Improving phosphorus removal in aerobic granular sludge processes through selective microbial management

New insight into the granule formation in the reactor for enhanced biological phosphorus removal

While granulated activated sludge exhibits high productivity, the processes of granule formation are incompletely studied. The processes of granule formation and succession of communities were investigated in a laboratory sequencing batch reactor (SBR) under conditions for enhanced biological phosphorus removal (EBPR) using microbiological and molecular techniques. Active consumption of acetate, primarily by the phosphate-accumulating organisms (PAO), commenced at day 150 of cultivation. This was indicated by the high ratio of molar P-released/acetate uptake (0.73-0.77 P-mol/C-mol), characteristic of PAO. During this period, two types of granule-like aggregates formed spontaneously out of the activated sludge flocs. The aggregates differed in morphology and microbial taxonomic composition. While both aggregate types contained phosphorus-enriched bacterial cells, PAO prevailed in those of morphotype I, and glycogen-accumulating organisms (GAOs) were predominant in the aggregates of morphotype II. After 250 days, the elimination of the morphotype II aggregates from the reactor was observed. The subsequent selection of the community was associated with the development of the morphotype I aggregates, in which the relative abundance of PAO increased significantly, resulting in higher efficiency of phosphorus removal. Metagenomic analysis revealed a predominance of the organisms closely related to Candidatus Accumulibacter IС and IIС and of Ca. Accumulibacter IIB among the PAO. Based on the content of the genes of the key metabolic pathways, the genomes of potential PAO belonging to the genera Amaricoccus, Azonexus, Thauera, Zoogloea, Pinisolibacter, and Siculibacillus were selected. The patterns of physicochemical processes and the microbiome structure associated with granule formation and succession of the microbial communities were revealed.

Achieving tetracycline removal enhancement with granules in marine matrices: Performance, adaptation, and mechanism studies

Using the aerobic granular sludge (AGS) to improve tetracycline (TET) removal in the treatment of mariculture wastewater was investigated in the present study. The AGS rapidly adapted to and was sustained in seawater matrices with a robust granule strength (k = 0.0014) and a more stable sludge yield than the activated sludge (AS) (0.14 vs 0.11 g-VSS/g-COD_rem). The compact structure provided the AGS with an anoxic environment, which favored the growth of N (37.3 %) and P removal bacteria (30.4 %) and the expression of functional genes (nos, nor, and nar), resulting in more than 62 % TN and TP removals, respectively. Similar abundances of aromatic compound-degrading bacteria (∼34 %) in both reactors (AGS and AS) led to comparable TET biodegradation efficiencies (∼0.045 mg/g-VSS). The greater size and surface area of the AGS expanded the boundary layer diffusion region, leading to 16 % increases in the granule's TET adsorption capacity.

Feasibility Study of Applying Anaerobic Step-Feeding Mode for the Treatment of High-Strength Wastewater in Granular Sequencing Batch Reactors (GSBRs)

This study investigated the feasibility of applying an anaerobic step-feeding strategy to enhance the performance of granular sequencing batch reactors (GSBRs) in terms of operational stability of the cultivated mature granules and nutrient removal efficiencies. Two identical 5 L reactors were operated with a total cycle time of 8 h. GSBRs were operated with high-strength synthetic wastewater (COD = 1250 ± 43, ammonium (NH4-N) = 115.2 ± 4.6, and orthophosphate (PO4-P) = 17.02 ± 0.9 mg/L) for 360 days through three stages: (1) Cultivation, 125 days (>2.1 mm); (2) Maturation, 175 days (>3 mm); (3) alternate feed loading strategy for R2 only for 60 days (anaerobic step-feeding). The granulation process, the physical properties of the granules, the nutrients, and the substrate removal performance were recorded during the entire operational period. For the cultivation and maturation stages, both reactors followed the fast single feeding mode followed by anaerobic mixing, and the results indicated a strong correlation between R1 and R2 due to the same working conditions. During the cultivation stage, adopting high organic loading rate (OLR) at the reactor start-up did not accelerate the formation of granules. Removal efficiency of PO4-P was less than 76% during the maturation period, while it exceeded 90% for COD, and was higher than 80% for NH4-N without effect of nitrite or nitrate accumulations due to simultaneous nitrification–denitrification. After changing filling mode for R2 only, there was unexpected deterioration in the performance and a rapid disintegration of the matured granules (poor settleability) accompanied by poor effluent quality due to high content of suspended solids because of applying selection pressure of short settling time. Consequently, GSBRs operation under the effect of fast single feeding mode followed by anaerobic mixing favors stable long-term granule stability.

Consistency between the metabolic performance of two aerobic granular sludge systems and the functional groups of bacteria detected by amplicon sequencing

Two sequential batch reactors (R1 and R2) of aerobic granular sludge (AGS) were inoculated with activated sludge of different origins. The objective was to investigate the granulation and the consistency between the structure of the microbial communities (16S rRNA amplicon sequencing) in each reactor and their metabolic performance (removal of C, N, and P). Both reactors were fed with acetate-based synthetic wastewater, targeting an anaerobic-aerobic cycle reputed to favor the phosphorus- and glycogen-accumulating organisms (PAO and GAO). Stable granulation was achieved in both reactors, where, instead of PAO, the dominant genera were ordinary heterotrophic organisms (OHO) such as Thauera, Paracoccus, and Flavobacterium known for their high capacity of aerobic storage of polyhydroxyalkanoates (PHA). Generally, there was good consistency between the metabolic behavior of each reactor and the bacterial genera detected. Both reactors showed high removals of C and complete nitrification (Nitrosomonas and Nitrospira detected) but a low level of simultaneous nitrification-denitrification (SND) during the aerated phase. The latter causes that nitrates were recycled to the initial phase, in detriment of PAO selection. Meanwhile, the study showed that selecting slow-growing OHOs (with aerobic storage capacity) favors stable granulation, revealing an alternative AGS technology for C and N removal.

Phosphorus Removal from Aerobic Granular Sludge: Proliferation of Polyphosphate-Accumulating Organisms (PAOs) under Different Feeding Strategies

Aerobic granular sludge (AGS) is known for high phosphorus removal from wastewaters, and phosphorus can be recovered from high phosphorus-containing waste sludge granules. This study aimed at determining the feeding strategy that provides the best performance in terms of the proliferation of polyphosphate-accumulating organisms (PAOs) and phosphorus removal. Using three AGS bioreactors, this study compared phosphorus removal and the proliferation dynamics of PAOs under three different feeding strategies: anaerobic slow feeding (R1), pulse feeding + anaerobic mixing (R2), and pulse feeding (R3). Results indicate that R1 and R2 achieved significantly higher phosphorus removal (97.6 ± 3% for R1 and 98.3 ± 1% for R2) than R3 (55 ± 11%). The anaerobic slow feeding procedure (R1) achieved the highest specific phosphorus release rate (SPRR) and specific phosphorus uptake rate (SPUR) as compared to the other two feeding conditions. 16S ribosomal ribonucleic acid (rRNA) gene sequencing assay of the microbial community for the three feeding strategies indicated that although the feeding strategy impacted reactor performance, it did not significantly alter the microbial community. The bacteria community composition maintained a similar degree of diversity. Proteobacteria, Bacteroidetes, and Verrucomicrobia were the dominant bacterial phyla in the system. Dominant PAOs were from the class Betaproteobacteria and the genera Paracoccus and Thauera. Glycogen-accumulating organisms were significantly inhibited while other less-known bacteria such as Wandonia and Hyphomonas were observed in all three reactors.

Enhanced biological phosphorus removal in aerobic granular sludge reactors by granular activated carbon dosing

This study investigated the effects of granular activated carbon (GAC) addition on the enrichment of polyphosphate accumulating organisms (PAOs), stratification of PAOs in the co-existing GAC-biofilms and granules and biological nutrient removal (BNR) in aerobic granular sludge (AGS) reactors. It was found that BNR increased in the GAC-augmented system. Establishment of enhanced biological phosphorus removal (EBPR) pathway was faster with about 1.7 to 2-fold higher P removal in GAC system than control. EBPR biomass grown in the presence of GAC was segregated into different size fractions for determining BNR and stratification of microbial groups. It was found that EBPR was majorly associated with the large biomass (>0.5 mm) fraction, corroborating with higher abundance of PAOs. Higher P removals of 60 to 70% with characteristic EBPR profiles were observed in 0.5 mm fraction. In contrast, P removals by 0.25 mm fraction were lower at 20 to 35% without EBPR profiles. EBPR biomass (>0.5 mm) fraction was segregated into granules and GAC-biofilms for determining the role of GAC in PAOs enrichment. P release (2.5-3.5 mg L^-1 P) and P uptake (5-7 mg L^-1 P) were higher in the P removal profiles exhibited by GAC-biofilms. In contrast, P release and P uptake were lower with the granules. These differences in P removal profiles resulted in distinct net P removal efficiencies of 70 ± 5% and 50 ± 6% for GAC-biofilms and granules, respectively. These differences in P removals were corroborated by higher abundance of PAOs in the GAC-biofilms than co-existing granules. PAO clade-level enrichment was found to be dependent on substrate wherein acetate feeding enriched PAO clade I, while acetate-propionate feeding caused enrichment of both PAO clade I and II. These results suggest that GAC addition to AGS reactors can aid in enrichment of PAOs, reduce the start-up period for EBPR, and increase P removal efficiencies.

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.

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.

Aerobic granular sludge treating low-strength municipal wastewater: Efficient carbon, nitrogen and phosphorus removal with hydrolysis-acidification pretreatment

Low organic load while high fraction of particulates still challenging the application of aerobic granular sludge process in low-strength municipal wastewater treatment. The feasibility of adopting short cycle length to increase organic load and hydrolysis-acidification pretreatment to enhance anaerobic COD uptake was evaluated. As the cycle length decreased from 4 h to 2 h, the organic loading rate increased from 0.98 to 1.3 g L^-1 d^-1 and granulation appeared after two weeks. Moreover, with the hydrolysis-acidification pretreatment, the average effluent TN and TP concentrations decreased respectively from 17.8 to 13.7 mg L^-1 and 0.76 to 0.41 mg L^-1, meeting the Grade IA of the effluent standards in China. Furthermore, cycle tests were conducted to reveal the underlying mechanism of the pretreatment effects. The results showed that the hydrolysis-acidification pretreatment enhanced the COD storage and phosphorus release in anaerobic phase, and improved the simultaneous nitrification-denitrification process, as well as the phosphorus uptake in aeration phase.

Start-up of an aerobic granular sludge system from stored granules: Evaluating the impact of storage period on biomass activity and stability and the effect of temperature on nitrification and phosphorus removal rates

Aerobic granular sludge (AGS) has been considered a breakthrough within the wastewater treatment sector. However, the long reactor start-up for the formation of granules is challenging and may hinder the spread of this technology. To circumvent this obstacle, inoculation of the reactors with pre-formed granules from existing plants is an interesting approach. In this context, issues related to biomass storage becomes very relevant. In this study, reactivation of aerobic granular biomass after storage was evaluated in a sequencing batch reactor (SBR) designed for achieving simultaneous organic matter, nitrogen and phosphorus removal. Two different scenarios, short (40 days) and long (180 days) storage periods, were assessed, and their influence on the granules physical properties and bioactivity was addressed. The results revealed that the granules stored for a shorter period showed higher resistance to breakage and underwent smooth color changes. On the other hand, the biomass stored for a longer period acquired a dark color and was more susceptible to disruption during reactivation. The granules stored for 6 months become swollen and exhibited an irregular morphology and fluffy structure within the first days of reactivation. Consequently, their settling properties were adversely affected, and some parameters such as the food-to-microorganism ratio had to be adjusted to prevent granules disintegration. Regarding the bioactivity of important microbial functional groups, COD removal was rapidly restored within a few days of SBR operation with the biomass stored for a shorter period. However, it took longer for the biomass stored for 180 days to reach the same performance observed for the granules stored for 40 days. A similar trend is valid for nitrification. In the experiments with sludge stored for a longer time, it took almost twice as long to reach effluent ammonium concentrations lower than 1 mg NH₄⁺-N L^-1 compared to the test using biomass stored for 40 days. Phosphate removal was strongly affected by biomass storage, especially after 180 days of inactivity, a condition found to be detrimental for polyphosphate-accumulating organisms. Finally, cycle tests were also conducted to assess substrate conversion rates for comparison between different trials and evaluate the influence of temperature (10-35 °C) on nitrification and phosphate removal rates.

Impact of the substrate composition on enhanced biological phosphorus removal during formation of aerobic granular sludge

Performance of enhanced biological phosphorus removal (EBPR) is often investigated with simple synthetic wastewater containing volatile fatty acids (VFAs). In this study, various (fermentable) substrates, individually and in mixtures, were examined during the application of a granulation strategy. In addition, the microbial community and N₂O formation were monitored. Sludge densification was observed in all systems. Stable EBPR, associated with the presence of Accumulibacter and an anaerobic P-release up to 21.9 mgPO₄^3--P.gVSS^-1, was only obtained when VFAs were present as sole substrate or in mixture. Systems fed with VFAs were strongly related to the formation of N₂O (maximum of 6.25% relative to the total available nitrogen). A moderate anaerobic dissolved organic carbon (DOC) uptake was observed when amino acids (64.27 ± 3.08%) and glucose (75.39 ± 5.79%) as sole carbon source were applied. The substrate/species-specific enrichment of Burkholderiaceae and Saccharimonadaceae respectively, resulted in unstable EBPR in those systems.

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.

Development of a double-layer EPS-ASM2d model to illustrate the effect on mainstream biological phosphorus system in side-stream phosphorus recovery process

In order to deeply investigate the influences of side-stream phosphorus (P) recovery operation on mainstream biological P removal system, an improved activated sludge model no. 2 (ASM2d) was established to illuminate the metabolic processes of P in a side-stream P recovery reactor. The improved ASM2d (named D-EPS-ASM2d) was established by extending of the P metabolic processes of double-layer extracellular polymeric substances (EPS) into conventional ASM2d model. The predicted effluent concentrations of COD, NH₄, and TP by the D-EPS-ASM2d had good fits with measured values in the side-stream P recovery process. Comparing with conventional ASM2d, the likelihood values of D-EPS-ASM2d related to COD, NH₄, and TP effluents were increased from 0.694, 0.837 and 0.762 to 0.868, 0.904 and 0.920, respectively, implying the simulation performances of D-EPS-ASM2d on nutrient removal processes were significantly improved. Besides, the calibrated values of f_PP,TEPS was 0.09, 0.102 and 0.123 as side-stream volume (SSV) increasing from 0.3 to 0.9, implying the fraction of P removal by tightly-bound EPS was enhanced with the increase of SSV.

Effect of calcium addition to aerobic granular sludge systems under high (conventional SBR) and low (simultaneous fill/draw SBR) selection pressure

This paper investigated the effect of calcium addition on the formation and properties of aerobic granules under high (conventional SBR) and low (simultaneous fill/draw SBR) selection pressure. Additionally, the simultaneous removal of carbon, nitrogen, and phosphorus, and the operational stability were assessed. The conventional SBRs showed earlier granule development (20 days) than the simultaneous fill/draw SBRs. The effect of calcium on granulation was more accentuated in conventional SBRs, forming larger granules in a shorter interval of time due to the higher EPS production. Additionally, higher amounts of calcium were found in the EPS matrix, mainly during the formation of granules. The operation regime and the addition of calcium did not affect the removal of carbon, nitrogen, and phosphorus. However, they both influenced the granulation time, settleability characteristics, size, and granule composition.

Modified Biochar—A Tool for Wastewater Treatment

Global deposits of concentrated phosphates, which are a necessary source for the production of phosphate fertilizers, are limited. These reserves keep getting thinner, and every day, large amounts of phosphorus end up in watercourses. In this study, we verified that modified biochar (saturated with FeCl3 solution and then neutralized with NaOH solution) can adsorb significant amounts of phosphorus from wastewater. Moreover, the agrochemical qualities of sludge water from a municipal wastewater treatment plant, struvite, phosphorus-saturated biochar, and iron(III) phosphate from a reused biochar filter were tested in this study. We determined the amount of mobile phosphorus as well as the amount of extractable phosphorus and its five fractions. It was found that modified biochar can hold one-third of the phosphorus amount contained in the commonly used agricultural fertilizer simple superphosphate (1 × 105 g of modified biochar captures up to 2.79 × 103 g of P). Moreover, plants can more easily access phosphorus biochar fractions than struvite, which is formed spontaneously during sludge management. The results of this research prove that the proposed method of recycling phosphorus from wastewater can be applied in technological practice.

Organic substrate diffusibility governs microbial community composition, nutrient removal performance and kinetics of granulation of aerobic granular sludge

Basic understanding of formation of aerobic granular sludge (AGS) has mainly been derived from lab-scale systems with simple influents containing only highly diffusible volatile fatty acids (VFA) as organic substrate. This study compares start-up of AGS systems fed by different synthetic and municipal wastewaters (WW), characterised by increasing complexity in terms of non-diffusible organic substrate. Four AGS reactors were started with the same inoculum activated sludge and operated for one year. The development of AGS, settling characteristics, nutrient and substrate removal performance as well as microbial community composition were monitored. Our results indicate that the higher the content of diffusible organic substrate in the WW, the faster the formation of AGS. The presence of non-diffusible organic substrate in the influent WW led to the formation of small granules and to the presence of 20-40% (% of total suspended solids) of flocs in the AGS. When AGS was fed with complex influent WW, the classical phosphorus and glycogen accumulating organisms (PAO, GAO) were outcompeted by their fermentative equivalents. Substrate and nutrient removal was observed in all reactors, despite the difference in physical and settling properties of the AGS, but the levels of P and N removal depended on the influent carbon composition. Mechanistically, our results indicate that increased levels of non-diffusible organic substrate in the influent lower the potential for microbial growth deep inside the granules. Additionally, non-diffusible organic substrates give a competitive advantage to the main opponents of AGS formation - ordinary heterotrophic organisms (OHO). Both of these mechanisms are suspected to limit AGS formation. The presented study has relevant implications for both practice and research. Start-up duration of AGS systems treating high complexity WW were one order of magnitude higher than a typical lab-scale system treating VFA-rich synthetic WW, and biomass as flocs persisted as a significant fraction. Finally, the complex synthetic influent WW - composed of VFA, soluble fermentable and particulate substrate - tested here seems to be a more adequate surrogate of real municipal WW for laboratory studies than 100%-VFA WW.

The regulation of N-acyl-homoserine lactones (AHLs)-based quorum sensing on EPS secretion via ATP synthetic for the stability of aerobic granular sludge

According to the relationship among microbial activity, quorum sensing (QS) and structural stability of aerobic granular sludge, the mechanism of QS regulation for microbial activity and granular stability was investigated in AGS process. Results showed that ATP content decreased sharply from 1.8 μmol/gVSS of stable granules to 0.8 μmol/gVSS of disintegrating granules, and the relative abundance of QS-activity microbes, Rhodobacter spp. and Xanthomonadaceae decreased in initially unstable granules compared with stable granules. The main AHLs were detected in this study, and C8-HSL, 3OHC8-HSL and 3OHC12-HSL decreased significantly when structure of granules changed from stability to disintegration. Accompanying with the decrease of AHLs level, the extracellular polymeric substances (EPS) content in initially unstable granules decreased sharply from 226.8 to 163.6 mg/gVSS with the ratio of extracellular protein to exopolysaccharide (PN/PS) decreasing from 3.6 to 2.2, despite EPS-secretion microbes enriched. The effect of QS on microbial activity was proved by AHL add-back study, results indicated that ATP and EPS content in sludge increased significantly (p < 0.05) with AHLs added, but EPS production was limited when ATP synthesis was disrupted. It was concluded that the AHLs-based QS favored the granular stability via the enhancement of ATP synthesis in microbes. This study provides a new perspective for QS regulation in aerobic granular sludge system, because the ATP regulated by QS could be the energy currency for cellular metabolism, such as nutrient removal, degradation of emerging pollutants, microbial growth and other aspects.

Investigation of rapid granulation in SBRs treating aniline-rich wastewater with different aniline loading rates

In this work, aerobic granules were cultivated in two reactors which were denoted as R_L and R_H under 0.6 and 1.8 kg m^-3 d^-1 of aniline loading rates, respectively. The aerobic granular sludge (AGS) in the two sequential batch reactors for treating aniline-rich wastewater was compared. The results showed that the AGS could be rapidly formed with sludge volume index below 30 mL g^-1. The AGS in R_L had more filamentous bacteria than that in R_H by microstructural observations while the secretion of protein in extracellular polymeric substances was improved in R_H and in turn increased relative hydrophobicity of AGS. Within 4-h cycle, the excellent removal of aniline and chemical oxygen demand (COD) were achieved in the two reactors. The removal efficiencies of aniline and COD were consistently over 99.7%, 89.6%, respectively in R_L and 98.6%, 86.6%, respectively in R_H. As for nitrogen removal, NH₄⁺-N released from aniline biodegradation could also be reduced efficiently via nitrification and no nitrite accumulation occurred in both the reactors. Total nitrogen removal performance in R_H was better, due to a more compact structure of AGS. The investigation of microbial community succession by pyrosequencing showed that the diversity of microorganisms decreased when AGS was developed. Proteobacteria especially Gammaproteobacteria significantly increased during aerobic granulation in both reactors. It was also found that the relative abundance of Actinobacteria was higher in R_H than that in R_L. Furthermore, the strains responsible for aniline biodegradation, nitrification, denitrification, and phosphorous accumulation were detected in the systems.

Short operational differences support granulation in a lab scale reactor in comparison to another conventional activated sludge reactor

This study explains how small operational differences support excellent granulation in aerobic granular reactors. Short settling time promoted granulation in AGS reactor. Gene expressions based on mRNA revealed much higher ammonium monooxygenase (amoA) in conventional reactor biomass than in the aerobic granular reactor (AGS) biomass during a complete cycle operation. The number of glycogen accumulating organisms in conventional was much higher than in the granular reactor. The denitrifying functional genes in the granular systems were upregulated in anaerobic and aerobic phases. The granular reactor removed 1.84 kg COD-m^-3day^-1, 0.09 kg NH₄⁺-N-m^-3day^-1, and 0.063 kg PO₄^3-P-m^-3day^-1. The conventional reactor removed 1.14 Kg-m^-3day^-1 COD, 0.05 kg-m^-3day^-1 NH₄⁺-N, and 0.028 kg-m^-3day^-1 PO₄^3--P. The granular reactor showed faster kinetics for nutrient and organics removal compared to the conventional reactor. Flocs in the conventional reactor had a lower abundance of Candidatus accumulibacter sp. and higher relative abundance of Candidatus competibacter.

Pollutants degradation performance and microbial community structure of aerobic granular sludge systems using inoculums adapted at mild and low temperature

Three aerobic granular sequencing batch reactors were inoculated using different inocula from Finland, Spain and a mix of both in order to investigate the effect over the degradation performance and the microbial community structure. The Finnish inoculum achieved a faster granulation and a higher depollution performance within the first two month of operation. However, after 90 days of operation, similar physico-chemical values were observed. On the other hand, the Real-time PCR showed that Archaea diminished from inoculum to granular biomass, while Bacteria and Fungi numbers remained stable. All granular biomass massive parallel sequencing studies were similar regardless of the inocula from which they formed, as confirmed by singular value decomposition principal coordinates analysis, expected effect size of OTUs, and β-diversity analyses. Thermoproteaceae, Meganema and a Trischosporonaceae members were the dominant phylotypes for the three domains studied. The analysis of oligotype distribution demonstrated that a fungal oligotype was ubiquitous. The dominant OTUs of Bacteria were correlated with bioreactors performance. The results obtained determined that the microbial community structure of aerobic granular sludge was similar regardless of their inocula, showing that the granulation of biomass is related to several phylotypes. This will be of future importance for the implementation of aerobic granular sludge to full-scale systems.

Effect of graphene oxide on the bioactivities of nitrifying and denitrifying bacteria in aerobic granular sludge

With the widespread application of graphene oxide (GO), it would be inevitably released into wastewater treatment plants (WWTPs) and get involved in the biochemical process. So far, there are controversies on the effects of low GO concentration (0.05-0.1 g/L) on the nitrogen removal process. Therefore, this study essentially investigates any potential effects of GO on wastewater microbial communities functions. In present study, the nitrifying and denitrifying batch tests were introduced to investigate the influence of 0.06 g/L of GO on bacteria. The results showed that GO could be easily combined with the aerobic granular sludge (AGS), and NH₄⁺-N was sharply absorbed, which directly promoted the bioactivities of ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) and extracellular polymeric substances (EPS) production. The influence of GO on the denitrifying bacteria was negligible, which resulted in the stable EPS production. Furthermore, as inferred from the near maximum chemical reaction rates, there were no obvious changes on the microbial community functions during nitrogen removal process.

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.

Cultivation of aerobic granular sludge in continuous flow under various selective pressure

Formation of aerobic granular sludge was examined in a novel continuous flow configuration, at 20 ± 1 °C. Synthetic proteinaceous wastewater with municipal primary effluent characteristics was used (i.e., COD = 370 ± 30 mg/L; TN = 43 ± 7 mg/L; and TP = 10 ± 2 mg/L). Various levels of selective pressure were applied after inoculation with flocculent sludge (i.e., estimated velocity gradients during settling between 1 and 9 1/s). Impeller rpm of 15 and below generated floccular-granular biomass, while 20 rpm and above generated large granules with a filamentous population. Effluent soluble COD, total inorganic nitrogen, and phosphate of 25 ± 7 mg/L, 11 ± 1 mg/L, and 0.1 ± 0.1 mg/L, respectively, were obtained. Observed yields were as low as 0.08-0.19 g-VSS/g-COD and whole sludge solids retention time was 18 ± 1 d. Famine conditions developed for 90% of the total aerobic volume and >45 ± 3% anaerobic substrate utilization was recorded. Aerobic granulation was demonstrated feasible under continuous flow providing adequate treatment with low biomass yields.

Microbial community structure and function in aerobic granular sludge

Aerobic granular sludge (AGS), a self-immobilized microbial consortium containing different functional microorganisms, is receiving growing attention, since it has shown great technological and economical potentials in the field of wastewater treatment. Microbial community is crucial for the formation, stability, and pollutant removal efficiency of aerobic granules. This mini-review systematically summarizes the recent findings of the microbial community structure and function of AGS and discusses the new research progress in the microbial community dynamics during the granulation process and spatial distribution patterns of the microbiota in AGS. The presented information may be helpful for the in-depth theoretical study and practical application of AGS technology in the future.

State of the art on granular sludge by using bibliometric analysis

With rapid industrialization and urbanization in the nineteenth century, the activated sludge process (ASP) has experienced significant steps forward in the face of greater awareness of and sensitivity toward water-related environmental problems. Compared with conventional flocculent ASP, the major advantages of granular sludge are characterized by space saving and resource recovery, where the methane and hydrogen recovery in anaerobic granular and 50% more space saving, 30-50% of energy consumption reduction, 75% of footprint cutting, and even alginate recovery in aerobic granular. Numerous engineers and scientists have made great efforts to explore the superiority over the last 40 years. Therefore, a bibliometric analysis was desired to trace the global trends of granular sludge research from 1992 to 2016 indexed in the SCI-EXPANDED. Articles were published in 276 journals across 44 subject categories spanning 1420 institutes across 68 countries. Bioresource Technology (293, 11.9%), Water Research (235, 9.6%), and Applied Microbiology and Biotechnology (127, 5.2%) dominated in top three journals. The Engineering (991, 40.3%), China (906, 36.9%), and Harbin Inst Technol, China (114, 4.6%) were the most productive subject category, country, and institution, respectively. The hotspot is the emerging techniques depended on granular reactors in response to the desired removal requirements and bio-energy production (primarily in anaerobic granular sludge). In view of advanced and novel bio-analytical methods, the characteristics, functions, and mechanisms for microbial granular were further revealed in improving and innovating the granulation techniques. Therefore, a promising technique armed with strengthened treatment efficiency and efficient resource and bio-energy recovery can be achieved.

Aerobic granular sludge technology: Mechanisms of granulation and biotechnological applications

Aerobic granular sludge (AGS) is a novel microbial community which allows simultaneous removal of carbon, nitrogen, phosphorus and other pollutants in a single sludge system. AGS is distinct from activated sludge in physical, chemical and microbiological properties and offers compact and cost-effective treatment for removing oxidized and reduced contaminants from wastewater. AGS sequencing batch reactors have shown their utility in the treatment of abattoir, live-stock, rubber, landfill leachate, dairy, brewery, textile and other effluents. AGS is extensively researched for wide-spread implementation in sewage treatment plants. However, formation of AGS takes relatively much longer time while treating low-strength wastewaters like sewage. Strategies like increased volumetric flow by means of short cycles and mixing of sewage with industrial wastewaters can promote AGS formation while treating low-strength sewage. This article reviewed the state of research on AGS formation mechanisms, bioremediation capabilities and biotechnological applications of AGS technology in domestic and industrial wastewater treatment.

Start-up and operation of an aerobic granular sludge system under low working temperature inoculated with cold-adapted activated sludge from Finland

An aerobic granular sludge system has been started-up and operated at 7°C temperature using cold-adapted activated sludge as inoculum. The system could form granular biomass due to batch operation allowing for just 5-3min of biomass sedimentation. Scanning electron microscopy showed that fungi helped in the granular biomass formation in the early stages of the granule formation. The removal performance of the system was of 92-95% in BOD5, 75-80% in COD, 70-76% in total nitrogen and 50-60% in total phosphorous. The bacterial community structure from cold-adapted activated sludge changed during the operational time, leading to a final configuration dominated by Microbacteriaceae members Microbacterium and Leucobacter, which were strongly correlated to biomass settling velocity and bioreactor performance, as suggested by multivariate redundancy analyses. This experiment showed that aerobic granular sludge systems could be successfully started-up and operated, with high performance, under low operational temperatures when using cold-adapted biomass as inoculum.

Formation of aerobic granular sludge during the treatment of petrochemical wastewater

In this study, petrochemical wastewater from the port of Antwerp was used for the development of aerobic granular sludge. Two different reactor setups were used, (1) a completely aerated sequencing batch reactor (SBR_ae) with a feast/famine regime and (2) a sequencing batch reactor operated with an anaerobic feast/aerobic famine strategy (SBR_an). The seed sludge showed poor settling characteristics with a sludge volume index (SVI) of 285mL.gMLSS^-1 and a median particle size by volume of 86.0µm±1.9µm. In both reactors, granulation was reached after 30days with a SVI of 71mL.gMLSS^-1 and median granule size of 264.7µm in SBR_an and a SVI of 56mL.gMLSS^-1 and median granule size of 307.4µm in SBR_ae. The chemical oxygen demand (COD) and dissolved organic carbon (DOC) removal was similar in both reactors and above 95%. The anaerobic DOC uptake increased from 0.13% to 43.2% in 60days in SBR_an.

Long-term stability and nutrient removal efficiency of aerobic granules at low organic loads

The feasibility of application of aerobic granular sludge cultivated with high organic loads for biological nutrient removal (BNR) from low-strength wastewater was studied. Granules obtained with high-strength (COD=1400mg/L) wastewater were fed with medium (COD=700mg/L) and then low-strength (COD=400mg/L) wastewater. The granules rapidly acclimated to the medium-strength wastewater. However, feeding with low-strength wastewater reduced the F/M ratio from 0.4 to 0.2gCOD/gVSSd and granules disintegration occurred. Re-granulation was obtained after poor settling biomass was washed out and the F/M ratio reached 0.4gCOD/gVSSd. Disintegration of granules coincided with the decrease in extracellular polymeric substances (EPS) content and protein-to-carbohydrate ratio and re-granulation was assisted with the increase in EPS and protein-to-carbohydrate ratio. The results indicated that cultivation of aerobic granules with high organic loads and its implication for BNR treatment of low-strength wastewater while balancing the F/M ratio can be an alternative to reduce start-up period.

Filamentous bulking caused by Thiothrix species is efficiently controlled in full-scale wastewater treatment plants by implementing a sludge densification strategy

Filamentous bulking caused by Thiothrix species is responsible for sludge washout and loss of performance in dairy wastewater treatment plants. A long-term study was conducted over 1.5 years to test three different mitigation strategies in a full-scale plant composed of two parallel sequential batch reactors (SBR1 and 2). Strategies based on polyaluminium chloride addition and volatile fatty acids reduction were ineffective to permanently solve the problem. On the contrary, modification of the reactor cycle based on the implementation of a periodic starvation proved efficient to solve the biomass wash-out and drastically reduce the sludge volume index in both reactors. Bacterial diversity analysis using 16S amplicon sequencing and quantitative PCR indicated a reduction of Thiothrix abundance from 51.9 to 1.0% in SBR1 and from 71.8 to 0.6% in SBR2. Simultaneously, the abundance of the glycogen-accumulating bacterium Candidatus Competibacter increased in both reactors. Microscopy analysis confirmed the transition between a bulking sludge towards a granular-like sludge. This study confirms the applicability of a periodic starvation to (1) solve recurring Thiothrix bulking, (2) convert loose aggregates into dense and compact granular-like structures and (3) considerably reduce energy demand for aeration.

Granulation of activated sludge under low hydrodynamic shear and different wastewater characteristics

Five reactors were operated with low upflow superficial air velocities (0.41cmmin^-1) in order to observe granulation on synthetic wastewaters with different characteristics: 1) 340mg-CODL^-1; 2) 630mg-CODL^-1; and 3) 1300mg-CODL^-1. Stable granulation was only observed under low hydrodynamic shear for low-strength wastewater. 55-70% of soluble chemical oxygen demand (COD) was utilized before aeration and 91% COD, 62% total nitrogen (TN), and 96% total phosphorus (TP) were removed from the low-strength wastewater. Although medium-strength wastewater did generate granules they rapidly acquired a filamentous surface layer that resulted in decreased performance and loss of nitrification. 94% COD, 30% TN, and 85% TP were removed from the medium-strength wastewater. The high-strength wastewater did not develop granules and 85% COD was removed. Results demonstrated that high shear force was not required for granulation. Rather, granulation depended on multiple parameters to out-select rapidly growing aerobic microorganisms.

Influence of feeding pattern and hydraulic selection pressure to control filamentous bulking in biological treatment of dairy wastewaters

In sequencing batch reactors (SBRs) treating dairy wastewaters, the overgrowth of filamentous bacteria is a frequent cause of operational problems. The present study aimed at understanding to what extent the operating conditions of a SBR can be optimized to convert a bulking activated sludge into a well-settling biomass at low aeration velocity. The abundance of filament morphotypes and floc-formers able to store biopolymers were analysed by PCR-DGGE and 16S amplicon sequencing. The results indicated that a combination of an anaerobic-microaerated feeding pattern with a low selective pressure was beneficial to supress filamentous overgrowth and to form aerobic granules, while increasing the efficiency of suspended solid removal. Average removal efficiencies for total chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) were 94±2%, 95±1% and 83±13%, respectively.

The impact of zinc oxide nanoparticles on phosphorus removal and the microbial community in activated sludge in an SBR

10 and 20 mg L⁻¹ ZnO NPs damaged the integrity of cell membranes and the microbial community and affected phosphorus removal.