Comparison of aerobic granulation in SBR and continuous-flow plants

Novel separate aeration self-circulating technology for continuous aerobic granular sludge process: Performance evaluation, hydrodynamic simulation and control strategy

The continuous aerobic granular sludge (AGS) process is promising for upgrading existing wastewater treatment facilities. However, this approach is still challenging because of its complicated structure and operation. To address this issue, a novel separate aeration self-circulating technology (abbreviated as Zier) was proposed, which is promising for cultivating AGS by its outstanding upflow velocity and circulation multiplier (more than 30 m/h and 200, respectively). This study elaborated on the Zier reactor's feasibility, optimization, and control strategy through computational fluid dynamics simulations, theoretical calculations, and experiments. An appropriate flow regime for efficient removal of pollutant and granulation of sludge was attained at a superficial gas velocity of 1.3 cm/s. Moreover, optimizing the aeration column diameter to half of the reaction column and increasing the height/diameter ratio to 20 dramatically boosted the nitrogen removal capacity over 1.6 kg N/m3/d. Utilizing a smaller circulation pipe diameter ensured granulation under a consistent flow regime. By judiciously regulating, multiple CSTRs and PFRs seamlessly integrated within the Zier reactor across a broad spectrum of particle sludge. The validity of these findings was further substantiated through experimental and theoretical validations. Drawing from these findings, a multi-scenario control strategy was proposed as Zier's map. With all the superiorities shown by the Zier reactor, this study could offer new insights into an efficient continuous AGS process.

Full-scale upgrade activated sludge to continuous-flow aerobic granular sludge: Implementing microaerobic-aerobic configuration with internal separators

Aerobic granular sludge (AGS) has been successfully used in sequencing batch reactors. However, their application to existing continuous-flow systems remains challenging. In this study, a novel microaerobic-aerobic configuration with internal separators was implemented in a full-scale municipal wastewater treatment facility with a nominal capacity of 2.5 × 104 m3 d-1. Sludge characteristics, pollutant removal and associated pathways, shifts in the microbial community, and underlying granulation mechanisms were investigated. Following a two-month operation period, the transition from flocculent-activated sludge to well-defined AGS with distinct boundaries and compact structures was successfully achieved. The average size of sludge increased from 31.9 to 138.5 μm, with granules larger than 200 μm constituting 28.9 % of the total sludge and SVI30 averaging 51.4 ± 8.2 mL g-1. The 95th percentile effluent COD, NH4+-N, and TN concentrations were 35.0, 1.2, and 13.3 mg L-1, respectively. The primary pathways for pollutant removal were identified as simultaneous nitrification, denitrification, and phosphorus removal within the microaerobic tanks. The enrichment of denitrifying phosphorus-accumulating organisms, including Hydrogenophaga, Accumulibacter, Azospira, Dechloromonas, and Pseudomonas, provides an essential microbial foundation. Furthermore, computational fluid dynamics modeling revealed that the incorporation of internal separators in aerobic tanks induced shifts in the flow pattern, transitioning from a single-circulation cell to multiple vortical cells. This alteration amplified the local velocity gradients, generating the required shear forces to drive granulation. Moreover, mass balance analysis revealed that the microaerobic and aerobic tanks operated under feast and famine conditions, respectively, creating a microbial selection pressure that favored granulation. This process eliminates the need for external clarifiers, resulting in a footprint reduction of 38.2 % and one-third energy savings for sludge reflux. This study offers valuable insights into the application of continuous-flow AGS to upgrade existing activated sludge systems with limited retrofitting requirements.

Current progress of continuous-flow aerobic granular sludge: A critical review

Aerobic granular sludge (AGS) is promising for water resource recovery. Despite the mature granulation strategies in sequencing batch reactor (SBR), the application of AGS-SBR in wastewater treatment is usually costly as it requires extensive infrastructure conversion (e.g., from continuous-flow reactor to SBR). In contrast, continuous-flow AGS (CAGS) that does not require such infrastructure conversion is a more cost-effective strategy to retrofit existing wastewater treatment plants (WWTPs). Formation of aerobic granules in both batch and continuous-flow mode depends on many factors, including selection pressure, feast/famine conditions, extracellular polymeric substances (EPS), and environmental conditions. Compared with AGS in SBR, creating proper conditions to facilitate granulation in continuous-flow mode is challenging. Researchers have been seeking to tackle this bottleneck by studying the impacts of selection pressure, feast/famine conditions, and operating parameters on granulation and granule stability in CAGS. This review paper summarizes the state-of-the-art knowledge regarding CAGS for wastewater treatment. Firstly, we discuss the CAGS granulation process and effective parameters (i.e., selection pressure, feast/famine conditions, hydrodynamic shear force, reactor configuration, the role of EPS, and other operating factors). Then, we evaluate CAGS performance in removing COD, nitrogen, phosphorus, emerging pollutants, and heavy metals from wastewater. Finally, the applicability of the hybrid CAGS systems is presented. At last, we suggest that integrating CAGS with other treatment methods such as membrane bioreactor (MBR) or advanced oxidation processes (AOP) can benefit the performance and stability of granules. However, future research should address unknowns including the relationship between feast/famine ratio and stability of the granules, the effectiveness of applying particle size-based selection pressure, and the CAGS performance at low temperatures.

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

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

Achieving rapid thiosulfate-driven denitrification (TDD) in a granular sludge system

Sulfur-oxidizing bacteria (SOB) can drive a high level of autotrophic denitrification (AD) activity with thiosulfate (S₂O₃^2-) as the electron donor. However, the slow growth of SOB results in a low biomass concentration in the AD reactor and unsatisfactory biological nitrogen removal (BNR). In this study, our goal was to establish a high-rate thiosulfate-driven denitrification (TDD) system via sludge granulation. Granular sludge was successfully cultivated by increasing the nitrogen loading rate stepwise in thiosulfate-oxidizing/nitrate-reducing conditions in an upflow anaerobic blanket reactor. In the mature-granular-sludge reactor, a nitrate removal rate of 280 mg N/L/h was achieved with a nitrate removal efficiency of 97.7%±1.0% at a hydraulic retention time of only 15 minutes, with no nitrite detected in the effluent. Extracellular polymeric substance (EPS) analysis indicated that the proteins in loosely bound and tightly bound EPS were responsible for maintaining the compact structure of the TDD granular sludge. The dynamics of the microbial-community shift were identified by 16S rRNA high-throughput pyrosequencing analysis. The Sulfurimonas genus was found to be enriched at 74.1% of total community and may play the most critical role in the high-rate BNR. The batch assay results reveal that no nitrite accumulation occurred during nitrate reduction because the nitrate reduction rate (75.90±0.67 mg N/g MLVSS/h) was almost equal to the nitrite reduction rate (66.06±1.28 mg N/g MLVSS/h) in the thiosulfate-driven granular sludge reactor. The results of this study provide support for the establishment of a high-rate BNR system that maintains its stability with a low sludge yield.

Activated sludge and other aerobic suspended culture processes

This paper provides an overview of activated sludge related to suspended growth processes for the year 2019. The review encompasses process modeling of activated sludge, microbiology of activated sludge, process kinetics and mechanism, nitrogen and phosphorus control, design, and operation in the activated sludge field. The fate and effect of xenobiotics in activated sludge, including trace organic contaminant and heavy metal xenobiotics, which had influence on the growth of suspended sludge, are covered in this review. Compared to past reviews, many topics show increase in activity in 2019. These include, biokinetics process of aerobic granular sludge formation, pyrolysis kinetic mechanism of granular sludge. These topics are referred to formation and disintegration of granular sludge. Other sections include activated sludge settling model, toxicity resistant microbial community, nitritation-anammox processes for nitrogen removal, and respirometry used in the operation of real wastewater treatment plant are especially highlighted in this review. PRACTITIONER POINTS: Biokinetics process of aerobic granular sludge formation Toxicity resistant microbial community in activated sludge Nitritation-anammox processes for nitrogen removal in activated sludge.

Effect of different operational modes on the performance of granular sludge in continuous-flow systems and the successions of microbial communities

Continuous flow reactors with time intermittent operational (TIO) mode and spatial intermittent operational (SIO) mode were operated to evaluate the effects of operational modes on the removal performances, the characteristics of granules and the dynamics of microbial communities in simultaneous nitrification, denitrification and phosphorus removal (SNDPR) granular system. The results showed that the removal efficiency of TP, TN were 81.3%, 86.7% under TIO mode, and 70.6%, 77.4% under SIO mode, respectively. Meanwhile, the PN and value of PN/PS in SIO were higher than those in TIO. Besides, results of high-throughput pyrosequencing illustrated that the combination of filamentous archaea (Methanothrix) and filamentous bacteria (Thiothrix) had resulted in the increase of EPS and SVI under SIO mode. Finally, functional bacterial and archaeal species, involving HMA, AMA, AOA, DPAOs etc., were identified to reveal the effects of operational modes on the mechanism of nutrients removal in granular SNDPR continuous-flow system.

The Impact of Different Powdered Mineral Materials on Selected Properties of Aerobic Granular Sludge

This study aimed to evaluate and compare the physical, chemical and biological properties of aerobic granular sludge from reactors with the addition of different powdered mineral materials. These properties have a significant impact on the efficiency of systems in which the biomass in granular form is used. Four identical granular sequencing batch reactors (GSBRs) were adopted for the research performed on a laboratory scale (R1—control reactor; R2, R3 and R4—with materials, PK, PG and PL respectively). The results indicate that the addition of powdered mineral materials improved the properties of biomass in reactors. The SVI₅/SVI₃₀ ratio values were significantly lower in the reactors with added materials (approx. 1.3 ± 0.3). The mean values of the sludge volume index at 30 min were the lowest in the R2 (39.8 ± 8.6 mL/g) and R4 (32.8 ± 10.7 mL/g) reactors. The settling velocity of biomass was the highest in the R2 reactor (15.4 ± 6.1 m/h). In the early days of the study, the highest extracellular polymeric substances (EPS) content was found in the biomass from the reactors to which the materials with higher Ca and Mg content were added (380.18–598.30 mg/g MLVSS). The rate of specific oxygen uptake (SOUR) by biomass indicated an insufficient biomass content in the R1 reactor—to 7.85 mg O₂/(g MLVSS∙h)—while in the reactors with materials, the SOUR values were at the higher levels.