Full scale performance of the aerobic granular sludge process for sewage treatment

Archaea communities in aerobic granular sludge: A mini-review

Recent research on the archaea community in aerobic granular sludge (AGS) has attracted considerable attention. This review summarizes the existing literature on composition, distribution, and related functions of archaea community in AGS. Furthermore, the effects of granulation, substrate, temperature, process types, and aeration models on the archaea community were discussed. Significantly, the layered structure of AGS facilitates the enrichment of archaea, including methanogenic archaea and ammonia-oxidizing archaea. Archaea engage in metabolic interactions with other microorganisms, enhancing the ecological functionalities of AGS and its tolerance to adverse conditions. Future investigations should focus on minimizing greenhouse gas emissions and exploring the roles and interactive mechanisms of archaea and other microorganisms within AGS.

Influence of residual anaerobic granular sludge (AnGS) from anaerobically digested molasses wastewater in aerobic granular sludge reactor

This study investigated the impact of residual anaerobic granular sludge (AnGS) from anaerobic digesters treating molasses wastewater on ammonium reduction in a downstream aerobic granular sludge (AGS) reactor. Two conditions were tested: raw (high AnGS concentration) and settled (low AnGS concentration) anaerobically digested molasses wastewaters were fed into the AGS reactor. With the introduction of raw wastewater, enhanced nitrite accumulation at 30 % and improved total inorganic nitrogen (TIN) removal at 11 % were observed compared to 1 % nitrite accumulation and 8 % TIN removal with the introduction of settled wastewater. However, AnGS adversely affected other aspects of reactor performance, increasing effluent solid content and decreasing soluble chemical oxygen demand removal efficiency from 20 % in the low AnGS condition to 11 % in the high AnGS condition. Despite the observed retention of AnGS in the reactor, no significant bioaugmentation effects on the microbial community of the AGS were observed. Aerobic granular sludge was consistently observed in both conditions. The study suggests that AnGS may act as a nucleus for granule formation, helping to maintain granule stability in a disturbed environment. This study offers a systematic understanding of the impact of AnGS on subsequent nitrogen removal process using AGS, aiding in the decision making in the treatment of high solid anaerobic digestate.

Two-way role of iron-carbon in biochemical reactions: Microelectrolysis and enhanced activity of aerobic granular sludge for efficient refractory wastewater treatment

Industrial wastewater contained a large amount of refractory organics, and single treatment processes had limitations. This study investigated the mechanism of refractory organics removal using iron-carbon built-in coupled activated sludge (ICAS) and explored the role and function of iron-carbon (IC) within the ICAS system. The aerobic granular sludge (AGS) cultivated with IC exhibited a loose surface and a tight interior structure. Iron in the AGS concentrated near the outer layer to form a crust, which protected the inner microorganisms. IC promoted EPS secretion and regulated the abundance of positive and negative signaling molecules to maintain AGS stability. Experiments using quinoline as a model refractory organic showed that both physical adsorption by IC and biological adsorption by sludge rapidly fixed a large amount of pollutants, providing a buffer capacity for the system. The iron mineral crust on the AGS surface enhanced quinoline adsorption. Hydroxylation was the first step in quinoline degradation, with IC upregulating the genes iorA/B, qorB, and wrbA involved in this process, and the relative abundances of quinoline-degrading bacteria. Both pyridine ring opening and benzene ring cleavage occurred in the single IC system, and the microelectrolysis process produced •OH and [H], which made degradation pathway for quinoline through IC more complex than microbial degradation. Although the IC-mediated pathway accounted for only a small part of overall quinoline removal in the ICAS system, the ICAS system not only preserved the microelectrolysis process but also enhanced microbial metabolic activity. This work provided insights into the synergistic removal of pollutants and maintenance of AGS stability by the ICAS process, ensuring efficient treatment of refractory organic wastewater.

Novel separate aeration self-circulating technology for continuous aerobic granular sludge process: Performance evaluation, hydrodynamic simulation and control strategy

The continuous aerobic granular sludge (AGS) process is promising for upgrading existing wastewater treatment facilities. However, this approach is still challenging because of its complicated structure and operation. To address this issue, a novel separate aeration self-circulating technology (abbreviated as Zier) was proposed, which is promising for cultivating AGS by its outstanding upflow velocity and circulation multiplier (more than 30 m/h and 200, respectively). This study elaborated on the Zier reactor's feasibility, optimization, and control strategy through computational fluid dynamics simulations, theoretical calculations, and experiments. An appropriate flow regime for efficient removal of pollutant and granulation of sludge was attained at a superficial gas velocity of 1.3 cm/s. Moreover, optimizing the aeration column diameter to half of the reaction column and increasing the height/diameter ratio to 20 dramatically boosted the nitrogen removal capacity over 1.6 kg N/m3/d. Utilizing a smaller circulation pipe diameter ensured granulation under a consistent flow regime. By judiciously regulating, multiple CSTRs and PFRs seamlessly integrated within the Zier reactor across a broad spectrum of particle sludge. The validity of these findings was further substantiated through experimental and theoretical validations. Drawing from these findings, a multi-scenario control strategy was proposed as Zier's map. With all the superiorities shown by the Zier reactor, this study could offer new insights into an efficient continuous AGS process.

Emerging biotechnological applications of anaerobic ammonium oxidation

Anaerobic ammonium oxidation (anammox) is an energy-efficient method for nitrogen removal that opens the possibility for energy-neutral wastewater treatment. Research on anammox over the past decade has primarily focused on its implementation in domestic wastewater treatment. However, emerging studies are now expanding its use to novel biotechnological applications and wastewater treatment processes. This review highlights recent advances in the anammox field that aim to overcome conventional bottlenecks, and explores novel and niche-specific applications of the anammox process. Despite the promising results and potential of these advances, challenges persist for their real-world implementation. This underscores the need for a transition from laboratory achievements to practical, scalable solutions for wastewater treatment which mark the next crucial phase in the evolution of anammox research.

Treatment of high ammonia anaerobically digested molasses wastewater using aerobic granular sludge reactor

This study addressed the treatment of high ammonia, low biodegradable chemical oxygen demand (bCOD) anaerobically digested molasses wastewater, utilizing an aerobic granular sludge (AGS) reactor. The AGS achieved 99 % ammonia removal regardless of the bCOD supplementation. By adding low ammonia (<60 mg/L), high bCOD raw molasses wastewater (before anaerobic digestion) as a carbon source, enhanced nitrogen removal, increasing from 10 % to 97 %, and improved sludge settleability via bio-induced calcite precipitation were observed. Functional genes prediction suggested two potential denitrification pathways, including heterotrophic denitrification by Paracoccus and Thauera, and autotrophic denitrification, specifically sulfide-oxidizing autotrophic denitrification by Thiobacillus. An increase in the relative abundance of microorganisms involved in heterotrophic denitrification was observed with the addition of high bCOD raw molasses wastewater. Consequently, incorporating raw molasses wastewater into the AGS presents a sustainable approach to achieve mixotrophic denitrification, maintain stable granular sludge and ensure stable treatment performance when treating anaerobically digested molasses wastewater.

Nutrients removal in overloaded WWTP by intermittently aerated IFAS: Effects of biofilm carrier and intermittent aeration cycle

Updating of the current Urban Waste Water Treatment Directive (91/271/EEC) will demand stricter regulations for nutrients removal. In this frame, wastewater treatment plants (WWTPs) of small-to-medium potential will face new challenges for achieving process intensification. Integrating intermittent aeration (IA) and integrated fixed-film activated sludge (IFAS) technologies could be a promising solution to meet such requirements. This study analyzed how IA cycles affected nutrients removal in IFAS reactors with different biofilm carriers (e.g., plastic and sponge media). The plants responses to different carbon/nitrogen/phosphorous (C/N/P) ratios were evaluated while operating under low sludge retention time (SRT) to simulate overloaded conditions. A short IA cycle (1 h) with an aeration/not aeration ratio of 2:1 enabled high organic carbon and nitrification performances when operating at high C/N/P (11.8/1/1), whereas low denitrification and phosphorous removal yields were obtained because of the short not-aerated phase. Decreasing C/N ratio (8.8/1/1) without changing the IA cycle resulted in nitrification worsening because of the reduced metabolic kinetics of biofilm. Under such load conditions, a higher IA cycle (2 h) was necessary to improve process performance. A longer not-aerated phase was also positive for denitrification and phosphorous removal because of the establishment of anoxic and anaerobic environments within the bulk and inner biofilm layers. Besides, results suggested that sponge carriers offered advantages over plastic ones, enabling a higher biofilm retention capacity, better nutrient removal, as well as robustness and resilience to operating condition changes. This would result in simpler management systems for implementing the IA process, thus reducing process complexity and costs.

Bibliometric analysis of research trends in biogranulation technology for wastewater treatment

Inadequate management and treatment of wastewater pose significant threats, including environmental pollution, degradation of water quality, depletion of global water resources, and detrimental effects on human well-being. Biogranulation technology has gained increasing traction for treating both domestic and industrial wastewater, garnering interest from researchers and industrial stakeholders alike. However, the literature lacks comprehensive bibliometric analyses that examine and illuminate research hotspots and trends in this field. This study aims to elucidate the global research trajectory of scientific output in biogranulation technology from 1992 to 2022. Utilizing data from the Scopus database, we conducted an extensive analysis, employing VOSviewer and the R-studio package to visualize and map connections and collaborations among authors, countries, and keywords. Our analysis revealed a total of 1703 journal articles published in English. Notably, China emerged as the leading country, Jin Rencun as the foremost author, Bioresource Technology as the dominant journal, and Environmental Science as the prominent subject area, with the Harbin Institute of Technology leading in institutional contributions. The most prominent author keyword identified through VOSviewer analysis was "aerobic granular sludge," with "sequencing batch reactor" emerging as the dominant research term. Furthermore, our examination using R Studio highlighted "wastewater treatment" and "sewage" as notable research terms within the field. These findings underscore a diverse research landscape encompassing fundamental aspects of granule formation, reactor design, and practical applications. This study offers valuable insights into biogranulation potential for efficient wastewater treatment and environmental remediation, contributing to a sustainable and cleaner future.

Potential for curdlan recovery from aerobic granular sludge wastewater treatment systems - A review

The aerobic granular sludge (AGS) biotechnology has been explored for wastewater treatment for over two decades. AGS is gaining increased interest due to its enhanced treatment performance ability and the potential for resource recovery from AGS-based wastewater treatment systems. Resource recovery from AGS is a promising approach to sustainable wastewater treatment and attaining a circular economy in the wastewater management industry. Currently, research is at an advanced stage on recovering value-added resources such as phosphorus, polyhydroxyalkanoates, alginate-like exopolysaccharides, and tryptophan from waste aerobic granules. Recently, other value-added resources, including curdlan, have been identified in the aerobic granule matrix, and this may increase the sustainability of biotechnology in the wastewater industry. This paper provides an overview of AGS resource recovery potential. In particular, the potential for enhanced curdlan biosynthesis in the granule matrix and its recovery from AGS wastewater treatment systems is outlined.

High methane potential of oxygenic photogranules decreases after starvation

Oxygenic photogranules (OPG) are granular biofilms that can treat wastewater without external aeration, making it an advantage over activated sludge. Excess of OPG biomass can serve as energy source through anaerobic digestion. Two sequencing batch photoreactors were operated over 400 days to grow OPG. Biochemical methane potentials (BMP) were obtained from near-infrared spectroscopy. OPGs had an average BMP of 356 mL CH4·gVS-1, much higher than typical BMP from activated sludge. A partial least squares analysis could relate BMP with reactor operating conditions, like light regime, load or biomass concentration. Since organic load was the most influential parameter on BMP, three starvation experiments were set up. An average decrease of BMP by 18.4 % was observed. However, the unexpected growth of biomass during starvation resulted in a higher total methane volume. In conclusion, starvation reduces the BMP of OPGs but anaerobic digestion of OPG biomass remains a promising route for biomass valorization.

Review on antibiotic pollution dynamics: insights to occurrence, environmental behaviour, ecotoxicity, and management strategies

Antibiotic contamination poses a significant global concern due to its far-reaching impact on public health and the environment. This comprehensive review delves into the prevalence of various antibiotic classes in environmental pollution and their interactions with natural ecosystems. Fluoroquinolones, macrolides, tetracyclines, and sulphonamides have emerged as prevalent contaminants in environmental matrices worldwide. The concentrations of these antibiotics vary across diverse environments, influenced by production practices, consumer behaviours, and socio-economic factors. Low- and low-middle-income countries face unique challenges in managing antibiotic contamination, with dominant mechanisms like hydrolysis, sorption, and biodegradation leading to the formation of toxic byproducts. Ecotoxicity reports reveal the detrimental effects of these byproducts on aquatic and terrestrial ecosystems, further emphasizing the gravity of the issue. Notably, monitoring the antibiotic parent compound alone may be inadequate for framing effective control and management strategies for antibiotic pollution. This review underscores the imperative of a comprehensive, multi-sectoral approach to address environmental antibiotic contamination and combat antimicrobial resistance. It also advocates for the development and implementation of tailored national action plans that consider specific environmental conditions and factors. Thus, an approach is crucial for safeguarding both public health and the delicate balance of our natural ecosystems.

Azithromycin removal using pine bark, oak ash and mussel shell

Adsorption is considered an interesting option for removing antibiotics from the environment because of its simple design, low cost, and potential efficiency. In this work we evaluated three by-products (pine bark, oak ash, and mussel shell) as bio-adsorbents for the antibiotic azithromycin (AZM). Furthermore, they were added at doses of 48 t ha-1 to four different soils, then comparing AZM removal for soils with and without bio-adsorbents. Batch-type experiments were used, adding AZM concentrations between 2.5 and 600 μmol L-1 to the different bio-adsorbents and soil + bio-adsorbent mixtures. Regarding the bio-adsorbents, oak ash showed the best adsorption scores (9600 μmol kg-1, meaning >80% retention), followed by pine bark (8280 μmol kg-1, 69%) and mussel shell (between 3000 and 6000 μmol kg-1, 25-50% retention). Adsorption data were adjusted to different models (Linear, Freundlich and Langmuir), showing that just mussel shell presented an acceptable fitting to the Freundlich equation, while pine bark and oak ash did not present a good adjustment to any of the three models. Regarding desorption, the values were always below the detection limit, indicating a rather irreversible adsorption of AZM onto these three by-products. Furthermore, the results showed that when the lowest concentrations of AZM were added to the not amended soils they adsorbed 100% of the antibiotic, whereas when the highest concentrations of AZM were spread, the adsorption decreased to 55%. However, when any of the three bio-adsorbents was added to the soils, AZM adsorption reached 100% for all the antibiotic concentrations used. Desorption was null in all cases for both soils with and without bio-adsorbents. These results, corresponding to an investigation carried out for the first time for the antibiotic AZM, can be seen as relevant in the search of low-cost alternative treatments to face environmental pollution caused by this emerging contaminant.

Effects of salinity on glycerol conversion and biological phosphorus removal by aerobic granular sludge

Industrial wastewater often has high levels of salt, either due to seawater or e.g. sodium chloride (NaCl) usage in the processing. Previous work indicated that aerobic granular sludge (AGS) is differently affected by seawater or saline water at similar osmotic strength. Here we investigate in more detail the impact of NaCl concentrations and seawater on the granulation and conversion processes for AGS wastewater treatment. Glycerol was used as the carbon source since it is regularly present in industrial wastewaters, and to allow the evaluation of microbial interactions that better reflect real conditions. Long-term experiments were performed to evaluate and compare the effect of salinity on granulation, anaerobic conversions, phosphate removal, and the microbial community. Smooth and stable granules as well as enhanced biological phosphorus removal (EBPR) were achieved up to 20 g/L NaCl or when using seawater. However, at NaCl levels comparable to seawater strength (30 g/L) incomplete anaerobic glycerol uptake and aerobic phosphate uptake were observed, the effluent turbidity increased, and filamentous granules began to appear. The latter is likely due to the direct aerobic growth on the leftover substrate after the anaerobic feeding period. In all reactor conditions, except the reactor with 30 g/L NaCl, Ca. Accumulibacter was the dominant microorganism. In the reactor with 30 g/L NaCl, the relative abundance of Ca. Accumulibacter decreased to ≤1 % and an increase in the genus Zoogloea was observed. Throughout all reactor conditions, Tessaracoccus and Micropruina, both actinobacteria, were present which were likely responsible for the anaerobic conversion of glycerol into volatile fatty acids. None of the glycerol metabolizing proteins were detected in Ca. Accumulibacter which supports previous findings that glycerol can not be directly utilized by Ca. Accumulibacter. The proteome profile of the dominant taxa was analysed and the results are further discussed. The exposure of salt-adapted biomass to hypo-osmotic conditions led to significant trehalose and PO43--P release which can be related to the osmoregulation of the cells. Overall, this study provides insights into the effect of salt on the operation and stability of the EBPR and AGS processes. The findings suggest that maintaining a balanced cation ratio is likely to be more important for the operational stability of EBPR and AGS systems than absolute salt concentrations.

Utilizing anaerobic substrate distribution for growth of aerobic granular sludge in continuous-flow reactors

The development of continuous flow reactors (CFRs) employing aerobic granular sludge (AGS) for the retrofit of existing wastewater treatment plants (WWTPs) using a continuous-flow activated sludge (CFAS) system has garnered increasing interest. This follows the worldwide adoption of AGS technology in sequencing batch reactors (SBRs). The better settleability of AGS compared to AS allows for process intensification of existing wastewater treatment plants without the difficult conversion of often relatively shallow CFRs to deeper AGS-SBRs. To retrofit existing CFAS systems with AGS, achieving both increased hydraulic capacity and enhanced biological nutrient removal necessitates the formation of granular sludge based on the same selective pressures applied in AGS-SBRs. Previous efforts have focussed mainly on the selective wasting of flocculent sludge and retaining granular sludge to drive aerobic granulation. In this study a pilot-scale CFR was developed to best mimic the implementation of the granulation mechanisms of full-scale AGS-SBRs. The pilot-scale reactor was fed with pre-settled municipal wastewater. We established metrics to assess the degree to which the proposed mechanisms were implemented in the pilot-scale CFR and compared them to data from full-scale AGS-SBRs, specifically with respect to the anaerobic distribution of granule forming substrates (GFS). The selective pressures for granular sludge formation were implemented through inclusion of anaerobic upflow selectors with a water depth of 2.5 meters, which yielded a sludge with properties similar to AGS from full-scale SBRs. In comparison to the CFAS system at Harnaschpolder WWTP treating the same pre-settled wastewater, a more than twofold increase in volumetric removal capacity for both phosphorus and nitrogen was achieved. The use of a completely mixed anaerobic selector, as opposed to an anaerobic upflow selector, caused a shift in EBPR activity from the largest towards the smallest size class, while nitrification was majorly unaffected. Anaerobic selective feeding via bottom-feeding is, therefore, favorable for the long-term stability of AGS, especially for less acidified wastewater. The research underlines the potential of AGS for enhancing the hydraulic and biological treatment capacity of existing CFAS systems.

Making waves: Riding the densification wave from current understanding to advancement

Densification is a novel intensification strategy with the potential to improve treatment capacity within existing continuous-flow (CF) water resource recovery facilities at low capital and operating costs and at relatively small particle sizes compared to typical aerobic granular sludge (AGS) systems. To achieve densification, biological selection principles derived from selector design and AGS concepts have been coupled with physical selection via hydrocyclones at full-scale CF facilities to promote the growth and retention of granules. This combination lowers the sludge volume index (SVI) through superior sludge settling and paves the way for optimized nutrient removal and energy efficiency in low dissolved oxygen conditions. This paper sheds light on the benefits of densification. It delves into areas of advancement to further its implementation: hydrocyclone design, selector zone design, operational guidelines, and the target range for particle sizes and granule fractions.

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

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

Effects of enhanced biological phosphorus removal on rapid control of sludge bulking and fast formation of aerobic granular sludge

This study investigated the effects of enhanced biological phosphorus removal (EBPR) on rapid sludge bulking control and fast aerobic granular sludge (AGS) formation by adding 20 % of EBPR activated sludge to the bulking activated sludge (BAS) reactor. The results indicate that activating EBPR activity swiftly improved BAS settleability within 16 days, thus resolving sludge bulking issues. Subsequently, a settling time-based selection was employed, resulting in the BAS granulation within another 16 days. The rapid achievement of EBPR activity improved the BAS settleability and facilitated the formation of sludge aggregates, thereby expediting BAS granulation. Inhibition of filamentous bacteria and enrichment of slow-growing organisms contributed to both sludge bulking control and aerobic granulation. Furthermore, the increase in proteins/polysaccharides ratio facilitated the granulation process. Additionally, total nitrogen removal increased from 59.4 % to 71.7 % because of the mature AGS formation. This study provided an approach to simultaneously control sludge bulking and promote aerobic granulation.

The fate and transport of neonicotinoid insecticides and their metabolites through municipal wastewater treatment plants in South China

Neonicotinoid insecticides (NEOs) have gained widespread usage as the most prevalent class of insecticides globally and are frequently detected in the environment, posing potential risks to biodiversity and human health. Wastewater discharged from wastewater treatment plants (WWTPs) is a substantial source of environmental NEOs. However, research tracking NEO variations in different treatment units at the WWTPs after being treated by the treatment processes remains limited. Therefore, this study aimed to comprehensively investigate the fate of nine parent NEOs (p-NEOs) and five metabolites in two municipal WWTPs using distinct treatment processes. The mean concentrations of ∑NEOs in influent (effluent) for the UNITANK, anaerobic-anoxic-oxic (A2/O), and cyclic activated sludge system (CASS) processes were 189 ng/L (195 ng/L), 173 ng/L (177 ng/L), and 123 ng/L (138 ng/L), respectively. Dinotefuran, imidacloprid, thiamethoxam, acetamiprid, and clothianidin were the most abundant p-NEOs in the WWTPs. Conventional wastewater treatment processes were ineffective in removing NEOs from wastewater (-4.91% to -12.1%), particularly major p-NEOs. Moreover, the behavior of the NEOs in various treatment units was investigated. The results showed that biodegradation and sludge adsorption were the primary mechanisms responsible for eliminating NEO. An anoxic or anaerobic treatment unit can improve the removal efficiency of NEOs during biological treatment. However, the terminal treatment unit (chlorination disinfection tank) did not facilitate the removal of most of the NEOs. The estimated total amount of NEOs released from WWTPs to receiving waters in the Pearl River of South China totaled approximately 6.90-42.6 g/d. These findings provide new insights into the efficiency of different treatment processes for removing NEOs in current wastewater treatment systems.

Study on mechanism of removal of sudden Tetracycline by compound modified biological sand filtration process

The removal of tetracycline from the sewage plant effluents through advanced treatment methods is key to controlling tetracycline levels in the water environment. In this study, modified quartz sands (QS) were used in a biological sand filter to remove tetracycline. The modified QS, with different surface characteristics, were prepared using glass etching technology combined with subsequent chemical modification methods, including hydroxylation treatment, metal ion modification, and amino modification. The adsorption efficiency of hydroxylated QS was higher than that of metal ion modified and amino modified QS, with adsorption efficiencies of 20.4331 mg/kg, 12.8736 mg/kg, and 10.1737 mg/kg, respectively. Results indicated that QS primarily reduce tetracycline through adsorption. Adsorption on ordinary QS fit the pseudo-first-order kinetic model, while adsorption on other modified QS and biofilm-coated QS fit the pseudo-second-order kinetics model. Biodegradation was identified as another mechanism for tetracycline reduction, which fit the zero-order kinetic model. Pseudomonas alcaligenes and unclassified Pseudomonas accounted for 96.6% of the total tetracycline-degrading bacteria. This study elucidates the effectiveness and mechanisms of five types of QS in treating tetracycline from sewage plant effluents. It provides a novel method for tetracycline reduction in real-world wastewater scenarios.

Resource recovery of high value-added products from wastewater: Current status and prospects

Wastewater resource recovery not only allows the extraction of value-added products and offsets the operational costs of wastewater treatment, but it is also conducive to alleviating adverse environmental issues due to energy and chemical inputs and associated emissions. A number of attractive compounds such as alginate-like polymers, struvite, polyhydroxyalkanoates, and sulfated polysaccharides, were found and successfully obtained from wastewater and have a wide range of application prospects. The aim of this work is to provide a comprehensive review of recent advances in recovery of these popular products from wastewater, and their physicochemical properties, main sources, and current recovery status are summarized. Various factors influencing the recovery performance of these materials are thoroughly discussed. Moreover, the research needs and future directions towards wastewater resource recovery are highlighted. This study can provide valuable insights for future research endeavors aiming to improve wastewater resource recovery through the retrieval of high value-added products.

Two-stage sequencing batch reactors with added iron shavings for nutrient removal and aerobic sludge granulation treating real wastewater with low carbon to nitrogen ratios

In response to the challenges of limited nutrient removal and the difficulty in forming aerobic granular sludge (AGS) with low carbon to nitrogen (C/N) ratios, a novel two-stage sequencing batch reactors (SBRs) (R1 and R2) system with added iron shavings was proposed and established. The results showed that AGS was developed and nitrogen (82.8 %) and phosphorus (94.7 %) were effectively removed under a C/N ratio at 1.7 ± 0.5. The average size of R1 and R2 increased from 45.3 μm to 138.7 μm and 132.8 μm. Under high biological selective pressure, phosphorus accumulating organisms like Comamonadaceae (14.8 %) and Chitinophagales (5.7 %) experienced enrichment in R1. Furthermore, R2 exhibited an increased abundance of nitrifying bacteria (2.3 %) and a higher proportion of nitrogen removal through autotrophic denitrification (＞17.5 %). Overall, this study introduces an innovative two-stage SBRs with added iron shavings, offering a novel approach for the treatment of low C/N ratios wastewater.

The water-soluble fraction of extracellular polymeric substances from a resource recovery demonstration plant: characterization and potential application as an adhesive

Currently, there is a growing interest in transforming wastewater treatment plants (WWTPs) into resource recovery plants. Microorganisms in aerobic granular sludge produce extracellular polymeric substances (EPS), which are considered sustainable resources to be extracted and can be used in diverse applications. Exploring applications in other high-value materials, such as adhesives, will not only enhance the valorization potential of the EPS but also promote resource recovery. This study aimed to characterize a water-soluble fraction extracted from the EPS collected at the demonstration plant in the Netherlands based on its chemical composition (amino acids, sugar, and fatty acids) and propose a proof-of-concept for its use as an adhesive. This fraction comprises a mixture of biomolecules, such as proteins (26.6 ± 0.3%), sugars (21.8 ± 0.2%), and fatty acids (0.9%). The water-soluble fraction exhibited shear strength reaching 36-51 kPa across a pH range of 2-10 without additional chemical treatment, suggesting a potential application as an adhesive. The findings from this study provide insights into the concept of resource recovery and the valorization of excess sludge at WWTPs.

Anionic extracellular polymeric substances extracted from seawater-adapted aerobic granular sludge

Anionic polymers, such as heparin, have been widely applied in the chemical and medical fields, particularly for binding proteins (e.g., fibroblast growth factor 2 (FGF-2) and histones). However, the current animal-based production of heparin brings great risks, including resource shortages and product contamination. Recently, anionic compounds, nonulosonic acids (NulOs), and sulfated glycoconjugates were discovered in the extracellular polymeric substances (EPS) of aerobic granular sludge (AGS). Given the prevalence of anionic polymers, in marine biofilms, it was hypothesized that the EPS from AGS grown under seawater condition could serve as a raw material for producing the alternatives to heparin. This study aimed to isolate and enrich the anionic fractions of EPS and evaluate their potential application in the chemical and medical fields. The AGS was grown in a lab-scale reactor fed with acetate, under the seawater condition (35 g/L sea salt). The EPS was extracted with an alkaline solution at 80 °C and fractionated by size exclusion chromatography. Its protein binding capacity was evaluated by native gel electrophoresis. It was found that the two highest molecular weight fractions (438- > 14,320 kDa) were enriched with NulO and sulfate-containing glycoconjugates. The enriched fractions can strongly bind the two histones involved in sepsis and a model protein used for purification by heparin-column. These findings demonstrated possibilities for the application of the extracted EPS and open up a novel strategy for resource recovery. KEY POINTS: • High MW EPS from seawater-adapted AGS are dominant with sulfated groups and NulOs • Fifty-eight percent of the EPS is high MW of 68-14,320 kDa • EPS and its fractions can bind histones and fibroblast growth factor 2.

A novel carbon emission evaluation model for anaerobic-anoxic-oxic urban sewage treatment

The proposal of the dual carbon goal and the blue economy in China has sparked a keen interest in carbon emissions reduction from sewage treatment. Carbon accounting in urban sewage plants serves as the foundation for carbon emission reduction in sewage treatment. This paper re-evaluated carbon accounting in the operational processes for urban sewage treatment plants to develop a novel carbon emission evaluation model for anaerobic-anoxic-oxic treatment plants. The results show that the carbon emissions generated by non-carbon dioxide gases far exceed the carbon emissions from carbon dioxide alone. Moreover, the recycling of sewage leads to carbon emissions reduction that offsets the carbon emissions generated during the operation of the sewage plant. Also, the carbon emissions generated by sewage treatment plants are lower than those generated by untreated sewage. The findings and insights provided in this paper provide valuable references for carbon accounting and the implementation of low-carbon practices in urban sewage treatment plants.

When aerobic granular sludge faces emerging contaminants: A review

The evolution of emerging contaminants (ECs) has caused greater requirements and challenges to the current biological wastewater treatment technology. As one of the most promising biological treatment technologies, the aerobic granular sludge (AGS) process also faces the challenge of ECs. This study summarizes the recent progress and characteristics of several representative ECs (persistent organic pollutants, endocrine disrupting chemicals, antibiotics, and microplastics) in AGS systems that have garnered widespread attention. Additionally, the biodegradation and adsorption mechanisms of ECs were discussed, and the interactions between various ECs and AGS was elucidated. The importance of extracellular polymeric substances for the stabilization of AGS and the removal of ECs is also discussed. Knowledge gaps and future research directions that may enable the practical application of AGS are highlighted. Overall, AGS processes show great application potential and this review provides guidance for the future implementation of AGS technology as well as elucidating the mechanism of its interaction with ECs.

Hierarchically porous biochar derived from aerobic granular sludge for high-performance membrane capacitive deionization

Membrane capacitive deionization (MCDI) is a cost-effective desalination technique known for its low energy consumption. The performance of MCDI cells relies on the properties of electrode materials. Activated carbon is the most widely used electrode material. However, the capacitive carbon available on the market is often expensive. Here, we developed hierarchically porous biochar by combining carbonization and activation processes, using easily acquired aerobic granular sludge (AGS) from biological sewage treatment plants as a precursor. The biochar had a specific surface area of 1822.07 m2 g-1, with a micropore area ratio of 58.65% and a micropore volume of 0.576 cm3 g-1. The MCDI cell employing the biochar as electrodes demonstrated a specific adsorption capacity of 34.35 mg g-1, comparable to commercially available activated carbon electrodes. Our study presents a green and sustainable approach for preparing highly efficient, hierarchically porous biochar from AGS, offering great potential for enhanced performance in MCDI applications.

Full-scale upgrade activated sludge to continuous-flow aerobic granular sludge: Implementing microaerobic-aerobic configuration with internal separators

Aerobic granular sludge (AGS) has been successfully used in sequencing batch reactors. However, their application to existing continuous-flow systems remains challenging. In this study, a novel microaerobic-aerobic configuration with internal separators was implemented in a full-scale municipal wastewater treatment facility with a nominal capacity of 2.5 × 104 m3 d-1. Sludge characteristics, pollutant removal and associated pathways, shifts in the microbial community, and underlying granulation mechanisms were investigated. Following a two-month operation period, the transition from flocculent-activated sludge to well-defined AGS with distinct boundaries and compact structures was successfully achieved. The average size of sludge increased from 31.9 to 138.5 μm, with granules larger than 200 μm constituting 28.9 % of the total sludge and SVI30 averaging 51.4 ± 8.2 mL g-1. The 95th percentile effluent COD, NH4+-N, and TN concentrations were 35.0, 1.2, and 13.3 mg L-1, respectively. The primary pathways for pollutant removal were identified as simultaneous nitrification, denitrification, and phosphorus removal within the microaerobic tanks. The enrichment of denitrifying phosphorus-accumulating organisms, including Hydrogenophaga, Accumulibacter, Azospira, Dechloromonas, and Pseudomonas, provides an essential microbial foundation. Furthermore, computational fluid dynamics modeling revealed that the incorporation of internal separators in aerobic tanks induced shifts in the flow pattern, transitioning from a single-circulation cell to multiple vortical cells. This alteration amplified the local velocity gradients, generating the required shear forces to drive granulation. Moreover, mass balance analysis revealed that the microaerobic and aerobic tanks operated under feast and famine conditions, respectively, creating a microbial selection pressure that favored granulation. This process eliminates the need for external clarifiers, resulting in a footprint reduction of 38.2 % and one-third energy savings for sludge reflux. This study offers valuable insights into the application of continuous-flow AGS to upgrade existing activated sludge systems with limited retrofitting requirements.

Sewage sludge minimisation by OSA-MBR: A pilot plant experiment

This study presents the excess sludge minimisation in a Membrane Bioreactor (MBR) system by an Oxic Settling Anaerobic (OSA) process. The pilot plant was fed with real wastewater and OSA was operated with two different hydraulic retention times (HRT), respectively 4 (Period II) and 6 h (Period III) and compared to an MBR (Period I). Multiple parameters/variables were monitored: sludge minimisation, nitrogen and carbon removal, membrane fouling, and biokinetic behaviour through respirometry. With respect to the current literature, greenhouse gas emissions were also here monitored, often neglected. Results demonstrated that combining MBR and OSA systems can significantly reduce excess sludge production (89.7%, in Period III and 59.7% in Period II, compared to Period I). However, Period III presented better PO4-P removal efficiencies but worse performances in the other parameters (COD, NH4 and Total Nitrogen). No substantial variation in membrane fouling was obtained over the experimental periods. Finally, the HRT increase in the anaerobic reactor promoted a N2O-N increase inside the unaerated reactors, highlighting the need for a trade-off between sludge minimisation and GHG emission.

Adaptation mechanisms of microalgal-bacterial granular sludge to outdoor light-limited conditions

Microalgal-bacterial granular sludge (MBGS) has attached attention for sustainable wastewater treatment, but it remains elusive whether it can adapt to outdoor light-limited conditions. This paper investigated the biological adaptation mechanisms of MBGS to outdoor light-limited diel conditions using real municipal wastewater. The results indicated that MBGS still had excellent pollutants removal performance, and that both the extracellular polymeric substances and glycogen content of MBGS increased significantly. The main functional microalgae and bacteria were revealed to be Leptolyngbyaceae and Rhodanobacteria, respectively. Further analyses indicated that the abundance of genes encoding PsbA, PsbD, PsbE, PsbJ, PsbP, Psb27, Psb28-2, PsaC, PsaE, PsaL, PsbX, PetB, PetA, and PetE increased in photosystem. Meanwhile, the abundance of gene encoding Rubisco decreased but the gene abundance regarding to crassulacean acid metabolism cycle increased. These suggested that MBGS could adjust the photosynthetic pathway to ensure the completion of photosynthesis. This study is anticipated to add fundamental insights for the MBGS process operated under outdoor light-limited conditions.

Model-based evaluation of simultaneous nitrification and denitrification in aerobic granular sludge systems

A lab-scale granular sludge sequencing batch reactor (G-SBR) system was operated using synthetic wastewater. The total nitrogen removal efficiency of 85% was obtained together with the achievement of complete total phosphorus removal with average granule diameter of 400 µm. Dual-step nitrification and denitrification model with fixed biofilm thickness was used for performance analysis. The denitrification mode only contributed to TN removal with 25% which can be calculated with process stoichiometry. The remaining nitrogen removal could be explained by simulating simultaneous nitrification and denitrification which was responsible for 75% denitrification during aerobic period. In addition, low NO3- concentration at the beginning of the fill period provided advantage for securing a prolonged anaerobic period for enhanced biological phosphorus removal (EBPR). The model parameters of boundary layer thickness (zBL = 50 µm) and half-saturation of O2 for nitrite-oxidizing bacteria (KO2,NOB = 0.5 gO2/m3) were tuned to fit NO2 and NO3 profiles in SBR cycle.

Metaproteomics, metagenomics and 16S rRNA sequencing provide different perspectives on the aerobic granular sludge microbiome

The tremendous progress in sequencing technologies has made DNA sequencing routine for microbiome studies. Additionally, advances in mass spectrometric techniques have extended conventional proteomics into the field of microbial ecology. However, systematic studies that provide a better understanding of the complementary nature of these 'omics' approaches, particularly for complex environments such as wastewater treatment sludge, are urgently needed. Here, we describe a comparative metaomics study on aerobic granular sludge from three different wastewater treatment plants. For this, we employed metaproteomics, whole metagenome, and 16S rRNA amplicon sequencing to study the same granule material with uniform size. We furthermore compare the taxonomic profiles using the Genome Taxonomy Database (GTDB) to enhance the comparability between the different approaches. Though the major taxonomies were consistently identified in the different aerobic granular sludge samples, the taxonomic composition obtained by the different omics techniques varied significantly at the lower taxonomic levels, which impacts the interpretation of the nutrient removal processes. Nevertheless, as demonstrated by metaproteomics, the genera that were consistently identified in all techniques cover the majority of the protein biomass. The established metaomics data and the contig classification pipeline are publicly available, which provides a valuable resource for further studies on metabolic processes in aerobic granular sludge.

Enhancement of nutrient removal in an activated sludge process using aerobic granular sludge augmentation strategy with ammonium-based aeration control

To enhance nutrient removal from low-strength municipal wastewater in a continuous-flow activated sludge (CFAS) process using aerobic granular sludge (AGS) augmentation strategy, a pilot-scale demonstration was configured with a mainstream reactor (anaerobic/aerobic process) and a sidestream sequencing batch reactor for AGS production. The aeration of the mainstream reactor was controlled based on dissolved oxygen (DO) and ammonium concentrations during Phases I and II-III, respectively. During Phase III, an anoxic zone was created in the mainstream aerobic tank. Throughout the demonstration period, excellent sludge settleability in the mainstream reactor (SVI30 ≤ 80 mL g-1) under long sludge retention time conditions (≥12 d) allowed the maintenance of a high mixed liquor suspended solids concentration (≥3000 mg L-1). The total nitrogen (TN) removal ratio improved significantly during Phases II and III (49.3 ± 4.1% and 50.1 ± 10.2%, respectively) compared to Phase I (43.2 ± 5.5%). Low DO concentration (< 0.5 mg L-1) by the ammonium-based aeration tended to increase the simultaneous nitrification and denitrification efficiency (> 40%), enhancing TN removal (> 50%). The reduction of DO and nitrate concentrations in the returning sludge liquor can stabilize phosphorus removal (approximately 80% of the 25th percentile). In addition, the aeration efficiency during Phase III decreased by 26-29% compared to Phase I. These results suggest that the introduction of ammonium-based aeration control to the CFAS using the AGS augmentation strategy could contribute to superior sewerage treatment, including nutrient removal and a low carbon footprint.

Towards a better understanding of the anaerobic/oxic/anoxic-aerobic granular sludge process (AOA-AGS) for simultaneous low-strength wastewater treatment and in situ sludge reduction from ambient to winter temperatures

The new anaerobic/oxic/anoxic-aerobic granular sludge (AOA-AGS) merits the advantages of effective carbon utilization and low-carbon treatment. However, low temperature poses stressing concerns and the resisting mechanism remains much unknown. Herein, an AOA-AGS process was configured for simultaneous nitrification, denitrification and phosphorus removal (SNDPR) with low-strength wastewater from ambient (>15 °C) to winter temperatures (<15 °C). Results showed that simultaneously advanced nutrients removal, and dramatic in situ sludge reduction (Yobs of 0.093 g MLSS/g COD) were gained regardless of seasonally decreasing temperatures. Winter temperatures even amplified Candidatus Competibacter predominating from 20.11% to 34.74%, which laid the core basis for endogenous denitrification, sludge minimization and temperature resistance. A removal model was thus proposed given the observed functional groups, and doubts were also raised for future investigations. This study would aid a better understanding on the microbial ecology and engineering aspects of the new AOA-AGS process treating low-strength wastewater at low temperatures.

Effect of biofilm thickness on the activity and community composition of phosphorus accumulating bacteria in a moving bed biofilm reactor

Can biofilms enhance the rates of phosphorus removal in wastewater treatment? In order to narrow the scientific gap on the effect of biofilm thickness on the activity and microbial community of phosphorus-accumulating bacteria, this study investigated biofilms of 30 to 1000 µm thickness in a moving bed biofilm reactor. Measurements on 5 different biofilm carriers showed that biomass-specific phosphorus release and uptake rates increased as a function of biofilm thickness for biofilms thinner than about 110 µm but were lower for thicker biofilms of about 550-1000 µm. The reduced phosphorus uptake and release rates in the thickest biofilms can result from substrate mass transfer limitations whereas the low activity in the thinnest biofilms can be related to a too high turnover rate in the biofilm due to heterotrophic growth. Additionally, the microbial ecology of the different biofilms confirms the observed phosphorus uptake and release rates. The results from the full-length 16S rRNA gene sequencing of the bacterial community showed that the thicker biofilms were characterized by higher relative abundance (40-58%) of potential phosphorus accumulating genera Zoogloea, Acinetobacter, Dechloromonas and Ca. Accumulibacter. In contrast, the thinner biofilms were dominated by the genus Ferribacterium (34-60%), which might be competing with phosphorus-accumulating bacteria as indicated by the relatively high acetate uptake rates in the thinner biofilms. It is concluded that there is an optimal biofilm thickness of 100-500 µm, at which the phosphorus accumulating bacteria have the highest activity.

Pilot-scale aerobic granular sludge reactors with granular activated carbon for effective nitrogen and phosphorus removal from domestic wastewater

Aerobic granular sludge (AGS) is a breakthrough biotechnology of 21st century and an innovative alternative to activated sludge for treating wastewater. Concerns on long-start up periods for development of AGS and stability of granules are impeding its widespread implementation for treating low-strength domestic wastewater especially in tropical climate conditions. Addition of nucleating agents have been shown to improve development of AGS while treating low-strength wastewaters. There are no previous studies on AGS development and biological nutrient removal (BNR) in the presence of nucleating agents during treatment of real domestic wastewater. This study investigated AGS formation and BNR pathways while treating real domestic wastewater in a 2 m3 pilot-scale granular sequencing batch reactor (gSBR) operated without and with granular activated carbon (GAC) particles. The gSBRs were operated under tropical climate (T ≈ 30 °C) for >4-years to evaluate the effect of GAC addition on granulation, granular stability and BNR at pilot-scale. Formation of granules was observed within 3 months. MLSS values of 4 and 8 g/L were recorded within 6 months in gSBRs without and with GAC particles, respectively. The granules had an average size of 1.2 mm and SVI5 of 22 mL/g. Ammonium was mainly removed through nitrate formation in the gSBR without GAC. But, ammonium was removed by short-cut nitrification via nitrite due to washout of nitrite oxidizing bacteria in the presence of GAC. Phosphorus removal was much higher in gSBR with GAC due to the establishment of enhanced biological phosphorus removal (EBPR) pathway. After 3 months, the phosphorus removal efficiencies were at 15 % and 75 %, respectively, without and with GAC particles. The addition of GAC led to moderation in bacterial community and enrichment of polyphosphate-accumulating organisms. This is the first ever report on pilot-scale demonstration of AGS technology in the Indian sub-continent and GAC addition on BNR pathways.

Antibiotic resistance genes in constructed wetlands: Driving indicators and risk assessment

Constructed wetlands (CWs) were responsible for the in-depth purification of wastewater, providing an ideal environment for the transport, acquisition, and dissemination of antibiotic resistance genes (ARGs). A better understanding of influencing factors and risks of ARGs in CWs was deemed indispensable. In this research, the abundance of ARGs and mobile genetic elements (MGEs) was determined to be higher in summer and spring, ranging from 53.7 to 8.51 × 106 and 30.9-6.02 × 106 copies/mL, respectively. Seasonal variation significantly influenced the abundance of ARGs and MGEs, as well as the co-occurrence patterns among ARGs, MGEs and bacteria. However, the environmental gradients, from the influent (CW01) to the effluent (CW10), did not impose significant effects on the abundance of ARGs and MGEs. Furthermore, the ratios of pathogenic bacteria to ARG hosts and ARG risks index decreased by 50.4% and 88.54% along with the environmental gradients, indicating that CWs could act as barriers to the transfer of ARGs. Partial least squares-path modeling (PLSPM) revealed that temperature was the main driving factor of ARGs, followed by MGEs, stable and differential bacteria. This finding effectively and innovatively explored the driving indicators for the variations and risks of ARGs caused by spatial-temporal variations, providing new insights into the evaluation and control of ARGs in CWs.

Integrated Treatment of Per- and Polyfluoroalkyl Substances in Existing Wastewater Treatment Plants-Scoping the Potential of Foam Partitioning

Foam fractionation is becoming increasingly popular as a treatment technology for water contaminated with per- and polyfluoroalkyl substances (PFAS). At many existing wastewater treatment facilities, particularly in aerated treatment steps, foam formation is frequently observed. This study aimed to investigate if foam fractionation for the removal of PFAS could be integrated with such existing treatment processes. Influent, effluent, water under the foam, and foam were sampled from ten different wastewater treatment facilities where foam formation was observed. These samples were analyzed for the concentration of 29 PFAS, also after the total oxidizable precursor (TOP) assay. Enrichment factors were defined as the PFAS concentration in the foam divided by the PFAS concentration in the influent. Although foam partitioning did not lead to decreased ∑PFAS concentrations from influent to effluent in any of the plants, certain long-chain PFAS were removed with efficiencies up to 76%. Moreover, ∑PFAS enrichment factors in the foam ranged up to 105, and enrichment factors of individual PFAS ranged even up to 106. Moving bed biofilm reactors (MBBRs) were more effective at enriching PFAS in the foam than activated sludge processes. Altogether, these high enrichment factors demonstrate that foam partitioning in existing wastewater treatment plants is a promising option for integrated removal. Promoting foam formation and removing foam from the water surface with skimming devices may improve the removal efficiencies further. These findings have important implications for PFAS removal and sampling strategies at wastewater treatment plants.

Sialylation and Sulfation of Anionic Glycoconjugates Are Common in the Extracellular Polymeric Substances of Both Aerobic and Anaerobic Granular Sludges

Anaerobic and aerobic granular sludge processes are widely applied in wastewater treatment. In these systems, microorganisms grow in dense aggregates due to the production of extracellular polymeric substances (EPS). This study investigates the sialylation and sulfation of anionic glyconconjugates in anaerobic and aerobic granular sludges collected from full-scale wastewater treatment processes. Size exclusion chromatography revealed a wide molecular weight distribution (3.5 to >5500 kDa) of the alkaline-extracted EPS. The high-molecular weight fraction (>5500 kDa), comprising 16.9-27.4% of EPS, was dominant with glycoconjugates. Mass spectrometry analysis and quantification assays identified nonulosonic acids (NulOs, e.g., bacterial sialic acids) and sulfated groups contributing to the negative charge in all EPS fractions. NulOs were predominantly present in the high-molecular weight fraction (47.2-84.3% of all detected NulOs), while sulfated glycoconjugates were distributed across the molecular weight fractions. Microorganisms, closely related to genera found in the granular sludge communities, contained genes responsible for NulO and sulfate group synthesis or transfer. The similar distribution patterns of sialylation and sulfation of the anionic glycoconjugates in the EPS samples indicate that these two glycoconjugate modifications commonly occur in the EPS of aerobic and anaerobic granular sludges.

Mechanistic study on the ferric chloride-based rapid cultivation and enhancement of aerobic granular sludge

Aerobic granular sludge (AGS) can achieve simultaneous carbon, nitrogen and phosphorus removal owing to its three-dimensional oxygen gradient structure. However, long start-up period and poor operational stability restrict its application and promotion. A novel rapid granulation strategy, viz., the short-term (7 days) addition of ferric chloride at the commissioning stage, was developed and verified in this study. The granulation period was shortened by 9 days, and the formed granules were compact and dense with an Fe3+ concentration of 250 mg L-1. The addition of flocculant not only maintained a high sludge concentration during the initial stages of granulation (5.3 g L-1), but also stimulated the secretion of TB-EPS and increased protein and polysaccharide contents, thereby expediting granule formation. Additionally, ferric chloride induced a diverse microbial community in granules, resulting in the emergence of new genera, such as Thaurea, Brevundimonas and Kinneretia, which improved pollutant removal performance and flocculent aggregation. The removal efficiencies of COD, PO43--P, and NH4+-N stabilized at 94.2, 62.4, and 71.3%, respectively. Therefore, it has been demonstrated that short-term ferric chloride dosing has a synergistic effect on aerobic granulation.

Construction and application of a new CRISPR/Cas12a system in Stenotrophomonas AGS-1 from aerobic granular sludge

Aerobic granular sludge (AGS) is a microbial aggregate with a biofilm structure. Thus, investigating AGS in the aspect of biofilm and microbial attachment at the genetic level would help to reveal the mechanism of granule biofilm formation. In this work, a two-plasmid clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated proteins (Cas)12a genome editing system was constructed to identify attachment genes for the first time in Stenotrophomonas AGS-1 from AGS. One plasmid contained a Cas12a cassette driven by an arabinose-inducible promoter, and another contained the specific crRNA and homologous arms (HAs). Acidaminococcus sp. Cas12a (AsCas12a) was adopted and proven to have mild toxicity (compared to Cas9) and strong cleavage activity for AGS-1. CRISPR/Cas12a-mediated rmlA knockout decreased attachment ability by 38.26%. Overexpression of rmlA in AGS-1 resulted in an increase of 30.33% in attachment ability. These results showed that the modulation of rmlA was an important factor for the biofilm formation of AGS-1. Moreover, two other genes (xanB and rpfF) were knocked out by CRISPR/Cas12a and identified as attachment-related genes in AGS-1. Also, this system could achieve point mutations. These data indicated that the CRISPR/Cas12a system could be an effective molecular platform for attachment gene function identification, which would be useful for the development of AGS in wastewater treatment.

Case study of aerobic granular sludge and activated sludge-Energy usage, footprint, and nutrient removal

This study demonstrates a comparison of energy usage, land footprint, and volumetric requirements of municipal wastewater treatment with aerobic granular sludge (AGS) and conventional activated sludge (CAS) at a full-scale wastewater treatment plant characterized by large fluctuations in nutrient loadings and temperature. The concentration of organic matter in the influent to the AGS was increased by means of hydrolysis and bypassing the pre-settler. Both treatment lines produced effluent concentrations below 5 mg BOD7 L-1 , 10 mg TN L-1 , and 1 mg TP L-1 , by enhanced biological nitrogen- and phosphorus removal. In this case study, the averages of volumetric energy usage over 1 year were 0.22 ± 0.08 and 0.26 ± 0.07 kWh m-3 for the AGS and CAS, respectively. A larger difference was observed for the energy usage per reduced population equivalents (P.E.), which was on average 0.19 ± 0.08 kWh P.E.-1 for the AGS and 0.30 ± 0.08 kWh P.E.-1 for the CAS. However, both processes had the potential for decreased energy usage. Over 1 year, both processes showed similar fluctuations in energy usage, related to variations in loading, temperature, and DO. The AGS had a lower specific area, 0.3 m2  m-3 d-1 , compared to 0.6 m2  m-3 d-1 of the CAS, and also a lower specific volume, 1.3 m3  m-3 d-1 compared to 2.0 m3  m-3 d-1 . This study confirms that AGS at full-scale can be compact and still have comparable energy usage as CAS. PRACTITIONER POINTS: Full-scale case study comparison of aerobic granular sludge (AGS) and conventional activated sludge (CAS), operated in parallel. AGS had 50 % lower footprint compared to CAS. Energy usage was lower in the AGS, but both processes had potential to improve the energy usage efficiency. Both processes showed low average effluent concentrations.

Aerobic granular sludge formation and stability in enhanced biological phosphorus removal system under antibiotics pressure: Performance, granulation mechanism, and microbial successions

Wastewater containing antibiotics can pose a significant threat to biological wastewater treatment processes. This study investigated the establishment and stable operation of enhanced biological phosphorus removal (EBPR) by aerobic granular sludge (AGS) under mixed stress conditions induced by tetracycline (TC), sulfamethoxazole (SMX), ofloxacin (OFL), and roxithromycin (ROX). The results show that the AGS system was efficient in removing TP (98.0%), COD (96.1%), and NH₄⁺-N (99.6%). The average removal efficiencies of the four antibiotics were 79.17% (TC), 70.86% (SMX), 25.73% (OFL), and 88.93% (ROX), respectively. The microorganisms in the AGS system secreted more polysaccharides, which contributed to the reactor's tolerance to antibiotics and facilitated granulation by enhancing the production of protein, particularly loosely bound protein. Illumina MiSeq sequencing revealed that putative phosphate accumulating organisms (PAOs)-related genera (Pseudomonas and Flavobacterium) were enormously beneficial to the mature AGS for TP removal. Based on the analysis of extracellular polymeric substances, extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, and microbial community, a three-stage granulation mechanism was proposed including adaption to the stress environment, formation of early aggregates and maturation of PAOs enriched microbial granules. Overall, the study demonstrated the stability of EBPR-AGS under mixed antibiotics pressure, providing insight into the granulation mechanism and the potential use of AGS for wastewater treatment containing antibiotics.

Anaerobic fermentation of aerobic granular sludge: Insight into the effect of granule size and sludge structure on hydrolysis and acidification

Aerobic granular sludge (AGS) has different physicochemical properties and microbial communities compared to conventional activated sludge (CAS), which may result in different behaviors during anaerobic fermentation and require further investigation. This study investigated the effect of granule size and sludge structure on the hydrolysis and acidification of AGS. Experimental results show that AGS exhibited significantly higher soluble chemical oxygen demand (SCOD) dissolution and total volatile fatty acids (TVFA) production (330.6-430.3 mg/gVSS and 231.0-312.5 mgCOD/gVSS) compared to conventional activated sludge (CAS) (167.0 mg/gVSS and 133.3 mgCOD/gVSS). This is because AGS (90.6-96.9 mg/gVSS) had higher extracellular polymeric substances (EPS) content than CAS (81.2 mg/gVSS). EPS can not only serve as substrates but also release the trapped hydrolases. Moreover, the relative abundances of hydrolytic/acidogenic bacteria and genes were higher in AGS (0.46%-3.60% and 3.01 × 10^-3%-4.04 × 10^-3%) than in CAS (0.30% and 1.23 × 10^-3%). The optimal granule size for AGS fermentation was found to be 500-1600 μm. The crushing of granule structure promoted the dissolution of small amounts of EPS and the release of some trapped hydrolases, thereby potentially enhancing the enzyme-substrate contacts and bacteria-substrate interactions. Therefore, the highest SCOD dissolution (510.6 mg/gVSS) and TVFA production (352.1 mgCOD/gVSS) from crushed 500-1600 μm AGS were observed. Overall, the findings of this study provide valuable insights into the recovery of organic carbon from AGS via anaerobic fermentation.

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.

Density measurements of aerobic granular sludge

Granular sludge processes are frequently used in domestic and industrial wastewater treatment. The granule buoyant density and biomass density are important parameters for the design and operation of granular sludge reactors. Different methods to measure the granule density include the pycnometer method, the Percoll density gradient method, the dextran blue method, and the settling velocity method. In this study, a comparison was made between these four methods to measure granule density for granules from a full-scale granular sludge plant treating domestic sewage. The effect of salinity on granule density was assessed as well. Three out of the four evaluated methods yielded comparable results, with granule buoyant densities between 1025.7 and 1028.1 kg/m³ and granule biomass densities between 71.1 and 71.5 g/L (based on volatile suspended solids (VSS)). The settling velocity method clearly underestimated the granule density, due to the complex relation between granule properties and settling velocity. The pycnometer method was the most precise method, but it was also quite susceptible to bias. The granule buoyant density increased proportionally with salinity, to 1049.2 kg/m³ at 36 g/L salinity. However, the granule biomass density, based on VSS, remained constant. This showed that the granule volume was not affected by salinity and that the buoyant density increase was the result of diffusion of salts into the granule pores. Overall, the granule density can be measured reliably with most methods, as long as the effect of salinity is considered. The results are discussed in light of operational aspects for full-scale granular sludge plants.

De novo granulation of sewage-borne microorganisms: A proof of concept on cultivating aerobic granular sludge without activated sludge and effective enhanced biological phosphorus removal

Long start-up periods for granulating activated sludge and concerns on granular stability are the bottlenecks reported during implementation of novel aerobic granular sludge (AGS) technology in municipal wastewater treatment plants. Here, de novo granulation of sewage-borne microorganisms without using activated sludge (AS) inoculum was investigated in bench-scale sequencing batch reactors (SBR). Data showed that formation of AGS from sewage-borne microorganisms was rapid and first granules appeared within one week. Granulation was indicated by appearance of biomass particles (size >0.12 mm), high biomass levels (∼8 g/L) and superior settling properties (SVI_30 min: 30 mL/g). Granulation process involved distinct stages like formation of aggregates, retention of aggregates, and growth of millimetre sized granules. Simultaneous COD, nitrogen and phosphorous removal was established within 10 days of start-up in the SBR without using AS inoculum. However, phosphorus removal became stable after 50 days of start-up. Total nitrogen (TN) and total phosphorus (TP) removals of 92% and 70%, respectively, were achieved from real domestic wastewater. Furthermore, addition of granular activated carbon (GAC) had improved both granulation and biological nutrient removals. Interestingly, phosphorus removal became quite stable within 10 days of start-up in the SBR operated with GAC particles. TN and TP removals were found to be higher at >98% and >94%, respectively, in GAC-augmented SBR. Removal of ammonia and phosphorus were mediated by nitritation-denitritation and enhanced biological phosphorus removal (EBPR) pathways, respectively. The bacterial diversity of AGS was lower than that of sewage. Quantitative PCR indicated enrichment of ammonia oxidizing bacteria, denitrifying bacteria and polyphosphate accumulating organisms during granulation. De novo granulation of sewage-borne microorganisms is a promising approach for rapidly cultivating AGS and establishing biological nutrient removal in sewage treatment plants.

Integrated resource recovery from aerobic granular sludge plants

The study evaluated the combined phosphorus, nitrogen, methane, and extracellular polymeric substances (EPS) recovery from aerobic granular sludge (AGS) wastewater treatment plants. About 30% of sludge organics are recovered as EPS and 25-30% as methane (≈260 ml methane/g VS) by integrating alkaline anaerobic digestion (AD). It was shown that 20% of excess sludge total phosphorus (TP) ends in the EPS. Further, 20-30% ends in an acidic liquid waste stream (≈600 mg PO₄-P/L), and 15% in the AD centrate (≈800 mg PO₄-P/L) as ortho-phosphates in both streams and is recoverable via chemical precipitation. 30% of sludge total nitrogen (TN) is recovered as organic nitrogen in the EPS. Ammonium recovery from the alkaline high-temperature liquid stream is attractive, but it is not feasible for existing large-scale technologies because of low ammonium concentration. However, ammonium concentration in the AD centrate was calculated to be 2600 mg NH₄-N/L and ≈20% of TN, making it feasible for recovery. The methodology used in this study consisted of three main steps. The first step was to develop a laboratory protocol mimicking demonstration-scale EPS extraction conditions. The second step was to establish mass balances over the EPS extraction process on laboratory and demonstration scales within a full-scale AGS WWTP. Finally, the feasibility of resource recovery was evaluated based on concentrations, loads, and integration of existing technologies for resource recovery.

Coupling physical selection with biological selection for the startup of a pilot-scale, continuous flow, aerobic granular sludge reactor without treatment interruption

This study removes two technical constraints for transitioning full-scale activated sludge infrastructure to continuous flow, aerobic granular sludge (AGS) facilities. The first of these is the loss of treatment capacity as a result of the rapid washout of flocculent sludge inventory and in turn the potential loss of nitrification during initial AGS reactor startup. The second is the physical selector design which currently is limited to either the complex sequencing batch reactor selection or sidestream hydrocyclones. Briefly, real wastewater data collected from this study suggested that by increasing the surface overflow rate (SOR) of an upflow clarifier to 10 m h ^- 1, the clarifier can be taken advantage of as a physical selector to separate flocculant sludge from AGS. Redirecting the physical selector underflow and overflow sludge to the feast and famine zones of a treatment train, respectively, can create a biological selection that not only promotes AGS formation but also safeguards the effluent quality throughout the AGS reactor startup period. This study provides a novel concept for economically implementing continuous flow AGS within existing full-scale, continuous flow treatment trains.

Impact of the anaerobic feeding mode on substrate distribution in aerobic granular sludge

There is a growing interest to implement aerobic granular sludge (AGS) in existing conventional activated sludge (CAS) systems with a continuous flow-through configuration. The mode of anaerobic contact of raw sewage with the sludge is an important aspect in the adaptation of CAS systems to accommodate AGS. It remains unclear how the distribution of substrate over the sludge by a conventional anaerobic selector compares to the distribution via bottom-feeding applied in sequencing batch reactors (SBRs). This study investigated the effect of the anaerobic contact mode on the substrate (and storage) distribution by operating two lab-scale SBRs; one with the traditional bottom-feeding through a settled sludge bed similar to full-scale AGS systems, and one where the synthetic wastewater was fed as a pulse at the start of the anaerobic phase while the reactor was mixed through sparging of nitrogen gas (mimicking a plug-flow anaerobic selector in continuous flow-through systems). The distribution of the substrate over the sludge particle population was quantified via PHA analysis, combined with the obtained granule size distribution. Bottom-feeding was found to primarily direct substrate towards the large granular size classes (i.e. large volume and close to the bottom), while completely mixed pulse-feeding gives a more equal distribution of substrate over all granule sizes (i.e. surface area dependant). The anaerobic contact mode directly controls the substrate distribution over the different granule sizes, irrespective of the solids retention time of a granule as an entity. Preferential feeding of the larger granules will enhance and stabilise the granulation compared to pulse-feeding, certainly under less advantageous conditions imposed by real sewage.

Challenges of aerobic granular sludge utilization: Fast start-up strategies and cationic pollutant removal

Aerobic granular sludge (AGS) is a self-aggregated microorganism consortium with pollutant removal properties. The aim of this work is to study and review the application of aerobic granules for water treatment with special focus on new applications and methodologies. Carbon-nitrogen containing pollutants are the classic targets of AGS technology. Carbon and nitrogen removal of AGS are classified as a biodegradation process. More recently, the AGS granules have been studied as sorbent materials for wastewater treatment. In particular, the sorption of cationic pollutants has been studied through biosorption and bioaccumulation mechanisms without distinguishing when one or the other process is involved. AGS conformation made them suitable for complex wastewater treatment. Indeed, several studies have demonstrated the removal of polyvalent cationic pollutants even with higher capacity than conventional sorbent materials. However, this was achieved almost exclusively for synthetic substrates, with single cation evaluation and using in some cases only qualitative measures. For successful industrial AGS application in complex substrates, it is necessary to evaluate and demonstrate the technology in real industrial conditions and reduce the currently long start-up times which limits its utility. Two new strategies have been proposed: autoinducer molecules and the production of artificial granular from common active sludge with commercial alginate. Finally, the increase of research on AGS cations assimilation properties will allow a new point of view, where granules will be materials for the recovery of valuable metals from industrial wastewater streams.