Flocs in disguise? High granule abundance found in continuous-flow activated sludge treatment plants

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.

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.

Novel anammox granules formation from conventional activated sludge for municipal wastewater treatment through flocs management

The direct integration of anammox process into municipal wastewater treatment has caused widespread concern, but the lack of anammox seeds limited its real application. This study successfully cultivated anammox granules (322.0 μm) from conventional activated sludge treating municipal wastewater. Through ultra-low floc sludge retention times of 8d, nitrifiers on flocs were eliminated and partial nitrification was realized. Furthermore, highly bacteria-enriched granules were initially formed, with Nitrosomonas and Ca. Competibacter 4-fold higher than that of flocs. Specific staining results revealed the microbial interaction with Ca. Brocadia, considering that Ca. Competibacter and Nitrosomonas correspondingly identified in the inner and outer layers of granules. The percentage of Ca. Brocadia present on the granules increased substantially from 0.0 % to 3.0 %, accompanied by a nitrogen removal rate of 0.3 kg·m-3·d-1. Our findings revealed a valuable reference for the anammox bacteria in-situ enrichment under mainstream conditions, which provides theoretical guidance for anammox-based processes practical application.

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.

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.

Roles of high/low nucleic acid bacteria in flocs and probing their dynamic migrations with respirogram

Bacterial migration is crucial for the stability of activated sludge but rarely reported. The static distribution was explored by changes in bacteria concentration with extracellular polymeric substances (EPS) extractions. Next, denitrification and aeration were conducted as normal running conditions for examining the bacterial migration between floc-attached and dispersed growth. Above observations were further explored by conducting copper ion (Cu2+) shock as an extreme running condition. After extracting EPS, low nucleic acid (LNA) bacteria migrated from the sludge to the supernatant primarily, and high nucleic acid (HNA) bacteria remained in the residual sludge, suggesting that HNA bacteria mainly distributed inside the sludge while LNA bacteria outside the sludge. During the denitrification process, LNA bacteria migrated out of flocs, which increased by 6.94 × 106 events/mL in the supernatant. During the feast phase of aeration, LNA bacteria grew attached to flocs, causing the increased flocs diameter from 45.60 to 47.40 μm. During the following aerobic famine phase, LNA bacteria grew dispersedly, but HNA bacteria remained unchanged. However, a further severe famine phase drove HNA bacteria to be dispersed, breaking flocs with the decreased diameter from 48.10 to 46.50 μm. When the Cu2+ shock was employed, LNA and HNA bacteria increased but the LNA/HNA ratio decreased in the supernatant, indicating more HNA bacteria migrating to the dispersed phase. From a structural perspective, HNA bacteria distributed inside the sludge and functioned as the backbone of flocs, undertaking the maintenance of flocs stability primarily; while LNA bacteria distributed outside the sludge and functioned as filling materials, having a secondary influence on flocs stability. These processes were also probed by respirogram exactly, correlating the system-scale measurement and microscale migrations and providing an early warning signal under abnormal circumstances. The processed HNA-backbone theory is promising for regulating the stability of activated sludge based on bacterial migrations.

Enhanced in-situ sludge reduction of the side-stream process via employing micro-aerobic approach in both mainstream and side-stream

Side-stream reactor (SSR), as an in-situ sludge reduction process with high sludge reduction efficiency (SRE) and less negative impact on effluent, has been widely researched. In order to reduce cost and promote large-scale application, the anaerobic/anoxic/micro-aerobic/oxic bioreactor coupled with micro-aerobic SSR (AAMOM) was used to investigate nutrient removal and SRE under short hydraulic retention time (HRT) of SSR. When HRT of SSR was 4 h, AAMOM system achieved 30.41% SRE, while maintaining carbon and nitrogen removal efficiency. Micro-aerobic in mainstream accelerated the hydrolysis of particulate organic matter (POM) and promoted denitrification. Micro-aerobic in side-stream increased cell lysis and ATP dissipation, thus increasing SRE. Microbial community structure indicated that the cooperative interactions among hydrolytic, slow growing, predatory and fermentation bacteria played key roles in improving SRE. This study confirmed that SSR coupled micro-aerobic was a promising and practical process, which could benefit nitrogen removal and sludge reduction in municipal wastewater treatment plants.

Long-term effects of hydrocyclone operation on activated sludge morphology and full-scale secondary settling tank wet-weather operation in long sludge age WWTP

This paper presents the study concerning long-term effects of a full scale hydrocyclone unit implemented in a continuous flow long sludge age system, on sedimentation, treatment efficiency and sludge morphology. The research concentrates on identifying the mechanisms of sludge behaviour within the system. The gravimetric selection of activated sludge via a hydrocyclone is a recent development for enhancing sludge separation, where heavier flocs are retained in the system, and lighter ones are discarded as waste sludge. The effects of implementing hydroclyclones were analysed with the use of SEM imagining and fractal dimensioning through the frequent assessment of sludge settling capabilities, effluent quality, and floc properties. Over the course of 60 weeks of hydrocyclone operation, sedimentation efficiency varied significantly. Sludge volume index values of 40 mL/g, achieved during the warm season, were not sustained when the temperature decreased and an overgrowth of filamentous bacteria occurred. Good settling efficiency was also observed in batch tests, where settling velocity of experimental sludge was app. 1 m/h higher than for the reference train at the same concentrations. This was confirmed during wet weather, as the experimental train sustained safe sludge blanket height in secondary clarifiers. SEM imaging and fractal dimension analysis revealed that the underflow that returned to the system had a more compact and spherical shape, which led to an increased content of granule-like particles in the reactor. The presence of flocs with a diameter exceeding 900 μm in the underflow, which is not observed in the feed, indicated agglomeration within the hydrocyclone. This is contradictory to most of the literature data coming from laboratory experiments. This phenomenon was attributed to differences in the size and geometry of the used hydrocyclones, and the potential process mechanism was presented.

Rapid start-up and advanced nutrient removal of simultaneous nitrification, endogenous denitrification and phosphorus removal aerobic granular sequence batch reactor for treating low C/N domestic wastewater

The rapid start-up and advanced nutrient removal of simultaneous nitrification, endogenous denitrification, and phosphorus (P) removal aerobic granular sequence batch reactor (SNEDPR-AGSBR) is a challenge in the treatment of low carbon/nitrogen (C/N) domestic sewage. In this study, the feasibility of the SNEDPR-AGSBR process was examined in an exceedingly single-stage anaerobic/aerobic/anoxic sequencing batch reactor for treating low C/N ratio (3.3-5.0) domestic sewage. The initial results showed that accompanied by the rapid formation of the mature aerobic granular sludge based on the selection for slow-growing organisms, the rapid start-up (38 d) of the SNEDPR-AGSBR process was successfully realized. The formed mature aerobic granules had a dense structure with an average diameter of 667.7 μm and SVI₃₀ of 30.0 mL/g. Two conditions for achieving the competitive balance between phosphorus-accumulating organisms/denitrifying phosphorus-accumulating organisms (PAOs/DPAOs) and glycogen accumulating organisms/denitrifying glycogen accumulating organisms (GAOs/DGAOs) were revealed by the long-term operation results. First, the dissolved oxygen (DO) concentration needed to be decreased to 3.0 mg/L in the aerobic phase, and then, the aerobic and anoxic phase hydraulic retention time (HRT) should be increased to 3.0 h. Notably, high removal efficiencies for NH₄⁺-N (100%), total nitrogen (84.3%), and P (91.8%) of the SNEDPR-AGSBR process were stably obtained with a low C/N ratio of 3.9 domestic sewage. Simultaneous nitrification and endogenous denitrification (SNED) efficiency of 61.6% was achieved during a long-term operation of 142 days. Finally, microbial community analysis confirmed that GAOs (Defluviicoccus)/DGAOs (Candidatus_Competibacter) were responsible for the removal N, and PAOs (Acinetobacter, Candidatus_Accumulibacter, Hypomicrobinm)/DPAOs (Pseudomonas and Dechloromonas) ensured P removal.

Enhanced sulfonamides removal via microalgae-bacteria consortium via co-substrate supplementation

Both co-cultivation and co-substrate addition strategies have exhibited massive potential in microalgae-based antibiotic bioremediation. In this study, glucose and sodium acetate were employed as co-substrate in the cultivation of microalgae-bacteria consortium for enhanced sulfadiazine (SDZ) and sulfamethoxazole (SMX) removal. Glucose demonstrated a two-fold increase in biomass production with a maximum specific growth rate of 0.63 ± 0.01 d^-1 compared with sodium acetate. The supplementation of co-substrate enhanced the degradation of SDZ significantly up to 703 ± 18% for sodium acetate and 290 ± 22% for glucose, but had almost no effect on SMX. The activities of antioxidant enzymes, including peroxidase, superoxide dismutase and catalase decreased with co-substrate supplementation. Chlorophyll a was associated with protection against sulfonamides and chlorophyll b might contribute to SDZ degradation. The addition of co-substrates influenced bacterial community structure greatly. Glucose enhanced the relative abundance of Proteobacteria, while sodium acetate improved the relative abundance of Bacteroidetes significantly.

On the mechanisms for aerobic granulation - model based evaluation

In this study a mathematical framework was developed to describe aerobic granulation based on 6 main mechanisms: microbial selection, selective wasting, maximizing transport of substrate into the biofilm, selective feeding, substrate type and breakage. A numerical model was developed using four main components; a 1D convection/dispersion model to describe the flow dynamics in a reactor, a reaction/diffusion model describing the essential conversions for granule growth, a setting model to track granules during settling and feeding, and a population model containing up to 100,000 clusters of granules to model the stochastic behaviour of the granulation process. With this approach the model can explain the dynamics of the granulation process observed in practice. This includes the presence of a lag phase and a granulation phase. Selective feeding was identified as an important mechanism that was not yet reported in literature. When aerobic granules are grown from activated sludge flocs, a lag phase occurs, in which not many granules are formed, followed by a granulation phase in which granules rapidly appear. The ratio of granule forming to non-granule forming substrate together with the feast/famine ratio determine if the transition from the lag phase to the granulation phase is successful. The efficiency of selective wasting and selective feeding both determine the rate of this transition. Brake-up of large granules into smaller well settling particles was shown to be an important source for new granules. The granulation process was found to be the combined result from all 6 mechanisms and if conditions for either one are not optimal, other mechanisms can, to some extent, compensate. This model provides a theoretical framework to analyse the different relevant mechanisms for aerobic granular sludge formation and can form the basis for a comprehensive model that includes detailed nutrient removal aspects.

Effects of sulphur amino acids on the size and structure of microbial communities of aerobic granular sludge bioreactors

Granular activated sludge has been described as a promising tool in treating wastewater. However, the effect of high concentrations of sulphur amino acids, cysteine and methionine, in the evolution, development and stability of AGS-SBRs (aerobic granular sludge in sequential batch reactors) and their microbial communities is not well-established. Therefore, this study aimed to evaluate microbial communities' size, structure and dynamics in two AGS-SBRs fed with two different concentrations of amino acids (50 and 100 mg L⁻¹ of both amino acids). In addition, the impact of the higher level of amino acids was also determined under an acclimatization or shock strategy. While N removal efficiency decreased with amino acids, the removal of the organic matter was generally satisfactory. Moreover, the abrupt presence of both amino acids reduced even further the removal performance of N, whereas under progressive adaptation, the removal yield was higher. Besides, excellent removal rates of cysteine and methionine elimination were found, in all stages below 80% of the influent values. Generally considered, the addition of amino acids weakly impacts the microbial communities' total abundances. On the contrary, the presence of amino acids sharply modulated the dominant bacterial structures. Furthermore, the highest amino acid concentration under the shock strategy resulted in a severe change in the structure of the microbial community. Acidovorax, Flavobacterium, Methylophilus, Stenotrophomonas and Thauera stood out as the prominent bacteria to cope with the high presence of cysteine and methionine. Hence, the AGS-SBR technology is valuable for treating influents enriched in sulphur Aa inclusively when a shock strategy was used.

Bacterial Community Composition and Function in a Tropical Municipal Wastewater Treatment Plant

Bacterial diversity and community composition are of great importance in wastewater treatment; however, little is known about the diversity and community structure of bacteria in tropical municipal wastewater treatment plants (WWTPs). Therefore, in this study, activated sludge samples were collected from the return sludge, anaerobic sludge, anoxic sludge, and aerobic sludge of an A2O WWTP in Haikou, China. Illumina MiSeq high-throughput sequencing was used to examine the 16S ribosomal RNA (rRNA) of bacteria in the samples. The microbial community diversity in this tropical WWTP was higher than in temperate, subtropical, and plateau WWTPs. Proteobacteria, Bacteroidota, Patescibacteria, and Chloroflexi were the dominant phyla. Nitrification bacteria Nitrosomonas, and Nitrospira were also detected. Tetrasphaera, instead of Candidatus Accumulibacter, were the dominant polyphosphate accumulating organisms (PAOs), while, glycogen accumulating organisms (GAOs), such as Candidatus Competibacter and Defluviicoccus were also detected. The bacterial community functions predicted by PICRUSt2 were related to metabolism, genetic information processing, and environmental information processing. This study provides a reference for the optimization of tropical municipal WWTPs.

MiDAS 4: A global catalogue of full-length 16S rRNA gene sequences and taxonomy for studies of bacterial communities in wastewater treatment plants

Microbial communities are responsible for biological wastewater treatment, but our knowledge of their diversity and function is still poor. Here, we sequence more than 5 million high-quality, full-length 16S rRNA gene sequences from 740 wastewater treatment plants (WWTPs) across the world and use the sequences to construct the ‘MiDAS 4’ database. MiDAS 4 is an amplicon sequence variant resolved, full-length 16S rRNA gene reference database with a comprehensive taxonomy from domain to species level for all sequences. We use an independent dataset (269 WWTPs) to show that MiDAS 4, compared to commonly used universal reference databases, provides a better coverage for WWTP bacteria and an improved rate of genus and species level classification. Taking advantage of MiDAS 4, we carry out an amplicon-based, global-scale microbial community profiling of activated sludge plants using two common sets of primers targeting regions of the 16S rRNA gene, revealing how environmental conditions and biogeography shape the activated sludge microbiota. We also identify core and conditionally rare or abundant taxa, encompassing 966 genera and 1530 species that represent approximately 80% and 50% of the accumulated read abundance, respectively. Finally, we show that for well-studied functional guilds, such as nitrifiers or polyphosphate-accumulating organisms, the same genera are prevalent worldwide, with only a few abundant species in each genus.

Microbial communities are responsible for biological wastewater treatment. Here, Dueholm et al. generate more than 5 million high-quality, full-length 16S rRNA gene sequences from wastewater treatment plants across the world to construct a database with a comprehensive taxonomy, providing insights into diversity and function of these microbial communities.

Insights into the minimization of excess sludge production in micro-aerobic reactors coupled with a membrane bioreactor: Characteristics of extracellular polymeric substances

The production of excess sludge by the activated sludge system of wastewater treatment plants is a problem. In this study, the EPS characteristics on production and degradation were investigated in the real-scale food processing wastewater treatment system (i.e., a micro-aerobic reactor coupled with a membrane bioreactor (MAR-MBR)) with a treatment capacity of 150 t d^-1, which could cater for the low production of excess sludge (i.e., 9 t·a^-1; 76% moisture content). The total organic carbon concentrations in the different EPS fractions were in the following order: soluble EPS (S-EPS) < loosely bound EPS (LB-EPS) < tightly bound EPS (TB-EPS). Although the components (e.g., protein and humic acid-like substances) of each EPS fraction changed significantly throughout the MAR-MBR process owing to the low production of excess sludge, the degrees of change in S-EPS, LB-EPS, and TB-EPS were significantly different from the corresponding change in their relative molecular weights. Furthermore, the microbial community composition was beneficial for the release and degradation of EPS, and the regulation of gene functions via the MAR-MBR enhanced this process.

Culturing partial denitrification biofilm in side stream incubator with ordinary activated sludge as inoculum: One step closer to mainstream Anammox upgrade

Recently, adding carriers into anoxic zone is proposed for mainstream Anammox upgrade, which relied on the denitrifiers responsible for partial denitrification (PD) to generate essential nitrite for Anammox bacteria. Still, their low abundance in the naturally formed biofilm leads to insufficient nitrite supply. This study investigated the sequential culturing of PD biofilm. By inoculating ordinary activated sludge, the PD process was quickly established within 54-day. During that, decreasing carbon to nitrogen ratio and anoxic duration in order might be effective strategies. Adding carriers shifted the microbial community, especially the proliferation of Flavobacterium. When solely using the mature PD biofilm, high nitrate to nitrite transformation ratio (>70%) was obtained. Meanwhile, both nitrate-reducing and nitrite-generating processes slowed down and lasted ∼90 min. In addition, abundant Simplicispira candidate for PD was detected in biofilm. This study also suggests that regularly harvesting PD-related functional bacteria from a side-stream incubator promotes mainstream Anammox upgrade.

Intensifying existing urban wastewater

[Figure: see text].

Biological process architecture in continuous-flow activated sludge by gravimetry: Controlling densified biomass form and function in a hybrid granule-floc process at Dijon WRRF, France

Full-scale demonstration of activated sludge conversion into a granule-floc hybrid process was implemented in Dijon (France) water resource recovery facility (WRRF). Biomass densification was achieved based on external gravimetric selection using hydrocyclones within continuous-flow anaerobic-anoxic-oxic (A² O) biological nutrient removal (BNR) bioreactor. The goal was to optimize settleability of biological sludge by lowering and stabilizing sludge volume index (SVI) to improve process robustness and resiliency. Process proved to stabilize operation and to uncouple the total solids residence time (SRT) between floc and granule morphologies. The densified biomass initially produced stable SVI < 100 ml/g for a period of 4 months and thereafter a steady state year-round SVI below 50 ml/g, including the winter period during which the control train SVI expansion >200 ml/g. The densified biomass successfully broke the vicious cycle of interannual bulking. Form and function interrelationship is proposed for the densified biomass (hybrid floc-granule). The concept of biological architecture is proposed as the purposeful control of granule and floc proportions, with a proposed "form factor" ratio of 1:2 granule to floc, that produce a "SRT uncoupling function factor" ratio of 4:1 granule to floc, further resulting in very stable settling and effluent functionalities. PRACTITIONER POINTS: Controlling granule-floc proportions allows for sludge volume index (SVI) operational adjustment, which further allows for increased clarified design accuracy. One-third aggregates dramatically improved settling characteristics: 20% and 35% of AGS ensures SVIs below 100 and 50 ml/g, respectively. Densified biomass enables new SRT and clarifier flux rates approaches for engineering and operation practices: Doubling typical surface loading rates from 6.0-8.5 to 15-20 kg m^-2  h^-1 and surface overflow rates from 0.6-0.8 to 1.5-2.4 m/h SRT uncoupling of 1:4 is achieved between floc and granule, enabling specific niche environment for fast and slow growing organisms.

Unaerated feeding alters the fate of dissolved methane during aerobic wastewater treatment

In municipal wastewater treatment plants, some dissolved methane can enter the aerobic bioreactors. This greenhouse gas originates from sewers and return flows from anaerobic sludge treatment. In well-mixed conventional activated sludge reactors, methane emissions are largely avoided because methane oxidizing bacteria consume a large fraction, even without optimizing for this purpose. In this work, the fate of dissolved methane is studied in aerobic granular sludge reactors, as they become increasingly popular. The influence of the characteristic design and operating conditions of these reactors are studied with a mathematical model with apparent conversion kinetics and stripping: the separation of feeding and aeration in time, a higher substrate transport resistance, a high retention time of granular biomass and a taller water column. Even for a best-case scenario combining an unrealistically low intragranule substrate transport resistance, a high retention time, a tall reactor, an extremely high influent methane concentration and no oxygen limitation, the methane conversion efficiency was only 12% when feeding and aeration were separated in time, which is lower than for continuous activated sludge reactors under typical conditions. A more rigorous model was used to confirm the limited conversion, considering the multi-species and multi-substrate biofilm kinetics, anoxic methane consumers and the high substrate concentration at the bottom during upward plug flow feeding. The observed limited methane conversion is mainly due to the high concentration that accumulates during unaerated feeding phases, which favours stripping more than conversion in the subsequent aeration phase. Based on these findings, strategies were proposed to mitigate methane emissions from wastewater treatment plants with sequentially operated reactors.

Inducing granulation within a full-scale activated sludge system to improve settling

Most cold-climate biological nutrient removal facilities experience poor settling mixed liquor during winter, resulting in treatment capacity throughput limitations. The Metro Wastewater Reclamation District in Denver, Colorado, operated two full-scale secondary treatment trains to compare the existing biological nutrient removal configuration (Control) to one that was modified to operate with an anaerobic selector and with hydrocyclone selective wasting (Test) to induce granulation. Results from this evaluation showed that the Test achieved significantly better settling behaviour than the Control. The difference in the mean diluted SVI₃₀ between the Test and Control were statistically significant (P < 0.05), with values of 77 ± 17 and 135 ± 25 mL/g observed for the Test and Control respectively. These settling results were accompanied by differences in the particle size distribution, with notably higher settling velocities commensurate with increasing particle size. The degree of granulation observed in the Test train was between 32 and 56% of the mass greater than ≥250 μm in particle size whereas 16% of the mixed liquor in the Control was ≥250 μm over the entire study period. The improved settling behaviour of the Test configuration may translate into an increase of secondary treatment capacity during winter by 32%.

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.

An investigation into the optimal granular sludge size for simultaneous nitrogen and phosphate removal

An aerobic granular sludge (AGS) pilot plant fed with a mixture of acetate amended centrate and secondary effluent was used to investigate the optimal granule size range for simultaneous nitrification and denitrification (SND) and ortho-phosphate removal. The anaerobic phase was mixed to understand how AGS will perform if integrated with a continuous flow activated sludge system that cannot feed the influent through the settled sludge bed. Five different granule size fractions were taken from the pilot (operated at DO setpoint of 2mgO₂/L) and each size was subjected to activity tests in a well-controlled lab-scale AGS reactor at four dissolved oxygen (DO) concentrations of 1, 2, 3, and 4 mgO₂/L. The size fractions were: 212 - 600 µm, 600 - 1000 µm, 1000 - 1400 µm, 1400 - 2000 µm, and >2000 µm. The smallest size range (212 - 600 µm) had the highest nitrification and phosphate removal rates at DO setpoints from 1 - 3 mgO₂/L, which was attributed to the higher aerobic volume fraction in small granules and hence a higher abundance of phosphorus accumulating organisms (PAO) and ammonia oxidizing bacteria (AOB). In comparison, large granules (>1000 µm) had 1.4 - 4.7 times lower ammonia oxidation rates than the smallest size range, which aligned with their lower AOB abundance relative to granule biomass. The granules with the highest anoxic volume fraction had the highest abundance of nitrite reductase genes (nir gene) but did not show the highest specific nitrogen removal rate. Instead, smaller granules (212 - 600 and 600 - 1000 µm), which had a lower nir gene abundance, had the highest specific nitrogen removal rates (1.2 - 3.1 times higher than larger granules) across all DO values except at 4 mgO₂/L. At a DO setpoint of 4 mgO₂/L, nitrite production by ammonia oxidation (ammonia monooxygenase) exceeded nitrite reduction by nitrite reductase in granules smaller than 1000 µm, in addition, some denitrifying heterotrophs switched to oxygen utilization in deeper layers hence suppressing denitrification activity. At the DO range of 2 - 4 mg/L, granular size had a greater effect on nutrient removal than DO. Therefore, for AGS developed at an average DO setpoint of 2 mgO₂/L, selecting for size fractions in the range of 212 - 1000 µm and avoiding DO values higher than 3 mgO₂/L can achieve both a higher nitrogen removal capacity and energy savings. This study is the first to investigate the influence of different DO values on SND and biological phosphorus removal performance of different aerobic granular sludge sizes.

Aerobic Granular Sludge-Membrane BioReactor (AGS-MBR) as a Novel Configuration for Wastewater Treatment and Fouling Mitigation: A Mini-Review

This mini-review reports the effect of aerobic granular sludge (AGS) on performance and membrane-fouling in combined aerobic granular sludge-membrane bioreactor (AGS-MBR) systems. Membrane-fouling represents a major drawback hampering the wider application of membrane bioreactor (MBR) technology. Fouling can be mitigated by applying aerobic granular sludge technology, a novel kind of biofilm technology characterized by high settleability, strong microbial structure, high resilience to toxic/recalcitrant compounds of industrial wastewater, and the possibility to simultaneously remove organic matter and nutrients. Different schemes can be foreseen for the AGS-MBR process. However, an updated literature review reveals that in the AGS-MBR process, granule breakage represents a critical problem in all configurations, which often causes an increase of pore-blocking. Therefore, to date, the objective of research in this sector has been to develop a stable AGS-MBR through multiple operational strategies, including the cultivation of AGS directly in an AGS-MBR reactor, the occurrence of an anaerobic-feast/aerobic-famine regime in continuous-flow reactors, maintenance of average granule dimensions far from critical values, and proper management of AGS scouring, which has been recently recognized as a crucial factor in membrane-fouling mitigation.

Aerobic granular sludge: Impact of size distribution on nitrification capacity

The relationship between ammonia oxidation rate, nitrifiers population, and modelled aerobic zone volume in different granule sizes was investigated using aerobic granular sludge from a pilot-scale reactor. The pilot was fed with centrate and secondary effluent amended with acetate as the main carbon source. The maximum specific ammonia oxidation rates and community composition of different aerobic granular sludge size fractions were evaluated by batch tests, quantitative PCR, and genomic analysis. Small (331µm) granules had a 4.72 ± 0.09 times higher maximum specific ammonia oxidizing rate per 1 gVSS, and a 4.05 ± 0.17 times higher specific amoA gene copy number than large (2225µm) granules per 1 gram of wet biomass. However, when related to surface area, small granules had 1.43 ± 0.01 times lower maximum specific ammonia oxidation rate and a 1.66 ± 0.04 times lower specific amoA gene copy number per unit surface than large granules. Experimental results aligned with modeling results in which smaller granules had a higher specific aerobic zone volume to biomass and lower specific aerobic zone volume to surface area. Aerobic granular sludge reactors having the same average diameter of granules may have very different proportions of granule size fractions and hence possess different nitrification rates. Therefore, instead of the commonly reported average granule diameter, a new method was proposed to determine the aerobic volume density per sample, which correlated well with the nitrification rate. This work provides a roadmap to control nitrification capacity by two methods: (a) crushing larger granules into smaller fractions, or (b) increasing the mixed liquor suspended solid concentration to increase the total aerobic zone volume of the system.

Interaction and removal of oxytetracycline with aerobic granular sludge

Aerobic granular sludge as a promising technology showed great resistance to adverse conditions. However, the interaction between oxytetracycline (OTC) and granular sludge was not studied sufficiently. This study therefore investigated OTC-tolerance ability of incomplete and complete granulation sludge from aspects of simultaneous nutrients removal, sludge characteristics, microbial activity, community changes, and vice versa OTC removal performance. Incomplete granulation sludge showed better denitrification performance and resistance. Whereas, denitrification and phosphorus removal of complete granulation sludge suffered a permanent collapse under 5 mg/L OTC. OTC could be removed by rapid adsorption and slow biodegradation via granular sludge. The EPS, especially TB-PS, played a significant role during the operational period subjected to OTC. The major genera of Lysobacter and Candidatus_Competibacter laid the biological basis for stability and functionality of granules, which acted as the putative contributors for resisting and removing OTC. This study showed that incomplete-granulated sludge qualified more promising application prospect.

Distribution and diversity of filamentous bacteria in wastewater treatment plants exhibiting foaming of Taihu Lake Basin, China

Foaming caused by filamentous bacteria in activated sludge (AS) is a common phenomenon in municipal wastewater treatment plants (WWTPs) in Taihu Lake Basin of South China. In this study, total bacterial and filamentous bacterial communities were comprehensively characterized in AS and foams from eight municipal WWTPs by high-throughput sequencing technology. Results showed that alpha diversities of total bacterial communities in foams were obviously lower than those in AS samples. The bacterial community structures were significantly different between WWTPs rather than sample types (AS vs. foam). For most WWTPs, the Actinobacteria phylum was highly enriched in foams and the most abundant genera in foams were common mycolata. Sixteen filamentous bacteria were identified against the improved bulking and foaming bacteria (BFB) database. Abundance and composition of BFB in different WWTPs and different sample types were significantly different. 'Nostocoida limicola' I Trichococcus and Microthrix were generally dominant in AS samples. The dominant BFB in foams were associated with Microthrix, Skermania, Gordonia, and Mycobacterium. A new Defluviicoccus spp. in cluster III was identified in severe and continuous foams. Moreover, dominant BFB in stable and continuous foams with light level in one typical WWTP were diverse, even, and dynamic. Bacterial co-occurrence network analysis implied that the bacterial community of AS was more sensitive to disturbance than that of foam.