Effect of algae growth on aerobic granulation and nutrients removal from synthetic wastewater by using sequencing batch reactors

Microalgal-bacterial system responses to nitrogen forms in dairy farm wastewater: Focusing on the phycosphere and nitrogen transformation

As an environmentally friendly medium, microalgae are often used in wastewater treatment. However, few studies have examined the effects of different nitrogen forms on microalgae and bacteria, particularly regarding material and gene transfer and nitrogen metabolic pathways in phycosphere and various extracellular polymeric substance layers. To address this research gap, Chlorella vulgaris was used to treat the dairy farm wastewater with different nitrogen additives. The results showed that the dry weight and chlorophyll a production were lowest at high ammonia nitrogen concentrations (0.460 g/L and 0.883 mg/L, respectively). The ability of microalgae to remove total phosphorus and ammonia nitrogen was significantly enhanced at appropriate nitrogen concentrations. There were clear differences in community abundance between phycosphere and different extracellular polymeric substance layers bacteria. Nitrate nitrogen promoted electron transfer in photosynthesis, while organic nitrogen facilitated the synthesis of siderophores. In high-ammonia nitrogen wastewater, ammonia nitrogen conversion primarily occurred through the action of nitrifying bacteria, whereas denitrification promoted nitrate nitrogen conversion. There is an interaction between nitrogen forms and microalgal-bacterial system. The study provided critical insights for microalgae treatment of dairy farm wastewater, contributing to environmental friendly and resource recycling wastewater management.

A review on recent advancement in algal-bacterial aerobic granular sludge: From inoculation approaches to operation strategies

In recent years, the environmental unsustainability of traditional wastewater treatment processes, alongside the growing emphasis on carbon neutrality and self-sufficiency in wastewater treatment plants (WWTPs), has drawn attention to algal-bacterial aerobic granular sludge (AGS) systems. This review synthesizes recent advances in algal-bacterial AGS, focusing on the latest research on its granulation mechanisms under both hydrostatic and hydrodynamic conditions and providing a comprehensive overview of cultivation strategies based on various sludges. We discuss the latest reports on the impacts of process operation strategies for algal-bacterial AGS and emphasize the operation performance and design of different bioreactor configurations. Challenges for commercialization and sustainable applications are highlighted; future application directions are also posted. In summary, algal-bacterial AGS exhibits significant potential in simultaneous efficient wastewater treatment and bioresource recovery, targeting circular bioeconomy and sustainable wastewater industry.

Microalgal-bacterial aggregates for wastewater treatment: Origins, challenges, and future directions

Microalgal-bacterial aggregates are promising for wastewater treatment because they remove organic matter, nitrogen, and phosphorus while producing biomass that settles quickly. This review details the development of microalgal-bacterial aggregates, identifies key challenges, and proposes future research directions. While many studies have been performed in the laboratory with synthetic wastewater and artificial lighting, more research is needed to better understand how to form and sustain aggregates at larger scales with real wastewater and natural lighting. While it appears that microalgal-bacterial aggregates are unlikely to replace or augment conventional activated sludge, they have the potential to improve resource recovery in existing microalgae-based wastewater treatment processes (e.g., high-rate algal ponds). Alternatively, attached-growth bioreactors utilizing microalgal-bacterial consortia may be able to compete directly with conventional activated sludge while providing the benefits that microalgae offer, although additional research is needed. PRACTITIONER POINTS: More pilot and full-scale research on microalgal-bacterial processes is needed. Microalgae cultivation with short retention times is challenging. Attached-growth processes may allow for competitive footprint requirements.

Removal of antibiotics and their impact on growth, nutrient uptake, and biomass productivity in semi-continuous cultivation of Auxenochlorella protothecoides

The prevalence of antibiotics in wastewater poses risks to human and animal health, contributing to antimicrobial resistance. Although various antibiotic removal methods exist, microalgae-based technology presents a cost-effective and eco-friendly alternative; however, limited research on its long-term integration in semi-continuous wastewater treatment trials hinders our understanding of its potential effectiveness. This investigation explored the antibiotic removal capabilities of the microalga Auxenochlorella protothecoides in photobioreactors with synthetic wastewater under semi-continuous conditions over one month. Additionally, the study assessed the impact of seven commonly used antibiotics (ciprofloxacin, clarithromycin, erythromycin, metronidazole, ofloxacin, sulfamethoxazole, and trimethoprim) on the microalgal system regarding growth, nutrient removal, and biomass productivity. The microalga effectively removed antibiotics, achieving maximum removal efficiencies ranging from 45.8% to 70.1% over 3-4 days of exposure. Remarkably, antibiotics stimulated algal growth, resulting in an 11.0% increase in biomass. Nutrient removal also improved significantly; ammonium removal rose from 78.0% to 86.4%, and phosphate removal increased from 85.1% to 90.3%. Furthermore, the biomass composition showed notable enhancements, with increases in pigments (12.9%), lipids (20.6%), proteins (45.8%), and carbohydrates (50.6%). These findings highlight the potential applicability of A. protothecoides as a valuable addition to conventional wastewater treatment plants. The study emphasises the importance of considering antibiotic presence in microalgae-based wastewater treatment technologies, as these compounds can have a stimulatory effect that enhances both growth and nutrient removal efficiency. Overall, this research contributes to the development of more effective strategies for managing antibiotic pollution in wastewater.

Overestimated role of inoculation bacteria-algae ratio in wastewater treatment

Microalgae-bacteria systems present a promising approach for CO2 reduction in wastewater treatment. The effect of inoculation bacteria-algae ratio on performance was investigated in this study. Different inoculation ratios (bacteria: algae 1:2, 1:1, 1:0.5, 1:0.25, 1:0.125, w/w) obtained comparable nutrients removal (p > 0.05). Over time, the bacteria-algae ratios converged into two groups (3:1 and 4:1), demonstrating self-adaption between bacteria and microalgae. Furthermore, principal component analysis (PCA) distinguished the performance of reactors into two groups, one group consisting of 1:2, 1:1, and 1:0.5 ratios and the other group consisting of 1:0.25 and 1:0.125 ratios, confirming their convergence in terms of nutrient removal and photosynthetic properties. The performance differed merely in sludge volume index (SVI) and nitrite accumulation, with 1:2 and 1:0.125 being the most prone to accumulate nitrite. This study implies that photobioreactor performance was not sensitive to inoculation ratio, whose role was overestimated, since microalgae and bacteria self-assemble to form niches. PRACTITIONER POINTS: Effect of inoculation bacteria-algae ratio on performance was overestimated Photosynthesis and nutrients removal were grouped at different inoculation ratios Different ratio showed similar nutrients removal efficiency Self-adaption made ratios of 1:2, 1:1, 1:0.5 converge into 3:1.

Harnessing diurnal dynamics: Understanding the influence of light-dark cycle on algal-bacterial symbiotic system under aniline stress

To assess the inherent effects of light-dark cycle on the aniline degradation and nitrogen removal in algal-bacterial symbiotic system, three groups with different photoperiods (0L:12D;6L:6D;12L:0D) were set up. The results revealed that the aniline degradation rate of the three systems all surpassed 99 %, the total nitrogen removal rate of Z2-6L:6D was approximately 36 % higher than Z1-0L:12D eventually, the Z1-0L:12D was restrained by NH4+-N assimilation and nitrification while anoxic denitrification in Z3-12L:0D. The disappearance of microalgae biomass was accompanied by the sharp decreased of polysaccharide in Z1 and longer illumination suppressed the secretion of extracellular polymeric substances, the Z3 yielded slightly superior biomass production despite the double illumination compared with Z2. Moreover, high throughput sequencing analysis illustrated that the microbial community structure in Z2 was more abundant and even than Z3, the TM7a, norank_f__norank_o__Saccharimonadales, Ellin6067 and Scenedesmus proliferated wildly and the photoinhibition to functional genus was effectively alleviated in Z2.

The response to shock loads of Ni-MOF and NiO NPs on aerobic granular sludge and algal-bacterial aerobic granular sludge

Currently, the increasing use of nickel metal-organic frameworks (Ni-MOF) and nickel oxide nanoparticles (NiO NPs) has raised concerns regarding their potential environmental impact on wastewater treatment systems. Herein, the responses of aerobic granular sludge (AGS) and algal-bacterial aerobic granular sludge (AB-AGS) to Ni-MOF and NiO NPs were investigated. The results showed that Ni-MOF concentrations of 50, 100, and 200 mg/L significantly reduced nutrient removal in both systems, particularly affecting ammonia, nitrite, and phosphorus removal, while denitrification processes remained stable. AB-AGS exhibited greater tolerance to nickel than AGS, likely due to its higher content of extracellular polymeric substances (EPSs), in which the algae were embedded, indicating a robust bacterial-algal symbiotic system. Conversely, NiO NPs had no adverse effects on bioreactor performance, likely due to their insolubility and integration into the sludge matrix. This research provides valuable insights into the potential future applications of AGS and AB-AGS technologies for treating wastewater contaminated with nickel and other heavy metals, highlighting the superior resilience of AB-AGS to nickel exposure.

Insight into recent advances in microalgae biogranulation in wastewater treatment

Microalgae-based technology is widely utilized in wastewater treatment and resource recovery. However, the practical implementation of microalgae-based technology is hampered by the difficulty in separating microalgae from treated water due to the low density of microalgae. This review is designed to find the current status of the development and utilization of microalgae biogranulation technology for better and more cost-effective wastewater treatment. This review reveals that the current trend of research is geared toward developing microalgae-bacterial granules. Most previous works were focused on studying the effect of operating conditions to improve the efficiency of wastewater treatment using microalgae-bacterial granules. Limited studies have been directed toward optimizing operating conditions to induce the secretion of extracellular polymeric substances (EPSs), which promotes the development of denser microalgae granules with enhanced settling ability. Likewise, studies on the understanding of the EPS role and the interaction between microalgae cells in forming granules are scarce. Furthermore, the majority of current research has been on the cultivation of microalgae-bacteria granules, which limits their application only in wastewater treatment. Cultivation of microalgae granules without bacteria has greater potential because it does not require additional purification and can be used for border applications.

Optimal algae species inoculation strategy for algal-bacterial granular sludge: Sludge characteristics, performance and microbial community

The algal-bacterial granular sludge (ABGS) system is emerging as a promising technology for future wastewater treatment. This study assessed the impact of different algae species inoculation on granulation, performance, and microbial communities within ABGS systems. The experimental setup included single-species inoculations (Chlorella sp. (R1), Scenedesmus sp. (R2), and Desmodesmus sp. (R3)) and a mixed-species inoculation strategy (R4). Results revealed that R4 achieved the fastest completed granulation process (15 days) with the largest average granule diameter (772.93 μm) and highest physical strength (2.24 ± 0.26%) in the end of the experiment. The relative abundance of extracellular polymeric substances secreting bacteria of R4 maintained high level in whole operation time. Algae assimilation capacity and the abundance of functional bacteria can also influence removal performance. In mature stage, only the average effluent total nitrogen (3.15 ± 2.87 mg/L), total phosphorus (0.37 ± 0.27 mg/L), chemical oxygen demand (25.25 ± 2.98 mg/L) concentration in R4 was lower than that of Grade I discharge standard of municipal wastewater treatment plants in China. The best inorganic carbon utilization and lipid production ability were observed in R4 and R3, respectively. The choice of inoculated algae species was identified as a key factor for bacterial community dynamics. Overall, above results demonstrated that mixed algae species inoculation can be selected as the optimal algae inoculation strategy due to its excellent granulation, performance, and acceptable carbon utilization and lipid production.

Interaction between microalgae and phycosphere bacteria in a binary cultivation system-based dairy farm wastewater treatment

The combination of microalgal culture and wastewater treatment is an emerging topic. This study investigated the use of different microalgae to treat different types of dairy farm wastewater. The results showed that the removal of ammonia nitrogen and total phosphorus by mixed microalgae was over 99% and 80%, respectively. The highest production of protein in biomass and extracellular polymeric substances was observed in high-concentration wastewater. In the phycosphere, the abundance of Proteobacteria and Cyanobacteria increased, while that of Bacteroidota decreased. Phycosphere bacteria were strongly correlated with microalgal growth and the composition of extracellular polymeric substances, especially with bound extracellular polymeric substances relative to soluble extracellular polymeric substances. Genes associated with photosynthesis and respiration in phycosphere bacteria were upregulated, contributing to the material exchange capacity in the microalgal-bacterial systems. The interaction between microalgae and phycosphere bacteria thus represents the core of the binary cultivation system-based wastewater treatment and requires further investigation.

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.

Mini critical review: Membrane fouling control in membrane bioreactors by microalgae

Membrane bioreactors (MBRs) hold significant promise for wastewater treatment, yet the persistent challenge of membrane fouling impedes their practical application. One promising solution lies in the synergy between microalgae and bacteria, offering efficient nutrient removal, reduced energy consumption, and potential mitigation of extracellular polymeric substances (EPS) concentrations. Inoculating microalgae presents a promising avenue to address membrane fouling in MBRs. This review marks the first exploration of utilizing microalgae for membrane fouling control in MBR systems. The review begins with a comprehensive overview of the evolution and distinctive traits of microalgae-MBRs. It goes further insight into the performance and underlying mechanisms facilitating the reduction of membrane fouling through microalgae intervention. Moreover, the review not only identifies the challenges inherent in employing microalgae for membrane fouling control in MBRs but also illuminates prospective pathways for future advancement in this burgeoning field.

A systematic review of antibiotics and antibiotic resistance genes (ARGs) in mariculture wastewater: Antibiotics removal by microalgal-bacterial symbiotic system (MBSS), ARGs characterization on the metagenomic

Antibiotic residues in mariculture wastewater seriously affect the aquatic environment. Antibiotic Resistance Genes (ARGs) produced under antibiotic stress flow through the environment and eventually enter the human body, seriously affecting human health. Microalgal-bacterial symbiotic system (MBSS) can remove antibiotics from mariculture and reduce the flow of ARGs into the environment. This review encapsulates the present scenario of mariculture wastewater, the removal mechanism of MBSS for antibiotics, and the biomolecular information under metagenomic assay. When confronted with antibiotics, there was a notable augmentation in the extracellular polymeric substances (EPS) content within MBSS, along with a concurrent elevation in the proportion of protein (PN) constituents within the EPS, which limits the entry of antibiotics into the cellular interior. Quorum sensing stimulates the microorganisms to produce biological responses (DNA synthesis - for adhesion) through signaling. Oxidative stress promotes gene expression (coupling, conjugation) to enhance horizontal gene transfer (HGT) in MBSS. The microbial community under metagenomic detection is dominated by aerobic bacteria in the bacterial-microalgal system. Compared to aerobic bacteria, anaerobic bacteria had the significant advantage of decreasing the distribution of ARGs. Overall, MBSS exhibits remarkable efficacy in mitigating the challenges posed by antibiotics and resistant genes from mariculture wastewater.

A review of emerging membrane-based microalgal-bacterial processes for wastewater treatment: Process configurations, biological and membrane performance, and perspectives

Microalgal-bacterial (MB) consortia create an excellent eco-system for simultaneous COD/BOD and nutrients (N and P) removals in a single step with significant reduction in or complete elimination of aeration and carbonation in the biological wastewater treatment processes. The integration of membrane separation technology with the MB processes has created a new paradigm for research and development. This paper focuses on a comprehensive and critical literature review of recent advances in these emerging processes. Novel membrane process configurations and process conditions affecting the biological performance of these novel systems have been systematically reviewed and discussed. Membrane fouling issues and control of MB biofilm formation and thickness associated with these emerging suspended growth or immobilized biofilm processes are addressed and discussed. The research gaps, challenges, outlooks of these emerging processes are identified and discussed in-depth. The findings from the literature suggest that the membrane-based MB processes are advanced biotechnologies with a significant reduction in energy consumption and process simplification and high process efficiency that are not achievable with current technologies in wastewater treatment. There are endless opportunities for research and development of these novel and emerging membrane-based MB processes.

Unveiling the role of ferrous ion in driving microalgae granulation from salt-tolerant strains for mariculture wastewater treatment

Development of microalgal-bacterial granular sludge (MBGS) from saline-adapted microalgae is a promising approach for efficient mariculture wastewater treatment, whereas the elusive mechanisms governing granulation have impeded its widespread adoption. In this study, spherical and regular MBGS were successfully developed from mixed culture of pure Spirulina platensis and Chlorella sp. GY-H4 at 10 mg/L Fe2+ concentration. The addition of Fe2+ was proven to induce the formation of Fe-precipitates which served as nucleation sites for microbial attachment and granulation initiation. Additionally, Fe2+ increased the prevalence of exopolysaccharide-producing cyanobacteria, i.e. Synechocystis and Leptolyngbya, facilitating microbial cell adhesion. Furthermore, it stimulated the secretion of extracellular proteins (particularly tryptophan and aromatic proteins), which acted as structural backbone for the development of spherical granule form microalgal flocs. Lastly, it fostered the accumulation of exogenous heterotrophic functional genera, resulting in the efficient removal of DOC (98 %), PO43--P (98 %) and NH4+-N (87 %). Nevertheless, inadequate Fe2+ hindered microalgal floc transformation into granules, excessive Fe2+ expanded the anaerobic zone within the granules, almost halved protein content in the TB-EPS, and inhibited the functional genes expression, ultimately leading to an irregular granular morphology and diminished nutrient removal. This research provides valuable insights into the mechanisms by which Fe2+ promotes the granulation of salt-tolerant microalgae, offering guidance for the establishment and stable operation of MBGS systems in mariculture wastewater treatment.

Allelochemicals-mediated interaction between algae and bacteria: Direct and indirect contact

Secondary metabolites with bioactivity are allelochemicals. This study adopted direct contact (R0) and indirect contact (separated by 0.45 µm membrane, R1-A for algae, R1-S for sludge) to reveal the role of metabolites especially allelochemicals on interaction of bacteria and algae. Direct contact exhibited better nutrients removal than indirect contact, due to less antibacterial allelochemicals and oxidative stress. Bacterial signaling molecules were not detected. The major algae-derived allelochemicals were 13-Docosenamide, 9-Octadecenamide, n-Hexadecanoic acid, erucic acid, octadecanoic acid, β-sitosterol, and E,E,Z-1,3,12-Nonadecatriene-5,14-diol. Furthermore, presence of 13-Docosenamide and 9-Octadecenamide was associated with succession of Flavobacterium and suppression of nitrifying bacteria (Nitrosomonas, Ellin6067, and Nitrospira). Direct contact stimulated denitrifying bacteria Saccharimonadales and algae Scenedesmus, whereas indirect contact is friendly to Dechloromonas, Competibacter, nitrifying bacteria, algae Desmodesmus and Dictyosphaerium. This study highlights the essentiality of cell contact of bacteria-algae in establishing synergy, as cell contact mitigates antagonistic effect induced by metabolites.

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.

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.

Revealing mechanisms of triclosan on the removal and distribution of nitrogen and phosphorus in microalgal-bacterial symbiosis system

Microalgal-bacterial symbiosis (MABS) system performs synergistic effect on the reduction of nutrients and carbon emissions in the water treatment process. However, antimicrobial agents are frequently detected in water, which influence the performance of MABS system. In this study, triclosan (TCS) was selected to reveal the effects and mechanisms of antimicrobial agents on MABS system. Results showed that the removal efficiencies of chemical oxygen demand, NH4+-N and total phosphorus decreased by 3.0%, 24.0% and 14.3% under TCS stress. In contrast, there were no significant decrease on the removal effect of total nitrogen. Mechanism analysis showed that both the growth rate of microorganisms and the nutrients retention capacity of extracellular polymeric substances were decreased. The intracellular accumulation for nitrogen and phosphorus was promoted due to the increased cytomembrane permeability caused by lipid peroxidation. Moreover, microalgae were dominant in MABS system with ratio between microalgae and bacteria of more than 5.49. The main genus was Parachlorella, with abundance of more than 90%. Parachlorella was highly tolerant to TCS, which might be conductive to maintain its survival. This study revealed the nutrients pathways of MABS system under TCS stress, and helped to optimize the operation of MABS system.

Photosynthetic oxygen-supported algal-bacterial aerobic granular sludge can facilitate carbon, nitrogen and phosphorus removal from wastewater: Focus on light intensity selection

Photosynthetic O2 is a promising alternative for mechanical aeration, the major energy-intensive unit in wastewater treatment plants. This study aimed to investigate the effects of light intensity varied from 190 to 1400 µmol·s-1·m-2 on photosynthetic O2-supported algal-bacterial aerobic granular sludge (AGS) system. Results indicate photosynthetic O2 can implement aerobic phosphorus (P) uptake and ammonia oxidation under the test illumination range even at dissolved oxygen concentration < 0.5 mg/L. An obvious O2 accumulation occurred after 60-90% nutrients being removed under 330-1400 µmol·s-1·m-2, and highly efficient ammonia removal, P uptake, and dissolved inorganic carbon removal were achieved under 670-1400 µmol·s-1·m-2. On the other hand, photosynthesis as O2 supplier showed little effect on major ions except for K+. This study provides a better understanding of the roles of light intensity on photosynthetic O2-supported algal-bacterial AGS system, targeting a sustainable wastewater industry.

Effects of light intensity and salinity on formation and performance of microalgal-bacterial granular sludge

Photosynthetic microorganisms in microalgal-bacterial granular sludge offer advantages in wastewater treatment processes. This study examined the effects of light intensity and salinity on microalgal-bacterial granular sludge formation and microbial changes. Activated sludge was inoculated into three bioreactors and operated in batch treatment mode for 100 days under different light intensities (0, 60, and 120 μmol m-2 s-1) and staged increases in salinity concentration (0, 1, 2, and 3%). Results showed that microalgal-bacterial granular sludge was successfully formed within 30 days, and high light exposure increased algal particle stability and inorganic nitrogen removal (63, 66, 71%), while chemical oxygen demand removal (>95%) was similar across groups. High-throughput sequencing results showed that the critical algae were Chlorella and diatoms, while the main bacteria included Paracoccus and Xanthomarina with high extracellular polymeric substance production. This study aims to enhance the comprehension of MBGS processes in saline wastewater treatment under varying light intensities.

A novel membrane bioreactor inoculated with algal-bacterial granular sludge for sewage reuse and membrane fouling mitigation: Performance and mechanism

In this study, a novel membrane bioreactor (MBR) inoculated with algal-bacterial granular sludge (ABGMBR) was established to improve pollutant removal and alleviate membrane fouling. The ABGMBR system showed higher pollutant removal rate and longer operation time (152 day) compared to the control MBR (AGMBR). Moreover, the contents of the pollutants such as granular sludges, extracellular polymeric substances (EPS), and soluble microbial products on the membrane were remarkably reduced, leading to the formation of a porous and loose cake layer on the membrane and a slow increase in transmembrane pressure. Standard blocking was the main mechanism of membrane fouling; however, the membrane pore blockage was significantly reduced in ABGMBR. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory suggested that the aggregation and adhesion of foulants on the membrane were greatly inhibited in ABGMBR. Furthermore, correlation analysis showed significant differences in membrane fouling characteristics between AGMBR and ABGMBR. The ABGMBR system effectively retarded sludge disintegration and increased the repulsion between the sludge and membrane owing to the favorable mixed liquor characteristics. This study showcases the superior operational efficiency and anti-fouling performance of ABGMBR, offering a novel perspective on sewage reuse and membrane fouling mitigation.

The role of light wavelengths in regulating algal-bacterial granules formation, protein and lipid accumulation, and microbial functions

High value-added products recovery from algal-bacterial granular sludge (ABGS) has received great attention recently. This study aimed to explore the role of different light wavelengths in regulating granule formation, protein and lipid production, and microbial functions. Bacterial granular sludge (BGS, R0) was most conducive to forming ABGS under blue (R2) light with the highest chlorophyll a (10.2 mg/g-VSS) and diameter (1800 μm), followed by red (R1) and white (R3) lights. R0-R3 acquired high protein contents (>164.8 mg/g-VSS) with essential amino acids above 44.4%, all of which were suitable for recycling, but R2 was the best. Also, blue light significantly increased total lipid production, while red light promoted the accumulation of some unsaturated fatty acids (C18:2 and C18:3). Some unique algae and dominant bacteria (e.g., Stigeoclonium, Chlamydomonas, and Flavobacteria) enrichment and some key functions (e.g., amino acid, fatty acid, and lipid biosynthesis) up-regulation in R2 might help to improve proteins and lipids quality. Combined, this study provides valuable guidance for protein and lipid recovery from ABGS.

Insight into the roles of microalgae on simultaneous nitrification and denitrification in microalgal-bacterial sequencing batch reactors: Nitrogen removal, extracellular polymeric substances, and microbial communities

This study explored the influence and mechanism of microalgae on simultaneous nitrification and denitrification (SND) in microalgal-bacterial sequencing batch reactors (MB-SBR). It particularly focused on nitrogen transformation in extracellular polymeric substances (EPS) and functional groups associated with nitrogen removal. The results showed that MB-SBR achieved more optimal performance than control, with an SND efficiency of 68.01% and total nitrogen removal efficiency of 66.74%. Further analyses revealed that microalgae changed compositions and properties of EPS by increasing EPS contents and improving transfer, conversion, and storage capacity of nitrogen in EPS. Microbial community analysis demonstrated that microalgae promoted the enrichment of functional groups and genes related to SND and introduced diverse nitrogen removal pathways. Moreover, co-occurrence network analysis elucidated the interactions between communities of bacteria and microalgae and the promotion of SND by microalgae as keystone connectors in the MB-SBR. This study provides insights into the roles of microalgae for enhanced SND.

Facile approach for nanoconfinement of multilayer graphene oxide with polyether polyurethane sponge as biological carrier for the establishment of microalgal-bacterial bioreactor

Physically precise and mechanically robust biocarrier is basic and urgent requirement of algal-bacterial wastewater treatment plants for homogenously biofilm growth. Herein, a highly efficient graphene oxide (GO) coordinated polyether polyurethane (PP) sponge was synthesized through GO incorporation into PP sponge to improve the GO coating, followed by UV-light treatment for industrial application. The resulted sponge showed remarkable physiochemical characteristics, excellent thermal (>0.02 Wm^-1 K^-1) and mechanical (>363.3 KPa) stability. To test the potential of sponge in real world scenarios, the activated sludge from real wastewater treatment plant was utilized. Interestingly, the GO-PP sponge enhanced the electron transfer between microorganisms and promoted the standardized microorganism's growth and biofilm formation (22.7 mg/d per gram sponge, 172.1 mg/g), providing the feasibility to accomplish a symbiotic system within specifically design upgraded algal-bacterial reactor. Furthermore, the continuous flow process by utilizing GO-PP sponge in algal-bacterial reactor demonstrated the effectiveness in treating low concentration antibiotic wastewater, presenting 86.7 % removal rate and >85 % after 20 cycles. Overall, this work illustrates an applicable strategy to develop a sophisticated modified pathway for the next-generation biological-based applications.

Enhancing phosphorus removal of photogranules by incorporating polyphosphate accumulating organisms

Photogranules are a novel wastewater treatment technology that can utilize the sun's energy to treat water with lower energy input and have great potential for nutrient recovery applications. They have been proven to efficiently remove nitrogen and carbon but show lower conversion rates for phosphorus compared to established treatment systems, such as aerobic granular sludge. In this study, we successfully introduced polyphosphate accumulating organisms (PAOs) to an established photogranular culture. We operated photobioreactors in sequencing batch mode with six cycles per day and alternating anaerobic (dark) and aerobic (light) phases. We were able to increase phosphorus removal/recovery by 6 times from 5.4 to 30 mg/L/d while maintaining similar nitrogen and carbon removal compared to photogranules without PAOs. To maintain PAOs activity, alternating anaerobic feast and aerobic famine conditions were required. In future applications, where aerobic conditions are dependent on in-situ oxygenation via photosynthesis, the process will rely on sunlight availability. Therefore, we investigated the feasibility of the process under diurnal cycles with a 12-h anaerobic phase during nighttime and six short cycles during the 12 h daytime. The 12-h anaerobic phase had no adverse effect on the PAOs and phototrophs. Due to the extension of one anaerobic phase to 12 h the six aerobic phases were shortened by 47% and consequently decreased the light hours per day. This resulted in a decrease of phototrophs, which reduced nitrogen removal and biomass productivity up to 30%. Finally, we discuss and suggest strategies to apply PAO-enriched photogranules at large-scale.

Algal-bacterial aerobic granular sludge for real municipal wastewater treatment: Performance, microbial community change and feasibility of lipid recovery

Despite various research works on algal-bacterial aerobic granular sludge for wastewater treatment and resource recovery processes, limited information is available on its application in real wastewater treatment in terms of performance, microbial community variation and resource recovery. This study investigated the performance of algal-bacterial aerobic granular sludge on real low-strength wastewater treatment in addition to the characterization of microbial community and fatty acid compositions for biodiesel production. The results demonstrated 71% COD, 77% NH₄⁺-N and 31% phosphate removal efficiencies, respectively. In addition, all the water parameters successfully met the effluent standard A, imposed by the Department of Environment (DOE) Malaysia. Core microbiome analyses revealed important microbial groups (i.e., Haliangium ochraceum, Burkholderiales and Chitinophagaceae) in bacterial community. Meanwhile the photosynthetic microorganisms, such as Oxyphotobacteria and Trebouxiophyceae dominated the algal-bacterial aerobic granular sludge, suggesting their important roles in granulation and wastewater treatment. Up to 12.51 mg/gSS lipid content was recovered from the granules. In addition, fatty acids composition showed high percetages of C16:0 and C18:0, demonstrating high feasibility to be used for biodiesel production application indicated by the cetane number, iodine value and oxidation stability properties.

Strategy for the formation of microalgae-bacteria aggregates in high-rate algal ponds

This work studied the formation of aggregates used for wastewater treatment in high-rate algal ponds (HRAP). For this, the establishment of microalgae-bacteria aggregates in these systems was evaluated, considering strategies for the inoculation and start-up. Two HRAP were operated in parallel, at first in batch mode and then in continuous flow. The wastewater treatment was efficient, with removal rates around 80% for COD and N-ammoniacal. Volatile suspended solids and chlorophyll for the culture grew continuously reached a concentration of 548 ± 11 mg L^-1 and 7.8 mg L^-1, respectively. Larger photogranules were observed when the system was placed in a continuous regime. The protein fraction of extracellular polymeric substances was identified as a determinant in photogranules formation. During the continuous regime, more than 50% of the biomass was higher than 0.2 mm, flocculation efficiency of 78 ± 6%, and the volumetric sludge index of 32 ± 5 mL g^-1. The genetic sequencing showed the growth of cyanobacteria in the aggregate and the presence of microalgae from the chlorophytes and diatoms groups in the final biomass.

Highly efficient carbon assimilation and nitrogen/phosphorus removal facilitated by photosynthetic O2 from algal-bacterial aerobic granular sludge under controlled DO/pH operation

Reducing CO₂ emission and energy consumption is crucial for the sustainable management of wastewater treatment plants (WWTPs). In this study, an algal-bacterial aerobic granular sludge (AGS) system was developed for efficient carbon (C) assimilation and nitrogen (N)/phosphorus (P) removal without the need for mechanical aeration. The photosynthetic O₂ production by phototrophic organisms maintained the dissolved oxygen (DO) level at 3-4 mg/L in the bulk liquid, and an LED light control system reduced 10-30% of light energy consumption. Results showed that the biomass assimilated 52% of input dissolved total carbon (DTC), and the produced O₂ simultaneously facilitated aerobic nitrification and P uptake with the coexisting phototrophs serving as a C fixer and O₂ supplier. This resulted in a stably high total N removal of 81 ± 7% and an N assimilation rate of 7.55 mg/(g-MLVSS∙d) with enhanced microbial assimilation and simultaneous nitrification/denitrification. Good P removal of 92-98% was maintained during the test period at a molar ∆P/∆C ratio of 0.36 ± 0.03 and high P release and uptake rates of 10.84 ± 0.41 and 7.18 ± 0.24 mg/(g- MLVSS∙h), respectively. Photosynthetic O₂ was more advantageous for N and P removal than mechanical aeration. This proposed system can contribute to a better design and sustainable operation of WWTPs using algal-bacterial AGS.

Bioremediation of coastal aquaculture effluents spiked with florfenicol using microalgae-based granular sludge - a promising solution for recirculating aquaculture systems

Aquaculture is a crucial industry in the agri-food sector, but it is linked to serious environmental problems. There is a need for efficient treatment systems that allow water recirculation to mitigate pollution and water scarcity. This work aimed to evaluate the self-granulation process of a microalgae-based consortium and its capacity to bioremediate coastal aquaculture streams that sporadically contain the antibiotic florfenicol (FF). A photo-sequencing batch reactor was inoculated with an autochthonous phototrophic microbial consortium and was fed with wastewater mimicking coastal aquaculture streams. A rapid granulation process occurred within ca. 21 days, accompanied by a substantially increase of extracellular polymeric substances in the biomass. The developed microalgae-based granules exhibited high and stable organic carbon removal (83-100%). Sporadically wastewater contained FF which was partially removed (ca. 5.5-11.4%) from the effluent. In periods of FF load, the ammonium removal slightly decreased (from 100 to ca. 70%), recovering 2 days after FF feeding ceased. A high-chemical quality effluent was obtained, complying with ammonium, nitrite, and nitrate concentrations for water recirculation within a coastal aquaculture farm, even during FF feeding periods. Members belonging to the Chloroidium genus were predominant in the reactor inoculum (ca. 99%) but were replaced from day-22 onwards by an unidentified microalga from the phylum Chlorophyta (>61%). A bacterial community proliferated in the granules after reactor inoculation, whose composition varied in response to feeding conditions. Bacteria from the Muricauda and Filomicrobium genera, Rhizobiaceae, Balneolaceae, and Parvularculaceae families, thrived upon FF feeding. This study demonstrates the robustness of microalgae-based granular systems for aquaculture effluent bioremediation, even during periods of FF loading, highlighting their potential as a feasible and compact solution in recirculation aquaculture systems.

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.

Effect of biomass retention time on performance and fouling of a stirred membrane photobioreactor

Co-culture of microalgae-activated sludge has the potential to purify wastewater while reduce energy demand from aeration. In this work, a mechanically stirred membrane photobioreactor (stirred-MPBR) was used to evaluate the impact of the biomass retention time (BRT) on the treatment performance and membrane fouling. Results showed that stirred-MPBR was affected by BRT during treating domestic wastewater at a flux of 16.5 L m^-2 h^-1. The highest productivity was attained at BRT 7d (102 mg L^-1 d^-1), followed by BRT 10d (86 mg L^-1 d^-1), BRT 5d (85 mg L^-1 d^-1), and BRT 3d (83 mg L^-1 d^-1). Statistical analysis results showed that BRT 7d had a higher COD removal rate than BRT 10d, however, there is no difference in total nitrogen removal rate. The highest TP removal occurred when the biomass operated at BRT as short as 3d. Reduced BRTs caused a change in the microalgae-activated sludge biomass fraction that encouraged nitrification activity while simultaneously contributing to a higher fouling rate. The bound protein concentrations dropped from 31.35 mg L^-1 (BRT 10d) to 10.67 mg L^-1 (BRT 3d), while soluble polysaccharides increased from 0.99 to 1.82 mg L^-1, respectively. The concentrations of extracellular polymeric substance fractions were significantly altered, which decreased the mean floc size and contributed to the escalating fouling propensity. At the optimum BRT of 7d, the stirred-MPBR showed sufficient access to light and nutrients exchange for mutualistic interactions between the microalgae and activated sludge.

Insights into nitrogen and phosphorus metabolic mechanisms of algal-bacterial aerobic granular sludge via metagenomics: Performance, microbial community and functional genes

Aiming to propose the potential mechanism for the enhancement of nitrogen (N) and phosphorus (P) removal of algal-bacterial aerobic granular sludge (A-AGS), metagenomic analysis was applied to identify the metabolic pathways. The results showed that chemical oxygen demand, ammonia nitrogen, total N, and total P removal of A-AGS could reach to 94.5%, 97.5%, 78.1%, and 88.5%, respectively. Algae enriched the content of extracellular polymeric substance, which significantly promoted the formation of A-AGS. Further investigations in functional genes suggested that nitrification process (amo, nxr, hao, etc.), denitrification process (nir, nap, nor, etc.), and polyphosphate accumulation (ppk, ppk2, etc.) were enhanced greatly in A-AGS. Notably, genus Thauera was the dominant source of functional genes, which penetrated both in N and P metabolism. The higher N and P removal performance in A-AGS could be attributed to synergistic effect between bacteria and microalgae, which may provide the basic for the application in wastewater treatment.

Microalgae-bacterial biomass outperforms PN-anammox biomass for oxygen saving in continuous-flow granular reactors facing extremely low-strength freshwater aquaculture streams

The dissolved oxygen (DO) concentration in water streams is one of the most important and critical quality parameters in aquaculture farms. The main objective of this study was to evaluate the potential of two Continuous Flow Granular Reactors, one based on Partial Nitrification-Anammox biomass (Aquammox CFGR) and the other on Microalgae-Bacteria biomass (AquaMab CFGR), for improving dissolved oxygen availability in the recirculation aquaculture systems (RAS). Both reactors treated the extremely low-strength effluents from a freshwater trout farm (1.39 mg NH₄⁺-N/L and 7.7 mg TOC/L). The Aquammox CFGR, removed up to 68% and 100% of ammonium and nitrite, respectively, but the DO concentration in the effluent was below 1 mg O₂/L while the anammox activity was not maintained. In the AquaMab CFGR, bioaugmentation of aerobic granules with microalgae was attained, producing an effluent with DO concentrations up to 9 mg O₂/L and removed up to 77% and 80% of ammonium and nitrite, respectively, which is expected to reduce the aeration costs in fish farms.

Effect of fungal pellets on denitrifying bacteria at low carbon to nitrogen ratio: Nitrate removal, extracellular polymeric substances, and potential functions

This research aims to elucidate the effect of fungal pellets (FP) on denitrifying bacteria regarding nitrate (NO₃^--N) removal, extracellular polymeric substances (EPS), and potential functions at a low carbon to nitrogen (C/N) ratio. A symbiotic system of FP and denitrifying bacteria GF2 was established. The symbiotic system showed 100% NO₃^--N removal efficiency (4.07 mg L^-1 h^-1) at 6 h and enhanced electron transfer capability at C/N = 1.5. The interactions between FP and denitrifying bacteria promoted the production of polysaccharides (PS) in EPS. Both the increased PS and the PS provided by FP as well as protein and humic acid-like substances in EPS could be consumed by denitrifying bacteria. FP acted as a protector and provided habitat and nutrients for denitrifying bacteria as well as improved the ability of carbohydrate metabolism, amino metabolism, and nitrogen metabolism of denitrifying bacteria. This study provides a new perspective on the relationship between FP and denitrifying bacteria.

Understanding the influence of free nitrous acid on microalgal-bacterial consortium in wastewater treatment: A critical review

Microalgal-bacterial consortium (MBC) constitutes a sustainable and efficient alternative to the conventional activated sludge process for wastewater treatment (WWT). Recently, integrating the MBC process with nitritation (i.e., shortcut MBC) has been proposed to achieve added benefits of reduced carbon and aeration requirements. In the shortcut MBC system, nitrite or free nitrous acid (FNA) accumulation exerts antimicrobial influences that disrupt the stable process performance. In this review, the formation and interactions that influence the performance of the MBC were firstly summarized. Then the influence of FNA on microalgal and bacterial monocultures and related mechanisms together with the knowledge gaps of FNA influence on the shortcut MBC were highlighted. Other challenges and future perspectives that impact the scale-up of the shortcut MBC for WWT were illustrated. A potential roadmap is proposed on how to maximize the stable operation of the shortcut MBC system for sustainable WWT and high-value biomass production.

Energy saving and rapid establishment of granular microalgae system from tiny microalgae cells: Effect of decrease in upflow air velocity under intermittent aeration condition

The novel type of microalgae granules (MGs) derived from tiny microalgae cells has received extensive attention due to its great potential for nutrient remediation and resource recovery in wastewater treatment whereas the long start-up time with increased labor expenses remains a bottleneck. In this study, an operation strategy at reduced upflow air velocity (UAV = 0.49 cm/s in R_A) under intermittent aeration mode was proposed and compared with R_B at a higher UAV (0.98 cm/s) in terms of MGs formation, maintenance, and energy consumption. Although the formation of MGs in R_A was delayed for 12 days compared to R_B, 40.78 % increase in chlorophyll-a content was detected in MGs in R_A along with more cost-effective carbon, nitrogen, and phosphorus removals due to efficient microalgae assimilation and energy reduction. Results from this study provide new insight into minimizing energy input for rapid establishment and stable operation of MG systems towards environmentally sustainable wastewater management.

Current Progress, Challenges and Perspectives in the Microalgal-Bacterial Aerobic Granular Sludge Process: A Review

Traditional wastewater treatment technologies have become increasingly inefficient to meet the needs of low-consumption and sustainable wastewater treatment. Researchers are committed to seeking new wastewater treatment technologies, to reduce the pressure on the environment caused by resource shortages. Recently, a microalgal-bacterial granular sludge (MBGS) technology has attracted widespread attention due to its high efficiency wastewater treatment capacity, low energy consumption, low CO₂ emissions, potentially high added values, and resource recovery capabilities. This review focused primarily on the following aspects of microalgal-bacterial granular sludge technology: (1) MBGS culture and maintenance operating parameters, (2) MBGS application in different wastewaters, (3) MBGS additional products: biofuels and bioproducts, (4) MBGS energy saving and consumption reduction: greenhouse gas emission reduction, and (5) challenges and prospects. The information in this review will help us better understand the current progress and future direction of the MBGS technology development. It is expected that this review will provide a sound theoretical basis for the practical applications of a MBGS technology in environmentally sustainable wastewater treatment, resource recovery, and system optimization.

Insights into activated sludge/Chlorella consortia under dark condition compared with light condition

Bacteria-algae consortia in the light bring the benefit of O₂ production and CO₂ reduction for wastewater treatment, while the bottleneck for application is how it behaves in the dark. In this study, inoculum ratio and sludge retention time (SRT) affected nutrient removal rather than chemical oxygen demand (COD) removal. Dark conditions (with a sludge/Chlorella inoculum ratio of 1:2 at a SRT of 15 d) achieved comparable performance to those of light conditions, due to bacteria contribution and mechanical aeration. Compared with light conditions, the ratio of Chla/Chlb decreased and Caro/(Chla + Chlb) increased to response oxidative stress. In the dark, algae were associated with Nitrosomonas and Dechloromonas. Flavobacterium disassociated with Chlorella in the dark but associated with Chlorella in the light. Moreover, nitritation genes (amo and Hao) and denitrifying gene (narH) were up-regulated, while P metabolism genes (PPX and PPK) were down-regulated. It is proposed to enrich Nitrosomonas in the night and denitrify polyphosphate accumulating organisms (DPAO) in the daytime to establish short-cut nitrification and denitrifying phosphorus removal in practical applications.

Future trends and patterns in leachate biological treatment research from a bibliometric perspective

Leachate has become a great deal of concern due to its complex properties which are primarily caused by the high concentrations of organics and ammonia. Thus, proper leachate treatment is required prior to its discharge. Leachate can be treated in various ways, and biological treatment is one of the approaches. This treatment has been shown to be both effective and cost-efficient while offering the possibility of resource recovery in the form of bioenergy. In this study, the underlying patterns in publications related to leachate biological treatment were uncovered through bibliometric analysis. This study also lays the groundwork for a deeper understanding of the past, current, and future trends of the leachate biological treatment. Research publications from 1974 to 2021 were retrieved from the Scopus database, and it was identified that 2013 articles were published in the span of 47 years. From the analyzed publications, China played a leading role in publishing leachate biological treatment research articles as well as having the most productive institutions and authors. Meanwhile, the USA was found to be the most active country in initiating international collaborations with 33 countries. The research hotspots were also successfully identified using keyword co-occurrences analysis. Anaerobic digestion and constructed wetland were revealed to be the research hotspots. The critical role of biological treatment in removing nitrogen from leachate was also highlighted. Besides, numerous research gaps were identified in the application of aerobic granular sludge (AGS) for leachate treatment. This can be a potential area for research in the future. Finally, future research should be encouraged to focus on the use of sustainable treatment systems in which energy recovery in the form of biogases is promoted.

Improved biohydrogen production via graphene oxide supported granular system based on algal hydrolyzate, secondary sewage sludge and bacterial consortia

Biohydrogen production using renewable sources has been regarded as one of the most sustainable ways to develop low-cost and green production technology. In order to achieve this objective, herein biohydrogen production has been conducted using the combination of untreated secondary sewage sludge (Sss), algal biomass hydrolyzate (Abh), graphene oxide (GO) and bacterial consortia that forms a granular system. Thus, naturally formed granular system produced cumulative H₂ of 1520mL/L in 168h with the maximum production rate of 13.4mL/L/h in 96h at initial pH 7.0, and optimum temperature of 37^oC. It is noticed that the combination of Abh, Sss and GO governed medium showed 42.05% higher cumulative H₂ production along with 22.71% higher production rate as compared to Abh and Sss based H₂ production medium. The strategy presented herein may find potential applications for the low-cost biohydrogen production using waste biomasses including Sss and Abh.

Adaptation responses of microalgal-bacterial granular sludge to polystyrene microplastic particles in municipal wastewater

Microplastics are frequently detected in wastewater treatment plants, but the knowledge of their effects on microalgal-bacterial granular sludge (MBGS) is still unknown. This study investigated the performance and adaptive response of MBGS exposed in municipal wastewater in the presence of polystyrene (PS) microplastic particles with different sizes (i.e., 100 nm, 5 μm, and 10 μm). Results indicated that the average removal efficiency of influent organics, ammonia, and phosphorus by MBGS process was stable at above 85%, showing insignificant difference between three sizes of microplastic particles. The community richness of MBGS was reduced by nano-sized (i.e., 100 nm) and micro-sized (i.e., 5 μm) PS microplastic particles, while the community diversity decreased in all types. Although filamentous cyanobacteria were broken by PS microplastic particles, the performance of MBGS process was insignificantly affected due to the stimulated extracellular polymeric substances, which could act as adaptive responses and protect MBGS from stress damage. This study proves that MBGS process can be operated in the presence of prevalent PS microplastic particles.

Growth of Microalgae-Bacteria Flocs for Nutrient Recycling from Digestate and Liquid Slurry and Methane Production by Anaerobic Digestion

Biogas production by anaerobic digestion from different wastes represents a growing interest in the panel of renewable energy. Digestate has already been a subject of numerous studies as part of microalgal culturing because it is still rich in nutrients. This study wants to use it as a reference to investigate the possibility to exploit Slurry for the same applications. The first part of this research aims to evaluate microalgae-bacterial flocs growth for nutrient recycling from liquid digestate and slurry, working at three different dilutions (10%, 30%, and 50%) of these two substrates, in order to determine the best value for nutrients and pollutants removal (ammonia and chemical oxygen demand removal rate) and microalgae-bacterial biomass production (autotrophic index). The best dilutions were 30% for digestate and 10% for slurry, allowing the highest ammonia and chemical oxygen demand removal rates. The second part evaluated methane production during anaerobic digestion at different ratios of substrate/inoculum (0.2, 0.5, and 0.8), using microalgae-bacterial flocs as a substrate and digestate or slurry as the inoculum. After 30 days, the anaerobic digestion without flocs showed the best performance compared to digestion with flocs (726.7 mL CH4·g−1 slurry, 245.6 mL CH4·g−1 digestate), whereas, for flocs digestion, the best ratio for both inocula was 0.2 substrate/inoculum with 317.2 mL CH4·g−1 slurry and 165.7 mL CH4·g−1 digestate. All solid masses are expressed in terms of volatile solids (VS).

Start-up strategies to develop aerobic granular sludge and photogranules in sequential batch reactors

In this study, start-up strategies to develop conventional aerobic granular sludge (AGS) and algal aerobic granular sludge (AAGS) (photogranules), were investigated. The granulation experiment was conducted in four sequencing batch reactors (SBR), of which two were conventional SBRs (RC1, RC2) used as control, and two were photo-SBRs (R1, R2). R1 and RC1 were operated with a 40-min feeding during the reactors´ anaerobic cycle period, whereas R2 and RC2 with a 60-min feeding. All the reactors were operated in two phases with a C:N = 4:1 in Phase I and 8:1 in Phase II. In Phase I, AGS in RC1 and RC2 was formed 15 days before the AAGS development in R1 and R2. However, the AAGS generally presented better stability and higher diameter. On the other hand, AGS presented greater abundance of extracellular polymeric substances producing organisms, such as Xanthomonadacea and Rhodocyclaceae. Chemical oxygen demand (COD) and NH₄⁺-N removal efficiencies were similar in all the four reactors of approximately 70% and 60%, respectively. In this phase, despite the good biomass structure, the reactors were not able to completely oxidize the high influent concentration of NH₄⁺-N (100 mg.L^-1) and COD (400 mg.L^-1). This can be associated to the short time of the aerobic phase and low biomass content. In Phase II in all the reactors, a good increase in COD and NH₄⁺-N removal efficiencies to values above 95% and 93%, respectively, was achieved under a higher C:N ratio of 8 with lower influent concentration of NH₄⁺-N (50 mg.L^-1). The 60-min anaerobic feeding period in R2 and RC2 resulted in greater removal efficiency of nitrogen, confirming that small variation on cycle periods can affect the biomass composition; the biomass presented more compact granules and larger diameters under 60 min-feeding when compared with those obtained with 40 min-feeding in Phase I.

Biogranulation process facilitates cost-efficient resources recovery from microalgae-based wastewater treatment systems and the creation of a circular bioeconomy

Energy self-sufficient wastewater treatment designs can reduce net energy consumption and achieve resources recovery. Microalgae are regarded as a promising candidate for developing a circular bioeconomy in wastewater treatment plants (WWTPs) due to its potential for simultaneous wastewater remediation and high value-added materials production. Much effort has been made to overcome the high production costs for microalgae; however, biomass harvesting still remains as the bottleneck for its large-scale application. In this study, the novel biogranulation system facilitating easier and faster microalgae harvesting was firstly compared with the conventional suspended culture for energy-efficiency and sustainability assessment on microalgae (Ankistrodesmus falcatus var. acicularis) cultivation using the synthetic anaerobic digestion liquor. Results demonstrated that the biogranulation system enhanced volumetric biomass productivity (223.17 ± 11.82 g/m³/day) by about 4.4 times compared to that from the suspended system (41.57 ± 2.08 g/m³/day) under the same environmental conditions. It was noticed that lipids, carbohydrates and proteins were accumulated in microalgae cells along with nutrients remediation, and the microalgae granules with much higher proteins content (313.28 ± 26.67 mg/g-VSS) could be easily harvested through 2 min gravity sedimentation with little impact on the contents of carbohydrates and lipids. In the whole cultivation and harvesting process, the biomass mass-based electricity consumption and footprint demand by the biogranulation system were reduced by 58% and 76%, respectively. Results from this study provide a cost-effective and sustainable approach for microalgae in the treatment of nutrients rich digestion liquor with simultaneous production of valuable biomaterials.

Effect of high ammonia on granular stability and phosphorus recovery of algal-bacterial granules in treatment of synthetic biogas slurry

The aim of the study was to investigate the application of algal-bacterial granules in treatment of high ammonia wastewater. Two identical cylindrical reactors, i.e., Rc and Rs was used to develop granular sludge system with synthetic biogas slurry. Rs was run under an artificial solar lamp controlled at 12 h power on and 12 h power off (∼10,000 lux); Rc was operated as control (no light). Results showed that algal-bacterial granules (ABGS) developed in Rs exhibited better structural stability in the face of high ammonia influent. Compared with aerobic granules (AGS), ABGS possessed high proteins (PN) content (145.3 mg/g-VSS) in extracellular polymeric substances (EPS) and better O₂ mass transfer inner granules. Higher phosphorus (P) removal capacity was obtained in Rs even under 400 mg/L NH₃–N which resulted in higher P content in ABGS biomass (56.4 mg/g-TSS). Bioavailable P in ABGS was 44 mg P/g-SS on day 160, approximately 1.53-times higher than that in AGS.

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.