Aerobic Granulation: Advances and Challenges

Powdered activated carbon facilitated degradation of complex organic compounds and tetracycline in stressed anaerobic digestion systems

Tetracycline exerts an inhibitory effect on anaerobic digestion, inducing stressed microbial activities and even system failure. Continuous-flow reactors (CFRs) and sequencing batch reactors (SBRs) were employed along with the dosage of powdered activated carbon (PAC) to enhance tetracycline removal during anaerobic digestion of complex organic compounds. PAC increased the maximum methane production rate by 15.6% (CFRs) and 13.8% (SBRs), and tetracycline biodegradation by 24.4% (CFRs) and 19.2% (SBRs). CFRs showed higher tetracycline removal and methane production rates than SBRs. Geobacter was enriched in CFRs, where Methanothrix was enriched with the addition of PAC. Desulfomicrobium harbored abundant propionate degradation-related genes, significantly correlating with tetracycline removal. The genes encoding carbon dioxide reduction in Methanothrix along with the detection of Geobacter might indicate direct interspecies electron transfer for methanogenesis in CFRs and PAC-added reactors. The study offers new insights into anaerobic digestion under tetracycline-stressed conditions and strategies for optimizing tetracycline removal.

Pilot-scale investigation of performance and microbial community in a novel system combining fixed and suspended activated sludge

The conventional activated sludge (CAS) process is a widely used method for wastewater treatment due to its effectiveness and affordability. However, it can be prone to sludge abnormalities such as sludge bulking/foaming and sludge loss, which can lead to a decrease in treatment efficiency. To address these issues, a novel bag-based fixed activated sludge (BBFAS) system utilizing mesh bags to contain the sludge was developed for low carbon/nitrogen ratio wastewater treatment. Pilot-scale experiments demonstrated that the BBFAS system could successfully avoid the sludge abnormalities. Moreover, it was not affected by mass transfer resistance and exhibited significantly higher nitrogen removal efficiency, surpassing that of the CAS system by up to 78%. Additionally, the BBFAS system demonstrated comparable organic matter removal efficiency to CAS system. 16S rRNA gene high-throughput sequencing revealed that the bacterial community structure within the BBFAS system was significantly different from that of the CAS system. The bacteria associated with ammonium removal were more abundant in the BBFAS system than in the CAS system. The abundance of Nitrospira in the BBFAS could reach up to 6% and significantly higher than that in the CAS system, and they were likely responsible for both ammonia-oxidizing and nitrite-oxidizing functions. Clear stratification of microbial communities was observed from the outer to inner layers of the bag components due to the gradients of dissolved oxygen and other substrates. Overall, this study presents a promising approach for avoiding activated sludge abnormalities while maintaining high pollutant removal performance.

Managing stability of aerobic granules by coordinating diameter and denitrification

Aerobic Granular Sludge (AGS) technology is a promising solution for wastewater treatment due to its structure and high biomass retention capacity. However, the stability of AGS is still a challenge for widespread use. This study investigated the relationships among granule stability, granule diameter, biomass retention capacity, and denitrification efficiency. The results showed that granule diameter did not necessarily indicate granule stability, nor was it associated with biomass retention capacity. For mature granules, promoting simultaneous nitrification and denitrification rather than anoxic denitrification was found to improve granule stability. The deterioration of clarification capacity caused by increased anoxic denitrification at high nitrate concentration was not indicated by diameters or the commonly used SVI5/SVI30. Therefore, ensuring coordination between diameter and denitrification control is crucial for the stability of AGS. These results provide a basis for further research and development of efficient and user-friendly methods for monitoring granular stability.

Granulation of partial denitrification sludge: Advances in mechanism understanding, technologies development and perspectives

The high-rate and stably efficient nitrite generation is vital and still challenges the wide application of partial denitrification (PD) and anammox technology. Increasing attention has been drawn to the granulation of PD biomass. However, the knowledge of PD granular sludge is still limited in terms of granules characterization and mechanisms of biomass aggregation for high nitrite accumulation. This work reviewed the performance and granulation of PD biomass for high nitrite accumulation via nitrate reduction, including the system start-up, influential factors, granular characteristics, hypothetical mechanism, challenges and perspectives in future application. The physiochemical characterization and key influential factors were summarized in view of nitrite production, morphology analysis, extracellular polymer substance structure, as well as microbial mechanisms. The PD granules exhibit potential advantages of a high biomass density, good settleability, high hydraulic loading rates, and strong shock resistance. A novel granular sludge-based PD combined with anammox process was proposed to enhance the capability of nitrogen removal. In the future, PD granules utilizing different electron donors is a promising way to broaden the application of anammox technology in both municipal and industrial wastewater treatment.

Effects of micro-bubble aeration on the pollutant removal and energy-efficient process in a floc-granule sludge coexistence system

To investigate energy-saving approaches in wastewater treatment plants and decrease aeration energy consumption, this study successfully established a floc-granule coexistence system in a sequencing batch airlift reactor (SBAR) employing micro-bubble aeration. The analysis focused on granule formation and pollutant removal under various aeration intensities, and compared its performance with a traditional floc-based coarse-bubble aeration system. The results showed that granulation efficiency was positively associated with aeration intensity, which enhanced the secretion of extracellular polymeric substances (EPSs) and facilitated granule formation. The SBAR with the micro-aeration intensity of 30 mL·min-1 showed the best granulation performance (granulation efficiency 52.6%). In contrast to the floc-based system, the floc-granule coexistence system showed better treatment performance, and the best removal efficiencies of NH4+-N, TN, and TP were 100.0, 77.0, and 89.5%, respectively. The floc-granule coexistence system also enriched higher abundance of nutrients removal microbial species, such as Nitrosomonas (0.05-0.14%), Nitrospira (0.14-2.32%), Azoarcus (2.95-12.17%), Thauera (0.43-1.95%), and Paracoccus (0.76-2.89%). The energy-saving potential was evaluated, which indicated it is feasible for the micro-aeration floc-granule coexistence system to decrease the aeration consumption by 14.4% as well as improve the effluent.

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

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

Anaerobic fermentation of aerobic granular sludge: Insight into the effect of granule size and sludge structure on hydrolysis and acidification

Aerobic granular sludge (AGS) has different physicochemical properties and microbial communities compared to conventional activated sludge (CAS), which may result in different behaviors during anaerobic fermentation and require further investigation. This study investigated the effect of granule size and sludge structure on the hydrolysis and acidification of AGS. Experimental results show that AGS exhibited significantly higher soluble chemical oxygen demand (SCOD) dissolution and total volatile fatty acids (TVFA) production (330.6-430.3 mg/gVSS and 231.0-312.5 mgCOD/gVSS) compared to conventional activated sludge (CAS) (167.0 mg/gVSS and 133.3 mgCOD/gVSS). This is because AGS (90.6-96.9 mg/gVSS) had higher extracellular polymeric substances (EPS) content than CAS (81.2 mg/gVSS). EPS can not only serve as substrates but also release the trapped hydrolases. Moreover, the relative abundances of hydrolytic/acidogenic bacteria and genes were higher in AGS (0.46%-3.60% and 3.01 × 10^-3%-4.04 × 10^-3%) than in CAS (0.30% and 1.23 × 10^-3%). The optimal granule size for AGS fermentation was found to be 500-1600 μm. The crushing of granule structure promoted the dissolution of small amounts of EPS and the release of some trapped hydrolases, thereby potentially enhancing the enzyme-substrate contacts and bacteria-substrate interactions. Therefore, the highest SCOD dissolution (510.6 mg/gVSS) and TVFA production (352.1 mgCOD/gVSS) from crushed 500-1600 μm AGS were observed. Overall, the findings of this study provide valuable insights into the recovery of organic carbon from AGS via anaerobic fermentation.

Celebrating 50 years of microbial granulation technologies: From canonical wastewater management to bio-product recovery

Microbial granulation technologies (MGT) in wastewater management are widely practised for more than fifty years. MGT can be considered a fine example of human innovativeness-driven nature wherein the manmade forces applied during operational controls in the biological process of wastewater treatment drive the microbial communities to modify their biofilms into granules. Mankind, over the past half a century, has been refining the knowledge of triggering biofilm into granules with some definite success. This review captures the journey of MGT from inception to maturation providing meaningful insights into the process development of MGT-based wastewater management. The full-scale application of MGT-based wastewater management is discussed with an understanding of functional microbial interactions within the granule. The molecular mechanism of granulation through the secretion of extracellular polymeric substances (EPS) and signal molecules is also highlighted in detail. The recent research interest in the recovery of useful bioproducts from the granular EPS is also emphasized.

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

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

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

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

Energy efficiency assessment of China wastewater treatment plants by unit energy consumption per kg COD removed

Unit energy consumption per kg pollutant removed (kWh/kgCOD_removed) is used for the first time in assessing and ranking the sustainability of main treatment technologies of 1215 wastewater treatment plants (WWTPs) in China. The metric better measures the sustainability of main treatment technologies in WWTPs than unit energy consumption per cubic meter treated (kWh/m³). The energy consumption data of these WWTPs were selected from the database of 1399 WWTPs to evaluate the energy efficiency of different treatment technologies. 80.3% of the WWTPs applied anaerobic-oxic plus anaerobic-anoxic-oxic, oxidation ditch, and sequencing batch reactor as main technologies. Statistical analysis shows that the unit energy consumption of WWTPs decreases with increasing design flow rate, operation loading rate, and influent COD concentration. For example, the average unit energy consumption of SBR decreases from 2.76 kWh/kgCOD_removed to 0.83 kWh/kgCOD_removed when the design flow rate increases from less than 10,000 m³/d to 100,000-200,000 m³/d. The mean unit energy consumption of SBR decreases from 1.71 kWh/kgCOD_removed to 1.32 kWh/ kgCOD_removed and 2.85 kWh/ kgCOD_removed to 0.63 kWh/kgCOD_removed as the operation loading rate and COD removal increase from 40% to 100% and from less than 150 mg/L to over 450 mg/L, respectively. SBR has the lowest unit energy consumption among all the technologies. Therefore, SBR might be the most appropriate technology in small and medium-scale WWTPs in China. Regression equations were developed to predict the unit energy consumption for sustainable design treatment trains by input variables such as design flow rate, operation loading rate, and influent COD concentration.

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.

Response of performance, sludge characteristics, and microbial communities of biological phosphorus removal system to salinity

The effects of salinity on highly enriched polyphosphate- or glycogen-accumulating organisms (PAOs or GAOs) have been revealed, which is meaningful but idealized. In this study, three salinity levels (0.5%, 1.0%, and 0.75%) were sequentially adopted in a PAOs and GAOs coexisted biological phosphorus removal (BPR) reactor within 150 days. Compared to a slight decrease of phosphorus removal efficiency (PRE) under 0.5% salinity (from 96.09% to 73.68%), doubled salinity (1.0%) resulted in a lengthy recovery period and a sharp PRE decline (13.89%), and the PRE was merely kept at 27.39% even through salinity was decreased to 0.75% hereafter. Salinity was also found to stimulate more extracellular protein secretion, resulting in sludge volume index reduction (<32.87 mL/g) and particle size enlargement (222.78 μm on average). Hyphomicrobium (0.96%-1.76%) and unclassified_f_Rhodobacteraceae (4.72%-13.33%) could resist certain salinity and conduct BPR, but better salt-tolerant Candidatus_Competibacter eventually became the predominant genus (>40%).

Filamentous Bacteria and Stalked Ciliates for the Stable Structure of Aerobic Granular Sludge Treating Wastewater

Aerobic granular sludge (AGS) is a promising technology for wastewater treatment. AGS formation belongs to microbial self-aggregation. Investigation of the formation and stability of AGS is widely paid attention to, in particular the structure stability of large size granules. Two types of AGS were developed in two sequencing batch reactors fed by two different wastewaters, respectively. Through confocal laser scanning microscope (CLSM) and scanning electron microscopy (SEM), the structure and composition of granules were analyzed. Filamentous bacteria were observed in granules from synthetic wastewater reactor, while filamentous bacteria and stalked ciliates (Epistylis sp.) were simultaneously found in granules from domestic wastewater reactor. The analytic results show that filamentous bacteria and stalked ciliates acting as skeletons play important roles in the formation and stability of granules. With the bonding of extracellular polymeric substances (EPS), the filamentous bacteria and stalked ciliates could build bridges and frames to promote the aggregation of bacteria; these microorganisms could create a space grid structure around the surface layer of granules to enhance the strength of granules, and the remnants of the stalks could serve as supports to fix the steadiness of granules.

Formation and stability of aerobic granular sludge in a sequential batch reactor for the simultaneous removal of organic matter and nutrients from low-strength domestic wastewater

Simultaneous removal of organic matter, nitrogen, and phosphorus, via simultaneous nitrification and denitrification (SND) and enhanced biological phosphorus removal processes, was evaluated in a pilot-scale sequential batch reactor. The focus was on granule's morphology, stability, microbiological composition, and reactor performance while treating diluted domestic wastewater with total chemical oxygen demand (COD_t) of ≈ 200 mg.L^-1. The applied organic loading rate was 0.9 ± 0.3 kg COD_t.m^-3.d^-1 in the experiment. Aerobic granular sludge developed gradually. After 87-day operation, granules (diameter ≥ 0.2 mm) were ≥ 50 % of the biomass, and after 168 days, complete granulation was obtained (≥ 80 % of biomass). In the third period (days 168-247, complete granulation), mixed liquor biomass reached a volatile suspended solids (VSS) concentration of 1.2 ± 0.3 g VSS.L^-1, with the granules remaining stable until the experimental end. In this period, low effluent concentrations of COD, nitrogen (NH₄⁺-N, NO₂^--N and NO₃^--N) and phosphate (PO₄^3-P) were obtained (mg.L^-1): 36 ± 11; 4 ± 5; 3 ± 3, 4 ± 5; and 0.9 ± 0.4, respectively. COD, NH₄⁺-N, and PO₄^3--P removal efficiencies (%) were 80 ± 11; 83 ± 20; and 55 ± 24, respectively. Heterotrophic nitrification and SND were observed, resulting in a process efficiency of 31 % even with dissolved oxygen applied to saturation. The phosphate removal was mainly attributed to denitrifying phosphorus accumulating organisms. Pseudomonas, the dominant genus found, acted in nitrogen and phosphorus removal. Pseudoxanthomonas also assisted in phosphorus removal. Bacterial communities in the flocs (≈ 20 % of biomass) during the last period were similar to those in the granules; therefore, they constituted the basis for granule formation, directly contributed to the simultaneous good removal of organic matter and nutrients.

Phosphorus Removal from Aerobic Granular Sludge: Proliferation of Polyphosphate-Accumulating Organisms (PAOs) under Different Feeding Strategies

Aerobic granular sludge (AGS) is known for high phosphorus removal from wastewaters, and phosphorus can be recovered from high phosphorus-containing waste sludge granules. This study aimed at determining the feeding strategy that provides the best performance in terms of the proliferation of polyphosphate-accumulating organisms (PAOs) and phosphorus removal. Using three AGS bioreactors, this study compared phosphorus removal and the proliferation dynamics of PAOs under three different feeding strategies: anaerobic slow feeding (R1), pulse feeding + anaerobic mixing (R2), and pulse feeding (R3). Results indicate that R1 and R2 achieved significantly higher phosphorus removal (97.6 ± 3% for R1 and 98.3 ± 1% for R2) than R3 (55 ± 11%). The anaerobic slow feeding procedure (R1) achieved the highest specific phosphorus release rate (SPRR) and specific phosphorus uptake rate (SPUR) as compared to the other two feeding conditions. 16S ribosomal ribonucleic acid (rRNA) gene sequencing assay of the microbial community for the three feeding strategies indicated that although the feeding strategy impacted reactor performance, it did not significantly alter the microbial community. The bacteria community composition maintained a similar degree of diversity. Proteobacteria, Bacteroidetes, and Verrucomicrobia were the dominant bacterial phyla in the system. Dominant PAOs were from the class Betaproteobacteria and the genera Paracoccus and Thauera. Glycogen-accumulating organisms were significantly inhibited while other less-known bacteria such as Wandonia and Hyphomonas were observed in all three reactors.

Assessing a co-culture fungal granule ability to remove pharmaceuticals in a sequencing batch reactor

Biodegradation of carbamazepine (CBZ), diclofenac sodium (DCF) and ibuprofen (IBP) was evaluated through fungal granules development in a sequencing batch reactor (SBR). Fungal granules were developed in co-culture of T. polyzona, A. niger, T. longibrachiatum, M. circinelloides and R. microsporus at a retention time (RT) of 2 days and 1 day. Ligninolytic enzymes [laccase (Lac), lignin peroxidase (LiP) and manganese peroxidase (MnP)] were determined. Removal of pharmaceuticals was assessed and metabolites identified using the SPE-UPLC-QToF/MS methods. A pH range of 3-4.6 was found to improve the granulation development from day 6 and the production of ligninolytic enzymes [MnP (253.00 ± 14.19 U/L), Lac (111.58 ± 10.00 U/L) and LiP (95.25 ± 8.22 U/L)]. At steady-state, a removal of 97.41±0.25%, 99.83±0.14%, and 99.91±0.08 were achieved at an RT of 2 days for CBZ, DCF, and IBP, respectively, and of 91.94±0.05%, 99.31±0.12% and 97.72±0.23% at an RT of 1 days for the same PhCs. A variety of chemical reactions have been proposed for degradation pathways catalysed by enzyme-producing fungi, generating fragment ions of intermediate compounds. This study is highly relevant for cost-effective and environmentally friendly wastewater treatment processes in water scare countries.

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

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

Distribution characteristics of phosphorus-containing substances in a long running aerobic granular sludge-membrane bioreactor with no sludge discharge

This work aimed at revealing the distribution characteristics of phosphorus (P) containing substances in an aerobic granular sludge-membrane bioreactor (AGS-MBR). During the long running period (180 days) with no sludge discharge, AGS was successfully cultivated on day 20, and the system performed well in removing organic pollutants and total nitrogen (TN). However, the removal of total P (TP) showed a fluctuant tendency, and P was found to distribute in all the phases of the system. In the intracellular phase, it occupied the largest ratio all through the period. In AGS, inorganic P (IP) was measured to be about 74.4-77.8% of TP, with non-apatite IP (NAIP) composing 57.5-69.6%, while in organic P (OP), the ratio of monoester and diester phosphate was in the range of 19-26.9% and 12-13.5%, respectively. The presence of highly releasable and bioavailable P (NAIP + OP) in AGS implied that it might be a potential P resource for utilization.

Underlying function regulators of anaerobic granular sludge: Starvation and dormancy

Anaerobic granular sludge (AnGS) is the core of anaerobic granular sludge bed system. In this study, the effect and its mechanism of stopping substrate supply on function of AnGS were investigated. The cutoff of exogenous substrate supply triggered AnGS to enter the dormant state. Some methanization microorganisms sporulated. The number and activity of methanization microorganisms based on 16S rDNA and 16S rRNA/16S rDNA ratio declined and stayed at 45.5% and 0.06% (bacteria), 48.7% and 0.39% (archaea) of the initial vegetative value, respectively. The resuming of exogenous substrate supply promoted AnGS to restore the vegetative state. The spores disappeared. The specific methanization activity of AnGS returned to the original level of 35.82 mL-CH4/g-VSS·d, but the delay time for gas production (DTGS) was prolonged from 9.54 to 18.04 h (0-132 d). The dormancy of methanization microorganisms was the main cause for the fluctuation of apparent function and the stability of intrinsic function of AnGS under starvation stress. The dormancy stabilized the structure and sustained the methanization community of AnGS via the reduction of EPS (structure binder/energy reserve) consumption.

Direct sludge granulation by applying mycelial pellets in continuous-flow aerobic membrane bioreactor: Performance, granulation process and mechanism

This study provides a sustainable manner for direct cultivation of aerobic granular sludge (AGS) by addition of mycelial pellets (MPs) into continuous-flow aerobic MBR. The results showed that the granulation time in MPs-MBR was shortened by at least 65 days, accounting for enhanced mean size of granules (0.68-0.76 mm), increased mixed liquor suspended solids (MLSS) concentration (12.8 g/L) and improved settling ability (78.1 mL/g), in comparison with that of 0.23-0.28 mm, 9.8 g/L and 102.1 mL/g in control MBR. MPs-MBR demonstrated significant advantages in terms of COD reduction (97.0-99.1%), NH₄⁺-N reduction (100%) and TN reduction (32.27-42.33%). MPs, extracellular polymeric substances (EPS) and filamentous bacteria acted as inducible nucleus, crosslinking matter and supporting skeleton, respectively, in favor of promoting the formation and stabilization of AGS with a four-layered structure. The relevant mechanism was underlined by rheological analysis, indicating that MPs addition enhanced non-Newtonian flow characteristics and network structure of sludge.

Simultaneous removal of organic matter and nitrogen compounds from landfill leachate by aerobic granular sludge

This study aimed at investigating the treatment of landfill leachate using the aerobic granular sludge process in a lab-scale sequential batch reactor (SBR-AGS). The leachate from a giant sanitary landfill localized in the State of São Paulo (Brazil) exhibited high concentration of organic matter (COD 5,300 ± 78 mg L^-1) and total nitrogen (TKN 2,630 ± 355 mg L^-1). Comparatively, the leachate was added to wastewater in three different volumetric ratios (5, 10 and 20%) and the mixtures were characterized over treatment. The results indicated that there were no significant changes in the behaviour of the biological process even at the highest leachate ratio. The granulation of the aerobic sludge occurred after 90 days of operation and the granules had a diameter of 485-1585 μm. SBR-AGS exhibited removal efficiency of 87-89% for organic matter and at least 98% for total nitrogen, regardless of the leachate ratio. The treated effluent that received 20% of leachate showed 2.7 mg L^-1 ammonia and 1.1 mg L-1 nitrate. This study shows that SBR-AGS was able to form large granules, thus promoting a simultaneous nitrification and denitrification (SND) process. We highlighted that SND occurred in low dissolved oxygen concentrations (< 1.5 mg L^-1) for 120 days, without compromising aerobic granule integrity. These results suggest that the aerobic granular sludge process is a promising alternative for the co-treatment of landfill leachate and domestic wastewater under tropical climate conditions and its use should be encouraged.

Isolation and characterization of a strain with high microbial attachment in aerobic granular sludge

Aerobic granule is a special microbial aggregate associated with biofilm structure. The formation of aerobic granular sludge is primarily depending on its bacterial community and relevant microbiological properties. In this experiment, a strain with high microbial attachment was isolated from aerobic granular sludge, and the detailed characteristics were examined. Its high attachment ability could reach 2.34 (OD_600nm), while other low attachment values were only around 0.06-0.32, which indicated a big variation among the different bacteria. The strain exhibited a very special morphology with many fibric fingers under SEM observation. A distinctive behaviour was to form a spherical particle by themselves, which would be very beneficial for the formation and development of granular sludge. The EPS measurement showed that its PN content was higher than low attachment bacteria, and 3D-EEM confirmed that there were some different components. Based on the 16S rRNA analysis, it was identified to mostly belong to Stenotrophomonas. Its augmentation to particle sludge cultivation demonstrated that the strain could significantly promote the formation of aerobic granule. Conclusively, it was strongly suggested that it might be used as a good and potential model strain or chassis organism for the aerobic granular sludge formation and development.

Carbamazepine removal from low-strength municipal wastewater using a combined UASB-MBR treatment system

An Upflow Anaerobic Sludge Blanket reactor combined with a two-stage membrane bioreactor were operated for 193 days in order to evaluate the biological removal of carbamazepine (CBZ) from low-strength municipal wastewater. The system worked in three different organic load stages (0.7 ± 0.1 kg COD·m^-3·d^-1, 0.4 ± 0.1 kg COD·m^-3·d^-1 and 0.1 ± 0.0 kg COD·m^-3·d^-1) to assess the impact of the influent OLR on operational parameters such as anaerobic and aerobic sludge retention time (SRT), acidity, volatile fatty acids (VFAs), biomass activity or biogas production. The highest carbamazepine removals were achieved during the anaerobic stage (UASB reactor), reaching averages of 48.9%, 48.0% and 38.2% operating at high, medium and low OLR, respectively. The aerobic treatment (MBR) served as post-treatment, improving the removals, and the global UASB-MBR system reached averages of 70.0%, 59.6% and 49.8% when the influent was at medium and low OLR, respectively. The results demonstrate the potential of combined biological systems on the removal of recalcitrant pharmaceuticals.

The effects of temperature shock on the treatment of high-concentration organic wastewater by an Fe0/GO-anaerobic system

Temperature changes are inevitable during the actual operation of anaerobic systems. Adding an Fe⁰/GO composite is shown to improve microbial activity and alleviate the adverse effects caused by temperature shock.

Thermoanaerobacterium sp. Strain RBIITD as a dominant species in accelerating thermophilic dark fermentation start up through pH and substrate concentration regulation

In this work, accelerated start-up of biological hydrogen production system fed with glucose and molasses at 55 °C by regulating pH and COD concentration was investigated in two groups. Then three reactors in each group were compared: controlling pH, controlling pH with COD, and controlling the COD. The reactors in group A presented best hydrogen yield of 1.84 mol H₂/mol glucose·day and worked stably at the 8th day. The highest hydrogen yield in group B was 2.13 mol H₂/mol molasses·day and steadily at the 11th day. It proved that controlling two key parameters of the inflow pH (8.0) and substrate concentration (4000 mg COD/L) could realize fast start-up of hydrogen production reactor. This study demonstrated that Thermoanaerobacterium sp. strain RBIITD could produce hydrogen and provide a new avenue for biological hydrogen production by dark fermentation using cheap substrate towards a more sustainable and feasible technology.

Impact of additive application on the establishment of fast and stable aerobic granulation

Aerobic granular sludge (AGS) is a microbial biofilm self-aggregation, which is effective for nutrient and pollutant removal, through the development of dense microbial layers bound together with extracellular polymeric substances (EPSs). However, long start-up times and granule disintegration are still challenges ahead. An array of external additives, including ion chelating agents, sludge-based enhancers, and magnetic influence have been tested to overcome these barriers. The application of such additives may promote enhanced EPS production, neutralization of charges on the bacterial surface, acts as a core-induced agent, or as a bridge to connect EPSs and cell surfaces. Although additives may improve the granule formation without reducing treatment efficiencies, there are still environmental concerns due to the fate and toxicity of discharged excess sludge. This mini-review identifies an array of external additives and their mechanisms to improve granulation properties, and proposes discussion about the technical and economic viability of these additives. KEY POINTS: • Additives reduce granulation time and repair granule disintegration. • Biopolymer-based additives fulfill technical and environmental requirements. • Sludge-based additives are cheap and in line with the resource recovery concept. • The need for environmental-friendly additives for aerobic granular sludge process. • External additives affect granular biomass size distribution.

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

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

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

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

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.

Simultaneous removal of As(III) and Cu(II) from real bottom ash leachates by manganese-oxidizing aerobic granular sludge: Performance and mechanisms

Manganese-oxidizing aerobic granular sludge (Mn-AGS) is a novel extension of AGS technology to treat arsenic (As) in organic wastewater. In this study, Mn-AGS was first applied to treat real wastewater (bottom ash leachates) containing high levels of As(III) and Cu(II) in a sequencing batch reactor (SBR) for 91 days. Influent and effluent As(III), As(V), Cu(II), as well as pH and chemical oxygen demand (COD) were monitored daily, and sludge was collected regularly for morphological observation, chemical characterization, and microbial analysis. The results indicated that As(III) and Cu(II) could be efficiently removed from wastewater (∼83% and ∼100%, respectively), but the performance was sensitive to pH variation, especially for As(III). The removed As and Cu were mostly bound to carbonates (60.2 ± 2.0% and 70.0 ± 0.6%, respectively) and Fe/Mn oxides (28.2 ± 1.6% and 14.6 ± 0.5%, respectively) in the final sludge. Influent As(III) was partially oxidized into As(V), and high fractions of As(V) were obtained in the Fe/Mn oxide-bound phase. Unexpectedly, microbial analysis revealed that community richness was only slightly changed when the influent was acidized (pH 4.0) but greatly reduced after the influent pH back to 6.0. It could be explained by that acid-fast bacteria rapidly grew after pH recovery and eliminated non-acid-fast bacteria. This work further supported the practical application of Mn-AGS to treat As(III)-containing organic wastewaters.

Enhanced simultaneous nitrification, denitrification and phosphorus removal through mixed carbon source by aerobic granular sludge

Aerobic granular sludge-based simultaneous nitrification, denitrification and phosphorus removal (SNDPR) systems were configured for the treatment of low-strength municipal wastewater. Granular characteristics, process performance, and the corresponding microbial ecology dynamics were comprehensively explored with sodium acetate and succinate as mixed carbon source. Results revealed that aerobic granules kept structural and functional resilience, while mixed carbon source largely altered and balanced the growth and competition of phosphorus/glycogen accumulating organisms (PAOs/GAOs). Appropriate ratio of mixed carbon source was vital for superb physiochemical behaviors and reliable removal performance by aerobic granules. Therefore, the aerobic granular SNDPR system could achieve deep-level nutrients removal through enhancing the anaerobic carbon uptake rate and strengthening the carbon usage efficiency. The present work could add some guiding sight into the application of aerobic granular SNDPR system for wastewater treatment.

Impact of hydraulic retention time and organic matter concentration on side-stream aerobic granular membrane bioreactor

This study investigated the effect of hydraulic retention time (HRT) and chemical oxygen demand (COD) concentration on membrane fouling in aerobic granular membrane bioreactor (AGMBR) in a systematic approach. Changes in HRT (7, 10, and 15 h) and COD (500, 1000 and 1500 mg/L) were applied in five operational phases, to determine the most significant parameters to control membrane fouling for enhanced AGMBR performance. Membrane permeability loss was dramatically intensified with increase in HRT from 7.5 to 15 h and COD from 500 to 1000 mg/L. The highest polysaccharide content of loosely bound EPS (0.41 mg PS/mg VSS) and soluble microbial products (SMPs) (27 mg PS/L) occurred alongside poor AGMBR performance. Variations in membrane fouling were accompanied with considerable changes in Flavobacterium, Thauera and Paracoccus populations. Analysis of variance (ANOVA) demonstrated that HRT and interaction between HRT and COD were the most significant parameters in controlling membrane fouling.

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.

Coaggregation of bacterial communities in aerobic granulation and its application on the biodegradation of sulfolane

Aerobic granulation is regarded as the future technology for wastewater treatment that can replace conventional activated sludge. In this study, two approaches of forming sulfolane degrading aerobic granules (SDAG) were successfully developed and evaluated. These include adaptation of pre-grown granules to sulfolane environment and coaggregation of pre-grown granules with bacterial culture native to sulfolane contaminated site. The adaption method required a longer period to form robust SDAG compared to coaggregation method where degradation of sulfolane was observed within 24 h. Electronic images revealed dominant filamentous bacteria on the surface of granules while DNA analysis unveiled the complexity of the dynamic change of microbial community during aerobic granule formation. The rate of sulfolane degradation by coaggregated granules reduced as the concentration of carbon source increased, nevertheless, the rate increased with increased biomass. In addition, the presence of co-contaminants can slightly impact the ability of newly cultivated granules to degrade sulfolane. Finally, the stability and settleability of the new aerobic granules was investigated under different environmental conditions. About 30% of the aerobic granules were lost after 14 d of operation without any continuous supply of carbon sources. The surviving SDAGs continued to display an intact structure coupled with good settleability.

Elucidating the effects of starvation and reactivation on anaerobic sulfidogenic granular sludge: Reactor performance and granular sludge transformation

In biological wastewater treatment, the bacteria starvation always challenges the stability of system operation. Yet, the effects of starvation and possibility of reactivation are less understood for anaerobic sulfidogenic system. Sulfidogenic systems use sulfate as electron acceptor for organic chemical oxygen demand (COD) degradation, so it will encounter two kind of starvations: (i) complete stoppage of wastewater flow (named complete food starvation) and (ii) remaining organic COD but with very low level sulfate in the influent (named sulfate starvation). In this study, the relative long-term starvation (over 30-day) and subsequent reactivation were studied in two lab-scale continuous gas recirculation sulfate-reducing upflow sludge bed (CGR-SRUSB) reactors. It was observed that the complete food starvation and sulfate starvation decreased the COD removal rate (in the similar range of 60-65%) and the specific sulfidogenic activity (about 45% and 61% respectively), as well as increasing the sludge flotation potential (SFP) from <15% to 58% and 35% respectively. Moreover, the following restoration experiments proved that the perturbed systems could be reactivated within 10-15 days for both cases. The results of investigating the mechanisms showed the performance deterioration were highly attributed to the starvation-induced granular sludge transitions, with respect to the changing of sludge physico-chemical properties (permeability, porosity, hydrophobicity and viscocity) and microbial stuctures (sulfate-reducting bacteria and extracellular polymeric substances). The outcomes of this study can provide useful information for dealing with the prolonged starvation problems in sulfidogenesis-based systems in industrial and municipal wastewater treatment.

Organic loading rate (OLR) regulation for enhancement of aerobic sludge granulation: Role of key microorganism and their function

According to unique growth characteristics of various environmental microorganism specially with different substrates and their levels, aerobic sludge granulation are studied under different operation mode of influent organic loading rate (OLR), and the EPS component, sludge surface characters and functional microbes are analyzed to achieve a novel process for stable sludge granulation. Results showed that activated sludge cultivated under gradient influent OLR decreasing from 5.5 to 3.5 kgCOD m^-3 d^-1 achieved complete granulation with average size of 438 μm and exopolysaccharide (PS) to protein (PN) ratio over 2.0. Meanwhile, these granules had excellent flocculability and hydrophobicity with Zeta potential and contact angle of -15 mV and 110°, respectively. Principal component analysis (PCA) illustrated that microbes with function of EPS secretion enriched with decreased OLR regulation for their suitable specific growth characteristics, then promoted other microbes aggregation and sludge granulation along with the improvement of cellular surface characters and microbial niche.

Optimization of organics to nutrients (COD:N:P) ratio for aerobic granular sludge treating high-strength organic wastewater

The present study attempted to optimize the nutrients required for biological growth and biomass synthesis in the treatment of high-strength organics wastewater using aerobic granular sludge (AGS). Three identical sequencing batch reactors (SBRs) were used to cultivate aerobic granules at COD concentration of ~5000 mg/L at COD:N:P ratios of 100:2.8:0.4, 100:4.4:0.5, and 100:5:0.7. Results indicated that the amount of nutrients needed for biomass growth does not follow the conventional organics to nutrients ratio (COD:N:P) of 100:5:1 when dealing with high-strength organics wastewater. The highest removal efficiency was achieved at COD:N:P ratio of 100:2.8:0.4, where COD, TN, and P removal was 98.8 ± 0.3%, 100.0 ± 0.0%, and 99.3 ± 1.0%, respectively. Moreover, the presence of high amounts of organics led to the dominance of the fast-growing heterotrophs in all SBRs, with the genus Thauera identified as the most abundant genera (23-40%), while autotrophic nitrifiers disappeared. The observed biomass yield at COD:N ratio of 100:2.8 suggested that heterotrophic nitrification may have occurred, while at COD:N ratios of 100:4.4 and 100:5, all the nitrogen was used for biomass synthesis. Moreover, at COD:N ratio of 100:5, almost 1/5 of the organics were utilized by the biomass cells to produce EPS as defensive action against the effects of free ammonia. Batch optimization experiments showed that the fastest rate of removal occurred at COD:N:P ratio of 100:1.1:0.4. After 4 h, the COD, TN, and P removal efficiencies were 95%, 99%, and 96%, achieving overall removal efficiencies of 98%, 100%, and 97% respectively, at HRT of 8 h. The bacterial behavior in consuming the organics was altered under nutrient-deficient conditions, where faster degradation rates were observed as the amounts of nutrients decreased, with higher relative abundance of heterotrophs and diazotrophic bacterial populations.

Aerobic granular sludge and naphthenic acids treatment by varying initial concentrations and supplemental carbon concentrations

Aerobic granular sludge (AGS) has previously been utilized in the treatment of toxic compounds due to its diverse and dense microbial structure. The present study subjected mature AGS to model naphthenic acids (NAs) representative of the Canadian oil sands. To this effect, three NA concentrations (10, 50 and 100 mg/L) and three supplemental carbon source concentrations (600, 1200 and 2500 mg/L) were studied in batch reactors for 5 days. The responding variables were chemical oxygen demand (COD), NA concentrations and nutrients. Cyclohexane carboxylic acid (CHCA), cyclohexane acetic acid (CHAA) and 1-adamantane carboxylic acid (ACA) were chosen to study structure-based degradation kinetics. The optimal COD according to the runs was 1200 mg/L. CHCA was removed completely with biodegradation rate constants increasing with lower NA concentrations and lower COD concentrations. CHAA was also removed completely, however, an optimal rate constant of 1.9 d^-1 was achieved at NA and COD concentrations of 50 mg/L and 1200 mg/L, respectively. ACA removal trends did not follow statistically significant regressions; however, degradation and sorption helped remove ACA up to 19.9%. Pseudomonas, Acinetobacter, Hyphomonas and Brevundimonas spp. increased over time, indicating increased AGS adaptability to NAs.

Understanding of aerobic sludge granulation enhanced by sludge retention time in the aspect of quorum sensing

Aerobic granular sludge (AGS) reactors with different sludge retention times (SRTs) were established for enhanced functional microorganism enrichment and granular formation. Results showed that higher total nitrogen (TN) removal efficiency and compact granules were achieved in the 6-day-SRT reactor. Also, Xanthomonadaceae, Rhodobacteraceae and Hyphomonadaceae with AHL-producing and EPS-secreting functions also enriched under 6-day SRT. For investigating the enhanced mechanism of sludge granulation, typical quorum sensing signals of acylated-homoserine-lactones (AHLs) and extracellular polymeric substances (EPS) were analyzed. Tryptophan-and-protein-like substances were major EPS components in granules formed at 6-day SRT. Meanwhile, most detected AHLs, i.e. C8-HSL and 3OHC8-HSL, were correlated positively with contents of tryptophan-and-protein-like substances. According to AHLs add-back test, AHLs especially those with 8-carbon sidechains, played important roles in aerobic sludge granulation via secreting special extracellular proteins by functional microbes enrichment.

Impact of food-to-microorganisms ratio on the stability of aerobic granular sludge treating high-strength organic wastewater

This work investigated the long-term stability of aerobic granular sludge treating high-strength organic wastewater in a semi-pilot scale sequential batch reactor (SBR). The reactor was operated for 316 days under different operational conditions. It was found that the F/M ratio is an important parameter affecting granules formation and stability. Three selection mechanisms were investigated: (1) cultivation and maturation at moderately high influent COD concentration (2500 mg/L) followed by increase in influent COD concentration to 7500 mg/L; (2) stressed cultivation and operation at high influent COD concentration of 4500 mg/L; and (3) alternate feed loading strategy (variable influent COD concentration across the daily schedule of cycles at 50%, 75%, and 100% of the peak concentration of 5000 mg/L). It was found that adopting high OLR at the reactor start-up accelerated the formation of granules. However, the overgrowth of biomass under high organics concentration negatively affected the stability of granules and led to disintegration due to the presence of methanogens in the granule core. Cultivation at high organics concentration resulted in a rapid loss of microbial diversity and reactor failure. Under alternate feed loading, adequate selection of microbial community was maintained and resulted in stable reactor performance. Moreover, a strong correlation between F/M ratio and the granules settling ability was observed. When F/M ratio exceeded 1.5 gCOD/gSS.d, granules showed poor settleability and under very high sludge loading rates (above 2.5), sludge bulking occurred and led to washout of sludge due to the strong selection pressure of short settling time. Operating the reactor at F/M ratio of 0.5-1.4 gCOD/gSS.d appears to favor stable long-term granule stability.

Recovery of dehydrated aerobic granules: A comparison

Dehydrated aerobic granules, if can be sufficiently recovered without significant loss of structural stability and biological activities, presents a promising long-time storage option in practical use. This study dehydrated aerobic granules by six protocols: air drying at 25 or 50 °C, freeze-dry, acetone or ethanol dehydration, and microwave heating, and then recovered them in liquid medium, with the measured characteristics being reported. The granule stability has no correlation with measured settleability, hydrophobicity or extracellular polymeric substances compositions; instead, is correlated with the functional strains presented in the recovered granules. Air dry dehydration minimally damage the functional strains including genus Brevundimonas and genus Comamonas and markedly deteriorated structural breaker such as Acinetobacter of Moraxellaceae to lead to stable and tough recovered granules.

Natural sunlight induced rapid formation of water-born algal-bacterial granules in an aerobic bacterial granular photo-sequencing batch reactor

Wastewater treatment by means of algal-bacterial granules has become a hot topic worldwide recently. Rapid granulation of algal-bacterial granules was achieved in an aerobic bacterial granular sequencing batch reactor (SBR) under natural sunlight exposure. Occurrence of abundant filamentous bacteria bridging the water-born algae, and overproduction of extracellular polymeric substances (EPS) (especially polysaccharides (PS), tryptophan & protein-like, and humic acid-like substances) were observed on the first 3 days, while the algae grew into the inner side of the granules and mature granules were obtained on day 7. The growth of the water-born algae slightly decreased the settleability, mean sizes of the granules, but stimulated the bioactivity significantly. Whereas, the biomass retention decreased before day 3, and got stable soon with the maturation period with symbiotic growth of algal-bacterial biomass. Illumina results revealed that the introduction of algae reduced the richness and diversity of bacterial community. Besides, few changes in structure and some compositions shifts in bacterial communities were found, while the predominant algae shifted from Diatomea to green algae Chlorophyceae. The possible mechanism for natural sunlight induced granulation of algal-bacterial granules was thus proposed based on the interactions between algae and bacteria.

Influence of the Hybrid Sewage Treatment Plant’s Exploitation on Its Operation Effectiveness in Rural Areas

The article evaluates the effectiveness of the removal of organic pollutants—nitrogen and phosphorus—from household sewage in a hybrid bioreactor with a submerged fixed bed. The experiment was carried out in two exploitation variants that were both conducted in a laboratory model of the hybrid bioreactor: (I) cycles of 120 min of aeration and 60 min of no aeration with a constant sewage dosage, and (II) cycles 60 min of aeration and 60 min of no aeration, with a periodic sewage dosage in the no-aeration phase. The experiment was carried out on real sewage primarily treated in a septic tank. The amount of pollution removal was calculated and compared with the mandatory standards according to Polish law. Moreover, the susceptibility of the sewage to the biological treatment, nitrification, and denitrification activity was determined. The research shows a higher effectiveness for the 60/60 model in comparison to the 120/60 model. High operation efficiency was observed regarding the removal of organic pollution and nitrate nitrogen. The tested structure showed very low nitrification activity combined with intense denitrification. These processes were observed in the 60/60 variant. The structure was often overloaded with the nitrate nitrogen, which was considered to be the nitrification process inhibitor. It was suggested that phosphorus was also removed by the denitrifying bacteria.

Effect of air flow rate on development of aerobic granules, biomass activity and nitrification efficiency for treating phenol, thiocyanate and ammonium

The impact of air flow rate on aerobic granulation was evaluated for treating toxic multiple pollutants; phenol (400 mg L^-1), thiocyanate (100 mg L^-1) and ammonia nitrogen (100 mg L^-1) by using three lab scale sequencing batch reactors (SBRs) (R1, R2 and R3). Larger granules (2938.67 ± 64.91 μm) with higher biomass concentration (volatile solids of 4.17 ± 0.09 g L^-1), higher granule settling velocity (55.56 ± 1.36 m h^-1) and lower sludge volume index (35.25 ± 1.71 mL gTSS^-1) were observed at optimal air flow rate of 2.5 L min^-1 (R2). Confocal laser scanning microscopic images illustrated the extended fluorescence for extracellular polymeric substances in R2. In R2, partial nitrification was achieved. Phenol was completely removed in all the reactors while partial removal of SCN^- and no nitrification were observed with a decrease (1.5 L min^-1) and an increase (3.5 L min^-1) in air flow rates (R1 and R3, respectively). This study provides an experimental contribution to examine the effect of optimal combination of aeration and toxic multiple pollutants, governing characteristics and nitrification efficiency of granules along with SBR performance in an economic way in terms of optimal air supply.

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

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

Aerobic sludge granulation for simultaneous anaerobic decolorization and aerobic aromatic amines mineralization for azo dye wastewater treatment

In this study, the capability of using aerobic granules to undergo simultaneous anaerobic decolorization and aerobic aromatic amines degradation was demonstrated for azo dye wastewater treatment. An integrated acclimation-granulation process was devised, with Mordant Orange 1 as the model pollutant. Performance tests were carried out in a batch column reactor to evaluate the effect of various operating parameters. The optimal condition was to use 1.0-1.7 mm (1.51 ± 0.33 mm) granules, 5 g/L biomass, and 4000 mg/L organics as nutrient; and supplement the wastewater with 1  mg/L dissolved oxygen. This led to a dye mineralization of 61 ± 2%, an anaerobic dye removal of 88 ± 1%, and an aerobic aromatic amines removal of 70 ± 3% within 48 h. This study showed that simultaneous anaerobic/aerobic process by aerobic granules could be a possible alternative to the conventional activated sludge process.