Optimizing granules size distribution for aerobic granular sludge stability: Effect of a novel funnel-shaped internals on hydraulic shear stress

Evaluation of the shear stability of aerobic granular sludge from a pilot-scale membrane bioreactor: Establishment of a quantitative method

This work established a quantitative method to access the shear stability of aerobic granular sludge (AGS) and validated its feasibility by using the mature AGS from a pilot-scale (50 tons/day) membrane bioreactor (MBR) for treating real municipal wastewater. The results showed that the changing rate (ΔS) of the peak area (S) of granule size distribution (GSD) exhibited an exponential relationship (R2≥0.76) with the shear time (y=a-b·cx), which was a suitable indicative index to reflect the shear stability of different AGS samples. The limiting granule size (LGS) was defined and proposed to characterize the equilibrium size for AGS after being sheared for a period of time, whose value in terms of Dv50 showed high correlation (R2=0.92) with the parameter a. The free Ca2+ (28.44-34.21 mg/L) in the influent specifically interacted with polysaccharides (PS) in the granule's extracellular polymeric substance (EPS) as a nucleation site, thereby inducing the formation of Ca precipitation to enhance its Young's modulus, while Ca2+ primarily interacted with PS in soluble metabolic product (SMP) during the initial granulation process. Furthermore, the Young's modulus significantly affected the parameter a related to shear stability (R2=0.99). Since the parameter a was more closely related (R2=1.00) to ΔS than that of the parameter b or c, the excellent correlation (R2=0.99) between the parameter a and the wet density further verified the feasibility of this method.

Influence of residual anaerobic granular sludge (AnGS) from anaerobically digested molasses wastewater in aerobic granular sludge reactor

This study investigated the impact of residual anaerobic granular sludge (AnGS) from anaerobic digesters treating molasses wastewater on ammonium reduction in a downstream aerobic granular sludge (AGS) reactor. Two conditions were tested: raw (high AnGS concentration) and settled (low AnGS concentration) anaerobically digested molasses wastewaters were fed into the AGS reactor. With the introduction of raw wastewater, enhanced nitrite accumulation at 30 % and improved total inorganic nitrogen (TIN) removal at 11 % were observed compared to 1 % nitrite accumulation and 8 % TIN removal with the introduction of settled wastewater. However, AnGS adversely affected other aspects of reactor performance, increasing effluent solid content and decreasing soluble chemical oxygen demand removal efficiency from 20 % in the low AnGS condition to 11 % in the high AnGS condition. Despite the observed retention of AnGS in the reactor, no significant bioaugmentation effects on the microbial community of the AGS were observed. Aerobic granular sludge was consistently observed in both conditions. The study suggests that AnGS may act as a nucleus for granule formation, helping to maintain granule stability in a disturbed environment. This study offers a systematic understanding of the impact of AnGS on subsequent nitrogen removal process using AGS, aiding in the decision making in the treatment of high solid anaerobic digestate.

Deciphering the pathogenic risks of microplastics as emerging particulate organic matter in aquatic ecosystem

Microplastics are accumulating rapidly in aquatic ecosystems, providing habitats for pathogens and vectors for antibiotic resistance genes (ARGs), potentially increasing pathogenic risks. However, few studies have considered microplastics as particulate organic matter (POM) to elucidate their pathogenic risks and underlying mechanisms. Here, we performed microcosm experiments with microplastics and natural POM (leaves, algae, soil), thoroughly investigating their distinct effects on the community compositions, functional profiles, opportunistic pathogens, and ARGs in Particle-Associated (PA) and Free-Living (FL) bacterial communities. We found that both microplastics and leaves have comparable impacts on microbial community structures and functions, enriching opportunistic pathogens and ARGs, which may pose potential environmental risks. These effects are likely driven by their influences on water properties, including dissolved organic carbon, nitrate, DO, and pH. However, microplastics uniquely promoted pathogens as keystone species and further amplified their capacity as hosts for ARGs, potentially posing a higher pathogenic risk than natural POM. Our research also emphasized the importance of considering both PA and FL bacteria when assessing microplastic impacts, as they exhibited different responses. Overall, our study elucidates the role and underlying mechanism of microplastics as an emerging POM in intensifying pathogenic risks of aquatic ecosystems in comparison with conventional natural POM.

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.

Smaller Aerobic Granules Significantly Reduce N2O Production by Ammonia-Oxidizing Bacteria: Evidences from Biochemical and Isotopic Analyses

The mitigation of nitrous oxide (N2O) is of primary significance to offset carbon footprints in aerobic granular sludge (AGS) systems. However, a significant knowledge gap still exists regarding the N2O production mechanism and its pathway contribution. To address this issue, the impact of varying granule sizes, dissolved oxygen (DO), and nitrite (NO2-) levels on N2O production by ammonia-oxidizing bacteria (AOB) during nitrification in AGS systems was comprehensively investigated. Biochemical and isotopic experiments revealed that increasing DO or decreasing NO2- levels reduced N2O emission factors (by 13.8 or 19.5%) and production rates (by 0.08 or 0.35 mg/g VSS/h) via weakening the role of the AOB denitrification pathway since increasing DO competed for more electrons required for AOB denitrification. Smaller granules (0.5 mm) preferred to diminish N2O production via enhancing the role of NH2OH pathway (i.e., 59.4-100% in the absence of NO2-), while larger granules (2.0 mm) induced conspicuously higher N2O production via the AOB denitrification pathway (approximately 100% at higher NO2- levels). Nitrifying AGS systems with a unified size of 0.5 mm achieved 42% N2O footprint reduction compared with the system with mixed sizes (0.5-2.0 mm) under optimal conditions (DO = 3.0 mg-O2/L and NO2- = 0 mg-N/L).

Retention and recycling of granules in continuous flow-through system to accomplish denitrification and perchlorate reduction

This study employed a completely anoxic reactor and a gravity-settling design for continuously separating from flocculated biomass and hydraulically recycling granules back to the main reactor. The average chemical oxygen demand (COD) removal in the reactor was 98 %. Average nitrate (NO₃^--N) and perchlorate (ClO₄^-) removal efficiencies of 99 % and 74 ± 19 % were observed, respectively. Preferential utilization of NO₃^- over ClO₄^- led to COD limiting conditions, which resulted in ClO₄^- in the effluent. The average granule diameter in continuous flow-through bubble-column (CFB) anoxic granular sludge (AxGS) bioreactor was 6325 ± 2434 µm, and the average SVI₃₀/SVI₁ was > 90% throughout its operation. 16s rDNA amplicon sequencing revealed Proteobacteria (68.53% - 88.57%) and Dechloromonas (10.46% - 54.77%) to be the most abundant phylum and genus present in reactor sludge representing the denitrifying and ClO₄^- reducing microbial community. This work represents a pioneering development of CFB-AxGS bioreactor.

Effects of superficial gas velocity on the performance of an air-lift internal circulation partial nitrification-anammox granular sludge reactor

The airlift internal circulation reactor for partial nitrification-anammox (PNA-ALR) has the advantages of a small footprint, high mass transfer efficiency, and the ease of formation of granular sludge, thus making it an effective biological treatment for ammonia-containing wastewater. Although superficial gas velocity (SGV) is an essential parameter for PNA-ALR, it is unclear how the magnitude of SGV impacts nitrogen removal performance. In this study, the nitrogen removal efficiencies of five PNA-ALRs with different SGV were measured during feeding with synthetic municipal wastewater. At an optimal SGV of 2.35 cm s-1, the PNA-ALR consistently maintained the total inorganic nitrogen (TIN) removal efficiency at 76.31% and the effluent TIN concentration was less than 10 mg L-1. By increasing or decreasing the SGV, the nitrogen removal efficiency decreased to a range between 30% and 50%. At lower SGV, the dead space in the PNA-ALR was increased by 21.15%, and the feast/famine ratio of sludge increased to greater than 0.5, which caused a disruption in the structure, and a large loss of, granular sludge. Computational fluid dynamics (CFD) simulations showed operation at a higher SGV, resulting in excessive shear stress of 3.25 N m-2 being generated from bubble rupture in the degassing section. Fluorescent staining determined a decrease of 26.5% in viable bacteria. These results have improved our understanding of the effects of SGV on a PNA-ALR during mainstream wastewater treatment.

Formation, application, and storage-reactivation of aerobic granular sludge: A review

It was an important discovery in wastewater treatment that the microorganisms in the traditional activated sludge can form aerobic granular sludge (AGS) by self-aggregation under appropriate water quality and operation conditions. With a typical three-dimensional spherical structure, AGS has high sludge-water separation efficiency, great treatment capacity, and strong tolerance to toxic and harmful substances, so it has been considered to be one of the most promising wastewater treatment technologies. This paper comprehensively reviewed AGS from multiple perspectives over the past two decades, including the culture conditions, granulation mechanisms, metabolic and structural stability, storage, and its diverse applications. Some important issues, such as the reproducibility of culture conditions and the structural and functional stability during application and storage, were also summarized, and the research prospects were put forward. The aggregation behavior of microorganisms in AGS was explained from the perspectives of physiology and ecology of complex populations. The storage of AGS is considered to have large commercial potential value with the increase of large-scale applications. The purpose of this paper is to provide a reference for the systematic and in-depth study on the sludge aerobic granulation process.

Adaptive regulation of activated sludge's core functional flora based on granular internal spatial microenvironment

A great deal of efforts has been put into studying the influence of the external macroenvironment for activated sludge to survive on microbial community succession, while granular internal spatial microenvironment should be given equal attention, because it is more directly involved in the information exchange and material transfer among microorganisms. This study systematically investigated the effects of granular microenvironment on spatial colonization and composition of sludge's core functional flora, and the corresponding difference of biological treatment performance. High content of extracellular-proteins (67.53 mg/gVSS) or extracellular-polysaccharide (65.02 mg/gVSS) stimulated the microbial flocculation and aggregation of 0.5-1.5 mm granules (GS) or 1.5-3.0 mm granules (GM), respectively, which was resulted from excellent cell hydrophobicity (59.26%) or viscosity (3.47 mPa s), therefore, constituted relatively dense porous frame. More hollow space existed in 3.0-5.0 mm granules (GL), which formed loose skeleton with 0.213 mL/g of total pore volume and 17.21 nm of average pore size. Combining scanning electron microscope images and fluorescent in-situ hybridization based microbiological analysis, aerobic nitrifiers were observed to wrap or surround anaerobic bacteria, or facultative/anaerobic bacteria were self-encapsulated, which created granule's unique microenvironment with alternating aerobic and anaerobic zones. GS has the most rich organic matter degrading bacteria and anaerobic heterotrophic denitrifiers, while GM and GL presented the greatest relative abundance of facultative and aerobic denitrifiers, respectively. The activity of dehydrogenase and nitrogen invertase of GM showed be 1.32-3.09 times higher than those of GS and GL, contributing to its higher carbon and nitrogen removal. These findings highlight the importance of granular microenvironment to adaptive regulation of activated sludge's core functional flora and corresponding pollutant removal performance.

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.

Identification of partial denitrification granulation enhanced by low C/N ratio in the aspect of metabolomics and quorum sensing

Partial denitrification granular sludge (PDGS) and denitrification granular sludge (DGS) play an important role in nitrogen removal from wastewater. However, the inherent cause of aggregation capacity related to the ratio of COD to nitrogen (COD/N) is still unclear. In this study, metabolomics analysis was combined with microbiological analyses, granular performance and extracellular polymeric substances (EPS) structure to explore the granulation mechanism at different influent COD/N ratios. The results showed that the higher COD/N ratio selectively enhanced the gluconeogenesis pathway, purine and pyrimidine metabolism pathway, resulting in more extracellular polysaccharide (PS) excretion and floc sludge. The absence of carbon source weakened tricarboxylic acid cycle (TCA) reaction, resulting in NAD⁺ and ADP decrease, nitrite accumulation and change of microbial community structure. The amino acids biosynthesis pathway was enhanced under low COD/N ratio, which promoted the hydrophobicity of EPS. PDGS had stronger Acyl-homoserine lactones (AHLs)-based quorum sensing (QS) than DGS during the operational period. CO8-HSL, C8-HSL and C6-HSL, as the main form of AHLs, played a dominating role in DGS and PDGS. Batch tests illustrated that adding AHLs obviously improved the synthesis of the amino acids, threonine (Thr), tryptophan (Trp), methionine (Met) and glycine (Gly). Dosing AHLs regulated PS synthesis only at a high COD/N ratio. The glucose-6P, glycerate-3p and UDP-Glc were up-regulated only in DSG, which increased the hydrophilic groups in EPS. The results not only provided the new insights into the metabolism of denitrifying granular sludge, but also indicated the application potential of the technologies regarding start-up and operation of granule sludge.

Alternating nitrogen feeding strategy induced aerobic granulation: Influencing conditions and mechanism

Effective cultivation of stable aerobic granular sludge (AGS) is a crucial step in the successful application of this technology, and the formation of AGS could be facilitated by some environmental stress conditions. Four identical sequencing batch reactors (SBRs) were established to investigate the aerobic granulation process under the same alternating ammonia nitrogen feeding strategy superimposed with different environmental conditions (inorganic carbon source, temperature, N/COD). Although various superimposed conditions induced a significant difference in the size, settling velocity, mechanic strength of AGS, mature aerobic granules could be successfully obtained in all four reactors after 70 days' operation, indicating the alternating ammonia nitrogen feeding strategy was the most critical factor for AGS formation. Based on the results of redundancy analysis, the presence of an inorganic carbon source could facilitate the cultivation of AGS with nitrification function, while the moderate temperature and fluctuant N/COD might benefit the cultivation of more stable AGS. In addition, superimposed stress conditions could result in the difference in the microbial population between four reactors, but the population diversity and abundance of microorganisms were not the determinants of AGS formation. This study provided an effective method for the cultivation of AGS by using alternating ammonia nitrogen feeding strategy.

Structural Characteristics of Aerobic Granular Sludge and Factors That Influence Its Stability: A Mini Review

Current extensive research on aerobic granular sludge (AGS) largely focuses on improving its microbial biodiversity, settlement behavior, nitrogen and phosphorus removal efficiency, and shock load resistance. Great challenges that have to be faced are the bottleneck of slow-speed granulation and easy disintegration after granulation, which are key to the extended application of AGS technology. In the present review, the typical morphological structures of AGS are firstly summarized as well as the granulation model hypotheses, and then, we analyze the dominant microflora and their spatial distribution features. The influencing factors on particle structure stability are discussed thereafter on a macro and micro scale. Prospects and future research trends are also discussed based on the current study results for AGS technology.

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

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

An investigation into the optimal granular sludge size for simultaneous nitrogen and phosphate removal

An aerobic granular sludge (AGS) pilot plant fed with a mixture of acetate amended centrate and secondary effluent was used to investigate the optimal granule size range for simultaneous nitrification and denitrification (SND) and ortho-phosphate removal. The anaerobic phase was mixed to understand how AGS will perform if integrated with a continuous flow activated sludge system that cannot feed the influent through the settled sludge bed. Five different granule size fractions were taken from the pilot (operated at DO setpoint of 2mgO₂/L) and each size was subjected to activity tests in a well-controlled lab-scale AGS reactor at four dissolved oxygen (DO) concentrations of 1, 2, 3, and 4 mgO₂/L. The size fractions were: 212 - 600 µm, 600 - 1000 µm, 1000 - 1400 µm, 1400 - 2000 µm, and >2000 µm. The smallest size range (212 - 600 µm) had the highest nitrification and phosphate removal rates at DO setpoints from 1 - 3 mgO₂/L, which was attributed to the higher aerobic volume fraction in small granules and hence a higher abundance of phosphorus accumulating organisms (PAO) and ammonia oxidizing bacteria (AOB). In comparison, large granules (>1000 µm) had 1.4 - 4.7 times lower ammonia oxidation rates than the smallest size range, which aligned with their lower AOB abundance relative to granule biomass. The granules with the highest anoxic volume fraction had the highest abundance of nitrite reductase genes (nir gene) but did not show the highest specific nitrogen removal rate. Instead, smaller granules (212 - 600 and 600 - 1000 µm), which had a lower nir gene abundance, had the highest specific nitrogen removal rates (1.2 - 3.1 times higher than larger granules) across all DO values except at 4 mgO₂/L. At a DO setpoint of 4 mgO₂/L, nitrite production by ammonia oxidation (ammonia monooxygenase) exceeded nitrite reduction by nitrite reductase in granules smaller than 1000 µm, in addition, some denitrifying heterotrophs switched to oxygen utilization in deeper layers hence suppressing denitrification activity. At the DO range of 2 - 4 mg/L, granular size had a greater effect on nutrient removal than DO. Therefore, for AGS developed at an average DO setpoint of 2 mgO₂/L, selecting for size fractions in the range of 212 - 1000 µm and avoiding DO values higher than 3 mgO₂/L can achieve both a higher nitrogen removal capacity and energy savings. This study is the first to investigate the influence of different DO values on SND and biological phosphorus removal performance of different aerobic granular sludge sizes.

Self-flocculation of enriched mixed microalgae culture in a sequencing batch reactor

Microalgae-based biodiesel has gained widespread interest as an alternative energy source. Low-cost microalgae harvesting technologies are important for economically feasible biodiesel production. This study investigated, for the first time, the impact of adaptation period and height to diameter (H/D) ratio of a reactor on the growth and self-flocculation of microalgae, without the addition of bacteria. Six reactors were grouped into three sets of experiments, and each reactor was operated for 30 days at similar operating conditions (volume exchange ratio = 25% and settling time = 30 min). In set 1, two 8-L reactors, H5a (H/D ratio: 5) and H8a (H/D ratio: 8), were operated under batch operation. In set 2, reactors H5b and H8b were operated as sequential batch reactors (SBRs) without an adaptation period. In set 3, the reactors H5c and H8c were operated as SBRs with an adaptation period. The findings showed a threefold improvement in biomass productivity for the higher H/D ratio (H8c) and a reduction in biomass loss for microalgae. The H8c reactor exhibited 95% settling efficiency within 5 days, in comparison to 30 days for the H5c reactor. This study demonstrated that a higher H/D ratio and the introduction of an adaptation period in SBR operation positively influences growth and self-flocculation of enriched mixed microalgae culture.

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

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

Use of sodium chloride to rapidly restore polyhydroxyalkanoates production from filamentous bulking without polyhydroxyalkanoates productivity impairment

Occurrence of filamentous bulking would lead to excess biomass washout to fail the polyhydroxyalkanoate (PHA) production process. A strategy using sodium chloride (NaCl) addition to enhance sludge settleability so that high PHA productivity could be retained was tested in two sequencing batch reactors (SBR #1 and #2) fed with different organic acid mixtures but at identical NaCl gradients (0, 5 and 10 g/L). Significant improved sludge settleability was observed at 10 g/L NaCl regardless of occurrence of filamentous bulking. The NaCl addition strategy suppressed the growth of principal filamentous bacterium, Meganema, and enriched the PHA producers (Paracoccus and Thauera) to reach enhanced PHA productivities of 0.244-0.298 g/L^-1 d^-1 for the present studied system.

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

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

Full-scale membrane bioreactor process WWTPs in East Taihu basin: Wastewater characteristics, energy consumption and sustainability

Based on the collection and analysis of essential data from wastewater treatment plants (WWTPs) in recent ten years, the work provides the wastewater characteristics and energy consumption performance in full-scale membrane bioreactor (MBR) process in East Taihu basin, China. High-quality effluent was achieved although the influent carbon source was not beneficial to total nitrogen and total phosphorus removal. The average specific energy consumption (SEC) was 0.52 kWh/m³, which was remarkably lower than that of full-scale MBR process in developed countries, however, the average SEC value was higher than that of conventional activated sludge (CAS) process in China. In addition, the SEC value was largely reduced in 2018, and the regulation of suction pump and aeration mode were considered as the main control methods. Energy consumption will increase along with the influent volume, while the elevation of COD and NH₄⁺-N reduction will bring about relatively low energy consumption. Furthermore, sustainability index was established to comprehensively evaluate the performance of full-scale MBR process, meaning that with relatively low permeate ratio of effluent, full-scale MBR process presented to be inferior to CAS process in sustainability and not feasible to be applied in the upgradation and construction of WWTPs.

Growth and Break-Up of Methanogenic Granules Suggests Mechanisms for Biofilm and Community Development

Methanogenic sludge granules are densely packed, small, spherical biofilms found in anaerobic digesters used to treat industrial wastewaters, where they underpin efficient organic waste conversion and biogas production. Each granule theoretically houses representative microorganisms from all of the trophic groups implicated in the successive and interdependent reactions of the anaerobic digestion (AD) process. Information on exactly how methanogenic granules develop, and their eventual fate will be important for precision management of environmental biotechnologies. Granules from a full-scale bioreactor were size-separated into small (0.6-1 mm), medium (1-1.4 mm), and large (1.4-1.8 mm) size fractions. Twelve laboratory-scale bioreactors were operated using either small, medium, or large granules, or unfractionated sludge. After >50 days of operation, the granule size distribution in each of the small, medium, and large bioreactor sets had diversified beyond-to both bigger and smaller than-the size fraction used for inoculation. Interestingly, extra-small (XS; <0.6 mm) granules were observed, and retained in all of the bioreactors, suggesting the continuous nature of granulation, and/or the breakage of larger granules into XS bits. Moreover, evidence suggested that even granules with small diameters could break. "New" granules from each emerging size were analyzed by studying community structure based on high-throughput 16S rRNA gene sequencing. Methanobacterium, Aminobacterium, Propionibacteriaceae, and Desulfovibrio represented the majority of the community in new granules. H2-using, and not acetoclastic, methanogens appeared more important, and were associated with abundant syntrophic bacteria. Multivariate integration (MINT) analyses identified distinct discriminant taxa responsible for shaping the microbial communities in different-sized granules.

Kinetics of aerobic granular sludge treating low-strength synthetic wastewater at high dissolved oxygen

Three parallel reactors (i.e. R1-R3) were operated with 340 mg-COD L^-1, 42 mg-TN L^-1, and 7 mg-TP L^-1 at 20 ± 1°C. A mature granular sludge developed in 40 d and was stable for the 120 d experimentation period at an average food to microorganism ratio of 0.25 ± 0.08 g-COD g-VSS^-1 d^-1. Reactor biomass had higher inorganic content (i.e. 0.78-0.80 g-VSS g-TSS^-1) than effluent biomass (i.e. 0.88-0.92 g-VSS g-TSS^-1). Average granule diameter was 0.7-1.0 mm. Maximum phosphorus uptake and release rates averaged 4 ± 3 and 4 ± 2 mg-P g-VSS^-1 h^-1, respectively. Maximum observed nitrification rates averaged 1.9 ± 0.6 mg-N g-VSS^-1 h^-1. Phosphorus kinetics were similar between R1-R3 (i.e. P = 0.5309-0.6870) while nitrification kinetics varied significantly (i.e. P = 0.0002) even though conditions were the same. Effluent phosphate was on average 0.2 ± 0.4 mg-P L^-1 while total inorganic nitrogen removal averaged 60 ± 10% resulting in an average effluent of 17 mg-N L^-1. Aerobic granular sludge was capable of reliable nutrient removal from low-strength wastewater without volatile fatty acid source and at high dissolved oxygen concentrations.

Growth Progression of Oxygenic Photogranules and Its Impact on Bioactivity for Aeration-Free Wastewater Treatment

Oxygenic photogranules (OPGs), spherical aggregates comprised of phototrophic and nonphototrophic microorganisms, treat wastewater without aeration, which currently incurs the highest energy demand in wastewater treatment. In wastewater-treatment reactors, photogranules grow in number as well as in size. Currently, it is unknown how the photogranules grow in size and how the growth impacts their properties and performance in wastewater treatment. Here, we present that the photogranules' growth occurs with changes in phototrophic community and granular morphology. We observed that as the photogranules grow larger, filamentous cyanobacteria become enriched while other phototrophic microbes diminish significantly. The photogranules greater than 3 mm in diameter showed the development of a layered structure in which a concentric filamentous cyanobacterial layer encloses noncyanobacterial aggregates. We observed that the growth of photogranules significantly impacts their capability of producing oxygen, the key element in OPG wastewater treatment. Among seven size classes investigated in this study, photogranules in the 0.5-1 mm size group showed the highest specific oxygen production rate (SOPR), 21.9 ± 1.3 mg O₂/g VSS-h, approximately 75% greater than the SOPR of mixed photogranular biomass. We discuss engineering the OPG process based on photogranules' size, promoting the stability of the granular process and enhancing efficiency for self-aerating wastewater treatment.

Characterization of aerobic granular sludge of different sizes for nitrogen and phosphorus removal

Granular size plays a key role in the performance of the aerobic granular sludge (AGS). As the diameter of the granule increases, stratification may begin to appear due to the increase in mass transfer resistance. Aerobic granules harvested from a lab-scale anaerobic-aerobic sequencing batch reactor (AO-SBR) were classified into three categories according to their size: (a) 0.15-0.28 mm, (b) 0.28-0.45 mm and (c) larger than 0.45 mm. In this study, the categories were called small-size, medium-size and large-size granules, respectively. A fraction of the different forms of phosphate and denitrification efficiency was investigated in each category. Results show that small-size granules present much more easily mobile phosphorus than other granules. Moreover, the denitrification performance has been tested by using dumping and trickling patterns for COD and NO3--N feeding. The results demonstrated that the large-size granules exhibit poor denitrification rates, as opposed to the medium-size granules. Therefore, medium-size granules, with a size of 0.28-0.45 mm, are regarded as the most suitable granular size for AGS in this experiment from the perspective of denitrification and phosphorus removal.

Process optimization of anammox-driven hydroxyapatite crystallization for simultaneous nitrogen removal and phosphorus recovery

Based on the requirements for advanced treatment and resource recovery of nitrogen and phosphorus pollutants in wastewater, the coupled anammox and hydroxyapatite crystallization (anammox-HAP) process was studied with an aim of achieving high efficiency and low energy consumption during simultaneous nitrogen and phosphorus removal. In the long-term experiments and batch tests, the effects of substrate conditions (nitrogen and phosphorus load, calcium concentration, etc.) on the nitrogen removal and phosphorus recovery efficiencies were investigated. The granular structure and crystal properties were analyzed together with microscopic characterization methods, and the formation mechanism of coupled anammox-HAP granules was verified. Based on these experiments, a theoretical model and technical method for realizing the coupled process were established, and a reference for practical engineering application was provided.

Optimized aeration strategies for nitrogen removal efficiency: application of end gas recirculation aeration in the fixed bed biofilm reactor

Aeration strategy played an important role in reactor performance. In this study, when superficial upflow air velocity (SAV) decreased from 0.16 to 0.08 cm s^-1, low dissolved oxygen concentration (DO) of 2.0 mg L^-1 occurred in reactor. The required depth for anoxic microenvironment in biofilm decreased from 902.3 to 525.9 μm, which enhanced the growth of denitrifying bacteria and total nitrogen (TN) removal efficiency. However, decreasing aeration intensity resulted in insufficient hydraulic shear stress, which led to weak biofilm matrix structure. Mass biofilm detachment and reactor deterioration then occurred after 87 days of operation. An end gas recirculation aeration strategy was proposed to separately manipulate DO and aeration intensity. Low DO and high aeration intensity were simultaneously achieved, which enhanced the metabolism of denitrifying bacteria (such as Flavobacterium sp., Pseudorhodobacter sp., and Dok59 sp.) and EPS-producing bacteria (such as Zoogloea sp. and Rhodobacter sp.). Consequently, high TN removal performance (82.1 ± 2.7%) and stable biofilm structure were achieved.

Stability of aerobic granular sludge in a pilot scale sequencing batch reactor enhanced by granular particle size control

Aerobic granulation was successfully achieved in a pilot scale sequencing batch reactor within 40 days. Then, stability of different particle size granules was explored according to their activity and resistance to ultrasonic crushing. Different particle size granules (0.3-0.6 mm, 0.6-1 mm, 1-1.43 mm, 1.43-2 mm, 2-3 mm and 3-4 mm) were exposed under different ultrasonic power separately. It was found that the granules with 2-3 mm always had the maximum granulation rates after ultrasonic crushing. Meanwhile, activity data showed that the 2-3 mm granules had the lowest specific oxygen utilization rates, which indicated that they were easier to maintain stability as the increase of their particle sizes was the slowest. So, 500 mL mixed liquid of the reactor were taken out and sieved to obtain the 2-3 mm granules, which were subsequently returned to the reactor to increase their proportion. Through the manual regulation, the proportion of 2-3 mm granules kept increasing which gradually became dominant in the reactor. Under the strategy of 86 days of operation, the aerobic granules were regular and compact, which had good removal effects of the real wastewater. The results indicated that the stability of the system could be greatly enhanced by the method, which provided a new strategy to maintain the granular stability.

Texture of anammox sludge bed: Composition feature, visual characterization and formation mechanism

Anammox granular sludge biotechnology has been successfully applied in the environmental engineering for nitrogen removal from wastewater. However, the startup and control of anammox process was difficult and lacks an easy-recognized indicator. In this work, the texture of anammox sludge bed in a long-term (>15 months) stable-operated and high-rate anammox reactor was investigated for the first time. The results showed that the anammox sludge bed took on the unique texture which was closely related to the biological loading rates of sludge bed at different heights of the reactor. The texture was ascribed to the pattern created by granule gradation (granules size distribution) and granule number density (spaces between granules). The sludge bed texture displayed the distinctive vision and could be characterized by the reflected light of sludge bed. The lightness indicator was found to have an excellent linear relationship with the height of anammox sludge bed. The texture of anammox sludge bed was revealed to originate from the periodic cycle of gas-driven segregation and density-based aggregation of granules. The results hint at a convenient and efficient strategy for operators to assess and control the working state of anammox system.

Impact of poly dimethyldiallylammonium chloride on membrane fouling mitigation in a membrane bioreactor

Poly dimethyldiallylammonium chloride (PDMDAAC) was applied in a membrane bioreactor (MBR) to study its effects on mitigation of MBR membrane fouling. Floc size, zeta potential, soluble microbial substances (SMP) and extracellular polymeric substances (EPS) secretion were studied with respect to PDMMAAC-dosing operations. Results demonstrated that a sustainable filtration cycle extended 3.3 times with the optimal PDMDAAC dosage of 90 mg L^-1. The addition of PDMDAAC could increase zeta potential of sludge floc, which led to the decrease in repulsive electrostatic interactions between flocs, as well as the facilitation of flocs-to-flocs aggregation. With the optimal dosage of PDMDAAC, the mean size of sludge was 3.23 ± 0.55 times higher than the control group, resulting in higher impact resistance and better adaptive capacity to the changing environment, which led to less SMP secretion. Moreover, a high contaminants removal rate was achieved in the reactor that was dosed with PDMDAAC. The average effluent concentrations of chemical oxygen demand and total nitrogen were less than 45.6 ± 2.85 and 5.23 ± 0.61 mg L^-1, respectively, and the corresponding removal rates were 93.1 ± 5.81% and 89.1 ± 9.61%.

Changes in extracellular polymeric substances (EPS) content and composition in aerobic granule size-fractions during reactor cycles at different organic loads

This study aimed to systematically investigate the effect of organic loading on granule diameters, and on the composition of extracellular polymeric substances (EPS) in granules in various size-fractions at the beginning and end of the cycle of granular sludge sequencing batch reactor (GSBR). The organic loadings were 0.78 kg COD/(m³·d) (GSBR1), 1.16 kg COD/(m³·d) (GSBR2) and 1.53 kg COD/(m³·d) (GSBR3). Granules with a diameter of 0.5-1 mm had the most stable EPS content and composition. The smallest granules had the largest amount of bound EPS. The amount of loosely-bound EPS increased as granule diameters decreased; it was lowest in the famine phase at end of the cycle. The proteins/polysaccharides ratio decreased below 1 only in soluble EPS in the famine period. In GSBR1, granules with a diameter <0.5 mm predominated, and the increase in soluble EPS at end of the cycle was most substantial resulting in the lowest COD removal.

Aerobic granular sludge: Cultivation parameters and removal mechanisms

Aerobic granular sludge (AGS) has been the focus of many investigations, and the main parameters responsible for AGS formation are hydrodynamic shear force, short periods and feast-famine cycles. However, some other parameters are associated with AGS maintenance after long periods of operation. This review evaluates the parameters responsible for AGS formation and maintenance and some reference values are proposed. In addition, some discussions are addressed about the main metabolic pathways that AGS uses for the removal of some compounds, such as nutrients, organic matter, dyes, recalcitrant compounds, among others. Finally, the main microbial groups present in the AGS and their respective functions are discussed. It is also highlighted that many parameters that are taken as reference currently for AGS cultivation and maintenance can be optimized for energy savings, implementation costs, among others, as well as a greater recovery of resources during wastewater treatment, within the scope of the biorefinery concept.

Stability of aerobic granular sludge under condition of low influent C/N ratio: Correlation of sludge property and functional microorganism

Aerobic granular sludge process treating domestic wastewater with low C/N ratio is necessary to be studied for rapid urbanization in China and other countries. In this study, two parallel reactors with different influent C/N ratio (15 in R1, 5 in R2) were established. Compared to the disintegrated granule in R1 with high influent C/N ratio, granules with large size (650 μm) and compact structure (integrity coefficient <0.1) were stable in R2 along with influent C/N ratio decreased to 5. High-through sequencing illustrated the functional microbes like Thauera and Paracoccus enriched under low influent C/N ratio, and principal component analysis further showed these microbes were positive correlation with tryptophan and protein-like substances in extracellular polymeric substances (EPS) and granular strength. It was indicated that under low influent C/N ratio, several resistant microbes like Thauera (19.5%) enriched and then secreted tryptophan and protein-like substances, and stable granules with multi-functional microbes could be formed finally.

Effects of high-concentration influent suspended solids on aerobic granulation in pilot-scale sequencing batch reactors treating real domestic wastewater

The aim of this study was to investigate the effect of high-influent-concentration suspended solids (SS) on the cultivation, structure and long-term stability of aerobic granular sludge (AGS). Cultivation and long-term stability of AGS were monitored in two pilot-scale sequencing batch reactors fed with raw (R1) and settled (R2) domestic wastewater, representing high and medium SS content, respectively. The real domestic wastewater had high chemical oxygen demand (COD) content (1100 ± 270 mg COD L^-1). Aerobic granular sludge was cultivated in 44 days (R1) and 25 days (R2) under the conditions of high settling velocity (18 m h^-1) and high organic loading rate (OLR) (2.1-2.4 kg COD m³ day). The AGS in both reactors had similar structural properties during long-term operation and remained structurally and functionally stable during the last five months of operation. Comparative evaluation of the results indicated that the high influent SS content of the real domestic wastewater had a positive influence on maintaining significantly lower SVI₃₀ and relatively lower effluent SS concentration. Moreover, a higher influent SS content resulted in smaller mature granules during the stable period. Microbial community analyses helped to understand the aerobic granular sludge structure and showed that the sludge retention time and OLR affected the granular sludge population. The high influent SS increased biomass detachment from the granular sludge surface and caused wash-out of some bacteria colonizing the exterior of the granular sludge.

State of the art on granular sludge by using bibliometric analysis

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

Stability of algal-bacterial granules in continuous-flow reactors to treat varying strength domestic wastewater

Stability of algal-bacterial granules was investigated in two continuous-flow systems to treat synthetic domestic wastewater using single (R₁) and series (R₂=R_2-1+R_2-2 with automatically internal recirculation) reactors by seeding 50% (w/w) algal-bacterial granules. Almost similar organics and phosphorus removal efficiencies were obtained from the two systems, with no significant difference found for each between the designed two operation stages. However, R₂ exhibited superior performance on total nitrogen (TN) removal (76%). When double increased strength influent fed to R₁, R₁ achieved better denitrification with TN removal increased from 29% to 80%, possibly due to the increased influent organics concentration favored the denitrification process. Most importantly, the two systems well maintained their granular stability, and all granules became algal-bacterial ones with very little change detected in algae content in granules after 120days' operation. At last, the mechanisms were proposed regarding the formation and enhanced stability of new algal-bacterial granules in continuous-flow reactors.