Effects of carbon source on the formation, stability, bioactivity and biodiversity of the aerobic granule sludge

Demystifying polyphosphate-accumulating organisms relevant to wastewater treatment: A review of their phylogeny, metabolism, and detection

Currently, the most cost-effective and efficient method for phosphorus (P) removal from wastewater is enhanced biological P removal (EPBR) via polyphosphate-accumulating organisms (PAOs). This study integrates a literature review with genomic analysis to uncover the phylogenetic and metabolic diversity of the relevant PAOs for wastewater treatment. The findings highlight significant differences in the metabolic capabilities of PAOs relevant to wastewater treatment. Notably, Candidatus Dechloromonas and Candidatus Accumulibacter can synthesize polyhydroxyalkanoates, possess specific enzymes for ATP production from polyphosphate, and have electrochemical transporters for acetate and C4-dicarboxylates. In contrast, Tetrasphaera, Candidatus Phosphoribacter, Knoellia, and Phycicoccus possess PolyP-glucokinase and electrochemical transporters for sugars/amino acids. Additionally, this review explores various detection methods for polyphosphate and PAOs in activated sludge wastewater treatment plants. Notably, FISH-Raman spectroscopy emerges as one of the most advanced detection techniques. Overall, this review provides critical insights into PAO research, underscoring the need for enhanced strategies in biological phosphorus removal.

Lab-scale evaluation of Microalgal-Bacterial granular sludge as a sustainable alternative for brewery wastewater treatment

Microalgal-bacterial granular sludge (MBGS) could offer a sustainable alternative to traditional aerobic methods in brewery wastewater (BWW) treatment. This study compared MBGS with conventional activated sludge (AS) in treating real BWW and highlighted its advantages and challenges. MBGS achieved comparable chemical oxygen demand removal efficiency (93%) compared to AS (89%). Additionally, MBGS exhibited higher phosphate removal capabilities than AS. Extra nitrogen was added to influent to balance C/N ratio of BWW. MBGS was robust in handling C/N ratio fluctuations with an 82% total nitrogen removal efficiency. Metagenomic analysis further indicated that most of the genes involved in carbon, nitrogen and phosphorus metabolism were up-regulated in MBGS compared to AS. Despite changes in the microbial community and settling ability due to high starch and sugar content in BWW, MBGS demonstrated high efficiency and sustainability. Further research should optimize MBGS operation strategies to fully realize its potential for sustainable BWW treatment.

Bibliometric analysis of research trends in biogranulation technology for wastewater treatment

Inadequate management and treatment of wastewater pose significant threats, including environmental pollution, degradation of water quality, depletion of global water resources, and detrimental effects on human well-being. Biogranulation technology has gained increasing traction for treating both domestic and industrial wastewater, garnering interest from researchers and industrial stakeholders alike. However, the literature lacks comprehensive bibliometric analyses that examine and illuminate research hotspots and trends in this field. This study aims to elucidate the global research trajectory of scientific output in biogranulation technology from 1992 to 2022. Utilizing data from the Scopus database, we conducted an extensive analysis, employing VOSviewer and the R-studio package to visualize and map connections and collaborations among authors, countries, and keywords. Our analysis revealed a total of 1703 journal articles published in English. Notably, China emerged as the leading country, Jin Rencun as the foremost author, Bioresource Technology as the dominant journal, and Environmental Science as the prominent subject area, with the Harbin Institute of Technology leading in institutional contributions. The most prominent author keyword identified through VOSviewer analysis was "aerobic granular sludge," with "sequencing batch reactor" emerging as the dominant research term. Furthermore, our examination using R Studio highlighted "wastewater treatment" and "sewage" as notable research terms within the field. These findings underscore a diverse research landscape encompassing fundamental aspects of granule formation, reactor design, and practical applications. This study offers valuable insights into biogranulation potential for efficient wastewater treatment and environmental remediation, contributing to a sustainable and cleaner future.

Insight into performance of nitrogen removal enhanced by adding lactic acid-rich food waste fermentation liquid as carbon source in municipal wastewater treatment

Lactic acid-rich fermentation liquid (LAFL) of food waste is found to act as a promising alternative carbon source for nitrogen removal in wastewater treatment. Here, LAFL was employed to investigate its impacts on nitrogen removal during raw municipal wastewater treatment with a comparison to sodium acetate (NaAc). Results indicated that nitrogen removals were comparable when incorporated with LAFL and NaAc (92.89 % v.s. 91.23 %). Unlike the utilization of NaAc, using LAFL could avoid suppressing the relative abundance of the nitrification genes and thus pose a negative risk to nitrogen removal during prolonged operation. The introduction of LAFL increased the stability and robustness of the functional microbial community and effectively reduced excess activated sludge (AS) generation by 109 % compared to NaAc addition, consequently enhancing nitrogen removal but diminishing the treatment cost. In general, LAFL exhibits prospective engineering application potentials and economic advantages in improving nitrogen removal by AS process.

Enhanced granulation of activated sludge in an airlift reactor for organic carbon removal and ammonia retention from industrial fermentation wastewater: A comparative study

Ammonia retention and recovery from high-nitrogenous wastewater are new concepts being used for nitrogen management. A microaerophilic activated sludge system was developed to convert organic nitrogen into ammonia and retain it for its recovery; however, the settleability of activated sludge remains a challenge. Therefore, this study proposed an aerobic granular sludge system as a potential solution. Two types of sequencing batch reactors-airlift and upflow reactors-were operated to investigate the feasibility of fast granule formation, the performance of organic carbon removal and ammonia retention, and the dynamics of microbial community composition. The operation fed with industrial fermentation wastewater demonstrated that the airlift reactor ensured a more rapid granule formation than the upflow reactor because of the high shear force, and it maintained a superior ammonia retention stability of approximately 85 %. Throughout the operational period, changes in hydraulic retention time (HRT), settling time, and exchange ratio altered the granular particle sizes and microbial community compositions. Rhodocyclaceae were replaced with Comamonadaceae, Methylophilaceae, Xanthomonadaceae, and Chitinophagaceae as core taxa instrumental in granulation, likely because of their extracellular polymeric substance secretion. As the granulation process progressed, a significant decrease in the relative abundances of nitrifying bacteria-Nitrospiraceae and Nitrosomonadaceae-was observed. The reduction of settling time and HRT enhanced granulation and inhibited the activity of nitrifying bacteria. The success in granulation for ammonia conversion and retention in this study accelerates the paradigm shift from ammonia removal to ammonia recovery from industrial fermentation wastewater.

Exploratory study on the metabolic similarity of denitrifying carbon sources

Mixed carbon sources have been developed for denitrification to eliminate the "carbon dependency" problem of single carbon. The metabolic correlation between different carbon sources is significant as guidance for the development of novel mixed carbon sources. In this study, to explore the metabolic similarity of denitrifying carbon sources, we selected alcohols (methanol, ethanol, and glycerol) and saccharide carbon sources (glucose, sucrose, and starch). Batch denitrification experiments revealed that methanol-acclimated sludge improved the denitrification rate of both methanol (14.42 mg-N/gMLVSS*h) and ethanol (9.65 mg-N/gMLVSS*h), whereas ethanol-acclimated sludge improved the denitrification rate of both methanol (7.80 mg-N/gMLVSS*h) and ethanol (22.23 mg-N/gMLVSS*h). In addition, the glucose-acclimated sludge and sucrose-acclimated sludge possibly improved the denitrification rate of glucose and sucrose, and the glycerol-acclimated sludge improved the denitrification rate of volatile fatty acids (VFAs), alcohols, and saccharide carbon sources. Functional gene analysis revealed that methanol, ethanol, and glycerol exhibited active alcohol oxidation and glyoxylate metabolism, and glycerol, glucose, and sucrose exhibited active glycolysis metabolism. This indicated that the similarity in the denitrification metabolism of these carbon sources was based on functional gene similarity, and glycerol-acclimated sludge exhibited the most diverse metabolism, which ensured its good denitrification effect with other carbon sources.

Insight into the effect of particulate organic matter on sludge granulation at the low organic load: Sludge characteristics, extracellular polymeric substances and microbial communities response

This paper investigated the effect of particulate organic matter (POM) on sludge granulation under low organic load. The results showed that POM promoted the formation of aerobic granular sludge (AGS) with a chemical oxygen demand (COD) fraction of 25%, and POM also enhanced the sludge settleability and biomass retention. However, when the COD fraction of POM increased to 50% and 75%, the AGS performance deteriorated. The analysis of extracellular polymeric substances revealed that the POM (accounted for ≤ 50% of COD in the influent) suppressed the secretion of extracellular protein. Analysis of the microbial community showed that species diversity was lower in the POM-fed system, with Rhodocyclaceae being the predominant bacteria responsible for carbon source degradation. Additionally, molecular ecological network analysis demonstrated that when the COD fraction of the POM exceeded 50%, the connectivity and modularity between microbial species decreased, which may explain the sludge performance deterioration.

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

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

Response of nitrite accumulation, sludge characteristic and microbial transition to carbon source during the partial denitrification (PD) process

Partial denitrification (PD, nitrate (NO3--N) → nitrite (NO2--N)) as a novel pathway for NO2--N production has been widely concerned, but the specific conditions for highly efficient and stable nitrite maintenance are not yet fully understood. In this study, the effects of carbon sources (acetate, R1; propionate, R2; glucose, R3) on NO2--N accumulation was discussed without seeding PD sludge and the mechanism analysis related to sludge characteristic and microbial evolution were elucidated. The optimal NO2--N, nitrate-to-nitrite transformation ratio (NTR) and nitrite removal efficiency (NRE) reached up to 32.10 mg/L, 98.01 %, and 86.95 % in R1. However, due to the complex metabolic pathway of glucose, the peak time of NO2--N production delayed from 30 min to 60 min. The sludge particle size decreased from 154.2 μm (R1), 130.8 μm (R2) to 112.6 μm (R3) with the increasing extracellular polymeric substances (EPS) from 80.75-85.44 mg/gVSS, 82.68-92.75 mg/gVSS to 106.31-110.25 mg/gVSS, where the ratio of proteins/polysaccharides (PN/PS) was proved to be closely associated with NO2--N generation. For the microbial evolution, Saccharimonadales (70.42 %) dominated the glucose system, while Bacillus (7.42-21.63 %) and Terrimonas (4.24-5.71 %) were the main contributors for NO2--N accumulation in the acetate and propionate systems. The achievement of PD showed many advantages of lower carbon demand, minimal sludge production, lesser greenhouse gas emission and prominent nutrient removal, offering an economically and technically attractive alternative for NO3--N containing wastewater treatment.

Simultaneous nitrate and sulfate biotransformation driven by different substrates: comparison of carbon sources and metabolic pathways at different C/N ratios

Nitrate (NO₃^-) and sulfate (SO₄^2-) often coexist in organic wastewater. The effects of different substrates on NO₃^- and SO₄^2- biotransformation pathways at various C/N ratios were investigated in this study. This study used an activated sludge process for simultaneous desulfurization and denitrification in an integrated sequencing batch bioreactor. The results revealed that the most complete removals of NO₃^- and SO₄^2- were achieved at a C/N ratio of 5 in integrated simultaneous desulfurization and denitrification (ISDD). Reactor Rb (sodium succinate) displayed a higher SO₄^2- removal efficiency (93.79%) with lower chemical oxygen demand (COD) consumption (85.72%) than reactor Ra (sodium acetate) on account of almost 100% removal of NO₃^- in both Ra and Rb. Ra produced more S^2- (5.96 mg L^-1) and H₂S (25 mg L^-1) than Rb, which regulated the biotransformation of NO₃^- from denitrification to dissimilatory nitrate reduction to ammonium (DNRA), whereas almost no H₂S accumulated in Rb which can avoid secondary pollution. Sodium acetate-supported systems were found to favor the growth of DNRA bacteria (Desulfovibrio); although denitrifying bacteria (DNB) and sulfate-reducing bacteria (SRB) were found to co-exist in both systems, Rb has a greater keystone taxa diversity. Furthermore, the potential carbon metabolic pathways of the two carbon sources have been predicted. Both succinate and acetate could be generated in reactor Rb through the citrate cycle and the acetyl-CoA pathway. The high prevalence of four-carbon metabolism in Ra suggests that the carbon metabolism of sodium acetate is significantly improved at a C/N ratio of 5. This study has clarified the biotransformation mechanisms of NO₃^- and SO₄^2- in the presence of different substrates and the potential carbon metabolism pathway, which is expected to provide new ideas for the simultaneous removal of NO₃^- and SO₄^2- from different media.

Algal-bacterial aerobic granular sludge for real municipal wastewater treatment: Performance, microbial community change and feasibility of lipid recovery

Despite various research works on algal-bacterial aerobic granular sludge for wastewater treatment and resource recovery processes, limited information is available on its application in real wastewater treatment in terms of performance, microbial community variation and resource recovery. This study investigated the performance of algal-bacterial aerobic granular sludge on real low-strength wastewater treatment in addition to the characterization of microbial community and fatty acid compositions for biodiesel production. The results demonstrated 71% COD, 77% NH₄⁺-N and 31% phosphate removal efficiencies, respectively. In addition, all the water parameters successfully met the effluent standard A, imposed by the Department of Environment (DOE) Malaysia. Core microbiome analyses revealed important microbial groups (i.e., Haliangium ochraceum, Burkholderiales and Chitinophagaceae) in bacterial community. Meanwhile the photosynthetic microorganisms, such as Oxyphotobacteria and Trebouxiophyceae dominated the algal-bacterial aerobic granular sludge, suggesting their important roles in granulation and wastewater treatment. Up to 12.51 mg/gSS lipid content was recovered from the granules. In addition, fatty acids composition showed high percetages of C16:0 and C18:0, demonstrating high feasibility to be used for biodiesel production application indicated by the cetane number, iodine value and oxidation stability properties.

Influence of operating regime on resource recovery in aerobic granulation systems under osmotic stress

Aerobic granular sludge (AGS) systems have great potential for biopolymers recovery, especially when subjected to adverse conditions. This work aimed to study the production of alginate-like exopolymers (ALE) and tryptophan (TRY) under osmotic pressure in conventional and staggered feeding regimes. The results revealed that systems operated with conventional feed accelerated the granulation, although less resistant to saline pressures. The staggered feeding systems favored better denitrification conditions and long-term stability. Salt addition gradient increase influenced biopolymers' production. However, staggered feeding, despite decreasing the famine period, did not influence the production of resources and extracellular polymeric substances (EPS). Sludge retention time (SRT), which was not controlled, proved to be an important operational parameter with negative influences on biopolymers' production in values greater than 20 days. Thus, the principal component analysis confirmed that the production of ALE at low SRT is related to better-formed granules with good sedimentation characteristics and good AGS performances.

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

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

Supplementary Information

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

Enhanced biological nitrogen and phosphorus removal from sewage driven by fermented glycerol: comparative assessment between sequencing batch- and continuously fed-structured fixed bed reactor

The nutrient biological removal from sewage, especially from anaerobic reactor effluents, still represents a major challenge in conventional sewage treatment plants. In this work, the nitrogen and phosphorus removal from anaerobic pre-treated domestic sewage in an up-flow anaerobic sludge blanket (UASB) reactor was assessed in a structured fixed bed reactor (SFBR) operated in a continuous and in a batch mode using polyurethane foam as material support for biomass and fermented glycerol as the exogenous carbon source. The SFBR was operated as a sequencing batch reactor with cycles of 90, 120, and 150 min under anaerobic, oxic, and anoxic conditions, respectively, reaching average efficiencies for total nitrogen and phosphorus removal of 88% and 56%, respectively. Fermented glycerol was added during the non-aerated periods. Under continuous feeding, the SFBR was operated with aeration/non-aeration periods of 2/1 (h) and 3/1 (h), hydraulic retention time of 12 h, and a recirculation ratio of 3. Without fermented glycerol addition, the maximum removal of total nitrogen (TN) reached 42%, while adding glycerol in the non-aerated period improved TN removal to 64.9% (2/1 h) and 69.5% (3/1 h). During continuous operation, no phosphorus removal was observed, which was released during the non-aerated period, remaining in the effluent. Optical microscopy analyses confirmed the presence of polyphosphate granules and of the phosphorus accumulating organisms in the reactor biofilm. It was concluded that the batch feeding method was determinant for phosphorus removal. The structured fixed bed reactor with polyurethane foam proved to be feasible in the removal of organic matter and nutrients remaining in the UASB reactor effluent.

Influence of salt addition to stimulating biopolymers production in aerobic granular sludge systems

The influence of salt addition to stimulating biopolymers production in aerobic granular sludge (AGS) systems was evaluated. The control systems (R1: acetate and R2: propionate) initially obtained less accumulation of mixed liquor volatile suspended solids (MLVSS), indicating that the osmotic pressure in the salt-supplemented systems (R3: acetate and R4: propionate) contributed to biomass growth. However, the salt-supplemented systems collapsed between days 110 and 130 of operation. R3 and R4 showed better performance regarding nutrients removal due to the greater abundance of nitrifying and denitrifying bacteria and phosphate-accumulating organisms. Salt also contributed to the higher production of biopolymers such as alginate-like exopolymers (ALE) per gram of volatile suspended solids (VSS) (R1: 397 mgALE∙gVSS^-1, R2: 140 mgALE∙gVSS^-1, R3: 483 mgALE∙gVSS^-1, R4: 311 mgALE∙gVSS^-1). Amino acids like tyrosine and tryptophan were better identified in extracellular polymeric substances extract from salt-operated reactors. This study brings important results in the context of resource recovery by treating saline effluents.

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

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

Aerobic granular sludge cultivated from Fe-loaded activated carbon as carrier working low-strength wastewater conditions by bioreactor

This study proposes a new method to promote the granulation process while accelerating the degradation efficiency of nutrients. The new strategy could involve preparing Fe-loaded activated carbon (FAC) before start-up of granular cultivation and then cultivating the process of aerobic granular sludge (AGS) with such materials. In addition, this experiment could further comprehend how the preparation and characteristics of FAC affect the formation and properties of AGS. The conclusions showed that compared with the control, FAC enhanced the sedimentation performance and significant removal efficiency. Meanwhile, the values of protein (PN) and polysaccharide (PS) also increased significantly in the addition of FAC, indicating the production of substances were induced by FAC. Molecular biology methods indicated that the rapid production of granulation and removal of nutrients were considered as the abundance of various microbes and denitrifying bacteria at the addition of FAC. This research showed that the presence of FAC is a useful strategy for the initiation of sludge particle formation to promote the treatment of wastewater, containing COD and NH₄⁺ at about 150-100 and 30 mg L^-1.

Nitrite accumulation, denitrification kinetic and microbial evolution in the partial denitrification process: The combined effects of carbon source and nitrate concentration

The combined effects of carbon source (HAc, HPr, Glu, Glu + HAc) and nitrate concentration (40, 80 mg/L labeling as R₄₀, R₈₀) on partial denitrification (PD) were discussed at C/N ratio of 2.5 (COD = 100, 200 mg/L). The optimal NO₂^--N and NTR reached to 67.03 mg/L, 99.14% in HAc-R₈₀ system, and denitrification kinetics revealed the same conclusion, corresponding to higher COD utilization rate (CUR: 58.46 mgCOD/(gVSS·h)), nitrate reduction rate (N_aRR: 29.94 mgN/(gVSS·h)) and nitrite accumulation rate (N_iAR: 29.68 mgN/(gVSS·h)). The preference order was HAc > HPr > Glu + HAc > Glu in both R₄₀ and R₈₀ systems due to different metabolic pathways, however, the NO₂^--N accumulation and kinetic parameters of R₈₀ group were dramatically higher than those in R₄₀ for the same carbon source. The R₈₀ group facilitated more concentrated biodiversity (607-808 OTUs) with Terrimonas and norank_f_Saprospiraceae responsible for high NO₂^--N accumulation in HAc and HPr served systems, while norank_f_norank_o_Saccharimonadales and OLB13 dominated the Glu containing systems.

Simultaneous nitrification-denitrification in biofilm systems for wastewater treatment: Key factors, potential routes, and engineered applications

Simultaneous nitrification-denitrification (SND) is an advantageous bioprocess that allows the complete removal of ammonia nitrogen through sequential redox reactions leading to nitrogen gas production. SND can govern nitrogen removal in single-stage biofilm systems, such as the moving bed biofilm reactor and aerobic granular sludge system, as oxygen gradients allow the development of multilayered biofilms including nitrifying and denitrifying bacteria. Environmental and operational conditions can strongly influence SND performance, biofilm development and biochemical pathways. Recent advances have outlined the possibility to reduce the carbon and energy consumption of the process via the "shortcut pathway", and simultaneously remove both N and phosphorus under specific operational conditions, opening new possibilities for wastewater treatment. This work critically reviews the factors influencing SND and its application in biofilm systems from laboratory to full scale. Operational strategies to enhance SND efficiency and hints to reduce nitrous oxide emission and operational costs are provided.

Carbon source affects the resource recovery in aerobic granular sludge systems treating wastewater

This study evaluated the influence of carbon sources on alginate-like exopolymers (ALE) and tryptophan (Trp) biosynthesis in the aerobic granular sludge (AGS). With acetate, the highest biopolymers levels, per gram of volatile suspended solids (VSS) (418.7 mgALE∙g^-1 and 4.1 mgTrp∙gVSS^-1), were found likely due to biomass loss throughout the operation, which resulted in lower sludge age (4-7 days) and shorter famine period. During granulation, encouraging results on ALE production were obtained with propionate (>250 mgALE∙gVSS^-1), significantly higher than those found with glycerol, glucose, and sucrose. Regarding tryptophan production, propionate and glycerol proved to be good substrates, although the content was still lower than acetate (1.6 mgTrp∙gVSS^-1). Granules fed with glucose showed the worst results compared to the other substrates (38.5 mgALE∙VSS^-1 and 0.6 mgTrp∙gVSS^-1) due to the filamentous microorganisms' abundance found. Therefore, this study provides insights to value the production of compounds of industrial interest in AGS systems.

Bioaugmentation for low C/N ratio wastewater treatment by combining endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) in the continuous A2/O - MBBR system

Endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) were combined in a novel A²/O - MBBR (Anaerobic Anoxic Oxic - Moving Bed Biofilm Reactor) system for low carbon/nitrogen (C/N) ratio wastewater treatment. The DPR performance was compared and the nutrient metabolism was elucidated based on the optimization of hydraulic retention time (HRT, 4-12 h) and nitrate recycling (R, 200%-600%). In the continuous-flow, the nitrate (NO₃^-) denitrification accompanied by nitrite (NO₂^-, via EPD) accumulation with the nitrate-to-nitrite transformation ratio (NTR) of 35.87%-43.31% in the anoxic zones. At HRT of 12 h with R of 500%, batch test initially revealed the DPR mechanism using both NO₃^- and NO₂^- as electron acceptor, where denitrifying phosphorus accumulation organisms (DPAOs) and denitrifying glycogen accumulation organisms (DGAOs) were the main contributors for EPD with incomplete denitrification (NO₃^- → NO₂^-). Furthermore, stoichiometry-based functional bacteria analysis displayed that higher bioactivity of DPAOs (NO₂^-→N₂, 57.30%; NO₃^-→N₂, 35.85%) over DGAOs (NO₃^-→N₂, 6.85%) facilitated the anoxic NO₃^- reduction. Microbial community analysis suggested that Cluster I of Defluviicoccus-GAO group (∼4%) was responsible for stable NO₂^- accumulation performance via EPD, while increased Accumulibacter-PAO group (by ∼15%) contributed to the advanced nutrient removal. Based on the achievement of NO₂^- accumulation, the application feasibility of integrated EPD - DPR - Anammox for deep-level nutrient removal was discussed.

Food waste hydrolysate as a carbon source to improve nitrogen removal performance of high ammonium and high salt wastewater in a sequencing batch reactor

The high ammonium and high salt (HAHS) wastewater generated from the anaerobic digestate of food waste is usually difficult to be treated by biological process because of its low C/N ratio. Herein, food waste hydrolysate (FWH) is rich in readily biodegradable organic matter, was utilized as carbon source to enhance the nitrogen removal of HAHS in the activated-sludge system. Results showed that compared with the control average total nitrogen removal efficiency increased from 73.4% to 94.9% and effluent declined from 281.4 mg/L to 53.9 mg/L by adding FWH at the C/N ratio of 6, satisfying the sewage discharge standard regulated by China. Besides, FWH utilization led to higher selectivity of the species responsible for nitrogen removal in related to glucose-adding group, which were dominated by Flavobacteriaceae, Melioribacteraceae, PHOS-HE36, and Rhodobacteraceae after a long-term operation. In general, FWH is an alternative carbon source to enhance nitrogen removal in HAHS wastewater treatment.

Effect of microaerophilic treatment on swine wastewater (SWW) treatment: Engineering and microbiological aspects

The microaerobic process on swine wastewater (SWW) treatment was investigated, evaluating its effect on organic matter hydrolysis and removal, biogas production, operational stability, and microbial community structure. UASB reactors operating under higher organic loading rates (OLRs) and lower hydraulic retention times (HRTs) than those found in the SWW treatment literature were also assessed. The microaerophilic reactor R2 presented a higher total and particulate organic matter removals and operational stability than the anaerobic reactor R1, reaching COD_P removals of 79.4 ± 4.6%. In the specific methanogenic activity (SMA) tests, the microaerobic sludge (R2) showed hydrolytic and acetogenic/methanogenic activity superior to inoculum and anaerobic sludge (R1). The microbiological evaluation of R2 revealed the high presence of hydrolytic microorganisms, therefore justifying the higher hydrolytic activity found in the SMA tests and higher particulate organic matter removal found in the microaerobic reactor.

Effects of wastewater type on stability and operating conditions control strategy in relation to the formation of aerobic granular sludge - a review

Currently, research trends on aerobic granular sludge (AGS) have integrated the operating conditions of extracellular polymeric substances (EPS) towards the stability of AGS systems in various types of wastewater with different physical and biochemical characteristics. More attention is given to the stability of the AGS system for real site applications. Although recent studies have reported comprehensively the mechanism of AGS formation and stability in relation to other intermolecular interactions such as microbial distribution, shock loading and toxicity, standard operating condition control strategies for different types of wastewater have not yet been discussed. Thus, the dimensional multi-layer structural model of AGS is discussed comprehensively in the first part of this review paper, focusing on diameter size, thickness variability of each layer and diffusion factor. This can assist in facilitating the interrelation between disposition and stability of AGS structure to correspond to the changes in wastewater types, which is the main objective and novelty of this review.

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

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

Pseudo-analytical solutions for multi-species biofilm model of aerobic granular sludge

This paper demonstrates modelling of the aerobic granular sludge (AGS) process with the pseudo-analytical solutions (PAS) of a biofilm model. A MATLAB programmed graphical user interface platform was developed to facilitate the model calculation and access. Model calibration and validation were carried out through using experimental data collected from a granular sludge sequencing batch reactor operation. The experimental and modelling results identified the distribution of heterotrophs and nitrifiers on the AGS and its contribution to the performance of wastewater treatment. The model could describe multi-species biofilms according to the distinguishing features among the three levels of PAS models. The models demonstrated increasing degrees of interaction (no interaction, competition for nitrogen and layering and protection) between heterotrophs and nitrifiers. Modelling the AGS process using PAS increases the accessibility of the simulation of multiple species in both biofilm and suspended biomass.

Effects of sulfamethoxazole on aerobic sludge granulation process

AIMS

Our purpose was to clarify the effect of sulfamethoxazole (SMX) on the start-up period, particle formation, and treatment efficiency of an aerobic granular sludge system.

METHODS AND RESULTS

We compared an R1 granular sequencing batch reactor (GSBR) started with 5 μg L^-1 SMX and an R2 GSBR started without SMX, as a control, to investigate the impact of a trace amount of SMX (5 μg L^-1 ) on aerobic granular sludge (AGS) characteristics and the removal of conventional contaminants. AGS granulation in the R1 system was not inhibited by SMX, but the granule particle size was smaller than that in the R2 system. Both systems had good performance removing conventional pollutants. Extracellular polymeric substance secretion in the R1 system was lower than that in the R2 system. After stabilizing reactor operations, the SMX removal efficiency in the R1 system (~73.93%) was higher than that in the R2 system (~70.66%). The start-up modes also determined the differences in the microbial community structure of the AGS systems.

CONCLUSIONS

SMX-activated AGS performed better than AGS without SMX.

SIGNIFICANCE AND IMPACT OF STUDY

The study can help engineers determine start-up modes with varieties of antibiotics in AGS processes and provide references for the optimization of water treatment processes.

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

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

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

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

Resource recovery in aerobic granular sludge systems: is it feasible or still a long way to go?

Lately, wastewater treatment plants are much often being designed as wastewater-resource factories inserted in circular cities. Among biological treatment technologies, aerobic granular sludge (AGS), considered an evolution of activated sludge (AS), has received great attention regarding its resource recovery potential. This review presents the state-of-the-art concerning the influence of operational parameters on the recovery of alginate-like exopolysaccharides (ALE), tryptophan, phosphorus, and polyhydroxyalkanoates (PHA) from AGS systems. The carbon to nitrogen ratio was identified as a parameter that plays an important role for the optimal production of ALE, tryptophan, and PHA. The sludge retention time effect is more pronounced for the production of ALE and tryptophan. Additionally, salinity levels in the bioreactors can potentially be manipulated to increase ALE and phosphorus yields simultaneously. Some existing knowledge gaps in the scientific literature concerning the recovery of these resources from AGS were also identified. Regarding industrial applications, tryptophan has the longest way to go. On the other hand, ALE production/recovery could be considered the most mature process if we take into account that existing alternatives for phosphorus and PHA production/recovery are optimized for activated sludge rather than granular sludge. Consequently, to maintain the same effectiveness, these processes likely could not be applied to AGS without undergoing some modification. Therefore, investigating to what extent these adaptations are necessary and designing alternatives is essential.

Microaeration improves the removal/biotransformation of organic micropollutants in anaerobic wastewater treatment systems

This work assessed the effect of increasing microaeration flow rates (1-6 mL min^-1 at 28 °C and 1 atm, equivalent to 0.025-0.152 L O₂ L^-1 feed) on the removal/biotransformation of seven organic micropollutants (OMPs) (three hormones, one xenoestrogen, and three pharmaceuticals), at 200 μg L^-1 each, in a lab-scale upflow anaerobic sludge blanket reactor operated at a hydraulic retention time (HRT) of 7.4 h. Additionally, the operational stability of the system and the evolution of its microbial community under microaerobic conditions were evaluated. Microaeration was demonstrated to be an effective strategy to improve the limited removal/biotransformation of the evaluated OMPs in short-HRT anaerobic wastewater treatment systems. The rise in the airflow rate considerably increased the removal efficiencies of all OMPs. However, there seems to be a saturation limit for the biochemical reactions. Then, the best results were obtained with 4 mL air min^-1 (0.101 L O₂ L^-1 feed) (~90%) because, above this flow rate, the efficiency increase was negligible. The long-term exposure to microaerobic conditions (249 days) led the microbiota to a gradual evolution. Consequently, there was some enrichment with species potentially associated with the biotransformation of OMPs, which may explain the better performance at the end of the microaerobic term even with the lowest airflow rate tested.

Shortcut nitrification-denitrification and biological phosphorus removal in acetate- and ethanol-fed moving bed biofilm reactors under microaerobic/aerobic conditions

This study investigated the feasibility of coupling simultaneous partial nitrification and denitrification (SPND) to biological phosphorus removal in continuous-flow intermittently-aerated moving bed biofilm reactors (MBBRs) fed with different carbon sources, i.e. ethanol and acetate. Bacterial cultivation at pH 8.2 (±0.2), 26-28 °C and SRT of 4 day and microaerobic/aerobic MBBR operation allowed to achieve average dissolved organic carbon (DOC), total inorganic nitrogen (TIN) and P-PO₄^3- removal efficiencies (REs) of 100%, 81-88% and 83-86% at HRT of 1 day, dissolved oxygen (DO) range of 0.2-3 mg L^-1 and feed C/N and C/P ratios of 3.6 and 11, respectively. Acetate supplementation favored a diversified microbial community, while overgrowth of heterotrophs was observed when increasing feed C/N ratio in ethanol-fed MBBR. Illumina sequencing displayed the presence of putative phosphorus accumulating organisms (PAOs) such as Hydrogenophaga and Pseudomonas in MBBR biofilm and suspended biomass, whereas no typical NOB was identified during the study.

Phosphate removal via biological process coupling with hydroxyapatite crystallization in alternating anaerobic/aerobic biofilter reactor

In this work, a laboratory-scale alternating anaerobic/aerobic biofilter (A/O BF) filled with self-made steel slag media was constructed, where the integrated biological and crystalline phosphorus removal process was realized to remove phosphorus and achieve phosphorus recovery from wastewater. Phosphorus accumulating organisms (PAOs) were successfully enriched within 30 days operation, the maximum phosphate removal efficiency was close to 80% under the optimal conditions with the anaerobic time of 34 h, HRT of 4 h and influent COD of 300 mg/L. The analysis of SEM-EDS and XRD indicated that hydroxyapatite (HAP) crystals were formed inside biofilms without addition of chemical reagents. The high phosphate environment created by PAOs and the release of Ca²⁺ from the steel slag media might be responsible for the generation of HAP. These findings have crucial implications for the application BF technology to remove and recover phosphorus from wastewater.

Effects of the antibiotics trimethoprim (TMP) and sulfamethoxazole (SMX) on granulation, microbiology, and performance of aerobic granular sludge systems

This work assessed the effect of the antibiotics trimethoprim (TMP) and sulfamethoxazole (SMX) on the granulation process, microbiology, and organic matter and nutrient removal of an aerobic granular sludge (AGS) system. In addition, after the maturation stage, the impact of the redox mediator anthraquinone-2,6-disulfonate (AQDS) (25 μM) on the biotransformation of the antibiotics was evaluated. The reactor R1 was maintained as a control, and the reactor R2 was supplemented with TMP and SMX (200 μg L^-1). The ability to remove C, N, and P was similar between the reactors. However, the structural integrity of the AGS was impaired by the antibiotics. Low TMP (∼30%) and SMX (∼60%) removals were achieved when compared to anaerobic or floccular biomass aerobic systems. However, when the system was supplemented with AQDS, an increase in the removal of TMP (∼75%) and SMX (∼95%) was observed, possibly due to the catalytic action of the redox mediator on cometabolic processes. Regarding the microbial groups, whereas Proteobacteria and Bacterioidetes increased, Planctomycetes decreased in both reactors. However, TMP and SMX presence seemed to inhibit or favor some genera during the formation of the granules, possibly due to their bactericidal action.

Effect of Aeration Mode on Microbial Structure and Efficiency of Treatment of TSS-Rich Wastewater from Meat Processing

The present study investigated the effect of aeration mode on microbial structure and efficiency of treatment of wastewater with a high concentration of suspended solids (TSS) from meat processing in sequencing batch reactors (R). R1 was constantly aerated, while in R2 intermittent aeration was applied. DNA was isolated from biomass and analyzed using next-generation sequencing (NGS) and real-time PCR. As a result, in R1 aerobic granular sludge was cultivated (SVI30 = 44 mL g−1 MLSS), while in R2 a very well-settling mixture of aerobic granules and activated sludge was obtained (SVI30 = 65 mL g−1 MLSS). Intermittent aeration significantly increased denitrification and phosphorus removal efficiencies (68% vs. 43%, 73% vs. 65%, respectively) but resulted in decomposition of extracellular polymeric substances and worse-settling properties of biomass. In both reactors, microbial structure significantly changed in time; an increase in relative abundances of Arenimonas sp., Rhodobacterace, Thauera sp., and Dokdonella sp. characterized the biomass of stable treatment of meat-processing wastewater. Constant aeration in R1 cycle favored growth of glycogen-accumulating Amaricoccus tamworthensis (10.9%) and resulted in 2.4 times and 1.4 times greater number of ammonia-oxidizing bacteria and full-denitrifiers genes in biomass, respectively, compared to the R2.

Nutrient metabolism, mass balance, and microbial structure community in a novel denitrifying phosphorus removal system based on the utilizing rules of acetate and propionate

The effect of acetate (HAc) and propionate (HPr) on denitrifying phosphorus removal (DPR) was evaluated in a novel two-sludge A²/O - MBBR (anaerobic/anoxic/oxic - moving bed biofilm reactor) system. Results showed that it was the carbon source transformation and utilization especially the composition of poly-β-hydroxyalkanoates (PHA) (mainly poly-β-hydroxybutyrate (PHB) and poly-bhydroxyvalerate (PHV)) decided DPR performance, where the co-exist of HAc and HPr promoted the optimal nitrogen (85.77%) and phosphorus (91.37%) removals. It facilitated the balance of PHB and PHV and removing 1 mg NO₃^- (PO₄^3-) consumed 3.04-4.25 (6.84-9.82) mgPHA, where approximately 40-45% carbon source was saved. Mass balance revealed the main metabolic pathways of carbon (M_An,C (consumed amount in anaerobic stage) and M_A-O,C (consumed amount in anoxic and oxic stages): 66.38-76.19%), nitrogen (M_DPR,N (consumed amount in DPR): 57.01-65.75%), and phosphorus (M_WS,P (discharged amount in waste sludge): 81.05-85.82%). Furthermore, the relative abundance and microbial distribution were assessed to elucidate DPR mechanism (e.g. Accumulibacter, Acinetobacter, Dechloromonas, Competibacter, and Defluviicoccus) in the A²/O reactor and nitrification performance (e.g. Nitrosomonas, Nitrosomonadaceae and Nitrospira) in the MBBR. Carbon source was demonstrated as the key point to stimulate the biodiversity and bioactivity related to DPR potential, and the operational strategy of carbon source addition was proposed based on the utilizing rules of HAc and HPr.

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

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

Pilot-scale aerobic granular sludge in the treatment of municipal wastewater: Optimizations in the start-up, methodology of sludge discharge, and evaluation of resource recovery

This work evaluated the formation, maintenance, performance, and microbiology of a pilot-scale aerobic granular sludge reactor treating low-strength municipal wastewater under tropical climate conditions. Additionally, different resource recovery possibilities (phosphorous, tryptophan, and alginate-like exopolysaccharides) were investigated from the produced sludge. Granulation occurred after 35 days without external carbon source supplementation (COD_inf ≈ 461 mg/L; COD/DBO₅ ≈ 3.2). Some protocols were implemented: (i) fat separation to decrease granule flotation; (ii) high exchange rates (60%) during rainy periods to increase the organic load; (iii) selective sludge discharge methodology. After granules formation, optimizations were done to improve reactor performance (COD, BOD, NH₄⁺, and PO₄^3- removals close to 90%), and energy demand reduced from 0.43 (start-up) to 0.25 kWh/m³ (after optimizations). The produced sludge had a high concentration of phosphorus (0.020 g P/g VSS), tryptophan (0.048 g tryptophan/g VSS), and alginate-like exopolysaccharides (0.219 g ALE/g VSS), indicating a good resource recovery possibility.

Bioaugmentation of low C/N ratio wastewater: Effect of acetate and propionate on nutrient removal, substrate transformation, and microbial community behavior

The effect of various acetate/propionate ratios (1:0, 2:1, 1:1, 1:2, and 0:1) in a two-sludge A2/O - MBBR process was investigated. Results showed that the increased propionic/acetic ratios exerted indistinctive impact on COD (91.21-93.44%) and P (92.23-93.87%) removals, but high P content (7.42%) accelerated sludge granulation proved by SEM and EDS. Acetate favored N removal (79.52%-82.92%) with higher PURA (3.53-4.06 mgP/(gVSS·h)), while the removal declined (75.14%) due to lower PHB/PHA ratio (52.3-57.8%) with propionate as sole carbon source. Based on the stoichiometry-based quantifications, PAOs were the major contributors to nutrient removal although certain GAOs and OHO participated. The mixture ratio of 1:1 facilitated microbial diversity (995 OTUs), Rhodobacteraceae (25.63%) was responsible for high-efficient denitrifying phosphorus removal, while Defluviicoccus (15.23%) contributed to nitrite accumulation was the main competitiveness with PAOs. Nitrospira, Nitrosomonas, and Nitrosomonadaceae responsible for nitrification accounted for 7.73%, 27.11%, and 38.76% in MBBR, but the biodiversity decreased owing to the enrichment and purification.

Limited simultaneous nitrification-denitrification (SND) in aerobic granular sludge systems treating municipal wastewater: Mechanisms and practical implications

Simultaneous nitrification-denitrification (SND) is, in theory, a key advantage of aerobic granular sludge systems over conventional activated sludge systems. But practical experience and literature suggests that SND and thus total nitrogen removal are limited during treatment of municipal wastewater using AGS systems. This study thus aims at quantifying the extent and understanding the mechanisms of SND during treatment of municipal wastewater with aerobic granular sludge (AGS) systems. Experiments (long-term and batch-tests) as well as mathematical modelling were performed. Our experimental results demonstrate that SND is significantly limited during treatment of low-strength municipal wastewater with AGS systems (14-39%), while almost full SND is observed when treating synthetic influent containing only diffusible substrate (90%). Our simulations demonstrate that the main mechanisms behind limited SND are (1) the dynamics of anoxic zone formation inside the granule, (2) the diffusibility and availability of electron-donors in those zones and (3) the aeration mode. The development of anoxic zones is driven by the utilisation of oxygen in the upper layers of the granule leading to transport limitations of oxygen inside the granule; this effect is closely linked to granule size and wastewater composition. Development of anoxic zones during the aerobic phase is limited for small granules at constant aeration at bulk dissolved oxygen (DO) concentration of 2 mgO₂ L^-1, and anoxic zones only develop during a brief period of the aerated phase for large granules. Modelling results further indicate that a large fraction of electron-donors are actually utilised in aerobic rather than anoxic redox zones - in the bulk or at the granule surface. Thus, full SND cannot be achieved with AGS treating low strength municipal wastewater if a constant DO is maintained during the aeration phase. Optimised aeration strategies are therefore required. 2-step and alternating aeration are tested successfully using mathematical modelling and increase TN removal to 40-79%, without compromising nitrification, and by shifting electron-donor utilisation towards anoxic redox conditions.

Cultivation of granules containing anaerobic decolorization and aerobic degradation cultures for the complete mineralization of azo dyes in wastewater

Granules which could efficiently mineralize azo dyes were cultivated through immobilization of aerobic degradation strains in a core composed of anaerobic decolorization cultures. The core was obtained in a up-flow anaerobic sludge blanket (UASB) reactor incubated with anaerobic decolorization bacteria. Aerobic degradation strains were then grown on the surface of the anaerobic core in a sequencing batch reactor (SBR). Three of the granules' surface layers demonstrated the occurrence of immobilization. The granulation process was monitored with 16S rDNA high throughput sequencing. Anaerobic decolorization cultures belonging to the genera of unclassified, Levilinea, and Petrimonas and the aerobic degradation genera of Thauera, unclassified, Thermomonas, and Ottowia were successfully fixed in the granules. The obtained granules were capable of decolorizing azo dyes under anaerobic situation, and the generated aromatic amines were then completely mineralized in aerated environment. Comparative studies on the relationship between removed contaminates and typical components concentrations in low to high strength azo dye wastewater showed that the granules have great potentials in treating wastewater with different complexity. The removal efficiency of COD and TOC was not restricted by loading concentrations.

Performance and population structure of two carbon sources granular enhanced biological phosphorus removal systems at low temperature

This study explored the effect of two carbon sources on performance and population structure of granular enhanced biological phosphorus removal systems at long-term low temperature by using two sequencing batch reactors, with acetate (SBR-1) and propionate (SBR-2) as carbon sources respectively. Results showed that highly efficient EBPR were successfully achieved, and the average PO₄^3--P and COD removal efficiency of SBR-1 and SBR-2 were 94.2%, 87.1% and 98.2%, 87.0%, respectively. Moreover, the acetate system preferred to utilize intracellular Mg/K-polyP to produce ATP for VFA uptake rather than glycogen. High-throughput sequencing analysis revealed that the abundance of Rhodocyclaceae were 31.7% (SBR-1) and 71.7% (SBR-2), and genus Dechloromonas was enriched to 60.5% with propionate, evidently higher than acetate (1.2%). Furthermore, in addition to oxygen, Dechloromonas could use nitrate as electron acceptors for phosphate uptake. The study further provides support to simultaneous nitrogen and phosphorus removal at low temperature.

Impact of cycle type on aerobic granular sludge formation, stability, removal mechanisms and system performance

This paper aimed to assess the impact of the cycle type on aerobic granular sludge (AGS) formation, stability and system performance. Six AGS reactors were operated either on A/O cycles (anaerobic followed by oxic phase) or A/O/A cycles (anaerobic, followed by oxic and anoxic phases), changing only the phase time distribution. Reactors with high percentage of aerobic phase (65% of the total cycle time) generated granules with better settleability and resistance, however denitrification was impaired. On the other hand, reactors with long anaerobic or anoxic phases presented excellent nutrients removals, but the granules were fluffy and unstable. The best results in terms of performance and stability were achieved in an A/O/A reactor with short anoxic phase (10% of the total cycle) and medium aerobic phase (55% of the total cycle). Therefore, in AGS reactors, it is indispensable to optimize the cycle, aiming at fast biomass formation, long-term granule stability and high-rate pollutants removal.

Effect of calcium addition on the formation and maintenance of aerobic granular sludge (AGS) in simultaneous fill/draw mode sequencing batch reactors (SBRs)

This work investigated the effect of Ca²⁺ (100 mg L^-1) addition on the formation and maintenance of aerobic granular sludge in a simultaneous fill/draw mode sequencing batch reactor (SBR), operated with a low liquid upflow velocity (0.92 m h^-1), in order to verify if Ca²⁺ presence compensates the low selection pressure imposed. Additionally, carbon and nutrients removals, granules characteristics and microbial community were evaluated. For this, two SBRs (R1, control, and R2, Ca²⁺-supplemented) were operated (6-h cycle). In general, Ca²⁺ supplementation affected positively the sludge settleability, although a larger fraction of inert solids was found in the granules. The total extracellular polymeric substances were the same for both reactors, and no remarkable differences were observed between their polysaccharides and proteins contents. Overall, Ca²⁺ addition in a simultaneous fill/draw mode SBR neither accelerated the granule formation nor improved the operational performance. The microbial community structure, especially in terms of bioactivity, was not affected as well. Therefore, the effect of divalent cations might be more pronounced in conventional SBRs, in which the selection pressure is higher.

Comparison of the dynamics, biokinetics and microbial diversity between activated sludge flocs and aerobic granular sludge

This work aimed to compare the dynamics, biokinetics, and microbial diversity between activated sludge flocs (ASF) and aerobic granular sludge (AGS) whose systems were operated under similar experimental conditions in terms of inoculum, feeding, substrate source, etc. Therefore, the kinetic parameters involved in the organic matter removal, nitrification, denitrification, and dephosphatation were determined, as well as the microbial changes were assessed by metagenomics analysis. Regarding the kinetic parameter yield coefficient (Y), values of 0.55 and 0.36 g VSS/g COD were found for ASF and AGS, respectively, showing a higher sludge production in ASF and the importance of feast/famine periods for lowering sludge production in AGS systems. AGS presented a lower sludge production and a higher endogenous consumption rate than ASF. The activity of phosphorus-accumulating bacteria was remarkably higher in AGS. Although both biomasses were aerobic, their kinetic parameters had significant differences.

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.

The cotreatment of landfill leachate and high-nitrate wastewater using SBRs: evaluation of denitrification performance and microbial analysis

Resourceful disposal of landfill leachate has always been an intractable worldwide problem. This study was conducted to investigate the feasibility of biologically treating a combined waste stream of landfill leachate and high-concentration nitrate nitrogen (high-nitrate) wastewater. Raw landfill leachate was pretreated using anaerobic fermentation and ammonia stripping to improve biodegradability. The control sequencing batch reactor (SBR, named R0) was fed only with synthetic high-nitrate wastewater with sodium acetate as the carbon source, whereas the other experimental SBR (named R1) was loaded with mixtures containing leachates. Excessive increase in leachate adversely affected the cotreatment, and it was concluded that the landfill leachate volume ratio should never exceed 7.5% of the total wastewater (14% of the initial COD) based on further batch experiments. The maximum specific denitrification rate of 58.05 mg NO₃⁻-N (gVSS h)⁻¹ was attained in R1, while that of 32.32 mg NO₃⁻-N (gVSS h)⁻¹ was obtained in R0. Illumina MiSeq sequencing revealed that adding landfill leachate did not change the fact that Pseudomonas, Thauera, and Pannonibacter dominant in the sodium acetate supported the denitrification systems, but led to the adjustment of their relative abundance. Moreover, the narG, nirK, nirS, and norB denitrifying genes exhibited increased abundance by 138–980% in the cotreated system, which was confirmed by q-PCR analyses. These findings reveal that the denitrification efficiency of activated sludge in SBR cotreated with landfill leachate and high-nitrate wastewater significantly improved, and this may contribute toward the understanding of the molecular mechanisms of biological denitrification under the blending treatment of leachate and high-nitrate wastewater.

Resourceful disposal of landfill leachate has always been an intractable worldwide problem.