Effects of Wash-Out Dynamics on Nitrifying Bacteria in Aerobic Granular Sludge During Start-Up at Gradually Decreased Settling Time

Celebrating 50 years of microbial granulation technologies: From canonical wastewater management to bio-product recovery

Microbial granulation technologies (MGT) in wastewater management are widely practised for more than fifty years. MGT can be considered a fine example of human innovativeness-driven nature wherein the manmade forces applied during operational controls in the biological process of wastewater treatment drive the microbial communities to modify their biofilms into granules. Mankind, over the past half a century, has been refining the knowledge of triggering biofilm into granules with some definite success. This review captures the journey of MGT from inception to maturation providing meaningful insights into the process development of MGT-based wastewater management. The full-scale application of MGT-based wastewater management is discussed with an understanding of functional microbial interactions within the granule. The molecular mechanism of granulation through the secretion of extracellular polymeric substances (EPS) and signal molecules is also highlighted in detail. The recent research interest in the recovery of useful bioproducts from the granular EPS is also emphasized.

Landfill leachate biological treatment: perspective for the aerobic granular sludge technology

Landfill leachates are high-strength complex mixtures containing dissolved organic matter, ammonia, heavy metals, and sulfur species, among others. The problem of leachate treatment has subsisted for some time, but an efficient and cost-effective universal solution capable of ensuring environmental resources protection has not been found. Aerobic granular sludge (AGS) has been considered a promising technology for biological wastewater treatment in recent years. Granules' layered structure, with an aerobic outer layer and an anaerobic/anoxic core, enables the presence of diverse microbial populations without the need for support media, allowing simultaneous removal of different pollutants in a single unit. Besides, its strong and compact arrangement provides higher tolerance to toxic pollutants and the ability to withstand large load fluctuations. Furthermore, its good that settling properties allow high biomass retention and better sludge separation. Nevertheless, AGS-related research has focused on carbon-nitrogen-phosphorus removal, mainly from sanitary sewage. This review aims to summarize and analyze the main findings and problems reported in the literature regarding AGS application to landfill leachate treatment and identify the knowledge gaps for future applications.

Self-Aggregation and Denitrifying Strains Accelerate Granulation and Enhance Denitrification

A long start-up period is one of the main factors limiting the practical application of aerobic granular sludge (AGS). Bioaugmentation could be a good strategy to accelerate aerobic granulation. In this research, four denitrifying strains were isolated from mature AGS. Mycobacterium senegalense X3-1 exhibited the strongest self-aggregation ability and good denitrification ability. Ensifer adhaerens X1 showed the strongest denitrification ability but poor self-aggregation ability. Additionally, strain X3-1 demonstrated the highest extracellular polymeric substances (EPS) contents accompanied by relatively high N-acyl-homoserine lactones (AHLs) concentrations, which could illustrate its predominant aggregation ability—AHLs produced by bacteria regulate EPS secretion to accelerate cell aggregation. Strain X3-1 and X1 were chosen as inoculated bacterium to verify the effects of bioaugmentation on AGS granulation and denitrification. Granulation was achieved in the sequential batch reactors (SBRs) added strain X3-1 10 days earlier than the control group. The particle morphology and TIN removal rate of X3-1 were both superior to the latter. The introduction of strains reduced the richness and diversity of the microbial community, but the key functional bacteria, Candidatus_Competibacter, proliferates in the SBR inoculated with X3-1. Conclusively, it is suggested Mycobacterium senegalense X31 could be a prospective strain for enhancing AGS formation and denitrification.

Simultaneous removal of organic matter and nitrogen compounds from landfill leachate by aerobic granular sludge

This study aimed at investigating the treatment of landfill leachate using the aerobic granular sludge process in a lab-scale sequential batch reactor (SBR-AGS). The leachate from a giant sanitary landfill localized in the State of São Paulo (Brazil) exhibited high concentration of organic matter (COD 5,300 ± 78 mg L^-1) and total nitrogen (TKN 2,630 ± 355 mg L^-1). Comparatively, the leachate was added to wastewater in three different volumetric ratios (5, 10 and 20%) and the mixtures were characterized over treatment. The results indicated that there were no significant changes in the behaviour of the biological process even at the highest leachate ratio. The granulation of the aerobic sludge occurred after 90 days of operation and the granules had a diameter of 485-1585 μm. SBR-AGS exhibited removal efficiency of 87-89% for organic matter and at least 98% for total nitrogen, regardless of the leachate ratio. The treated effluent that received 20% of leachate showed 2.7 mg L^-1 ammonia and 1.1 mg L-1 nitrate. This study shows that SBR-AGS was able to form large granules, thus promoting a simultaneous nitrification and denitrification (SND) process. We highlighted that SND occurred in low dissolved oxygen concentrations (< 1.5 mg L^-1) for 120 days, without compromising aerobic granule integrity. These results suggest that the aerobic granular sludge process is a promising alternative for the co-treatment of landfill leachate and domestic wastewater under tropical climate conditions and its use should be encouraged.

Influence of static mixer on the development of aerobic granules for the treatment of low-medium strength domestic wastewater

The present work investigates the feasibility of aerobic granulation for the treatment of low-medium strength domestic wastewater for long-term operation and effects of a static mixer on the properties and removal performances of the aerobic granules formed. The static mixer was installed in a sequential batch reactor to provide higher hydrodynamic shear force in enhancing the formation of the aerobic granules. Aerobic granules were successfully formed in the domestic wastewater, and the granulation treatment system was sustained for a period of 356 days without granules disintegration. Subsequent to the installation, aerobic granules with a low SVI₃₀ of 41.37 mL/g_TSS, average diameter 1.11 mm, granular strength with integrity coefficient 10.4% and regular shape with minimum filamentous outgrowth were formed. Mineral concentrations such as Fe, Mg, Ca and Na as well as composition of protein and polysaccharide in tightly bound-extracellular polymeric substance of the aerobic granules were found to be higher under the effect of the static mixer. However, no significant improvement was observed on the TCOD, NH₄⁺-N and TSS removal performance. Good TCOD and TSS removal performance of above 85% and 90%, respectively and moderate NH₄⁺-N removal performance of about 60% were observed throughout the study. Higher simultaneous nitrification and denitrification (SND) efficiency of 56% was observed after the installation of the static mixer, as compared to 21% prior. Therefore, it may be concluded that the installation of the static mixer significantly improved the properties of aerobic granules formation and SND efficiency but not the TCOD, NH₄⁺-N and TSS removal performance.

Effect of the co-treatment of synthetic faecal sludge and wastewater in an aerobic granular sludge system

The co-treatment of two synthetic faecal sludges (FS-1 and FS-2) with municipal synthetic wastewater (WW) was evaluated in an aerobic granular sludge (AGS) reactor. After characterisation, FS-1 showed the following concentrations, representative for medium-strength FS: 12,180 mg TSS L^-1, 24,300 mg total COD L^-1, 93.8 mg PO₃-P L^-1, and 325 mg NH₄-N L^-1. The NO₃-N concentration was relatively high (300 mg L^-1). For FS-2, the main difference with FS-1 was a lower nitrate concentration (18 mg L^-1). The recipes were added consecutively, together with the WW, to an AGS reactor. In the case of FS-1, the system was fed with 7.2 kg total COD m^-3d^-1 and 0.5 kg Nitrogen m^-3d^-1. Undesired denitrification occurred during feeding and settling resulting in floating sludge and wash-out. In the case of FS-2, the system was fed with 8.0 kg total COD m^-3d^-1 and 0.3 kg Nitrogen m^-3d^-1. The lower NO₃-N concentration in FS-2 resulted in less floating sludge, a more stabilised granular bed and better effluent concentrations. To enhance the hydrolysis of the slowly biodegradable particulates from the synthetic FS, an anaerobic stand-by period was added and the aeration period was increased. Overall, when compared to a control AGS reactor, a lower COD consumption (from 87 to 35 mg g^-1 VSS h^-1), P-uptake rates (from 6.0 to 2.0 mg P g VSS^-1 h^-1) and NH₄-N removal (from 2.5 to 1.4 mg NH₄-N g VSS^-1 h^-1) were registered after introducing the synthetic FS. Approximately 40% of the granular bed became flocculent at the end of the study, and a reduction of the granular size accompanied by higher solids accumulation in the reactor was observed. A considerable protozoa Vorticella spp. bloom attached to the granules and the accumulated particles occurred; potentially contributing to the removal of the suspended solids which were part of the FS recipe.

The differential proliferation of AOB and NOB during natural nitrifier cultivation and acclimation with raw sewage as seed sludge

Nitrifier immigration from sewers to wastewater treatment systems is attracting increasing attention for understanding nitrifier community assembly mechanisms, and improving process modeling and operation. In this study, nitrifiers in raw sewage were cultivated and acclimated in a sequencing batch reactor (SBR) for 90 days to investigate the characteristics of the influent nitrifiers after immigration. During the experiment, specific nitrite utilization rate (SNUR) exceeded specific ammonia utilization rate (SAUR) when floc size reached 224 ± 46 μm, and nitrogen loss occurred at the same time. The ratio of nitrite oxidizing bacteria (NOB) to ammonia oxidizing bacteria (AOB) increased from 0.84 to 2.14 after cultivation. The Illumina MiSeq sequencing showed that the dominant AOB was Nitrosomonas sp. Nm84 and unidentified species, and the three most abundant Nitrospira were Nitrospira defluvii, Nitrospira calida, and unidentified Nitrospira spp. in both raw sewage and cultivated activated sludge. The shared reads of raw sewage and activated sludge were 48.76% for AOB and 89.35% for Nitrospira. These indicated that nitrifiers, especially NOB, immigrated from influent can survive and propagate in wastewater systems, which may be a significant hinder to suppress NOB in the application of advanced nitrogen remove process based on partial nitrification in the mainstream.

Enhanced granulation process, a more effective way of aerobic granular sludge cultivation in pilot-scale application comparing to normal granulation process: From the perspective of microbial insights

Aerobic granular sludge (AGS) could be cultivated from only flocs (called normal granulation (NG) process) or mixture of flocs and crushed AGS (called enhanced granulation (EG) process), which might lead to different system performances such as granulation speed and pollutants removal efficiencies. However, the differences of mechanisms between NG and EG processes at microbial community level are still unknown. In this study, the NG and EG processes were implemented successively in a pilot-scale sequencing batch reactor (SBR) with certain amounts of additional carbon sources. Illumina MiSeq sequencing and quantitative PCR were applied to investigate the dynamics of bacterial communities during NG and EG processes and explore the possible explanations for faster EG process. The results showed that significant distinctions in bacterial diversities and community structures were observed between NG and EG processes. The major contributor to NG process was bacterial communities with 32.04% contribution. While EG process was more dependent on the interactions (73.16% contribution) between the bacterial communities and environmental variables (operational parameters and self-adaptive variable). EG process had higher relative abundances of functional bacteria than NG process. Glycogen accumulating organisms (GAOs) related bacteria with a total relative abundance of maximum 65.43% might be mainly responsible for the faster EG process. This study provided microbial insights for practical application of AGS technology that inoculating crushed AGS might be an effective way to cultivate AGS.

Sustainable Approach to Eradicate the Inhibitory Effect of Free-Cyanide on Simultaneous Nitrification and Aerobic Denitrification during Wastewater Treatment

Simultaneous nitrification and aerobic denitrification (SNaD) is a preferred method for single stage total nitrogen (TN) removal, which was recently proposed to improve wastewater treatment plant design. However, SNaD processes are prone to inhibition by toxicant loading with free cyanide (FCN) possessing the highest inhibitory effect on such processes, rendering these processes ineffective. Despite the best efforts of regulators to limit toxicant disposal into municipal wastewater sewage systems (MWSSs), FCN still enters MWSSs through various pathways; hence, it has been suggested that FCN resistant or tolerant microorganisms be utilized for processes such as SNaD. To mitigate toxicant loading, organisms in SNaD have been observed to adopt a diauxic growth strategy to sequentially degrade FCN during primary growth and subsequently degrade TN during the secondary growth phase. However, FCN degrading microorganisms are not widely used for SNaD in MWSSs due to inadequate application of suitable microorganisms (Chromobacterium violaceum, Pseudomonas aeruginosa, Thiobacillus denitrificans, Rhodospirillum palustris, Klebsiella pneumoniae, and Alcaligenes faecalis) commonly used in single-stage SNaD. This review expatiates the biological remedial strategy to limit the inhibition of SNaD by FCN through the use of FCN degrading or resistant microorganisms. The use of FCN degrading or resistant microorganisms for SNaD is a cost-effective method compared to the use of other methods of FCN removal prior to TN removal, as they involve multi-stage systems (as currently observed in MWSSs). The use of FCN degrading microorganisms, particularly when used as a consortium, presents a promising and sustainable resolution to mitigate inhibitory effects of FCN in SNaD.

Combined Deterministic and Stochastic Processes Control Microbial Succession in Replicate Granular Biofilm Reactors

Granular sludge is an efficient and compact biofilm process for wastewater treatment. However, the ecological factors involved in microbial community assembly during the granular biofilm formation are poorly understood, and little is known about the reproducibility of the process. Here, three replicate bioreactors were used to investigate microbial succession during the formation of granular biofilms. We identified three successional phases. During the initial phase, the successional turnover was high and α-diversity decreased as a result of the selection of taxa adapted to grow on acetate and form aggregates. Despite these dynamic changes, the microbial communities in the replicate reactors were similar. The second successional phase occurred when the settling time was rapidly decreased to selectively retain granules in the reactors. The influence of stochasticity on succession increased and new niches were created as granules emerged, resulting in temporarily increased α-diversity. The third successional phase occurred when the settling time was kept stable and granules dominated the biomass. Turnover was low, and selection resulted in the same abundant taxa in the reactors, but drift, which mostly affected low-abundant community members, caused the community in one reactor to diverge from the other two. Even so, performance was stable and similar between reactors.

The key role of inoculated sludge in fast start-up of sequencing batch reactor for the domestication of aerobic granular sludge

Two types of inoculated sludges, granular sludge that had been stored at -20°C and activated sludge, were investigated for the domestication of aerobic granular sludges (AGSs) in sequencing batch reactors (SBRs). The results showed that using the stored granular sludge as inoculation sludge could effectively shorten the domestication time of AGS and yielded mature granular sludge after 22 days of operation. The AGS domesticated by stored granular sludge had better biomass and sedimentation properties; its MLSS and SVI reached 8.55 g/L and 35.27 mL/g, respectively. The removal efficiencies for chemical oxygen demand (COD), ammonium nitrogen (NH₄⁺-N) and total phosphorus (TP) reached 90.76%, 97.39% and 96.40%, respectively. By contrast, 54 days were needed to obtain mature granules using activated sludge. The microbial community structure was probed by using scanning electron microscopy (SEM) and high-throughput sequencing. The results showed that the diversity of the microbial community in mature granules was reduced when stored granular sludge rather than activated sludge was employed as inoculation sludge, and the dominant microbes were changed. The dominant species in mature granules domesticated using stored granular sludge were Zoogloea, Acidovorax and Tolumonas at the genus classification level, while the dominant species were Zoogloea and TM7-genera in granules developed from activated sludge.

Natural continuous influent nitrifier immigration effects on nitrification and the microbial community of activated sludge systems

Two sequencing batch reactors (SBRs) were operated for 100days under aerobic conditions, with one being fed with unsterilized municipal wastewater (USBR), and the other fed with sterilized municipal wastewater (SSBR). Respirometric assays and fluorescence in situ hybridization (FISH) results show that active nitrifiers were present in the unsterilized influent municipal wastewater. The maximum ammonia utilization rate (AUR) and nitrite utilization rate (NUR) of the unsterilized influent were 0.32±0.12mg NH₄⁺-N/(L·hr) and 0.71±0.18mg NO₂^--N/(L·hr). Based on the maximum utilization rates, the estimated seeding intensity for the ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) of the USBR was 0.08g AOB/(g AOB·day) and 0.20g NOB/(g NOB·day) respectively. The fraction of nitrifiers/total bacteria in the influent was 5.35%±2.1%, the dominant AOB was Nitrosomonas spp., Nitrosococcus mobilis hybridizated with Nsm156, and the dominant NOB was Nitrospira hybridizated with Ntspa662. The influent nitrifiers potentially seeded the activated sludge of the bioreactor and hence demonstrated a mitigation of the acclimatization times and instability during start-up and early operation. The AUR and NUR in the USBR was 15% and 13% higher than the SSBR respectively during the stable stage, FISH results showed that nitrifiers population especially the Nitrospira in the USBR was higher than that in the SSBR. These results indicate that the natural continuous immigration of nitrifiers from municipal influent streams may have some repercussions on the modeling and design of bioreactors.

The mechanisms of granulation of activated sludge in wastewater treatment, its optimization, and impact on effluent quality

Granular activated sludge has gained increasing interest due to its potential in treating wastewater in a compact and efficient way. It is well-established that activated sludge can form granules under certain environmental conditions such as batch-wise operation with feast-famine feeding, high hydrodynamic shear forces, and short settling time which select for dense microbial aggregates. Aerobic granules with stable structure and functionality have been obtained with a range of different wastewaters seeded with different sources of sludge at different operational conditions, but the microbial communities developed differed substantially. In spite of this, granule instability occurs. In this review, the available literature on the mechanisms involved in granulation and how it affects the effluent quality is assessed with special attention given to the microbial interactions involved. To be able to optimize the process further, more knowledge is needed regarding the influence of microbial communities and their metabolism on granule stability and functionality. Studies performed at conditions similar to full-scale such as fluctuation in organic loading rate, hydrodynamic conditions, temperature, incoming particles, and feed water microorganisms need further investigations.

Shifts in bacterial community composition and abundance of nitrifiers during aerobic granulation in two nitrifying sequencing batch reactors

Shifts in bacterial community composition and abundance of nitrifiers during aerobic granulation, and the effects of wastewater composition on them were investigated using Illumina sequencing and quantitative PCR. The bacterial diversity decreased sharply during the post-granulation period. Although cultivated with different wastewater types, aerobic granular sludge (AGS) formed with similar bacterial structure. The bacterial structure in AGS was completely different from that of seed sludge. The minor genera in seed sludge, e.g., Arcobacter, Aeromonas, Flavobacterium and Acinetobacter, became the dominant genera in AGS. These genera have the potential to secrete excess extracellular polymer substances. Whereas, the dominant genera in seed sludge were found in less amount or even disappeared in AGS. During aerobic granulation, ammonia-oxidizing archaea were gradually washed-out. While, ammonia-oxidizing bacteria, complete ammonia oxidizers and nitrite-oxidizing bacteria were retained. Overall, in this study, the bacterial genera with low relative abundance in seed sludge are important for aerobic granulation.

Salt-tolerance aerobic granular sludge: Formation and microbial community characteristics

The salt-tolerance aerobic granular sludge (SAGS) dominated by moderately halophilic bacteria was successfully cultured in a 9% (w/v) salty, lab-scale sequence batch reactor (SBR) system. Influence of high salinity (0-9% w/v NaCl) on the formation, performance and microbial succession of the SAGS were explored. Crystal nucleus hypothesis, selection pressure hypothesis and compressed double electric layers hypothesis were used to discuss the formation mechanism of SAGS. Notably, salinity could be seen as a kind of selection pressure contributed to the formation of SAGS, while salinity also declined the performance of SAGS system. High throughput 16S rRNA gene analysis showed that the salinity had great influence on the species succession and community structure of SAGS. Moreover, Salinicola and Halomonas were dominant at 9% salt concentration, therefore moderate halophiles were identified as functional groups for the tolerance of hypersaline stress.

Comparison of the bacterial community composition in the granular and the suspended phase of sequencing batch reactors

Granulation of activated sludge is an increasingly important area within the field of wastewater treatment. Granulation is usually achieved by high hydraulic selection pressure, which results in the wash-out of slow settling particles. The effect of the harsh wash-out conditions on the granular sludge ecosystem is not yet fully understood, but different bacterial groups may be affected to varying degrees. In this study, we used high-throughput amplicon sequencing to follow the community composition in granular sludge reactors for 12 weeks, both in the granular phase and the suspended phase (effluent). The microbiome of the washed out biomass was similar but not identical to the microbiome of the granular biomass. Certain taxa (e.g. Flavobacterium spp. and Bdellovibrio spp.) had significantly (p < 0.05) higher relative abundance in the granules compared to the effluent. Fluorescence in situ hybridization images indicated that these taxa were mainly located in the interior of granules and therefore protected from erosion. Other taxa (e.g. Meganema sp. and Zooglea sp.) had significantly lower relative abundance in the granules compared to the effluent, and appeared to be mainly located on the surface of granules and therefore subject to erosion. Despite being washed out, these taxa were among the most abundant members of the granular sludge communities and were likely growing fast in the reactors. The ratio between relative abundance in the granular biomass and in the effluent did not predict temporal variation of the taxa in the reactors, but it did appear to predict the spatial location of the taxa in the granules.

Microbial Population Dynamics and Ecosystem Functions of Anoxic/Aerobic Granular Sludge in Sequencing Batch Reactors Operated at Different Organic Loading Rates

The granular sludge process is an effective, low-footprint alternative to conventional activated sludge wastewater treatment. The architecture of the microbial granules allows the co-existence of different functional groups, e.g., nitrifying and denitrifying communities, which permits compact reactor design. However, little is known about the factors influencing community assembly in granular sludge, such as the effects of reactor operation strategies and influent wastewater composition. Here, we analyze the development of the microbiomes in parallel laboratory-scale anoxic/aerobic granular sludge reactors operated at low (0.9 kg m^-3d^-1), moderate (1.9 kg m^-3d^-1) and high (3.7 kg m^-3d^-1) organic loading rates (OLRs) and the same ammonium loading rate (0.2 kg NH₄-N m^-3d^-1) for 84 days. Complete removal of organic carbon and ammonium was achieved in all three reactors after start-up, while the nitrogen removal (denitrification) efficiency increased with the OLR: 0% at low, 38% at moderate, and 66% at high loading rate. The bacterial communities at different loading rates diverged rapidly after start-up and showed less than 50% similarity after 6 days, and below 40% similarity after 84 days. The three reactor microbiomes were dominated by different genera (mainly Meganema, Thauera, Paracoccus, and Zoogloea), but these genera have similar ecosystem functions of EPS production, denitrification and polyhydroxyalkanoate (PHA) storage. Many less abundant but persistent taxa were also detected within these functional groups. The bacterial communities were functionally redundant irrespective of the loading rate applied. At steady-state reactor operation, the identity of the core community members was rather stable, but their relative abundances changed considerably over time. Furthermore, nitrifying bacteria were low in relative abundance and diversity in all reactors, despite their large contribution to nitrogen turnover. The results suggest that the OLR has considerable impact on the composition of the granular sludge communities, but also that the granule communities can be dynamic even at steady-state reactor operation due to high functional redundancy of several key guilds. Knowledge about microbial diversity with specific functional guilds under different operating conditions can be important for engineers to predict the stability of reactor functions during the start-up and continued reactor operation.