Sludge Aggregation Research Articles

Response of anammox to organics with different degradation characteristics and exposure time: Performance, sludge characteristics and bacterial community

The effects of typical organic compounds including easily degradable organic matters sodium acetate, yeast and methanol, and refractory organic matter (ROM) humic acid on anaerobic ammonium oxidation (anammox) systems in short-term and medium-term exposure time were studied. During short-term experiments, nitrogen removal activity (NRA) was inhibited at sodium acetate level of 150 mg L−1 total organic carbon (TOC) and methanol level of 30–150 mg L−1 TOC, but humic acid and yeast (≤150 mg L−1 TOC) enhanced nitrogen removal in anammox systems. The greatest NRA of 30.10 mg TN g−1 VSS h−1 was recorded at yeast level of 90 mg L−1 TOC. In medium-term experiments, organics significantly inhibited the nitrogen removal ability. As a ROM, humic acid enhanced sludge aggregation and biological diversity, but decreased the bioactivity and extracellular polymeric substances levels. Due to the endogenous denitrification, the relative abundance of anammox bacteria (AnAOB) was decreased. Candidatus Kuenenia is still dominant in sludge with methanol and humid acid, but AnAOB are not dominant due to the addition of sodium acetate and yeast. This research would be beneficial for the full-scale application of the anammox process in treating real wastewater with organics and ammonia.

Illuminating the role of powder carrier materials in shaping sludge aggregation in wastewater treatment: Insights from extended operation performance to microbial response mechanism

This study uncovered the response of novel micro-granule wastewater treatment technology to different powder carrier materials. Characteristics and distinctions among different systems were assessed based on process performance, sludge aggregation capacity, and microbial metabolism. Zeolite carrier system exhibited remarkable nitrogen removal efficiency of 89.6 ± 0.9 %, while diatomite carriers, in conjunction with intermittent aeration, enhanced simultaneous nitrification and denitrification from 2.6 % to 27.1 %. Iron-based carriers demonstrated efficient phosphorus removal (94.7 ± 1.2 %) through both chemical and microbial pathways. Specific surface area, pore structure and biocompatibility of powder carriers determined the formation and size of micro-granules. Tryptophan-like substances, C-(C/H), and Npr in extracellular polymeric substances strongly correlated with sludge hydrophobicity and granulation. Significant enrichment in norank_Comamonadaceae and Nitrosomonas in zeolite powder carrier system promoted partial nitrification and endogenous denitrification. Differences in metabolic pathways elucidated the up-regulation of amino acid synthesis, energy metabolism, and membrane transport as potential mechanisms driving micro-granule formation and efficient treatment performance.

Carbon fixation via volatile fatty acids recovery from sewage sludge through electrochemical–pretreatment–based anaerobic digestion

Capturing the carbon in volatile fatty acids (VFA) produced from the anaerobic digestion (AD) of sewage sludge has the potential to not only provide economic benefits but also reduce greenhouse gas production. This study demonstrates a chemical–free method to collect VFA from an AD instead of methane that involves electrochemical pretreatment (EPT) of sludge. Experimental results show that applying 15 V EPT for 45 min enhances acidogenesis and selectively inhibits methanogenesis, leading to a substantial VFA accumulation (2563.1 ± 307.9 mg COD/L) and achieving 2.5 times more carbon fixation than via methane production. Interfacial thermodynamic analysis shows that EPT induces a decrease in both the repulsive electrostatic energy (from 152.9 kT to 12.2 kT) and the energy barrier (from 57.0 kT to 2.6 kT) in the sludge, leading to increased sludge aggregation and entrapment of microorganisms. Molecular docking sheds lights on how the methanogens interacts with the organic matter released from EPT (e.g., alanine–tRNA ligase), showing that these interactions potentially interfere with the proteins that are associated with the activities of the methanogens and the electron transfer pathways, thereby impeding methanogenesis. Integrating EPT into AD therefore facilitates the recovery of valuable VFA and the capture of carbon from freshwater sludge, providing notable economic and environmental benefits in sewage sludge treatment.

A novel process based on powder carriers demonstrates robustness in nitrogen and phosphorus removal from real municipal wastewater

The development of efficient and low-consumption wastewater upgrading process is currently at the forefront of the wastewater treatment field. In this study, a novel wastewater treatment process based on powder carriers was proposed. Three systems, namely the activated sludge (AS) system, powder carrier (PC) system, and moving bed biofilm reactor (MBBR) system, were established and operated for over 140 days to treat real municipal wastewater. The characteristics and differences between the three systems were comprehensively investigated. The results suggested that the PC system exhibited notable advantages in nitrogen and phosphorus removal, especially under high influent load and low aeration conditions. The PC system, characterized by a higher nitrification rate compared to the MBBR system and a higher denitrification rate compared to the AS system, contributed to the stable nitrogen removal performance. The particle size of the zoogloea increased under the linkage of the powder carriers, and the mean size of micro-granules reached 170.88 μm. Large number of hydrophobic functional groups on sludge surface, coupled with increased protein content in EPS, further promoted sludge aggregation. Micro-granules formation improved settling performance and enhanced the abundance and activity of functional microbes. A significant enrichment in denitrifying bacteria and denitrifying phosphorus accumulating bacteria was observed in PC system. Up-regulation of the napA, narG, and nosZ genes was responsible for efficient nitrogen removal of the PC system. Moreover, a higher abundance in polyphosphate phosphotransferase (2.11 %) was found in PC system compared with AS and MBBR systems. The increase in the enzymes associated with poly-β-hydroxybutyrate (PHB) synthesis metabolism in PC system provided the energy for denitrification and phosphorus removal processes.

Extracellular polymeric substances and microbial community shift during the start-up of a single-stage partial nitritation/anammox process.

A sequencing batch reactor (SBR) was operated to investigate variations of extracellular polymeric substances (EPS) and microbial community during the start-up of the single-stage partial nitritation/anammox (SPN/A) process at intermittent aeration mode. The SPN/A system was successfully started on day 34, and the nitrogen removal efficiency and total nitrogen loading rate were 82.29% and 0.31 kg N/(m3 ·d), respectively. Furthermore, the relationship between the protein secondary structures and microbial aggregation was strongly related. The α-helix/ (β-sheet + random coil) ratios increased obviously from 0.20 ± 0.03 to 0.23 ± 0.01, with the sludge aggregation mean size increased from 56 to 107 μm during the start-up of SPN/A. During the start-up of SPN/A, Candidatus Kuenenia was the primary anammox bacteria, whereas Nitrospira was the main functional bacteria of nitrite-oxidizing bacteria. Correlation between the microbial community and EPS components was performed. The EPS and microbial community played important roles in keeping stable nitrogen removal and the formation of sludge granules.

Rapid, stable, and highly-efficient development of salt-tolerant aerobic granular sludge by inoculating magnetite-assisted mycelial pellets

Long cultivation time hinders the industrial applications of aerobic granular sludge (AGS) in treatment of hypersaline wastewater. Mycelial pellets (MPs) have been used to efficiently strengthen the flocculent sludge aggregation and accelerate the formation of AGS. However, the MPs-based AGS was easily crushed or fragmented into several small pieces/granules that brought the uncertainty and extended the transition process to form mature AGS. In this study, magnetite was used to strengthen MPs (halotolerant fungus Cladosporium tenuissimum NCSL-XY8), and co-culture and adsorption type of magnetite-assisted mycelial pellets (CMMPs and AMMPs) were prepared and used for acceleration of salt-tolerant aerobic granular sludge (SAGS) cultivation under 3% salinity conditions. Compared to inoculating MPs, the inoculation of either CMMPs or AMMPs could stably transition to mature SAGS without evident fragmentation, which obviously increased the certainty and stability of SAGS formation. Also, highly-efficient simultaneous nitrogen and carbon removal (∼98% TOC and ∼80% TN removal) could be reached in 8 days. Typically, the granules maintained perfect characteristics (D50 > 1300 μm, D10 > 350 μm, SVI30 < 45 mL/g, and SVI30/SVI5 = 1.0) during the whole cultivation/transition processes (Day 0–55) by using the inoculum of CMMPs. ITS rDNA sequencing revealed the inoculated fungus Cladosporium tenuissimum played key roles in the formation of SAGS. All the phenomena indicated the rapid, stable, and highly-efficient start-up of SAGS could be successfully realized by inoculating CMMPs.

The culprit for the declining performance of anaerobic reactors caused by calcification: Bioavailability deterioration

Calcification is a critical challenge for achieving anaerobic reactors’ high-efficiency. However, the aggregation caused by calcification at both granular sludge and reactor levels remain to be fully understood. Herein, this study investigated the characteristics of calcification in an anaerobic reactor (RH) operated with high calcium-containing wastewater for over 200-day. It was found that the COD removal efficiency in RH dropped from 98.00 ± 2.06% to 41.29 ± 3.79%, which was much lower than that of 95.50 ± 1.55% in the control reactor. Morphological analysis revealed that the high influent calcium caused granular sludge aggregation, which would further led to the rapid deterioration in bioavailability, as confirmed by both mass transfer tests and theoretical simulations. Moving forward, statistical analysis showed that the proportion of bioavailability deterioration zones in RH system (61.68%) was similar to the decreased COD removal efficiency (57.87%), proving that bioavailability deterioration was the culprit for the performance decline of anaerobic reactor.

Absence of nitrogen and phosphorus in activated sludge: Impacts on flocculation characteristics and the microbial community

A sequencing batch reactor operated in anoxic/aerobic mode was used to investigate the effects of simultaneous nitrogen (N) and phosphorus (P) absence in activated sludge. Through controlling the N and P content in the influent water, we investigated changes in sludge concentration, settling performance, morphological structure, surface properties, extracellular polymer content, and microbial community structure and function. When N and P were simultaneously absent, the relative hydrophobicity of the activated sludge and the sludge concentration decreased. The specific oxygen uptake rate also decreased, the activated sludge did not bulk, and the flocs were larger and more concentrated. The content of extracellular polysaccharides did not significantly change. An increase in the concentration of extracellular proteins, particularly loose-binding layer proteins, improved activated sludge aggregation. The structure and function of the activated sludge microbial community changed significantly in the absence of both N and P, with a significant decrease in the relative abundance of species related to N and P metabolism. Tax4Fun analysis showed that the absence of both N and P decreased metabolism and genetic information processing of the microbial community. These results elucidate the effects of simultaneous N and P absence on activated sludge from the perspective of microbial community structure and function. These results will be useful for treatment of N and P deficient wastewater by the activated sludge method.

Potential role of quorum quenching activity of silver nanoparticles in controlling non-filamentous bulking within activated sludge process

The effective prevention and control of non-filamentous bulking is a significant challenge. In this study, the underlying effect of quorum sensing (QS) on inducing non-filamentous bulking and the maintenance effect of silver nanoparticles (AgNPs) on sludge floc stability, aggregation and settleability based on the quorum quenching (QQ) activity during non-filamentous bulking were investigated. The results showed that the concentration of N-acyl homoserine lactone (AHL) increased significantly in the activated sludge system at a high organic load rate (OLR), triggering the AHL-mediated QS. Additionally, the triggered QS promoted exopolysaccharide secretion, reducing the surface charge and hydrophobicity of the sludge aggregates, and further deteriorating the settleability of the sludge aggregates. AgNPs, a quorum sensing inhibitor (QSI), inhibited the AHL-QS based on QQ activity under high OLR, which maintained the physicochemical properties of extracellular polymeric substances (EPS). AgNPs-QQ maintained the surface energy barrier and electrostatic barrier of sludge aggregates and the gel properties of exopolysaccharides, which is favorable for microbial aggregation. The appropriate concentrations of AgNPs (≤10 mg/L) had no negative effect on biological nutrient removal in the sequencing batch reactors (SBRs) at the high organic loading. Therefore, AgNPs effectively prevent and control non-filamentous bulking by their QQ activity in the activated sludge process. Thus, the present study provided new insights into controlling non-filamentous bulking during the activated sludge process.

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

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

Heat treatment-enhanced bioleaching of new electroplating sludge containing high concentration of CuS and its mechanisms

Electroplating sludge by CaO/Ca(OH)2 precipitation craft represents a typical toxic heavy metals-bearing hazardous waste worldwide. Recently, sulfide electroplating sludge is growingly produced as a substitute for Ca(OH)2-based hydroxides sludge since sulfide precipitation process sharply reduces the production of sludge, posing an urgent challenge to the traditional treatment ways suitable for hydroxides sludge. In this work, bioleaching was firstly utilized to extract Cu from the new sulfide electroplating sludge containing high concentration of CuS, and a thermal pre-treatment was used to promote the bioleaching performance. The results showed that a low Cu dissolution of 53% from the original sulfide sludge by bioleaching was gained because of the refractory nature of sulfides. However, a thermal treatment evidently enhanced the bioleaching efficiency of sulfide electroplating sludge. The Cu release efficiency sharply rose from 59% to 100% when the roast temperature lifted from 200 °C to 600 °C. However, a higher temperature of 800 °C strongly injured the bioleaching, gaining the lowest Cu release of 46%. A full conversion of refractory CuS into highly water soluble CuSO4 occurred with the 600℃-roast, concomitant with the formation of a finer particle which witnessed a great rise in the specific surface area from 3.64 to 12.06 m2/g, accounting for 100% Cu liberation by bioleaching. In contrast, the calcination at 800 °C prompted the decomposition of CuSO4 to form CaSO4 which not only covered the sludge as coating layer but also induced sludge aggregation to form a larger particle with a smaller specific surface area of 1.22 m2/g, inhibiting the bioleaching. This present works revealed the internal mechanisms for a perfect combined process of the thermal treatment and bioleaching for tackling the sulfide electroplating sludge.

Comprehensively understanding fouling properties of cake and bulk sludge in an anammox membrane bioreactor: Focusing on the composition, interfacial thermodynamics and microbial community

A more comprehensive investigation into fouling properties of cake and bulk sludge played an essential role in better understanding membrane fouling caused by the bio-cake formation in an anammox membrane bioreactor (MBR). In this work, the cake and bulk sludge from a stable anammox MBR were analyzed with the composition, interfacial thermodynamics and microbial community. Physicochemical analysis indicated that cake sludge agglomerated into larger flocs with a denser structure and a higher protein/polysaccharide (PN/PS) ratio as compared with bulk sludge. It also revealed that cake sludge was found to have looser protein secondary structures and more hydrophobic components than those of bulk sludge, which means the differences in surface hydrophobicity between two sludges might play a key role in revealing anammox sludge aggregation and further cake layer formation process. The dominant role of hydrophobic attraction energies between cake sludge and membrane surface at a shorter separation distance (< 2 nm), after the analysis of the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) method, confirmed cake layer formation process related with surface hydrophobicity. Furthermore, biological analysis demonstrated that the relative microbial abundances of Candidatus_Brocadia and Candidatus_Kuenenia in cake sludge were much greater than those of bulk sludge, implying a higher biofouling potential for them. Overall, this study fills the gap in understanding the mechanism between anammox sludge properties and cake formation, which provides basic knowledge for membrane fouling control in anammox MBR.

Effect of different forms and components of EPS on sludge aggregation during granulation process of aerobic granular sludge.

The characteristics of three forms of extracellular polymeric substances (EPS) and their component proteins (PN) and polysaccharides (PS) during the granulation process of aerobic granular sludge (AGS) and their effects on the aggregation performance of AGS were studied. Meanwhile, Pearson correlation analysis was conducted to explore the correlations between different forms of EPS and their components and sludge properties in the granulation process. The results showed that the AGS reactor could be started within 40 days by using the strategy of decreasing sedimentation time, and the sludge granulation degree was 90.22%. It was found that the content of tightly bound EPS (TB-EPS) accounted for 72.81-93.11% of the total EPS. Compared with dissolved EPS (S-EPS) and loosely bound EPS (LB-EPS), TB-EPS had a great impact on the zeta potential and hydrophobicity of sludge surface. During the granulation process, the content and relative hydrophobicity of PN in TB-EPS increased from 30.38±0.12mg/g VSS to 83.99±2.05mg/g VSS and 37.63%-71.25%, respectively. Through the flocculation experiment, it showed that the contribution rate of TB-EPS to the flocculation capacity of sludge cells increased from 13.30% to 35.37% and that of PN to flocculation capacity was 76.93%, which was 6.8 times that of PS. It indicated that TB-EPS played the most critical role in the formation of AGS and the PN in EPS was the major contributor to promoting the formation of AGS and enhance its aggregation performance. This research could further understand the role of various forms of EPS and its components in the process of sludge granulation, reveal the sludge granulation mechanism, and provide theoretical support for the rapid start-up of AGS reactor.

Heterotrophic bio-reduction process of hexavalent chromium: Toxic effects, concentration-adaptation and sustainable sludge-based bio-augmentation strategy

For high hexavalent chromium (Cr(VI)) industrial wastewater, heterotrophic bio-reduction is a promising process. Nevertheless, a high concentration of Cr(VI) (e.g., more than 100 mg L−1) may still devastate the sustainability of the bioprocess. In this paper, the Cr(VI) toxic effects and resistance on concentration-adaptation and a sludge-based bio-augmentation were studied. The results suggested: 1) The Cr(VI) exposure took 6–24 h to cause significantly the toxic inhibition and lethal effects on the sludge communities. 2) Although adaptation to 50 mg L−1 influent Cr(VI) improved Cr(VI) reduction by 2 times, the stepwise adaptation was ineffective for 100 mg L−1 Cr(VI). Lowering the Cr(VI) loading could not recover the dysfunctional sludge. 3) A bio-augmentation strategy by exchanging Cr(VI) stressed sludge with Cr(VI) free sludge at intervals (e.g., 0.8 gMLVSS L−1 every 3.5 d) was proposed and treated successfully 100 mg L−1 Cr(VI). The mechanism behind might be that exchanging sludge at intervals maintained a relatively high Cr(VI) reduction bacteria activity and a stable live/dead bacteria ratio. 4) The variation of Cr(VI) reductase distribution, morphology and microbial community during the concentration-adaption and bio-augmentation implied the sludge might use multiple toxic resistance mechanisms such as sludge aggregation, the enrichment of Cr(VI) reduction bacteria. Therefore, through comprehensively testing, this work not only revealed the draws of concentration-adaptation for Cr(VI), but also developed a sustainable, convenient and robust sludge-based bio-augmentation strategy to overcome the highly toxic Cr(VI) wastewater.

A hindered settling velocity model related to the fractal dimension and activated sludge flocs characteristics: Application to a sludge with a previous fragmentation and flocculation process

The modelling of the activated sludge hindered settling velocity as a function of the flocs characteristics and the incorporation of these models to the secondary settling tanks simulation is a hardly studied subject. Commonly used empirical models cannot describe the changes in the settling velocity caused by changes in the flocs characteristics. In this paper, a model for the hindered settling velocity as a function of the fractal dimension and other flocs characteristics is proposed. The model was used to describe the settling velocity after a flocculation process originated by activated sludge fragmentation. The model reproduces the observed abrupt decrease of hindered settling velocity in a small range of suspended solids concentration. It also enables to relate the flocs characteristics and the settling velocity to the fragmentation and sludge aggregation mechanisms.

Influence of nitrogen-poor wastewater on activated sludge aggregation and settling: Sequential responses of extracellular proteins and exopolysaccharides

To explore the mechanism by which the degree of wastewater nitrogen deficiency affects the aggregation and sedimentation of activated sludge, the structure, aggregation, sedimentation and thermodynamic properties of activated sludge and the composition, structure and properties of extracellular polymers under different influent nitrogen contents were examined. Hydrophobicity was the main factor affecting sludge aggregation. When nitrogen was limited (C/N = 100:2), the sludge aggregation ability decreased to 38.1 ± 1.1%, but with increases in extracellular protein (PN) content (89.8% increase) and hydrophobicity, the sludge aggregation ability returned to the level in the control phase. Under nitrogen deficiency (C/N = 100:0), the PN/exopolysaccharide (PS) decreased to 1.36 ± 0.09, and the increased hydrophilicity of PS led to a decrease in sludge aggregation ability (48.5 ± 1.0%) and a loose structure, which reduced sludge settling performance. In addition, PN and PS sequentially responded to changes in influent nitrogen content and played key roles in sludge aggregation. This study clarifies the mechanism by which the degree of wastewater nitrogen deficiency affects the aggregation and sedimentation of activated sludge from the perspective of extracellular polymers (EPS), which will help promote the further application of activated sludge processes.

New insight into filamentous sludge bulking: Potential role of AHL-mediated quorum sensing in deteriorating sludge floc stability and structure

The microcosmic mechanisms underlying filamentous bulking remain unclear. The role of extracellular polymeric substances (EPS) governed by quorum sensing (QS) in deteriorating sludge floc stability and structure during filamentous bulking and the feasibility of using quorum quenching (QQ) to maintain sludge floc stability and structure and sludge settling were investigated in this study. The results indicated that the concentration of C6HSL increased from 22.08±3.22 ng/g VSS to 81.42±5.98 ng/g VSS during filamentous bulking. The filamentous bacteria gradually evolved the hdtS gene related to the synthesis of C6HSL with increases in the population density. Triggered QS by filamentous bacteria proliferation induced variation in the composition and structure of EPS within the sludge flocs. The proteins (PN) content of the EPS increased evidently from 40.06 ± 2.41 mg/g VSS to 110.32 ± 4.32 mg/g VSS, and the polysaccharides (PS) content slightly increased during filamentous bulking. The upregulated proteins in the EPS led to a decrease in the relative hydrophobicity of the sludge and an increase in negative surface charge. The α-helix/(β-sheet+random coil) ratio evidently increased from 0.76 to 0.99 during filamentous bulking, revealing that the proteins were tightly structured, which prevented the exposure of inner hydrophobic groups. The total energy of the interaction (WT) between bacteria increased during sludge bulking, which resulted in the weakening of sludge aggregation. Variation in the physicochemical properties of EPS induced by QS in the filamentous bacteria markedly restrained adhesion between the filamentous bacteria and floc-forming bacteria. The production of PN in the EPS and the expression of the hdtS gene were inhibited by vanillin, which served as a QS inhibitor. The WT between bacteria with 50 mg/L of vanillin basically did not change. Filamentous bulking was significantly inhibited by the addition of vanillin. Therefore, QQ is a potential strategy for the prevention and control of filamentous bulking. This study provides new information regarding the microcosmic mechanisms of filamentous bulking.

Rapid start-up of photo-granule process in a photo-sequencing batch reactor under low aeration conditions: Effect of inoculum AGS size

The photo-granule process is an effective and economically feasible alternative for wastewater treatment, but little information is available regarding how to speed up the photo-granulation process. In this study, the effect of inoculum aerobic granular sludge (AGS) size on the start-up of the photo-granule process was investigated under low aeration conditions (superficial gas velocity of 0.5 cm/s). For this purpose, the inoculum AGS was sorted into various size-categories (0.4–0.8 mm, 0.8–1.4 mm, 1.4–2.2 mm, and > 2.2 mm) to serve as individual inoculum sludge. The excellent settling properties (SVI5 of 39.3 mL/g), strong mechanical strength, efficient nutrient removal (COD: 94.2–97.1%; TN: 80.1–84.8%; TP: 60.4–91.5%), and high biodiesel yields (12.11 mg/g MLSS) were rapidly achieved in the system inoculated with 0.8–1.4 mm AGS. The granulation process was facilitated by filamentous algae as the nucleus, extracellular polymeric substances as the backbone, and the enrichment of functional bacteria (such as Thauera and Sphingorhabdus). Furthermore, the inherent influencing mechanisms of inoculum AGS size on the photo-granulation were revealed from cellular hydrophobicity, surface thermodynamics, and sludge aggregation behavior. This study provides a novel start-up approach of the photo-granule process by inoculating with the optimal AGS size, which is convenient, practically feasible and significantly reduced the aeration consumption.

Phosphorus deficiency leads to the loosening of activated sludge: The role of exopolysaccharides in aggregation

Whether phosphorus deficiency in influent will affect the aggregation and sedimentation of activated sludge needs to be further clarified. This paper systematically studied the structure, aggregation and settlement of activated sludge, and the composition, properties and chemical structure of extracellular polymers and microbial community structure of sludge under different influent phosphorus contents to determine the causes of sludge aggregation and structural deterioration. The results show that phosphorus deficiency in influent leads to a decrease in the aggregation capacity and a loose structure of activated sludge, and the reduction of hydrophobic interactions is the main factor of sludge aggregation and structural deterioration. The content, functional groups and protein secondary structure of extracellular protein were almost unchanged. An increase in the content and hydrophilicity of extracellular polysaccharide (PS) results in a decrease in sludge hydrophobicity. Under phosphorus deficiency, the changes in microbial species related to PS secretion were the reasons for the increase in PS content and hydrophilicity. The negative effects of PS content and hydrophilicity on sludge aggregation and structure are important findings of this work and are expected to be useful for better understanding the restoration of activated sludge performance in the treatment of phosphorus-deficient wastewater.

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

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