Simultaneous nitrification, denitrification and phosphorus removal: What have we done so far and how do we need to do in the future?

Aerobic biodegradation of micropollutants by nitrifiers and heterotrophs: Changes in biodegradation rate constant depending on levels of growth substrates and microbial activities

This study investigated how the biodegradation rate constant (kbio) of 11 micropollutants (MPs) responded to changes in the concentrations of growth substrates and microbial activities while considering the predominant microbial degraders of MPs. Metformin, losartan, valsartan, and cimetidine (group A) were biodegraded predominantly by nitrifiers. MPs of group A showed a positive correlation with kbio and nitrifying activity, while a negative correlation was observed with the initial concentration of ammonium, possibly due to competitive inhibition. Atenolol, caffeine, and naproxen (group B) were biodegraded predominantly by heterotrophs, with kbio remaining stable despite changes in organic matter concentration or heterotrophic activity. Olmesartan, candesartan, diclofenac, and sulfamethoxazole (group C) showed low kbio regardless of growth substrate concentration and microbial activity, which could be attributed to their chemical structures. These findings suggest that the kbio of MPs in WWTPs could respond differently to growth substrate concentration and microbial activity depending on their predominant degraders.

Advanced treatment of tannery effluent from Fez City (Morocco) using a sequence of aerobic and sono-photo-Fenton processes

This work aims to purify real tannery wastewater (TWW) after a physicochemical characterization. A pretreatment using air stripping (aerobic pretreatment; AP) was first applied and compared to anaerobic pretreatment (ANP). The COD and BOD5 were highly removed by AP, reaching 86% and 88% compared to ANP, which only achieved 48% and 55%, respectively. Following the AP, the sono-photo-Fenton (SPF) process was applied as post-treatment. The optimal conditions pH = 3, [H2O2] = 1834 mg/L, and [Fe2+] = 1281 mg/L improved the COD, color and BOD5 removal of 96%, 98%, and 98%, respectively. Turbidity, N-NO3-, and N-NO2- were completely removed (100%) by the combined processes (AP+SPF), while Cr, Cl-, and N-NH4+ were reduced to 99%, 97%, and 99%, respectively. Finally, phytotoxicity tests were performed to confirm the efficiency of the sequential processes. The highest germination percentage, germination rate index, and seedling vigor index for the grains wheat and Medicago sativa were observed using the TWW treated by the AP+SPF, followed by those treated by AP alone. In contrast, no germination indicators were noticed in raw TWW. These findings highlight the significant purification effectiveness of the sequential processes of AP and SPF post-treatment, which suggests the potential use of this combination for the efficient treatment of real liquid effluents.

Elucidating the factors that impact the abundance of aerobic and denitrifying polyphosphate-accumulating organisms in an integrated fixed film activated sludge-enhanced biological phosphorus removal process

The integrated fixed-film activated sludge (IFAS)-enhanced biological phosphorus removal (EBPR) (IFAS-EBPR) system is significant for its ability to enhance nitrification and phosphorus removal by leveraging fixed-film carriers for nitrifiers and suspended growth for heterotrophs, but limited understanding of microbial dynamics and variable performance continues to drive interest in optimizing its application. This study investigated the key factors that affect the distribution and abundance of denitrifying (DPAOs) and non-denitrifying types of Accumulibacter-like polyphosphate-accumulating organisms (non-DPAOs) in a lab-scale continuous-flow IFAS-EBPR. The increase of nitrate recycles ratios from 1.0, 1.5, to 2.5Q affected both nitrogen (N) removal efficiency and EBPR activities, with P removal profile transitioning from having continuous P release to P uptake in anoxic zone. Population abundance analysis revealed that Clade I Accumulibacter (preconceived DPAO-Accumulibacter clade) correlated positively with the nitrate recycle flows (r = 0.96) and anoxic residual nitrate concentration (r = 0.99), while Clade II Accumulibacter (non-DPAO clade) showed negative correlation (r = -0.93), indicating the response of DPAO to the operational condition, particularly the nitrate recycle ratio, in our system. Furthermore, the relative abundance of DPAOs correlated  negatively with the anoxic residual biodegradable COD (r = -0.99), anoxic hydraulic retention time (HRT) (r = -0.99), and nitrate and nitrite levels in the anoxic zone (r = -0.95, -0.99). SUMO model simulations revealed DPAO denitrification rates of 8.2, 7.8, and 4.1 gN/m3/d from 1.0, 1.5, to 2.5Q, accounting for only 5.8%, 9.5%, and 4.0% of nitrate removal in the anoxic zone, respectively. This study demonstrates that DPAO and non-DPAO populations are dynamically affected by operating conditions in a continuous-flow EBPR system, and the actual contribution of DPAO to the overall denitrification is relatively small (<10%). This study contributed to our better understanding of DPAO population dynamics and providing insights for optimizing DPAO in a EBPR processes for more sustainable wastewater treatment. PRACTITIONER POINTS: Accumulibacter-like DPAO subgroups and non-DPAOs coexist in IFAS-EBPR systems, with abundances controlled by nitrate recycle flows. DPAOs contribute <10% to overall denitrification, but their enrichment significantly impacts carbon reduction in EBPR processes. Optimizing nitrate recycle ratio (around 1.5) balances nitrate recycling and oxygen inhibition, enhancing DPAO performance in wastewater treatment. Understanding DPAO subpopulation dynamics is crucial for designing efficient BNR systems with reduced external carbon requirements.

Achieving synchronous nitrogen and phosphorus removal by aerobic enrichment of electrotrophic/heterotrophic bacteria and denitrifying polyphosphate-accumulating organisms in repeatedly oxygen-rich microbial fuel cells

Realizing the enrichment of functional bacteria in microbial fuel cells (MFCs) for wastewater treatment holds substantial research significance. This study explored a novel method of repeatedly oxygen-rich anode environment to enrich electrotrophic/heterotrophic bacteria (EHB) and denitrifying polyphosphate-accumulating organisms (DPAOs) in membrane-less single-chamber air-cathode (AC) MFCs to treat household wastewater. Repeated accumulation of higher dissolved oxygen (DO) was conducive to enhancing the growth of EHB and DPAOs. The systems achieved the maximum removal of 99% of ammonium, 78% of total inorganic nitrogen and 55% of total phosphorus. Repeated oxygen-rich conditions favored the selection of nitrogen-oxidizing bacteria on both electrodes, such as unclassified_f_Xanthomonadaceae, unclassified_p_Bacteroidota, Nitrosomonas and Nitrospira, thereby increasing nitrate availability for DPAOs like Candidatus Contendobacter, unclassified_c_Actinomycetia as well as other denitrifiers such as Anaerolineales, unclassified_p_Chloroflexi, unclassified_o_Rhodospirillales. The genes nxrAB, narGH and nasC, associated with nitrification and denitrification, and the genes gcd, phoD, ugpQ, glpQ, involved in phosphate metabolism, were up-regulated in presence of repeated DO accumulation, thereby enhancing pollutants removal. This study presents a novel approach for the synchronous removal of nitrogen and phosphorus from domestic wastewater through the enrichment of functional bacteria in the repeatedly oxygen-rich ACMFCs.

Inoculation with Acinetobacter indicus CZH-5 in internal circulation airlift zeolite spheres sequencing batch reactor to augment simultaneous removal of nitrogen, phosphorus, and tetracycline

Inoculating functional bacterial strains is a cost-effective strategy for enhancing treatment of anaerobic digestion liquids in swine wastewater. This study systematically evaluated inoculation of heterotrophic nitrification aerobic denitrification strain Acinetobacter indicus CZH-5 in an internal circulation airlift zeolite sphere-based sequencing batch reactor (IR) for aerobic removal of nitrogen (N), phosphorus (P), and tetracycline (TEC). Inoculation with CZH-5 promoted secretion of quorum sensing signaling molecules, specifically N-acyl-homoserine lactones (C6-HSL and C10-HSL). These signaling molecules enhance quorum sensing and reinforce cooperation among functional bacteria. Under optimal conditions, average removal efficiencies of total nitrogen, total phosphate, and TEC were 92.8 %, 88.4 %, and 93.1 %, respectively. The removal performance in IR exceeded that of the control by 26 %-71 %. N removal involved complete nitrification-denitrification, while accumulated P was transformed into phosphate monoesters within biofilm. Metagenomic analysis identified Thauera and Acinetobacter as the dominant genera, and Acinetobacter indicus as predominant species. Inoculation enhanced microbial richness and diversity to improve system operational stability. The abundance of functional genes associated with N, P, and TEC transformations significantly increased compared to the control. This study aimed to investigate the characteristics and mechanisms of inoculating a heterotrophic nitrification aerobic denitrification strain into an aerated biofilm system for swine wastewater remediation.

Role of intracellular storage polymers in simultaneous biological nutrient removal and resources recovery

Simultaneous biological nutrient removal (SBNR) using an anaerobic-anoxic-oxic phase is the key feature of advanced wastewater treatment plants (WWTPs). Removing ammonia, total nitrogen, and phosphorus concurrently with organic matter and suspended solids from wastewater is essential to meeting stringent effluent discharge standards via SBNR in WWTPs. More insight into the mechanisms of SBNR, i.e., simultaneous nitrification-denitrification (SND) and enhanced biological phosphorus removal (EBPR) processes, the intracellular carbon reserves, i.e., polyhydroxyalkanoates (PHA) and specifically poly-β-hydroxybutyrates (PHB), will play a critical role in nutrients removal and resource recovery in WWTPs. Volatile fatty acids (VFA) in wastewater are the preferable source of PHA formation. However, municipal wastewater could not supply sufficient VFA fractions owing to short sewer lines; therefore, developing pre-fermentation chambers and other technological integration in the WWTPs can play an effective role in VFA production from raw sewage, resulting in the effective formation of PHA. On the other hand, PHA is a value-added biochemical, i.e., a potential substitute for fossil fuel plastics. WWTPs complying with SBNR are the bio-refineries for PHA (bioplastic precursors) production using diverse microbial populations. This review enlightens three dimensions of progressive systems and engineering-based viewpoints: (i) Increasing the SBNR by optimizing operational conditions subject to the substrate storage mechanisms of treatment systems; (ii) Technical solutions to enhance the VFA availability in sewage in WWTPs to achieve effective SBNR; and (iii) production of PHB (PHA) in WWTPs.

Performance of a pilot-scale microbial electrolysis cell coupled with biofilm-based reactor for household wastewater treatment: simultaneous pollutant removal and hydrogen production

The septic tank is the most commonly used decentralized wastewater treatment systems for household wastewater treatment in on-site applications. The removal rate of various pollutants is lower in different septic tank configurations. The integration of a microbial electrolysis cells (MEC) into septic tank or biofilm-based reactors can be a green and sustainable technology for household wastewater treatment and energy production. In this study, a 50-L septic tank was converted into a 50-L MEC coupled with biofilm-based reactor for simultaneous household wastewater treatment and hydrogen production. The biofilm-based reactor was integrated by an anaerobic packed-bed biofilm reactor (APBBR) and an aerobic moving bed biofilm reactor (aeMBBR). The MEC/APBBR/aeMBBR was evaluated at different organic loading rates (OLRs) by applying voltage of 0.7 and 1.0 V. Result showed that the increase of OLRs from 0.2 to 0.44 kg COD/m3 d did not affect organic matter removals. Nutrient and solids removal decreased with increasing OLR up to 0.44 kg COD/m3 d. Global removal of chemical oxygen demand (COD), biochemical oxygen demand (BOD), total nitrogen (TN), ammoniacal nitrogen (NH4+), total phosphorus (TP) and total suspended solids (TSS) removal ranged from 81 to 84%, 84 to 85%, 53 to 68%, 88 to 98%, 11 to 30% and 76 to 88% respectively, was obtained in this study. The current density generated in the MEC from 0 to 0.41 A/m2 contributed to an increase in hydrogen production and pollutants removal. The maximum volumetric hydrogen production rate obtained in the MEC was 0.007 L/L.d (0.072 L/d). The integration of the MEC into biofilm-based reactors applying a voltage of 1.0 V generated different bioelectrochemical nitrogen and phosphorus transformations within the MEC, allowing a simultaneous denitrification-nitrification process with phosphorus removal.

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.

Acid-tolerant Pseudomonas citronellolis YN-21 exhibits a high heterotrophic nitrification capacity independent of the amo and hao genes

Heterotrophic nitrifying bacteria are found to be promising candidates for implementation in wastewater treatment systems due to their tolerance to extreme environments. A novel acid-resistant bacterium, Pseudomonas citronellolis YN-21, was isolated and reported to have exceptional heterotrophic nitrification capabilities in acidic condition. At pH 5, the highest NH4+ removal rate of 7.84 mg/L/h was displayed by YN-21, which was significantly higher than the NH4+ removal rates of other strains in neutral and alkaline environments. Remarkably, a distinct accumulation of NH2OH and NO3- was observed during NH4+ removal by strain YN-21, while traditional amo and hao genes were not detected in the genome, suggesting the possible presence of alternative nitrifying genes. Moreover, excellent nitrogen removal performance was displayed by YN-21 even under high concentrations of metal ion stress. Consequently, a broad application prospect in the treatment of leather wastewater and mine tailwater is offered by YN-21.

Digitalization of phosphorous removal process in biological wastewater treatment systems: Challenges, and way forward

Phosphorus in wastewater poses a significant environmental threat, leading to water pollution and eutrophication. However, it plays a crucial role in the water-energy-resource recovery-environment (WERE) nexus. Recovering Phosphorus from wastewater can close the phosphorus loop, supporting circular economy principles by reusing it as fertilizer or in industrial applications. Despite the recognized importance of phosphorus recovery, there is a lack of analysis of the cyber-physical framework concerning the WERE nexus. Advanced methods like automatic control, optimal process technologies, artificial intelligence (AI), and life cycle assessment (LCA) have emerged to enhance wastewater treatment plants (WWTPs) operations focusing on improving effluent quality, energy efficiency, resource recovery, and reducing greenhouse gas (GHG) emissions. Providing insights into implementing modeling and simulation platforms, control, and optimization systems for Phosphorus recovery in WERE (P-WERE) in WWTPs is extremely important in WWTPs. This review highlights the valuable applications of AI algorithms, such as machine learning, deep learning, and explainable AI, for predicting phosphorus (P) dynamics in WWTPs. It emphasizes the importance of using AI to analyze microbial communities and optimize WWTPs for different various objectives. Additionally, it discusses the benefits of integrating mechanistic and data-driven models into plant-wide frameworks, which can enhance GHG simulation and enable simultaneous nitrogen (N) and Phosphorus (P) removal. The review underscores the significance of prioritizing recovery actions to redirect Phosphorus from effluent to reusable products for future considerations.

Sustainable adsorbent frameworks based on bio-resourced materials and biodegradable polymers in selective phosphate removal for waste-water remediation

Phosphorus to an optimum extent is an essential nutrient for all living organisms and its scarcity may cause food security, and environmental preservation issues vis-à-vis agroeconomic hurdles. Undesirably excess phosphorus intensifies the eutrophication problem in non-marine water bodies and disrupts the natural nutrient balance of the ecosystem. To overcome such dichotomy, biodegradable polymer-based adsorbents have emerged as a cost-effective and implementable approach in striking a "desired optimum-undesired excess" balance pertaining to phosphate in a sustainable manner. So far, the reports on adopting such adsorbent-approach for wastewater remediation remained largely scattered, unstructured, and poorly correlated. In this background, the contextual review comprehensively discusses the current state-of-the-art in utilizing biodegradable polymeric frameworks as an adsorbent system for phosphate removal and its efficient recovery from the aquatic ecosystem, while highlighting their characteristics-specific functional efficiency vis-à-vis easiness of synthetic and commercial viability. The overview further delves into the sources and environmental ramifications of excessive phosphorus in water bodies and associated mechanistic pathways of phosphorus removal via adsorption, precipitation, and membrane filtration enabled by biodegradable (natural and synthetic) polymeric substrates. Finally, functionality optimization, degradability tuning, and adsorption selectivity of biodegradable polymers are highlighted, while aiming to strike a balance in "removal-recovery-reuse" dynamics of phosphate. Thus, the current review not only paves the way for future exploration of biodegradable polymers in sustainable cost-effective adsorbents for phosphorus removal but also can serve as a guide for researchers dealing with this critical issue.

Enhanced ammonia removal in tidal flow constructed wetland by incorporating steel slag: Performance, microbial community, and heavy metal release

Utilizing alkaline solid wastes, such as steel slag, as substrates in tidal flow constructed wetlands (TFCWs) can effectively neutralize the acidity generated by nitrification. However, the impacts of steel slag on microbial communities and the potential risk of heavy metal release remain poorly understood. To address these knowledge gaps, this study compared the performance and microbial community structure of TFCWs filled with a mixture of steel slag and zeolite (TFCW-S) to those filled with zeolite alone (TFCW-Z). TFCW-S exhibited a much higher NH4+-N removal efficiency (98.35 %) than TFCW-Z (55.26 %). Additionally, TFCW-S also achieved better TN and TP removal. The steel slag addition helped maintain the TFCW-S effluent pH at around 7.5, while the TFCW-Z effluent pH varied from 3.74 to 6.25. The nitrification and denitrification intensities in TFCW-S substrates were significantly higher than those in TFCW-Z, consistent with the observed removal performance. Moreover, steel slag did not cause excessive heavy metal release, as the effluent concentrations were below the standard limits. Microbial community analysis revealed that ammonia-oxidizing bacteria, ammonia-oxidizing archaea, and complete ammonia-oxidizing bacteria coexisted in both TFCWs, albeit with different compositions. Furthermore, the enrichment of heterotrophic nitrification-aerobic denitrification bacteria in TFCW-S likely contributed to the high NH4+-N removal. In summary, these findings demonstrate that the combined use of steel slag and zeolite in TFCWs creates favorable pH conditions for ammonia-oxidizing microorganisms, leading to efficient ammonia removal in an environmentally friendly manner.

Enhanced biological phosphorus removal sustained by aeration-free filamentous microalgal-bacterial granular sludge

The microalgal-bacterial granular sludge (MBGS) based enhanced biological phosphorus removal (EBPR) (MBGS-EBPR) was recently proposed as a sustainable wastewater treatment process. Previous work showed the possibility of obtaining an MBGS-EBPR process starting from mature MBGS and phosphate-accumulating organisms (PAOs) enriched aerobic granular sludge (AGS) and validated the effectiveness of removing carbon/nitrogen/phosphorus with mechanical aeration. The present work evaluated whether the same could be achieved starting from conventional activated sludge and operating under aeration-free conditions in an alternating dark/light photo-sequencing batch reactor (PSBR). We successfully cultivated filamentous MBGS with a high settling rate (34.5 m/h) and fast solid-liquid separation performance, which could be attributed to the proliferation of filamentous cyanobacteria and stimulation of extracellular polymeric substances (EPS) production. The process achieved near-complete steady-state removal of carbon (97.2 ± 1.9 %), nitrogen (93.9 ± 0.7 %), and phosphorus (97.7 ± 1.7 %). Moreover, improved phosphorus release/uptake driven by photosynthetic oxygenation under dark/light cycles suggests the enrichment of PAOs and the establishment of MBGS-EBPR. Batch tests showed similar phosphorus release rates in the dark but significantly lower phosphorus uptake rates in the presence of light when the filamentous granules were disrupted. This indicates that the filamentous structure of MBGS has minor limitations on substrate mass transfer while exerting protective effects on PAOs, thus playing an important role in sustaining the function of aeration-free EBPR. Microbial assays further indicated that the enrichment of filamentous cyanobacteria (Synechocystis, Leptoolybya, and Nodosilinea), putative PAOs and EPS producers (Hydrogenophaga, Thauera, Flavobacterium, and Bdellovibrio) promoted the development of filamentous MBGS and enabled the high-efficient pollutant removal. This work provides a feasible and cost-effective strategy for the startup and operation of this innovative process.

Elucidating phosphorus removal dynamics in a denitrifying woodchip bioreactor

Denitrifying woodchip bioreactors (DBRs) are an established nitrate mitigation technology, but uncertainty remains on their viability for phosphorus (P) removal due to inconsistent source-sink behaviour in field trials. We investigated whether iron (Fe) redox cycling could be the missing link needed to explain P dynamics in these systems. A pilot-scale DBR (Aotearoa New Zealand) was monitored for the first two drainage seasons (2017-2018), with supplemental in-field measurements of reduced solutes (Fe2+, HS-/H2S) and their conjugate oxidised species (Fe3+/SO42-) made in 2021 to constrain within-reactor redox gradients. Consistent with thermodynamics, the dissolution of Fe3+(s) to Fe2+(aq) within the DBR sequentially followed O2, NO3- and MnO2(s) reduction, but occurred before SO42- reduction. Monitoring of inlet and outlet chemistry revealed tight coupling between Fe and P (inlet R2 0.94, outlet R2 0.85), but distinct dynamics between drainage seasons. In season one, outlet P exceeded inlet P (net P source), and coincided with elevated outlet Fe2+, but at ⁓50 % lower P concentrations relative to inlet Fe:P ratios. In season 2 the reactor became a net P sink, coinciding with declining outlet Fe2+ concentrations (indicating exhaustion of Fe3+(s) hydroxides and associated P). In order to characterize P removal under varying source dynamics (i.e. inflows vs in-situ P releases), we used the inlet Fe vs P relationship to estimate P binding to colloidal Fe (hydr)oxide surfaces under oxic conditions, and the outlet Fe2+ concentration to estimate in-situ P releases associated with Fe (hydr)oxide reduction. Inferred P-removal rates were highest early in season 1 (k = 0.60 g P m3 d-1; 75-100 % removal), declining significantly thereafter (k = 0.01 ± 0.02 g P m3 d-1; ca. 3-67 % removal). These calculations suggest that microbiological P removal in DBRs can occur at comparable magnitudes to nitrate removal by denitrification, depending mainly on P availability and hydraulic retention efficiency.

Promotion of anaerobic biodegradation of azo dye RR2 by different biowaste-derived biochars: Characteristics and mechanism study by machine learning

The addition of biochar resulted in a 31.5 % to 44.6 % increase in decolorization efficiency and favorable decolorization stability. Biochar promoted extracellular polymeric substances (EPS) secretion, especially humic-like and fulvic-like substances. Additionally, biochar enhanced the electron transfer capacity of anaerobic sludge and facilitated surface attachment of microbial cells. 16S rRNA gene sequencing analysis indicated that biochar reduced microbial species diversity, enriching fermentative bacteria such as Trichococcus. Finally, a machine learning model was employed to establish a predictive model for biochar characteristics and decolorization efficiency. Biochar electrical conductivity, H/C ratio, and O/C ratio had the most significant impact on RR2 anaerobic decolorization efficiency. According to the results, the possible mechanism of RR2 anaerobic decolorization enhanced by different types of biochar was proposed.

Achieving simultaneous nitrification and endogenous denitrifying phosphorus removal in anaerobic/intermittently-aerated moving bed biofilm reactor for low carbon-to-nitrogen ratio wastewater treatment

In this study, an anaerobic/intermittently-aerated moving bed biofilm reactor (AnIA-MBBR) was proposed to realize simultaneous nitrification and endogenous denitrifying phosphorus removal (SNEDPR) in treating low carbon-to-nitrogen (C/N) ratio wastewater. The effect of different intermittent aeration modes (short and long aeration) on nutrients' removal was investigated. With the C/N ratio around 3, the removal efficiencies of total nitrogen and phosphorus were 90% and 74%, 88% and 59%, respectively, for short aeration and long aeration. The different aeration time also altered the nutrients' degradation pathway, biofilm characteristics, microbial community, and functional metabolic pathways. The results confirmed the occurrence of aerobic denitrifiers, anoxic denitrifiers, phosphorus accumulating organisms, glycogen accumulating organisms in AnIA-MBBR systems and their synergistic performance induced the SNEDPR. These results indicated that the application of AnIA in MBBR systems was an effective strategy to achieve SNEDPR, providing better simultaneous removal performance of nitrogen and phosphorus from low C/N ratio wastewater.

Hydraulic retention time optimization achieved unexpectedly high nitrogen removal rate in pilot-scale anaerobic/aerobic/anoxic system for low-strength municipal wastewater treatment

Applications of post-denitrification processes are subjected to low reaction rates caused by a lack of carbon resources. To offer a solution for reaction rate promotion, this research found a pilot-scale anaerobic/aerobic/anoxic bioreactor treating 55-120 m3/d low-strength municipal wastewater for 273 days. A short hydraulic retention time (HRT, 5-6 h) and a high nitrogen removal rate (63.2 ± 9.3 g-N/m3·d) were achieved using HRT optimization. The effluent total nitrogen concentration was maintained at 5.8 ± 1.4 mg/L while operating at a high nitrogen loading rate of 86.2 ± 12.8 g-N/m3·d. The short aeration (1.25-1.5 h) minimized the Glycogen loss. The endogenous denitrification rate increased to above 1.0 mg/(g-VSS·h). The functional genus Ca. Competibacter enriched to 2.3 %, guaranteeing the efficient post-denitrification process. Dechloromonas rose to 1.1 %, aiding in the synchronous phosphorus removal. These findings offered fresh insights into AOA processes to achieve energy/cost-saving wastewater treatment.

Highly efficient phosphorous removal in constructed wetland with iron scrap: Insights into the microbial removal mechanism

Excessive phosphorus (P) in surface water can lead to serious eutrophication and economic losses. Iron-based constructed wetland (CW) is considered as a promising solution to eliminate P effectively due to the advantage of low-cost. However, there is limited available information on the microbial removal mechanism of P in iron-based CW up to now. Therefore, CW with iron scrap was constructed to investigate the treatment performance and microbial removal mechanism in this study. Results showed that efficient and stable P removal (97.09 ± 1.90%) was achieved in iron scrap-based CW during the experiment period, which was attributed to the precipitation of iron and P and improved microbially mediated P removal. Metagenomic analysis showed that microbial diversity was enhanced and phosphate accumulating organisms (e.g., Dechloromonas and Tetrasphaera) were enriched in CW with iron scrap, which explained higher P removal reasonably. In addition, the abundance of genes involved in the P starvation (e.g., phoB), uptake and transport (e.g., pstB) were enhanced in iron scrap-based CW. Enrichment analysis demonstrated that phosphotransferase pathway was also significantly up-regulated in CW with iron scraps, indicating that the energy supply of microbial P removal was enhanced. These findings provide a better understanding of the microbial removal mechanism of P in iron-based CW.

Effect of food-to-microorganisms ratio on aerobic granular sludge settleability: Microbial community, potential roles and sequential responses of extracellular proteins and polysaccharides

The food-to-microorganism ratio (F/M) is an important parameter in wastewater biotreatment that significantly affects the granulation and settleability of aerobic granular sludge (AGS). Hence, understanding the long-term effects and internal mechanisms of F/M on AGS settling performance is essential. This study investigated the relationship between F/M and the sludge volume index (SVI) within a range of 0.23-2.50 kgCOD/(kgMLVSS·d). Thiothrix and Candidatus_Competibacter were identified as two dominant bacterial genera influencing AGS settling performance. With F/M increased from 0.27 kgCOD/(kgMLVSS·d) to 1.53 kgCOD/(kgMLVSS·d), the abundance of Thiothrix significantly increased from 0.20% to 27.02%, and the hydrophobicity of extracellular proteins (PN) decreased, which collectively reduced AGS settling performance. However, under high-F/M conditions, the gel-like polysaccharides (PS) effectively retained the granular biomass by binding to the highly abundant Thiothrix (53.65%). The progressive increment in biomass led to a concomitant reduction in F/M, resulting in the recovery of AGS settleability. In addition, two-dimensional correlation infrared spectroscopy analysis revealed the preferential responses of PN and PS to the increase and decrease of F/M, and the content and characteristics of PN and PS played important roles in granular settling. The study provides insight into the microbial composition and the potential role of extracellular polymer substances in the AGS sedimentation behavior, offering valuable theoretical support for stable AGS operation.

An ecological explanation for carbon source-associated denitrification performance in wastewater treatment plants

The underlying mechanism associated with the roles of dosed carbon source in denitrification performance remains largely unknown. In this study, three denitrifying consortia (DNC) were constructed via evolutionary top-down enrichment method with well-defined conditions and specific carbon sources (acetate, glucose and their mixture). The reactor operation shows that nearly complete nitrate removal was achieved; however, the glucose feeding resulted in much higher concentrations of biomass and non-settable flocs. The 16S rRNA sequencing suggests that the bacterial diversity of the acetate-fed DNC was significantly higher than those of acetate/glucose-fed and glucose-fed DNCs. The dentrifying population in the acetate-fed DNC was dominated by Propionivibrio (16.1 %) and Thauera (3.4 %); whereas those of acetate/glucose- and glucose-fed DNCs were dominated by Pleomorphomonas (21.5 % and 26.3 %, respectively). Interestingly, the supernatant of acetate-fed DNC contained a high abundance of genera Thauera (averaged at 85.1 %), indicating the free-living nature of Thauera. Both PICURSt2 analysis of 16S rRNA sequencing and metagenomic analysis indicate that the acetate-fed DNC contained higher abundances of denitrifying genes; the acetate/glucose-fed and glucose-fed DNCs, in comparison, enriched genes related to glucose transportation and metabolism. Additionally, the acetate-fed DNC had better network stability than other two groups. This study adds important knowledge regarding the ecological traits of DNC, providing important clues for rational addition of carbon sources in wastewater treatment plants.

Synergistic analysis of performance, functional genes, and microbial community assembly in SNDPR process under Zn(II) stress

One of the most prevalent heavy metals found in rural sewage is Zn(II), while its effect on simultaneous nitrification, denitrification and phosphorus removal (SNDPR) remains unclear. In this work, the responses of SNDPR performance to long-term Zn(II) stress were investigated in a cross-flow honeycomb bionic carrier biofilm system. The results indicated that Zn(II) stress at 1 and 5 mg L^-1 could increase nitrogen removal. Maximum ammonia nitrogen, total nitrogen, and phosphorus removal efficiencies of up to 88.54%, 83.19%, and 83.65% were obtained at Zn(II) concentration of 5 mg L^-1. The functional genes, such as archaeal amoA, bacterial amoA, NarG, NirS, NapA, and NirK, also reached the highest value at 5 mg L^-1 Zn(II), with the absolute abundances of 7.73 × 10⁵, 1.57 × 10⁶, 6.68 × 10⁸, 1.05 × 10⁹, 1.79 × 10⁸, and 2.09 × 10⁸ copies·g^-1 dry weight, respectively. The neutral community model demonstrated that deterministic selection was responsible for the system's microbial community assembly. Additionally, response regimes with extracellular polymeric substances and cooperation among microorganisms facilitated the stability of the reactor effluent. Overall, the findings of this paper contribute to improving the efficiency of wastewater treatment.

Research landscape and hotspots of selective catalytic reduction (SCR) for NOx removal: insights from a comprehensive bibliometric analysis

Selective catalytic reduction (SCR) has been one of the most efficient and widely used technologies to remove nitrogen oxides (NO_x). SCR research has developed rapidly in recent years, which can be reflected by the dramatic increase of related academic publications. Herein, based on the 10,627 documents from 2001 to 2020 in Web of Science, the global research landscape and hotspots in SCR are investigated based on a comprehensive bibliometric analysis. The results show that SCR research has developed positively; the annul number of articles increase sharply from 246 in 2001 to 1092 in 2020. People's Republic of China and Chinese Academy of Sciences are the most productive country and institution, respectively. The global collaboration is extensive and frequent, while People's Republic of China and USA have the most frequent research cooperation. Applied Catalysis B-Environmental is the leading publication source with 711 records. Five major research areas on SCR are identified and elaborated, including catalyst, reductant, deactivation, mechanism, and others. Zeolite is the most widely studied SCR catalyst, while copper, silver, platinum, and iron are the most popular metal elements in catalyst. Ammonia (NH₃) is dominated among various SCR reductants, while hydrocarbon reductant has gained more attention. Sulfur dioxide (SO₂) and vapor are the two most concerned factors leading to catalyst deactivation, and catalyst regeneration is also an important research topic. Density functional theory (DFT), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and kinetics are the most widely used methods to conduct mechanism study. The studies on "low temperature," "atomic-scale insight," "elemental mercury," "situ DIRFTS investigation," "arsenic poisoning," "SPOA-34," "Cu-CHA catalyst," "TiO₂ catalyst," and "Ce catalyst" have been the hotspots in recent years.

Study on the removal characteristics and degradation pathways of highly toxic and refractory organic pollutants in real pharmaceutical factory wastewater treated by a pilot-scale integrated process

Introduction

Pharmaceutical wastewater frequently contains high levels of toxic pollutants. If they are discharged untreated, they pose a threat to the environment. The traditional activated sludge process and the advanced oxidation process do not sufficiently remove toxic and conventional pollutants from pharmaceutical wastewater treatment plants (PWWTPs).

Methods

We designed a pilot-scale reaction system to reduce toxic organic pollutants and conventional pollutants from pharmaceutical wastewater during the biochemical reaction stage. This system included a continuous stirred tank reactor (CSTR), microbial electrolysis cells (MECs), an expanded sludge bed reactor (EGSB), and a moving bed biofilm reactor (MBBR). We used this system to further investigate the benzothiazole degradation pathway.

Results and discussion

The system effectively degraded the toxic pollutants (benzothiazole, pyridine, indole, and quinoline) and the conventional chemicals (COD, NH4+-N, TN). During the stable operation of the pilot-scale plant, the total removal rates of benzothiazole, indole, pyridine, and quinoline were 97.66, 94.13, 79.69, and 81.34%, respectively. The CSTR and MECs contributed the most to the removal of toxic pollutants, while the EGSB and MBBR contributed less to the removal of the four toxic pollutants. Benzothiazoles can be degraded via two pathways: the benzene ring-opening reaction and the heterocyclic ring-opening reaction. The heterocyclic ring-opening reaction was more important in degrading the benzothiazoles in this study.

Conclusion

This study provides feasible design alternatives for PWWTPs to remove both toxic and conventional pollutants at the same time.

Nutrients and Environmental Factors Cross Wavelet Analysis of River Yi in East China: A Multi-Scale Approach

The accumulation of nutrients in rivers is a major cause of eutrophication, and the change in nutrient content is affected by a variety of factors. Taking the River Yi as an example, this study used wavelet analysis tools to examine the periodic changes in nutrients and environmental factors, as well as the relationship between nutrients and environmental factors. The results revealed that total phosphorus (TP), total nitrogen (TN), and ammonia nitrogen (NH4+-N) exhibit multiscale oscillation features, with the dominating periods of 16-17, 26, and 57-60 months. The continuous wavelet transform revealed periodic fluctuation laws on multiple scales between nutrients and several environmental factors. Wavelet transform coherence (WTC) was performed on nutrients and environmental factors, and the results showed that temperature and dissolved oxygen (DO) have a strong influence on nutrient concentration fluctuation. The WTC revealed a weak correlation between pH and TP. On a longer period, however, pH was positively correlated with TN. The flow was found to be positively correct with N and P, while N and P were found to be negatively correct with DO and electrical conductance (EC) at different scales. In most cases, TP was negatively correlated with 5-day biochemical oxygen demand (BOD₅) and permanganate index (COD_Mn). The correlation between TN and COD_Mn and BOD₅ was limited, and no clear dominant phase emerged. In a nutshell, wavelet analysis revealed that water temperature, pH, DO, flow, EC, COD_Mn, and BOD₅ had a pronounced influence on nutrient concentration in the River Yi at different time scales. In the case of the combination of environmental factors, pH and DO play the largest role in determining nutrient concentration.