Aerobic granular processes: Current research trends

Enhancing Wastewater Treatment with Aerobic Granular Sludge: Impacts of Tetracycline Pressure on Microbial Dynamics and Structural Stability

This study evaluated the efficiency of aerobic granular sludge (AGS) technology in treating wastewater contaminated with tetracycline (TC), a common antibiotic. AGS was cultivated under a TC pressure gradient ranging from 5 mg/L to 15 mg/L and compared with conventional wastewater conditions. The results demonstrated that AGS achieved high removal efficiencies and exhibited robust sedimentation performance, with significant differences in average particle sizes observed under both conditions (618.6 μm in TC conditions vs. 456.4 μm in conventional conditions). Importantly, exposure to TC was found to alter the composition and production of extracellular polymeric substances (EPSs), thereby enhancing the structural integrity and functional stability of the AGS. Additionally, the selective pressure of TC induced shifts in the microbial community composition; Rhodanobacter played a crucial role in EPS production and biological aggregation, enhancing the structural integrity and metabolic stability of AGS, while Candida tropicalis demonstrated remarkable resilience and efficiency in nutrient removal under stressful environmental conditions. These findings underscore the potential of AGS technology as a promising solution for advancing wastewater treatment methods, thus contributing to environmental protection and sustainability amid growing concerns over antibiotic contamination.

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.

Characteristics, Performance and Microbial Response of Aerobic Granular Sludge for Treating Tetracycline Hypersaline Pharmaceutical Wastewater

Salt-tolerant aerobic granular sludge(AGS) was successfully cultivated under the dual stress of tetracycline and 2.5% salinity, resulting in an average particle size of 435.0 ± 0.5 and exhibiting a chemical oxygen demand(COD) removal rate exceeding 80%, as well as excellent sedimentation performance. The analysis of metagenomics technology revealed a significant pattern of succession in the development of AGS. The proportion of Oleiagrimonas, a type of salt-tolerant bacteria, exhibited a gradual increase and reached 38.07% after 42 days, which indicated that an AGS system based on moderate halophilic bacteria was successfully constructed. The expression levels of targeted genes were found to be reduced across the entire AGS process and formation, as evidenced by qPCR analysis. The presence of int1 (7.67 log10 gene copies g-1 in 0 d sludge sample) enabled microbes to horizontally transfer ARGs genes along the AGS formation under the double pressure of TC and 2.5% salinity. These findings will enhance our understanding of ARG profiles and the development in AGS under tetracycline pressure, providing a foundation for guiding the use of AGS to treat hypersaline pharmaceutical wastewater.

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.

Resistance of aerobic granular sludge microbiomes to periodic loss of biomass

Granular sludge is a biofilm process used for wastewater treatment which is currently being implemented worldwide. It is important to understand how disturbances affect the microbial community and performance of reactors. Here, two acetate-fed replicate reactors were inoculated with acclimatized sludge and the reactor performance, and the granular sludge microbial community succession were studied for 149 days. During this time, the microbial community was challenged by periodically removing half of the reactor biomass, subsequently increasing the food-to-microorganism (F/M) ratio. Diversity analysis together with null models show that overall, the microbial communities were resistant to the disturbances, observing some minor effects on polyphosphate-accumulating and denitrifying microbial communities and their associated reactor functions. Community turnover was driven by drift and random granule loss, and stochasticity was the governing ecological process for community assembly. These results evidence the aerobic granular sludge process as a robust system for wastewater treatment.

Low-temperature-resistance granulation of activated sludge and the microbial responses to the granular structural stabilization

Completely loss of granular structural stability and reliable start-up of aerobic granular sludge (AGS) system are considered as the biggest challenges for its engineering application under seasonal temperature variation, especially extremely low temperatures. In this study, two identical sequencing batch reactors (SBR) were successfully start-up at 10 °C (R1) and 25 °C (R2), respectively, and then operated under a strategy of stepwise change of temperatures to investigate the stability of the granular sludge by examining its microbial characteristics, bis (3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP), extracellular polymeric substance (EPS) and sludge physiochemical properties. The results showed that AGS formed under the low temperature preferentially secrete EPS and c-di-GMP for stable granulation and improvement of its resistance to temperature changes. Meanwhile, R1 successfully obtained aerobic granulation with high biomass concentration and superior settleability, as well as high pollutant removal performance. In comparison, R2 took a longer time for granulation and was subjected to serious disintegration of AGS. The matrix structure partially formed by filamentous bacteria during the start-up stage in R1 was one of major reasons for its own superiority beyond R2 in granulation. Slow-growing organisms such as autotrophic nitrifying and Anammox bacteria, phosphorus accumulation organisms, EPS-producing genera, and c-di-GMP pathway-dependent genera, were exclusively enriched in the R1 and resulted in higher pollutants removal efficiencies and stable structure, whereas Sphaerotilus dominated in R2 that related closely with its unstable performance. Therefore, the strategy based on the stepwise change of temperatures from extremely low temperatures may be one feasible way for the sustainable application of AGS system, which is of significance to address the challenging problems of AGS applications.

Aerobic granulation and microbial community succession in sequencing batch reactors treating the low strength wastewater: The dual effects of weak magnetic field and exogenous signal molecule

The application of magneto-biological effects in wastewater treatment has been brought under the spotlight recently. This work explored the dual effects of magnetic field (MF) and exogenous N-hexanoyl-l-homoserine lactone (C₆-HSL) on activated sludge granulation. Results showed that exposure to MF and C₆-HSL obviously accelerated the aerobic granulation process and promoted the secretion of extracellular polymeric substances, especially polysaccharides, humic acid-like substances, aromatic proteins, and tryptophan-like substrates. Illumina MiSeq sequencing results indicated that the introduction of MF and C₆-HSL can increase the diversity and richness of microbial community without antagonism, and the biological basis for rapid granulation process in this study was the enrichment of slow-growing bacteria Candidatus_Competibacter. Besides, the overgrowth of filamentous bacteria Thiothrix could be suppressed due to the presence of MF, improving the stabilities of aerobic granular sludge. This study provides a new understanding of the MF and C₆-HSL effects on rapid aerobic granulation when treating the low-strength wastewater.

Distinct responses of aerobic granular sludge sequencing batch reactors to nitrogen and phosphorus deficient conditions

The nutrients availability determines efficiency of biological treatment systems, along with the structure and metabolism of microbiota. Herein nutrients deficiencies on aerobic granular sludge were comparatively evaluated, treating wastewater with mass ratios of chemical oxygen demand : nitrogen : phosphorus being 200:20:4, 200:2:4, and 200:20:0.4 (deemed as nutrient-balanced, nitrogen-deficient, and phosphorus-deficient), respectively. Results revealed that both nitrogen and phosphorus deficiencies significantly raised the effluent qualities especially nitrogen removal. However, nitrogen deficiency aroused considerable growth of filamentous bacteria, while granules kept compact structure under phosphorus deficient condition. Extracellular polymeric substances (EPS) also varied in contents and structures in response to different wastewaters. Microbial community structure analysis demonstrated that nitrogen deficiency led to lower richness and higher diversity, while the reverse was observed under phosphorus deficient condition. Nitrogen deficiency mainly induced decrease of nitrifying bacteria, while similarly phosphorus deficiency led to loss of phosphorus accumulating organisms. Dramatic enrichment Candidatus_Competibacter and filamentous Thiothrix were found under nutrients deficiencies, in which the latter explained and indicated filamentous bulking potential especially under nitrogen limited condition. Bacterial metabolism patterns verified the functions of microbial community responding to nutrients via PICRUSt2 prediction mainly by up-regulating cell motility, and cellular processes and signaling. This study could aid understanding of long-term stability of aerobic granular sludge for low-strength wastewater treatment.

Sustainable production of algae-bacteria granular consortia based biological hydrogen: New insights

Microbes recycling nutrient and detoxifying ecosystems are capable to fulfil the future energy need by producing biohydrogen by due to the coupling of autotrophic and heterotrophic microbes. In granules microbes mutualy exchanging nutrients and electrons for hydrogen production. The consortial biohydrogen production depend upon constituent microbes, their interdependence, competition for resources, and other operating parameters while remediating a waste material in nature or bioreactor. The present review deals with development of granular algae-bacteria consortia, hydrogen yield in coculture, important enzymes and possible engineering for improved hydrogen production.

Dynamics of microbial community and their effects on membrane fouling in an anoxic-oxic gravity-driven membrane bioreactor under varying solid retention time: A pilot-scale study

Membrane fouling in a membrane bioreactor (MBR) is highly influenced by the characteristics of the influent, the mixed liquor microbial community and the operational parameters, all of which are environment specific. Therefore, we studied the dynamics of microbial community during the treatment of real municipal wastewater in a pilotscale anoxic-oxic (A/O) MBR equipped with a gravity-driven membrane filtration system. The MBR was operated at three different solid retention times (SRTs): 25, 40 and 10 days for a total period of 180 days in Nordic environmental conditions. Analysis of microbial community dynamics revealed a high diversity of microbial species at SRT of 40 days, whereas SRT of 25 days was superior with microbial richness. Production of soluble microbial products (SMP) and extracellular polymeric substances (EPS) was found to be intensely connected with the SRT and food to microorganism (F/M) ratio. Relatively longer operational period with the lowest rate of membrane fouling was observed at SRT of 25 days, which was resulted from the superior microbial community, lowest production of SMP and loosely bound EPS as well as the lower filtration resistance of larger sludge flocs. Abundance of quorum quenching (QQ) bacteria and granular floc forming bacterial genera at SRT of 25 days provided relatively lower membrane fouling tendency and larger floc formation, respectively. On the other hand, substantial amount of various surface colonizing and EPS producing bacteria was found at SRT of 10 days, which promoted more rapid membrane fouling compared with the fouling rate seen at other tested SRTs. To sum up, this research provides a realistic insight into the impact of SRT on microbial community dynamics and resulting characteristics of mixed liquor, floc size distribution and membrane fouling for improved MBR operation.

Discovering future research trends of aerobic granular sludge using bibliometric approach

The advantageous characteristics of aerobic granular sludge (AGS) have led to their increasing popularities among academics and industrial players. However, there has been no bibliometric report on current and future research trends of AGS. This study utilized the available reports of AGS in the Scopus database for comprehensive bibliometric analyses using VOSviewer software. A total of 1203 research articles from 1997 to 2020 were analyzed. The dominance of the Netherlands and China were revealed by the high number of publications and citations. Nevertheless, the Netherlands exhibited higher average citation per article at 76.4. A recent process of AGS involving biochar and algal addition were also identified. Meanwhile, the application of AGS for antibiotic containing wastewater as well as possibility of resource recovery were recently reported and was expected to expand in the future. It was suggested that application of AGS would develop further along with the development of sustainable wastewater treatment process.

Effects of wastewater type on stability and operating conditions control strategy in relation to the formation of aerobic granular sludge - a review

Currently, research trends on aerobic granular sludge (AGS) have integrated the operating conditions of extracellular polymeric substances (EPS) towards the stability of AGS systems in various types of wastewater with different physical and biochemical characteristics. More attention is given to the stability of the AGS system for real site applications. Although recent studies have reported comprehensively the mechanism of AGS formation and stability in relation to other intermolecular interactions such as microbial distribution, shock loading and toxicity, standard operating condition control strategies for different types of wastewater have not yet been discussed. Thus, the dimensional multi-layer structural model of AGS is discussed comprehensively in the first part of this review paper, focusing on diameter size, thickness variability of each layer and diffusion factor. This can assist in facilitating the interrelation between disposition and stability of AGS structure to correspond to the changes in wastewater types, which is the main objective and novelty of this review.

Research on rapid cultivation of aerobic granular sludge (AGS) with different feast-famine strategies in continuous flow reactor and achieving high-level denitrification via utilization of soluble microbial product (SMP)

The mixture of partial AGS and flocculent sludge in continuous flow reactors were operated with periodic famine (PF) strategy and continuous feast (CF) strategy to reveal the impact of the feast-famine strategies on cultivation of AGS and the dynamics of microbial communities. The experimental results showed that the mature AGS were cultivated with PF and CF strategy on the 31st and the 71st days respectively, which was the result of good extracellular polymeric substance (EPS) secretion with PF strategy. It could accelerate the formation of microbial aggregates due to the conditions of periodic famine. High-level denitrification with PF strategy via utilization of SMP was examined by EEM-PARAFAC on cycle test. The high-throughput pyrosequencing showed that the dominant bacteria with PF strategy involved functional bacteria of nutrient removal and EPS secreting bacteria, while the dominant bacteria were fast-growing heterotrophic organisms with CF strategy.

Salinity is the major driver of the global eukaryotic community structure in fish-canning wastewater treatment plants

Fish-canning wastewater is characterized frequently by a high content of salt (NaCl), making its treatment particularly difficult; however, the knowledge of the effect of NaCl on eukaryotic communities is very limited. In the present study, the global diversity of eukaryotes in activated sludges (AS) from 4 different wastewater treatment plants (WWTPs) treating fish-canning effluents varying in salinity (0.47, 1.36, 1.72 and 12.76 g NaCl/L) was determined by sequencing partial 18S rRNA genes using Illumina MiSeq. A greater diversity than previously reported was observed in the AS community, which comprised 37 and 330 phylum-like and genera-like groups, respectively. In this sense, the more abundant genus-like groups (average relative abundance (RA) > 5%) were Adineta (6.80%), Lecane (16.80%), Dictyostelium (7.36%), Unclassified_Fungi7 (6.94%), Procryptobia (5.13) and Oocystis (5.07%). The eukaryotic communities shared a common core of 25 phylum-like clades (95% of total sequences); therefore, a narrow selection of the eukaryotic populations was found, despite the differences in the abiotic characteristics of fish-canning effluents and reactor operational conditions inflicted. The differences in NaCl concentration were the main factor that influenced the structure of the eukaryotic community, modulating the RAs of the different phylum-like clades of the common core. Higher levels of salt increased the RAs of Ascomycota, Chlorophyta, Choanoflagellata, Cryptophyta, Mollusca, Nematoda, Other Protists and Unclassified Fungi. Among the different eukaryotic genera here found, the RA of Oocystis (Chlorophyta) was intimately correlated to increasing NaCl concentrations and it is proposed as a bioindicator of the global eukaryotic community of fish-canning WWTPs.

Resource recovery in aerobic granular sludge systems: is it feasible or still a long way to go?

Lately, wastewater treatment plants are much often being designed as wastewater-resource factories inserted in circular cities. Among biological treatment technologies, aerobic granular sludge (AGS), considered an evolution of activated sludge (AS), has received great attention regarding its resource recovery potential. This review presents the state-of-the-art concerning the influence of operational parameters on the recovery of alginate-like exopolysaccharides (ALE), tryptophan, phosphorus, and polyhydroxyalkanoates (PHA) from AGS systems. The carbon to nitrogen ratio was identified as a parameter that plays an important role for the optimal production of ALE, tryptophan, and PHA. The sludge retention time effect is more pronounced for the production of ALE and tryptophan. Additionally, salinity levels in the bioreactors can potentially be manipulated to increase ALE and phosphorus yields simultaneously. Some existing knowledge gaps in the scientific literature concerning the recovery of these resources from AGS were also identified. Regarding industrial applications, tryptophan has the longest way to go. On the other hand, ALE production/recovery could be considered the most mature process if we take into account that existing alternatives for phosphorus and PHA production/recovery are optimized for activated sludge rather than granular sludge. Consequently, to maintain the same effectiveness, these processes likely could not be applied to AGS without undergoing some modification. Therefore, investigating to what extent these adaptations are necessary and designing alternatives is essential.

New Advances in Aerobic Granular Sludge Technology Using Continuous Flow Reactors: Engineering and Microbiological Aspects

Aerobic granular sludge (AGS) comprises an aggregation of microbial cells in a tridimensional matrix, which is able to remove carbon, nitrogen and phosphorous as well as other pollutants in a single bioreactor under the same operational conditions. During the past decades, the feasibility of implementing AGS in wastewater treatment plants (WWTPs) for treating sewage using fundamentally sequential batch reactors (SBRs) has been studied. However, granular sludge technology using SBRs has several disadvantages. For instance, it can present certain drawbacks for the treatment of high flow rates; furthermore, the quantity of retained biomass is limited by volume exchange. Therefore, the development of continuous flow reactors (CFRs) has come to be regarded as a more competitive option. This is why numerous investigations have been undertaken in recent years in search of different designs of CFR systems that would enable the effective treatment of urban and industrial wastewater, keeping the stability of granular biomass. However, despite these efforts, satisfactory results have yet to be achieved. Consequently, it remains necessary to carry out new technical approaches that would provide more effective and efficient AGS-CFR systems. In particular, it is imperative to develop continuous flow granular systems that can both retain granular biomass and efficiently treat wastewater, obviously with low construction, maintenance and exploitation cost. In this review, we collect the most recent information on different technological approaches aimed at establishing AGS-CFR systems, making possible their upscaling to real plant conditions. We discuss the advantages and disadvantages of these proposals and suggest future trends in the application of aerobic granular systems. Accordingly, we analyze the most significant technical and biological implications of this innovative technology.

Treatment of real domestic sewage in a pilot-scale aerobic granular sludge reactor: Assessing start-up and operational control

Aerobic granular sludge (AGS) has been considered a breakthrough in the wastewater treatment sector given its key characteristics, such as excellent settleability, simultaneous removal of organic and nutrient pollutants, and compactness. However, the formation of granules often delays the start-up of granular-based systems, especially in large-scale settings. This study addressed the start-up of a pilot-scale AGS sequencing batch reactor (SBR) treating domestic sewage, monitored for over 280 days. The challenges faced during aerobic granulation using a mixture of activated sludge and anaerobic granular sludge as inoculum and the performance of the reactor on organic matter, nitrogen, and phosphorus removal were discussed. Results showed that robust and stable granules were formed after an initial period of around six months, with the settling time playing a key role on granules development. At least 80% of granules had a diameter greater than 0.2 mm and 60% >1 mm. In general, the reactor achieved high nitrogen removal efficiency, as well as satisfactory removal of soluble COD. However, total COD abatement was impaired by the various episodes of suspended solids loss with the effluent. Overall, this study demonstrated that the reactor was efficient in the treatment of domestic sewage, but its performance was adversely affected from sudden changes in the influent quality. PRACTITIONER POINTS: Aerobic granular sludge (AGS) applied to small-scale domestic sewage treatment. The control of sludge age in AGS can be a problem due to short sedimentation times. High DO to maintain aerobic granulation can economically make the process economically unfeasible in tropical countries. A sludge with excellent sedimentation properties was obtained. However, maintaining the granule over time is a challenge.

Nitrification characteristics of long-term idle aerobic activated sludge during domestication

Nitrite accumulation usually occurred when domesticating the idle aerobic activated sludge. A sequencing batch reactor (SBR) was used to investigate whether the short-cut nitrification sludge could be cultivated using the idle sludge as inoculated sludge. The results showed that the nitrification process consisted of three stages. In the first stage, the activity of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were very low with almost no nitrification performance. In the second stage, the activity of AOB started to recover with the effluent NH₄⁺-N gradually decreased to 0.29 mg L^-1, while NOB was alternately inhibited by free ammonia (FA), free nitrous acid (FNA), and nitrite. The effluent NO_x^--N was mainly NO₂^--N with an average nitrite accumulation ratio of 74.00%. In the third stage, the nitrification altered from short-cut nitrification to complete nitrification, and the nitrification kinetics of AOB and NOB were both well-fitted to the Monod equation (R² > 0.92). The variations of effluent pH and ORP between cycles could indicate the recovery stage of the nitrifying ability. Through monitoring the curves of effluent pH and ORP, when the domestication process is between the pH peak and ORP plateau, the short-cut nitrification sludge could be cultivated. This study revealed the mechanism of nitrite accumulation during the domestication of long-term idle aerobic activated sludge, and established a control strategy to accelerate the domestication.

A Bioreactor Designed for Restricting Oversize of Aerobic Granular Sludge

Aerobic granular sludge (AGS) with oversized diameter commonly affects its stability and pollutant removal. In order to effectively restrict the particle size of AGS, a sequencing batch reactor (SBR) with a spiny aeration device was put forward. A conventional SBR (R1) and an SBR (R2) with the spiny aeration device treating tannery wastewater were compared in the laboratory. The result indicates that the size of the granular sludge from R2 was smaller than that from R1 with sludge granulation. The spines and air bubbles could effectively restrict the particle size of AGS by collision and abrasion. Nevertheless, there was no significant change in mixed liquor suspended solids (MLSS) and the sludge volume index (SVI) in either bioreactors. The removal (%) of chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) in these two bioreactors did not differ from each other greatly. The analysis of biological composition displays that the proportion of Proteobacteria decreased slightly in R2. The X-ray fluorescence (XRF) analysis revealed less accumulation of Fe and Ca in smaller granules. Furthermore, a pilot-scale SBR with a spiny aeration device was successfully utilized to restrict the diameter of granules at about 300 μm.

MOF based engineered materials in water remediation: Recent trends

The significant upsurge in the demand for freshwater has prompted various developments towards water sustainability. In this context, several materials have gained remarkable interest for the removal of emerging contaminants from various freshwater sources. Among the currently investigated materials for water treatment, metal organic frameworks (MOFs), a developing class of porous materials, have provided excellent platforms for the separation of several pollutants from water. The structural modularity and the striking chemical/physical properties of MOFs have provided more room for target-specific environmental applications. However, MOFs limit their practical applications in water treatment due to poor processability issues of the intrinsically fragile and powdered crystalline forms. Nevertheless, growing efforts are recognized to impart macroscopic shapability to render easy handling shapes for real-time industrial applications. Furthermore, efforts have been devoted to improve the stabilities of MOFs that are subjected to fragile collapse in aqueous environments expanding their use in water treatment. Advances made in MOF based material design have headed towards the use of MOF based aerogels/hydrogels, MOF derived carbons (MDCs), hydrophobic MOFs and magnetic framework composites (MFCs) to remediate water from contaminants and for the separation of oils from water. This review is intended to highlight some of the recent trends followed in MOF based material engineering towards effective water regeneration.

Effects of phenol on extracellular polymeric substances and microbial communities from aerobic granular sludge treating low strength and salinity wastewater

The effects of phenol on aerobic granular sludge including extracellular polymeric substances (EPS) and microbial community were investigated for low strength and salinity wastewater treatment. Elevated phenol over 20 mg/L stimulated biological phosphorus removal mainly via co-metabolism with nearly complete phenol degradation, whereas resulted in significant accumulation of nitrate around 4 mg/L. Aerobic granules kept structural stability via enhancing production of extracellular polymeric substances (EPS), especially folds of polysaccharides (PS) and varying functional groups identified through EEM, FTIR and XPS spectral characterizations at increasing phenol loads. Illumina MiSeq sequencing results indicated that elevated phenol decreased the bacterial diversity and richness, and caused remarkable variations in structural and compositions of microbial population. Multiple halophilic bacteria including Stappia, Luteococcus, and Formosa laid the biological basis for stability of aerobic granules and efficient biological nutrients and phenol removal. Redundancy analysis (RDA) suggested the key role of phenol in shaping the relative abundances and predominant genera. This study proved that aerobic granular sludge was feasible for low-saline and phenol-laden low-strength wastewater treatment.

Optimizing granulation of a sulfide-based autotrophic denitrification (SOAD) sludge: Reactor configuration and mixing mode

Challenges such as long-term cultivation and sludge floatation are common in flocculent sulfide-oxidizing autotrophic denitrification (SOAD) systems. The present study aims to optimize the granulation of SOAD sludge by mainly adjusting the reactor configuration and mixing mode. Three liquid-lift upflow reactors viz. a reactor equipped with a three-phase separator (Reactor A), a modified version of Reactor A equipped with a hydraulic regulator (Reactor B), and a reactor with a mounted baffle and intermittent mechanical mixing (Reactor C). These reactors were operated for more than 160 days. The results showed that dense and compact granules with 200 μm of diameter developed within 40 days and gradually increased to approximately 400 μm in Reactor C, which had a volatile suspended solids (VSS) concentration of 7500 mg VSS/L of sludge concentration; this Reactor C was also subject to modified reactor configuration and operating conditions. In comparison, filamentous granules formed in Reactor A due to a low substrate loading and granules formed in Reactor B but with significant biomass loss caused by sludge flotation. Both of the reactors only have ≤1000 mg VSS/L VS 7500 mg VSS/L in Reactor C. Also, Reactor C having 0.77 h of hydraulic retention time (HRT) and 0.94 kgNO₃^--N/m³ d & 1.87 kgS^2--S/m³ d of nitrogen and sulfide loading rate, respectively, showed a better performance in terms of nitrate removal (89%) and sulfur conversion (above 70%) due to its enrichment by the typical autotrophic denitrifiers (39.0% of Thiobacillus, 22.4% of Sulfurimonas) in the granules. Our findings provide a method to optimize the design and operation of granulation reactors that can be extended to similar processes treating organic-deficient wastewaters.

Characterization of aerobic granules formed in an aspartic acid fed sequencing batch reactor under unfavorable hydrodynamic selection conditions

Granules initiation and development is the backbone of aerobic granular sludge technology. Feed composition can notably affect initiation and development of aerobic granules, and yield aerobic granules with distinct microbial community, morphology and structure. This paper reports an unexpected formation of aerobic granules in an aspartic acid fed SBR under unfavorable hydrodynamic selection conditions. Detailed characteristics of these aerobic granules were investigated in terms of morphology, structure, bioactivity and EPS. The results showed that due to the absence of favorable hydrodynamic selection pressure, the formed aerobic granules had an irregular shape with a rough outline and loose internal structure, which was quite different from mature aerobic granules. Bacteria in these aerobic granules were mainly presented in the form of microcolony with calcium and β-polysaccharides responsible for its mechanical stability. The high N/C ratio of aspartic acid enabled the enrichment of significant amount of nitrifiers within aerobic granules and thus resulted in high nitrification activity of these aerobic granules. The negatively charged and hydrophilic aspartic acid also induced the bacteria to secrete more exopolysaccharides for contributing to more neutral and hydrophilic surface of the aerobic granules, which was beneficial for aspartic acid capture. As a result, polysaccharides, rather than proteins, became the major components of EPS in these aerobic granules. This paper provides us a foundation to better understand the granulation potential of proteinaceous substrates that is frequently encountered in industrial wastewaters.

Coupling of Fe-C and aerobic granular sludge to treat refractory wastewater from a membrane manufacturer in a pilot-scale system

A novel pilot-scale system based on aerobic granular sludge (AGS) as a biological treatment step was proposed to treat refractory wastewater from a membrane manufacturer. The components of the system included a microelectrolysis Fe-C filter, a hydrolysis acidification bioreactor (HA), sequence batch reactor 1 (AGS SBR1), sequence batch reactor 2 (AGS SBR2), and a membrane bioreactor (MBR). The Fe-C filter effectively improved the biodegradability of the wastewater components and introduced some byproducts (such as Fe²⁺, Fe³⁺, and Fe minerals) that are beneficial for the cultivation and stability of the AGS. Ideal conditions for aerobic granulation were maintained in the SBR, such as alternating feast and famine conditions. A selection pressure, including a hydraulic shear force and settling time, was also created therein. The results showed that the AGS was formed successfully in both SBR1 and SBR2, the sludge volume index after 30 min (SVI₃₀) and mean particle size reached 34.2 mL/g and 720 µm, and 36.7 mL/g and 610 µm, respectively, and a satisfactory nutrient removal capacity was achieved in the system. During the entire experimental period, the microbial community changed significantly; enrichment of microbes with the secretion of extracellular polymeric substances (EPS), granule stabilization functions in the AGS, and the differentiation of microbes corresponding to the function of each unit were observed. The use of Fe-C, application of SBRs, and use of dewatered sludge as an inoculant played key roles in the cultivation and stability of the AGS.

Successful granulation and microbial differentiation of activated sludge in anaerobic/anoxic/aerobic (A2O) reactor with two-zone sedimentation tank treating municipal sewage

A continuous pilot-scale A²O reactor with a two-zone sedimentation tank (A²O-TST) was constructed for the formation of aerobic granular sludge (AGS) to treat real municipal sewage. The characteristics of sludge, nutrient removal performance and the corresponding microbial ecology dynamics were studied during granulation process. Experimental results indicated that AGS with a mean particle size of 210 μm and sludge volume index after 30 min of 47.5 mL/g was successfully formed with effluent COD, total nitrogen and total phosphorus concentrations in the reactor reaching 22.8, 3.5 and 0.2 mg/L, respectively. Furthermore, high throughput data indicated that granules in settling tank-1 (ST-1) harbored slow-growing autotrophic organisms like Nitrosomonas and Nitrospira, while the flocs in settling tank-2 (ST-2) were dominated by fast-growing heterotrophic organisms including Ca. Accumulibacter, Dechloromonas, Flavobacterium, Arcobacter and Halomonas. Simulation results using computational fluid dynamics and discrete element method (CFD-DEM) modeling verified that the selection pressure created by the TST separator contributed to the retention of heavy granules (>1.011 kg/m³ density) in ST-1 zone and the withdrawal of light flocs (<1.011 kg/m³ density) from ST-2 zone. Therefore, the segregation of biomass using the TST system provides an opportunity to select for desired microbial populations and to optimize the nitrogen and phosphorus removal performance of the A²O-TST reactor. This study could add a guiding sight into the application of two-sludge system based on AGS technology for upgrading traditional A²O process.

Impact of additive application on the establishment of fast and stable aerobic granulation

Aerobic granular sludge (AGS) is a microbial biofilm self-aggregation, which is effective for nutrient and pollutant removal, through the development of dense microbial layers bound together with extracellular polymeric substances (EPSs). However, long start-up times and granule disintegration are still challenges ahead. An array of external additives, including ion chelating agents, sludge-based enhancers, and magnetic influence have been tested to overcome these barriers. The application of such additives may promote enhanced EPS production, neutralization of charges on the bacterial surface, acts as a core-induced agent, or as a bridge to connect EPSs and cell surfaces. Although additives may improve the granule formation without reducing treatment efficiencies, there are still environmental concerns due to the fate and toxicity of discharged excess sludge. This mini-review identifies an array of external additives and their mechanisms to improve granulation properties, and proposes discussion about the technical and economic viability of these additives. KEY POINTS: • Additives reduce granulation time and repair granule disintegration. • Biopolymer-based additives fulfill technical and environmental requirements. • Sludge-based additives are cheap and in line with the resource recovery concept. • The need for environmental-friendly additives for aerobic granular sludge process. • External additives affect granular biomass size distribution.

Elevated salinity deteriorated enhanced biological phosphorus removal in an aerobic granular sludge sequencing batch reactor performing simultaneous nitrification, denitrification and phosphorus removal

Hypersaline wastewater may pose threats to biological wastewater treatment processes. An aerobic granular sludge-based sequencing batch reactor (SBR) performing simultaneous nitrification, denitrification and phosphorus removal (SNDPR) was evaluated with increased salinity from 1 to 2 % (w/v). Nitrogen removal performance was unaffected by salinity up to 20 g/L in terms of reliable and efficient nitrification and denitrification. Enhanced biological phosphorus removal (EBPR) process was completely deteriorated at salinity up to 2 %, in contrast to excellent phosphorus removal at 1 %. Profiles of phosphorus over one cycle demonstrated that higher salinity not only inhibited anaerobic phosphorus release but also impeded aerobic/anoxic phosphorus uptake. Illumina MiSeq sequencing revealed multiple halophilic and non-halophilic bacteria within aerobic granules with family Anaerolineaceae being the predominant potential salt adapter. Besides, ammonia oxidizing bacteria (AOB), glycogen accumulating organisms (GAOs) were more tolerant to salt than nitrite oxidizing bacteria (NOB) and phosphorus accumulating organisms (PAOs) and denitrifying PAOs (DNPAOs). These results deciphered the resilience of aerobic granular sludge-based biological nitrogen and phosphorus removal processes to hypersaline stress.

Rapid aerobic sludge granulation in an integrated oxidation ditch with two-zone clarifiers

Aerobic granular sludge (AGS) was rapidly cultivated in an integrated oxidation ditch with two-zone clarifiers by using a novel external sludge treatment and return mode to treat low concentrations of actual domestic sewage. The selective pressure created by the two-zone clarifiers can retain the well-settling granules and discharge light flocs with poor settleability. The granules stayed in the reactor, which induced bacterial attachment to the granules that acted as nuclei, while the discharged flocs can stimulate microorganisms to secrete large amounts of extracellular polymeric substances (EPS) under the external conditioning of CaCl₂ and natural air drying. Then, this surplus sludge was returned to the reactor to create more small granules that combined with each other through the action of hydraulic shear forces to achieve rapid granulation. The results showed that AGS was formed successfully in the reactor on day 18, and after 51 days of continuous operation, the biomass concentration and settling ability were further improved (the mixed liquor suspended solids (MLSS) and sludge volume index at 5 min (SVI₅) were stable at approximately 3500 mg/L and 40.0 mL/g, respectively). During the whole experimental period, the biological sludge activity was greatly improved, and the EPS and microbial community changed significantly, including an enrichment of microbes with EPS secretion and granule stabilization functions. The study results reveal that the pollutant removal efficiency improved after granulation. Furthermore, this approach required less energy and is eco-friendly for potential full-scale implementation.

Cultivation of granules containing anaerobic decolorization and aerobic degradation cultures for the complete mineralization of azo dyes in wastewater

Granules which could efficiently mineralize azo dyes were cultivated through immobilization of aerobic degradation strains in a core composed of anaerobic decolorization cultures. The core was obtained in a up-flow anaerobic sludge blanket (UASB) reactor incubated with anaerobic decolorization bacteria. Aerobic degradation strains were then grown on the surface of the anaerobic core in a sequencing batch reactor (SBR). Three of the granules' surface layers demonstrated the occurrence of immobilization. The granulation process was monitored with 16S rDNA high throughput sequencing. Anaerobic decolorization cultures belonging to the genera of unclassified, Levilinea, and Petrimonas and the aerobic degradation genera of Thauera, unclassified, Thermomonas, and Ottowia were successfully fixed in the granules. The obtained granules were capable of decolorizing azo dyes under anaerobic situation, and the generated aromatic amines were then completely mineralized in aerated environment. Comparative studies on the relationship between removed contaminates and typical components concentrations in low to high strength azo dye wastewater showed that the granules have great potentials in treating wastewater with different complexity. The removal efficiency of COD and TOC was not restricted by loading concentrations.

Metagenomics revealed the phase-related characteristics during rapid development of halotolerant aerobic granular sludge

Efforts to produce aerobic granular sludge (AGS) for high-efficient and stable nutrient removal in high saline wastewaters have gained much attention recently. This study was undertaken to describe the phase-related characteristics of the rapid formation of glucose-fed salt-tolerant AGS (SAGS) generated from common municipal activated sludge using metagenomic approaches. The time needed for SAGS formation is about 11 days in a multi-ion matrix salinity of 3%. There were three distinct developmental phases during sludge maturation which were designated: I) the salinity adaptation phase (days 1-2), II) the particle-size transition phase (days 3-5) and III) the maturation and steady-state phase (days 6-11), respectively. Genome-based analysis revealed that during the phase I, members of the genus Mangrovibacter, which has the potential to secrete extracellular polymeric substances (EPS), dominated during the formation of initial SAGS aggregates. During phase II, fungi of the class Saccharomycetes, in particular the genus Geotrichum, became dominant and provided a matrix for bacterial attachment. This mutualistic interaction supported the rapid development and maintenance of mature SAGS. This work characterizes a robust approach for the rapid development of SAGS for efficient saline sewage treatment and provides unique insight into the granulation mechanism occurring during the development process.

Formation of partial-denitrification (PD) granular sludge from low-strength nitrate wastewater: The influence of loading rates

Granular sludge has been believed to be a promising technology in wastewater biological treatment. However, the formation of granules at low substrate concentration is a difficult task that has seldom been achieved. This study aimed at forming the granules in the recently developed partial-denitrification (PD, NO₃^--N→NO₂^--N) for nitrite production. Two sequencing batch reactors (SBRs) were operated at a low nitrate of 30 mg N/L with nitrate loading rate (NLR) of 0.12 (R1) and 0.24 kg N/m³/d (R2). Results showed that the granulation of PD sludge experienced a developing and matured process with the progressive increase in size followed by maintaining a stable value. Higher NLR resulted in a more rapid granulation with the larger and looser structure. While the granules under lower NLR appeared to be denser and more compact with better settling ability. Microbial communities of two SBRs were revealed to show little difference, with the PD functional bacteria of Thauera (50.7% in R1 and 55.4% in R2) dominated during the granulation process. The Flavobacterium, likely to be closely related with sludge granulation, accounted for a higher proportion in R2 (10.16%) than R1 (5.91%), which might result in a larger granule formed in R2. This study clearly confirmed the feasibility of granulation of PD sludge under low nitrogen loads, shedding new light on the low-strength nitrate wastewater treatment with an efficient and economical way.

Robustness of an aerobic granular sludge sequencing batch reactor for low strength and salinity wastewater treatment at ambient to winter temperatures

Acclimation performances and characteristics of aerobic granular sludge to salt and temperature were investigated in a sequencing batch reactor (SBR) performing simultaneous nitrification, denitrification and phosphorus removal (SNDPR). The aerobic granular SNDPR system was firstly subjected to low salinity (0.5%, w/v) at moderate temperature (> 15 ℃) and subsequent low temperature (< 15 ℃). The shock loading of salinity temporarily deteriorated biological phosphorus removal, while dual stresses of salinity and low temperature induced temporary inhibition on both nitrogen and phosphorus removal. Both salinity and low temperature stimulated the settleability of aerobic granules, accompanied with decreased ratios of protein to polysaccharide (PN/PS). Illumina MiSeq sequencing revealed that salinity rarely affected bacterial richness, but significantly decreased the diversity. Whereas low temperature strengthened both bacterial richness and diversity. Phyla Proteobacteria, Chloroflexi and their sub-groups acted as the main halophilic bacteria while Proteobacteria was also psychrophilic. The functional bacteria such as nitrifiers, denitrifiers, and phosphorus removal bacteria exhibited greater tolerance to salt and low temperature than glycogen accumulating organisms (GAOs). Overall, the present study demonstrated the resilience and robustness of aerobic granular sludge toward salinity and low temperature, which could aid the knowledge of saline wastewater treatment by aerobic granular sludge.

Chronic responses of aerobic granules to the presence of graphene oxide in sequencing batch reactors

The chronic responses of aerobic granular sludge (AGS) to the presence of graphene oxide nanoparticles (GO NPs) (5, 15, 25, 35, 45, 55, 65, 75, 85, and 95 mg/L of GO NPs for 7 days) during biological wastewater treatment processes were investigated. Bioreactor performance, extracellular polymeric substance (EPS) secretion, and microbial community characteristics were assessed. The results showed that the effects of GO NPs on bioreactor performances were dependent on the dose applied and the duration for which it was applied. At concentrations of 55, 75, and 95 mg/L, GO NPs considerably inhibited the efficiency of organic matter and ammonia removal; however, nitrite and nitrate removal rates were unchanged. Biological phosphorus removal decreased even when only low concentrations of GO NPs were used. The secretion of EPS, which could alleviate the toxicity of GO NPs, also changed. The increased amount of nanoparticles also resulted in significant changes to the bacterial community structure. Based on the amplicon sequencing of 16S rRNA genes, Paracoccus sp., Klebsiella sp., and Acidovorax species were identified as the most tolerant strains.

Metal oxyanion removal from wastewater using manganese-oxidizing aerobic granular sludge

As, Sb, and Cr are redox-sensitive and toxic heavy metal(loid)s, and redox reactions are usually involved in the treatment of substrates containing these elements. In this study, manganese-oxidizing aerobic granular sludge (Mn-AGS) was obtained by continuously adding Mn(II) to the sludge in a sequencing batch reactor (SBR). Morphological observations, and analyses of extracellular polymeric substances (EPS), Mn valence-states, and microbial communities were performed on the resulting sludge. After 50 days of cultivation, biogenic Mn(III,IV) oxides (bio-MnO_x) accumulated up to approximately 25 mg Mn/g suspended solids (SS). X-ray photoelectron spectroscopy (XPS) revealed that the percentage of Mn(III,IV) was 87.6%. The protein (PN) component in EPS increased from 80.3 to 87.8 mg/g volatile suspended solids (VSS) during cultivation, which might be favorable for sludge granulation and heavy metal(loid) removal. Batch experiments showed that Mn-AGS was better at oxidizing As(III)/Sb(III) into less toxic As(V)/Sb(V) than traditional AGS. Remarkably, the results indicated that Mn-AGS did not oxidize Cr(III) but was able to reduce Cr(VI) into relatively harmless Cr(III). This work provided a new promising method with which to treat As(III), Sb(III), and Cr(VI) in wastewaters.

Understanding the granulation of partial denitrification sludge for nitrite production

Partial-denitrification (PD) has previously been demonstrated to be another pathway for nitrite production, which provides a cost-effective approach for nitrate (NO₃^--N) removal through combing with anammox. In this study, the formation of PD granules was firstly investigated in a sequencing batch reactor (SBR) with influent nitrate of 60 mg N/L. The granulation process was explored via the physicochemical and biological characterization. Sludge granulation initiated within the first 20 days with an average size of 93.7 μm in diameter, it experienced a developing, shaping and matured periods, with a maximum size of 709.3 μm obtained. High nitrite production of PD was always maintained during the granulation with a mean nitrate-to-nitrite transformation ratio (NTR) of 88.3%, and in-situ maximum NO3-N reduction rate of 84.9 mg N/h/g VSS was obtained. Mature PD granules hold an excellent settling property with 5-min sludge volume index (SVI₅) of 32.0 mL/g MLSS obtained and smooth surface with large amounts of rod bacteria covered. CaCO₃ precipitates formed in the PD process played a vital role in the initial granulation, acting as the nucleus for cell attachment. Extracellular polymeric substances (EPS), mainly the proteins (PN) content, was found to be of supreme importance in granules developing and maintaining its structural stability. Besides, the abundance of Flavobacterium and norank_p__Gracilibacteria were revealed to be in accordance with the change of granules size, seemed to contribute to sludge granulation. The developed granule-based PD integrated with anammox process provides an engineering-feasible and economic-favorable solution for industrial nitrate wastewater treatment.

Novel aerobic granular sludge culture strategy: Using granular sludge Anammox process effluent as a biocatalyst

The effluent from granular sludge Anammox process was used as a biocatalyst to improve the culture rate of aerobic granular sludge, and the internal causes of this effect were studied. In this study, in sequencing batch reactor, the formation and changes of AGS was monitored with granular sludge Anammox process effluent added before and after. The community structure of AGS was analyzed by molecular biology methods. The results showed ammonia utilizing rate increased from 3.41 to 5.96 mgNH₄⁺-N/(g VSS·h), NO₂^--N maintained a high accumulation rate, and the denitrification performance remained stable. On 40th day, the diameter of the AGS reached 3.5 mm, and the concentrations of PN and PS reached 330.5 and 62.9 mg/gVSS, respectively. The community structure has changed. Nitrosomonas (31.7%) became the new dominant bacteria. Signal molecules contained in the effluent as inter-species signal molecules could enhance the formation of AGS.

Characterization of aerobic granular sludge of different sizes for nitrogen and phosphorus removal

Granular size plays a key role in the performance of the aerobic granular sludge (AGS). As the diameter of the granule increases, stratification may begin to appear due to the increase in mass transfer resistance. Aerobic granules harvested from a lab-scale anaerobic-aerobic sequencing batch reactor (AO-SBR) were classified into three categories according to their size: (a) 0.15-0.28 mm, (b) 0.28-0.45 mm and (c) larger than 0.45 mm. In this study, the categories were called small-size, medium-size and large-size granules, respectively. A fraction of the different forms of phosphate and denitrification efficiency was investigated in each category. Results show that small-size granules present much more easily mobile phosphorus than other granules. Moreover, the denitrification performance has been tested by using dumping and trickling patterns for COD and NO3--N feeding. The results demonstrated that the large-size granules exhibit poor denitrification rates, as opposed to the medium-size granules. Therefore, medium-size granules, with a size of 0.28-0.45 mm, are regarded as the most suitable granular size for AGS in this experiment from the perspective of denitrification and phosphorus removal.

Biological wastewater treatment (anaerobic-aerobic) technologies for safe discharge of treated slaughterhouse and meat processing wastewater

Slaughterhouse industry generates considerable amount of wastewater rich in proteins, lipids, fibres, and carbohydrates. Numerous technologies such as electrocoagulation, membrane separation, advanced oxidation, physico-chemical processes, and biological treatment have been implemented for reducing the concentrations of these compounds. Nevertheless, this review aims to provide extensive information solely on the biological treatment (anaerobic and aerobic) of slaughterhouse wastewater. The advantages of anaerobic treatment are excellent organic matter removal, less sludge production, low energy requirement, execution of higher loading rates, and considerable production of biogas. Aerobic treatment on the other hand is a less sensitive process, possess lower start-up period, and efficient nutrient removal process. Numerous case studies are described to bestow maximum understanding of the wastewater characteristics, kind of treatment employed, and complications involved in managing and treating of slaughterhouse effluent. Additionally, role of microbial community involved in the treatment of slaughterhouse waste is also discussed. Sequential anaerobic and aerobic reactors are also reviewed in order to present their advantages over single bioreactors. Intermittent sequencing batch reactor is a promising technology than other high rate digesters in the removal of carbon, nitrogen, and phosphorous.

Strategies to improve aerobic granular sludge stability and nitrogen removal based on feeding mode and substrate

A systemic strategy was proposed to improve aerobic granular sludge (AGS) stability and nitrogen (N) removal efficiency by optimizing feeding mode and substrate aiming at complicated wastewater characteristics. Key functional groups at the genus level identified by high-throughput sequencing were evaluated as well. The results showed that anaerobic feeding mode and acetate promoted the compact AGS formation with excellent total nitrogen (TN) removal efficiency (averaging 91.7% ± 4.1%) at various dissolved oxygen conditions. While the aerobic feeding mode led to a loose AGS structure with a vulnerable anaerobic core and poor TN removal efficiency (averaging 58.8% ± 7.4%). Simultaneous nitrification and denitrification process played the dominant role in N removal in compact AGS over the alternating nitrification and denitrification process. High-concentration glucose undermined feast-famine condition with filamentous bacteria growth out of granule and decreased TN removal efficiency to 67.3% ± 15.2%. Lower food to microorganism ratio may result in a lower N removal rate attributed to the sharply increased biomass concentration fed by glucose. Ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, denitrifying bacteria, and denitrifying phosphorus accumulation organisms enriched during AGS granulation also contributed to the efficient N removal. The proposed strategy provided insights into the relationship between various factors and stable AGS formation, and systemic operation methods for various complicated wastewater treatment.

Towards mechanical characterization of granular biofilms by optical coherence elastography measurements of circumferential elastic waves

Microbial granular biofilms are spherical, multi-layered aggregates composed of communities of bacterial cells encased in a complex matrix of hydrated extracellular polymeric substances (EPS). While granular aggregates are increasingly used for applications in industrial and municipal wastewater treatment, their underlying mechanical properties are poorly understood. The challenges of viscoelastic characterization for these structures are due to their spherical geometry, spatially heterogeneous properties, and their delicate nature. In this study, we report a model-based approach for nondestructive characterization of viscoelastic properties (shear modulus and shear viscosity) of alginate spheres with different concentrations, which was motivated by our measurements in granular biofilms. The characterization technique relies on experimental measurements of circumferential elastic wave speeds as a function of frequency in the samples using the Optical Coherence Elastography (OCE) technique. A theoretical model was developed to estimate the viscoelastic properties of the samples from OCE data through inverse analysis. This work represents the first attempt to explore elastic waves for mechanical characterization of granular biofilms. The combination of the OCE technique and the theoretical model presented in this paper provides a framework that can facilitate quantitative viscoelastic characterization of samples with curved geometries and the study of the relationships between morphology and mechanical properties in granular biofilms.

Rapid control of activated sludge bulking and simultaneous acceleration of aerobic granulation by adding intact aerobic granular sludge

The feasibility of rapidly controlling activated sludge bulking and accelerating aerobic sludge granulation was evaluated by adding intact aerobic granular sludge (AGS) to the bulking activated sludge (BAS) reactor. Two ratios of AGS to BAS (0.2 in the first reactor (R1), and 0.4 in the second reactor (R2)) were tested. The results indicate that the addition of AGS immediately improved the settling ability of BAS (sludge volume index at 30 min (SVI₃₀) in R1 and R2 decreased from 173.1 mL/g to 130.8 and 91.3 mL/g, respectively) and gradually increased the biomass concentration (mixed liquor suspended solids (MLSS) in R1 and R2 increased to 4722 and 5190 mg/L, respectively), thus resolving the sludge bulking problem. Meanwhile, adding AGS not only promoted the BAS growth in aggregates, but also facilitated the selection of well-settling aggregates at an early stage. Consequently, the granulation process was significantly accelerated. The granulation time in R1 and R2 was 14 and 10 days, respectively, indicating that the higher ratio of AGS to BAS can result in the faster granulation. Partial nitrification could be maintained during the BAS granulation process when the initial inoculation of nitritation sludge was large enough. Additionally, the microbial community changed during the BAS granulation process. The genera Thauera and Zoogloea belonging to family Rhodobacteraceae were speculated to play an important role in the BAS granulation.

Characterization of partial-denitrification (PD) granular sludge producing nitrite: Effect of loading rates and particle size

The granule-based partial-denitrification (PD) reactor can achieve an efficient nitrite production for anammox process. In this study, the PD granules were successfully cultivated in a sequencing batch reactor (SBR), the physicochemical properties and microbial activities were characterized at increasing nitrate loading rate (NLR). Results indicated that high NLR benefited the PD for nitrite production, and a more dense and compact granule can be developed at the NLR of 0.24-0.48 Kg N/m³/d. Whereas the settling ability decreased when the NLR increased to 0.96 Kg N/m³/d, which was likely caused by the decrease of proteins (PN) in extracellular polymeric substances (EPS) content and the increase of loosely bound EPS (L-EPS) fraction. Besides, the characterization of size-fractionated PD granules revealed that a better settling ability was generally obtained with larger granules except for the size of d > 3.35 mm, which the settling velocity was reduced, likely attributed to the excess L-EPS fraction (56.3%) and presence of empty capsule inside granules. The smaller granules exhibited higher microbial activities due to the favorable mass transfer, the nitrate reduction rate was as high as 152.6 mg N/h/g VSS with granules of d < 1.0 mm. Results obtained in this study provided a better understanding of the properties of PD granules and would be helpful for the future development of granule-based PD reactor in achieving an efficient and stable nitrite production.

Enhanced aerobic granulation by applying the low-intensity direct current electric field via reactive iron anode

A novel granulation strategy by applying the low-intensity direct current (DC) electric field via reactive iron anode into the aerobic granular sludge (AGS) system was systematically investigated in this study. Three identical sequencing batch reactors (SBRs, namely R1, R2, and R3) were operated for 100 days. Comparatively, the R1 and R3 were continuously subjected to the 1.0 V DC electric field via a reactive Fe anode and an inert Ti-Ir/Rh anode, respectively, while the R2 without DC exposure. The results showed that the sludge granulation processes were accelerated in order as follows: R2<R3<R1, and the properties of mature granules were improved in order as follows: R3<R2<R1. Interestingly, at the end of experiment, total phosphorus (TP) removal efficiency in R1 dramatically increased to 80.52%, which was 2.15 and 1.96 folds than that in R2 and R3, respectively. Further investigations revealed that this novel strategy could simultaneously improve the secretion of EPS and the release of iron ions in R1, which cooperatively enhanced the granulation process. Moreover, in R1, mineral precipitation of phosphate remarkably improved the capability of phosphorus removal. The observed effective and stable performance highlights the feasibility and potential of this novel strategy for the rapid start-up and stable operation of AGS system.

Rapid reformation of larger aerobic granular sludge in an internal-circulation membrane bioreactor after long-term operation: Effect of short-time aeration

The investigation aimed at revealing the influence of an external disturbance on the rapid reformation of larger aerobic granular sludge (AGS) in an internal-circulation membrane bioreactor (IC-MBR) after long-term operation. The used IC-MBR was continuously operated well for more than one year, in which, the biomass was still in the state of AGS with a balanced average size at around 200 μm and an even size distribution. By providing short-time aeration to the biomass within this bioreactor, the characteristics of biomass were totally changed in a very short time, including the surface hydrophilicity, physic-chemical properties, and the structure of microbial community, which created suitable conditions for the growth of filamentous bacteria (Saccharibacteria). Such a variation was very beneficial to the reformation of larger AGS, which resulted in the average size of AGS increased to nearly 400 μm with a compact structure and clear edge in no more than one month.

Hydrodynamic shear force shaped the microbial community and function in the aerobic granular sequencing batch reactors for low carbon to nitrogen (C/N) municipal wastewater treatment

The lab-scale aerobic granules process was applied for low carbon to nitrogen (C/N < 4) wastewater treatment under different hydrodynamic shear forces. Results revealed that aerobic granules exhibited strong adaptability and stability. The aerobic granules might adopt an extracellular polymeric substances (EPS) regulating mechanism to address the changes in operational conditions, especially through growing secretion of fluorescence protein. The hydrodynamic shear force determinedly shaped and regulated the diversity and structure of dominant microbial community, briefly, reduced aeration intensity with increased time led to higher microbial richness, lower diversity and evenness, and shifts of predominant microorganisms. Phylogenetic classification of the key functional groups including bacteria related to carbon and nutrients removal, EPS production and quorum sensing (QS) presented much more differences among the reactors subject to different conditions. Therefore, the present work adds insight into the comprehensive understanding of the effect of aeration induced hydrodynamic shear force on aerobic granules.