Stability of aerobic granules during long-term bioreactor operation

Deciphering the influence of salinity stress on the biological aniline degradation system: Pollutants degradation performance and microbial response

In order to evaluate the impact of salinity gradients on the aniline biodegradation system, six reactors at salinity concentrations (0%-5%) were established. The results presented the salinity except for 5% imposed negligible effects on aniline degradation performance. Nitrification had prominent resistance to salinity (0%-1.5%) while were significantly restrained when salinity increased. The total nitrogen (TN) removal efficiency of Z4 (1.5%) was 20.5% higher than Z1 (0%) during the stable operation phase. Moreover, high throughput sequencing analysis showed that halophilic bacterium, such as Halomonas, Rhodococcus, remained greater survival advantages in high salinity system. The substantial enrichment of Flavobacterium, Dokdonella, Paracoccus observed in Z4 ensured its excellent nitrogen removal performance. The close cooperation among dominant functional bacteria was strengthened when salt content was below 1.5% while exceeding 1.5% led to the collapse of metabolic capacity through integrating the toxicity of aniline and high osmotic pressure.

Reduction of greenhouse gas emissions from closed activated sludge- to aerobic granular sludge-based biosystems via gas circulation

Greenhouse gas (GHG) emissions from biological treatment units are challenging wastewater treatment plants (WWTPs) due to their wide applications and global warming. This study aimed to reduce GHG emissions (especially N2O) using a gas circulation strategy in a closed sequencing-batch reactor when the biological unit varies from activated sludge (AS) to aerobic granular sludge (AGS). Results show that gas circulation lowers pH to 6.3 ± 0.2, facilitating regular granules but elevating total N2O production. From AS to AGS, N2O emission factor increased (0.07-0.86 %) due to decreasing ammonia-oxidizing rates while the emissions of CO2 (0.3 ± 0.1 kg-CO2/kg-chemical oxygen demand) and CH4 remained in the closed biosystem. The gas circulation decreased N2O emission factor by 63 ± 15 % after granulation higher than 44 ± 34 % before granulation, which is implemented by heterotrophic denitrification. This study provides a feasible strategy to enhance heterotrophic N2O elimination in the biological WWTPs.

Effects of enhanced biological phosphorus removal on rapid control of sludge bulking and fast formation of aerobic granular sludge

This study investigated the effects of enhanced biological phosphorus removal (EBPR) on rapid sludge bulking control and fast aerobic granular sludge (AGS) formation by adding 20 % of EBPR activated sludge to the bulking activated sludge (BAS) reactor. The results indicate that activating EBPR activity swiftly improved BAS settleability within 16 days, thus resolving sludge bulking issues. Subsequently, a settling time-based selection was employed, resulting in the BAS granulation within another 16 days. The rapid achievement of EBPR activity improved the BAS settleability and facilitated the formation of sludge aggregates, thereby expediting BAS granulation. Inhibition of filamentous bacteria and enrichment of slow-growing organisms contributed to both sludge bulking control and aerobic granulation. Furthermore, the increase in proteins/polysaccharides ratio facilitated the granulation process. Additionally, total nitrogen removal increased from 59.4 % to 71.7 % because of the mature AGS formation. This study provided an approach to simultaneously control sludge bulking and promote aerobic granulation.

Impact of food waste addition in energy efficient municipal wastewater treatment by aerobic granular sludge process

Recently, one of the main purposes of wastewater treatment plants is to achieve a neutral or positive energy balance while meeting the discharge criteria. Aerobic granular sludge (AGS) technology is a promising technology that has low energy and footprint requirements as well as high treatment performance. The effect of co-treatment of municipal wastewater and food waste (FW) on the treatment performance, granule morphology, and settling behavior of the granules was investigated in the study. A biochemical methane potential (BMP) test was also performed to assess the methane potential of mono- and co-digestion of the excess sludge from the AGS process. The addition of FW into wastewater enhanced the nutrient treatment efficiency in the AGS process. BMP of the excess sludge from the AGS process fed with the mixture of wastewater and FW (195 ± 17 mL CH4/g VS) was slightly higher than BMP of excess sludge from the AGS process fed with solely wastewater (173 ± 16 mL CH4/g VS). The highest methane yield was observed for co-digestion of excess sludge from the AGS process and FW, which was 312 ± 8 mL CH4/g VS. Integration of FW as a co-substrate in the AGS process would potentially enhance energy recovery and the quality of effluent in municipal wastewater treatment.

High carriage of plasmid-mediated quinolone resistance (PMQR) genes by ESBL-producing and fluoroquinolone-resistant Escherichia coli recovered from animal waste dumps

BACKGROUND

There has been a rise in the consumption of fluoroquinolones in human and veterinary medicine recently. This has contributed to the rising incidence of quinolone resistance in bacteria. This study aimed at the determination of the antibiotic resistance profile of ESBL-producing and fluoroquinolone-resistant E. coli (FQEC) isolated from animal waste obtained from the waste dumps of an agricultural farm and their carriage of genes encoding PMQR.

METHODS AND RESULTS

Isolation of ESBL-producing E. coli from animal waste samples was done on CHROMagar ESBL, while presumptive isolates were purified, and identified via the detection of uidA gene. Susceptibility to a panel of ten antibiotics was done using the disc diffusion method, and detection of PMQR genes (qnrA, qnrB, qnrS, aac(6')-lb-cr, qepA and oqxAB) was done using monoplex and duplex PCR. Twenty-five ESBL-producing and FQEC were obtained from the cattle (6), piggery (7) and poultry (12) waste dumps of the farm. There was 100% resistance to cefpodoxime, cefotaxime, enrofloxacin, trimethoprim-sulfamethoxazole and penicillin by the isolates. The resistance to the other antibiotics was streptomycin (48%), ceftazidime (24%), while no isolate resisted amoxicillin-clavulanate and imipenem. The frequencies of PMQR genes detected were; qnrA (96%), oqxAB (96%), qnrB (92%), while  qnrS was detected in 88% (22) of the isolates. Aminoglycoside acetyltransferase (aac(6')-lb-cr) and quinolone efflux pump (qepA) were each detected in 20 (80%) of the isolates.

CONCLUSIONS

This study showed that animal wastes disposed indiscriminately into dumps could be a budding 'hotspot' for multidrug resistant, ESBL-producing and fluoroquinolone-resistant E. coli carrying multiple genes encoding resistance to fluoroquinolone antibiotics.

Unifying concepts in methanogenic, aerobic, and anammox sludge granulation

The retention of dense and well-functioning microbial biomass is crucial for effective pollutant removal in several biological wastewater treatment technologies. High solids retention is often achieved through aggregation of microbial communities into dense, spherical aggregates known as granules, which were initially discovered in the 1980s. These granules have since been widely applied in upflow anaerobic digesters for waste-to-energy conversions. Furthermore, granular biomass has been applied in aerobic wastewater treatment and anaerobic ammonium oxidation (anammox) technologies. The mechanisms underpinning the formation of methanogenic, aerobic, and anammox granules are the subject of ongoing research. Although each granule type has been extensively studied in isolation, there has been a lack of comparative studies among these granulation processes. It is likely that there are some unifying concepts that are shared by all three sludge types. Identifying these unifying concepts could allow a unified theory of granulation to be formed. Here, we review the granulation mechanisms of methanogenic, aerobic, and anammox granular sludge, highlighting several common concepts, such as the role of extracellular polymeric substances, cations, and operational parameters like upflow velocity and shear force. We have then identified some unique features of each granule type, such as different internal structures, microbial compositions, and quorum sensing systems. Finally, we propose that future research should prioritize aspects of microbial ecology, such as community assembly or interspecies interactions in individual granules during their formation and growth.

The influence of different nitrate concentrations on aerobic sludge granulation and the role of extracellular polymeric substances

This study investigated the influence of nitrate on aerobic granular sludge (AGS) granulation. The introduction of nitrate at 5, 15 and 20 mg L-1 promoted AGS granulation, and the promoting effect was positively correlated with nitrate concentrations. Meanwhile, exogenous nitrate significantly increased denitrification rate in the AGS system. However, granular disintegration appeared at a long-term addition of nitrate. An in-deep analysis showed that nitrate stimulated the secretion of extracellular polymeric substances (EPS), especially the content of proteins, which might be the main reason for the AGS granulation. However, the rapid and excessive increase in EPS might cause granular disintegration, as excessive EPS blocked the transmission of substrates, leading to the increase of dead cells in the granules. Besides, nitrate also altered the hydrophobicity of EPS and the content of α-helix, 3-turned helix and polymeric chain that favored aggregation, which also affected AGS granulation. From the microbial community level, nitrate induced the enrichment of denitrifying bacteria, including those that also functioned as EPS producers, such as Micropruina and Flavobacterium, resulting in the rapid increase of functional enzymes associated with amino acid synthesis, thereby promoting the secretion of proteins in EPS. Conversely, disintegration caused by mass transfer blockage might lead to the loss of EPS producing bacteria and subsequent decrease in EPS content, further accelerating granular disintegration.

Full dynamic control of dairy wastewater treatment by aerobic granular sludge using electric conductivity and oxygen uptake rate

The objective of the current study was to determine the applicability of a sensor-based dynamic control strategy for the treatment of real variable dairy wastewater by aerobic granular sludge (AGS) performing enhanced biological phosphorus removal (EBPR). Two parallel sequencing batch reactors (SBRs) were set up that used only an anaerobic feast/aerobic famine microbial selection strategy to successfully obtain sludge granulation. SBR-STA used a fixed cycle length, while the duration of the reaction steps in SBR-DYN was variable. The control strategy was based solely on (derived) signals from low-cost and common sensors. The profile of the electric conductivity during the anaerobic reaction step was related to the microbial release of phosphate (PO4-P) and the associated uptake of dissolved organic carbon (DOC) by polyphosphate-accumulating organisms (PAOs). Control of the aerobic reaction step was based on the oxygen uptake rate (OUR). This resulted in a dynamic reactor operation with significant efficiency gains, such as 32% shorter cycle times and 42% higher sludge loading rates without impairing the effluent quality. These results extend the existing potential of indirect control strategies to full biological nutrient removal processes, which may be of great assistance to the operators and designers of industrial installations.

Effect of food-to-microorganisms ratio on aerobic granular sludge settleability: Microbial community, potential roles and sequential responses of extracellular proteins and polysaccharides

The food-to-microorganism ratio (F/M) is an important parameter in wastewater biotreatment that significantly affects the granulation and settleability of aerobic granular sludge (AGS). Hence, understanding the long-term effects and internal mechanisms of F/M on AGS settling performance is essential. This study investigated the relationship between F/M and the sludge volume index (SVI) within a range of 0.23-2.50 kgCOD/(kgMLVSS·d). Thiothrix and Candidatus_Competibacter were identified as two dominant bacterial genera influencing AGS settling performance. With F/M increased from 0.27 kgCOD/(kgMLVSS·d) to 1.53 kgCOD/(kgMLVSS·d), the abundance of Thiothrix significantly increased from 0.20% to 27.02%, and the hydrophobicity of extracellular proteins (PN) decreased, which collectively reduced AGS settling performance. However, under high-F/M conditions, the gel-like polysaccharides (PS) effectively retained the granular biomass by binding to the highly abundant Thiothrix (53.65%). The progressive increment in biomass led to a concomitant reduction in F/M, resulting in the recovery of AGS settleability. In addition, two-dimensional correlation infrared spectroscopy analysis revealed the preferential responses of PN and PS to the increase and decrease of F/M, and the content and characteristics of PN and PS played important roles in granular settling. The study provides insight into the microbial composition and the potential role of extracellular polymer substances in the AGS sedimentation behavior, offering valuable theoretical support for stable AGS operation.

Towards a better understanding of the anaerobic/oxic/anoxic-aerobic granular sludge process (AOA-AGS) for simultaneous low-strength wastewater treatment and in situ sludge reduction from ambient to winter temperatures

The new anaerobic/oxic/anoxic-aerobic granular sludge (AOA-AGS) merits the advantages of effective carbon utilization and low-carbon treatment. However, low temperature poses stressing concerns and the resisting mechanism remains much unknown. Herein, an AOA-AGS process was configured for simultaneous nitrification, denitrification and phosphorus removal (SNDPR) with low-strength wastewater from ambient (>15 °C) to winter temperatures (<15 °C). Results showed that simultaneously advanced nutrients removal, and dramatic in situ sludge reduction (Yobs of 0.093 g MLSS/g COD) were gained regardless of seasonally decreasing temperatures. Winter temperatures even amplified Candidatus Competibacter predominating from 20.11% to 34.74%, which laid the core basis for endogenous denitrification, sludge minimization and temperature resistance. A removal model was thus proposed given the observed functional groups, and doubts were also raised for future investigations. This study would aid a better understanding on the microbial ecology and engineering aspects of the new AOA-AGS process treating low-strength wastewater at low temperatures.

Pineapple wastewater as co-substrate in treating real alkaline, non-biodegradable textile wastewater using biogranulation technology

This study was to develop biogranules using a sequencing batch reactor (SBR) and to evaluate the effect of pineapple wastewater (PW) as a co-substrate for treating real textile wastewater (RTW). The biogranular system cycle was 24 h (2 stages of phase), with an anaerobic phase (17.8 h) followed by an aerobic phase (5.8 h) for every stage of the phase. The concentration of pineapple wastewater was the main factor studied in influencing COD and color removal efficiency. Pineapple wastewater with different concentrations (7, 5, 4, 3, and 0% v/v) makes a total volume of 3 L and causes the OLRs to vary from 2.90 to 0.23 kg COD/m3day. The system achieved 55% of average color removal and 88% of average COD removal at 7%v/v PW concentration during treatment. With the addition of PW, the removal increased significantly. The experiment on the treatment of RTW without any added nutrients proved the importance of co-substrate in dye degradation.

Positive effects of lignocellulose on the formation and stability of aerobic granular sludge

Introduction

Lignocellulose is one of the major components of particulate organic matter in sewage, which has a significant influence on biological wastewater treatment process. However, the effect of lignocellulose on aerobic granular sludge (AGS) system is still unknown.

Methods

In this study, two reactors were operated over 5 months to investigate the effect of lignocellulose on granulation process, structure stability and pollutants removal of AGS.

Results and discussion

The results indicated that lignocellulose not only promoted the secretion of tightly bound polysaccharide in extracellular polymeric substances, but also acted as skeletons within granules, thereby facilitating AGS formation, and enhancing structural strength. Lignocellulose imposed little effect on the removal efficiency of pollutants, with more than 95, 99, and 92% of COD, NH4+-N, and PO43--P were removed in both reactors. However, it did exhibit a noticeable influence on pollutants conversion processes. This might be due to that the presence of lignocellulose promoted the enrichment of functional microorganisms, including Candidatus_Accumulibacter, Candidatus_Competibacter, Nitrosomonas, and Nitrospira, etc. These findings might provide valuable insights into the control strategy of lignocellulose in practical AGS systems.

Granule formation mechanism, key influencing factors, and resource recycling in aerobic granular sludge (AGS) wastewater treatment: A review

The high-efficiency and additionally economic benefits generated from aerobic granular sludge (AGS) wastewater treatment have led to its increasing popularity among academics and industrial players. The AGS process can recycle high value-added biomaterials including extracellular polymeric substances (EPS), sodium alginate-like external polymer (ALE), polyhydroxyfatty acid (PHA), and phosphorus (P), etc., which can serve various fields including agriculture, construction, and chemical while removing pollutants from wastewaters. The effects of various key operation parameters on formation and structural stability of AGS are comprehensively summarized. The degradable metabolism of typical pollutants and corresponding microbial diversity and succession in the AGS wastewater treatment system are also discussed, especially with a focus on emerging contaminants removal. In addition, recent attempts for potentially effective production of high value-added biomaterials from AGS are proposed, particularly concerning improving the yield, quality, and application of these biomaterials. This review aims to provide a reference for in-depth research on the AGS process, suggesting a new alternative for wastewater treatment recycling.

Aerobic granular sludge formation and stability in enhanced biological phosphorus removal system under antibiotics pressure: Performance, granulation mechanism, and microbial successions

Wastewater containing antibiotics can pose a significant threat to biological wastewater treatment processes. This study investigated the establishment and stable operation of enhanced biological phosphorus removal (EBPR) by aerobic granular sludge (AGS) under mixed stress conditions induced by tetracycline (TC), sulfamethoxazole (SMX), ofloxacin (OFL), and roxithromycin (ROX). The results show that the AGS system was efficient in removing TP (98.0%), COD (96.1%), and NH₄⁺-N (99.6%). The average removal efficiencies of the four antibiotics were 79.17% (TC), 70.86% (SMX), 25.73% (OFL), and 88.93% (ROX), respectively. The microorganisms in the AGS system secreted more polysaccharides, which contributed to the reactor's tolerance to antibiotics and facilitated granulation by enhancing the production of protein, particularly loosely bound protein. Illumina MiSeq sequencing revealed that putative phosphate accumulating organisms (PAOs)-related genera (Pseudomonas and Flavobacterium) were enormously beneficial to the mature AGS for TP removal. Based on the analysis of extracellular polymeric substances, extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, and microbial community, a three-stage granulation mechanism was proposed including adaption to the stress environment, formation of early aggregates and maturation of PAOs enriched microbial granules. Overall, the study demonstrated the stability of EBPR-AGS under mixed antibiotics pressure, providing insight into the granulation mechanism and the potential use of AGS for wastewater treatment containing antibiotics.

Potential of direct granulation and organic loading rate tolerance of aerobic granular sludge in ultra-hypersaline environment

Salt-tolerant aerobic granular sludge (SAGS) technology has shown potentials in the treatment of ultra-hypersaline high-strength organic wastewater. However, the long granulation period and salt-tolerance acclimation period are still bottlenecks that hinder SAGS applications. In this study, "one-step" development strategy was used to try to directly cultivate SAGS under 9% salinity, and the fastest cultivation process was obtained under such high salinity compared to the previous papers with the inoculum of municipal activated sludge without bioaugmentation. Briefly, the inoculated municipal activated sludge was almost discharged on Day 1-10, then fungal pellets appeared and it gradually transitioned to mature SAGS (particle size of ∼4156 μm and SVI₃₀ of 57.8 mL/g) from Day 11 to Day 47 without fragmentation. Metagenomic revealed that fungus Fusarium played key roles in the transition process probably because it functioned as structural backbone. RRNPP and AHL-mediated systems might be the main QS regulation systems of bacteria. TOC and NH₄⁺-N removal efficiencies maintained at ∼93.9% (after Day 11) and ∼68.5% (after Day 33), respectively. Subsequently, the influent organic loading rate (OLR) was stepwise increased from 1.8 to 11.7 kg COD/m³·d. It was found that SAGS could maintain intact structure and low SVI₃₀ (< 55 mL/g) under 9% salinity and the OLR of 1.8-9.9 kg COD/m³·d with adjustment of air velocity. TOC and NH₄⁺-N (TN) removal efficiencies could maintain at ∼95.4% (below OLR of 8.1 kg COD/m³·d) and ∼84.1% (below nitrogen loading rate of 0.40 kg N/m³·d) in ultra-hypersaline environment. Halomonas dominated the SAGS under 9% salinity and varied OLR. This study confirmed the feasibility of direct aerobic granulation in ultra-hypersaline environment and verified the upper OLR boundary of SAGS in ultra-hypersaline high-strength organic wastewater treatment.

Environmentally relevant level of perfluorooctanoic acid affect the formation of aerobic granular sludge

The growing increasing occurrence of perfluorooctanoic acid (PFOA) in wastewater has raised concerns about its potential impact on the environment. Nevertheless, the impact of PFOA at environmentally relevant level on the formation of aerobic granular sludge (AGS) is still a 'black box'. This study thus aims to fill this gap by comprehensive investigation of sludge properties, reactor performance and microbial community during the formation of AGS. It was found that 0.1 mg/L PFOA delayed the formation of AGS, causing relatively lower proportion of large size AGS at the end of operation process. Interestingly, the microorganisms contribute to the reactor's tolerance to PFOA by secreting more extracellular polymeric substances (EPS) to slow or block the entry of toxic substances into the cells. During the granule maturation period, the reactor nutrient removal especially chemical oxygen demand (COD) and total nitrogen (TN) were affected by PFOA, decreasing the corresponding removal efficiencies to ∼81.2% and ∼69.8%, respectively. Microbial analysis further revealed that PFOA decreased the abundances of Plasticicumulans, Thauera, Flavobacterium and Cytophagaceae_uncultured, but it has promoted Zoogloea and Betaproteobacteria_unclassified growth, which maintained the structures and functions of AGS. The above results revealed that the intrinsic mechanism of PFOA on the macroscopic representation of sludge granulation process was revealed, and it is expected to provide theoretical insights and practical support for direct adoption of municipal or industrial wastewater containing perfluorinated compounds to cultivate AGS.

Non-filamentous sludge bulking induced by exopolysaccharide variation in structure and properties during aerobic granulation

The forming mechanism of non-filamentous sludge bulking during aerobic granulation were investigated basing on three feeding strategies (R1 direct aeration after fast feeding, R2 anaerobic stirring after fast feeding and R3 anaerobic plug-flow slow feeding). Results showed that strong selection stress (shortening settling time) led to a sharp flocs washout and the subsequent increase of food to microorganisms (F/M) in R1 and R3 reactors, but not found in R2 due to the different strategies of feeding modes. With the increase of F/M, zeta potential and hydrophobicity of sludge surfaces significantly decreased and thus leading to an enhanced repulsive force and energy barriers for sludge aggregation. Particularly, when F/M exceeded 1.2 kgCOD/(kgMLSS·d), non-filamentous sludge bulking was ultimately triggered in R1 and R3. Further analysis showed that massive extracellular exopolysaccharide (PS) accumulated on the surfaces of non-filamentous bulking sludge due to the increased abundance of the microorganisms related to PS secretion during sludge bulking. In addition, significantly increased intracellular second messenger (c-di-GMP), a key substance regulating PS biosynthesis, was confirmed via its concentration determination as well as microbial function prediction analysis, which played a critical role in sludge bulking. Combing with the systematic detection from surface plasmon resonance system, rheometer and size-exclusion chromatography-multiangle laser light detection-refractive index system, higher molecular weight, compact conformation, higher viscosity and higher hydrophilicity was determined in sludge bulking PS relative to PS extracted from non-filamentous bulking sludge. Clearly, the changes of PS (content, structures and properties) driven by c-di-GMP are the dominant mechanism for the formation of non-filamentous sludge bulking during aerobic granulation. This work could provide theoretical support for successful start-up and application of aerobic granular sludge technology.

Hierarchical stringent response behaviors of activated sludge system to stressed conditions

The stringent response of activated sludge systems to either stressed or harmful environments is important for the stable operation of activated sludge, which is examined by taking copper ion (Cu²⁺) as a stress model in this study. When weak stress was employed (Cu²⁺ ≤ 2.5 mg/L), the N-acyl-homoserine lactones (AHLs) of C6-, C8-, and C10-HSL increased by 30 %, 13 %, and 127 %, respectively, while the redox sensor green (RSG) intensity decreased by 28 %. Encountering the increased stress (2.5 mg/L < Cu²⁺ ≤ 5 mg/L), bacteria concentration in the supernatant increased by 87 %. However, the respiration rates of autotrophic and heterotrophic bacteria (SOUR_a and SOUR_h) and adenosine triphosphate decreased by 52 %, 18 %, and 27 %, respectively, and the flocs disintegrated with a diameter decreasing from 57 to 51 μm. When the stress became more serious (Cu²⁺ > 5 mg/L), the respiration rates continued to decline, but the quasi-endogenous respiration ratio (R_q/t) increased from 31 % to 47 %. Negligible changes occurred in the endogenous respiration rate (SOUR_e), adenosine diphosphate, and adenosine monophosphate. Based on these results, a hierarchical stringent response model of the activated sludge system to stressed conditions was proposed, and these responses were evaluated by respirogram. The initial response to weak stress was related to the most sensitive signals of quorum sensing and RSG intensity, well described by the quasi-endogenous respiration rate. The adaptive response to increased stress was the proactive migrations of low- and high-nucleic-acid bacteria to the supernatant, causing the looseness and even disintegration of sludge flocs, well described by SOUR_a, SOUR_h, and R_q/t. The lethal response to lethal stress was related to endogenous metabolic processes, well described by SOUR_e. This work provides new insights into understanding the stringent response of activated sludge systems to some stressed conditions. It helps to regulate the stability of activated sludge systems with respirogram technology.

Wastewater treatment by high density algal flocs for nutrient removal and biomass production

The present work investigated the efficiency of algal flocs biomass for the treatment of primary and secondary effluent in static and mixing conditions under different hydraulic retention time (HRT). Primary effluent fed cultures created a high-density biomass of 2.8 and 3.8 g L-1 under static and mixing conditions, respectively. Secondary effluent was more effective in order to create even higher density biomass of 7.8 and 6 g L-1 under static and mixing conditions, respectively. The algal floc biomass developed was quite effective for organic matter and nutrient removal. Primary effluent fed cultures seemed to be more efficient for chemical oxygen demand (COD) and ammonia nitrogen removal, while secondary effluent fed cultures for nitrates removal. At an HRT of 8 days using primary effluent, the removal of COD, ammonia nitrogen and total phosphorus was 86.2, 100 and 97.4%, respectively. The cultures fed with secondary effluent, even at low HRT of 1.2 days, achieved removals of 88.4, 77.5, 100 and 98.6% for COD, nitrates, ammonia, and total phosphorus, respectively.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10811-023-02931-2.

Potential causes of partial-denitrification (PD) granular sludge breakdown under high nitrate loading rates

The granule instability has been frequently reported during the operation of high loading rates. While, there no research was performed on the recently developed anoxic partial-denitrification (PD) granules, a novel pathway in producing nitrite from nitrate for anammox process. Herein, this work, for the first time, investigated the influence of nitrate loading rates on the instability of PD granules and identified the key causes. Two lab-scale sequencing batch reactors (SBRs) were operated with nitrate loading rates (NLR) increased from 0.48 to 3.84 kg N/m³/d (R1, 8 cycles/d), and 0.96 to 7.68 kg N/m³/d (R2, 16 cycles/d) by gradually elevating the influent nitrate concentration. Results showed that nitrite production rates increased with the NLRs, with a maximal value of 5.26 kg N/m³/d obtained. However, the compact regular PD granules were not stable and broke down when NLR was above 3.84 kg N/m³/d, which resulted in serious sludge washing out from SBR. The high NLRs led to the extracellular polymeric substances (EPS) transformation in terms of its composition and structure, which the protein content in the EPS and the tightly bound EPS (T-EPS) fraction was significantly decreased, this was supposed to be the major reason causing the breakdown of PD granules. Besides, it was found the PD granule in R2 was more deteriorated than that in R1 under the same high NLR, suggesting the short starvation (idle) times in SBR cycle was likely another reason impairing the stability of PD granules. Overall, this research provides useful information in development of granule-based PD systems and sheds light on achieving high-rate nitrite production in SBR with great stability.

Pb(II) bioremediation using fresh algal-bacterial aerobic granular sludge and its underlying mechanisms highlighting the role of extracellular polymeric substances

Lead (Pb) discharged from rural industries poses a significant threat to the environment and human health. Algal-bacterial aerobic granular sludge (A-B AGS) is a promising alternative for sewage treatment with high efficiency and good settleability. In this study, Pb(II) biosorption using fresh A-B AGS was investigated for the first time. The important role of extracellular polymeric substances (EPS) was revealed with the involved mechanisms being clarified. The desorbents for Pb recovery from Pb-loaded A-B AGS were also screened. Results showed that A-B AGS has an excellent maximum Pb adsorption capacity of 72.4 mg·g^-1 at pH 6.0. EPS plays an important role in keeping microbial activity, Pb bonding, and providing metal ions (Ca, Na and Mg) for Pb ion exchanges. Electrostatic interaction, ion exchange, and bonding to functional groups may occur orderly in the Pb biosorption process and the formation of pyromorphite (Pb₅(PO₄)₃Cl) contributes to Pb biosorption. About 66 % of the adsorbed Pb was accumulated in the A-B AGS microbial cells. Na₂EDTA (0.05 M) can recover 60.3 % of the loaded Pb with the highest microbial activity of granules being remained. All the findings will provide the theoretical basis for the large-scale application of A-B AGS to bioremediate Pb(II)-containing wastewater.

Achieving robust nitritation in a modified continuous-flow reactor: From micro-granule cultivation to nitrite-oxidizing bacteria elimination

In this study, a modified continuous-flow nitrifying reactor was successfully operated for rapid cultivation of micro-granules and achieving robust nitritation. Results showed that sludge granulation with mean size of ca. 100 µm was achieved within three weeks by gradually increasing settling velocity-based selection pressure from 0.48 to 0.9 m/hr. Though Nitrospira like nitrite-oxidizing bacteria (NOB) were enriched in the micro-granules with a ratio between ammonia-oxidizing bacteria (AOB) and NOB of 5.7%/6.5% on day 21, fast nitritation was achieved within one-week by gradually increasing of influent ammonium concentration (from 50 to 200 mg/L). Maintaining ammonium in-excess was the key for repressing NOB in the micro-granules. Interestingly, when the influent ammonium concentration switched back to 50 mg/L still with the residual ammonium of 15-25 mg/L, the nitrite accumulation efficiency increased from 90% to 98%. Experimental results suggested that the NOB repression was intensified by both oxygen and nitrite unavailability in the inner layers of micro-granules. Unexpectedly, continuous operation with ammonium in excess resulted in overproduction of extracellular polysaccharides and overgrowth of some bacteria (e.g., Nitrosomonas, Arenimonas, and Flavobacterium), which deteriorated the micro-granule stability and drove the micro-granules aggregation into larger ones with irregular morphology. However, efficient nitritation was stably maintained with extremely high ammonium oxidation potential (> 50 mg/g VSS/hr) and nearly complete washout of NOB was obtained. This suggested that smooth and spherical granule was not a prerequisite for achieving NOB wash-out and maintaining effective nitritation in the granular reactor. Overall, the micro-granules exhibited a great practical potential for high-rate nitritation.

Understanding the N-acylated homoserine lactones(AHLs)-based quorum sensing for the stability of aerobic granular sludge in the aspect of substrate hydrolysis enhancement

Efficient substrate metabolism is the premise for stable operation of aerobic granular sludge and can be regulated by quorum sensing (QS). In this study, starch and acetate were selected to represent complex and simple substrates to provide comparable amount of metabolic energy for granules cultivation. Starch-fed granules were larger in size and contained higher EPS content than acetate-fed granules, though both granules exhibited similar substrate-degradation rates during sequencing batch reactor (SBR) cycle. Three N-acylhomoserine lactones (AHLs), including C8-HSL, 3OHC8-HSL and 3OHC12-HSL, were detected as dominant autoinducers in granules. They accumulated more in starch-fed granules than acetate-fed granules. The batch experiments were implemented to investigate QS regulation for granular stability in terms of substrate hydrolysis and transformation. The addition of three AHLs increased the activity of α-amylase, the main starch hydrolase, 4-6 times, significantly (p < 0.01) higher than the control treatment without AHLs amendment. While activity of dehydrogenase, the main simple substrate degradation enzyme, was increased only 1-2 times. Higher enzyme activity, especially α-amylase, significantly (p < 0.05) promoted the substrate-degradation rate (65 % than control group) and EPS yield in starch-fed system. Overall, QS can facilitate complex substrate uptake via hydrolysis enhancement and EPS secretion, which together promote sludge granulation and stability.

Quorum sensing unveils the sludge floccule-assisted stabilization of aerobic granules in granule-dominated sequencing batch reactors

Floccules are another major form of microbial aggregates in aerobic granular sludge systems. Previous studies mainly attributed the persistence of floccules to their relatively faster nutrient uptake and higher growth rate over aerobic granules; however, they failed to unravel the underlying mechanism of the long-term coexistence of these two aggregates. In this work, the existence and function of the floccules in an aerobic granule-dominated sequencing batch reactor were investigated from the view of quorum sensing (QS) and quorum quenching (QQ). The results showed that though the floccules were closely associated with the granules in terms of similar community structures (including the QS- and QQ-related ones), they exhibited a relatively higher QQ-related activity but a lower QS-related activity. A compatible proportion of floccules might be helpful to maintain the QS-related activity and keep the granules stable. In addition, the structure difference was demonstrated to diversify the QS- and QQ-related activities of the floccules and the aerobic granules. These findings could broaden our understanding of the interactions between the coexistent floccules and granules in aerobic granule-dominated systems and would be instructive for the development of the aerobic granular sludge process.

Positive effects of magnetic Fe3O4@polyaniline on aerobic granular sludge: Aerobic granulation, granule stability and pollutants removal performance

The magnetic material has been determined to have a positive effect on sludge granulation and wastewater treatment performance. In this study, the effect of magnetic Fe₃O₄@polyaniline (Fe₃O₄@PANI) on aerobic granulation, granule stability, and pollutants removal performance was evaluated by adding it into a sequencing batch reactor to cultivate aerobic granular sludge (AGS). The results indicated that the composite combined the advantages of PANI and Fe₃O₄ to promote the formation of AGS during the granulation period. The Fe₃O₄@PANI stimulated the granules to secrete extracellular polymeric substances with a higher proteins/polysaccharides ratio, thus enhancing the stability of the AGS. In addition, microbial community analysis revealed that the great performance of the AGS on denitrification and phosphorus removal could be attributed to the enrichment of denitrifying bacteria, phosphorus accumulating organisms (PAO), and denitrifying PAO by Fe₃O₄@PANI. Thus, Fe₃O₄@PANI has been demonstrated to have a positive effect on the formation and stability of AGS.

Coupling high-rate activated sludge process with aerobic granular sludge process for sustainable municipal wastewater treatment

Achieving a neutral/positive energy balance without compromising discharge standards is one of the main goals of wastewater treatment plants (WWTPs) in terms of sustainability. Aerobic granular sludge (AGS) technology promises high treatment performance with low energy and footprint requirement. In this study, high-rate activated sludge (HRAS) process was coupled to AGS process as an energy-efficient pre-treatment option in order to increase energy recovery from municipal wastewater and decrease the particulate matter load of AGS process. Three different feeding strategies were applied throughout the study. AGS system was fed with raw municipal wastewater, with the effluent of HRAS process, and with the mixture of the effluent of HRAS process and raw municipal wastewater at Stage 1, Stage 2 and Stage 3, respectively. Total suspended solids (TSS), chemical oxygen demand (COD), ammonia nitrogen (NH₄⁺-N), and total phosphorus (TP) concentrations in the effluent were less than 10 mg/L, 60 mg/L, 0.4 mg/L, and 1.3 mg/L respectively at all stages. Fluctuations were observed in the denitrification performance due to changes in the influent COD/total nitrogen (TN) ratio. This study showed that coupling HRAS process with AGS process by feeding the AGS process with the mixture of HRAS process effluent and raw municipal wastewater could be an appropriate option for both increasing the energy recovery potential of WWTPs and enabling high effluent quality.

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.

Recovery of structural extracellular polymeric substances (sEPS) from aerobic granular sludge: Insights on biopolymers characterization and hydrogel properties for potential applications

Nowadays, wastewater treatment plants (WWTPs) are transforming into water resource recovery facilities (WRRFs) where the resource recovery from waste streams is pivotal. Aerobic granular sludge (AGS) is a novel technology applied for wastewater treatment. Extracellular polymeric substances (EPS) secreted by microorganisms promote the aggregation of bacterial cells into AGS and the structural fraction of EPS (sEPS) is responsible for the mechanical properties of AGS. sEPS can be extracted and recovered from waste AGS by physico-chemical methods and its characterization is to date of relevant concern to understand the properties in the perspective of potential applications. This study reports on: characterization of sEPS extracted and recovered from AGS; - formation and characterization of sEPS-based hydrogels. Briefly, sEPS were extracted by a thermo-alkaline process followed by an acidic precipitation. sEPS-based hydrogels were formed by a cross-linking process with a 2.5% w/w CaCl₂ solution. The following key-findings can be drawn: i) hydrogels can be formed starting from 1% w/w sEPS on, by diffusion of Ca²⁺ into sEPS network; ii) the Ca/C molar ratio of hydrogels decreased with increasing concentration of sEPS from 1 to 10% w/w; iii) the thermogravimetric and spectroscopic behaviours of sEPS show that the cross-linking reaction mainly involves the polysaccharidic fraction of biopolymers; iv) water-holding capacity up to 99 gH₂O/g_sEPS was registered for 1% w/w sEPS-based hydrogels, suggesting applications in several industrial sectors (i.e. chemical, paper, textile, agronomic, etc.); v) rheological results highlighted a solid-like behaviour (G'≫G") of sEPS-based hydrogels. The power-law fitting of G' vs. sEPS concentration suggests that the expansion of the sEPS network during cross-linking occurs through a percolative mechanism involving the initial formation of sEPS oligomers clusters followed by their interconnection towards the formation of 3D network. These findings provide additional information about the mechanisms of sEPS-based hydrogel formation and reveal the peculiar physico-chemical characteristics of sEPS which nowadays are increasingly gaining interest in the context of resource recovery.

Effect of flow regime on mass transfer diffusion and stability of aerobic granular sludge (AGS) in view of interfacial thermodynamic

Aerobic granular sludge (AGS) technology has been widely studied as "The Next Generation Wastewater Treatment technology". The effect of hydraulic conditions on the structural stability of AGS has been widely studied. However, the function of flow regime on the AGS stability, especially dissolved oxygen (DO) mass transfer, is still unknown. In this study, we used the Reynolds number (Re) to quantify the flow regime and selected different stages of AGS as experimental subjects. Results showed that the relatively suitable Re (Re = 150) could create lower DO mass transfer limitation (Lc = 27.4 μm) and increase protein (PN) contents and the abundance of hydrophobic functional groups in AGS. At this condition (Re = 150), the interfacial Gibbs free energy of sludge-water (ΔG_LS^a) was at a lower state (-129.75 ± 2.15 mJ·m^-2), which favored the stability of AGS. Principal component analysis (PCA) and correlation analysis indicated that the response of ΔG_LS^a was affected by Lc, PN, and hydrophobic groups. In addition, results obtained for unstable AGS further verified that suitable Re regulates the structural stability of AGS. This study deepens the understanding of Re as an important hydraulic parameter for structural stability of AGS, which is also of great significance for energy saving of sequential batch reactors (SBRs) with agitation in practical engineering.

Assessing the effect of SiO2 and TiO2 nanoparticles on granule stability and microbial community shift in aerobic granular sludge process

The widely used SiO₂ and TiO₂ nanoparticles (NPs) can accumulate in industrial wastewaters, thereby posing challenge to biological wastewater treatment processes. In this work, the performance of aerobic granular sludge (AGS) reactors fed with wastewater containing 50 mg L^-1 SiO₂ and TiO₂ NPs were investigated. The results showed that the granules could resist the NPs in wastewater (no disintegration of granules was observed). SiO₂ NPs had a negative effect on the settleability of granules, with the SVI₃₀ increased by 64.5% and protein secretion decreased by 29.9%. To the contrary, the settleability of granules was improved in the presence of TiO₂ NPs due to the increase of the protein secretion. Possibly because of the compact and layered structure of granules, in the presence of both types of NPs, no obvious reduction of the overall removal efficiency of organics was found, and nanoparticle-resistant strains were enriched. The overall nitrification and denitrification efficiencies were hardly affected by SiO₂ NPs while significantly inhibited by TiO₂ NPs. Some functional genera, such as Hyphomicrobium and Acidovorax, showed growth inhibition with TiO₂ NPs, which might be responsible for the reduction of nitrification and denitrification efficiencies.

The biofilm life cycle: expanding the conceptual model of biofilm formation

Bacterial biofilms are often defined as communities of surface-attached bacteria and are typically depicted with a classic mushroom-shaped structure characteristic of Pseudomonas aeruginosa. However, it has become evident that this is not how all biofilms develop, especially in vivo, in clinical and industrial settings, and in the environment, where biofilms often are observed as non-surface-attached aggregates. In this Review, we describe the origin of the current five-step biofilm development model and why it fails to capture many aspects of bacterial biofilm physiology. We aim to present a simplistic developmental model for biofilm formation that is flexible enough to include all the diverse scenarios and microenvironments where biofilms are formed. With this new expanded, inclusive model, we hereby introduce a common platform for developing an understanding of biofilms and anti-biofilm strategies that can be tailored to the microenvironment under investigation.

An accelerated approach of biogas production through a two-stage BioH2/CH4 continuous anaerobic digestion system from Napier grass

Napier grass found to be greatest potential for gaseous bioenergy production. The biohydrogen and biomethane productions from untreated Napier grass in single and two-stage continuous bioreactors was evaluated using anaerobic digestion technology. The bioreactors were fed Napier grass with organic loading rate (OLR) of 0.5, 1.0 and 2.0 kg VS/m³-d, respectively. The hydrogen, methane, and energy yields were evaluated. The methane yield of single-stage system was 282.08 CH₄/g vS with OLR of 0.5 kg VS/m³-d. For two-stage system, the biohydrogen and biomethane yields were 90.06 mL H₂/g vS and 367.00 mL CH₄/g vS with OLRs of 1.0 and 0.5 kg VS/m³-d, respectively. The energy yields of single and two-stage systems were 13.14 and 10.10 kJ/g vS respectively. The peak OLR of Napier grass was 0.5 kg VS/m³-d for the two-stage system whereas the total energy recovery was 30 % higher than the single-stage system.

Unveiling significance of Ca2+ ion for start-up of aerobic granular sludge reactor by distinguishing its effects on physicochemical property and bioactivity of sludge

Almost all of the aerobic granular sludge (AGS) reactors were fed on certain amounts of Ca²⁺ ion, but whether and why it was necessary for reactor start-up remain unknown. Herein, this study conducted a set of comparative experiments in three AGS reactors, which were operated in parallel with Ca²⁺ addition in R3, hydroxyapatite (HAP) addition in R1, and without any forms of Ca addition in R2. Results showed that R3 not only achieved the complete granulation of sludge, but exhibited superior performance of COD and nutrient removal. In contrast, R1 had a slightly quicker granulation rate than R3 (R1: 0.07 day^-1; R3: 0.06 day^-1), but the formed granules could not efficiently degrade pollutants. In R2, both sludge granulation and pollutants removal did not proceed normally. Further investigations found that the Ca²⁺ ion acted in three ways: (1) it increased inorganic composition of sludge to promote granulation; (2) the transformed HAP strengthened stability of granular structure; (3) it ensured bioactivity of granules by driving enrichment of functional microbes and synthesis of metabolism enzymes. Overall, this study systemically proved significance of Ca²⁺ ion for the start-up of AGS reactors and its influencing mechanisms on different properties of granules.

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

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

Is building up substrate during anaerobic feeding necessary for granulation?

For a successful granulation process in activated sludge systems, the stimulation of slow growing organisms such as glycogen accumulating microorganisms (GAOs) is a key factor. Here we show that the introduction of an anaerobic feast followed by an aerobic famine phase successfully transforms bulking sludge, caused by the abundance of genus Kouleothrix, to a hybrid floccular-granular sludge. Two sequencing batch reactors (SBRs) were operated for 228 days treating the same industrial wastewater derived from the cleaning of trucks transporting liquid food (the cargo consists of approximately 70% chocolate and 30% beer). By respectively applying a fast and slow feeding in two parallel SBRs, different degrees of substrate build-up were achieved in the two reactors during the feast phase. The F/M ratio over the feeding time was 1.41 ± 0.48 and 0.57 ± 0.16 kg COD·(kg VSS*d)^-1 for the fast-fed and the slow-fed SBR respectively. Our results demonstrate that substrate build-up during the anaerobic selection step is not necessary to obtain well-settling granular-like sludge.

A combined system for asbestos-cement waste degradation by dark fermentation and resulting supernatant valorization in anaerobic digestion

The use of biological processes for the treatment of asbestos cement waste (ACW) has gained interest in recent years. Nevertheless, this methodology is not yet consolidated because of the incomplete ACW conversion during the biological treatment and the consequent need for further treatments that generally require a high amount of energy and chemicals. In this study, the efficiency of both mesophilic and thermophilic dark fermentation (DF) fed with glucose in fed-batch conditions was assessed for ACW biological treatment. Both thermophilic and mesophilic DF of glucose resulted in a partial conversion of glucose into organic acids that successfully degraded all the asbestos fibers contained in an ACW sample. A hydrogen-rich biogas was produced as well: at the end of the mesophilic DF treatment 0.14 L_H2 g_glucose^-1 were obtained. In addition, the anaerobic digestion (AD) of the DF supernatants led to the production of 0.38 L_CH4 g_COD^-1.

A comparative study on simultaneous recovery of phosphorus and alginate-like exopolymers from bacterial and algal-bacterial aerobic granular sludges: Effects of organic loading rate

The effects of organic loading rate (OLR) on simultaneous phosphorus (P) and alginate-like exopolymers (ALE) recovery from bacterial aerobic granular sludge (AGS) and algal-bacterial AGS were examined and compared during 70 days' operation. With the increase of OLR (0.6-1.2 g COD/(L·day)), both AGS showed good settleability and granular strength with P bioavailability > 92% (Stage III). The moderate increase in OLR had a positive influence on simultaneous recovery of P and ALE. On day 60, the contents of ALE and guluronic acid/guluronic acid (GG) blocks reached the highest in algal-bacterial AGS, about 13.37 and 2.13 mg/g-volatile suspended solids (VSS), respectively. Meanwhile, about daily 0.55 kg of P is estimated to be recovered from the wastewater treatment plant with a treatment capacity of 10,000 m³/day. P mass balance analysis during ALE extraction from both AGS was conducive to further evaluation of P removal pathway and its application potentials.

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.

Effect of high ammonia on granular stability and phosphorus recovery of algal-bacterial granules in treatment of synthetic biogas slurry

The aim of the study was to investigate the application of algal-bacterial granules in treatment of high ammonia wastewater. Two identical cylindrical reactors, i.e., Rc and Rs was used to develop granular sludge system with synthetic biogas slurry. Rs was run under an artificial solar lamp controlled at 12 h power on and 12 h power off (∼10,000 lux); Rc was operated as control (no light). Results showed that algal-bacterial granules (ABGS) developed in Rs exhibited better structural stability in the face of high ammonia influent. Compared with aerobic granules (AGS), ABGS possessed high proteins (PN) content (145.3 mg/g-VSS) in extracellular polymeric substances (EPS) and better O₂ mass transfer inner granules. Higher phosphorus (P) removal capacity was obtained in Rs even under 400 mg/L NH₃–N which resulted in higher P content in ABGS biomass (56.4 mg/g-TSS). Bioavailable P in ABGS was 44 mg P/g-SS on day 160, approximately 1.53-times higher than that in AGS.

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

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

How can the addition of extracellular polymeric substances (EPS)-based bioflocculant affect aerobic granular sludge (AGS)?

The ability of extracellular polymeric substances (EPS) recovered from aerobic granular sludge (AGS) to act as bioflocculant was tested in a pilot-scale sequencing batch reactor (SBR), fed with low-strength municipal wastewater. EPS were compared with the addition of Na-alginate as a standardized biopolymer. The optimal dosage of both biopolymers was determined through jar-test assays (400 mg L^-1 of Na-alginate in a 250 mg Ca L^-1 and 50 mg L^-1 of EPS in pH of 2 ± 0.2). The addition of Na-alginate (Operational Period I- OP-I) and EPS (Operational Period II - OP-II) led to increased adhesion of particles with 2.9 ± 0.45 and 1.3 ± 0.3 g TSS L^-1 during OP-I and OP-II, respectively, and fast settling biomass (SVI₃₀ between 68 and 78 mL g^-1). Granule predominance occurred at early stages of OP-I (day 37) and OP-II (day 44), presenting diameters mainly within the 212-600 μm range. The reactor showed removal efficiencies of 85% for biochemical oxygen demand (BOD) and above 50% for N-NH₄ during the study periods. Furthermore, the addition of EPS as a bioflocculant promoted a substantial increase in polysaccharides (PS = 153.01 ± 121 mg g_VSS^-1) and proteins (PN = 121.96 ± 69 mg g_VSS^-1), while the addition of Na-alginate affected mostly the PS content (87 ± 24 mg g_VSS^-1). The microbial community shifted mainly from Betaproteobacteria (45%) during OP-I to Alphaproteobacteria (64%) in OP-II. Therefore, EPS affected both physical-chemical and microbial features of the AGS biomass without any change in treatment efficiencies. EPS is a promising resource to be recovered from aerobic granular sludge and to be used as an alternative and sustainable bioflocculant.

Uncover the secret of the stability and interfacial Gibbs free energy of aerobic granular sludge

Interfacial Gibbs free energy (IGFE) as a thermodynamic indicator characterize the stability of the natural system. For aerobic granular sludge (AGS), how IGFE determines the stability of sludge remains to be determined. The Gibbs free energy change at the AGS-water interface (ΔG_sw^a) and AGS interfaces (ΔG_s^c) were selected as the main interfacial thermodynamic factors. Results indicated that the stable AGS was guaranteed with ΔG_s^c at the range of -31 to - 46 J m^-2. Pearson correlation coefficients between ΔG_sw^a/ ΔG_s^c and relative hydrophobicity, water content, SVI₃₀, integrity coefficient were -0.9, 0.8, 0.85, and 0.84, which illustrated that the IGFE could be a more comprehensive thermodynamic indicator. Microbial community and EPS analysis verified the importance of denitrifiers, Amide III, protein-like substances for AGS stability. This work offers a new insight into the development of AGS stability based on IGFE.

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.