Start-up of aerobic granular biofilm at low temperature: Performance and microbial community dynamics

Large-sized aerobic granular biofilm: stable biotechnology to improve nitrogen removal and reduce sludge yield

Three parallel sequencing batch reactors (control, small-sized polyurethane sponge (PUS) (3.0 mm), and large-sized PUS (10.0 mm)) were used to investigate aerobic granular biofilm (AGB) characteristics. Results show that 10.0 mm PUS facilitated rapid formation of large-sized AGB (AGBL), which exhibited higher biomass concentration (8.5 g/L) and faster settling velocity (69.2-159.3 m/h) than aerobic granular sludge (AGS) (3.2 g/L and 38.6-80.0 m/h). The AGBL system also maintained long-term structural stability with a lower instability coefficient (0.004-0.018 min-1) than AGS (0.053-0.090 min-1). Additionally, during long-term operation, the AGBL system achieved excellent removal efficiencies for NH4+-N (99.6 ± 0.4 %) and total nitrogen (92.3 ± 2.6 %), and exhibited a lower sludge yield (0.05 gVSS/gCOD) than AGS (0.14 gVSS/gCOD). The larger size and compact structure of AGBL increased anoxic/anaerobic zones, enriching denitrifying and hydrolytic/fermentative bacteria. These findings highlight AGBL with large PUS as a more promising biotechnology for practical applications than conventional AGS.

Effect of gradual increase of salt on performance and microbial community during granulation process

Salinity was considered to have effects on the characteristics, performance microbial communities of aerobic granular sludge. This study investigated granulation process with gradual increase of salt under different gradients. Two identical sequencing batch reactors were operated, while the influent of Ra and Rb was subjected to stepwise increments of NaCl concentrations (0-4 g/L and 0-10 g/L). The presence of filamentous bacteria may contribute to granules formed under lower salinity conditions, potentially leading to granules fragmentation. Excellent removal efficiency achieved in both reactors although there was a small accumulation of nitrite in Rb at later stages. The removal efficiencies of chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) in Ra were 95.31%, 93.70% and 88.66%, while the corresponding removal efficiencies in Rb were 94.19%, 89.79% and 80.74%. Salinity stimulated extracellular polymeric substances (EPS) secretion and enriched EPS producing bacteria to help maintain the integrity and stability of the aerobic granules. Heterotrophic nitrifying bacteria were responsible for NH4+-N and NO2--N oxidation of salinity systems and large number of denitrifying bacteria were detected, which ensure the high removal efficiency of TN in the systems.

Enhanced aerobic granular sludge by thermally-treated dredged sediment in wastewater treatment under low superficial gas velocity

The positive contributions of carriers to aerobic granulation have been wildly appreciated. In this study, as a way resource utilization, the dredged sediment was thermally-treated to prepared as carriers to promote aerobic granular sludge (AGS) formation and stability. The system was started under low superficial gas velocity (SGV, 0.6 cm/s)for a lower energy consumption. Two sequencing batch reactors (SBR) labeled R1 (no added carriers) and R2 (carriers added), were used in the experiment. R2 had excellent performance of granulation time (shortened nearly 43%). The maximum mean particle size at the maturity stage of AGS in R2 (0.545 mm) was larger compared to R1 (0.296 mm). The sludge settling performance in R2 was better. The reactors exhibited high chemical oxygen demand (COD) and ammonia nitrogen (NH3-N) removal rates. The total phosphorus (TP) removal rate in R2 was higher than R1 (almost 15% higher) on stage II (93-175d). R2 had a higher microbial abundance and dominant bacteria content. The relative abundance of dominant species was mainly affected by the carrier. However, the enrichment of dominant microorganisms and the evolution of subdominant species were more influenced by the increase of SGV. The results indicated that the addition of carriers induced the secretion of extracellular polymeric substances (EPS) by microorganisms and accelerated the rapid formation of initial microbial aggregates. This work provided a low-cost method and condition to enhance aerobic granulation, which may be helpful in optimizing wastewater treatment processes.

Deciphering the role of micro/nano-hydroxyapatite in aerobic granular sludge system: Effects on treatment performance and enhancement mechanism

Hydroxyapatite (HAP), a mineral nucleus identified within aerobic granular sludge (AGS), plays a vital role in enhancing the AGS systems. However, the microscopic mechanism underlying their roles remains largely unexplored. Herein, a systematic investigation was carried out to elucidate the impact and enhanced mechanisms associated with HAP of different sizes, i.e. micro-HAP (mHAP) and nano-HAP (nHAP), on the aerobic granulation, nutrient removal and microbial diversity of AGS. Results showed that the presence of nHAP and mHAP significantly shortened the granulation process to 15 and 20 days, respectively. This might be ascribed to the fact that the large specific surface area of nHAP aggregates was conducive to microbial adhesion, biomass accumulation and sludge granulation. Compared with mHAP, the granules with nHAP showed better settlement performance, mechanical strength and larger diameter. The X-ray diffraction (XRD) and Raman spectrometer analysis confirmed the presence of HAP within the granules, which was found to stimulate the secretion of extracellular polymeric substance, improve the compactness of granule structure and suppress the growth of filamentous bacteria, thereby contributing to a stable AGS system. The presence of HAP, especially nHAP, effectively enriched the functional microorganisms, such as nitrifying and denitrifying bacteria (e.g. Candidatus_Competibacter) and phosphorus accumulating organisms (e.g. Flavobacterium), leading to the improved nutrient removal efficiencies (COD > 96%, TN > 76%, and TP > 74%). Further analysis revealed the up-regulation of functional enzymes (e.g. nitrite oxidoreductase and polyphosphate kinase) involved in nutrient metabolism, underlying the inherent mechanisms for the excellent nutrient removal. This study deepens the understanding of granulation mechanisms from the perspective of mineral cores, and proposes an economically feasible strategy for rapid initiation and stabilization of AGS reactors.

Efficient nitrogen removal from municipal wastewater using an integrated fixed-film activated sludge process in a novel air-lifting loop reactor: A pilot-scale demonstration

A novel air-lifting loop reactor combines anoxic, oxic, and settling zones to achieve organic and nutrient removal, as well as solid-liquid separation. To address sludge settling ability and operation stability issues caused by low dissolved oxygen in aerobic zones, this study proposes using modified polypropylene carriers to establish a fixed-film activated sludge (IFAS) system. A pilot-scale demonstration of the IFAS-based air-lifting loop reactor is conducted, and the results show successful operation for approximately 300 days. The pilot-scale reactor achieves a maximum aerobic granulation ratio of 16% in the bulk liquid. The IFAS system contributes to efficient removal of organic matter (96%) and nitrogen (94%) by facilitating simultaneous nitrification and denitrification, as well as fast solid-liquid separation with a low sludge volume index of 34 mL/g. Microbial analysis reveals enrichment of functional bacteria involved in nitrification, denitrification, and flocculation throughout the operation process.

Biochar inoculated with Rhodococcus biphenylivorans altered microecological regulation by promoting quorum sensing and electron transfer: Up-regulation of related genes and enhancement of phenol and ammonia degradation

Bioaugmentation is an efficient method for improving the efficiency of coking wastewater removal. Nevertheless, how different immobilization approaches affect the efficiency of bioaugmentation remains unclear, as does the corresponding mechanism. With the assistance of immobilized bioaugmentation strain Rhodococcus biphenylivorans B403, the removal of synthetic coking wastewater was investigated (drying agent, alginate agent, and absorption agent). The reactor containing the absorption agent exhibited the highest average removal efficiency of phenol (99.74 %), chemical oxygen demand (93.09 %), and NH4+-N (98.18 %). Compared to other agents, the covered extracellular polymeric substance on the absorption agent surface enhanced electron transfer and quorum sensing, and the promoted quorum sensing benefited the activated sludge stability and microbial regulation. The phytotoxicity test revealed that the wastewater's toxicity was greatly decreased in the reactor with the absorption agent, especially under high phenol concentrations. These findings showed that the absorption agent was the most suitable for wastewater treatment bioaugmentation.

Simultaneous removal of nitrate nitrogen and orthophosphate by electroreduction and electrochemical precipitation

Electrochemical methods can effectively remove nitrate nitrogen (NO3-N) and orthophosphate phosphorus (PO4-P) from wastewater. This work proposed a process for the simultaneous removal of NO3-N and PO4-P by combining electroreduction with electrochemically-induced calcium phosphate precipitation, and its performance and mechanisms were studied. For the treatment of 100 mg L-1 NO3-N and 5 mg L-1 PO4-P, NO3-N removal of 60-90% (per cathode area: 0.25-0.38 mg h-1 cm-2) and 80-90% (per cathode area: 0.33-0.38 mg h-1 cm-2) could be acquired within 3 h in single-chamber cell (SCC) and dual-chamber cell (DCC), while P removal was 80-98% (per cathode area: 0.10-0.12 mg h-1 cm-2) in SCC after 30 min and 98% (per cathode area: 0.37 mg h-1 cm-2) in DCC within 10 min. The faster P removal in DCC was due to the higher pH and more abundant Ca2+ in the cathode chamber of DCC, which was caused by the cation exchange membrane (CEM). Interestingly, NO3-N reduction enhanced P removal because more OH- can be produced by nitrate reduction than hydrogen evolution for an equal-charge reaction. For 10 mg L-1 PO4-P in SCC, when the initial NO3-N was 0, 20, 100, and 500 mg L-1, the P removal efficiencies after 1 h treatment were < 10%, 45-55%, 86-99%, and above 98% respectively. An increase in Ca2+ concentration also promoted P removal. However, Ca and P inhibited nitrate reduction in SCC at the relatively low initial Ca/P, as CaP on the cathode limited the charge or mass transfer process. The removal efficiency of NO3-N in SCC after 3 h reaction can reduce by about 17%, 40%, and 34% for Co3O4/Ti, Co/Ti, and TiO2/Ti. The degree of inhibition of P on NO3-N removal was related to the content and composition of CaP deposited on the cathode. On the cathode, the lower the deposited Ca and P, and the higher the deposited Ca/P molar ratio, the weaker the inhibition of P on NO3-N removal. Especially, P had little or even no inhibition on nitrate reduction when treated in DCC instead of SCC or under high initial Ca/P. It is speculated that under these conditions, a high local pH and local high concentration Ca2+ layer near the cathode led to a decrease in CaP deposition and an increase in Ca/P molar ratio on the cathode. High initial concentrations of NO3-N might also be beneficial in reducing the inhibition of P on nitrate reduction, as few CaP with high Ca/P molar ratios were deposited on the cathode. The evaluation of the real wastewater treatment was also conducted.

Characterization of a novel micro-pressure double-cycle reactor for low temperature municipal wastewater treatment

To solve the deterioration of effluent caused by low temperature in urban sewage treatment plant in cold areas, a new type of reactor was proposed, the biochemical environmental and low-temperature operating characteristics of the reactor were studied. Through analysis of flow simulation and dissolved oxygen (DO) distribution when the aeration rate was 0.6 m³/h, it showed that there were many different DO environments in the reactor at the same time, which provided favourable conditions for various biochemical reactions. The operation test showed that the average effluent removal rate of COD, TN, NH₄⁺-N and TP was 92.53%, 74.57%, 89.61% and 96.04%, respectively. And there were a variety of functional bacteria related to nitrogen and phosphorus removal in the system, most of them with strong adaptability at low temperatures. Among the dominant microorganisms, Flavobacterium and Rhodobacter were related to denitrification, Aeromonas and Thiothrix were related to phosphorous removal. Denitrifying phosphorus removal was the main way of phosphorus removal. Picrust2 results showed that the reactor operated well at low temperature, and the regional difference distribution of nitrification genes further confirmed the existence of functional zones in the reactor. The results showed that the Micro-pressure Double-cycle reactor worked well at low temperature, which provided a new idea and way for the upgrading of urban sewage treatment plants in cold areas.

Low-temperature-resistance granulation of activated sludge and the microbial responses to the granular structural stabilization

Completely loss of granular structural stability and reliable start-up of aerobic granular sludge (AGS) system are considered as the biggest challenges for its engineering application under seasonal temperature variation, especially extremely low temperatures. In this study, two identical sequencing batch reactors (SBR) were successfully start-up at 10 °C (R1) and 25 °C (R2), respectively, and then operated under a strategy of stepwise change of temperatures to investigate the stability of the granular sludge by examining its microbial characteristics, bis (3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP), extracellular polymeric substance (EPS) and sludge physiochemical properties. The results showed that AGS formed under the low temperature preferentially secrete EPS and c-di-GMP for stable granulation and improvement of its resistance to temperature changes. Meanwhile, R1 successfully obtained aerobic granulation with high biomass concentration and superior settleability, as well as high pollutant removal performance. In comparison, R2 took a longer time for granulation and was subjected to serious disintegration of AGS. The matrix structure partially formed by filamentous bacteria during the start-up stage in R1 was one of major reasons for its own superiority beyond R2 in granulation. Slow-growing organisms such as autotrophic nitrifying and Anammox bacteria, phosphorus accumulation organisms, EPS-producing genera, and c-di-GMP pathway-dependent genera, were exclusively enriched in the R1 and resulted in higher pollutants removal efficiencies and stable structure, whereas Sphaerotilus dominated in R2 that related closely with its unstable performance. Therefore, the strategy based on the stepwise change of temperatures from extremely low temperatures may be one feasible way for the sustainable application of AGS system, which is of significance to address the challenging problems of AGS applications.

Efficiency of various biofilm carriers and microbial interactions with substrate in moving bed-biofilm reactor for environmental wastewater treatment

In a moving bed-biofilm reactor (MBBR), the fluidization efficiency, immobilization of microbial cells, and treatment efficiency are directly influenced by the shape and pores of biofilm carriers. Moreover, the efficacy of bioremediation mainly depends on their interaction interface with microbes and substrate. This review aims to comprehend the role of different carrier properties such as material shapes, pores, and surface area on bioremediation productivity. A porous biofilm carrier with surface ridges containing spherical pores sizes > 1 mm can be ideal for maximum efficacy. It provides diverse environments for cell cultures, develops uneven biofilms, and retains various cell sizes and biomass. Moreover, the thickness of biofilm and controlled scaling shows a significant impact on MBBR performance. Therefore, the effect of these parameters in MBBR is discussed detailed in this review, through which existing literature and technical strategies that focus on the surface area as the primary factor can be critically assessed.

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

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

Stability and nutrients removal performance of a Phanerochaete chrysosporium-based aerobic granular sludge process by step-feeding and multi A/O conditions

A Phanerochaete chrysosporium-based aerobic granular sludge (PC-AGS) was developed by inoculating fungal mycelial pellets into a lab-scale aerobic granular sequencing batch reactor (AGSBR). A strategy using step-anaerobic feeding coupled with multi A/O conditions was adopted. The results showed that the removal efficiencies for total phosphorus (TP) and total inorganic nitrogen (TIN) were 94.56 ± 2.92% and 75.20 ± 7.74%, respectively, under relatively low aeration time. Compared with original AGS, the content of extracellular proteins for PC-AGS obviously increased from 18.61 to 41.44 mg/g MLSS by the end of phase I. Moreover, the mature granules had a larger size and better stability during the 100 days operation. Furthermore, the analysis of microbial diversity detected many key functional groups in PC-AGS granules that were beneficial to nutrients removal. This work demonstrated that the addition of fungal pellets not only enhanced the removal performance, but also improved the stability of the AGS system.

Simultaneous enhanced biological phosphorus removal and semi-nitritation (EBPR-SN) followed by anammox process treating municipal wastewater at seasonal temperatures: From summer to winter

This work investigated the feasibility of a novel simultaneous enhanced biological phosphorus removal and semi-nitritation (EBPR-SN) plus anammox process treating real municipal wastewater from summer to winter (28.1- 15.3 °C). Two lab-scale sequential reactors were used in this study, namely EBPR-SN and Anammox sequencing batch reactors (SBRs). Long-term operation suggested that ammonium oxidizing bacteria abundance decreased from 1.67% to 0.89% whereas nitrite oxidizing bacteria decreased to nearly undetected in the EBPR-SN SBR, maintaining the stable nitritation (nitrite accumulation ratio: 98.3 ± 1.0%). Lowering airflow rate was effective to retain nitritation with temperature decrease. Reliable nutrient removal was still maintained in winter (16.4 ± 0.7 °C), i.e. the removal efficiencies for nitrogen and phosphorus were 80.0 ± 3.5% and 95.4 ± 5.2%, respectively, with short aerobic HRT (6.4 h) and low dissolved oxygen (0.2-1.5 mg/L). The percentage of anammox contribution to nitrogen-removal increased with temperature decrease, although Candidatus Brocadia abundance decreased. Additionally, the protection of extracellular polymeric substances was important to the successful performance.

Insight into formation and biological characteristics of Aspergillus tubingensis-based aerobic granular sludge (AT-AGS) in wastewater treatment

The long start-up time and facile biomass loss of aerobic granular sludge (AGS) impede its application for actual wastewater treatment. The present study investigates a novel assist-aggregation strategy based on Aspergillus tubingensis (AT) mycelium pellets to accelerate sludge granulation, and engineered Fe₃O₄ nanoparticles (NPs) were used to further enhance flocculent sludge (FS) aggregation. The AT mycelium pellets, modified by 0.5 g/L Fe₃O₄@SiO₂-QC NPs (AT-V), had a more compact internal structure than the unmodified group (AT-I). The content of extracellular polymeric substances (EPS) and the zeta potential values were observed to increase from 39.86 mg/gVSS and -9.19 mv for AT-I to 69.64 mg/gVSS and 2.35 mv for AT-V, respectively. In optimized cultivation conditions, the aggregated sludge biomass of AT-V reached 1.54 g/g. An original AT-based AGS (AT-AGS) with a high biological activity (64.45 mgO₂/gVSS·h as specific oxygen uptake rate) and enhanced velocity (58.22 m/h) was developed in only 9 days. The removal efficiencies of total nitrogen (TN) and total phosphorus (TP) of the AT-AGS were 12.24% and 16.29% higher than those of the inoculated FS under high feeding load. Additionally, the analysis of cyclic diguanylate (c-di-GMP) and con-focal microscope images implied that polysaccharide (PS) of EPS played an important role in maintaining the stability of the AT-AGS. Finally, the dominant functional species contributing to sludge aggregation and pollutants removal of the AT-AGS showed a larger richness and diversity than those of the inoculated FS. This study might provide a novel high-efficiency strategy for the fast formation of AGS.

The effect of supporting matrix on sludge granulation under low hydraulic shear force: Performance, microbial community dynamics and microorganisms migration

Granular sludge usually takes extracellular polymers (EPS) as matrices for colonizing microorganisms and maintaining structural stability. However, the low strength of EPS threatens the disintegration of granules, especially under low hydraulic shear force. To accelerate the formation and enhance the stability of granules, micro-sized melamine (ME) sponges (RA) and polyurethane (PU) sponges (RB) were screened out as matrix substitutes for developing aerobic granular biofilm (AGB) in this study. The superficial gas velocity was 0.8 cm s^-1. Both reactors achieved over 95% ammonium nitrogen removal efficiency within 10 days. During stabilization period, the chemical oxygen demand, total nitrogen and total phosphorus removal efficiencies were 90.5%, 70% and 95% in RA and 87.8%, 83% and 88% in RB, respectively. Confocal laser scanning microscopy (CLSM) detection revealed that β-polysaccharide was more concentrated in the outer layer in PU-AGB but uniformly dispersed in ME-AGB. The denitrifying phosphorus accumulating organisms (Flavobacterium) was dominant in RA, while the denitrifying glycogen accumulating organisms (Candidatus_Competibacter) was dominant in RB. Fluorescence in situ hybridization (FISH) analysis indicated that the microbial distribution in ME-AGB was relatively uniform, while there was a significant migration of functional microorganisms in PU-AGB. The super-hydrophilicity of ME and the high hydrophobicity of PU may be the main reasons for these differences. Overall, this study indicated that ME sponge is a more suitable material for supporting AGB than PU sponge.