Feasibility of partial-denitrification/ anammox for pharmaceutical wastewater treatment in a hybrid biofilm reactor

Metabolic evolution and bottleneck insights into simultaneous autotroph-heterotroph anammox system for real municipal wastewater nitrogen removal

When biological nitrogen removal (BNR) systems shifted from treating simulated wastewater to real wastewater, a microbial succession occurred, often resulting in a decline in efficacy. Notably, despite their high nitrogen removal efficiency for real wastewater, anammox coupled systems operating without or with minimal carbon sources also exhibited a certain degree of performance reduction. The underlying reasons and metabolic shifts within these systems remained elusive. In this study, the simultaneous autotrophic/heterotrophic anammox system demonstrated remarkable metabolic resilience upon exposure to real municipal wastewater, achieving a nitrogen removal efficiency (NRE) of 82.83 ± 2.29 %. This resilience was attributed to the successful microbial succession and the complementary metabolic functions of heterotrophic microorganisms, which fostered a resilient microbial community. The system's ability to harness multiple electron sources, including NADH oxidation, the TCA cycle, and organics metabolism, allowed it to establish a stable and efficient electron transfer chain, ensuring effective nitrogen removal. Despite the denitrification channel's nitrite supply capability, the analysis of the interspecies correlation network revealed that the synergistic metabolism between AOB and AnAOB was not fully restored, resulting in selective functional bacterial and genetic interactions and the system's PN/A performance declined. Additionally, the enhanced electron affinity of PD increased interconversion of NO3--N and NO2--N, limiting the efficient utilization of electrons and thereby constraining nitrogen removal performance. This study elucidated the metabolic mechanism of nitrogen removal limitations in anammox-based systems treating real municipal wastewater, enhancing our understanding of the metabolic functions and electron transfer within the symbiotic bacterial community.

Enhanced and robust nitrogen removal using an integrated zeolite and partial denitrification anammox process

Anammox has received increased attention due to its enhanced and cost-efficient approach to nitrogen removal. However, its practical application is complicated by strict influent NO2--N to NH4+-N ratio demands and an 11% nitrate production from the anammox process. This study was the first known research to propose and verify a system of zeolite integrated with partial denitrification and anammox (Z-PDA) in an up-flow anaerobic sludge bed (UASB) reactor. The enhanced and robust nitrogen removal resulted in an ultra-high nitrogen removal efficiency (NRE, 93.0 ± 2.0%). Zeolite adsorption and biological desorption of ammonium contributed to robust nitrogen removal with fluctuating influent NO2--N to NH4+-N ratios. Applying 16S rRNA gene sequencing found that Candidatus Brocadia and Thauera were the key bacteria responsible for anammox and partial denitrification (PD), respectively. Zeolite also acted as a biological carrier. This significantly enriched anammox bacteria with a higher relative abundance of Candidatus Brocadia, reaching 49.2%. Metagenomic analysis demonstrated that the multiple functional genes related to nitrogen removal (nrfA/H, narG/H/I) and the metabolic pathways (Biosynthesis of cofactors, the Two-component system, the Biosynthesis of nucleotide sugars, and Purine metabolism) ensured the resilience of the Z-PDA system despite influent fluctuations. Overall, this study provided novel insights into the impacts of zeolite in the PDA system. It described the fundamental mechanism of zeolite based on adsorption and biological desorption, and demonstrated a meaningful application of the anammox process in sewage treatment.

Increasing biomass concentration facilitates simultaneous nitrogen removal and sludge reduction under low C/N conditions

To overcome the issues of limited carbon source and high sludge production in partial denitrification/anammox (PD/A) process, the effects of mixed liquor suspended solids (MLSS) and carbon/nitrogen ratio (C/N) on PD/A were investigated through parallel experiments. Nitrogen removal efficiencies decreased significantly when C/N was reduced (1.5 → 0.75). When MLSS was doubled, the nitrogen removal efficiencies in the two parallel reactors increased from 75.3 %, 72.9 % to 86.9 %, 89.7 %, respectively, and sludge yields decreased obviously. Combining with in-situ test, it was speculated when MLSS increased, fermentation was enhanced, providing substrate for partial denitrification. Thauera, involved in partial denitrification, decreased obviously with reduced C/N, but increased from 9.93 % to 38.16 % when MLSS doubled, which could promote the PD/A process. Terrimonas and Ignavibacterium (fermentative bacteria) increased from 1.26 %, 5.22 % to 6.62 %, 6.30 %, respectively. These results proved that increasing MLSS under low C/N ratios promoted fermentation in PD/A system, facilitating efficient nitrogen removal and sludge reduction.

Achieving rapid start-up and efficient nitrogen removal of partial-denitrification/anammox process using organic matter in brewery wastewater as carbon source

To find a cost-efficient carbon source for the partial denitrification/anaerobic ammonium oxidation (anammox) (PD/A) process, the practicability of using the organic matter contained in brewery wastewater as carbon source was investigated. Quick self-enrichment of denitrifying bacteria was achieved by supplying brewery wastewater as organic carbon source and using the mature anammox sludge as the seeding sludge. The PD/A process was successfully established after 33-day operation and then the average total nitrogen removal efficiency reached 92.29% when the influent CODCr: NO3--N: NH4+-N ratio was around 2.5: 1.0: 0.67. The relative abundance of Thauera increased from 0.03% in the seeding sludge to 54.29% on day 110, whereas Candidatus brocadia decreased from 30.66% to 2.08%. The metagenomic analysis indicated that the sludge on day 110 contained more nar and napA (total of 41.24%) than nirK and nirS (total of 11.93%). Thus NO2--N was accumulated efficiently in the process of denitrification and sufficient NO2--N was supplied for anammox bacteria in the PD/A process. Using brewery wastewater as carbon source not only saved the cost of nitrogen removal but also converted waste into resource and reduced the treatment expense of brewery wastewater.

Effects of a novel sawdust-modified carrier on performance, bioaccumulation and microbial community of sequencing batch reactor

The impact of a novel sawdust-modified carrier on the performance of aerobic sequencing batch reactor (SBR) was examined. Compared with the conventional polyethylene (PE) carrier, the sawdust-modified carrier had coarse surface and porous side wall, which was beneficial for the rapid formation of biofilm. The biomass of sawdust-modified carrier was 3.4 ± 0.7 times more than those of PE carrier at the end of this study. The biofilm gotten from suspended carrier had higher extracellular polymeric substances (EPS) concentrations than activated sludge (AS). The EPS from biofilm contained higher proportions of polysaccharides compared to those from AS. The SBR with addition of sawdust-modified carrier exhibited higher ammonia nitrogen removal efficiency (84.8%) than the one with addition of conventional PE carrier (73.1%) in a typical cycle at 12 h. The volumetric nitrification rates of modified carrier were higher than those of conventional PE carrier. High throughput sequencing revealed that sawdust-modified carriers exhibited greater microbial richness and diversity compared with traditional PE carriers. Saccharimonadales was the most predominant genus that removed organic matter under aerobic condition, whereas Nitrospira was the dominant nitrifying genus. The present study verifies the advantage of sawdust-modified carrier, which has the potential for the full-scale application in the future.

Starvation resilience in anammox-based bioreactors: A stable nitrogen removal route on partial denitrification/anammox (PD/A)

This study investigates the performance, resilience and microbial community dynamics of two anaerobic processes, i.e. pure anammox (R1) and partial denitrification/anammox (PD/A) (R2), following a 30-day starvation period. The tolerance to starvation was assessed by comparing nitrogen removal efficiency and microbial activity across both reactors. Results show that the PD/A process recovery to pre-starvation performance levels within just one day, as compared to the pure anammox process. Notably, although the activity of anammox bacteria decreased in both processes during starvation, the decay rate in R1 was 69.59 % higher than in R2, potentially explaining the quicker recovery of R2. Furthermore, enhanced secretion of extracellular polymeric substance (EPS) during starvation served as a protective mechanism. The potential functions and genes in microorganisms, as well as the pathway of nitrogen cycling, were demonstrated through analyses using the KEGG database. This research reveals essential mechanistic insights and strategic guidance for the effective implementation of anammox-based biological nitrogen removal processes.

Synergy between Nitrogen Removal and Fermentation Bacteria Ensured Efficient Nitrogen Removal of a Mainstream Anammox System at Low Temperatures

Simultaneous partial nitrification, anammox, denitrification, and fermentation (SNADF) is a novel process achieving simultaneous advanced sludge reduction and nitrogen removal. The influence of low temperatures on the SNADF reactor was explored to facilitate the application of mainstream anammox. When temperature decreased from 32 to 16 °C, efficient nitrogen removal was achieved, with a nitrogen removal efficiency of 81.9-94.9%. Microbial community structure analysis indicated that the abundance of Candidatus Brocadia (dominant anaerobic ammonia oxidizing bacteria (AnAOB) in the system) increased from 0.03% to 0.18%. The abundances of Nitrospira and Nitrosomonas increased from 1.6% and 0.16% to 2.5% and 1.63%, respectively, resulting in an increase in the ammonia-oxidizing bacteria (AOB) to nitrite-oxidizing bacteria (NOB) abundance ratio from 0.1 to 0.64. This ensured sufficient nitrite for AnAOB, promoting nitrogen removal. In addition, Candidatus Competibacter, which plays a role in partial denitrification, was the dominant denitrification bacteria (DNB) and provided more nitrite for AnAOB, facilitating AnAOB enrichment. Based on the findings from microbial correlation network analysis, Nitrosomonas (AOB), Thauera, and Haliangium (DNB), and A4b and Saprospiraceae (fermentation bacteria), were center nodes in the networks and therefore essential for the stability of the SNADF system. Moreover, fermentation bacteria, DNB, and AOB had close connections in substrate cooperation and resistance to adverse environments; therefore, they also played important roles in maintaining stable nitrogen removal at low temperatures. This study provided new suggestions for mainstream anammox application.

Characterizing algal-bacterial symbiotic biofilms: Insights into coexistence of algae and anaerobic microorganisms

This study constructed an integrated algae/partial nitrification/anammox biofilm system and operated it for 240 days. The total nitrogen removal efficiency exceeded 90 %. The structure, compositions, and function of this symbiotic biofilm, which played a pivotal role in the system, were analyzed in detail. Microscope photos and fluorescence in situ hybridization both showed that bacteria and algae were well integrated. The dissolved oxygen gradient further confirmed that different functional microorganisms grew at varying depths within biofilm. Algae formed an oxygen-producing zone (0-0.48 mm), followed by ammonia oxidizing bacteria (AOB) consuming oxygen to form an oxygen-consuming zone (0.48-0.86 mm), and anaerobic ammonia oxidizing bacteria (AnAOB) removed nitrogen in anaerobic zone (>0.86 mm). Chlorella, Nitrosomonas and Candidatus_Kuenenia were identified as the dominant algae, AOB and AnAOB, with relative abundances of 11.80 %, 19.77 % and 3.07 %, respectively. This layered biofilm benefitted providing a suitable environment for various microorganisms to survive within a complex biofilm.

Attached indigenous microalgal-bacterial consortium with greater stress-resistance facilitated recovery of integrated fixed-film system after experiencing short-term stagnation inhibition

Stability of integrated fixed-film indigenous microalgal-bacterial consortium (IF-IMBC) requires further investigation. This study focused on the influence of short-term stagnation (STS), caused by influent variations or equipment maintenance, on IF-IMBC. Results showed that the IF-IMBC system experienced initial inhibition followed by subsequent recovery during STS treatment. Enhanced organics utilization was believed to contribute to system recovery. It is proposed that the attached IMBC possessed greater stress resistance. On the one hand, a higher increase in bacteria potentially participating in organic degradation was observed. Moreover, the dominant eukaryotic species significantly decreased in suspended IMBC while its abundance remained stable in the attached state. On the other hand, increased abundance for most functional enzymes was primarily observed in the attached bacteria. This fundamental research aims to bridge the knowledge gap regarding the response of IMBC to variations in operational conditions.

Treatment of low-C/N nitrate wastewater using a partial denitrification-anammox granule system: Granule reconstruction, stability, and microbial structure analyses

Industrial wastewater discharged into sewer systems is often characterized by high nitrate contents and low C/N ratios, resulting in high treatment costs when using conventional activated sludge methods. This study introduces a partial denitrification-anammox (PD/A) granular process to address this challenge. The PD/A granular process achieved an effluent TN level of 3.7 mg/L at a low C/N ratio of 2.3. Analysis of a typical cycle showed that the partial denitrification peaked within 15 min and achieved a nitrate-to-nitrite transformation ratio of 86.9%. Anammox, which was activated from 15 to 120 min, contributed 86.2% of the TN removal. The system exhibited rapid recovery from post-organic shock, which was attributed to significant increases in protein content within TB-EPS. Microbial dispersion and reassembly were observed after coexistence of the granules, with Thauera (39.12%) and Candidatus Brocadia (1.25%) identified as key functional microorganisms. This study underscores the efficacy of PD/A granular sludge technology for treating low-C/N nitrate wastewater.

Food wastewater treatment using a hybrid biofilm reactor: nutrient removal performance and functional microorganisms on filler biofilm and suspended sludge

In this study, a laboratory-scale hybrid biofilm reactor (HBR) was constructed to treat food wastewater (FWW) before it is discharged into the sewer. The chemical oxygen demand (COD) of 29 860 mg L-1 in FWW was degraded to 200-350 mg L-1 using the HBR under the operating parameters of COD load 1.68 kg m-3 d-1, hydraulic retention time (HRT) of 426.63 h, dissolved oxygen (DO) of 8-9 mg L-1, and temperature of 22-23 °C. The biomass of biofilm on the surface of filler was 2.64 g L-1 for column A and 0.91 g L-1 for column O. Microbial analysis revealed richer and more diverse microorganisms in filler biofilms compared to those in suspended sludge. The hybrid filler was conducive to the development of functional microbial species, including phyla Firmicutes, Actinobacteriota, and Chloroflexi, and genus level norank_f_JG30-KF-CM45, which will improve FWW treatment efficiency. Moreover, the microorganisms on the filler biofilm had more connections and relationships than those in the suspended sludge. The combination of an up-flow anaerobic sludge bed (UASB) and HBR was demonstrated to be an economical strategy for practical applications as a shorter HRT of 118.34 h could be obtained. Overall, this study provides reliable data and a theoretical basis for the application of HBR and FWW treatments.

Unravelling biosynthesis and biodegradation potentials of microbial dark matters in hypersaline lakes

Biosynthesis and biodegradation of microorganisms critically underpin the development of biotechnology, new drugs and therapies, and environmental remediation. However, most uncultured microbial species along with their metabolic capacities in extreme environments, remain obscured. Here we unravel the metabolic potential of microbial dark matters (MDMs) in four deep-inland hypersaline lakes in Xinjiang, China. Utilizing metagenomic binning, we uncovered a rich diversity of 3030 metagenome-assembled genomes (MAGs) across 82 phyla, revealing a substantial portion, 2363 MAGs, as previously unclassified at the genus level. These unknown MAGs displayed unique distribution patterns across different lakes, indicating a strong correlation with varied physicochemical conditions. Our analysis revealed an extensive array of 9635 biosynthesis gene clusters (BGCs), with a remarkable 9403 being novel, suggesting untapped biotechnological potential. Notably, some MAGs from potentially new phyla exhibited a high density of these BGCs. Beyond biosynthesis, our study also identified novel biodegradation pathways, including dehalogenation, anaerobic ammonium oxidation (Anammox), and degradation of polycyclic aromatic hydrocarbons (PAHs) and plastics, in previously unknown microbial clades. These findings significantly enrich our understanding of biosynthesis and biodegradation processes and open new avenues for biotechnological innovation, emphasizing the untapped potential of microbial diversity in hypersaline environments.

Synergistic Integration of Anammox and Endogenous Denitrification Processes for the Simultaneous Carbon, Nitrogen, and Phosphorus Removal

The feasibility of a synergistic endogenous partial denitrification-phosphorus removal coupled anammox (SEPD-PR/A) system was investigated in a modified anaerobic baffled reactor (mABR) for synchronous carbon, nitrogen, and phosphorus removal. The mABR comprising four identical compartments (i.e., C1-C4) was inoculated with precultured denitrifying glycogen-accumulating organisms (DGAOs), denitrifying polyphosphate-accumulating organisms, and anammox bacteria. After 136 days of operation, the chemical oxygen demand (COD), total nitrogen, and phosphorus removal efficiencies reached 88.6 ± 1.0, 97.2 ± 1.5, and 89.1 ± 4.2%, respectively. Network-based analysis revealed that the biofilmed community demonstrated stable nutrient removal performance under oligotrophic conditions in C4. The metagenome-assembled genomes (MAGs) such as MAG106, MAG127, MAG52, and MAG37 annotated as denitrifying phosphorus-accumulating organisms (DPAOs) and MAG146 as a DGAO were dominated in C1 and C2 and contributed to 89.2% of COD consumption. MAG54 and MAG16 annotated as Candidatus_Brocadia (total relative abundance of 16.5% in C3 and 4.3% in C4) were responsible for 74.4% of the total nitrogen removal through the anammox-mediated pathway. Functional gene analysis based on metagenomic sequencing confirmed that different compartments of the mABR were capable of performing distinct functions with specific advantageous microbial groups, facilitating targeted nutrient removal. Additionally, under oligotrophic conditions, the activity of the anammox bacteria-related genes of hzs was higher compared to that of hdh. Thus, an innovative method for the treatment of low-strength municipal and nitrate-containing wastewaters without aeration was presented, mediated by an anammox process with less land area and excellent quality effluent.

Refractory dissolved organic matters in sludge leachate trigger the combination of anammox and denitratation for advanced nitrogen removal

The cost-effective treatment of sludge leachate (SL) with high nitrogen content and refractory dissolved organic matter (rDOM) has drawn increasing attention. This study employed, for the first time, a rDOM triggered denitratation-anammox continuous-flow process to treat landfill SL. Moreover, the mechanisms of exploiting rDOM from SL as an inner carbon source for denitratation were systematically analyzed. The results demonstrated outstanding nitrogen and rDOM removal performance without any external carbon source supplement. In this study, effluent concentrations of 4.27 ± 0.45 mgTIN/L and 5.58 ± 1.64 mgTN/L were achieved, coupled with an impressive COD removal rate of 65.17 % ± 1.71 %. The abundance of bacteria belonging to the Anaerolineaceae genus, which were identified as rDOM degradation bacteria, increased from 18.23 % to 35.62 %. As a result, various types of rDOM were utilized to different extents, with proteins being the most notable, except for lignins. Metagenomic analysis revealed a preference for directing electrons towards NO3--N reductase rather than NO2--N reductase, indicating the coupling of denitratation bacteria and anammox bacteria (Candidatus Brocadia). Overall, this study introduced a novel synergy platform for advanced nitrogen removal in treating SL using its inner carbon source. This approach is characterized by low energy consumption and operational costs, coupled with commendable efficiency.

Novel and innovative approaches to partial denitrification coupled with anammox: A critical review

The partial denitrification (PD) coupled with anaerobic ammonium oxidation (Anammox) (PD/A) process is a unique biological denitrification method for sewage that concurrently removes nitrate (NO3--N) and ammonium (NH4+-N) in sewage. Comparing PD/A to conventional nitrification and denitrification technologies, noticeable improvements are shown in energy consumption, carbon source demand, sludge generation and emissions of greenhouse gasses. The PD is vital to obtaining nitrites (NO2--N) in the Anammox process. This paper provided valuable insight by introduced the basic principles and characteristics of the process and then summarized the strengthening strategies. The functional microorganisms and microbial competition have been discussed in details, the S-dependent denitrification-anammox has been analyzed in this review paper. Important factors affecting the PD/A process were examined from different aspects, and finally, the paper pointed out the shortcomings of the coupling process in experimental research and engineering applications. Thus, this research provided insightful information for the PD/A process's optimization technique in later treating many types of real and nitrate-based wastewater. The review paper also provided the prospective economic and environmental position for the actual design implementation of the PD/A process in the years to come.

Optimizing carbon sources regulation in the biochemical treatment systems for coal chemical wastewater: Aromatic compounds biodegradation and microbial response strategies

The complex composition of coal chemical wastewater (CCW), marked by numerous highly toxic aromatic compounds, induces the destabilization of the biochemical treatment system, leading to suboptimal treatment efficacy. In this study, a biochemical treatment system was established to efficiently degrade aromatic compounds by quantitatively regulating the dosage of co-metabolized substrates (specifically, the chemical oxygen demand (COD) Glucose: COD Sodium acetate = 3:1, 1:3, and 1:1). The findings demonstrated that the system achieved optimal performance under the condition that the ratio of COD Glucose to COD Sodium acetate was 3:1. When the co-metabolized substrate was added to the system at an optimal ratio, examination of pollutant removal and cumulative effects revealed that the removal efficiencies for COD and total organic carbon (TOC) reached 94.61 % and 86.40 %, respectively. The removal rates of benzene series, nitrogen heterocyclic compounds, polycyclic aromatic hydrocarbons, and phenols were 100 %, 100 %, 63.58 %, and 94.12 %, respectively. Research on the physiological response of microbial cells showed that, under optimal ratio regulation, co-metabolic substrates led to a substantial rise in microbial extracellular polymeric substances (EPS) secretion, particularly extracellular proteins. When the system reached the end of its operation, the contents of loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) for proteins in the optimal group were 7.12 mg/g-SS and 152.28 mg/g-SS, respectively. Meanwhile, the ratio of α-Helix / (β-Sheet + Random coil) and the proportion of intermolecular interaction forces were also increased in the optimal group. At system completion, the ratio of α-Helix / (β-Sheet + Random coil) reached 0.717 (LB-EPS) and 0.618 (TB-EPS), respectively. Additionally, the proportion of intermolecular interaction forces reached 74.83 % (LB-EPS) and 55.03 % (TB-EPS). An in-depth analysis of the metabolic regulation of microorganisms indicated that the introduction of optimal ratios of co-metabolic substrates contributed to a noteworthy upregulation in the expression of Catechol 2,3-dioxygenase (C23O) and Dehydrogenase (DHA). The expression levels of C23O and DHA were measured at 0.029 U/mg Pro·g MLSS and 75.25 mg TF·(g MLSS·h)-1 (peak value), respectively. Correspondingly, enrichment of aromatic compound-degrading bacteria, including Thauera, Saccharimonadales, and Candidatus_Competibacter, occurred, along with the upregulation of associated functional genes such as Catechol 1,2-dioxygenase, Catechol 2,3-dioxygenase, Protocatechuate 3,4-dioxygenase, and Protocatechuate 4,5-dioxygenase. Considering the intricate system of multiple coexisting aromatic compounds in real CCW, this study not only obtained an optimal ratio for carbon source addition but also enhanced the efficient utilization of carbon sources and improved the capability of the system to effectively degrade aromatic compounds. Additionally, this paper established a theoretical foundation for metabolic regulation and harmless treatment within the biochemical treatment of intricate systems, exemplified by real CCW.

Selective genes expression and metabolites transformation drive a robust nitrite accumulation during nitrate reduction under alternating feast-famine condition

Nitrite production via denitrification has been regarded as a key approach for survival of anaerobic ammonium oxidation (anammox) bacteria. Despite the important carbon substrate, little is known about the role of differential genes expression and extracellular metabolite regulation among diverse microbial communities. In this study, a novel alternating feast-famine strategy was proposed and demonstrated to efficiently accumulate nitrite in a low-nitrogen loading rate (NLR) (0.2∼0.8 kg N/m3/d) denitrification system. Highly selective expression of denitrifying genes was revealed as key regulators. Interestingly, in absence of carbon source (ACS) condition, the expression of narG and narI/V genes responsible for reduction of nitrate to nitrite jumped to 2.5 and 5.1 times higher than that in presence of carbon source (PCS) condition with carbon to nitrate ratio of 3.0. This fortunately facilitated a rapid nitrite accumulation once acetate was added, despite a significantly down-regulated narG and narI/narV and up-regulated nirS/nirK. This strategy selected Thauera as the most dominant denitrifier (50.2 %) with the highest contribution to narG and narI/narV genes, responsible for the high nitrite accumulation. Additionally, extracellular xylose, pyruvate, and glucose jointly promoted carbon-central metabolic pathway of key denitrifiers in ACS stage, playing an important role in the process of self-growth and selective enrichment of functional bacteria. The relatively rapid establishment and robust performance obtained in this study shows an engineering-feasible and economically-favorable solution for the regulation of partial denitrification in practical application.

Co-occurrence, toxicity, and biotransformation pathways of metformin and its intermediate product guanylurea: Current state and future prospects for enhanced biodegradation strategy

Accumulation of metformin and its biotransformation product "guanylurea" are posing an increasing concern due to their low biodegradability under natural attenuated conditions. Therefore, in this study, we reviewed the unavoidable function of metformin in human body and the route of its release in different water ecosystems. In addition, metformin and its biotransformation product guanylurea in aquatic environments caused certain toxic effects on aquatic organisms which include neurotoxicity, endocrine disruption, production of ROS, and acetylcholinesterase disturbance in aquatic organisms. Moreover, microorganisms are the first to expose and deal with the release of these contaminants, therefore, the mechanisms of biodegradation pathways of metformin and guanylurea under aerobic and anaerobic environments were studied. It has been reported that certain microbes, such as Aminobacter sp. and Pseudomonas putida can carry potential enzymatic pathways to degrade the dead-end product "guanylurea", and hence guanylurea is no longer the dead-end product of metformin. However, these microbes can easily be affected by certain geochemical cycles, therefore, we proposed certain strategies that can be helpful in the enhanced biodegradation of metformin and its biotransformation product guanylurea. A better understanding of the biodegradation potential is imperative to improve the use of these approaches for the sustainable and cost-effective remediation of the emerging contaminants of concern, metformin and guanylurea in the near future.

Research progress of anaerobic ammonium oxidation (Anammox) process based on integrated fixed-film activated sludge (IFAS)

The integrated fixed-film activated sludge (IFAS) process is considered one of the cutting-edge solutions to the traditional wastewater treatment challenges, allowing suspended sludge and attached biofilm to grow in the same system. In addition, the coupling of IFAS with anaerobic ammonium oxidation (Anammox) can further improve the efficiency of biological denitrification. This paper summarises the research progress of IFAS coupled with the anammox process, including partial nitrification anammox, simultaneous partial nitrification anammox and denitrification, and partial denitrification anammox technologies, and describes the factors that limit the development of related processes. The effects of dissolved oxygen, influent carbon source, sludge retention time, temperature, microbial community, and nitrite-oxidising bacteria inhibition methods on the anammox of IFAS are presented. At the same time, this paper gives an outlook on future research focus and engineering practice direction of the process.

Simultaneous ammonia and nitrate removal by novel integrated partial denitrification-anaerobic ammonium oxidation-bioelectrochemical system

The current study explored the performance of an integrated partial denitrification-anaerobic ammonium oxidation (anammox)-bioelectrochemical system on simultaneous removal of ammonia nitrogen and nitrate nitrogen. Different operational conditions were selected to optimize critical parameters of the process for improving nitrogen removal. The results indicated that more than 90 % of total inorganic nitrogen removal efficiency was achieved under the optimal conditions: ammonia nitrogen/nitrate nitrogen ratio of 1:2, external resistance of 200 Ω and inoculation volume ratio of anammox bacteria/denitrifying at 2:1. Improved nitrogen removal under the optimal conditions were confirmed by microbial community changes (Candidatus Brocadia and Thiobacillus) and enhanced of nitrogen metabolism-related genes (hao, hzsA/C and hdh). Increases of Limnobacter indicated an enhanced electron transfer efficiency. Overall, high-efficiency and stable nitrogen removal efficiency without nitrite nitrogen accumulation could be achieved by the integrated system under the optimal conditions, providing novel insights for simultaneous treatment of domestic wastewater and groundwater.

Enrichment of anammox biomass during mainstream wastewater treatment driven by achievement of partial denitrification through the addition of bio-carriers

Anammox is widely considered as the most cost-effective and sustainable process for nitrogen removal. However, how to achieve the enrichment of anammox biomass remains a challenge for its large-scale application, especially in mainstream wastewater treatment. In this study, the feasibility of enrichment of anammox biomass was explored through the realization of partial denitrification and the addition of bio-carriers. By using ordinary activated sludge, a sequencing batch reactor (SBR) followed by an up-flow anaerobic sludge bed (UASB) was operated at 25 ± 2°C for 214 days. The long-term operation was divided into five phases, in which SBR and UASB were started-up in Phases I and II, respectively. By eliminating oxygen and adjusting the inflow ratios in Phases III-V, advanced nitrogen removal was achieved with the effluent total nitrogen being 4.7 mg/L and the nitrogen removal efficiency being 90.5% in Phase V. Both in-situ and ex-situ activity tests demonstrated the occurrence of partial denitrification and anammox. Moreover, 16S rRNA high-throughput sequencing analysis revealed that Candidatus Brocadia was enriched from below the detection limit to in biofilms (0.4% in SBR, 2.2% in UASB) and the floc sludge (0.2% in SBR, 1.3% in UASB), while Thauera was mainly detected in the floc sludge (8.1% in SBR, 8.8% in UASB), which might play a key role in partial denitrification. Overall, this study provides a novel strategy to enrich anammox biomass driven by rapid achievement of partial denitrification through the addition of bio-carriers, which will improve large-scale application of anammox processes in mainstream wastewater treatment.

Iron cycle-enhanced anaerobic ammonium oxidation in microaerobic granular sludge

Granule-based partial nitritation and anaerobic ammonium oxidation (PN/A) is an energy-efficient approach for treating ammonia wastewater. When treating low-strength ammonia wastewater, the stable synergy between PN and anammox is however difficult to establish due to unstable dissolved oxygen control. Here, we proposed, the PN/A granular sludge formed by a micro-oxygen-driven iron redox cycle with continuous aeration (0.42 ± 0.10 mg-O2/L) as a novel strategy to achieve stable and efficient nitrogen (N) removal. 240-day bioreactor operation showed that the iron-involved reactor had 37 % higher N removal efficiency than the iron-free reactor. Due to the formation of the microaerobic granular sludge (MGS), the bio(chemistry)-driven iron cycle could be formed with the support of anaerobic ammonium oxidation coupled to Fe3+ reduction. Both ammonia-oxidizing bacteria and generated Fe2+ could scavenge the oxygen as a defensive shield for oxygen-sensitive anammox bacteria in the MGS. Moreover, the iron minerals derived from iron oxidation and Fe-P precipitates were also deposited on the MGS surface and/or embedded in the internal channels, thus reducing the size of the channels that could limit oxygen mass transfer inside the MGS. The spatiotemporal assembly of diverse functional microorganisms in the MGS for the realization of stable PN/A could be achieved with the support of the iron redox cycle. In contrast, the iron-free MGS could not optimize oxygen mass transfer, which led to an unstable and inefficient PN/A. This work provides an alternative iron-related autotrophic N removal for low-strength ammonia wastewater.

Rapid start-up of an innovative pilot-scale staged PN/A continuous process for enhanced nitrogen removal from mature landfill leachate via robust NOB elimination and efficient biomass retention

The start-up and stable operation of partial nitritation-anammox (PN/A) treatment of mature landfill leachate (MLL) still face challenges. This study developed an innovative staged pilot-scale PN/A system to enhance nitrogen removal from MLL. The staged process included a PN unit, an anammox upflow enhanced internal circulation biofilm (UEICB) reactor, and a post-biofilm unit. Rapid start-up of the continuous flow PN process (full-concentration MLL) was achieved within 35 days by controlling dissolved oxygen and leveraging free ammonia and free nitrous acid to selectively suppress nitrite-oxidizing bacteria (NOB). The UEICB was equipped with an annular flow agitator combined with the enhanced internal circulation device of the guide tube, which achieved an efficient enrichment of Candidatus Kuenenia in the biofilm (relative abundance of 33.4 %). The nitrogen removal alliance formed by the salt-tolerant anammox bacterium (Candidatus Kuenenia) and denitrifying bacteria (unclassified SBR1031 and Denitratisoma) achieved efficient nitrogen removal of UEICB (total nitrogen removal percentage: 90.8 %) and at the same time effective treatment of the refractory organic matter (ROM). The dual membrane process of UEICB fixed biofilm combined with post-biofilm is effective in sludge retention, and can stably control the effluent suspended solids (SS) at a level of less than 5 mg/L. The post-biofilm unit ensured that effluent total nitrogen (TN) remained below the 40 mg/L discharge standard (98.5 % removal efficiency). Compared with conventional nitrification-denitrification systems, the staged PN/A process substantially reduced oxygen consumption, sludge production, CO2 emissions and carbon consumption by 22.8 %, 67.1 %, 87.1 % and 87.1 %, respectively. The 195-day stable operation marks the effective implementation of the innovative pilot-scale PN/A process in treating actual MLL. This study provides insights into strategies for rapid start-up, robust NOB suppression, and anammox biomass retention to advance the application of PN/A in high-ammonia low-carbon wastewater.

Insights into current bio-processes and future perspectives of carbon-neutral treatment of industrial organic wastewater: A critical review

With the rise of the concept of carbon neutrality, the current wastewater treatment process of industrial organic wastewater is moving towards the goal of energy conservation and carbon emission reduction. The advantages of anaerobic digestion (AD) processes in industrial organic wastewater treatment for bio-energy recovery, which is in line with the concept of carbon neutrality. This study summarized the significance and advantages of the state-of-the-art AD processes were reviewed in detail. The application of expanded granular sludge bed (EGSB) reactors and anaerobic membrane bioreactor (AnMBR) were particularly introduced for the effective treatment of industrial organic wastewater treatment due to its remarkable prospect of engineering application for the high-strength wastewater. This study also looks forward to the optimization of the AD processes through the enhancement strategies of micro-aeration pretreatment, acidic-alkaline pretreatment, co-digestion, and biochar addition to improve the stability of the AD system and energy recovery from of industrial organic wastewater. The integration of anaerobic ammonia oxidation (Anammox) with the AD processes for the post-treatment of nitrogenous pollutants for the industrial organic wastewater is also introduced as a feasible carbon-neutral process. The combination of AnMBR and Anammox is highly recommended as a promising carbon-neutral process for the removal of both organic and inorganic pollutants from the industrial organic wastewater for future perspective. It is also suggested that the AD processes combined with biological hydrogen production, microalgae culture, bioelectrochemical technology and other bio-processes are suitable for the low-carbon treatment of industrial organic wastewater with the concept of carbon neutrality in future.

Multipathway of Nitrogen Metabolism Revealed by Genome-Centered Metatranscriptomics from Pyrite-Guided Mixotrophic Partial Denitrification/Anammox Installations

Further reducing the organic requirements is essential for the sustainable development of partial denitrification/anammox technology. Here, an innovative mixotrophic partial denitrification/anammox (MPD/A) installation fed with pyrite and few organics was realized, and the average nitrogen and phosphorus removal rates were as high as 96.24 ± 0.11% and 79.23 ± 2.06%, respectively, with a C/N ratio of 0.5. To understand the nature by which MPD/A achieves efficient nitrogen removal and organic conservation, the electron transfer-dependent nitrogen escape and energy metabolism were first elucidated using multiomics analysis. Apart from heterotrophic denitrification and anammox, the results revealed some unexpected metabolic couplings of MPD/A systems, in particular, putative nitrate-dependent organic and pyrite oxidation among nominally heterotrophic Denitratisoma (PRO3) strains, which accelerated nitrate gasification with a low-carbon supply. Additionally, Candidatus Brocadia (AMX) employed extracellular solid-state electron acceptors as terminal electron sinks for high-rate ammonium removal. AMX transported ammonium electrons to extracellular γFeO(OH) (generated from pyrite oxidation) through the transient storage of menaquinoline pools, cytoplasmic migration via multiheme cytochrome(s), and OmpA protein/nanowires-mediated electron hopping on cell surfaces. Further investigation observed that extracellular electron flux resulted in the transfer of more energy from the increased oxidation of the electron donor to the ATP, supporting nitrite-independent ammonium removal.

Granulation of partial denitrification sludge: Advances in mechanism understanding, technologies development and perspectives

The high-rate and stably efficient nitrite generation is vital and still challenges the wide application of partial denitrification (PD) and anammox technology. Increasing attention has been drawn to the granulation of PD biomass. However, the knowledge of PD granular sludge is still limited in terms of granules characterization and mechanisms of biomass aggregation for high nitrite accumulation. This work reviewed the performance and granulation of PD biomass for high nitrite accumulation via nitrate reduction, including the system start-up, influential factors, granular characteristics, hypothetical mechanism, challenges and perspectives in future application. The physiochemical characterization and key influential factors were summarized in view of nitrite production, morphology analysis, extracellular polymer substance structure, as well as microbial mechanisms. The PD granules exhibit potential advantages of a high biomass density, good settleability, high hydraulic loading rates, and strong shock resistance. A novel granular sludge-based PD combined with anammox process was proposed to enhance the capability of nitrogen removal. In the future, PD granules utilizing different electron donors is a promising way to broaden the application of anammox technology in both municipal and industrial wastewater treatment.

Hydroxyapatite (HAP) formation in acetate-driven partial denitrification process: Enhancing sludge granulation and phosphorus removal

Partial denitrification/anammox (PD/A) processes have emerged as a promising technology for efficient nitrogen removal from wastewater. However, these processes fail to remove phosphorus (P), a key pollutant that contributes to water eutrophication. To address this issue, the potential of inducing hydroxyapatite (HAP) precipitation in PD processes to achieve simultaneous P removal was investigated for the first time. Specifically, three SBRs (R1-R3) for PD were operated with adding varying concentrations of external Ca (30, 60, and 120 mg/L, respectively). Results demonstrated significant P reduction in all three SBRs, particularly in R3 with high Ca, which achieved an 80 % removal efficiency. Notably, sludge granulation was observed during operation, with the granule size in R3 with high Ca reaching 906.1 μm during the stable period, exceeding those in R2 (788.7 μm) and R1 (707.1 μm). This led to good settle ability of the PD sludge, as demonstrated by the lowest SVI5 (20 mL/g MLSS). Moreover, the decrease in the MLVSS/MLSS ratio suggested that the inorganic content accumulated, as observed by confocal laser scanning microscopy in the interior of the granules. Elemental composition analysis suggested that PD granules contained high P and Ca, while the X-ray diffraction (XRD) results confirmed the formation of HAP. Overall, this study demonstrated that PD-HAP coupled granular sludge process has potential as a robust and efficient method for nitrite production, as well as effective P removal and recovery, thereby advancing the application of anammox processes in wastewater treatment.

Response of biofilm-based systems for antibiotics removal from wastewater: Resource efficiency and process resiliency

Biofilm-based systems have efficient stability to cope-up influent shock loading with protective and abundant microbial assemblage, which are extensively exploited for biodegradation of recalcitrant antibiotics from wastewater. The system performance is subject to biofilm types, chemical composition, growth and thickness maintenance. The present study elaborates discussion on different type of biofilms and their formation mechanism involving extracellular polymeric substances secreted by microbes when exposed to antibiotics-laden wastewater. The biofilm models applied for estimation/prediction of biofilm-based systems performance are explored to classify the application feasibility. Further, the critical review of antibiotics removal efficiency, design and operation of different biofilm-based systems (e.g. rotating biological contactor, membrane biofilm bioreactor etc.) is performed. Extending the information on effect of various process parameters (e.g. hydraulic retention time, pH, biocarrier filling ratio etc.), the microbial community dynamics responsible of antibiotics biodegradation in biofilms, the technological problems, related prospective and key future research directions are demonstrated. The biofilm-based system with biocarriers filling ratio of ∼50-70% and predominantly enriched with bacterial species of phylum Proteobacteria protected under biofilm thickness of ∼1600 μm is effectively utilized for antibiotic biodegradation (>90%) when operated at DO concentration ≥3 mg/L. The C/N ratio ≥1 is best suitable condition to eliminate antibiotic pollution from biofilm-based systems. Considering the significance of biofilm-based systems, this review study could be beneficial for the researchers targeting to develop sustainable biofilm-based technologies with feasible regulatory strategies for treatment of mixed antibiotics-laden real wastewater.

Microbial interactions play a keystone role in rapid anaerobic ammonium oxidation sludge proliferation and biofilm formation

Two mature anaerobic ammonium oxidation (anammox) consortia with high/low relative abundance of anammox bacteria were inoculated for the rapid sludge proliferation and biofilm formation in this study, named up-flow anaerobic sludge blanket reactor (UASB1) (high) and UASB2 (low), respectively. Results showed that the nitrogen removal efficiency of UASB2 reached 90.94% after the 120-day operation, which was 13% higher than that of UASB1. Moreover, its biomass amounts were 22.18% (biofilm) and 40.96% (flocs) higher than that of UASB1, respectively. Ca. Kuenenia possessed relative abundances of 29.32% (flocs), 27.42% (biofilm) and 31.56% (flocs), 35.20% (biofilm) in the UASB1 and UASB2, respectively. The relative abundances of genes involved in anammox transformation (hzs, nir) and carbon metabolism (fdh, lgA/B/C, acs) were higher in the UASB2, indicating that Ca. Kuenenia might produce acetate and glycogen to enhance microbial interactions. These findings emphasized the importance of microbial interactions in anammox sludge proliferation and biofilm formation.

Pilot-scale demonstration of self-enrichment of anammox bacteria in a two-stage nitrification-denitrification suspended sludge system treating municipal wastewater under extremely low nitrogen loading rate

In suspended sludge system, efficient enrichment and retention of anammox bacteria are crucial obstacles in mainstream wastewater treatment by anammox process. In this study, anammox bacteria was self-enriched in a pilot-scale suspended sludge system of two-stage nitrification-denitrification process serving municipal wastewater treatment. With the low ammonia (NH4+-N) of 9.3 mg/L, nitrate (NO3--N) of 15.6 mg/L and COD/NO3--N of 2.2 under extremely low nitrogen loading rate of 0.012 kg N/m3/d, anammox activity bloomed after its abundance increasing from 5.9 × 107 to 4.6 × 109 copies/g dry sludge. Significant NH4+-N removal was occurred and maintained stably in the denitrification reactor with anammox bacteria accounting for 1.13%, even under temperature decreasing to 20.0℃. The adequately anoxic environment, efficient retention with the static settlement, and NO2- production via NO3- reduction provided favorable environment for anammox bacteria. This study demonstrated the feasibility and great potential in mainstream anammox application without seeding specific sludge.

Response of nitrite accumulation, sludge characteristic and microbial transition to carbon source during the partial denitrification (PD) process

Partial denitrification (PD, nitrate (NO3--N) → nitrite (NO2--N)) as a novel pathway for NO2--N production has been widely concerned, but the specific conditions for highly efficient and stable nitrite maintenance are not yet fully understood. In this study, the effects of carbon sources (acetate, R1; propionate, R2; glucose, R3) on NO2--N accumulation was discussed without seeding PD sludge and the mechanism analysis related to sludge characteristic and microbial evolution were elucidated. The optimal NO2--N, nitrate-to-nitrite transformation ratio (NTR) and nitrite removal efficiency (NRE) reached up to 32.10 mg/L, 98.01 %, and 86.95 % in R1. However, due to the complex metabolic pathway of glucose, the peak time of NO2--N production delayed from 30 min to 60 min. The sludge particle size decreased from 154.2 μm (R1), 130.8 μm (R2) to 112.6 μm (R3) with the increasing extracellular polymeric substances (EPS) from 80.75-85.44 mg/gVSS, 82.68-92.75 mg/gVSS to 106.31-110.25 mg/gVSS, where the ratio of proteins/polysaccharides (PN/PS) was proved to be closely associated with NO2--N generation. For the microbial evolution, Saccharimonadales (70.42 %) dominated the glucose system, while Bacillus (7.42-21.63 %) and Terrimonas (4.24-5.71 %) were the main contributors for NO2--N accumulation in the acetate and propionate systems. The achievement of PD showed many advantages of lower carbon demand, minimal sludge production, lesser greenhouse gas emission and prominent nutrient removal, offering an economically and technically attractive alternative for NO3--N containing wastewater treatment.

Effect of free ammonia on partial denitrification: Long-term performance, mechanism, and feasibility of PD/Anammox-FBBR for mature landfill leachate treatment

As a stable and effective approach for NO2--N accumulation, partial denitrification (PD) could significantly cut down operation cost, and PD/Anammox (PD/A) is a promising nitrogen removal process in wastewater treatment. The biotoxicity of free ammonia (FA) to nitrifying bacteria and anammox bacteria has been demonstrated, while whether FA affects PD bacteria is an open question. Here, long-term operation of PD-fixed bed biofilm reactor (PD-FBBR) treating synthetic wastewater and mature landfill leachate was conducted to reveal the mechanism concerning the effect of FA on PD bacteria. Stable NO2--N accumulation was achieved with NO3--N to NO2--N transformation ratio (NTR) of 60-70% during 280-day operation with FA ranged from 0 to 20.71 ± 0.23 mg/L, while NTR decreased and maintained at ∼30% when FA reached 40.59 ± 0.19 mg/L. Specific NOx--N reduction rate improved at low FA concentration (< 12 mg/L), while high FA level (> 25 mg/L) had inhibitory effect on PD bacteria. Under FA stress, more extracellular polymeric substances (EPS) were secreted, and the glnA gene abundance, glutamine synthase concentration, and glutamine concentration in cell and EPS significantly increased, indicating the enhancement of glutamine biosynthesis in PD bacteria for ammonia assimilation played an important role in response to FA stress. Metagenomic sequencing showed that FA stimulated the upregulation of narK (NO3--N/NO2--N antiporter) gene abundance and enhanced uptake of NO3--N and extrusion of NO2--N. Comamonas, unclassified_f__Comamonadaceae and Thauera were highly enriched in biofilm, which played a key role in the stable NO2--N accumulation. Furthermore, a novel two stage PD/A-FBBR was applied to mature landfill leachate treatment, and satisfactory total inorganic nitrogen removal efficiency ranged from 81.38 ± 3.56% to 89.16 ± 1.57% was obtained at relatively low COD/NO3--N of 2.57-2.84. Overall, these findings demonstrated how PD bacteria respond to FA stress and confirmed the feasibility of PD/A process in high FA wastewater treatment.

New insights into the effects of wetland plants on nitrogen removal pathways in constructed wetlands with low C/N ratio wastewater: Contribution of partial denitrification-anammox

Nitrogen (N) removal in constructed wetlands (CWs) was often challenged by limited denitrification due to the lack of carbon source, and wetland plants would be more important in carbon (C) and N cycling in CWs with influent of low carbon to nitrogen (C/N) ratio. In this study, the underlying mechanisms of nitrate nitrogen (NO3--N) removal under different low C/N ratios were revealed by constructing microcosm CWs, and the unplanted group was set as the control to explore the role of plants in N removal. The results showed that plants and the concentration of influent carbon significantly affected NO3--N and total nitrogen (TN) removal (p < 0.05). The presence of plants significantly increased the concentration of DO and wetland plant-derived DOM (p < 0.05). The enhanced NO3--N and TN removal with increased C/N ratio attributed to high denitrification activity reflected in the abundance of denitrification microbes and genes. However, the contribution of partial denitrification-anammox (PDN/AMX) to N removal in CWs decreased from more than 75.3% at the C/N ratio of 0 to 70.4% and 22.3% with the C/N ratio increased to 1.5 and 3, respectively. Furthermore, the PDN/AMX process was negatively correlated with favorable oxygen environment in the planted group and plants roots carbon secretion, but the overall N removal efficiency of the CWs was enhanced by increased abundance of N removal-related functional genes in the presence of plants. Abovementioned results provided new insights to explain the mechanism of N removal in CWs under low C/N ratio.

Responses of Bacillus sp. under Cu(II) stress in relation to extracellular polymeric substances and functional gene expression level

The production and composition of extracellular polymeric substances (EPS), as well as the EPS-related functional resistance genes and metabolic levels of Bacillus sp. under Cu(II) stress, were investigated. EPS production increased by 2.73 ± 0.29 times compared to the control when the strain was treated with 30 mg L^-1 Cu(II). Specifically, the polysaccharide (PS) content in EPS increased by 2.26 ± 0.28 g CDW^-1 and the PN/PS (protein/polysaccharide) ratio value increased by 3.18 ± 0.33 times under 30 mg L^-1 Cu(II) compared to the control. The increased EPS secretion and higher PN/PS ratio in EPS strengthened the cells' ability to resist the toxic effect of Cu(II). Differential expression of functional genes under Cu(II) stress was revealed by Gene Ontology pathway enrichment analysis and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis. The enriched genes were most obviously upregulated in the UMP biosynthesis pathway, the pyrimidine metabolism pathway, and the TCS metabolism pathway. This indicates an enhancement of EPS regulation-related metabolic levels and their role as a defense mechanism for cells to adapt to Cu(II) stress. Additionally, seven copper resistance genes were upregulated while three were downregulated. The upregulated genes were related to the heavy metal resistance, while downregulated genes were related to cell differentiation, indicating that the strain had initiated an obvious resistance to Cu(II) despite its severe cell toxicity. These results provided a basis for promoting EPS-regulated associated functional genes and the application of gene-regulated bacteria in heavy metal-containing wastewater treatment.

Prokaryotic community interchange between distinct microhabitats causes community pressure on anammox biofilm development

Biofilms are an efficient way to underpin the biological process of wastewater treatment. However, little is known about the driving forces of biofilm formation and development in industrial settings. Long-term observation of anammox biofilms indicated the interplay between different microhabitats (biofilm, aggregate, plankton) was important in sustaining biofilm formation. SourceTracker analysis showed that 88.77 ± 2.26% of initial biofilm originated from the aggregate, however, independent evolution was led by anammox species in the later stage (182d and 245d). Noticeably, the source proportion of aggregate and plankton increased when temperature varied, suggesting an interchange of species between different microhabitats could be helpful to biofilm recovery. The microbial interaction pattern and community variation displayed similar trends, but the unknown source proportion of interaction was very high during the entire incubation (7-245d), thereby the same species may develop different relationships within the distinct microhabitats. The core phyla, Proteobacteria and Bacteroidota, accounted for ∼80% of interactions in all lifestyles, which is consistent with the fact that Bacteroidota played important role in the early stage of biofilm assembly. Although anammox species evolved few links with other OTUs, Candidatus Brocadiaceae still outcompeted the NS9 marine group to dominate the homogeneous selection process in the later stage (56-245d) of biofilm assembly, implying that the functional species may be decoupled from the core species in the microbial network. The conclusions will shed a light on the understanding of biofilm development in large-scale biosystems of wastewater treatment.

Insights to Fe(II) on the fate of humic acid and humic acid Fe complex with biogeobattery effect in simultaneous partial nitritation, anammox and denitrification (SNAD) system

The role of Fe(II) on the humic acid (HA) transformation and the effects of humic acid Fe (HA-Fe) on simultaneous partial nitrification, anammox and denitrification (SNAD) system were investigated. After adding Fe(II), the HA content decreased and the HA inhibition on the SNAD system was released. Results showed that Fe(II) and HA formed the lower water-soluble HA-Fe, promoting the HA removal. HA-Fe with stronger electron transfer capacity constituted the interface with microorganisms to forming the biogeobattery effect. This accelerated the microbial electron transfer, as well as improved the key enzymes and ATP, indicating that HA-Fe stimulated the microbial activity of the SNAD system. Microbial community and quorum sensing analysis further demonstrated that HA-Fe enhanced the mutual symbiosis between electroactive and nitrogen removal bacteria, to ensure the stability of the SNAD system. The study provided references for efficient HA removal and revealed the biogeobattery effect of HA-Fe in the SNAD system.

Starting-up performances and microbial community shifts in the coupling process (SAPD-A) with sulfide autotrophic partial denitrification (SAPD) and anammox treating nitrate and ammonium contained wastewater

A novel coupling process (SAPD-A) with sulfide autotrophic partial denitrification (SAPD) (NO₃^--N→NO₂^--N) and anaerobic ammonium oxidation (Anammox) was developed using anaerobic sequencing batch reactor (ASBR) in this work. The integrated process comprised two stages. Firstly, the starting-up of SAPD process succeeded by gradually increasing the influent nitrate and sulfide in 95 days. The average nitrate removal efficiency (NRE) and NO₂^--N accumulation rates were 71.24% ± 0.21% and 46.44% ± 0.53% at SAPD process (days 75-95). Then, successful coupling process (SAPD-A) was implemented in two stages (stage I and stage II of SAPD-A). In stage I, it is feasible to promote the successful construction of SAPD-A process by elevating influent ammonium only based on SAPD system, making the NRE increased from 44.45% ± 0.46% (day 95) to 64.62% ± 0.12% at the end of stage I in SAPD-A system (day 126). Meanwhile, the ammonium nitrogen removal efficiency (ARE) and total nitrogen removal efficiency (TN-RE) also rose up to 42.46% ± 2.02% and 63.28% ± 0.54% respectively. Furthermore, the average ARE, NRE and TN-RE during the stage II in the bioreactor could reach 65.17% ± 1.45%, 74.50% ± 0.81% and 77.81% ± 0.37% by loading some biofilters (with of approximate 10% of the volume of the bioreactor) attached anaerobic ammonium oxidation bacteria (AnAOB). High-throughput sequencing results showed that the dominant genera concerning nitrogen removal were norank_f_norank_o_Fimbriimonadates (with the abundance of 2.88-8.54%), norank_ o_ norank _ c_ OM190 (2.48-4.41%), norank_f_norank_o_norank_c_WWE3 (11.01-17.69%), subgroup_10 (1.97-3.81%), Limnobacter（2.17-3.49%）, norank_f_n orank_ o_norank_ c_OLB14 (2.03-5.23%), norank-f-PHOS-HE36 (2.18-5.5%), Ellin6067 (1.34-2.24%) and Candidatus_ Brocadia (1.95-2.42%) during the whole starting-up period of coupling SAPD-A process. Batch experiments revealed that the sulfide was fully oxidized within 2 h, with the maximum reaction rate of 38.30 ± 1.53 mg (L h)^-1 in the first 1 h. Simultaneously, the concentration of nitrate sharply decreased from 53.08 ± 0.23 mg L^-1 to 24.16 ± 0.42 mg L^-1 with the reaction rate of 66.41 ± 2.12 mg (L h)^-1 in 0.5 h. Also, the ammonium concentration significantly declined from 47.88 ± 0.34 mg L^-1 to 10.98 ± 0.39 mg L^-1 in 8 h. Anammox process was responsible for the dominant nitrogen removal in the coupling SAPD-A system.

Biofilm-based technology for industrial wastewater treatment: current technology, applications and future perspectives

The microbial community in biofilm is safeguarded from the action of toxic chemicals, antimicrobial compounds, and harsh/stressful environmental circumstances. Therefore, biofilm-based technology has nowadays become a successful alternative for treating industrial wastewater as compared to suspended growth-based technologies. In biofilm reactors, microbial cells are attached to static or free-moving materials to form a biofilm which facilitates the process of liquid and solid separation in biofilm-mediated operations. This paper aims to review the state-of-the-art of recent research on bacterial biofilm in industrial wastewater treatment including biofilm fundamentals, possible applications and problems, and factors to regulate biofilm formation. We discussed in detail the treatment efficiencies of fluidized bed biofilm reactor (FBBR), trickling filter reactor (TFR), rotating biological contactor (RBC), membrane biofilm reactor (MBfR), and moving bed biofilm reactor (MBBR) for different types of industrial wastewater treatment. Besides, biofilms have many applications in food and agriculture, biofuel and bioenergy production, power generation, and plastic degradation. Furthermore, key factors for regulating biofilm formation were also emphasized. In conclusion, industrial applications make evident that biofilm-based treatment technology is impactful for pollutant removal. Future research to address and improve the limitations of biofilm-based technology in wastewater treatment is also discussed.

Insights into a novel nitrogen removal process based on simultaneous anammox and denitrification (SAD) following nitritation with in-situ NOB elimination

Simultaneous anammox and denitrification (SAD) is an efficient approach to treat wastewater having a low C/N ratio; however, few studies have investigated a combination of SAD and partial nitritation (PN). In this study, a lab-scale up-flow blanket filter (UBF) and zeolite sequence batch reactor (ZSBR) were continuously operated to implement SAD and PN advantages, respectively. The UBF achieved a high total nitrogen (TN) removal efficiency of over 70% during the start-up stage (days 1-50), and reached a TN removal efficiency of 96% in the following 90 days (days 51-140) at COD/NH₄⁺-N ratio of 2.5. The absolute abundance of anammox bateria increased to the highest value of 1.58 × 10⁷ copies/µL DNA; Comamonadaceae was predominant in the UBF at the optimal ratio. Meanwhile, ZSBR was initiated on day 115 as fast nitritation process to satisfy the influent requirement for the UBF. The combined process was started on day 140 and then lasted for 30 days, during the combined process, between the two reactors, the UBF was the main contributor for TN (66.5% ± 4.5%) and COD (71.8% ± 4.9%) removal. These results demonstrated that strong SAD occurred in the UBF when following a ZSBR with in-situ NOB elimination. This research presents insights into a novel biological nitrogen removal process for low C/N ratio wastewater treatment.

Feasibility of Bio-Coagulation Dewatering Followed by Bio-Oxidation Process for Treating Swine Wastewater

The unsatisfactory performance of the conventional swine wastewater treatment is drawing increasing attention due to the large amount of refractory chemical oxygen demand (COD), nitrogen, and phosphorus attached to the suspended solids (SS). In this study, for the first time, a novel process based on bio-coagulation dewatering followed by a bio-oxidation (BDBO) system was developed to treat swine wastewater containing high-strength SS, COD, TN, and TP. Firstly, after the bio-coagulation process, the removal efficiencies of SS, COD, NH3-N, and TP reached as high as 99.94%, 98.09%, 61.19%, and 99.92%, respectively. Secondly, the filtrate of the bio-coagulation dewatering process was introduced into the subsequent bio-oxidation process, in which the residual COD and NH3-N were further biodegraded in a sequence batch reactor. In addition, the dewatering performance of the concentrated swine slurry was substantially improved, with the specific resistance to filtration decreasing from 17.0 × 1012 to 0.3 × 1012 m/kg. Moreover, the concentrated swine slurry was pressed and filtered into a semi-dry cake after pilot-scale bio-coagulation dewatering treatment. Finally, the concentrations of COD and NH3-N in the effluent after the BDBO process, ranging between 150-170 mg/L and 75-90 mg/L, met the relevant discharge standard. Compared to traditional treatments, the BDBO system has excellent large-scale potential for improving the treatment efficiency, shortening the operation period, and reducing the processing costs, and is emerging as a cost-effective alternative for the treatment of wastewater containing high concentrations of SS, COD, TN, and TP.

A novel simultaneous partial nitrification, anammox, denitrification and fermentation process: Enhancing nitrogen removal and sludge reduction in a single reactor

This study verified the feasibility of simultaneous partial nitrification, anammox, denitrification and fermentation process under intermittent aeration in a single reactor, and explored the impact of dissolved oxygen (DO) on the synergy between fermentation and nitrogen removal. An advanced nitrogen removal efficiency of 92.8 % and a low observed sludge yield of 0.0268-0.1474 kgMLSS/kgCOD were achieved. In-situ test showed that nitrate and ammonium decreased synchronously in the absence of organic matter, indicating the possibility of simultaneous partial denitrification, anammox and fermentation. Additionally, the abundance of functional genes for acetate production was 66,894 hits, while the key genes relevant to methanogenesis were only 348 hits, which suggested that fermentation might stop at the acid-producing stage and promote partial denitrification-anammox reaction, achieving simultaneous sludge reduction and advanced nitrogen removal performance. When DO increased from 0.1-0.3 to 0.4-0.6 mg/L, the nitrogen removal efficiency was increased (63.9 %→92.8 %) while sludge reduction was negatively affected.

Metagenomic and extracellular polymeric substances analysis reveals the mechanism of exogenous N-hexanoyl-L-homoserine lactone in alleviating the inhibition of perfluorooctanoic acid on anammox process

To alleviate the negative effects of perfluorooctanoic acid (PFOA) on nitrogen removal via anaerobic ammonia oxidation (anammox), an exogenous signaling factor (N-hexanoyl-L-homoserine lactone, C₆-HSL) was introduced into an anammox reactor. Results showed that 2 μmol/L C₆-HSL promoted the nitrogen removal efficiency of the anammox reactor under PFOA stress, with the removal efficiencies of ammonia and nitrite increasing from 79.7 ± 4.8 % and 80.8 ± 3.8 %, to 94.4 ± 4.3 % and 97.1 ± 3.8 %. Exogenous C₆-HSL enhanced the compactness of the extracellular proteins, and improved the sludge hydrophobicity. Meanwhile, C₆-HSL resulted in a microbial shift, with the relative abundance of Planctomycetes increasing from 30.2 % to 49.5 %. Candidatus Kuenenia stuttgartiensis replaced Candidatus Brocadia sp. BL1 as the dominant species, while the available space for other nitrogen-removing bacteria was reduced. Exogenous C₆-HSL promoted the expression of anammox-related genes, such as hzsB and hdh, while denitrifying genes were down-regulated. In addition, the relative abundance of HdtS, which synthesizes AHLs, increased by 0.02446%.

Simultaneous removal of nitrate and ammonium by hydrogen-based partial denitrification coupled with anammox in a membrane biofilm reactor

Hydrogen-based membrane biofilm reactors (MBfRs) are effective for nitrogen removal. However, the safety of hydrogen limited the application of MBfR. Here, a hydrogen-based partial denitrification system coupled with anammox (H₂-PDA) was constructed in an MBfR for reducing hydrogen demand significantly. The metabolomics and structures of microbial communities were analyzed to determine the phenotypic differences and drivers underlying denitrification, anammox, and H₂-PDA. These findings indicated that total nitrogen (TN) removal increased from 57.1% in S1 to 93.7% in S2. During the H₂-PDA process, partial denitrification and anammox contributed to TN removal by 93.7% and 6.3%, respectively. Community analysis indicated that the H₂-PDA system was dominated by the genus Meiothermus, which is involved in partial denitrification. Collectively, these findings confirmed the feasibility of incorporating the H₂-PDA process in a MBfR and form a foundation for the establishment of novel and practical methods for efficient nitrogen removal.

Rapid enrichment of anammox bacteria and transformation to partial denitrification/anammox with nitrification/denitrification sludge

The stable nitrite (NO₂^--N) generation and rapid startup of anammox-based process are the main bottlenecks hindering its application in mainstream municipal wastewater treatment. In this study, a Partial-Denitrification (PD) system reducing nitrate (NO₃^--N) to NO₂^--N was rapidly developed within 40 days, using the nitrification/denitrification sludge from wastewater treatment plant. The NO₃^--N to NO₂^--N transformation ratios achieved 80.6 %. Significantly, a fast self-enrichment of anammox bacteria in this system was subsequently obtained, resulting in the successful transformation to an efficient PD/Anammox (PD/A) process after 79-day operation. The total nitrogen removal efficiency increased from 12.4 % to 90.0 % with influent ammonia and nitrate of 45.9 mg N/L and 62.2 mg N/L, corresponding to the anammox activity significantly increasing to 6.0 mgNH₄⁺-N/g VSS/h without seeding anammox sludge. Abundance of anammox increased from 6.7 × 10⁸ to 2.0 × 10¹¹ copies/g dry sludge. High-throughput sequencing results showed that Candidatus Brocadia was the only known anammox genus and accounted for 1.08 % during the PD/A stage. Functional bacteria for PD, assumed to be the Thauera, was enriched from 1.99 % to 60.06 % but decreased to 32.49 % during the improvement of anammox activity. It demonstrated that the PD system with stable NO₂^--N accumulation enabled a rapid self-enrichment of anammox bacteria and sufficient nitrogen removal with ordinary nitrification/denitrification sludge. This provides new insights into the scaling application of anammox by integrating PD with shortened startup periods and improved TN removal efficiency.

The organics-mediated microbial dynamics and mixotrophic metabolisms in anammox consortia under micro-aerobic conditions

The engineering applications of mainstream anaerobic ammonium oxidation (anammox) have raised increasing attention due to its energy-efficient, however, the organics-mediated microbial dynamics and mixotrophic metabolisms in anammox consortia under micro-aerobic conditions are still elusive. Here, the response of the anammox process to sodium acetate and glucose at a C/N ratio ranging from 0 to 0.5 was investigated under micro-aerobic conditions, respectively. Results showed that the additional glucose could promote the nitrogen removal efficiency (NRE) and nitrogen removal rate (NRR) of anammox processes at a low C/N ratio (0.3), representing 84.00% and 0.53 N kg·m^-3·d^-1. The introduced organics could regulate the diversity of the microbial community and simplify the microbial relationship in anammox consortia. Anammox could not benefit from the introduced sodium acetate, while glucose could effectively enhance the anammox activity and microbial interactions in anammox consortia. Glucose might also stimulate the mixotrophic mechanism of Ca. Kuenenia, further promotes the proliferation of anammox sludge under micro-aerobic conditions. This study reveals that glucose could positively mediate microbial interactions and mixotrophic metabolism in anammox consortia under micro-aerobic conditions, which raises a new horizon for the proliferation of anammox sludge for mainstream engineering applications.

Biological nitrogen removal from low carbon wastewater

Nitrogen has traditionally been removed from wastewater by nitrification and denitrification processes, in which organic carbon has been used as an electron donor during denitrification. However, some wastewaters contain low concentrations of organic carbon, which may require external organic carbon supply, increasing treatment costs. As a result, processes such as partial nitrification/anammox (anaerobic ammonium oxidation) (PN/A), autotrophic denitrification, nitritation-denitritation and bioelectrochemical processes have been studied as possible alternatives, and are thus evaluated in this study based on process kinetics, applicability at large-scale and process configuration. Oxygen demand for nitritation-denitritation and PN/A is 25% and 60% lower than for nitrification/denitrification, respectively. In addition, PN/A process does not require organic carbon supply, while its supply for nitritation-denitritation is 40% less than for nitrification/denitrification. Both PN/A and nitritation-denitritation produce less sludge compared to nitrification/denitrification, which saves on sludge handling costs. Similarly, autotrophic denitrification generates less sludge compared to heterotrophic denitrification and could save on sludge handling costs. However, autotrophic denitrification driven by metallic ions, elemental sulfur (S) and its compounds could generate harmful chemicals. On the other hand, hydrogenotrophic denitrification can remove nitrogen completely without generation of harmful chemicals, but requires specialized equipment for generation and handling of hydrogen gas (H2), which complicates process configuration. Bioelectrochemical processes are limited by low kinetics and complicated process configuration. In sum, anammox-mediated processes represent the best alternative to nitrification/denitrification for nitrogen removal in low- and high-strength wastewaters.

Advancements of sequencing batch biofilm reactor for slaughterhouse wastewater assisted with response surface methodology

Slaughterhouse wastewater (SWW) contains a significant volume of highly polluted organic wastes. These include blood, fat, soluble proteins, colloidal particles, suspended materials, meat particles, and intestinal undigested food that consists of higher concentrations of organics such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), nitrogen and phosphorus hence an efficient treatment is required before discharging into the water bodies. The effluent concentrations and performance of simultaneous sequential batch biofilm reactor (SBBR) with recycled plastic carrier media support are better than the local single-stage sequential batch reactor (SBR), which is lacking in the literature in terms of COD, NH₃, NO₃, and PO₄ treatment efficiency. The present study reports a novel strategy to remove the above mentioned contaminants using an intermittently aerated SBBR with recycled plastic carrier media support along with simultaneous nitrification and denitrification. The central composite design was evaluated to optimize the treatment performance of seven different process variables including; different alternating conditions (Oxic/anoxic) for aeration cycles (3/2 h in a 6 h cycle, 6/5 h in a 12 h cycle and 9/8 h in an 18 h cycle) and hydraulic retention time (6, 12 and 18 h). The average removal efficiencies are 94.5% for NH₃, 93% for NO₃ and 90.1% for PO₄, and 99% for COD. The study reveals that the denitrification in the post-anoxic phase was more efficient than the pre-anoxic phase for pollutant removal and maintaining higher quality effluent. The effluent concentrations and performance of simultaneous SBBR with recycled polyethylene carrier support media were better than local SBR system in terms of COD, NH₃, NO₃ and PO₄ treatment efficiency. Results stipulated the suitability of SBBR for wastewater treatment and reusability as a sustainable approach for wastewater management under optimum conditions.

A review of anammox-based nitrogen removal technology: From microbial diversity to engineering applications

The anaerobic ammonium oxidation (anammox) process has the advantages of high efficiency and low energy consumption, so it has broad application prospects in biological denitrification of wastewater. However, the application of anammox technology to existing wastewater treatment is still challenging. The main problems are the insufficient supply of nitrite and the susceptibility of anammox bacteria to environmental factors. In this paper, from the perspective of the diversity of anammox bacteria, the habitats and characteristics of anammox bacteria of different genera were compared. At the same time, laboratory research and engineering applications of anammox technology in treating wastewater from different sources were reviewed, and the progress of and obstacles to the practical application of anammox technology were clarified. Finally, a focus for future research was proposed to intensively study the water quality barrier factors of anammox and its regulation strategies. Meanwhile, a combined process was developed and optimized on this basis.

Insight into the microbial interactions of Anammox and heterotrophic bacteria in different granular sludge systems: effect of size distribution and available organic carbon source

Microbial interactions between Anammox and heterotrophic bacteria in different granule distributions in an Anammox system (AMX) and partial denitrification coupled with Anammox system (PDA) were analyzed in this paper. Candidatus Brocadia was the main Anammox microorganism in granules of 1.0 > d > 0.5 mm with the highest abundance of 21.5 % in AMX, significantly higher than the maximum proportion of 2.3 % in PDA sludge > 2.0 mm. However, the total nitrogen (TN) removal of 77.9 % in AMX was lower than PDA (94.0 %) because of the excessive NO3--N generated by nitrite-oxidizing bacteria (NOB). Anammox activity could be stimulated by heterotrophs via simple organic carbon, which decreased with the increasing size of sludge in AMX but increased in PDA. This highlighted that regulation of the distribution of sludge size and organic carbon source had an essential effect on efficient nitrogen removal of Anammox technology.