effectively improved the community richness and caused the enrichment of microorganisms related to nitrogen and phosphorus removal and EPS secreting. These results could provide valuable information for ecological restoration and decontamination performance enhancement of CWs exposed to PFOA.

Mature landfill leachate treatment using granular sludge-based reactor (GSR) via nitritation/denitritation: Process startup and optimization

Mature landfill leachate wastewater (LLW) was characterized by high ammonia, refractory chemical oxygen demand (COD) and heavy metal contents, which limits the nitrogen removal in conventional activated sludge systems. Granular sludge is known to be more resistant to toxic compounds because of its dense structure and diverse microbial community. Here, granular sludge-based reactor (GSR) was applied with nitritation/denitritation (Nit/DNit) process for effective ammonia-rich mature LLW treatment at 20 °C. After a short startup period, the efficiencies of ammonia removal and total inorganic nitrogen removal stabilized at 99 % and 93 %, respectively, under a hydraulic retention time (HRT) of 6 h. High ammonia oxidation rate (~ 0.64 g N/g VSS/d) was achieved, with ~93 % ammonia conversing to nitrite before being reduced to nitrogen gas. Microbial analysis results revealed that Nitrosomonas (ammonia oxidizing bacteria) and Thauera (denitrifiers) were the dominant bacteria with key functional genes involved in the Nit/DNit. With an increase in the LLW loading, increased ammonia oxidation rates and biomass retention were also observed. This study demonstrated that granular sludge-based technology is feasible for mature LLW treatment.

Unravelling gradient layers of microbial communities, proteins, and chemical structure in aerobic granules

Most bacteria live in microbial assemblages like biofilms and granules, and each layer of these assemblages provides a niche for certain bacteria with specific metabolic functions. In this study, a gentle (non-destructive) extraction approach based on a cation exchange resin and defined shear was employed to gradually disintegrate biomass and collect single layers of aerobic granules from a full-scale municipal wastewater treatment plant. The microbial community composition of granule layers was characterized using next-generation sequencing (NGS) targeting the 16S rRNA gene, and protein composition was investigated using metaproteomics. The chemical composition of eroded layers was explored using Fourier Transformed Infrared Spectroscopy. On the surface of the granules, the microbial structure (flocculation-supporting Nannocystis sp.) as well as composition of extracellular polymers (extracellular DNA) and proteome (chaperonins and binding proteins) favored microbial aggregation. Extracellular polymeric substances in the granules were composed of mostly proteins and EPS-producers, such as Tetrasphaera sp. and Zoogloea sp., were evenly distributed throughout the granule structure. The interior of the granules harbored several denitrifiers (e.g., Thauera sp.), phosphate-accumulating denitrifiers (Candidatus Accumulibacter, Dechloromonas sp.) and nitrifiers (Candidatus Nitrotoga). Proteins associated with glycolytic activity were identified in the outer and middle granule layers, and proteins associated with phosphorus conversions, in the deeper layers. In conclusion, the use of an existing cation-exchange resin for gradual biomass disintegration, combined with NGS and metaproteomic analysis was demonstrated as a promising approach for simultaneously investigating the identity and functions of microbes in multilayered biofilm structures.

Ordinary heterotrophic organisms with aerobic storage capacity provide stable aerobic granular sludge for C and N removal

The study investigated the mechanisms and microbial communities underlying the long-term stability and removal performances shown by aerobic granular sludge (AGS) reactor involving polyhydroxyalkanoates (PHA) aerobic-storing bacteria. The characteristics of the sludge, removal performances and bacterial community structure were determined. The prevailing metabolic phenotype was similar in the parent conventional activated sludge (CAS) reactor and its upgraded AGS version, showing high COD and NH₄ uptake, versus low P and N reduction. Polyphosphate and glycogen accumulating organisms, PAO and GAO, were not enriched in the reactors despite initial targeting of anaerobic-aerobic cycle. Instead, PHA-aerobic storing bacteria (Thauera and Paracoccus) were dominant, but revealing a stable AGS system for BOD and N removal. The PAO/GAO failed selection and Thauera overgrowth were analyzed for beneficial use in developing alternative AGS technology for BOD and N removal applications.

Dynamics of microbial community and their effects on membrane fouling in an anoxic-oxic gravity-driven membrane bioreactor under varying solid retention time: A pilot-scale study

Membrane fouling in a membrane bioreactor (MBR) is highly influenced by the characteristics of the influent, the mixed liquor microbial community and the operational parameters, all of which are environment specific. Therefore, we studied the dynamics of microbial community during the treatment of real municipal wastewater in a pilotscale anoxic-oxic (A/O) MBR equipped with a gravity-driven membrane filtration system. The MBR was operated at three different solid retention times (SRTs): 25, 40 and 10 days for a total period of 180 days in Nordic environmental conditions. Analysis of microbial community dynamics revealed a high diversity of microbial species at SRT of 40 days, whereas SRT of 25 days was superior with microbial richness. Production of soluble microbial products (SMP) and extracellular polymeric substances (EPS) was found to be intensely connected with the SRT and food to microorganism (F/M) ratio. Relatively longer operational period with the lowest rate of membrane fouling was observed at SRT of 25 days, which was resulted from the superior microbial community, lowest production of SMP and loosely bound EPS as well as the lower filtration resistance of larger sludge flocs. Abundance of quorum quenching (QQ) bacteria and granular floc forming bacterial genera at SRT of 25 days provided relatively lower membrane fouling tendency and larger floc formation, respectively. On the other hand, substantial amount of various surface colonizing and EPS producing bacteria was found at SRT of 10 days, which promoted more rapid membrane fouling compared with the fouling rate seen at other tested SRTs. To sum up, this research provides a realistic insight into the impact of SRT on microbial community dynamics and resulting characteristics of mixed liquor, floc size distribution and membrane fouling for improved MBR operation.

Changes in BNR Microbial Community in Response to Different Selection Pressure

This study investigated structural changes in microbial community of biological nutrient removal (BNR) in response to changes in substrate composition (ammonium and phosphate), redox condition, and morphological characteristics (flocs to granules), with a focus on nitrification and phosphate removal. Analyzing treatment performance and 16S rRNA phylogenetic gene sequencing data suggested that heterotrophic nitrification (HN) and autotrophic nitrification (AN) potentially happened in aerobic organic-rich (HN_AS) and aerobic organic-deficient (AN_AS) activated sludge batch reactors, respectively. However, phosphate release and uptake were not observed under alternating anaerobic/aerobic regime. Phosphate release could not be induced even when anaerobic phase was extended, although Accumulibacter existed in the inoculum (5.1% of total bacteria). Some potential HN (e.g., Thauera, Acinetobacter, Flavobacterium), AN (e.g., Nitrosomonas (3.2%) and Nitrospira), and unconventional phosphate-accumulating organisms (PAOs) were identified. Putative HN bacteria (i.e., Thauera (29–36%) and Flavobacterium (18–25%)) were enriched in aerobic granular sludge (AGS) regardless of the granular reactor operation mode. Enrichment of HN organisms in the AGS was suspected to be mainly due to granulation, possibly due to the floc-forming ability of HN species. Thus, HN is likely to play a role in nitrogen removal in AGS reactors. This study is supposed to serve as a starting point for the investigation of the microbial communities of AS- and AGS-based BNR processes. It is recommended that the identified roles for the isolated bacteria are further investigated in future works.

Efficient treatment of digested piggery wastewater via an improved anoxic/aerobic process with Myriophyllum spicatum and bionic aquatic weed

The traditional anoxic/aerobic process (A/O) process is widely used for treating digested piggery wastewater, but the lack of carbon sources leads to poor efficiency. Therefore, the process needs optimization to achieve high-efficiency and low-cost operation mode. In this study, an improved A/O system with bionic aquatic weed and Myriophyllum sp. was established to decontaminate digested piggery wastewater. The average removal efficiencies of chemical oxygen demand (COD), NH₄⁺-N, and total nitrogen (TN) by the improved A/O system was satisfactory. The average removal efficiencies of COD, NH₄⁺-N, and TN were 62.1%, 87.5%, and 61.9%, respectively. High-throughput sequencing identified a number of dominant microorganisms. The relative abundance of Nitrosomonas (ammonia-oxidizing bacteria) and Nitrospira (nitrite-oxidizing bacteria) was 0.07%-3.52% and 0.32%-1.30%, respectively. Combining bionic aquatic weed and Myriophyllum sp. altered the microbial community structure and metabolic pathways. The results demonstrate a cost-effective method for treating digested piggery wastewater.

A Distinct, Flocculent, Acidogenic Microbial Community Accompanies Methanogenic Granules in Anaerobic Digesters

The formation of dense, well-settling methanogenic granules is essential for the operation of high-rate, up-flow anaerobic bioreactors used for wastewater treatment. Granule formation (granulation) mechanisms have been previously proposed, but an ecological understanding of granule formation is still lacking. Additionally, much of the current research on granulation only examines the start-up phase of bioreactor operation, rather than monitoring the fate of established granules and how new granules emerge over time. This paper, therefore, attempts to provide an insight into the microbial ecology of granule formation outside the start-up phase of bioreactor operation and develop an ecological granulation model. The microbial communities of granules actively undergoing growth, breakage, and reformation were examined, and an ecological granulation model was proposed. A distinct pregranular microbial community, with a high proportion of acidogenic organisms, such as the Streptococcaceae, was identified and suggested to have a role in initiating granulation by providing simpler substrates for the methanogenic and syntrophic communities which developed during granule growth. After initial granule formation, deterministic influences on microbial community assembly increased with granule size and indicated that microbial community succession was influenced by granule growth, leading to the formation of a stepwise ecological model for granulation. IMPORTANCE Complex microbial communities in engineered environments can aggregate to form surface-attached biofilms. Others form suspended biofilms, such as methanogenic granules. The formation of dense, methanogenic granules underpins the performance of high-rate, anaerobic bioreactors in industrial wastewater treatment. Granule formation (granulation) has been well studied from a physico-chemical perspective, but the ecological basis is poorly understood. We identified a distinct, flocculent, microbial community, which was present alongside granules, comprising primary consumers likely key in providing simpler substrates to granules. This flocculent community is understudied in anaerobic digestion and may initiate, or perpetuate, granule formation. We propose that it may be possible to influence bioreactor performance (e.g., to regulate volatile fatty acid concentrations) by manipulating this community. The patterns of microbial community diversity and assembly revealed by the study indicate that cycles of granule growth and breakage lead to overall diversification of the bioreactor meta-community, with implications for bioreactor process stability.

Effects of wastewater type on stability and operating conditions control strategy in relation to the formation of aerobic granular sludge - a review

Currently, research trends on aerobic granular sludge (AGS) have integrated the operating conditions of extracellular polymeric substances (EPS) towards the stability of AGS systems in various types of wastewater with different physical and biochemical characteristics. More attention is given to the stability of the AGS system for real site applications. Although recent studies have reported comprehensively the mechanism of AGS formation and stability in relation to other intermolecular interactions such as microbial distribution, shock loading and toxicity, standard operating condition control strategies for different types of wastewater have not yet been discussed. Thus, the dimensional multi-layer structural model of AGS is discussed comprehensively in the first part of this review paper, focusing on diameter size, thickness variability of each layer and diffusion factor. This can assist in facilitating the interrelation between disposition and stability of AGS structure to correspond to the changes in wastewater types, which is the main objective and novelty of this review.

Efficiency of sulfamethoxazole removal from wastewater using aerobic granular sludge: influence of environmental factors

The effects of adsorption, sulfamethoxazole (SMX) content, chemical oxygen demand (COD), and dissolved oxygen (DO) are recognized to be crucial for SMX removal in the aerobic granular sludge (AGS) system. Therefore, we investigated the impact of adsorption and these three different environmental factors on the SMX removal loading rate and removal efficiency of an AGS system, and determined the differences in microbial community composition under different environmental conditions. Adsorption was not the main SMX removal mechanism, as it only accounted for 5% of the total removal. The optimal SMX removal conditions were obtained for AGS when the COD, DO, and SMX concentrations were 600 mg/L, 8 mg/L, and 2,000 µg/L, respectively. The highest SMX removal efficiency was 93.53%. Variations in the three environmental factors promoted the diversity and changes of microbial communities in the AGS system. Flavobacterium, Thauera, and norank_f_Microscillaceae are key microorganisms in the AGS system. Thauera, and norank_f_Microscillaceae were sensitive to increases in SMX concentrations and beneficial for degrading high SMX concentrations. In particular, Flavobacterium abundances gradually decreased with increasing SMX concentrations.

Effect of the gradual increase of Na2SO4 on performance and microbial diversity of aerobic granular sludge

Aerobic granular sludge (AGS) is a promising technology in treating saline wastewater. The effects of sodium sulfate on contaminant removal performance and sludge characteristics of AGS were studied. The results showed that under the stress of sodium sulfate, AGS kept good removal performance of ammonia nitrogen (NH+ 4-N), chemical oxygen demand (COD), and total nitrogen (TN), with removal efficiency reaching 98.7%, 91.5% and 62.7%, respectively. When sodium sulfate reached 14700 mg/L, nitrite oxidizing bacteria (NOB) were inhibited and nitrite accumulation occurred, but it had little impact on total phosphorus (TP) removal. Under the stress of sodium sulfate, compactness and settling performance of AGS was enhanced. The microbial community greatly varied and the microbial diversity of aerobic granular sludge has decreased under the stress of sodium sulfate. The study reveals that AGS has great potential in application on treating saline wastewater.

Sulfur autotrophic denitrification filter and heterotrophic denitrification filter: Comparison on denitrification performance, hydrodynamic characteristics and operating cost

Sulfur autotrophic denitrification (SAD) process, as an alternative to heterotrophic denitrification (HD) filter, receives growing interest in polishing the effluent from secondary sewage treatment. Although individual studies have indicated several advantages of SAD over HD, rare study has compared these two systems under identical condition and by using real secondary effluent. In this study, two small pilot scale filters (SAD and HD) were designed with identical configuration and operated parallelly by feeding the real secondary effluent from a WWTP. The results showed SAD filter can be started up without the addition of soluble electron donor, although the time (14 days) was about 3 times longer than that of HD filter. The nitrate removal rate of SAD filter at HRT of 1.4 h was measured as 0.268 ± 0.047 kg N/(m³∙d). Similar value was observed in HD filter with supplementing 90 mg/L COD. The COD concentration of effluent always kept lower than that of influent in SAD filter but not in HD filter. In addition, SAD filter could maintain a stable denitrification performance without backwash for 15 days, while decline of nitrate removal rate was observed in HD filter just 2 days after stopping the backwash. This different behavior was further confirmed as the SAD filter had a better hydraulic flow pattern. Analysis according to high-throughput 16S rRNA gene-based Illumina MiSeq sequencing clearly showed the microbial community evolution and differentiation among the samples of seed sludge, SAD and HD filters. Finally, the economic assessment was carried out, showing the operation cost of SAD filter was over 50% lower than that of HD filter.

Effect of voltage intensity on the nutrient removal performance and microbial community in the iron electrolysis-integrated aerobic granular sludge system

The effects of voltage intensity on the nutrient removal performance and microbial community in the iron electrolysis-integrated aerobic granular sludge (AGS) system were investigated over a period of 15 weeks. Results revealed that the application outcomes of iron electrolysis for AGS systems relied on voltage intensity. When a constant voltage of 1.5 V was applied, the sludge granulation was most obviously accelerated with a specific growth rate of the sludge diameter of 0.078 day^-1, and the removal efficiencies of total nitrogen (TN) and total phosphorus (TP) increased by 14.1% and 20.2%, respectively, compared to the control reactor (without the iron electrolysis-integration). Moreover, the AGS developed at different voltages included different microbial communities, whose shifts were driven by the Fe content and the average diameter of AGS. Both heterotrophic nitrifiers and mixotrophic denitrifiers were significantly enriched in the AGS developed at 1.5 V, which effectively enhanced TN removal. Together with the response of the functional genes involved in Fe, N, and P metabolism, the electrolytic iron-driven nutrient degradation pathway was further elaborated. Overall, this study clarified the optimum voltage condition when iron electrolysis was integrated into the AGS system, and revealed the enhancement mechanism of this coupling technology on nutrient removal during the treatment of low-strength municipal wastewater.

A Bioreactor Designed for Restricting Oversize of Aerobic Granular Sludge

Aerobic granular sludge (AGS) with oversized diameter commonly affects its stability and pollutant removal. In order to effectively restrict the particle size of AGS, a sequencing batch reactor (SBR) with a spiny aeration device was put forward. A conventional SBR (R1) and an SBR (R2) with the spiny aeration device treating tannery wastewater were compared in the laboratory. The result indicates that the size of the granular sludge from R2 was smaller than that from R1 with sludge granulation. The spines and air bubbles could effectively restrict the particle size of AGS by collision and abrasion. Nevertheless, there was no significant change in mixed liquor suspended solids (MLSS) and the sludge volume index (SVI) in either bioreactors. The removal (%) of chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) in these two bioreactors did not differ from each other greatly. The analysis of biological composition displays that the proportion of Proteobacteria decreased slightly in R2. The X-ray fluorescence (XRF) analysis revealed less accumulation of Fe and Ca in smaller granules. Furthermore, a pilot-scale SBR with a spiny aeration device was successfully utilized to restrict the diameter of granules at about 300 μm.

Development of simultaneous nitrification-denitrification and anammox and in-situ analysis of microbial structure in a novel plug-flow membrane-aerated sludge blanket

This study proposed a novel one-stage plug-flow microaerobic sludge blanket (PMSB) with membrane aerated for treating low carbon to nitrogen (C/N) ratio municipal sewage. The performance of simultaneous nitrification, denitrification, and anammox in the reactor was investigated. The results illustrated that the removal efficiencies of ammonium and total nitrogen (TN) were 93.2% and 87.1% with a C/N ratio of 4. High throughput sequencing revealed that aerobic bacteria, anaerobic bacteria and facultative anaerobe could co-exist at the same time in the sludge blanket. Meanwhile, a notable correlation between the oxygen concentration and the distance of the membrane module was analyzed. It was shown that the microbial community of functional bacteria developed in different aeration sites due to the oxygen concentration gradient. Microbial community structure was analyzed depending on the sludge stratification in the sludge blanket.

Aerobic granular sludge treating anaerobically pretreated brewery wastewater at different loading rates

In this study, three different aerobic granular sludge (AGS) reactors fed with anaerobically pre-treated brewery wastewater were studied. The AGS reactors were operated under different conditions including organic loading rates (OLR) between 0.8 and 4.1 kg COD m^-3 d^-1, C:N:P ratios (100:10:1 and 100:6:1) and food to microorganism ratios (F/M) between 0.8 ± 0.6 and 1.2 ± 0.5 and 0.9 ± 0.3 kg-TCOD kg-VSS^-1d^-1. Stable granulation was achieved within two weeks and the size of the granules increased according to the OLR applied. The results indicated that low C:N:P and F/M ratios were favorable to achieve stable aerobic granules in the long term. The carbon removal rate was load-independent in the range examined (TCOD removal >80%), whereas TN removals were inversely proportional to the OLRs. Overall, a longer aeration reaction time with a lower OLR was beneficial to granular structure, which exhibited a compact and defined architecture. Performance results within the other conditions studied further indicated that the microbial community and its complex functionality in nutrient removal was efficient at operational parameters of OLR at 0.8 ± 0.2 kg-TCOD m^-3d^-1 and F/M ratio at 0.5 ± 0.2 kg-TCOD VSS^-1d^-1. Moreover, the protein to polysaccharide ratio increased as OLR decreased, leading to a stable granular structure.

Longer persistence of quorum quenching bacteria over quorum sensing bacteria in aerobic granules

Involvements of quorum sensing (QS) in the formation of aerobic granules for wastewater treatment have been well recognized. In previous studies the evolution of the QS-related activities and communities during bioreactor start-up period has been extensively studied, while the variation of QS in long-term reactor operation remains unrevealed. Furthermore, information about the roles of quorum quenching (QQ) in bioreactors is very limited. In this work, both QS and QQ during the start-up and successive long-term operation period of an aerobic granule bioreactor were explored. The QS activity and communities increased in the start-up but gradually decreased in the long-term operation, while the QQ activity and communities remained stable. These results indicate the longer persistence of QQ than QS in the granules and the minor contribution of QS in the long-term operation. This work provides a new insight into the roles of QQ and QS in wastewater treatment bioreactors.

Recovering partial nitritation in a PN/A system during mainstream wastewater treatment by reviving AOB activity after thoroughly inhibiting AOB and NOB with free nitrous acid

Starting up or recovering partial nitritation is a major challenge for achieving or maintaining stable partial nitritation/anammox (PN/A) during mainstream wastewater treatment. This study presents a novel strategy for recovering the nitrite pathway by selectively reviving ammonium oxidizing bacteria (AOB) after thoroughly inhibiting AOB and nitrite oxidizing bacteria (NOB) using free nitrous acid (FNA). A sequencing batch reactor was operated for PN/A to treat real domestic wastewater for 423 days, during which twice FNA treatment was temporarily implemented. Results showed that with a single 0.45 mg/L FNA treatment on flocculent sludge, the NO₃^--N concentration during the aerobic period showed an uptrend again and the partial nitritation performance was deteriorated. In contrast, 1.35 mg/L FNA treatment induced the inhibition of both AOB and NOB leading to regressive ammonium oxidation, but a subsequently higher DO (1.5 mg/L) and longer aeration duration recovered partial nitritation. For the relative abundances of the acquired biomass related to nitrogen conversion, Nitrosomonas, Nitrospira and Nitrolancea increased to 9.65%, 10.27% and 4.35%, respectively, at the beginning of the 1.35 mg/L FNA treatment, and Nitrospira and Nitrolancea decreased to 2.80% and 0.03% whereas Nitrosomonas declined to 8.71% after 76 days. Ca. Brocadia showed less resilience after the 1.35 mg/L FNA treatment, with the relative abundance decreasing from 13.38% to 0.62% due to insufficient nitrite. Molecular ecological network analysis indicates that among anammox taxa, Ca. Kuenenia and Ca. Brocadia formed important links with other N cycle processes. Moreover, the proposed strategy shows operational flexibility because it can be easily used to control NOB in mainstream PN/A applications offered by flocculent sludge systems.

Performance and bacterial community structure of a novel inverse fluidized bed bioreactor (IFBBR) treating synthetic municipal wastewater

The performance of a lab-scale integrated anoxic and aerobic inverse fluidized bed bioreactors (IFBBR) for biological nutrient removal from synthetic municipal wastewater was studied at chemical oxygen demand (COD) loading rates of 0.34-2.10 kg COD/(m³-d) and nitrogen loading rates of 0.035-0.213 kg N/(m³-d). Total COD removal efficiencies of >84% were achieved, concomitantly with complete nitrification. The overall nitrogen removal efficiencies were >75%. Low biomass yields of 0.030-0.101 g VSS/g COD were achieved. Compared with other FBBR systems, the energy consumption for this IFBBR system was an average 59% less at organic loading rates (OLRs) of 1.02 and 2.10 kg COD/(m³-d). Bacterial community structures of attached and suspended biomass revealed that the dominant phyla were Proteobacteria, Bacteroidetes, and Epsilonbacteraeota, etc. The relative abundance of ammonia-oxidizing bacteria (AOBs) and nitrite-oxidizing bacteria (NOBs) in the aerobic attached biomass were 0.451% and 0.110%, respectively. COD mass balance in the anoxic zone was closed by consideration of sulfate reduction, which was confirmed by the presence of genus Chlorobium (sulfate-reducing bacteria) in the anoxic attached biofilm with a relative abundance of 0.32%.

The abundance of nirS-type denitrifiers and anammox bacteria in rhizospheres was affected by the organic acids secreted from roots of submerged macrophytes

Excessive nitrogen has been a global concern to cause lake eutrophication. The denitrification and anammox processes are considered to be effective biological pathways for nitrogen removal. Submerged macrophytes also play a key role in the nitrogen cycle of lakes. However, the mechanism of submerged macrophytes on regulating biological nitrogen removal pathways has not been well quantified. Therefore, this study investigated the impacts of submerged macrophytes on the community structures and abundance of the nirS-type denitrifiers and anammox bacteria in the rhizospheres. The qPCR results indicated that the abundance of two bacteria in the near-rhizospheres of submerged macrophytes was significantly lower than the root compartments and non-rhizospheres, while the concentrations of organic acids in the near-rhizospheres were higher than those of the root compartments and non-rhizospheres. Redundancy analysis results illustrated that concentrations of NO₃^--N, NO₂^--N, citric acid and oxalic acid were the key environmental indicators which had the significant impact on the microbial community. The concentrations of citric acid and oxalic acid were negatively correlated with the nirS-type denitrifiers abundance, and the oxalic acid concentrations were negatively correlated with the anammox bacteria abundance. These results indicated that submerged macrophytes could reduce the abundance of nirS-type denitrifiers and anammox bacteria by releasing organic acids. In addition, the highest diversity of denitrifier community were found in the rhizosphere of the Hydrilla verticillata, while the highest diversity of anammox community were found in the Potamogeton maackianus rhizosphere. These results indicate that the impacts of submerged macrophytes on the biological nitrogen removal pathways were species-dependent.

Pilot-scale investigation on nutrient removal characteristics of mineral-rich aerobic granular sludge: Identification of uncommon mechanisms

This study investigated nutrient removal characteristics and the related pathways in aerobic granular reactors using three pilot-scale granular sequencing batch reactors (GSBRs) treating wastewaters of diverse carbon and nutrient strength. The GSBRs were operated with alternating (AN/O/AX/O_SBR and AN/O_SBR) and purely-aerobic (O_SBR) operation modes. Mineral-rich aerobic granules with hydroxyapatite (HAp) core were cultivated in all the three GSBRs. The highest nitrogen removal efficiency (75%) was achieved in AN/O/AX/O_SBR and O_SBR and the lowest (22%) in AN/O_SBR, establishing a quasi-linear relationship with organic loading rate (OLR). Phosphorus removal efficiencies of 55-63% were achieved in the GSBRs despite different influent PO₄-P concentrations. Heterotrophic nitrification and biologically-induced phosphate precipitation (BIPP) became the dominant nutrient depletion pathways, contributing 61-84% and 39-96% to overall ammonium nitrogen and phosphorus removal, respectively. A direct relation was noted between heterotrophic nitrification efficiency (η_{Heterotrophic nitrification}) and nutrient availability, as nitrification efficiencies of 18 and 64% were observed for COD:N_inf of 5 and 20, respectively. Whereas, BIPP efficiency (η_BIPP) established inverse relation with (COD:P)_inf and (Ca:P)_inf and direct relation with phosphorus concentration beyond microbial growth requirement. Core heterotrophic nitrifiers and bio-calcifying species were identified as {Thauera and Flavobacterium} and {Flavobacterium, Acinetobacter, Pseudomonas, and Corynebacterium}, respectively. Ca-P crystallization was proposed to be via phosphate precipitation on calcite surfaces. Granulation mechanism was proposed as crystallization on bio-aggregates' periphery and then crystal growth toward the core.

Chronic responses of aerobic granules to the presence of graphene oxide in sequencing batch reactors

The chronic responses of aerobic granular sludge (AGS) to the presence of graphene oxide nanoparticles (GO NPs) (5, 15, 25, 35, 45, 55, 65, 75, 85, and 95 mg/L of GO NPs for 7 days) during biological wastewater treatment processes were investigated. Bioreactor performance, extracellular polymeric substance (EPS) secretion, and microbial community characteristics were assessed. The results showed that the effects of GO NPs on bioreactor performances were dependent on the dose applied and the duration for which it was applied. At concentrations of 55, 75, and 95 mg/L, GO NPs considerably inhibited the efficiency of organic matter and ammonia removal; however, nitrite and nitrate removal rates were unchanged. Biological phosphorus removal decreased even when only low concentrations of GO NPs were used. The secretion of EPS, which could alleviate the toxicity of GO NPs, also changed. The increased amount of nanoparticles also resulted in significant changes to the bacterial community structure. Based on the amplicon sequencing of 16S rRNA genes, Paracoccus sp., Klebsiella sp., and Acidovorax species were identified as the most tolerant strains.

Simultaneous decarburization, nitrification and denitrification (SDCND) in coking wastewater treatment using an integrated fluidized-bed reactor

There are two problems in biological treatment of coking wastewater (CWW): incapability of pre-anaerobic treatment to eliminate the toxicity in wastewater, and the lack of carbon source for subsequent denitrification in pre-aerobic treatment. To achieve simultaneous decarburization, nitrification and denitrification (SDCND) in CWW treatment, biological carrier materials was used to build an integrated fluidized-bed reactor (Reactor B, RB). A conventional fluidized-bed reactor (Reactor A, RA) was used as a control reactor under the same condition. The results showed that RB was more advantageous since its removal efficiencies of COD and TN were 90% and 87%, respectively, which were significantly higher than these in RA (82% and 45%), at a hydraulic retention time (HRT) of 60 h. Microelectrode measurement indicated that oxygen transfer was limited inside the carrier where the formation of a dissolved oxygen (DO) concentration gradient was observed. Microbial community analysis showed that the aerobic and anoxic microenvironments in RB promoted the co-existence of a wider variety of bacteria, thus achieving SDCND. These results indicated the integrated fluidized-bed reactor exhibited promising feasibility for simultaneous carbon and nitrogen removal in CWW treatment under the same aeration driven conditions. The SDCND process realized by fluidized-bed reactor provided a reference for the treatment of toxic industrial wastewater with high carbon to nitrogen ratio.

Impact of hydraulic retention time and organic matter concentration on side-stream aerobic granular membrane bioreactor

This study investigated the effect of hydraulic retention time (HRT) and chemical oxygen demand (COD) concentration on membrane fouling in aerobic granular membrane bioreactor (AGMBR) in a systematic approach. Changes in HRT (7, 10, and 15 h) and COD (500, 1000 and 1500 mg/L) were applied in five operational phases, to determine the most significant parameters to control membrane fouling for enhanced AGMBR performance. Membrane permeability loss was dramatically intensified with increase in HRT from 7.5 to 15 h and COD from 500 to 1000 mg/L. The highest polysaccharide content of loosely bound EPS (0.41 mg PS/mg VSS) and soluble microbial products (SMPs) (27 mg PS/L) occurred alongside poor AGMBR performance. Variations in membrane fouling were accompanied with considerable changes in Flavobacterium, Thauera and Paracoccus populations. Analysis of variance (ANOVA) demonstrated that HRT and interaction between HRT and COD were the most significant parameters in controlling membrane fouling.

Effect of chemical oxygen demand load on the nitrification and microbial communities in activated sludge from an aerobic nitrifying reactor

In this study, we explored the effect of chemical oxygen demand (COD) load on the nitrification and microbial communities in activated sludge isolated from an aerobic nitrifying tank. The activated sludge was cultured in three different COD groups: L-COD, 200 mg/L; M-COD, 1200 mg/L; H-COD, 4200 mg/L. The results indicated that the COD exerts a negligible effect on the nitrogen removal ability within the first 24 h. However, the nitrification rate decreased with culture time; the ammonium degradation rates were found to be 80.26%, 57.56%, and 43.43% at 72 h in the three COD groups, respectively. These values correspond to decreases of 19.40%, 41.83%, and 51.48%, respectively, in relation to those observed at 24 h. The activated sludge in the different COD groups exhibited similar community compositions after 24 h, as assessed by Illumina high-throughput sequencing, while a significant difference in the relative abundances of some organisms occurred after 48 and 72 h. Proteobacteria was the main phylum, with a relative abundance of >51.45%. The genera Aridibacter, Paracoccus, Nitrospira, and Nitrosomonas were suppressed by COD load over time. This study may contribute to our knowledge about the nitrification ability and microbial communities in activated sludge at different COD load levels.

Fast start-up strategies of MBBR for mariculture wastewater treatment

Moving bed biofilm reactor (MBBR) is widely used for ammonia removal in saline recirculating aquaculture systems but often faces a slow start-up problem. The aim of this study was to develop a strategy for the rapid start-up of MBBR treating synthetic mariculture wastewater. Changes in nitrification performance, biofilm characteristics and bacterial community were assessed in response to various start-up strategies: R1 as the control; R2 with step-decrease of inlet NH₄⁺-N; R3 with step-increase of inlet salinity; R4 added with particulate organic matter (POM) and R5 inoculated with nitrifying bacteria. Results show that nitrification was completed on day 63 for R3, 16-18 days faster than the other strategies. The highest protein (28.2 ± 5.1 mg/g·VS) and polysaccharide (59.4 ± 0.4 mg/g·VS) contents were observed in R3, likely linked to the faster biofilm formation. Fourier Transform infrared spectroscopy (FTIR) analysis confirmed the typical constituents of carbohydrates, proteins, lipids and DNA in biofilms. Moreover, along with the biofilm development in R3, the intensity of the peak at 1400 cm^-1 (assigned to specific amides) decreased. Pyrosequencing of 16s rRNA revealed that Gammaproteobacteria was the predominating microbial community at class level (35.6%) in R3. qPCR analysis further verified the significantly higher gene copies of amoA (1.57 × 10⁴ copies/μL) and nxrB (5.51 × 10³ copies/μL) in R3. Results obtained make the elevated salinity strategy a promising alternative for the rapid nitrification start-up of saline wastewater.

Enhanced nitrogen removal in an aerobic granular sequencing batch reactor under low DO concentration: Role of extracellular polymeric substances and microbial community structure

In this study, the role of extracellular polymeric substances (EPSs) in nitrogen removal and the microbial community structure of aerobic granular sludge (AGS) were analyzed under different dissolved oxygen (DO) conditions (6-7, 4-5, and 2-3 mg·L^-1). The EPSs transported and retained nitrogen in the denitrification process, and the total inorganic nitrogen (TIN) in the EPSs decreased from 6.09 to 5.54 mg·g^-1 MLSS when the DO concentration decreased from 6-7 to 2-3 mg·L^-1. The microbial community showed different core denitrifying bacterial populations involved in nitrogen removal in the AGS system under different DO conditions, with more species when they were higher relative abundances of denitrifying bacteria participating in the nitrogen removal process in AGS under low DO conditions, including Hydrogenophilaceae, Thauera, Enterobacter, Xanthomonadaceae_unclassified, Comalmonadaceae_unclassified, Nitrosomonas and Paracoccus. This study provides a more comprehensive understanding of the DO effect on the TIN removal mechanism by AGS.

Influence of operation mode and wastewater strength on aerobic granulation at pilot scale: Startup period, granular sludge characteristics, and effluent quality

This study attempted to investigate the influence of operation mode and wastewater strength on startup period, aerobic granular sludge (AGS) characteristics, and system effluent quality at pilot scale. Granulation was monitored in three pilot-scale granular sequencing batch reactors (GSBRs). Comparative evaluation of AN/O/AX/O_SBR and O_SBR, fed with wastewater of the same composition but run with completely different SBR reaction phase arrangements (alternating vs. purely aerobic), revealed the effect of SBR operation mode. Comparative study of the GSBRs operated with alternating SBR reaction phases (AN/O/AX/O_SBR and AN/O_SBR) and fed with wastewater of different strength (high- vs. medium-strength) determined the effect of wastewater composition. Granulation time and granule size were regulated by wastewater strength and the resulting organic and sludge loading conditions. Whereas, AGS morphology, granule structure, and floccular proportion of AGS were attributed to SBR operation mode. Effluent clarity in terms of suspended solid concentration depended on wastewater strength. Subtle but distinct microbial selection strategies were in effect during granulation which were also imposed by wastewater strength. Due to strong correlation between the effluent and AGS microbial structures, demonstrated by biodiversity analysis, differences in the microbial composition of effluent biomass and washout patterns of the GSBRs could be explained by wastewater strength as well. Limited nutrient removal efficiencies, restricted by organic matter concentration, could be due to involvement of unorthodox nutrient removal pathways which warrants further investigation.

Effective removal of nitrate by denitrification re-enforced with a two-stage anoxic/oxic (A/O) process from a digested piggery wastewater with a low C/N ratio

The combined process of a long-term biogas digester and double anoxic/oxic tanks is very commonly used in piggery wastewater treatment in South China, but the effluent does not meet the discharge standard of total nitrogen (TN) and chemical oxygen demand (COD_Cr) due to a low C/N ratio and insufficient organic carbon in digested piggery wastewater. Thus, a typical two-stage anoxic/oxic (A1/O1/A2/O2) process, which is widely used to treat digested piggery wastewater in the engineering application, was selected for study on a laboratory scale. Finally, the average removal efficiency of ammonia nitrogen in the two-stage AO process was 98.7%; at the same time, the content of nitrate increased to 180-190 mg/L. To further eliminate nitrogen, an anaerobic tank (S1), which was equipped the sludge that was acclimated in our laboratory by a high nitrogen loading slurry, was employed to treat the effluent from the two-stage AO process and contributed more than 70% removal efficiency. Further analysis showed that ammonia-oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) in the O1 and O2 tanks together contributed to the conversion of ammonia nitrogen to nitrate, but the process of heterotrophic denitrification was inhibited in the A1 and A2 tanks because of insufficient carbon sources. In addition, most of the nitrate concentration was reduced under conditions with insufficient carbon sources, while Thauera-dominated the bacterial population in the sludge sample of the S1 tank.

Denitrification performance and microbial versatility in response to different selection pressures

This study investigated functional dynamics of microbial community in response to different selection pressures, with a focus on denitrification. Suspended-biomass experiments demonstrated limited aerobic and relatively higher anoxic nitrate and nitrite reduction capabilities; the highest NO₂-N and NO₃-N removal rates were 1.3 ± 0.1 and 0.74 ± 0.01 in aerobic and 1.4 ± 0.05 and 3.4 ± 0.1 mg/L.h in anoxic media, respectively. Key potential denitrifiers were identified as: (i) complete aerobic denitrifiers: Dokdonella, Flavobacterium, and Ca. Accumulibacter; (ii) complete anoxic denitrifiers: Acinetobacter, Pseudomonas, Arcobacter, and Comamonas; (iii) incomplete nitrite denitrifier: Diaphorobacter (aerobic/anoxic), (iv): incomplete nitrate denitrifiers: Thauera (aerobic/anoxic) and Zoogloea (strictly-aerobic). Granular biomass removed 72 mg/L NH₄-N with no NO_x^- accumulation. Heterotrophic nitrification and aerobic denitrification were proposed as the principal nitrogen removal pathway in granular reactors, potentially performed by two key organisms Thuaera and Flavobacterium. Biodiversity analysis suggested that the selection pressure of nourishment condition was the decisive factor for microbial selection and nitrogen removal mechanism.

Selection of seeding strategy for fast start-up of Anammox process with low concentration of Anammox sludge inoculum

The long start-up time and large demand of Anammox seed sludge limit the practical application of Anammox process. In this study, the seeding strategy of fast start-up of Anammox process using 2.0 g VSS L^-1 of anaerobic granular sludge (AGS) or activated sludge (AS) with various low concentration of Anammox sludge as inoculum was investigated. In laboratory scale, the start-up (achieving 70% TN removal) was shortened from 21 days to 5 days when Anammox sludge concentration increased from 0.02 g VSS L^-1 to 0.2 g VSS L^-1 with 2 g VSS L^-1 AS as inoculum, and 16S rDNA analysis indicated the enrichment of Anammox bacteria, while the start-up failed with AGS. In pilot scale, the start-up was achieved in 10 days using 0.02 g VSS L^-1 of Anammox sludge and 2.0 g VSS L^-1 of AS, confirming the fast start-up of Anammox process with low concentration of Anammox sludge.

Fabrication of Novel Cyanuric Acid Modified g-C3N4/Kaolinite Composite with Enhanced Visible Light-Driven Photocatalytic Activity

A novel kind of cyanuric-acid-modified graphitic carbon nitride (g-C3N4)/kaolinite (m-CN/KA) composite with enhanced visible light-driven photocatalytic performance was fabricated through a facile two-step process. Rhodamine B (RhB) was taken as the target pollutant to study the photocatalytic performance of the synthesized catalysts. It is indicated that the cyanuric acid modification significantly enhanced photocatalytic activity under visible light illumination in comparison with the other reference samples. The apparent rate constant of m-CN/KA is almost 1.9 times and 4.0 times those of g-C3N4/kaolinite and bare g-C3N4, respectively. The superior photocatalytic performance of m-CN/KA could be ascribed, not only to the generation of abundant pore structure and reactive sites, but also to the efficient separation of the photogenerated electron-hole pairs. Furthermore, the possible photocatalytic degradation mechanism of m-CN/KA was also presented in this paper. It could be anticipated that this novel and efficient, metal-free, mineral-based photocatalytic composite has great application prospects in organic pollutant degradation.

Variation in bacterial community structure of aerobic granular and suspended activated sludge in the presence of the antibiotic sulfamethoxazole

The treatment performance and bacterial community structure of conventional activated sludge and aerobic granules exposed to antibiotic sulfamethoxazole (SMX) was studied. For three months, two sets of sequencing batch reactors inoculated with conventional and granular biomass were fed with a synthetic municipal wastewater containing 2 μg/L SMX. The presence of SMX had no significant impacts on treatment performance of the reactors as well as stability of the granules. Results confirmed different bacterial community structure of flocs and granules. During the operation, variations in bacterial community structure of suspended and granular sludge were observed in all reactors. The variations in bacterial community composition due to the exposure to 2 μg/L SMX were found after two months in both suspended and granular biomass. Nitrosomonas, Pseudomonas, and Acinetobacter were detected as the genes capable of degrading SMX in both biomass types. Also, Rikenellaceae, Oscillospira, Rhodocyclaceae, Zoogloea, and Shewanella varied in abundance over the operation time. Rikenellaceae and Oscillospira were vulnerable to SMX and decreased in abundance the operation time; while Rhodocyclaceae, Zoogloea, Shewanella, and Aeromonas were found as SMX resistance genes.

Removal of tetracycline by aerobic granular sludge and its bacterial community dynamics in SBR

In this study, the removal efficiency and mechanism of tetracycline by aerobic granular sludge (AGS) in SBR were investigated. The removal of tetracycline present in livestock and poultry wastewater and the effect on conventional pollutants, such chemical oxygen demand, and nitrogen and phosphorous removal performance have been assessed demonstrating that AGS was able to remove tetracycline by adsorption and biodegradation processes. The removal rate of tetracycline was more than 90%, and conventional pollutants were also efficiently removed. The high-throughput sequencing technology was applied to decipher the species succession and community structure of tetracycline-resistance granular sludge. The Chryseobacterium, Actinotignum, Lactococcus, Shinella and Clavibacter were gradually dominant and considered as the functional bacteria for the removal of tetracycline. The numbers of functional genes including amino acid, carbohydrate and inorganic ion transport and metabolism, as well as energy production and conversion, and secondary metabolites biosynthesis, were also increased. These functional genes played an important role in the biodegradation of tetracycline.

Bioaugmentation of sidestream nitrifying-denitrifying phosphorus-accumulating granules in a low-SRT activated sludge system at low temperature

Sidestream granular activated sludge grown on anaerobic digester dewatering centrate was bioaugmented and selectively retained to enable high nitrification performance of a 2.5-day aerobic SRT non-nitrifying flocculent activated sludge system at 12 °C. Sidestream-grown granules performed enhanced biological phosphorus removal (EBPR) and short-cut nitrogen removal via nitrite. After bioaugmentation, EBPR continued in the mainstream but ammonia oxidation was eventually to nitrate. Low effluent NH₃-N concentrations from 0.6 to 1.7 mg/L were achieved with nitrification solely by granules, thus enabling denitrification and nitrogen removal. Molecular microbial analyses of flocs and granules also suggested that nitrifying organisms persisted on granules with minimal nitrifier loss to flocs. Mainstream granule mass at the end of bioaugmentation testing was 1.7 times the amount of sidestream granules added, indicating mainstream granular growth. Nitrite and nitrate availability during the unaerated feeding period encouraged significant growth of ordinary heterotrophs in mainstream granules, but nevertheless mainstream nitrification capacity was sustained.

Characteristics and performance of aerobic algae-bacteria granular consortia in a photo-sequencing batch reactor

The characteristics and performance of algae-bacteria granular consortia which cultivated with aerobic granules and targeted algae (Chlorella and Scenedesmus), and the essential difference between granular consortia and aerobic granules were investigated in this experiment. The result indicated that algae-bacteria granular consortia could be successfully developed, and the algae present in the granular consortia were mainly Chlorella and Scenedesmus. Although the change of chlorophyll composition revealed the occurrence of light limitation for algal growth, the granular consortia could maintain stable granular structure, and even showed better settling property than aerobic granules. Total nitrogen and phosphate in the algal-bacterial granular system showed better removal efficiencies (50.2% and 35.7%) than those in the aerobic granular system (32.8% and 25.6%) within one cycle (6 h). The biodiesel yield of aerobic granules could be significantly improved by algal coupled process, yet methyl linolenate and methyl palmitoleate were the dominant composition of biodiesel obtained from granular consortia and aerobic granules, respectively. Meanwhile, the difference of dominant bacterial communities in the both granules was found at the order level and family level, and alpha diversity indexes revealed the granular consortia had a higher microbial diversity.

A novel single-stage process integrating simultaneous COD oxidation, partial nitritation-denitritation and anammox (SCONDA) for treating ammonia-rich organic wastewater

In this study, simultaneous carbon oxidation, partial nitritation, denitritation and anammox (SCONDA) was successfully integrated into a one-stage sequencing biofilm batch reactor for treating ammonia-rich organic wastewater with carbon to nitrogen (C/N) ratio of 3. The results showed that 94.3% of COD removal together with 92.6% NH₄⁺-N and 88% TN removal were achieved via SCONDA. High-throughout sequencing analysis further revealed that the microbial community developed in the proposed system was primarily dominated by heterotrophic bacteria (e.g. Thauera, Azovibrio, Ohtaekwangia, Azospira), autotrophic bacteria (e.g. Nitrosomona) and unclassified genus of anammox bacterium, which were all essential for COD and N removal via SCONDA. The observed spatial distributions of the functional species in stratified biofilms were found to be crucial for successful SCONDA at the low dissolved oxygen of 1.3 mg/L. The integrated SCONDA system is expected to offer a promising alternative for advanced nitrogen and organic removal from high-ammonia organic wastewater.

Microbial community structure and function in aerobic granular sludge

Aerobic granular sludge (AGS), a self-immobilized microbial consortium containing different functional microorganisms, is receiving growing attention, since it has shown great technological and economical potentials in the field of wastewater treatment. Microbial community is crucial for the formation, stability, and pollutant removal efficiency of aerobic granules. This mini-review systematically summarizes the recent findings of the microbial community structure and function of AGS and discusses the new research progress in the microbial community dynamics during the granulation process and spatial distribution patterns of the microbiota in AGS. The presented information may be helpful for the in-depth theoretical study and practical application of AGS technology in the future.

State of the art on granular sludge by using bibliometric analysis

With rapid industrialization and urbanization in the nineteenth century, the activated sludge process (ASP) has experienced significant steps forward in the face of greater awareness of and sensitivity toward water-related environmental problems. Compared with conventional flocculent ASP, the major advantages of granular sludge are characterized by space saving and resource recovery, where the methane and hydrogen recovery in anaerobic granular and 50% more space saving, 30-50% of energy consumption reduction, 75% of footprint cutting, and even alginate recovery in aerobic granular. Numerous engineers and scientists have made great efforts to explore the superiority over the last 40 years. Therefore, a bibliometric analysis was desired to trace the global trends of granular sludge research from 1992 to 2016 indexed in the SCI-EXPANDED. Articles were published in 276 journals across 44 subject categories spanning 1420 institutes across 68 countries. Bioresource Technology (293, 11.9%), Water Research (235, 9.6%), and Applied Microbiology and Biotechnology (127, 5.2%) dominated in top three journals. The Engineering (991, 40.3%), China (906, 36.9%), and Harbin Inst Technol, China (114, 4.6%) were the most productive subject category, country, and institution, respectively. The hotspot is the emerging techniques depended on granular reactors in response to the desired removal requirements and bio-energy production (primarily in anaerobic granular sludge). In view of advanced and novel bio-analytical methods, the characteristics, functions, and mechanisms for microbial granular were further revealed in improving and innovating the granulation techniques. Therefore, a promising technique armed with strengthened treatment efficiency and efficient resource and bio-energy recovery can be achieved.

Chronic toxicity of graphene and graphene oxide in sequencing batch bioreactors: A comparative investigation

The present study investigates the chronic toxicity of graphene (G) and graphene oxide (GO) in activated sludge. Sequencing batch bioreactors were fed with influents containing 0, 1 and 5mgL^-1 of GO or G (12h cycles) for ten days. Reduction in performance of the bioreactors in relation to chemical oxygen demand, ammonia and phosphate removals was observed after three days in the bioreactors fed with 5mgL^-1 of nanomaterials. After about eight days, these reactors reached a steady state nutrient removal, which corresponded to recovery of certain groups of ammonia oxidizing bacteria and phosphate accumulating bacteria despite the increasing accumulation of nanomaterials in the sludge. These results suggested that biological treatment can be affected transiently by initial exposure to the nanomaterials, but certain groups of microorganisms, less sensitive to these nanomaterials, can potentially strive in the presence of these nanomaterials. Results of 16S rRNA gene deep sequencing showed that G and GO affected differently the microbial communities in the activated sludge. Between the two nanomaterials investigated, GO presented the highest impact in nutrient removal, gene abundance and changes in microbial population structures.