Cultivating aerobic granular sludge in a developed continuous-flow reactor with two-zone sedimentation tank treating real and low-strength wastewater

Making waves: Riding the densification wave from current understanding to advancement

Densification is a novel intensification strategy with the potential to improve treatment capacity within existing continuous-flow (CF) water resource recovery facilities at low capital and operating costs and at relatively small particle sizes compared to typical aerobic granular sludge (AGS) systems. To achieve densification, biological selection principles derived from selector design and AGS concepts have been coupled with physical selection via hydrocyclones at full-scale CF facilities to promote the growth and retention of granules. This combination lowers the sludge volume index (SVI) through superior sludge settling and paves the way for optimized nutrient removal and energy efficiency in low dissolved oxygen conditions. This paper sheds light on the benefits of densification. It delves into areas of advancement to further its implementation: hydrocyclone design, selector zone design, operational guidelines, and the target range for particle sizes and granule fractions.

Microplastics shaped performance, microbial ecology and community assembly in simultaneous nitrification, denitrification and phosphorus removal process

The widespread use of microplastics (MPs) has led to an increase in their discharge to wastewater treatment plants. However, the knowledge of impact of MPs on macro-performance and micro-ecology in simultaneous nitrification, denitrification, and phosphorus removal (SNDPR) systems is limited, hampering the understanding of potential risks posed by MPs. This study firstly comprehensively investigated the performance, species interactions, and community assembly under polystyrene (PS) and polyvinyl chloride (PVC) exposure in SNDPR systems. The results showed under PS (1, 10 mg/L) and PVC (1, 10 mg/L) exposure, total nitrogen removal was reduced by 3.38-10.15 %. PS and PVC restrained the specific rates of nitrite and nitrate reduction (SNIRR, SNRR), as well as the activities of nitrite and nitrate reductase enzymes (NIR, NR). The specific ammonia oxidation rate (SAOR) and activity of ammonia oxidase enzyme (AMO) were reduced only at 10 mg/L PVC. PS and PVC enhanced the size of co-occurrence networks, niche breadth, and number of key species while decreasing microbial cooperation by 5.85-13.48 %. Heterogeneous selection dominated microbial community assembly, and PS and PVC strengthened the contribution of stochastic processes. PICRUSt prediction further revealed some important pathways were blocked by PS and PVC. Together, the reduced TN removal under PS and PVC exposure can be attributed to the inhibition of SAOR, SNRR, and SNIRR, the restrained activities of NIR, NR, and AMO, the changes in species interactions and community assembly mechanisms, and the suppression of some essential metabolic pathways. This paper offers a new perspective on comprehending the effects of MPs on SNDPR systems.

Response mechanism of non-biodegradable polyethylene terephthalate microplastics and biodegradable polylactic acid microplastics to nitrogen removal in activated sludge system

The issue of microplastics (MPs) has gained more attention among researchers and the public; however, there is still a lot to be studied about its impact on biological wastewater treatment. In this study, the effects of non-biodegradable polyethylene terephthalate (PET) and biodegradable polylactic acid (PLA) on wastewater treatment by sequencing batch reactor (SBR) were compared. The results showed that PET and PLA reduced the removal efficiency of NH4+-N by 1.7 % and 21.2 %, respectively. Structural equation functional model (SEM) analysis was used to infer the potential mechanism of PLA affecting ammonia oxidation. PLA primarily inhibits the activity of ammonia monooxygenase (AMO), while promoting an increase in reactive oxygen species (ROS) and antioxidant enzyme activity. Accordingly, the toxic effect of PLA further reduced the abundance of ammonia-oxidizing bacteria. This study showed that biodegradable MPs have a greater potential impact on wastewater treatment than non-biodegradable MPs, which warrants further investigation.

New insights into syntrophic ethanol oxidation: Effects of operational modes and solids retention times

Anaerobic ethanol oxidation relies on syntrophic interactions among functional microorganisms to become thermodynamically feasible. Different operational modes (sequencing batch reactors, SBRs, and continuous flow reactors, CFRs) and solids retention times (SRT, 25 days and 10 days) were employed in four ethanol-fed reactors, named as SBR25d, SBR10d, CFR25d, and CFR10d, respectively. System performance, syntrophic relationships, microbial communities, and metabolic pathways were examined. During the long-term operation, 2002.7 ± 56.0 mg COD/L acetate was accumulated in CFR10d due to the washout of acetotrophic methanogens. Microorganisms with high half-saturation constants were enriched in reactors of 25-day SRT. Moreover, ethanol oxidizing bacteria and acetotrophic methanogens with high half-saturation constants could be acclimated in SBRs. In SBRs, Syner-01 and Methanothrix dominated, and the low SRT of 10 days increased the relative abundance of Geobacter to 38.0%. In CFRs, the low SRT of 10 days resulted in an increase of Desulfovibrio among syntrophic bacteria, and CFR10d could be employed in enriching hydrogenotrophic methanogens like Methanobrevibacter.

A systemic evaluation of aerobic granular sludge among granulation, operation, storage, and reactivation processes in an SBR

As a biological promising wastewater treatment technology, aerobic granular sludge (AGS) technology had been widely studied in sequencing batch reactors (SBRs) for the decades. Presently, the whole processes of its granulation, long-term operation, storage, and reactivation have not been thoroughly evaluated, and also the relationships among microbial diversity, granular size, and characteristics were still not that clear. Hence, they were systematically evaluated in an AGS-SBR in this work. The results demonstrated that Proteobacteria and Bacteroidetes were the dominant phyla, Flavobacterium, Acinetobacter, Azoarcus, and Chryseobacterium were the core genera with discrepant abundances in diverse stages or granular size. Microbial immigration was significant in various stages due to microbial diversity had a line relationship with COD/MLVSS ratio (R2 = 0.367). However, microbial diversity had no line relationship with granular size (R2 = 0.001), indicating the microbial diversity in different-sized AGS was similar, although granular size had a line relationship with settleability (R2 = 0.978). Overall, compared to sludge traits (e.g., sludge size, settleability), COD/MLVSS played a key role on microbial evolution. This study revealed the relationships between granule characteristics and microbial community, and contributed to the future AGS-related studies.

Synergistic analysis of performance, microbial community, and metabolism in aerobic granular sludge under polyacrylonitrile microplastics stress

Aerobic granular sludge (AGS) has proved to be a promising biotechnology for microplastics wastewater treatment. However, polyacrylonitrile microplastics (PAN MPs), the most widely used plastic in textile materials, have not been investigated. Therefore, the effect of the neglected PAN MPs on AGS at different concentrations (1, 10, and 100 mg/L) was evaluated. The results indicated that PAN MPs with 1 and 10 mg/L concentrations had no obvious effect on granular stability and nutrient removal performance, but greatly promoted the secretion of EPS. Remarkably, the granule structure was severely damaged under 100 mg/L PAN MPs. Moreover, microbial community analysis showed that phylum Proteobacteria played a dominant role in resistance to PAN MPs. Metabolic analysis further revealed that genes related to denitrification pathway (nasA, nirK, nirS and norB) and membrane transport were significantly inhibited under PAN MPs stress. This study may provide additional information on the treatment of microplastics wastewater using AGS.

Current progress of continuous-flow aerobic granular sludge: A critical review

Aerobic granular sludge (AGS) is promising for water resource recovery. Despite the mature granulation strategies in sequencing batch reactor (SBR), the application of AGS-SBR in wastewater treatment is usually costly as it requires extensive infrastructure conversion (e.g., from continuous-flow reactor to SBR). In contrast, continuous-flow AGS (CAGS) that does not require such infrastructure conversion is a more cost-effective strategy to retrofit existing wastewater treatment plants (WWTPs). Formation of aerobic granules in both batch and continuous-flow mode depends on many factors, including selection pressure, feast/famine conditions, extracellular polymeric substances (EPS), and environmental conditions. Compared with AGS in SBR, creating proper conditions to facilitate granulation in continuous-flow mode is challenging. Researchers have been seeking to tackle this bottleneck by studying the impacts of selection pressure, feast/famine conditions, and operating parameters on granulation and granule stability in CAGS. This review paper summarizes the state-of-the-art knowledge regarding CAGS for wastewater treatment. Firstly, we discuss the CAGS granulation process and effective parameters (i.e., selection pressure, feast/famine conditions, hydrodynamic shear force, reactor configuration, the role of EPS, and other operating factors). Then, we evaluate CAGS performance in removing COD, nitrogen, phosphorus, emerging pollutants, and heavy metals from wastewater. Finally, the applicability of the hybrid CAGS systems is presented. At last, we suggest that integrating CAGS with other treatment methods such as membrane bioreactor (MBR) or advanced oxidation processes (AOP) can benefit the performance and stability of granules. However, future research should address unknowns including the relationship between feast/famine ratio and stability of the granules, the effectiveness of applying particle size-based selection pressure, and the CAGS performance at low temperatures.

Anammox-based granulation cycle for sustainable granular sludge biotechnology from mechanisms to strategies: A critical review

Anaerobic ammonium oxidation (anammox) granular sludge is a promising biotechnological process for treating low-carbon nitrogenous wastewater, and is featured with low energy consumption and footprint. Previous theoretical and experimental research on anammox granular sludge processes mainly focused on granulation (flocs → granules), but pay little attention to the granulation cycle including granulation and regeneration. This work reviewed the previous studies from the perspective of anammox granules lifecycle and proposed various sustainable formation mechanisms of anammox granules. By reviewing the anaerobic, aerobic, and anammox granulation mechanisms, we summarize the mechanisms of thermodynamic theory, heterogeneous growth, extracellular polymeric substance (EPS)-based adhesion, quorum sensing (QS)-based regulation, biomineralization-based growth, and stratification of microorganisms to understand anammox granulation. In the regeneration process, the formation of precursors for re-granulation is explained by the mechanisms of physical crushing, quorum quenching and dispersion cue sensing. Based on the granulation cycle mechanism, the rebuilding of the normal regeneration process is considered essential to avoid granule floatation and the wash-out of granules. This comprehensive review indicates that future research on anammox granulation cycle should focus on the effects of filamentous bacteria in denitrification-anammox granulation cycle, the role of QS/ quorum quenching (QQ)-based autoinducers, development of diversified mechanisms to understand the cycle and the cycle mechanisms of stored granules.

Aerobic granular sludge with filamentous bacteria immobilized by string carriers to treat simulated municipal wastewater in a continuous flow reactor

Aerobic granular sludge with filamentous bacteria (FAGS) has displayed many desirable properties. However, the selective pressure based on settling speed cannot effectively separate FAGS from water in sequencing batch reactor (SBR), which limits FAGS development. In this study, a new selection pressure was created by adding string carriers. Strings were used as crystal nuclei to form immobilized FAGS to achieve rapid separation from water. The immobilization of FAGS was achieved in both SBR and continuous flow reactor (CFR). The immobilization and long-term operation of FAGS in CFR were explored. NH₄⁺ and COD removal efficiency remained above 90 % and 85 %, respectively. Sphaerotilus, denitrifying microorganisms and EPS-secreting microorganisms were the main microorganisms in the immobilized FAGS. The selection pressure provided by the strings, the operating characteristics of the CFR, and the properties of Sphaerotilus may play key role in the immobilization of FAGS.

Fast Granulation by Combining External Sludge Conditioning with FeCl3 Addition and Reintroducing into an SBR

The separation of light and heavy sludge, as well as the aggregation rate of floccular sludge, are two critical aspects of the rapid granulation process in sequencing batch reactors (SBRs) in the early stages. In this study, we investigated the impact of a method to improve both sludge separation and granulation by coupling effluent sludge external conditioning with FeCl₃ addition and then reintroducing it into the SBR. By supplementation with 0.1 g Fe³⁺ (g dried sludge (DS))⁻¹, the concentration of extracellular polymeric substances (EPS) and sludge retention efficiency greatly increased, whereas the moisture content and specific oxygen uptake rate (SOUR) sharply decreased within 24 h external conditioning. Aggregates (1.75 ± 0.05 g·L⁻¹) were reintroduced into the bioreactor once daily from day 13 to day 15. Afterwards, on day 17, aerobic granules with a concentration of mixed liquor suspended solids (MLSS) of 5.636 g/L, a sludge volume index (SVI₃₀) of 45.5 mL/g and an average size of 2.5 mm in diameter were obtained. These results suggest that the external conditioning step with both air-drying and the addition of Fe³⁺ enhanced the production of EPS in the effluent sludge and improved rapid aggregation and high sludge retention efficiency. Consequently, the reintroduced aggregates with good traits shortened the time required to obtain mature aerobic granular sludge (AGS) and properly separate light and heavy sludge. Indeed, this method jump-started the aggregation, and rapid granulation processes were successful in this work. Additionally, while the removal efficiency of chemical oxygen demand (COD) and nitrogen from ammonium (NH₄⁺-N) decreased when reintroducing the treated sludge into the SBR, such properties increased again as the AGS matured in the SBR, up to removal efficiencies of 96% and 95%, respectively.

Effect of Seed Sludge Type on Aerobic Granulation, Pollutant Removal and Microbial Community in a Sequencing Batch Reactor Treating Real Textile Wastewater

The aerobic granulation, pollutant removal, and microbial community in real textile wastewater (TWW) treatment were compared using conventional activated sludge (CAS) and preformed aerobic granular sludge (AGS) in synthetic wastewater as seed in two reactors, reactor-1 (R1) and reactor-2 (R2), respectively. The results showed that complete granulation was achieved in R1 (sludge volume index at 5 min (SVI₅) and 30 min (SVI₃₀): 19.4 mL/g; granule size: 210 μm) within 65 days, while it only required 28 days in R2 (SVI₅ and SVI₃₀: 27.3 mL/g; granule size: 496 μm). The removal of COD, NH₄⁺-N and TN in R1 (49.8%, 98.8%, and 41.6%) and R2 (53.6%, 96.9%, and 40.8%) were comparable in 100% real TWW treatment, but stable performance was achieved much faster in R2. The real TWW had an inhibitory effect on heterotrophic bacteria activity, but it had no inhibition on ammonia-oxidizing bacteria activity. AGS with a larger particle size had a higher microbial tolerance to real TWW. Furthermore, filamentous Thiothrix in the AGS in R2 disappeared when treating real TWW, leading to the improvement of sludge settleability. Thus, seeding preformed AGS is suggested as a rapid start-up method for a robust AGS system in treating real TWW.

Exploring the Function of Quorum Sensing Regulated Biofilms in Biological Wastewater Treatment: A Review

Quorum sensing (QS), a type of bacterial cell–cell communication, produces autoinducers which help in biofilm formation in response to cell population density. In this review, biofilm formation, the role of QS in biofilm formation and development with reference to biological wastewater treatment are discussed. Autoinducers, for example, acyl-homoserine lactones (AHLs), auto-inducing oligo-peptides (AIPs) and autoinducer 2, present in both Gram-negative and Gram-positive bacteria, with their mechanism, are also explained. Over the years, wastewater treatment (WWT) by QS-regulated biofilms and their optimization for WWT have gained much attention. This article gives a comprehensive review of QS regulation methods, QS enrichment methods and QS inhibition methods in biological waste treatment systems. Typical QS enrichment methods comprise adding QS molecules, adding QS accelerants and cultivating QS bacteria, while typical QS inhibition methods consist of additions of quorum quenching (QQ) bacteria, QS-degrading enzymes, QS-degrading oxidants, and QS inhibitors. Potential applications of QS regulated biofilms for WWT have also been summarized. At last, the knowledge gaps present in current researches are analyzed, and future study requirements are proposed.

Enhancing the Stability of Aerobic Granular Sludge Process Treating Municipal Wastewater by Adjusting Organic Loading Rate and Dissolved Oxygen Concentration

Aerobic granular sludge (AGS) application in treating municipal wastewater has been greatly restricted due to its low stability. It has been found that operation parameters have a great impact on stability. The organic loading rate (OLR) and dissolved oxygen (DO) concentration are two very important parameters that impact stability. In this study, the organic loading rate (OLR) and aeration rate were studied to verify their influence on AGS system stability, which is indicated by determining pollutant removal performance, including chemical oxygen demand (COD), ammonia nitrogen, and total nitrogen (TN). The physical and chemical property changes of AGS and the effects of pollutant removal during the formation of AGS were systematically investigated. The AGS was formed after about 25 days and remained stable for about 45–50 days. The AGS was light-yellow globular sludge with an average particle size of 1.25 mm and a sludge volume index (SVI) of 33.9 mL/g. The optimal condition was obtained at an OLR of 4.2 kg COD/m3·d, aeration rate of 4 L/min, and a hydraulic retention time (HRT) of 4 h. The corresponding removal efficiencies of COD, ammonia nitrogen, and TN were 94.1%, 98.4% and 74.1%, respectively. The study shows that the AGS system has great potential for pollutant removal from wastewater.

Role of rotating speed on the stability of a self-sustaining algal-bacterial photo-granules process

The effects of rotating speed (120, 130, 140 and 150 rpm) of a photo-reactor on the self-sustaining algal-bacterial photo-granules sludge (ABPG) was investigated in this study. The ABPG process maintained good granular stability at 140 rpm with an integrity coefficient of 3.0% and excellent nutrient removal. The increases of the extracellular polymeric substances (EPS) content, the portions of α-helix (32.37%) in the secondary protein structure, and the relative abundance of functional bacteria (e.g. Candidatus_Competibacter), contributed to the maintenance of granular structure stability. However, the lower rotating speed (120 and 130 rpm) resulted in slow bacterial growth and reproduction while a higher rotating speed (150 rpm) caused the disintegration of photo-granules. Overall, the results reveal the influencing mechanisms of photo-reactor rotating speed on the self-sustaining ABPG process and provide an effective approach to maintain the system stability.

Emerging biological wastewater treatment using microalgal-bacterial granules: A review

Aiming at deepening the understanding of the formation and evolution of emerging microalgal-bacterial granule (MBG)-based wastewater treatment systems, the recent advances regarding the formation processes, transfer phenomena, innovative bioreactors development and wastewater treatment performance of MBG-based systems are comprehensively reviewed in this work. Particularly, the successful establishments of MBG-based systems with various inocula are summarized. Besides, as the indispensable factors for biochemical reactions in MBGs, the light and substrates (organic matters, inorganic nutrients, etc) need to undergo complicated and multi-scale transfer processes before being assimilated by microorganisms within MBGs. Therefore, the involved transfer phenomena and mechanisms in MBG-based bioreactors are critically discussed. Subsequently, some recent advances of MBG-based bioreactors, the application of MBG-based systems in treating various synthetic and real wastewater, and the future development directions are discussed. In short, this review helps in promoting the development of MBG-based systems by presenting current research status and future perspectives.

Enhancing robustness of halophilic aerobic granule sludge by granular activated carbon at decreasing temperature

High salinity seriously inhibits the growth and metabolism of microorganisms, resulting in poor settleability, excessive biomass loss and low treatment efficiency of biological wastewater treatment systems. The development of halophilic aerobic granular sludge (HAGS) is a feasible strategy for addressing this challenge. However, there are problems with the granulation of HAGS and the stability of granules at decreasing temperatures. In this study, granular activated carbon (GAC) with a large specific surface area and good biocompatibility was used to enhance the robustness of HAGS. The results showed that the addition of GAC shortened the granulation time from 60 d (control system) to 35 d (GAC-addition system). The proteins contents of extracellular polymeric substances (EPS) in the GAC-addition system was significantly higher (p < 0.05) than that in the control system during granulation. Satisfactory NH₄⁺-N and chemical oxygen demand (COD) removal efficiencies reached more than 96% in both systems at 18-26 °C. When the operating temperature was lower than 15 °C, the GAC-addition system exhibited better NH₄⁺-N removal performance (>80%) than the control system (<60%). Moreover, the abundance of almost all nitrogen metabolism-related genes in the GAC-addition system was higher than that in the control system. During the granulation process, the enrichment of functional microorganisms, including family Flavobacteriaceae, Rhodobacteraceae, and Cryomorphaceae, may promote the production of EPS by significantly upregulating (p < 0.05) the metabolic pathway "Signaling Molecules and Interaction" in the GAC-addition system. The overexpression of the nitrogen assimilation gene glnA in heterotrophic bacteria (Halomonas and Marinobacterium) may promote the conversion of inorganic nitrogen to extracellular proteins to adapt to the decreased operational temperature. Our findings confirm that GAC addition is a simple but effective strategy to accelerate granulation and enhance the robustness of HAGS in saline wastewater treatment.

Rapid granulation of aerobic granular sludge and maintaining its stability by combining the effects of multi-ionic matrix and bio-carrier in a continuous-flow membrane bioreactor

The present investigation aimed at providing a novel approach to promote the rapid granulation and stability of aerobic granular sludge (AGS) in a continuous-flow membrane bioreactor (MBR). By operating two identical MBRs with or with no bio-carrier for 125 days, it was found that the combination of multi-ionic matrix and bio-carrier could promote the rapid formation and maintain the long-term stability of AGS. The primary AGS was first observed inside the reactor on day 14, and the mature AGS appeared soon and kept stable for more than 4 months (its average size still was about 800 μm on day 125). Suitable filling ratio of bio-carrier was beneficial to form a stable and regular circulating water flow inside, and adding divalent metal ions quickly reduced the negative charges of tiny sludge particles, which were two essential factors leading to the rapid granulation of AGS and maintaining its stability. The multi-ionic matrix not only enhanced the biological aggregation process, but also facilitated the expansion of the cultivated AGS into a new multi-habitat system of Mn-AGS, in which, complex microbial communities with rich bio-diversity robustly promoted the efficient removal of organic pollutants and nutrients.

Distinct granulation pathways of aerobic granular sludge under poly aluminum chloride enhancement

Aerobic granular sludge (AGS), a novel strategy for nutrient removal which exhibits compact structure, good settleability, and resilience against high organic load, has been considered as a highly potential wastewater treatment technology. However, the long start-up period for granulation prevented its widespread development. In this study, the distinct pathways of PAC-enhanced AGS granulation were systematically investigated. Four identical sequencing batch reactors (SBR) with different PAC dosages (with 0, 50, 100, 400 mg/L effective Al³⁺ respectively) were applied. It was observed that the presence of PAC accelerated granules formation, promoted mechanical strength as well as denitrification rate of granules, and thus notably enhanced removal efficacies of COD, NH₄⁺-N, NO₂^- and NO₃^-. According to the dissolved oxygen (DO) distribution inside the sludge and the denitrification rate (SDNR) measurements, distinguishing structures of granules under different PAC addition were discovered. Comparatively, AGS under low PAC addition (i.e., 50 mg/L) resulted in the largest granule size, the biggest anaerobic zone and the highest denitrification rate. Presumably, for the system with the low PAC addition (50 mg/L), appropriate aluminum ions (Al³⁺) neutralized part of the negative charge on the microorganism surface, thereby promoting cells aggregation. In contrast, a high dosage of PAC (400 mg/L) induced excessive Al³⁺ absorbed on the cell surface after neutralization, which increased the repulsive force between microorganisms, leading to more cavities and channels existed inside the granules. Therefore, granules under low PAC dosage (i.e., 50 mg/L) presented large anaerobic zone and high denitrification rate, thus favored the best internal structure and nutrients removal efficiencies.

Microbial community structure analysis in a hybrid membrane bioreactor via high-throughput sequencing

Compared to the suspended culture (sludge) of a conventional bioreactor, hybrid membrane bioreactors (HMBRs) are more effective at removing pollutants owing to the addition of fillers addition of fillers that facilitate biofilm formation. However, the microbial community structure and composition in an HMBR remain unclear. We ran three laboratory-scale HMBRs at room temperature under different sludge retention times (SRTs; 10 d, 20 d, and 30 d) to compare the microbial diversity and community structure among the membrane surface, suspended filler surface, and mixed liquor using high-throughput sequencing. The results showed that SRT can markedly affect microbial community structure in the HMBR, and different trends appeared in the three functional units. The largest number of mutual operational taxonomic units was found in the activated sludge mixture and suspended carrier with an SRT of 20 d and 30 d. Species belonging to Proteobacteria in the 10 d SRT group had the greatest contribution to between-group differences. A longer SRT could mitigate membrane fouling by decreasing the relative abundance of Thauera and Sphaerotilus attached on the membrane surface. Comamonadaceae, a family of denitrifying bacteria, offer high denitrification potential for bioreactors operating under a long SRT.

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

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

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.

The impacts of calcium oxide nanoparticles on the anaerobic granule formation: CO2 sequestration and dosing

Lab experiments were conducted to investigate the effects of calcium oxide nanoparticles (CaO NPs) dosing on granule formation, granule development, and carbon oxide sequestration. The results showed that dosing CaO NPs adversely affected granulation due to the formation of precipitates and hydrolyzates with poor settleability. However, the optimal dosage of CaO NPs 4.5 g/l could benefit granule formation and stability by improving the embedded extracellular polymeric substances (EPS) and physical adhesion aggregation leads for CO₂ sequestration. The network of granules like Methanosarcina and in pore size 0.55 mm obtained in the reactor was 6.25 mm in average diameter, had a wet density 46 cm², sludge volume index 0.935 ml/g, and CO₂ sequestration 96.7% at 4.5 g/l CaO NP. The proposed study can provide a good prediction for the growth of granules stable texture in regular, dense, rigid, upper part smooth with below surface rough and granule yield showed CH₄ production 4.6 m³/d and CO₂ sequestration 4.75 l/gVS granules (w/v) granules. This study is a useful tool for studying the growth of granule growth characteristics and the mechanism of anaerobic granules for CO₂ sequestration from wastewater.

Rapid start-up of an aerobic granular sludge system for nitrogen and phosphorus removal through seeding chitosan-based sludge aggregates

This study presents a novel strategy to accelerate the start-up of aerobic granular sludge (AGS) system and ensure the nutrient removal during cultivation. This new method consists of preparing the chitosan-based sludge aggregates outside the reactor and then seeding the reactor with such sludge aggregates. To prepare chitosan-based sludge aggregates, chitosan was dissolved in acetic acid solution acting as a cationic flocculant to bind negatively charged sludge together, and then the dissolved chitosan was in situ precipitated by readjusting pH to form stable sludge aggregates. The chitosan-induced charge neutralization and water-insolubility of chitosan were the two main reasons for the super-rapid formation of chitosan-based sludge aggregates. The as-prepared chitosan-based sludge aggregates had a much lower sludge volume index at 30 min (SVI₃₀) (90.1 mL/g) than the original sludge (SVI₃₀ = 328.0 mL/g). They also had some AGS-like characteristics such as large particle size (1300 μm) and fast settling velocity (23.8 m/h). Consequently, short settling time can be achieved and excessive biomass wash-out can be avoided in the rapid start-up of AGS system with chitosan-based sludge aggregates as inoculant, which was beneficial to accelerating sludge granulation while maintaining nutrient removal. Additionally, the abundances of filamentous bacteria and Candidatus Accumulibacter and the content of extracellular polymeric substances increased during cultivation, which could also contribute to the AGS formation. By seeding chitosan-based sludge aggregates in the anaerobic/oxic sequencing batch reactor, complete granulation was rapidly achieved in 10 days, and good removals of nitrogen and phosphorus was obtained after 14-18 days of cultivation.

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.

A settling model for full-scale aerobic granular sludge

The settling behavior of aerobic granular sludge (AGS) in full-scale reactors is different from the settling of normal activated sludge. Current activated sludge models lack the features to describe the segregation of granules based on size during the settling process. This segregation plays an important role in the granulation process and therefore a better understanding of the settling is essential. The goal of this study was to model and evaluate the segregation of different granule sizes during settling and feeding in full-scale aerobic granular sludge reactors. Hereto the Patwardhan and Tien model was used. This model is an implementation of the Richardson and Zaki model, allowing for multiple classes of particles. To create the granular settling model, the most relevant parameters were identified using aerobic granular sludge from different full-scale Nereda® reactors. The settling properties of individual granules were measured as was the bulk behavior of granular sludge beds with uniform granular sludge particles. The obtained parameters were combined in a model containing multiple granule classes, which then was validated for granular sludge settling in a full-scale Nereda® reactor. In practice a hydraulic selection pressure is used to select for granular sludge. Under the same hydraulic selection pressure the model predicted that different stable granular size distributions can occur. This indicates that granular size distribution control would need a different mechanism then the hydraulic selection pressure alone. This model can be used to better understand and optimize operational parameters of AGS reactors that depend on granular sludge size, like biological nutrient removal. Furthermore insights from this model can also be used in the development of continuously fed AGS systems.

Successful granulation and microbial differentiation of activated sludge in anaerobic/anoxic/aerobic (A2O) reactor with two-zone sedimentation tank treating municipal sewage

A continuous pilot-scale A²O reactor with a two-zone sedimentation tank (A²O-TST) was constructed for the formation of aerobic granular sludge (AGS) to treat real municipal sewage. The characteristics of sludge, nutrient removal performance and the corresponding microbial ecology dynamics were studied during granulation process. Experimental results indicated that AGS with a mean particle size of 210 μm and sludge volume index after 30 min of 47.5 mL/g was successfully formed with effluent COD, total nitrogen and total phosphorus concentrations in the reactor reaching 22.8, 3.5 and 0.2 mg/L, respectively. Furthermore, high throughput data indicated that granules in settling tank-1 (ST-1) harbored slow-growing autotrophic organisms like Nitrosomonas and Nitrospira, while the flocs in settling tank-2 (ST-2) were dominated by fast-growing heterotrophic organisms including Ca. Accumulibacter, Dechloromonas, Flavobacterium, Arcobacter and Halomonas. Simulation results using computational fluid dynamics and discrete element method (CFD-DEM) modeling verified that the selection pressure created by the TST separator contributed to the retention of heavy granules (>1.011 kg/m³ density) in ST-1 zone and the withdrawal of light flocs (<1.011 kg/m³ density) from ST-2 zone. Therefore, the segregation of biomass using the TST system provides an opportunity to select for desired microbial populations and to optimize the nitrogen and phosphorus removal performance of the A²O-TST reactor. This study could add a guiding sight into the application of two-sludge system based on AGS technology for upgrading traditional A²O process.

Elucidating sludge characteristic, substrate transformation and microbial evolution in a two-sludge denitrifying phosphorus removal system under the impact of HRT

Granule formation has been recognized as a promising biotechnology in denitrifying phosphorus removal (DPR) systems by facilitating phosphorus accumulation organisms (PAOs) especially denitrifying PAOs (DPAOs), and hydraulic selection made this a more difficult task in continuous operation. This study aimed at exploring the microscopic mechanism and putting forward an effective strategy for DPR granulation under the impact of hydraulic retention time (HRT) (12 h, 10 h, 8 h) in a novel Anaerobic Anoxic Oxic - Moving Bed Biofilm Reactor (A²/O - MBBR) system. With the reduction of intracellular carbon storage (COD_intra) efficiency (88.58%-78.53%), nitrogen (N) (85.45%-79.11%) and phosphorus (P) (96.55%-92.47%) removals both dropped, but it exhibited a growth of anoxic phosphorus uptake rate (PUR_A) (3.79-5.68 mg P/(gMLVSS·h)). The batch tests associating with substrate transformation of poly-β-hydroxyalkanoates (PHA), glycogen (Gly) agreed well with the corresponding stoichiometry of phosphorus release rate (PRR) (4.83-7.53 mg P/(gMLVSS·h)), PUR_A (3.55-5.43 mg P/(gMLVSS·h)), oxic phosphorus uptake rate (PUR_O) (6.08-6.21 mg P/(gMLVSS·h)), and DPAOs/PAOs ratios (57.17%-89.31%), indicating a shift of microbial community. DPR granules gradually stabilized with low sludge volume index (SVI₅/SVI₃₀ ratio = 1.1-1.2), dense and compact structure, higher P content (11.63%), more extracted extracellular polymeric substances (EPS) (111.40-160.31 mg/gMLVSS) as proteins/polysaccharides (PN/PS) ratios (1.70-3.47) increased, leading to better sludge settleability and cell hydrophobicity. Fluorescence in situ hybridization (FISH) results showed that PAOs (mainly Cluster I: 20.20%) were the dominant bacteria in the A²/O reactor although a small amount of Defluviicoccus (3.18-3.48%) was responsible for nitrite accumulation, while ammonium-oxidizing bacteria (AOB) (mainly Nitrosomonas: 10.75%) and nitrite-oxidizing bacteria (NOB) (mainly Nitrospira: 15.06%) were enriched in the MBBR.

Rapid aerobic sludge granulation in an integrated oxidation ditch with two-zone clarifiers

Aerobic granular sludge (AGS) was rapidly cultivated in an integrated oxidation ditch with two-zone clarifiers by using a novel external sludge treatment and return mode to treat low concentrations of actual domestic sewage. The selective pressure created by the two-zone clarifiers can retain the well-settling granules and discharge light flocs with poor settleability. The granules stayed in the reactor, which induced bacterial attachment to the granules that acted as nuclei, while the discharged flocs can stimulate microorganisms to secrete large amounts of extracellular polymeric substances (EPS) under the external conditioning of CaCl₂ and natural air drying. Then, this surplus sludge was returned to the reactor to create more small granules that combined with each other through the action of hydraulic shear forces to achieve rapid granulation. The results showed that AGS was formed successfully in the reactor on day 18, and after 51 days of continuous operation, the biomass concentration and settling ability were further improved (the mixed liquor suspended solids (MLSS) and sludge volume index at 5 min (SVI₅) were stable at approximately 3500 mg/L and 40.0 mL/g, respectively). During the whole experimental period, the biological sludge activity was greatly improved, and the EPS and microbial community changed significantly, including an enrichment of microbes with EPS secretion and granule stabilization functions. The study results reveal that the pollutant removal efficiency improved after granulation. Furthermore, this approach required less energy and is eco-friendly for potential full-scale implementation.

Development of a dynamic feeding strategy for continuous-flow aerobic granulation and nitrogen removal in a modified airlift loop reactor for municipal wastewater treatment

This study investigated the aerobic sludge granulation and nitrogen removal performance in a modified airlift loop reactor treating municipal wastewater under different operation conditions. Dynamic feeding and aeration control were applied to create feast/famine conditions to facilitate microbial aggregation. Experimental results demonstrated that aerobic granular sludge could be cultivated in continuous-flow reactors fed with an optimized dynamic feeding condition. Fresh granules sizing 0.4-0.6 mm were observed in the reactors after a 61-day operation, then turned to matured granules after another 33-day operation with a compact structure, a stable size of 2-4 mm, and a low SVI of ~35 mL/g. Extracellular polymeric substances (EPS) analysis results showed that both EPS contents and the ratio of protein to polysaccharides increased with the granulation process, leading to an increase of cell hydrophobicity. Granular sludge exhibited a good nitrogen removal ability with a comparable level of specific nitrification rate and denitrification rate with those measured in state-of-the-art sequential batch reactors. Microbial population analysis showed an increase in the relative abundance of functional microbes, including Zoogloea, Nitrospira, Dechloromonas, and Thauera in the cultivated granules, suggesting a potentially crucial role of these microbes in sludge granulation and nitrogen removal. The dynamic feeding strategy and the reactor configuration are considered as critical factors for aerobic granulation under continuous-flow conditions for creating feast/famine conditions and allow sludge backflow without structure damage.

Startup and stable operation of advanced continuous flow reactor and the changes of microbial communities in aerobic granular sludge

In this study, the granular sludge was operated under low aeration condition in sequencing batch reactor (SBR) and advanced continuous flow reactor (ACFR), respectively. Through increasing the sludge retention time (SRT) from 22 days to 33 days, the ACFR was successful startup in 30 days and achieved long term stable operation. Under SBR operation condition, the aerobic granular sludge (AGS) showed good nitrogen (60%), phosphorus (96%) and COD removal performance. During stable operation of continuous-flow, the nitrogen removal efficiency was increasing to 70%, however, the phosphorus removal efficiency could only be restored to 65%. Meanwhile, the sludge discharge volume from ACFR was about half of that in SBR. Results of high-throughput pyrosequencing illustrated that methanogenic archaea (MA), ammonia oxidizing archaea (AOA), denitrifying bacteria (DNB), denitrifying polyphosphate-accumulating organisms (DPAOs) played an important role in the removal of nutrients in ACFR. This study could have positive effect on the practical application of AGS continuous flow process for simultaneous biological nutrient removal (SBNR).

Improving aerobic sludge granulation in sequential batch reactor by natural drying: Effluent sludge recovery and feeding back into reactor

One of the main problems in treating high volumes of wastewater is the long startup time required aerobic granular sludge (AGS), and this issue significantly limits the broad application of advanced AGS technology. To promote rapid AGS formation in the startup phase, a method was developed involving the recovery and natural drying of effluent sludge prior to feeding it back into the sequencing batch reactor (SBR). An analysis of the process shows that supplemented naturally dried sludge swiftly promoted sludge aggregation and granular sludge formation in the reactor, and feeding the SBR with naturally dried sludge aggregates (1.75 ± 0.05 g/L seven times) significantly shortened the granulation time in the startup phase by 14 days. In addition, MLSS, SVI₃₀, SVI₃₀/SVI₅, and the average granule size of AGS in the reactor were maintained at 4.66 g/L, 47.4 mL/g, 0.93, and 2.8 mm, respectively. When fed back into the bioreactor, the aggregates acted as nuclei/carriers in the rapid granulation and played a significant role in rendering the SBR operation stable. This approach could be used to eliminate the random granules aggregation-disintegration mechanism that occurs in the initial stage of AGS formation. The study results reveal that the removal rate of COD and NH₄⁺-N were above 95% and 96%, respectively. Furthermore, this approach requires less energy and significantly reduces the amount of sludge produced (as the effluent sludge is reused).

Exposure to polyamide 66 microplastic leads to effects performance and microbial community structure of aerobic granular sludge

Microplastic polyamide 66 (PA66) was used to explore its mechanism of influence on the contaminants removal from aerobic granular sludge (AGS) and the corresponding change to the microbial community. Results showed that the removal pollution efficiency of the experimental groups with PA66 were inhibited during the early treatment stage. However, as the experiment progressed, the removal efficiencies of chemical oxygen demand (COD) (92.66%, 93.10%, 93.11%, 93.79%) and ammonia nitrogen (94.25%, 94.58%, 95.61%, 94.73%) were similar in the addition 0 g/L (A), 0.1 g/L (B), 0.2 g/L (C) and 0.5 g/L (D) PA66 beakers at the last 10 days. On the first day, the intensity of fluorescence peaks representing tryptophan protein-like and aromatic protein-like substances of loosely-bound extracellular polymeric substances (LB-EPS) indicated that the PA66 microplastic caused damage to the sludge structure, and the intensity of fluorescence peaks representing fulvic acid-like and humic acid-like substances were stronger than those in the control beaker (A). Microbial community analysis showed that the main phyla were Firmicutes (49.11%, 59.77%, 44.33%, 41.21%), Proteobacteria (26.44%, 11.96%, 31.44%, 19.4%) and Bacteroidetes (9.24%, 13.05%, 11.89%, 14.71%) in the four beakers. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, genes representing [T] Signal transduction mechanisms illustrated that adding PA66 microplastic resulted in more signaling molecules in the AGS.

Promoting the granulation process of aerobic granular sludge in an integrated moving bed biofilm-membrane bioreactor under a continuous-flowing mode

This investigation demonstrated that aerobic granular sludge (AGS) could be cultivated rapidly in a single continuous-flowing membrane bioreactor (MBR) by introducing freely moved bio-carriers with a filling ratio of 10%. By operating the bioreactor for 28 days, AGS was successfully cultivated and kept stable for >2 months with a compact structure and clear shape, in which, extracellular polymeric substances played a key role in maintaining the stability of granular sludge structure. The microbial composition between AGS and attached biofilm was quite different, which indicated that the introduced bio-carriers improved the biodiversity within the bioreactor. Additionally, an explicit internal circulation was formed by the introduced bio-carriers, which was the main reason leading to the rapid formation of AGS. This is an interesting discovery and a novel approach to promote the rapid granulation of biomass in an MBR. Moreover, combining the biodegradation of AGS and filtration of membrane module, the bio-reactor achieved an excellent performance in removing COD_Cr (>90%) and TN (>85%) during the whole process.

Application of aerobic granules-continuous flow reactor for saline wastewater treatment: Granular stability, lipid production and symbiotic relationship between bacteria and algae

In this study a continuous flow reactor (CFR) was employed to compare the feasibility of bacterial aerobic granular sludge (AGS-CFR) and algal-bacterial granular sludge (ABGS-CFR) for treating 1-4% saline wastewater. High salinity was found to enhance algae growth in ABGS-CFR, which exhibited slightly higher total nitrogen and phosphorus removal efficiencies at 1-3% salinity. ABGS-CFR maintained good granular stability at 1-4% salinity, while AGS-CFR gradually disintegrated at 4% salinity with 39.3% less accumulation of alginate-like exopolysaccharides in the extracellular polymeric substances. Indole-3-acetic acid (IAA) and superoxide dismutase (SOD) analysis suggested that bacteria and algae (Nitzschia) in ABGS-CFR formed a good symbiotic relationship under high salinity conditions, achieving rapid algae growth and 2 times lipid production. High salinity was conducive to enriching Halomonas and Nitzschia but unfavorable for Nitrosomonas and Flavobacterium. Results from this study could provide useful information on interactions between bacteria and algae in ABGS-CFR for its future practical application.

Effect of aeration modes on simultaneous nitrogen and phosphorus removal and microbial community in a continuous flow reactor with granules

In this study, a continuous flow reactor with simultaneous nitrification, denitrification and phosphorus removal (SNDPR) granular sludge was operated in the continuous aeration (CA) and intermittent aeration (IA) modes to examine the effect of aeration on the performance of continuous-flow system. Then the experimental results showed that the IA₁ mode (4 h aeration and 1 h non-aeration) could improve the simultaneous nitrogen and phosphorus removal and the settleability of granules in continuous flow system. Results of high-throughput pyrosequencing illustrated that the methanogens, AOA, ANAMMOX, DNB, denitrifying polyphosphate-accumulating organisms (DPAOs) were the important participant of simultaneous biological nutrients removal (SBNR), meanwhile, the IA₁ mode could effectively inhibit the growth of filamentous microorganisms (Thiothrix and Acinetobacter). Finally, a conceptual model of the SNDPR granular microbial ecosystem under IA₁ mode was proposed as a base for analyzing the mechanism of simultaneous nutrient removal in continuous flow system.

Insight into the fenton-induced degradation process of extracellular polymeric substances (EPS) extracted from activated sludge

Although EPS in microbial aggregates are importance in successful implementation of biological wastewater treatment systems, they also exhibit detrimental role on certain circumstance, such as excess sludge dewatering. Extensive efforts have been put into the disruption of EPS for improving the dewaterability of excess sludge and Fenton's reagent treatment has been demonstrated to be a very promising sludge conditioning method for EPS destruction. However, the information regarding detailed degradation process of EPS during Fenton's reagent treatment is limited. In this study, EPS were extracted from activated sludge and treated with different concentrations of Fenton's reagent. The physicochemical characteristic changes of EPS under different treatment were investigated in terms of components, EEM, molecular weight (MW), UV-Vis and FTIR. The results showed that EPS were prone to be disintegrated, but hard to be fully mineralized. Humic substances in EPS were more resistant to Fenton's reagent than other components. Low MW components of EPS were preferentially degraded prior to the disruption of high MW components. Besides, the disintegration of EPS into lower MW ones was accompanied by the formation of higher MW compounds caused by the bridge interaction of Fe ions. The cleavage of protein's backbone in EPS was mainly through destruction of amide II (N-H and C-N) in -CO-NH-. Fenton's reagent treatment also led to a significant increase of oxygen-containing functional groups in EPS molecules. This paper may pave a path to deeply understand the mechanisms of dewatering improvements of excess sludge by Fenton's conditioning.

Aerobic granular sludge operation and nutrients removal mechanism in a novel configuration reactor combined sequencing batch reactor and continuous-flow reactor

A novel aerobic granular sludge (AGS) system called SBR (sequencing batch reactor)-CF (continuous-flow) system merging the advantages of sequencing batch reactors (SBRs) and continuous flow (CF) reactors was developed. The AGS was successfully operated in the SBR-CF system which consisted of four same SBRs (each served as settling tank/anaerobic feeding tank/aerobic reacting tank in turn). The effects of aeration intensity and hydraulic retention time (HRT) on the SBR-CF system were studied. The results showed strong aeration intensity (9.74 h^-1 in this study) and appropriate HRT (9 h in this study) were more favorable to the nutrients removal. The EEM-PARAFAC analysis was applied to characterize the LB-EPS, TB-EPS and domestic wastewater, as results TB-EPS was found play an important role in the biosorption in COD removal of the SBR-CF system. In addition, a preliminary conceptual reaction process model in the SBR-CF system was built using high-throughput pyrosequencing and phylogenetic assignment.

Rapid control of activated sludge bulking and simultaneous acceleration of aerobic granulation by adding intact aerobic granular sludge

The feasibility of rapidly controlling activated sludge bulking and accelerating aerobic sludge granulation was evaluated by adding intact aerobic granular sludge (AGS) to the bulking activated sludge (BAS) reactor. Two ratios of AGS to BAS (0.2 in the first reactor (R1), and 0.4 in the second reactor (R2)) were tested. The results indicate that the addition of AGS immediately improved the settling ability of BAS (sludge volume index at 30 min (SVI₃₀) in R1 and R2 decreased from 173.1 mL/g to 130.8 and 91.3 mL/g, respectively) and gradually increased the biomass concentration (mixed liquor suspended solids (MLSS) in R1 and R2 increased to 4722 and 5190 mg/L, respectively), thus resolving the sludge bulking problem. Meanwhile, adding AGS not only promoted the BAS growth in aggregates, but also facilitated the selection of well-settling aggregates at an early stage. Consequently, the granulation process was significantly accelerated. The granulation time in R1 and R2 was 14 and 10 days, respectively, indicating that the higher ratio of AGS to BAS can result in the faster granulation. Partial nitrification could be maintained during the BAS granulation process when the initial inoculation of nitritation sludge was large enough. Additionally, the microbial community changed during the BAS granulation process. The genera Thauera and Zoogloea belonging to family Rhodobacteraceae were speculated to play an important role in the BAS granulation.

Comparison of aerobic granulation in SBR and continuous-flow plants

Up to now, aerobic granulation of activated sludge is only realised in SBRs, where the discontinuous feed and sedimentation allow the formation of dense granules with excellent settling properties. However, aerobic granulation in continuous-flow plants (CFP) is gaining more and more interest in order to exploit the advantages of these excellent sludge properties to construct compact and efficient WWTP. Within the scope of this project, a SBR and CFP were operated in parallel to investigate the aerobic granulation of activated sludge and to compare the biomass in terms of their structure and settling behavior. CFP operation included two experimental phases with different reactor designs. The use of synthetic wastewater during phase I led to a biomass with a SVI of 42 ml g^-1, whereby the SVI declined only to 85 ml g^-1 in the second phase and the use of municipal sewage. After the start-up period, microscopic images of the biomass from CFP comprised small compact granules with a high flocculent fraction. Particle size distribution for phase II confirm, that 72% of the particles had a size over 200 μm. A strong correlation was observed between the appearance of NO_x-N in the first reactor and the SVI. The results illustrate, that the anaerobic conditions during feeding are essential to keep stable granules.

Determining the effects of aeration intensity and reactor height to diameter (H/D) ratio on granule stability based on bubble behavior analysis

Aerobic granular sludge was considered as a leading wastewater technology in the next century. However, the loss of granule stability limited the application of this promising biotechnology. Increasing aeration intensity and height to diameter (H/D) ratio were conventional strategies to enhance granule stability. In this study, hydraulic effects of aeration intensity and H/D ratio were explored basing on bubble behavior analysis. However, results revealed that due to viscous resistance, increasing aeration intensity and H/D ratio had limited effects on enhancing hydraulic shear stress, not to mention the extra operation and construction cost. A deflector component was further applied to regulate hydraulic shear stress on large granules under low aeration intensity and H/D ratio. Hydraulic shear stress of large granules was constantly around 3.0 times higher than that in the conventional reactor, resulting in higher percentage of granules within optimal size range (81.95 ± 5.13%). A high abundance of denitrifying bacteria was observed in reactors, which led to high TN removal efficiency of 88.6 ± 3.8%.

The characteristics of heat-driven ammonium adsorption in aerobic granular sludge

Adsorption is an important step during the migration of ammonium from the aqueous phase to biomass in biological nitrogen removal processes. A deeper understanding of the adsorption mechanisms is encouraged in constructing nitrogen conversion models. In this study, the ammonium adsorption in aerobic granular sludge was investigated at different conditions. Analysis of kinetic data indicated that ammonium adsorption was a fast process and followed pseudo-second-order kinetics (adsorption rate constant k₂ was between 0.031 and 0.065 g/(mg · min)). The maximum adsorption capacity and half saturation constant K_L in the Langmuir isotherm model were 4.95 mgNH₄ ⁺-N/g total suspended solids and 0.0126 L/mg, respectively. Effects of environmental conditions such as temperature, pH and competitive cations were also estimated. The optimum pH was 7 and the effects of competitive cations were in the order Ca²⁺ > Mg²⁺ > K⁺ > Na⁺. Values of thermodynamic parameters (ΔH^Ɵ = -14.697 kJ/mol, ΔS^Ɵ = -6.65 J/(mol · K)) indicated that the adsorption process was spontaneous and exothermic. Desorption tests showed that the process was reversible and low temperature had a negative effect on ammonium desorption. These findings could be useful for completing the mathematical model of the nitrogen removal process in bioreactors.

Novel system configuration with activated sludge like-geometry to develop aerobic granular biomass under continuous flow

A novel continuous flow system with "flat geometry" composed by two completely mixed aerobic tanks in series and a settler was used to promote the formation of aerobic granular sludge. Making similarities of this system with a typical sequencing batch reactor (SBR), for aerobic granules cultivation, the value of the tank 1/tank 2 vol ratio and the biomass recirculation rate would correspond with the feast/famine length ratio and the length of the operational cycle, respectively, while the settler upflow liquid velocity imposed would be related to the settling time. From the three experiments performed the best results were obtained when the tank 1/tank 2 vol ratio was of 0.28, the sludge recycling ratio of 0.25 and the settler upflow velocity of 2.5 m/h. At these conditions the aggregates had settling velocities between 29 and 113 m/h, sludge volume index at 10 min (SVI₁₀) of 70 mL/g TSS and diameters between 1.0 and 5.0 mm.

Nitrifying aerobic granular sludge fermentation for releases of carbon source and phosphorus: The role of fermentation pH

The effect of fermentation pH (uncontrolled, 4 and 10) on the releases of carbon source and phosphorus from nitrifying aerobic granular sludge (N-AGS) was investigated. Meanwhile, metal ion concentration and microbial community characterization were explored during N-AGS fermentation. The results indicated that N-AGS fermentation at pH 10 significantly promoted the releases of soluble chemical oxygen demand (SCOD) and total volatile fatty acids (TVFAs). However, SCOD and TVFA released from N-AGS were inhibited at pH 4. Moreover, acidic condition promoted phosphorus release (mainly apatite) from N-AGS during anaerobic fermentation. Nevertheless, alkaline condition failed to increase phosphorus concentration due to the formation of chemical-phosphate precipitates. Compared with the previously reported flocculent sludge fermentation, N-AGS fermentation released more SCOD and TVFAs, possibly due to the greater extracellular polymeric substances content and some hydrolytic-acidogenic bacteria in N-AGS. Therefore, N-AGS alkaline fermentation facilitated the carbon source recovery, while N-AGS acidic fermentation benefited the phosphorus recovery.

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

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

State of the art of aerobic granulation in continuous flow bioreactors

In the wake of the success of aerobic granulation in sequential batch reactors (SBRs) for treating wastewater, attention is beginning to turn to continuous flow applications. This is a necessary step given the advantages of continuous flow treatment processes and the fact that the majority of full-scale wastewater treatment plants across the world are operated with aeration tanks and clarifiers in a continuous flow mode. As in SBRs, applying a selection pressure, based on differences in either settling velocity or the size of the biomass, is essential for successful granulation in continuous flow reactors (CFRs). CFRs employed for aerobic granulation come in multiple configurations, each with their own means of achieving such a selection pressure. Other factors, such as bioaugmentation and hydraulic shear force, also contribute to aerobic granulation to some extent. Besides the formation of aerobic granules, long-term stability of aerobic granules is also a critical issue to be addressed. Inorganic precipitation, special inocula, and various operational optimization strategies have been used to improve granule long-term structural integrity. Accumulated studies reviewed in this work demonstrate that aerobic granulation in CFRs is capable of removing a wide spectrum of contaminants and achieving properties generally comparable to those in SBRs. Despite the notable research progress made toward successful aerobic granulation in lab-scale CFRs, to the best of our knowledge, there are only three full-scale tests of the technique, two being seeded with anammox-supported aerobic granules and the other with conventional aerobic granules; two other process alternatives are currently in development. Application of settling- or size-based selection pressures and feast/famine conditions are especially difficult to implement to these and similar mainstream systems. Future research efforts needs to be focused on the optimization of the granule-to-floc ratio, enhancement of granule activity, improvement of long-term granule stability, and a better understanding of aerobic granulation mechanisms in CFRs, especially in full-scale applications.