A novel strategy for rapid development of a self-sustaining symbiotic algal-bacterial granular sludge: Applying algal-mycelial pellets as nuclei

Deciphering the role of micro/nano-hydroxyapatite in aerobic granular sludge system: Effects on treatment performance and enhancement mechanism

Hydroxyapatite (HAP), a mineral nucleus identified within aerobic granular sludge (AGS), plays a vital role in enhancing the AGS systems. However, the microscopic mechanism underlying their roles remains largely unexplored. Herein, a systematic investigation was carried out to elucidate the impact and enhanced mechanisms associated with HAP of different sizes, i.e. micro-HAP (mHAP) and nano-HAP (nHAP), on the aerobic granulation, nutrient removal and microbial diversity of AGS. Results showed that the presence of nHAP and mHAP significantly shortened the granulation process to 15 and 20 days, respectively. This might be ascribed to the fact that the large specific surface area of nHAP aggregates was conducive to microbial adhesion, biomass accumulation and sludge granulation. Compared with mHAP, the granules with nHAP showed better settlement performance, mechanical strength and larger diameter. The X-ray diffraction (XRD) and Raman spectrometer analysis confirmed the presence of HAP within the granules, which was found to stimulate the secretion of extracellular polymeric substance, improve the compactness of granule structure and suppress the growth of filamentous bacteria, thereby contributing to a stable AGS system. The presence of HAP, especially nHAP, effectively enriched the functional microorganisms, such as nitrifying and denitrifying bacteria (e.g. Candidatus_Competibacter) and phosphorus accumulating organisms (e.g. Flavobacterium), leading to the improved nutrient removal efficiencies (COD > 96%, TN > 76%, and TP > 74%). Further analysis revealed the up-regulation of functional enzymes (e.g. nitrite oxidoreductase and polyphosphate kinase) involved in nutrient metabolism, underlying the inherent mechanisms for the excellent nutrient removal. This study deepens the understanding of granulation mechanisms from the perspective of mineral cores, and proposes an economically feasible strategy for rapid initiation and stabilization of AGS reactors.

Novel insights into the biological state in algal-bacterial granular sludge granulation: Armor-like protection provided by the algal barrier

Algal-bacterial granular sludge (ABGS) composed of microalgae and aerobic granular sludge, is a sustainable and promising technology for wastewater treatment. However, the formation mechanism of ABGS has not been clearly defined, and the direct formation of ABGS in saline wastewater has rarely been investigated. This study proposed novel insights into the granulation process of ABGS by assembling the algal barrier, which was successfully cultivated directly in saline wastewater. The results concluded that ABGS with the algal barrier maintained a higher biomass (MLSS of 7046 ± 61 mg/L), larger particle sizes (1.21 ± 0.06 mm), and better settleability (SVI30 of 46 ± 1 mL/g), enabling efficient pollutants removal. Soluble microbial products (SMP) were found to be closely related to the emergence of the algal barrier. In addition, under salinity stress, the high production of extracellular polymeric substances (EPS, 133.70 ± 1.40 mg/g VSS), specifically TB-EPS (90.29 ± 1.12 mg/g VSS), maintained a crucial role in the formation of ABGS. Further analysis indicated that biofilm producing bacteria Pseudofulvimonas and filamentous eukaryote Streptophyta were the key players in ABGS formation with the algal barrier. Furthermore, the enhancement of key genes and enzymes involved in nitrogen metabolism, TCA cycle, and polysaccharide metabolism suggested a more robust protective effect provided by the algal barrier. This study is expected to advance the application of simultaneous ABGS formation and pollutant removal in wastewater.

Mechanism Involved in Polyvinyl Chloride Nanoplastics Induced Anaerobic Granular Sludge Disintegration: Microbial Interaction Energy, EPS Molecular Structure, and Metabolism Functions

Nanoplastics (NPs) are emerging pollutants and have been reported to cause the disintegration of anaerobic granular sludge (AnGS). However, the mechanism involved in AnGS disintegration was not clear. In this study, polyvinyl chloride nanoplastics (PVC-NPs) were chosen as target NPs and their long-term impact on AnGS structure was investigated. Results showed that increasing PVC-NPs concentration resulted in the inhibition of acetoclastic methanogens, syntrophic propionate, and butyrate degradation, as well as AnGS disintegration. At the presence of 50 μg·L-1 PVC-NPs, the hydrophobic interaction was weakened with a higher energy barrier due to the relatively higher hydrophilic functional groups in extracellular polymeric substances (EPS). PVC-NPs-induced ROS inhibited quorum sensing, significantly downregulated hydrophobic amino acid synthesis, whereas it highly upregulated the genes related to the synthesis of four hydrophilic amino acids (Cys, Glu, Gly, and Lys), resulting in a higher hydrophily degree of protein secondary structure in EPS. The differential expression of genes involved in EPS biosynthesis and the resulting protein secondary structure contributed to the greater hydrophilic interaction, reducing microbial aggregation ability. The findings provided new insight into the long-term impact of PVC-NPs on AnGS when treating wastewater containing NPs and filled the knowledge gap on the mechanism involved in AnGS disintegration by PVC-NPs.

New insights into algal-bacterial sludge granulation based on the tightly-bound extracellular polymeric substances regulation in response to N-Methylpyrrolidone

Algal-bacterial granular sludge (ABGS) system is promising in wastewater treatment for its potential in energy-neutrality and carbon-neutrality. However, traditional cultivation of ABGS poses significant challenges attributable to its long start-up period and high energy consumption. Extracellular polymeric substances (EPS), which could be stimulated as a self-defense strategy in cells under toxic contaminants stress, has been considered to contribute to the ABGS granulation process. In this study, photogranulation of ABGS by EPS regulation in response to varying loading rates of N-Methylpyrrolidone (NMP) was investigated for the first time. The results indicated the formation of ABGS with a maximum average diameter of ∼3.3 mm and an exceptionally low SVI5 value of 67 ± 2 mL g-1 under an NMP loading rate of 125 mg L-1 d-1, thereby demonstrating outstanding settleability. Besides, almost complete removal of 300 mg L-1 NMP could be achieved at hydraulic retention time of 48 h, accompanied by chemical oxygen demand (COD) and total nitrogen (TN) removal efficiencies higher than 90 % and 70 %, respectively. Moreover, possible degradation pathway and metabolism mechanism in the ABGS system for enhanced removal of NMP and nitrogen were proposed. In this ABGS system, the mycelium with network structure constituted by filamentous microorganisms was a prerequisite for photogranulation, instead of necessarily leading to granulation. Stress of 100-150 mg L-1 d-1 NMP loading rate stimulated tightly-bound EPS (TB-EPS) variation, resulting in rapid photogranulation. The crucial role of TB-EPS was revealed with the involved mechanisms being clarified. This study provides a novel insight into ABGS development based on the TB-EPS regulation by NMP, which is significant for achieving the manipulation of photogranules.

Sustainable treatment of aquaculture water employing fungi-microalgae consortium: Nutrients removal enhancement, bacterial communities optimization, emerging contaminants elimination, and mechanism analysis

Fungi-microalgae consortium (FMC) has emerged as a promising system for advanced wastewater treatment due to its high biomass yield and environmental sustainability. This study aimed to investigate the nutrients removal, bacterial community shift, emerging contaminants elimination, and treatment mechanism of a FMC composed of Cordyceps militaris and Navicula seminulum for aquaculture pond water treatment. The fungi and microalgae were cultured and employed either alone or in combination to evaluate the treatment performance. The results demonstrated that the FMC could improve water quality more significantly by reducing nutrient pollutants and optimizing the bacterial community structures. Furthermore, it exhibited stronger positive correlation between the enrichment of functional bacteria for water quality improvement and pollutants removal performance than the single-species treatments. Moreover, the FMC outperformed other groups in eliminating emerging contaminants such as heavy metals, antibiotics, and pathogenic Vibrios. Superiorly, the FMC also showed excellent symbiotic interactions and cooperative mechanisms for pollutants removal. The results collectively corroborated the feasibility and sustainability of using C. militaris and N. seminulum for treating aquaculture water, and the FMC would produce more mutualistic benefits and synergistic effects than single-species treatments.

Hydrodynamic cultivation of aeration-free oxygenic photogranules is favored by sufficient amounts of organic carbon

Oxygenic photogranules (OPGs) have great potential for the aeration-free treatment of various wastewater, however, the effects of wastewater carbon composition on OPGs remain unknown. This study investigated the hydrodynamic photogranulation in three types of wastewater with the same total carbon concentration but different inorganic/organic carbon compositions, each operated at two replicated reactors. Results showed that photogranulation failed in reactors fed with only inorganic carbon. In reactors with equal inorganic and organic carbon, loose-structured OPGs formed but then disintegrated. Comparatively, reactors treating organic carbon-based wastewater obtained regular and dense OPGs with better settleability, lower effluent turbidity, excellent structural stability, and higher carbon assimilation rate. Sufficient amounts of organic carbon were crucial for the formation and stability of OPGs as they promoted the secretion of extracellular polymeric substances (EPS) and the growth of filamentous cyanobacteria. This study provides a basis for the startup of OPGs process and facilitates its large-scale application.

Mechanistic insights into different illumination positions control algae production in anaerobic dynamic membrane filtration (AnDM) during decentralized wastewater treatment

Sunlight illumination has the potential to control the stability and sustainability of dynamic membrane (DM) systems. In this study, an up-flow anaerobic sludge blanket (UASB) reactor was combined with DM under different illumination positions (direct, indirect and no illumination) to treat wastewater. Results indicated that the UASB achieved a COD removal up to 87.05 % with an average methane production of 0.28 L/d. Following treatment by the UASB, it was found that under illumination, the removal of organic substances by DM exhibited poor performance due to algal proliferation. However, the DM systems demonstrated efficient removal of ammonia nitrogen, ranging from 96.21 % to 97.67 % after stabilization. Total phosphorus removal was 45.72 %, and membrane flux remained stable when directly illuminated. Conversely, the DM system subjected to indirect illumination showed unstable membrane flux and severe fouling resistance. These findings offer valuable insights into optimizing illumination positions in DM systems under anaerobic conditions.

Influence of aeration modes and DO on simultaneous nitrification and denitrification in treatment of hypersaline high-strength nitrogen wastewater using sequencing batch biofilm reactor (SBBR)

Sequencing batch biofilm reactor (SBBR) has the potential to treat hypersaline high-strength nitrogen wastewater by simultaneous nitrification-denitrification (SND). Dissolved oxygen (DO) and aeration modes are major factors affecting pollutant removal. Low DO (0.35-3.5 mg/L) and alternative anoxic/aerobic (A/O) mode are commonly used for municipal wastewater treatment, however, the appropriate DO concentration and operation mode are still unknown under hypersaline environment because of the restricted oxygen transfer in denser extracellular polymeric substances (EPS) barrier and the decreased carbon source consumption during the anoxic phase. Herein, two SBBRs (R1, fully aerobic mode; R2, A/O mode) were used for the treatment of hypersaline high-strength nitrogen wastewater (200 mg/L NH4+-N, COD/N of 3 and 3% salinity). The results showed that the relatively low DO (2 mg/L) could not realize effective nitrification, while high DO (4.5 mg/L) evidently increased nitrification efficiency by enhancing oxygen transfer in denser biofilm that was stimulated by high salinity. A stable SND was reached 16 days faster with a ∼10% increase of TN removal under A/O mode. Mechanism analysis found that denser biofilm with coccus and bacillus were present in A/O mode instead of filamentous microorganisms, with the secretion of more EPS. Corynebacterium and Halomonas were the dominant genera in both SBBRs, and HN-AD process might assist partial nitrification-denitrification (PND) for highly efficient TN removal in biofilm systems. By using the appropriate operation mode and parameters, the average NH4+-N and TN removal efficiency could respectively reach 100% and 70.8% under the NLR of 0.2 kg N·m-3·d-1 (COD/N of 3), which was the highest among the published works using SND-based SBBRs in treatment of saline high-strength ammonia nitrogen (low COD/N) wastewater. This study provided new insights in biofilm under hypersaline stress and provided a solution for the treatment of hypersaline high-strength nitrogen (low COD/N) water.

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.

Bidirectional Enhancement of Nitrogen Removal by Indigenous Synergetic Microalgal-Bacterial Consortia in Harsh Low-C/N Wastewater

Conventional microalgal-bacterial consortia have limited capacity to treat low-C/N wastewater due to carbon limitation and single nitrogen (N) removal mode. In this work, indigenous synergetic microalgal-bacterial consortia with high N removal performance and bidirectional interaction were successful in treating rare earth tailing wastewaters with low-C/N. Ammonia removal reached 0.89 mg N L-1 h-1, 1.84-fold more efficient than a common microalgal-bacterial system. Metagenomics-based metabolic reconstruction revealed bidirectional microalgal-bacterial interactions. The presence of microalgae increased the abundance of bacterial N-related genes by 1.5- to 57-fold. Similarly, the presence of bacteria increased the abundance of microalgal N assimilation by 2.5- to 15.8-fold. Furthermore, nine bacterial species were isolated, and the bidirectional promotion of N removal by the microalgal-bacterial system was verified. The mechanism of microalgal N assimilation enhanced by indole-3-acetic acid was revealed. In addition, the bidirectional mode of the system ensured the scavenging of toxic byproducts from nitrate metabolism to maintain the stability of the system. Collectively, the bidirectional enhancement system of synergetic microalgae-bacteria was established as an effective N removal strategy to broaden the stable application of this system for the effective treatment of low C/N ratio wastewater.

Communication mediated interaction between bacteria and microalgae advances photogranulation

Recently, photogranules composed of bacteria and microalgae for carbon-negative nitrogen removal receive extensive attention worldwide, yet which type of bacteria is helpful for rapid formation of photogranules and whether they depend on signaling communication remain elusive. Varied signaling communication was analyzed using metagenomic method among bacteria and microalgae in via of two types of experimentally verified signaling molecule from bacteria to microalgae, which include indole-3-acetic acid (IAA) and N-acyl homoserine lactones (AHLs) during the operation of photo-bioreactors. Signaling communication is helpful for the adaptability of bacteria to survive with algae. Compared with non-signaling bacteria, signaling bacteria more easily adapt to the varied conditions, evidenced by the increased abundance in the operated reactors. Signaling bacteria are easier to enter the phycosphere, and they dominate the interactions between bacteria and algae rather than non-signaling bacteria. The co-abundance groups (CAGs) with signaling bacteria possess higher abundance than that without signaling bacteria (22.27 % and 6.67 %). Importantly, signaling bacteria accessibly interact with microalgae, which possess higher degree centralities and 32.50 % of them are keystone nodes in the network, in contrast to only 18.66 % of non-signaling bacteria. Thauera carrying both IAA and AHLs synthase genes are highly enriched and positively correlated with nitrogen removal rate. Our work not only highlights the essential roles of signaling communication between microalgae and bacteria in the development of photogranules, but also enriches our understanding of microbial sociobiology.

Oxygen affinity and light intensity induced robust phosphorus removal and fragile ammonia removal in a non-aerated bacteria-algae system

Non-aerated bacteria-algae system gaining O2 through photosynthesis presents an alternative for costly mechanical aeration. This study investigated oxygen supply and performance of nutrients removal at low and high light intensity (LL and HL). The results showed that P removal was high and robust (LL 97 ± 1.8 %, HL 95 % ± 2.9 %), while NH4+-N removal fluctuated dramatically (LL 66 ± 14.7 %, HL 84 ± 8.6 %). Oxygen generated at illumination of 200 μmol m-2 s-1, 6 h was sufficient to sustain aerobic phase for 2.25 g/L MLSS. However, O2 produced by algae was preferentially captured in the order of heterotrophic bacteria (HB), ammonia oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB). Oxygen affinity coupled with light intensity led to NOB suppression with stable nitrite accumulation ratio of 57 %. Free nitrous acid (FNA) and light stimulated the abundance of denitrifying polyphosphate accumulating organism (DPAO) of Flavobacterium, but with declined P-accumulating metabolism (PAM) of P release, P/C, K/P and Mg/P ratios. Flavobacterium and cyanobacteria Leptolyngbya, along with biologically induced CaP in extracellular polymeric substances was the key to robust P removal. AOB of Ellin6067 and DPAO of Flavobacteria offer a promising scenario for partial nitrification-denitrifying phosphorus removal.

Coupling of polyhydroxybutyrate and zero-valent iron for enhanced treatment of nitrate pollution within the Permeable Reactive Barrier and its downgradient aquifer

Permeable Reactive Barriers (PRBs) have been utilized for mitigating nitrate pollution in groundwater systems through the use of solid carbon and iron fillers that release diverse nutrients to enhance denitrification efficiency. We conduct laboratory column tests to evaluate the effectiveness of PRBs in remediating nitrate pollution both within the PRB and in the downgradient aquifer. We use an iron-carbon hydrogel (ICH) as PRB filler, which has different weight ratios of polyhydroxybutyrate (PHB) and microscale zero-valent iron (mZVI). Results reveal that denitrification in the downgradient aquifer accounts for at least 19.5 % to 32.5 % of the total nitrate removal. In the ICH, a higher ratio of PHB to mZVI leads to higher contribution of the downgradient aquifer to nitrate removal, while a lower ratio results in smaller contribution. Microbial community analysis further reveals that heterotrophic and mixotrophic bacteria dominate in the downgradient aquifer of the PRB, and their relative abundance increases with a higher ratio of PHB to mZVI in the ICH. Within the PRB, autotrophic and iron-reducing bacteria are more prevalent, and their abundance increases as the ratio of PHB to mZVI in the ICH decreases. These findings emphasize the downgradient aquifer's substantial role in nitrate removal, particularly driven by dissolved organic carbon provided by PHB. This research holds significant implications for nutrient waste management, including the prevention of secondary pollution, and the development of cost-effective PRBs.

Combined Phototrophic Simultaneous Nitrification-Endogenous Denitrification with Phosphorus Removal (P-SNDPR) System Treating Low Carbon to Nitrogen Ratio Wastewater for Potential Carbon Neutrality

Conventional biological nutrient removal processes rely on external aeration and produce significant carbon dioxide (CO2) emissions. This study constructed a phototrophic simultaneous nitrification-denitrification phosphorus removal (P-SNDPR) system to treat low carbon to nitrogen (C/N) ratios wastewater and investigated the impact of sludge retention time (SRT) on nutrient removal performance, nitrogen conversion pathway, and microbial structure. Results showed that the P-SNDPR system at SRT of 15 days had the highest nutrient removal capacity, achieving over 85% and 98% removal of nitrogen and phosphorus, respectively, meanwhile maintaining minimal CO2 emissions. Nitrogen removal was mainly through assimilation at SRTs of 5 and 10 days, and nitrification-denitrification at SRTs of 15 and 20 days. Stable partial nitrification was facilitated by photoinhibition and low DO levels. Flow cytometry sorting technique results revealed SRT drove community structural changes in translational activity (BONCAT+) microbes, where BONCAT+ microbes were mainly simultaneous nitrogen and phosphorus removal bacteria (Candidatus Accumulibacter), denitrifying bacteria (Candidatus Competibacter and Plasticicumulans), ammonia-oxidizing bacteria (Nitrosomonas), and microalgae (Chlorella and Dictyosphaerium). The P-SNDPR system represents a novel, carbon-neutral process for efficient nutrient removal from low C/N ratio wastewater without aeration and external carbon source additions.

Influence mechanism of sludge bed position on microalgal-bacterial granular sludge process

Sludge bed position in the reactor is one of the key parameters for microalgal-bacterial granular sludge (MBGS) process, which lacks of study. To fill this gap, this study investigated the influence of sludge bed position on MBGS. The sludge bed located closer to the bottom of the bioreactor demonstrated the optimal pollutant removal performance due to a close synergistic effect between microalgae and bacteria, resulting in the high growth rate as well as agglomeration rate of MBGS. Specifically, organics and ammonia removals were closely related to the sludge bed position. For the bottom bed position, the removals of organic matter, ammonia, and phosphate were 75.1 %, 73.1 %, and 82.5 %, whereas for the top bed position, they were only 13.2 %, 9.6 %, and 68.9 %, respectively. Additionally, a significant correlation between the position of the sludge bed and the relative abundance of Rotifera (R2 = 0.931) and Chlorophyta (R2 = 0.733) was observed, while the microbial communities at the lower sludge bed positions underwent rapider succession. It can be inferred that that a sludge bed located closer to the bottom of the bioreactor ensures that the light source and substrate matrix are transmitted in the same direction, thereby resulting in a close synergistic effect between microalgae and bacteria for achieving the excellent performance of MBGS. These results can provide basis knowledge for engineering application of MBGS process.

Microalgal-bacterial biofilms for wastewater treatment: Operations, performances, mechanisms, and uncertainties

Microalgal-bacterial biofilms have been increasingly considered of great potential in wastewater treatment due to the advantages of microalgal-bacterial synergistic pollutants removal/recovery, CO2 sequestration, and cost-effective biomass-water separation. However, such advantages may vary widely among different types of microalgal-bacterial biofilms, as the biofilms could be formed on different shapes and structures of attachment substratum, generating "false hope" for certain systems in large-scale wastewater treatment if the operating conditions and pollutants removal properties are evaluated based on the general term "microalgal-bacterial biofilm". This study, therefore, classified microalgal-bacterial biofilms into biofilms formed on 2D substratum, biofilms formed on 3D substratum, and biofilms formed without substratum (i.e. microalgal-bacterial granular sludge, MBGS). Biofilms formed on 2D substratum display higher microalgae fractions and nutrients removal efficiencies, while the adopted long hydraulic retention times were unacceptable for large-scale wastewater treatment. MBGS are featured with much lower microalgae fractions, most efficient pollutants removal, and acceptable retention times for realistic application, yet the feasibility of using natural sunlight should be further explored. 3D substratum systems display wide variations in operating conditions and pollutants removal properties because of diversified substratum shapes and structures. 2D and 3D substratum biofilms share more common in eukaryotic and prokaryotic microbial community structures, while MGBS biofilms are more enriched with microorganisms favoring EPS production, biofilm formation, and denitrification. The specific roles of stratified extracellular polymeric substances (EPS) in nutrients adsorption and condensation still require in-depth exploration. Nutrients removal uncertainties caused by microalgal-bacterial synergy decoupling under insufficient illumination, limited microbial community control, and possible greenhouse gas emission exacerbation arising from microalgal N2O generation were also indicated. This review is helpful for revealing the true potential of applying various microalgal-bacterial biofilms in large-scale wastewater treatment, and will provoke some insights on the challenges to the ideal state of synergistic pollutants reclamation and carbon neutrality via microalgal-bacterial interactions.

Effect of different inocula on the granulation process, reactor performance and biodiesel production of algal-bacterial granular sludge (ABGS) under low aeration conditions

The algal-bacterial granular sludge (ABGS) system is a prospective wastewater treatment technology, but few studies focused on the effects of different inoculum types on the establishment of the ABGS system under low aeration conditions (step-decrease superficial gas velocity from 1.4 to 0.5 cm/s). Results from this study indicated that compared with other inocula, the ABGS formed by co-inoculating aerobic granular sludge (AGS) and targeted algae (Chlorella) exhibited a shorter granulation period (shortened by 15 days), higher total nitrogen (89.4%) and PO43--P (95.0%) removal efficiencies, and a greater yield of fatty acid methyl esters (FAMEs) (9.04 mg/g MLSS). This was possibly attributed to that the functional bacteria (e.g. Thauera, Gemmobacter and Rhodobacter) in the inoculated AGS facilitated the ABGS granulation. The inoculated algae promoted their effective enrichment under illumination conditions and enhanced the production of extracellular polymeric substances, thus improving the stability of ABGS. The enriched algae were attached to the outer layer of the granules, which could provide sufficient oxygen for bacterial metabolism, revealing the inherent mechanisms for the good stability of ABGS under low aeration intensity. Overall, the rapid granulation of ABGS can be achieved by inoculating optimal inocula under low aeration conditions, which is convenient and economically feasible, and motivates the application of algal-bacterial consortia.

Advances, challenges, and prospects in microalgal-bacterial symbiosis system treating heavy metal wastewater

Heavy metal (HM) pollution, particularly in its ionic form in water bodies, is a chronic issue threatening environmental security and human health. The microalgal-bacterial symbiosis (MABS) system, as the basis of water ecosystems, has the potential to treat HM wastewater in a sustainable manner, with the advantages of environmental friendliness and carbon sequestration. However, the differences between laboratory studies and engineering practices, including the complexity of pollutant compositions and extreme environmental conditions, limit the applications of the MABS system. Additionally, the biomass from the MABS system containing HMs requires further disposal or recycling. This review summarized the recent advances of the MABS system treating HM wastewater, including key mechanisms, influence factors related to HM removal, and the tolerance threshold values of the MABS system to HM toxicity. Furthermore, the challenges and prospects of the MABS system in treating actual HM wastewater are analyzed and discussed, and suggestions for biochar preparation from the MABS biomass containing HMs are provided. This review provides a reference point for the MABS system treating HM wastewater and the corresponding challenges faced by future engineering practices.

Cross-Feeding between Filamentous Cyanobacteria and Symbiotic Bacteria Favors Rapid Photogranulation

Photogranules are dense algal-bacterial aggregates used in aeration-free and carbon-negative wastewater treatment, wherein filamentous cyanobacteria (FC) are essential components. However, little is known about the functional role of symbiotic bacteria in photogranulation. Herein, we combined cyanobacterial isolation, reactor operation, and multiomics analysis to investigate the cyanobacterial-bacterial interaction during photogranulation. The addition of FC to the inoculated sludge achieved a 1.4-fold higher granule size than the control, and the aggregation capacity of FC-dominant photogranules was closely related to the extracellular polysaccharide (PS) concentration (R = 0.86). Importantly, we found that cross-feeding between FC and symbiotic bacteria for macromolecular PS synthesis is at the heart of photogranulation and substantially enhanced the granular stability. Chloroflexi-affiliated bacteria intertwined with FC throughout the photogranules and promoted PS biosynthesis using the partial nucleotide sugars produced by FC. Proteobacteria-affiliated bacteria were spatially close to FC, and highly expressed genes for vitamin B1 and B12 synthesis, contributing the necessary cofactors to promote FC proliferation. In addition, Bacteroidetes-affiliated bacteria degraded FC-derived carbohydrates and influenced granules development. Our metabolic characterization identified the functional role of symbiotic bacteria of FC during photogranulation and shed light on the critical cyanobacterial-bacterial interactions in photogranules from the viewpoint of cross-feeding.

Enhanced ammonia nitrogen and phenol removal by immobilized bacteria through composite mycelium pellet-driven quinone redox cycle

The composite mycelium pellet (CMP) was coupled with Pseudomonas sp. Y1 (CMP-Y1) to remove phenol and ammonia nitrogen (NH4+-N). The CMP was formed by the self-assembly of fungal mycelium with sponge iron (SIO), gallic acid (GA), and oxalic acid. The results showed that CMP with abundant pore size and successful internal loading of sponge iron containing iron nanoparticles. CMP could induce GA redox cycle to form Fenton-like reaction and thus achieve efficient phenol removal (93.32%, 24 h). Meanwhile, the removal efficiencies of phenol, NH4+-N, and chemical oxygen demand (COD) using CMP-Y1 at 12 h were 93.71, 92.40, and 89.00%, respectively. The increase in the electron transfer activity of strain Y1 by the addition of CMP could facilitate the nitrogen removal processes. In addition, high-throughput sequencing results indicated the abundance of antioxidant and repair genes was increased, which might be a strategy of strain Y1 to cope with oxidative stress. This strategy provided the possibility for the practical application of the combination of advanced oxidation and biological treatment, and offered new insights into the symbiotic system of fungi and bacteria.

Filamentous cyanobacteria and hydrophobic protein in extracellular polymeric substances facilitate algae-bacteria aggregation during partial nitrification

In algae-bacteria symbiotic wastewater treatment, the excellent settling performance of algae-bacteria aggregates is critical for biomass separation and recovery. Here, the composition of extracellular polymeric substances (EPS), microbial profiles, and functional genes of algae-bacteria aggregates were investigated at different solid retention times (SRTs) (10, 20, and 40 d) during partial nitrification in photo sequencing bioreactors (PSBRs). Results showed that SRTs greatly influenced the nitrogen transformation and the formation and morphological structure of algae-bacteria aggregates. The highest nitrite accumulation, the largest particle size (~1.54 mm) and the best settling performance were observed for the algae-bacteria aggregates in the PSBR with an SRT of 10 d, where the abundant occurrence of filamentous cyanobacteria with the highest ratio of chlorophyll a/b and the lowest EPS amount with the highest protein-to-polysaccharide ratio were observed. In particular, the EPS at 10 d of SRT contained a higher amount of protein-related hydrophobic groups and a lower ratio of α-helix/(β-sheet + random coil), indicating a looser protein structure, which might facilitate the formation and stabilization of algae-bacteria aggregates. Moreover, algal-bacterial aggregation greatly depended on the composition and evolution of filamentous cyanobacteria (unclassified _o__Oscillatoriales and Phormidium accounted for 56.29 % of the identified algae at SRT 10 d). The metagenomic analysis further revealed that functional genes related to amino acid metabolism (e.g., genes of phenylalanine, tyrosine, and tryptophan biosynthesis) were expressed at high levels within 10 d of SRT. Overall, this study demonstrates the influence of EPS structures and filamentous cyanobacteria on algae-bacteria aggregation and reveals the biological mechanisms driving photogranule structure and function.

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

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

Anammox bacteria adapt to long-term light irradiation in photogranules

Photogranules composed of algae, nitrifiers, and anammox bacteria are promising for nitrogen removal from wastewater with reduced aeration and carbon emissions. However, it is difficult to be achieved as the potential inhibition of anammox bacteria by light. In this study, a syntrophic algal-partial nitrification/anammox granular sludge process was developed, with a nitrogen removal rate of 294.5 mg N/(L·d). We found the symbiosis in the community promoted the adaptation of anammox bacteria under light, and cross-feeding played an important role. Microalgae in the outer layers of photogranules sheltered most of the light and supplied cofactors and amino acids to promote nitrogen removal. In particular, Myxococcota MYX1 degraded the extracellular proteins produced by microalgae, providing amino acids to the entire bacterial community, which helped anammox bacteria save metabolic energy and adapt to light. Notably, the anammox bacteria Candidatus Brocadia exhibited unique light-sensing potential and adaptations to light irradiation compared with Candidatus Jettenia, including diverse DNA repair, scavenging of reactive oxygen species, cell movement. The phytochrome-like proteins encoded by Candidatus Brocadia further facilitated their spatial positioning and niche partitioning in photogranules. This study provides insights into the response of anammox bacteria in the algae-bacteria symbiosis system and suggests its potential application for carbon-negative nitrogen removal.

Insight into the effect of nitrate on AGS granulation: Granular characteristics, microbial community and metabolomics response

As a promising wastewater treatment technology, aerobic granular sludge (AGS) process is still hindered by slow granule formation and easy disintegration in the application. While nitrate, one of the target pollutants in wastewater, showed a potential effect on AGS granulation process. Herein, this study attempted to reveal the role of nitrate in AGS granulation. By adding exogenous nitrate (10 mg L^-1), the AGS formation was markedly improved and accomplished at 63 d, while the control group achieved AGS formation at 87 d. However, a disintegration was observed under a long-term nitrate feeding. A positive correlation was observed among granule size, extracellular polymeric substances (EPS) and intracellular c-di-GMP level in both formation and disintegration phases. The subsequent static biofilm assays indicated that nitrate might upregulate c-di-GMP via denitrification-derived NO, and c-di-GMP further upregulated EPS, thereby promoting AGS formation. However, excessive NO probably caused disintegration by downregulating c-di-GMP and EPS. Microbial community showed that nitrate favored the enrichment of denitrifiers and EPS producing microbes, which were responsible for the regulation of NO, c-di-GMP and EPS. Metabolomics analysis showed that amino acid metabolism was the most affected metabolism by nitrate. Some amino acids, such as Arg, His and Asp, were upregulated in the granule formation phase and downregulated in the disintegration phase, indicating the potential contribution to EPS biosynthesis. This study provides metabolic insight into how nitrate promotes/inhibits granulation, which may contribute to unwrapping the mystery of granulation and overcoming the limitations of AGS application.

Metagenomic analysis reveals the responses of microbial communities and nitrogen metabolic pathways to polystyrene micro(nano)plastics in activated sludge systems

Microplastics (MPs) and nanoplastics (NPs) are prevalent in sewage and pose a potential threat to nitrogen biotransformation in wastewater treatment systems. However, investigations on how MPs and NPs affect the microbial nitrogen conversion and metabolism of the activated sludge are still scanty. Herein, the responses of microbiomes and functional genes to polystyrene MPs and NPs in activated sludge systems were investigated by metagenomic analysis. Results indicated that 1 mg/L MPs and NPs had marginal impacts on the nitrogen removal performance of the activated sludge systems, whereas high concentrations of MPs and NPs (20 and 100 mg/L) decreased the total nitrogen removal efficiency (13.4%-30.6%) by suppressing the nitrogen transformation processes. Excessive reactive oxygen species induced by MPs and NPs caused cytotoxicity, as evidenced by impaired cytomembranes and decreased bioactivity. Metagenomic analysis revealed that MPs and NPs diminished the abundance of denitrifiers (e.g. Mesorhizobium, Rhodobacter and Thauera), and concurrently reduced the abundance of functional genes (e.g. napA, napB and nirS) encoding for key enzymes involved in the nitrogen transformations, as well as the genes (e.g. mdh) related to the electron donor production, thereby declining the nitrogen removal efficiency. Network analysis further clarified the attenuate association between denitrifiers and denitrification-related genes in the plastic-exposed systems, elucidating that MPs and NPs restrained the nitrogen removal by inhibiting the contributions of microorganisms to nitrogen transformation processes. This study provides vital insights into the responses of the microbial community structure and nitrogen conversion processes to micro(nano)plastics disturbance in activated sludge systems.

Nitrogen removal by algal-bacterial consortium during mainstream wastewater treatment: Transformation mechanisms and potential N2O mitigation

This work investigated nitrogen transformation pathways of the algal-bacterial consortium as well as its potential in reducing nitrous oxide (N₂O) emission in enclosed, open and aerated reactors. The results confirmed the superior ammonium removal performance of the algal-bacterial consortium relative to the single algae (Chlorella vulgaris) or the activated sludge, achieving the highest efficiency at 100% and the highest rate of 7.34 mg N g MLSS^-1 h^-1 in the open reactor with glucose. Enhanced total nitrogen (TN) removal (to 74.6%) by the algal-bacterial consortium was achieved via mixotrophic algal assimilation and bacterial denitrification under oxygen-limited and glucose-sufficient conditions. Nitrogen distribution indicated that ammonia oxidation (∼41.8%) and algal assimilation (∼43.5%) were the main pathways to remove ammonium by the algal-bacterial consortium. TN removal by the algal-bacterial consortium was primarily achieved by algal assimilation (28.1-40.8%), followed by bacterial denitrification (2.9-26.5%). Furthermore, the algal-bacterial consortium contributed to N₂O mitigation compared with the activated sludge, reducing N₂O production by 35.5-55.0% via autotrophic pathways and by 81.0-93.6% via mixotrophic pathways. Nitrogen assimilation by algae was boosted with the addition of glucose and thus largely restrained N₂O production from nitrification and denitrification. The synergism between algae and bacteria was also conducive to an enhanced N₂O reduction by denitrification and reduced direct/indirect carbon emissions.

Start-up and maintenance of indigenous microalgae-bacteria consortium treating toilet wastewater through partial nitrification and nitrite-type denitrification

Microalgae-bacteria consortium (MBC) provides an alternative to sustainable treatment of human toilet wastewater (TWW) and resource recovery. This study compared the conventional activated sludge system and wastewater indigenous MBC system (IMBC) for nitrogen removal in TWW through the coupled partial nitrification (PN) and nitrite-type denitrification process. PN was firstly established by alternating FA and FNA. Subsequently, the successful PN maintenance with the nitrite accumulation rate ranging between 90.1-95.3% was achieved using two strategies: light irradiation with the appropriate specific light energy density at 0.0188-0.0598 kJ/mg VSS and the timely nitrite-type denitrification with the algae-secreted organics as the carbon source, eventually resulting in the nitrite accumulation rate ranging between 90.1-95.3%. In the IMBC-PN system, bacterial metabolism contributed to 91.5% of nitrogen removal and the rest was through microalgal assimilation. This study offers a sustainable hybrid IMBC-PN process for high NH₄⁺-N strength wastewater treatment (e.g., TWW), which theoretically saves 23.5% aeration and 34.2% carbon source as well as reduces 17.0% sludge production.

Rapid establishment of algal-bacterial granular sludge system by applying mycelial pellets in a lab-scale photo-reactor under low aeration conditions: Performance and mechanism analysis

Light-driven algal-bacterial granular sludge (ABGS) is an innovative low-carbon technology with significant merits in treating municipal wastewater, but how to shorten the photogranulation process, especially under low aeration conditions, is largely unknown. Herein, two strategies were proposed to accelerate the start-up of the ABGS system in photo-sequencing batch reactors (PSBRs) with a low superficial gas velocity of 0.5 cm/s. Compared to directly dosing mycelial pellets (MPs), applying MPs to flocculate algae and using the formed algal-mycelial pellets (AMPs) as carriers enhanced the establishment of the algal-bacterial symbiosis. The ABGS system developed rapidly within 20 days, with a large particle diameter (mean diameter of 321 μm) and excellent settleability (SVI₃₀ of 55.4 mL/g). More importantly, this system could be stably operated for at least 100 days, mainly attributed to the reinforced secretion of protein with unique secondary structure and elevated hydrophobic functional groups. As for the reactor performance, the average removal efficiencies of the ABGS system were 97.8% for organic matter, 80.0% for total nitrogen, and 84.4% for phosphorus. The enrichment of functional bacteria and algae, and the up-regulation of functional genes and enzymes involved in electron production and transport processes likely drove the transformation of the pollutants, underlining the inherent mechanism for the excellent nutrient removal performance. This study provides a promising approach to solve the problem of a long ABGS start-up period and unstable granular structure under low aeration conditions, which is significant for achieving effective wastewater treatment without energy intensive aeration.

Performance and Bacterial Characteristics of Aerobic Granular Sludge in Treatment of Ultra-Hypersaline Mustard Tuber Wastewater

Mustard tuber wastewater (MTW) is an ultra-hypersaline high-strength acid organic wastewater. Aerobic granular sludge (AGS) has been demonstrated to have high tolerance to high organic loading rate (OLR), high salinity, and broad pH ranges. However, most studies were conducted under single stress, and the performance of AGS under multiple stresses (high salinity, high OLR, and low pH) was still unclear. Herein, mature AGS was used to try to treat the real MTW at 9% salinity, pH of 4.1–6.7, and OLR of 1.8–7.2 kg COD/m3·d. The OLR was increased, and the results showed that the upper OLR boundary of AGS was 5.4 kg COD/m3·d (pH of 4.2) with relatively compact structure and high removal of TOC (~93.1%), NH4+-N (~88.2%), and TP (~50.6%). Under 7.2 kg COD/m3·d (pH of 4.1), most of the AGS was fragmented, primarily due to the multiple stresses. 16S rRNA sequencing indicated that Halomonas dominated the reactor during the whole process with the presence of unclassified-f-Flavobacteriaceae, Aequorivita, Paracoccus, Bradymonas, and Cryomorpha, which played key roles in the removal of TOC, nitrogen, and phosphorus. This study investigated the performance of AGS under multiple stresses, and also brought a new route for highly-efficient simultaneous nitrification–denitrifying phosphorus removal of real MTW.

Overlooked pathways of endogenous simultaneous nitrification and denitrification in anaerobic/aerobic/anoxic sequencing batch reactors with organic supplementation

The anaerobic/aerobic/anoxic (A/O/A) process is a promising biotechnology to intensify denitrification in low carbon/nitrogen (C/N) wastewater treatment, but the neglected typical rate-limiting step-nitrification-would hinder its wider application. Heterotrophic nitrification driven by intracellular carbon (PHAs) could enhance nitrification and achieve endogenous simultaneous nitrification and denitrification (ESND) in the A/O/A process, but its feasibility remains unexamined. Here we established four A/O/A-SBRs at different C/N ratios (3, 7.5, 12, and 16.5) to address the above-mentioned knowledge gaps. The results showed that organic supplementation promoted both nitrification and denitrification (performance and relevant enzymatic activities) until organic overdose (C/N = 16.5) exacerbated niche competitions from other non-functional heterotrophs. qPCR and batch tests indicated that high C/N ratios inhibited autotrophic nitrifiers, and heterotrophic nitrifiers (HNB) dominated in the enhanced nitrification. Given the high HNB contribution (43.7%) and low COD variation (< 10 mg L^-1) in the SND (76.4%) of CN12, we proposed a potential SND pathway based on heterotrophic nitrification and denitrification driven by PHAs and verified it with batch tests. Microbial and functional analyses suggested that CN12 favored the intracellular carbon transformation and harbored the minimum autotrophic nitrifiers, supporting the dominance of ESND in the enhanced SND. Our findings expand the understanding of the relationships between intracellular carbon transformation and SND and provide a novel nitrogen removal pathway for the practical application of the A/O/A process.

Direct start-up of aerobic granular sludge system with dewatered sludge granular particles as inoculant

Aerobic granular sludge (AGS) is a promising technology for engineering applications in the biological treatment of sewage. New objective is to skip the conventional granulation step to integrate it into a continuous-flow reactor directly. This study proposed a method for integrating spherical pelletizing granular sludge (SPGS) into a new patented aerobic granular sludge bed (AGSB), a continuous up-flow reactor. AGSB system could be startup directly, and after 120 days of operation, the SPGS maintained a relatively intact spherical structure and stability. With an initial high chemical oxygen demand (COD) volume loading of over 2.0 kg/(m³·d), this system achieved the desired effect as the same as a mature AGS system. The final mixed liquid suspended solids, and the ratio of 30 min-5 min sludge volume index (SVI₃₀/SVI₅) were 20,000 mg/L, and 0.84, respectively. Although hydraulic elution and filamentous bacteria (FBs) had a slightly negative impact on initial phase pollutant removal, the final removal rates for COD, total nitrogen (TN), ammonia nitrogen (NH₄⁺-H), and total phosphorus (TP) were 90%, 70%, 95%, and 85%, respectively. The presence of specific functional microorganisms promoted the secretion of extracellular polymeric substances (EPS), from 90.65 to 209.78 mg/gVSS. The maturation process of SPGS altered the microbial community structures and reduced the species abundance of microbes in sludge.

Enhanced ammonia nitrogen removal from actual rare earth element tailings (REEs) wastewater by microalgae-bacteria symbiosis system (MBS): Ratio optimization of microalgae to bacteria and mechanism analysis

Microalgae-bacteria symbiosis system (MBS) appear to be a promising way for treating the rare earth elements (REEs) wastewater due to the natural symbiotic interactions between microalgae and bacteria. Herein, we investigated the effect of different inoculation ratios of microalgae and bacteria including 3:1 (MB_1), 1:1 (MB_2) and 1:3 (MB_3) on NH₄⁺-N removal from REEs wastewater and analyzed the corresponding biological mechanism. The NH₄⁺-N removal rate with MB_3 reached 17.69 ± 0.45 mg NH₄⁺-N/L d^-1, which was 2.58 times higher than that in single microalgae system. The results were further verified in continuous feeding photobioreactors and kept stable for 100 days. Metagenomic analysis revealed that the abundance of genes related to microalgae assimilation increased by 14 %-50 % in answer to photosynthesis and NH₄⁺-N absorption, while that related to nitrification apparently dropped, indicating that MBS was a sustainable method capable of enhancing NH₄⁺-N removal from REEs wastewater.

Evaluating the role of algae in algal-bacterial granular sludge: Nutrient removal, microbial community and granular characteristics

Algal-bacterial granular sludge (ABGS) and bacterial granular sludge (BGS, control group) were operated over 240 days to investigate the role of algae in treating synthetic municipal wastewater. The results showed that algae significantly improved the removal efficiency of total nitrogen (TN). The nitrogen removal load of ABGS was 2.6 mg-N/g-VSS/day (22.8 %, light) and 1.1 mg-N/g-VSS/day (9.6 %, dark) higher than that of BGS, respectively, which was attributed to algae enhanced NH₃-N removal capacity in the anaerobic stage and increased the utilization efficiency of organics in denitrification. Algae increased the relative abundance of denitrifying bacteria, and ABGS (28.83 %) was higher than BGS (14.28 %). Moreover, the dominant phylum of algae was Chlorophyta (98.39 %), the chlorophyll-a was sustained at 1.28 ± 0.26 mg/g-VSS. Algae significantly increased the content of extracellular polymeric substances (EPS), and the increased polysaccharide came from the tightly bound EPS. This study expands the understanding of the role of algae in ABGS.

Formation, application, and storage-reactivation of aerobic granular sludge: A review

It was an important discovery in wastewater treatment that the microorganisms in the traditional activated sludge can form aerobic granular sludge (AGS) by self-aggregation under appropriate water quality and operation conditions. With a typical three-dimensional spherical structure, AGS has high sludge-water separation efficiency, great treatment capacity, and strong tolerance to toxic and harmful substances, so it has been considered to be one of the most promising wastewater treatment technologies. This paper comprehensively reviewed AGS from multiple perspectives over the past two decades, including the culture conditions, granulation mechanisms, metabolic and structural stability, storage, and its diverse applications. Some important issues, such as the reproducibility of culture conditions and the structural and functional stability during application and storage, were also summarized, and the research prospects were put forward. The aggregation behavior of microorganisms in AGS was explained from the perspectives of physiology and ecology of complex populations. The storage of AGS is considered to have large commercial potential value with the increase of large-scale applications. The purpose of this paper is to provide a reference for the systematic and in-depth study on the sludge aerobic granulation process.

Using carbon dioxide-added microalgal-bacterial granular sludge for carbon-neutral municipal wastewater treatment under outdoor conditions: Performance, granule characteristics and environmental sustainability

Microalgal-bacterial granular sludge (MBGS) process has a gorgeous prospect for municipal wastewater treatment, but the research on the treatment of complex organic wastewater by MBGS process with CO₂ addition under outdoor conditions is not enough. Therefore, this paper evaluated the feasibility of CO₂-added MBGS process for complex organic wastewater disposal under natural day-night cycles. The results showed that the addition of CO₂ overall improved the removal efficiency of pollutants. Typically, the removal efficiency of total phosphorus increased averagely from 88.5 % to 95.0 % in 12-h day cycle and from 26.2 % to 45.3 % in 12-h night cycle. The addition of CO₂ increased the size of MBGS from 1.0 mm to 16.5 mm within 30 days due to extracellular polymeric substances secretion and the dominant filamentous microalgae on granules. The decrease of catalase activity and malondialdehyde content indicated that CO₂ reduced oxidative damage and maintained the normal growth of MBGS. Further estimates of the collected gas showed that CO₂-added MBGS process could reduce global CO₂ emissions by one hundred million tons per year. This study is expected to contribute to the goal of carbon neutrality in the area of wastewater treatment by MBGS process.

Light-introduced partial nitrification in an algal-bacterial granular sludge bioreactor: Performance evolution and microbial community shift

This objective of study was to evaluate the influence of light on the achievement of partial nitrification algal-bacterial granular bioreactor and its related nitrite accumulation mechanism. After 150-days operation, partial nitrification algal-bacterial granulation bioreactor was achieved under the 200 μmol/(m²·s) illuminance condition. The effluent NH₄⁺-N, NO₂^--N, NO₃^--N concentrations were average at 1.1, 61.7 and 8.0 mg/L (n = 21), respectively. The average sphericity of algal-bacterial aerobic granular sludge (AB-AGS) increased from 82.7% to 91.1%, accompanied by the significantly increased diameter. Additionally, extracellular protein increased by 1.5 times and 0.5 times higher in LB-EPS and TB-EPS of AB-AGS, respectively. According to typical cycles, N₂O emission amount reactor accounted for 2.4% of the removed nitrogen. Under the combined inhibition of light and free ammonia (FA), Nitrosomonas-related AOB (0.2% to 2.1%) were the predominant functional bacteria, whereas Nitrospira-related NOB (0.07% to below 0.01%) was fully inhibited.

Revealing the influencing mechanisms of polystyrene microplastics (MPs) on the performance and stability of the algal-bacterial granular sludge

Algal-bacterial granular sludge (ABGS) is an energy-saving and environment-friendly wastewater treatment technology; however, the effects of microplastics (MPs) on the performance and stability of the ABGS system remain unknown. Herein, the influencing mechanisms of polystyrene MPs (50 μm) on the ABGS were systematically investigated. The ABGS exhibited a high removal efficiency of MPs (over 96%) at 1 mg/L and 20 mg/L. Although the biomass content, sludge settling and particle size were not obviously affected by MPs, the COD and total phosphorus (TP) removal efficiencies were inhibited by 2.6%-4.1% and 2.9%-5.8%, respectively. Meanwhile, the structural stability of ABGS was damaged by MPs, owing to the excessive oxidative stress, low content of protein-like substance (especially tryptophan and tyrosine), and the large portion of loose protein secondary structure. Microbial community analysis revealed that the relative abundance of some functional bacteria (Candidatus_Competibacter and Rhodobacter) and algal species (Tetradesmus) were decreased under the MPs stress.