Stability of aerobic granules during long-term bioreactor operation

Multidisciplinary characterization of nitrogen-removal granular sludge: A review of advances and technologies

Nitrogen-removal granular sludge (NRGS) is a promising technology in wastewater treatment, with advantages of efficient nitrogen removal, less footprint, lower sludge production and energy consumption, and is a way for wastewater treatment plants to achieve carbon-neutrality. Aerobic granular sludge (AGS) and anammox granular sludge (AnGS) are two typical NRGS technologies that have attracted extensive attention. Mounting evidence has shown strong associations between NRGS properties and the status of NRGS systems; however, a holistic view is still missing. The aim of this article is to provide an overview of NRGS with an emphasis on characterization. Specifically, the integrated nitrogen transformation pathways inside NRGS and the performance of NRGS treating various wastewaters are discussed. NRGS properties are categorized as physical-, chemical-, biological- and systematical ones, presenting current advances and corresponding characterization technologies. Finally, the future prospects for furthering the mechanistic understanding and engineering application of NRGS are proposed. Overall, the technological advancements in characterization have greatly contributed to understanding NRGS properties, which are potential factors for optimizing the performance and evaluating the working status of NRGS. This review will provide guidance in characterizing NRGS properties and boost the introduction of novel characterization technologies.

Granulation strategies applied to industrial wastewater treatment: from lab to full-scale

About one third of the industrial activated sludge (AS) plants worldwide suffer from bad settling sludge, often caused by filamentous bulking phenomena. The present study investigated the effectiveness of a sludge granulation/densification strategy, based only on a metabolic selection mechanism, to eliminate sludge bulking in a sequencing batch reactor (SBR) treating real industrial wastewater. The wastewater originated from a tank truck cleaning company transporting chocolate and beer. The proposed strategy involved the introduction of a slow unaerated/anaerobic feeding step in the SBR operation. On lab-scale, the new feeding strategy resulted in (1) excellent settling with a sludge volume index (SVI) decreasing from more than 300 mL·g^-1 to 100 mL·g^-1 and lower, (2) the elimination of sludge bulking genera and (3) the significant enrichment of glycogen-accumulating organisms (GAO), mainly Defluviicoccus and Candidatus Competibacter, and this in less than 80 days. The feeding strategy was then applied to the full-scale installation, yielding similar results: a stable average SVI of 37 mL·g^-1 was reached after approximately 150 days. Full granulation was however not reached, which warrants further optimization. The present study shows that the proposed strategy can easily be applied in existing SBR systems to solve the problem of sludge bulking.

Enhanced aerobic granulation for treating low-strength wastewater in an anaerobic-aerobic-anoxic sequencing batch reactor by selecting slow-growing organisms and adding carriers

The aerobic granular sludge (AGS) process is a promising technology for wastewater treatment. However, a long start-up period for granulation and instability during long-term operation still hinder the application of AGS technology, especially for low-strength wastewater. To solve these two problems, this study tested a novel strategy involving the selection of slow-growing organisms and the addition of carriers in an anaerobic-aerobic-anoxic sequencing batch reactor (AN/O/AX_SBR). Three identical AN/O/AX_SBRs (R_Ctrl, R_CCM, and R_GAC), fed with low-strength wastewater, were operated for 120 days. R_Ctrl had no carriers, R_CCM contained cell culture microcarriers (CCM), and R_GAC contained granular activated carbon (GAC). Mature AGS was achieved within 80 days in all reactors. The carriers could reduce the maturation period of AGS by approximately 10 days (76, 66, and 69 days in R_Ctrl, R_CCM, and R_GAC, respectively) and improve the physical strength of the AGS. AGS showed a strong structure without excessive proliferation of filamentous bacteria, full-grown size (900-1100 μm), and good settleability (SVI₅ was 15.4-19.4 mL/g). Microbiological analysis showed that AN/O/AX_SBRs can provide a metabolic selective pressure to select slow-growing organisms such as nitrifying bacteria (norank_f__NS9_marine_group, Ellin6067, and Nitrospira), glycogen and phosphorus accumulating organisms (GAOs: Candidatus_Competibacter and Defluviicoccus; PAOs: Candidatus_Accumulibacter and Flavobacterium). All reactors showed good performance for simultaneous nitrification, endogenous denitrification, and phosphorus removal. The removal efficiencies of total nitrogen and total phosphorous were above 70% and 80%, respectively. The cycle test showed intermediate PAO-GAO metabolism prevailed in the system, and endogenous denitrification was primarily carried out by denitrifying GAOs.

A comparison between exogenous carriers enhanced aerobic granulation under low organic loading in the aspect of sludge characteristics, extracellular polymeric substances and microbial communities

In this study, polymeric ferric sulfate (PFS), aluminum sulfate (AS) and diatomite were added to enhance the aerobic granulation under low organic loading rate (OLR) of 0.6 kg·COD/(m³·d), and their effects of aerobic granule formation, extracellular polymeric substances (EPS) secretion and microbial community were investigated. The results showed that adding carriers could facilitated the growth of aerobic granules and improve the sludge settleability and biomass retention. Nutrient removal efficiencies were also enhanced. Compared with diatomite, adding PFS and AS resulted in more significant increase in EPS production, especially for the extracellular proteins. For microbial community, the dominated bacteria (Zoogloea, 18.47-23.95%) in the mature granular consortia were similar. Moreover, the introduction of PFS and diatomite contributed to the enrichment of Paracoccus, which was responsible for denitrification. Adding carriers potentially activated the functional genes related to metabolism and genetic information processing, and PFS had the most significant effects.

Promoting the granulation process of aerobic sludge via a sustainable strategy of effluent reflux in view of AHLs-mediated quorum sensing

Aerobic granular sludge (AGS) has excellent performance in wastewater treatment. However, the formation and mechanism of AGS by effluent reflux are not fully understood in sequential batch reactors (SBRs). In this study, two reactors were constructed, among which R1 was the control group, and the R2 reactor refluxed one-fourth of the supernatant of the effluent to the influent water. In the reactor of R2, the granules had better COD and TN removal efficiencies and resistance to external shocks, and AGS produced more extracellular polymeric substances (EPS). Analysis of microbial community indicated that AHLs-mediated microbes, denitrifying microbes, and EPS producers were enriched. At the same time, the correlation between 3OC6-HSL, C8-HSL, C12-HSL and PN was 0.89*, 0.94** and 0.92* respectively, the possible mechanism of enhanced granulation was mainly the promotion of AHLs by effluent reflux. Therefore, the effluent reflux strategy could be an innovative and sustainable strategy that validates the function of AHLs-mediated QS to accelerate aerobic sludge granulation and maintain its structural stability.

Monitoring morphological changes from activated sludge to aerobic granular sludge under distinct organic loading rates and increasing minimal imposed sludge settling velocities through quantitative image analysis

Quantitative image analysis (QIA) was used for monitoring the morphology of activated sludge (AS) during a granulation process and, thus, to define and quantify, unequivocally, structural changes in microbial aggregates correlated with the sludge properties and granulation rates. Two sequencing batch reactors fed with acetate at organic loading rates of 1.1 ± 0.6 kg_COD m^-3 d^-1 (R1) and 2.0 ± 0.2 kg_COD m^-3 d^-1 (R2) and three minimal imposed sludge settling velocities (0.27 m h^-1, 0.53 m h^-1, and 5.3 m h^-1) induced distinct granulation processes and rates. QIA results evidenced the turning point from flocculation to granulation processes by revealing the differences in the aggregates' stratification patterns and quantifying the morphology of aggregates with equivalent diameter (Deq) of 200 μm ≤ Deq ≤ 650 μm. Multivariate statistical analysis of the QIA data allowed to distinguish the granulation status in both systems, by clustering the observations according to the sludge aggregation and granules maturation status, and successfully predicting the sludge volume index measured at 5 min (SVI₅) and 30 min (SVI₃₀). These results evidence the possibility of defining unequivocally the granulation rate and anticipating the sludge settling properties at early stages of the process using QIA data. Hence, QIA could be used to predict episodes of granules disruption and hindered settling ability in aerobic granulation sludge processes.

Stability and nutrients removal performance of a Phanerochaete chrysosporium-based aerobic granular sludge process by step-feeding and multi A/O conditions

A Phanerochaete chrysosporium-based aerobic granular sludge (PC-AGS) was developed by inoculating fungal mycelial pellets into a lab-scale aerobic granular sequencing batch reactor (AGSBR). A strategy using step-anaerobic feeding coupled with multi A/O conditions was adopted. The results showed that the removal efficiencies for total phosphorus (TP) and total inorganic nitrogen (TIN) were 94.56 ± 2.92% and 75.20 ± 7.74%, respectively, under relatively low aeration time. Compared with original AGS, the content of extracellular proteins for PC-AGS obviously increased from 18.61 to 41.44 mg/g MLSS by the end of phase I. Moreover, the mature granules had a larger size and better stability during the 100 days operation. Furthermore, the analysis of microbial diversity detected many key functional groups in PC-AGS granules that were beneficial to nutrients removal. This work demonstrated that the addition of fungal pellets not only enhanced the removal performance, but also improved the stability of the AGS system.

Dependence of poly-β-hydroxybutyrate accumulation in sludge on biomass concentration in SBRs

The combination of wastewater treatment with polyhydroxyalkanoate production has attracted increasing interest in the context of the circular economy. Recent studies have thus attempted to optimize the conditions for polyhydroxyalkanoate accumulation in sludge when treating wastewater. The effects of biomass concentration and sludge morphologies in reactors on PHB storage, however, were neglected in the literature. Therefore, in this study settling time and organic loading rate were manipulated to adjust sludge morphology and biomass concentration in sequential batch reactors (SBRs) to investigate their influence on PHB storage in the feast phase. Our study shows that reducing settling times in SBRs from 10 to 0 min under organic loading rate of 3 g L^-1 d^-1 resulted in the decrease in biomass concentration at steady states from 4.2 to 1.0 g L^-1 and the change of sludge morphology from well-settled granules to poorly settled pinpoint flocs, but PHB content in sludge at the end of feast phase increased from 7.7 to 26.7%. The well-fitted regression lines between PHB content, SRT, feast/famine and food/microorganisms ratios and biomass concentration under different settling times suggest that PHB was highly dependent on biomass concentration but independent on sludge morphology. Under settling time of 0 min, the increase in OLR from 3 to 7.5 g L^-1 d^-1 resulted in an increased biomass concentration from 1.0 to 2.1 g L^-1 and an increase in PHB content from 26.7 to 33.8%. The batch and fed-batch experiments with different biomass concentrations also showed the influence of biomass concentration on PHB accumulation in sludge. The conclusion of the dependence of PHB content on biomass concentration under a fixed OLR and varied OLRs drawn from this study enables sludge PHB content as high as possible by adjusting biomass concentration in SBRs apart from the selective enriching strategies for PHB accumulating organisms when treating VFA-rich wastewater.

Features of aerobic granular sludge formation treating fluctuating industrial saline wastewater at pilot scale

A pilot-scale sequencing batch reactor, with a working volume of 3 m³, was installed in a fish cannery to develop aerobic granular sludge treating the produced effluents. Depending on the nitrogen (N) and organic matter (COD) concentration, the effluents were named in this study as medium-low-strength (Stage I) and high-strength (Stage II) wastewater. The composition of the wastewater was found to be a crucial factor to select granule-forming organisms. With medium-low-strength wastewater as feeding, the first granules were observed after 30 days, but the extremely high COD/N ratios of the wastewater provoked the overgrowth of filamentous bacteria after 4 months of operation (Stage I). When treating high-strength wastewater, stable aggregates with good settleability appeared, but well-shaped granules were not observed since the granulation process was not completed. The system was able to remove both COD (70-95%) and N (30-90%) treating both types of effluents. Biomass growth was the main N removal pathway. The reactor was found to be robust against factory production stops and, thus, a suitable alternative to treat wastewater from industries with discontinuous operation.

A new approach to evaluate and improve the stability of aerobic sludge systems based on maintenance coefficient

Stability is a key issue of wastewater treatment plants using either aerobic granular (AGS) or conventional activated sludge (CAS). The two forms of aerobic sludge were cultivated under different conditions to study the main factors affecting their stability. It was found that maintenance coefficient (m) describing the fraction of non-growth energy of granules increased significantly when the system became more stable during processes with the enhancement of granulation and the periodic short-term shock load. The yield coefficient (Y_H) was the main factor affecting the m value, and the inhibition in Y_H value was able to promote the maintenance potential according to the kinetic equation. Therefore, strategies that promote the maintenance coefficient could be applied to improve the stability of sludge systems, including inhibiting the yield rate and taking periodic short-term shock. Evaluation of stability based on the maintenance coefficient is a promising tool for ensuring the stable operation of wastewater treatment processes.

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.

Study on Start-Up Membraneless Anaerobic Baffled Reactor Coupled with Microbial Fuel Cell for Dye Wastewater Treatment

In this study, the antitoxicity performance of the traditional anaerobic baffled reactor (ABR) and the newly constructed membraneless anaerobic baffled reactor coupled with microbial fuel cell (ABR-MFC) was compared for the treatment of simulated printing and dyeing wastewater under the same hydraulic residence time. The sludge performances of ABR-MFC and ABR were evaluated on the dye removal rate, extracellular polymer (EPS) content, sludge particle size, methane yield, and the surface morphology of granular sludge. It was found that the maximum power density of the ABR-MFC reactor reached 1226.43 mW/m³, indicating that the coupled system has a good power generation capacity. The concentration of the EPS in the ABR-MFC reactor was about 3 times that in the ABR, which could be the result of the larger average particle size of sludge in the ABR-MFC reactor than in the ABR. The dye removal rate of the ABR-MFC reactor (91.71%) was higher than that of the ABR (1.49%). The methane production and microbial species in the ABR-MFC system were higher than those in the ABR. Overall, the MFC embedded in the ABR can effectively increase the resistance of the reactor, promote the formation of granular sludge, and improve the performance of the reactor for wastewater treatment.

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

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

Potential role of nanobubbles in dynamically modulating the structure and stability of anammox granular sludge within biological nitrogen removal process

The generation of visible macrobubbles considerably affects the structure and function of anammox granules in the anammox granular sludge (AnGS) system. However, the existence of nanobubbles (NBs) and their role in maintaining the AnGS structure and stability are unclear because of the complexity of the system and lack of effective analytical methods. In this study, methods for NB analysis and assessment of their effects were developed to investigate the formation and characteristics of NBs in an AnGS system and the effects of NBs on the properties and function of AnGS. The results indicated that dissolved gas supersaturation caused by AnGS generated NBs of 2.75 × 10⁸ bubbles/mL inside an AnGS reactor after running for 300 min at 30 °C. The increasing absolute value of the zeta potential of NBs with time indicated that the NBs in the AnGS system were gradually stable. The size of the stable NBs ranged from 150 nm to 400 nm. NB formation also increased the space and pressure between cells, leading to the breakage of the cell cluster and causing structural changes in granules. Changes in the local granular microstructure caused by NBs were favorable for the porous structure of granules to avoid granular disintegration and flotation caused by the excessive secretion of extracellular polymeric substances blocking gas channels. The formation and stability of NBs penetrating the cell clusters played a crucial role in the formation and stability of nanopores around or inside the cell clusters, further providing a basis for the formation of high-porosity structures and efficient mass transfer of AnGS.

Recent advancements in the biological treatment of high strength ammonia wastewater

The estimated global population growth of 81 million people per year, combined with increased rates of urbanization and associated industrial processes, result in volumes of high strength ammonia wastewater that cannot be treated in a cost-effective or sustainable manner using the floc-based conventional activated sludge approach of nitrification and denitrification. Biofilm and aerobic granular sludge technologies have shown promise to significantly improve the performance of biological nitrogen removal systems treating high strength wastewater. This is partly due to enhanced biomass retention and their ability to sustain diverse microbial populations with juxtaposing growth requirements. Recent research has also demonstrated the value of hybrid systems with heterogeneous bioaggregates to mitigate biofilm and granule instability during long-term operation. In the context of high strength ammonia wastewater treatment, conventional nitrification-denitrification is hampered by high energy costs and greenhouse gas emissions. Anammox-based processes such as partial nitritation-anammox and partial denitrification-anammox represent more cost-effective and sustainable methods of removing reactive nitrogen from wastewater. There is also growing interest in the use of photosynthetic bacteria for ammonia recovery from high strength waste streams, such that nitrogen can be captured and concentrated in its reactive form and recycled into high value products. The purpose of this review is to explore recent advancements and emerging approaches related to high strength ammonia wastewater treatment.

Integrated review of resource recovery on aerobic granular sludge systems: Possibilities and challenges for the application of the biorefinery concept

Aerobic Granular Sludge (AGS) is a biological treatment technology that has been extensively studied in the last decade. The possibility of resource recovery has always been highlighted in these systems, but real-scale applications are still scarce. Therefore, this paper aimed to present a systematic review of resources recovery such as water, energy, chemicals, raw materials, and nutrients from AGS systems, also analyzing aspects of engineering and economic viability. In the solid phase, sludge application in agriculture is an interesting possibility. However, the biosolids' metal concentration (the granules have high adsorption capacity due to the high concentration of extracellular polymeric substances, EPS) may be an issue. Another possibility is the recovery of Polyhydroxyalkanoates (PHAs) and Alginate-like exopolymers (bio-ALE) in the solid phase, emphasizing the last one, which has already been made in some Wastewater Treatment Plants (WWTPs), named and patented as Kaumera® process. The Operational Expenditure (OPEX) can be reduced by 50% in the WWTP when recovery of ALE is made. The ALE recovery reduced sludge yield by up to 35%, less CO₂ emissions, and energy saving. Finally, the discharged sludge can also be evaluated to be used for energetic purposes via anaerobic digestion (AD) or combustion. However, the AD route has faced difficulties due to the low biodegradability of aerobic granules.

Enhancing stability of aerobic granules by microbial selection pressure using height-adjustable influent strategy

Optimizing granules size distribution is critical for both reactor performance and stability. In this research, an optimal size range of 1800 to 3000 μm was proposed regarding mass transfer and granules stability based on granules developed at DO around 8.0 mg L^-1 with the feed COD:N:P at 100:5:1. A height-adjustable influent strategy was applied to facilitate the nutrient storage of granules at optimum size range via microbial selective pressure. Results suggested insufficient hydraulic shear stress led to overgrowth of granules size. High abundance of filamentous bacteria (Thiothrix sp.) was observed in oversized granules, which detached and affected the remaining granules, resulting in severe sludge bulking. Strong hydraulic shear stress suppressed uncontrolled growth of granules. However, fewer abundance of simultaneous nitrification and denitrification (SND) bacterium was acquired, which led to unfavored SND effect and total nitrogen (TN) removal efficiency. The height-adjustable influent strategy facilitated the poly-β-hydroxybutyrate (PHB) storage of granules at optimum size range, while limiting the overgrowth of granules size. Additionally, more than 87.51% of total granules situated in optimal sizes range, which led to higher abundance of SND bacterium and higher TN removal efficiency.

Fluctuation and Re-Establishment of Aerobic Granules Properties during the Long-Term Operation Period with Low-Strength and Low C/N Ratio Wastewater

Long-term structure stability of aerobic granules is critical to maintaining stable wastewater treatment performance. In this study, granulation and long-term stability of sludge-treating synthetic wastewater with a low chemical oxygen demand to nitrogen (COD/N) ratio of 4:1 and COD concentration of 400 mg/L in anoxic-oxic conditions were investigated for over 300 days. Inoculated suspended sludge gradually transformed into granules-dominant sludge on day 80. Due to the improved sludge volume index after 30 min settling (SVI30), mixed liquor suspended solids (MLSS) reached 5.2 g/L on day 140. Without any external intervention or disturbance, aerobic granules started to disintegrate from day 140, causing the increase in SVI and the decrease in biomass concentration until day 210, with the average sludge size reduced to 243 µm. From day 210, granular sludge started to be re-established by re-granulation, and the average granule size increased to 500 µm on day 302. During these disintegration and re-granulation periods, there was no obvious difference in terms of COD removal and nitrification, but microbial species were found more diverse after the re-granulation, with Thauera and Sphingomonas dominant. Although there was no external intervention, the food to microorganisms ratio (F/M) varied significantly due to the changes in biomass concentration caused by strong selective pressure and the change of sludge-settling ability in the reactor. F/M ratios should be controlled between 0.3 and 1.0 gCOD/gSS·d to maintain the stable structure of granules to minimize the fluctuation of sludge properties under the conditions used in this study. Although aerobic granular sludge is able to re-establish itself after disintegration, controlling F/M ratios in a certain range would benefit long-term stability. The findings in this study are significant to deepen the understanding of granule stability with low-strength and low COD ratio wastewater and, thus, provide guidance for maintaining the long-term stability of granules.

Support Vector Regression Modelling of an Aerobic Granular Sludge in Sequential Batch Reactor

Support vector regression (SVR) models have been designed to predict the concentration of chemical oxygen demand in sequential batch reactors under high temperatures. The complex internal interaction between the sludge characteristics and their influent were used to develop the models. The prediction becomes harder when dealing with a limited dataset due to the limitation of the experimental works. A radial basis function algorithm with selected kernel parameters of cost and gamma was used to developed SVR models. The kernel parameters were selected by using a grid search method and were further optimized by using particle swarm optimization and genetic algorithm. The SVR models were then compared with an artificial neural network. The prediction results R² were within >90% for all predicted concentration of COD. The results showed the potential of SVR for simulating the complex aerobic granulation process and providing an excellent tool to help predict the behaviour in aerobic granular reactors of wastewater treatment.

Reactivation of Frozen Stored Microalgal-Bacterial Granular Sludge under Aeration and Non-Aeration Conditions

In this paper, reactivation of microalgal-bacterial granular sludge (MBGS) stored at −20 °C for 6 months was investigated under respective aeration (R1) and non-aeration (R2) conditions. Results showed that the granular activity could be fully recovered within 21 days. The average removal efficiency of ammonia was higher in R1 (92.78%), while R2 showed higher average removal efficiencies of organics (84.97%) and phosphorus (85.28%). It was also found that eukaryotic microalgae growth was stimulated under aeration conditions, whereas prokaryotic microalgae growth and extracellular protein secretion were favored under non-aeration conditions. Sequencing results showed that the microbial community underwent subversive evolution, with Chlorophyta and Proteobacteria being dominant species under both conditions. Consequently, it was reasonable to conclude that the activity and structure of frozen stored MBGS could be recovered under both aeration and non-aeration conditions, of which aeration-free activation was more feasible on account of its energy-saving property. This study provides important information for the storage and transportation of MBGS in wastewater treatment.

Responses of aerobic granular sludge to fluoroquinolones: Microbial community variations, and antibiotic resistance genes

In this study, aerobic granular sludge (AGS) was operated under high levels of ammonium for removing three fluoroquinolones (FQs), i.e., ciprofloxacin (CFX), ofloxacin (OFX), and norfloxacin (NFX) at 3, 300, and 900 µg/L, respectively. Two key objectives were to investigate the differential distribution of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) in sludge fractions and to evaluate correlations between ARGs and MGEs to nitrifying and denitrifying bacteria. AGS showed excellent stability under the exposure of FQs, with nitrite-oxidizing bacteria (NOB) more sensitive to FQs than ammonium-oxidizing bacteria (AOB). Specific oxygen utilization rates (SOUR) showed a reduction of 26.9% for NOB but only 4.0% of the reduced activity of AOB by 3 μg/L FQs. AGS performed better removal efficiencies for CFX and NFX than OFX, and the efficiencies increased with their elevated concentrations, except at 900 μg/L FQs. The elevated FQ concentrations led to a significant enrichment of intI1 and genus Thauera, while qnrD and qnrS showed no accumulation. Compared to nitrifiers, FQs relevant ARGs and the intI1 gene preferred to exist in denitrifiers, and the abundance of denitrifiers behaved a decreasing trend with the sludge size. Two quinoline-degrading bacteria were found in the AGS system, i.e., Alicycliphilus and Brevundimonas, possibly carrying qnrS and qnrD, respectively. Their relative abundance increased with the sludge size, which was 2.18% in sludge <0.5 mm and increased to 3.70% in sludge >2.0 mm, suggesting that the AGS may be a good choice in treating FQs-containing wastewater.

Impact of fungal pellets dosage on long-term stability of aerobic granular sludge

The effects of fungal pellets (FPs) dosage on both structural and functional stability of aerobic granular sludge (AGS) were investigated during 200-day operation. Results showed that the AGS induced by low (a dry mass ratio of FPs to seed sludge, 30%) and high FPs dosage (60%) exhibited good morphology integrity during the entire phase of operation, while the filamentous overgrowth and AGS breakup were observed in the control reactor (0% FPs). Moreover, the granules developed at high FPs dosage demonstrated excellent nutrients removal (COD: 93%; NH4⁺-N: 100%; TN: 77%) and stable bioactivity with a maximum specific oxygen uptake rate (SOUR) of 52.6 ± 2.6 mg O₂/(gVSS·h), a value being 12.2% and 26.7% higher than that of 30% and 0% dosage. The microbial community analysis revealed 60% FPs dosage enriched various functional bacteria involved in nutrients removal. This study suggests a sustainable strategy for improving structural and functional stability of AGS.

The Use of Hydromulching as an Alternative to Plastic Films in an Artichoke (Cynara cardunculus cv. Symphony) Crop: A Study of the Economic Viability

The use of mulching in agriculture suppresses the weeds around crop plants, enhances the nutrients status of soil, controls the soil structure and temperature, and reduces soil water evaporation. Excessive use of low-density polyethylene mulches is contributing to the accumulation of high amounts of plastic wastes, an environmental problem for agricultural ecosystems. Fragments of plastic from such wastes can be found in soils, in water resources, and in organisms, including humans. The objective of this work was to study the economic viability of the use of different hydromulches in an artichoke crop. Three blends were prepared by mixing paper pulp (recycled from used paper) and cardboard (from paper mills) with different additives: wheat straw (WS), rice hulls (RH), and substrate used for mushroom cultivation (MS). These were compared with low-density polyethylene (Pe), a treatment without mulching on bare soil where hand weeding was performed (HW), and a treatment without mulching on bare soil where herbicide was applied (H). The results indicate that the use of hydromulch in an artichoke crop represents a good alternative for reducing plastic waste in agriculture. The net profits of the hydromulch treatments (MS, WS, RH) were higher than for HW and H, and slightly lower than for Pe. The most profitable treatment was Pe (€0.69 m−3), followed by RH (€0.59 m−3), WS (€0.58 m−3), MS (€0.47 m−3), HW (€0.36 m−3), and H (€0.32 m−3). A sensitivity analysis showed a probability of negative results of 0.04 in Pe, 0.13 in SM, 0.08 in WS, and 0.07 in RH, so the probability that the grower will make a profit is greater than 0.9 with the use of mulch (except mushroom substrate) or polyethylene.

Rapid formation of granules coupling n-DAMO and anammox microorganisms to remove nitrogen

Granular sludge exhibits unique features, including rapid settling velocity, high loading rate and relative insensitivity against inhibitors, thus being a favorable platform for the cultivation of slow-growing and vulnerable microorganisms, such as anaerobic ammonium oxidation (anammox) bacteria and nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) microorganisms. While anammox granules have been widely applied, little is known about how to speed up the granulation process of n-DAMO microorganisms, which grow even slower than anammox bacteria. In this study, we used mature anammox granules as biotic carriers to embed n-DAMO microorganisms, which obtained combined anammox + n-DAMO granules within 6 months. The results of whole-granule 16S rRNA gene amplicon sequencing showed the coexistence of anammox bacteria, n-DAMO bacteria and n-DAMO archaea. The microbial stratification along granule radius was further elucidated by cryosection-16S rRNA gene amplicon sequencing, showing the dominance of n-DAMO archaea and anammox bacteria at inner and outer layers, respectively. Moreover, the images of cryosection-fluorescence in situ hybridization (FISH) verified this stratification and also indicated a shift in microbial stratification. Specifically, n-DAMO bacteria and n-DAMO archaea attached to the anammox granule surface initially, which moved to the inner layer after 4-months operation. On the basis of combined anammox + n-DAMO granules, a practically useful nitrogen removal rate (1.0 kg N/m³/d) was obtained from sidestream wastewater, which provides new avenue to remove nitrogen from wastewater using methane as carbon source.

Effect of voltage intensity on the nutrient removal performance and microbial community in the iron electrolysis-integrated aerobic granular sludge system

The effects of voltage intensity on the nutrient removal performance and microbial community in the iron electrolysis-integrated aerobic granular sludge (AGS) system were investigated over a period of 15 weeks. Results revealed that the application outcomes of iron electrolysis for AGS systems relied on voltage intensity. When a constant voltage of 1.5 V was applied, the sludge granulation was most obviously accelerated with a specific growth rate of the sludge diameter of 0.078 day^-1, and the removal efficiencies of total nitrogen (TN) and total phosphorus (TP) increased by 14.1% and 20.2%, respectively, compared to the control reactor (without the iron electrolysis-integration). Moreover, the AGS developed at different voltages included different microbial communities, whose shifts were driven by the Fe content and the average diameter of AGS. Both heterotrophic nitrifiers and mixotrophic denitrifiers were significantly enriched in the AGS developed at 1.5 V, which effectively enhanced TN removal. Together with the response of the functional genes involved in Fe, N, and P metabolism, the electrolytic iron-driven nutrient degradation pathway was further elaborated. Overall, this study clarified the optimum voltage condition when iron electrolysis was integrated into the AGS system, and revealed the enhancement mechanism of this coupling technology on nutrient removal during the treatment of low-strength municipal wastewater.

A Bioreactor Designed for Restricting Oversize of Aerobic Granular Sludge

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

Response and recovery of mature algal-bacterial aerobic granular sludge to sudden salinity disturbance in influent wastewater: Granule characteristics and nutrients removal/accumulation

The impact of sudden salinity (1-3%) disturbance in influent wastewater on mature algal-bacterial aerobic granular sludge (AGS) was investigated, in addition to its recovery possibility when salinity disturbance was removed. Results show that the mature algal-bacterial AGS with less filamentous could maintain its good settleability with sludge volume index below 41 mL/g when wastewater salinity was increased to 3%, in which loosely bound extracellular polymeric substances might play an important role. Under this condition, the granule system achieved slightly lower dissolved organic carbon removal (from 97% to 94%), while the removals of ammonia nitrogen, total nitrogen and total phosphorus were remarkably decreased from ~100%, 66% and 70% to 23%, 16% and 38%, respectively. However, the organics and nutrients removals could be recovered immediately when the salinity disturbance was removed from the influent. P bioavailability, on the other hand, kept almost stable (93-97%) in the AGS during the examination period.

Development of an ensemble of machine learning algorithms to model aerobic granular sludge reactors

Machine learning models provide an adaptive tool to predict the performance of treatment reactors under varying operational and influent conditions. Aerobic granular sludge (AGS) is still an emerging technology and does not have a long history of full-scale application. There is, therefore, a scarcity of long-term data in this field, which impacted the development of data-driven models. In this study, a machine learning model was developed for simulating the AGS process using 475 days of data collected from three lab-based reactors. Inputs were selected based on RReliefF ranking after multicollinearity reduction. A five-stage model structure was adopted in which each parameter was predicted using separate models for the preceding parameters as inputs. An ensemble of artificial neural networks, support vector regression and adaptive neuro-fuzzy inference systems was used to improve the models' performance. The developed model was able to predict the MLSS, MLVSS, SVI₅, SVI₃₀, granule size, and effluent COD, NH₄-N, and PO₄^3- with average R², nRMSE and sMAPE of 95.7%, 0.032 and 3.7% respectively.

Uncover the secret of granule calcification and deactivation in up-flow anaerobic sludge bed (UASB) reactor with long-term exposure to high calcium

Calcification of anaerobic granular sludge causing the decrease of microbial activity has been focused for several decades, but the mechanism of calcification and deactivation of calcified granule are still lacking. In this study, the calcification process of anaerobic granular sludge was analyzed in the UASB reactor with long-term exposure to a high calcium level of 2 gCa²⁺L^-1. Greyish-white calcified granules with larger size and density first appeared at the bottom of sludge bed. Characterization of granular surface demonstrated that calcified granules had smaller specific surface area and pore volume with the disappearing channels in granules visualized over calcification. The coverage of calcite on granular surface with a thickness of 500-600 µm was observed by a microscope, which is responsible for the blockage of channels for substrate diffusion and transport. The evaluation of mass transfer showed that the 'effectiveness factor' η of calcified granules was higher than 1.23, proving the existence of mass transfer resistance. Furthermore, the interactions between calcium and extracellular polymeric substances (EPS) revealed the possible mechanism of calcite location at the outer layer of anaerobic granular sludge: The bulk calcite directly binds the EPS such as humic acids via charging functional groups, and bulk CO₃^2- could react easily with Ca²⁺ bound to EPS according to thermodynamic analysis.

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

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

Evolution of extracellular polymeric substances (EPS) in aerobic sludge granulation: Composition, adherence and viscoelastic properties

Aerobic granular sludge (AGS) is a promising wastewater treatment innovation, but its instability hinders its broader applications. Understanding the granulation process is vital to address this issue. Extracellular polymeric substances (EPS) play an essential role in sludge granulation. However, one crucial aspect of EPS, the adhesive and viscoelastic properties, has been neglected in AGS studies. In this study, we set up two reactors fed with COD/N ratios of 100: 5 (R1) and 100: 10 (R2) for comparison, to investigate the adhesive and viscoelastic properties of sludge EPS during the sludge granulation. We found that R2 showed a more rapid sludge granulation with more stable granules formed, contained a higher abundance of amoA gene, and had a higher production of polysaccharides than R1. We also found a sharp decrease in polysaccharide production and β-sheets abundance accompanied by granule size decrease in R1 on Day 80, indicating their essential roles in sludge granulation and granule stability. QCM-D (quartz crystal microbalance with dissipation monitoring) results showed that EPS became less adhesive and inclined to form unstable layers on the mineral surfaces along with the sludge granulation process. In contrast, they showed the opposite behavior and became more adhesive on the PVDF sensors. Our results suggested that higher polysaccharides, a higher β-sheets band in proteins, and lower mineral surface-adhesive and viscoelastic properties benefited the aerobic sludge granulation process and the granule maintenance.

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.

Impact of aerobic availability of readily biodegradable COD on morphological stability of aerobic granular sludge

Operational disturbances in aerobic granular sludge (AGS) systems can result in aerobic availability of readily biodegradable COD (rbCOD). Different from activated sludge, morphological consequences on the short and long term are not well described in literature. This study investigated the effect of incomplete anaerobic uptake of acetate on the morphological and process stability of AGS using a lab-scale reactor. A fraction of the total acetate load was dosed aerobically, which was increased stepwise while monitoring granular morphology. A good granular morphology and an SVI of 40 ml/g were obtained during initial enrichment and maintained for ≤20% aerobic acetate load dosed at 4 mg COD/g VSS/h. Biological phosphorus removal efficiency was initially unaffected, but the aerobic acetate dosage rate did decrease the aerobic phosphate uptake rate. This led to loss of phosphorus removal for >20% aerobic acetate load dosed at 8 mg COD/g VSS/h over the course of 12 days. Subsequently, significant outgrowth formed on the granular surfaces and developed over time into finger-like structures. Under these high aerobic acetate loads the SVI increased to 80 ml/g and resulted in significant biomass washout due to deteriorating settling properties of the sludge. The sludge settleability and biological phosphorus removal recovered 10 days after aerobic feeding of acetate was stopped. Aerobic presence of rbCOD can be tolerated if mostly anaerobic acetate uptake is maintained, thereby ensuring stable granular morphology and good settleability. The high enrichment of phosphate accumulating organisms in the granular sludge through bottom-feeding and selective wasting of flocs makes aerobic granular sludge resilient to morphological deterioration in aerobic presence of rbCOD.

Positive effects of zeolite powder on aerobic granulation: Nitrogen and phosphorus removal and insights into the interaction mechanisms

Aerobic granular sludge is considered one of the most promising biological wastewater treatment technologies of the 21st century. However, the long granulation time and poor treatment effect on N and P have severely limited its popularity and large-scale application. In this study, we systematically examine the strengthening effects of zeolite powder on granulation, N and P removal, and their interaction mechanisms. The addition of zeolite powder decreased sludge granulation time to 18 d, and improved average N and P removal rates by 4.48% and 2.22%, respectively. The multi-pore and nutrient-rich environment of the zeolite powder is beneficial for maintaining microbial activity and granular stability. Moreover, its adsorption to N and P enriches their respective removal strains, improving their removal efficiency.

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

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

Intertidal wetland sediment as a novel inoculation source for developing aerobic granular sludge in membrane bioreactor treating high-salinity antibiotic manufacturing wastewater

This study proposed a novel approach of cultivating aerobic granular sludge (AGS) using intertidal wetland sediment (IWS) as inoculant in MBR for saline wastewater treatment. Granulation was observed in IWS-MBR during start-up, with increased sludge particle size (3.1-3.3 mm) and improved settling property (23.8 ml/g). The abundant inorganic particulates (acted as nuclei) and distinctive microbial community in IWS contributed to the granules formation. With the help of AGS, IWS-MBR system exhibited excellent TOC reduction of 90.3 ± 6.1% and significant TN reduction of 31.2 ± 5.0%, while the control MBR (Co-MBR) only showed 58.9 ± 7.2% and 10.4 ± 2.7%, respectively. Meanwhile, membrane fouling was mitigated in IWS-MBR, with a longer filtration cycle of 21.5 d, as compared with that of 8.9 d for Co-MBR. Microbial community analysis revealed that abundant functional bacteria associated with granulation and pollutants removal were enriched from IWS and set the basis for AGS formation and the superior treatment performance.

Evaluation of the production of alginate-like exopolysaccharides (ALE) and tryptophan in aerobic granular sludge systems

The engineering and microbiological aspects involved in the production of alginate-like exopolysaccharides (ALE) and tryptophan (TRY) in aerobic granular sludge systems were evaluated. The inclusion of short anoxic phase (A/O/A cycle-anaerobic, oxic, and anoxic phase) and the control of sludge retention time (SRT ≈ 10 days) proved to be an important strategy to increase the content of these bioproducts in granules. The substrate concentration also has a relevant impact on the production of ALE and TRY. The results of the microbiological analysis showed that slow-growing heterotrophic microbial groups (i.e., PAOs and GAOs) might be associated with the production of ALE, and the EPS-producing fermentative bacteria might be associated with the TRY production. The preliminary economic evaluation indicated the potential of ALE recovery in AGS systems in decreasing the OPEX (operational expenditure) of the treatment, especially for larger sewage treatment plants or industrial wastewaters with a high organic load.

The effect of Np-magnetite on the granulation process of an SBR reactor used for domestic wastewater treatment

This study investigated the effect of magnetite nanoparticles (Np-magnetite) added to a pilot-scale sequencing batch reactor (SBR) treating domestic wastewater, to improve aerobic granular sludge (AGS) formation and the effects of granule disintegration. Np-magnetite additions (75 mg L^-1) were made during the start-up of the reactor and repeated after 100 and 170 days, when granule disintegration was observed. From the first Np-magnetite addition, SVI₅ was reduced from 1315 to 85 mL g^-1. The granular biomass was observed on the 56th day, when 57% of the granules presented diameters bigger than 212 µm. The 100-day disintegration episode disturbed the granular biomass, reducing the volatile suspended solids by 51%, increasing the SVI values to above 200 mL g^-1. Np-magnetite addition recovered all the granular biomass parameters to the values observed before disintegration. The treatment efficiency was stable during operation of the reactor for nutrients (52.8 ± 23.4% NH₄⁺-N; 54.5 ± 12.2% PO₄^3--P) and carbonaceous organic matter (71.7 ± 12.7% BOD₅; 77.5 ± 10.0% COD_t). Np-magnetite addition changed the microbial community of the granular sludge, analysed via high-throughput 16S RNA sequencing, and recovered the treatment efficiency previously disturbed by the disintegration processes. These results indicate the potential of Np-magnetite as an agent for sludge aggregation in an aerobic granular reactor.

Response of an aerobic granular and conventional flocculated reactors against changing feed composition from simple composition to more complex

This research evaluated the effect of changing feed composition on the performances of a conventional activated sludge (CAS) and an aerobic granular sludge (AGS) reactor operated simultaneously. Both reactors were initially fed with 100% synthetic feed. In a stepwise manner, the feed composition was slowly changed to real primary effluent collected from a local wastewater treatment plant. After an initial stabilization period, both reactors could achieve more than 90% NH₄⁺-N removal. However, PO₄^3--P removal eventually reached to a maximum of 92% in the AGS and 88% in the CAS. COD removal in both reactors was least affected, with the lowest percent removal of 81 ± 3% achieved in AGS and 62 ± 4% in CAS respectively when fed with 100% real wastewater. Despite granule breakage the AGS reactor was able to remove the pollutants (COD, N, P). The abundance of Candidatus Accumulibacter, a polyphosphate accumulating organism, in the AGS system increased over the operational phases: II (6.2%), III (10.32%), and IV (11.9%). While in CAS, it increased from phase I to phase II (12.6%), but decreased in phase III to 9.9%. Genus-based classification revealed a successive increase in the relative abundance of Nitrospira to 11.05% during Phase III and 10.3% during Phase IV in the AGS. In contrast with its presence in the CAS, which was, 3.4% during Phase III and 9.5% during Phase IV.

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

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

Aggregate size-dependence of bacterial community separation in a full-scale membrane bioreactor plant

The size of bacterial aggregates can determine both nutrient removal and sludge/water separation in activated sludge processes. In this study, the bacterial community structures and network associations of different sized aggregates obtained from a full-scale membrane bioreactor plant over a one-year period were investigated. Our results showed that biodiversity of larger sized aggregates was significantly higher than that of smaller ones and that the bacterial compositions of different sized aggregates differed significantly from each other. Bacteria related to nutrient removal (e.g. denitrification, hydrolysis and fermentation) were found to be significantly more abundant in larger aggregates than smaller ones. Network analysis revealed significant difference in species-species interactions, topological roles of individual OTU and keystone populations among different sized aggregates. Moreover, the occurrence of keystone OTUs affiliated with denitrifiers (Thermomonas) in networks of large and medium aggregates may suggest that denitrification influences bacterial interactions in large and medium aggregates. Overall, our results indicate the aggregates size-dependence of bacterial community separation in activated sludge. The current findings not only can provide guidance for process design and operation optimization, but also highlight the necessity for paying more attentions to the aggregate-based community, metabolic function and gene expression of activated sludge in future studies.

The effect of supporting matrix on sludge granulation under low hydraulic shear force: Performance, microbial community dynamics and microorganisms migration

Granular sludge usually takes extracellular polymers (EPS) as matrices for colonizing microorganisms and maintaining structural stability. However, the low strength of EPS threatens the disintegration of granules, especially under low hydraulic shear force. To accelerate the formation and enhance the stability of granules, micro-sized melamine (ME) sponges (RA) and polyurethane (PU) sponges (RB) were screened out as matrix substitutes for developing aerobic granular biofilm (AGB) in this study. The superficial gas velocity was 0.8 cm s^-1. Both reactors achieved over 95% ammonium nitrogen removal efficiency within 10 days. During stabilization period, the chemical oxygen demand, total nitrogen and total phosphorus removal efficiencies were 90.5%, 70% and 95% in RA and 87.8%, 83% and 88% in RB, respectively. Confocal laser scanning microscopy (CLSM) detection revealed that β-polysaccharide was more concentrated in the outer layer in PU-AGB but uniformly dispersed in ME-AGB. The denitrifying phosphorus accumulating organisms (Flavobacterium) was dominant in RA, while the denitrifying glycogen accumulating organisms (Candidatus_Competibacter) was dominant in RB. Fluorescence in situ hybridization (FISH) analysis indicated that the microbial distribution in ME-AGB was relatively uniform, while there was a significant migration of functional microorganisms in PU-AGB. The super-hydrophilicity of ME and the high hydrophobicity of PU may be the main reasons for these differences. Overall, this study indicated that ME sponge is a more suitable material for supporting AGB than PU sponge.