Study on the flow characteristics and the wastewater treatment performance in modified internal circulation reactor

CFD simulation of anaerobic granular sludge reactors: A review

Anaerobic digestion processes can generate renewable energy in the form of biogas while treating organic wastewater. The generation of biogas within anaerobic digestion systems is directly linked to the mixing conditions inside the reactors. In high-rate reactors such as the upflow anaerobic sludge blanket (UASB) reactor, the expanded granular sludge bed (EGSB) reactor and the internal circulation (IC) reactor, the hydrodynamic behaviour will depend on the interactions between the wastewater, the biogas, and the biomass granules. Over the past few years, various researchers have used computational fluid dynamics (CFD) to study the hydrodynamic behaviour in these types of reactors. This review aims to present and critically discuss the state of the art in the use of CFD applied to anaerobic granular sludge reactors (AGSRs). It briefly introduces and discusses the various aspects of modelling. It also reviews the various papers which used CFD to model these reactors and critically analyses the models used for the simulations in terms of general approaches and single-phase vs multiphase studies. The methods used in the validation of the CFD models are also described and discussed. Based on the findings, the challenges and future perspectives for the CFD modelling of AGSRs are discussed and gaps in the knowledge are identified.

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.

Exogenous N-acyl-homoserine lactones accelerate resuscitation of starved anaerobic granular sludge after long-term stagnation

So as to accelerate the resuscitation of starved anaerobic granular sludge after long-term stagnation, an innovative method was tried derived from the regulation of N-acyl-homoserine lactones (AHLs)-mediated quorum sensing (QS). The mixture of four AHLs was added to the starved anaerobic granular sludge system in this research. The results confirmed that the exogenous AHLs shortened the recovery time of the granular sludge, and improved the treatment performance and methanogenic capacity of the recovered anaerobic sludge to the level before stagnation. At the same time, exogenous AHLs enhanced the synthesis of extracellular polymeric substances (EPS) during the resuscitation period of starved anaerobic granular sludge. The outcomes of microbial composition detection showed that the change of bacterial and methanogenic bacteria communities towards accelerated performance recovery was significantly correlated with exogenous AHLs. This exploration provided a new technical idea for speeding up the recovery of starved anaerobic granular sludge.

Bacterial community composition of internal circulation reactor at different heights for large-scale brewery wastewater treatment

This study analyzed bacterial community structure for large-scale brewery wastewater treatment at different heights in internal circulation (IC) reactor. Proteobacteria, Bacteroidetes and Chloroflexi were dominant bacteria, which accounted for 64.17%, 64.04%, 59.87% and 55.79% in phylum level, respectively. The unidentified bacteria were accounted for a large proportion in genus level, available data showed that Longilinea, Desulfomicrobium, Caldithrix, Geobacter and Syntrophorhabdus were relatively abundant. Organic fermentation, hydrolysis, and acidification were mainly completed at the bottom, and production of hydrogen and methane were completed in the upper and middle part of reactor. Alpha diversity and cluster distance analysis showed the bacterial community could be divided into bottom, middle and upper part of IC reactor. The IC reactor possessed the COD_Cr removal efficiency of 80% - 84.09%, and BOD₅ of 77.50% - 86% for brewery wastewater. This study would provide bacterial analysis references of IC reactor for industrial wastewater treatment in future.

Comparison of long-term energy efficiency and microbial community dynamics of different reactors in response to increased loadings of water hyacinth juice

Water hyacinth is considered to be among the worst invasive weed species globally, causing detrimental environmental and social problems worldwide. It rapidly grows, and therefore has significant potential as a resource. Due to its high moisture content (approximately 95%), the by-product obtained by dehydrating water hyacinth yields a considerable amount of water hyacinth juice (WHJ). In this study, we performed a comparative assessment of long-term energy efficiency, maximum treatment capacity limits, and microbial community dynamics of modified internal circulation (MIC) and up-flow anaerobic sludge blanket (UASB) reactors in response to increasing loadings of WHJ. The MIC reactor exhibited a higher energy recovery rate and stronger performance compared with the UASB reactor. The optimal organic loading rates of the MIC and UASB reactors were 17.93 and 8.85 kg chemical oxygen demand (COD)/m³/d, with methane conversion rates of 0.21 and 0.15 m³ CH₄/kg COD, respectively. Furthermore, the engineering costs and project floor space required by the MIC reactor are less than those in the case of the UASB reactor. The high-throughput sequencing analysis indicated that the dominant phyla (e.g. Firmicutes and Bacteroidetes) were more abundant using the MIC reactor than with the UASB reactor, which may indicate WHJ degradation efficiency. Both reactors had similar predominant methanogens, suggesting that acetoclastic methanogenesis was the predominant metabolic pathway of methane formation. The results of this study provide new insights into the sustainable management of water hyacinth as a resource by establishing a regional ecosystem with biogas engineering applications.

Semi-continuous feeding combined with traditional domestication improved anaerobic performance during treatment of cassava stillage

The effects of feeding pattern were studied during anaerobic digestion of cassava stillage. Continuous feeding and semi-continuous feeding, were adopted in two internal circulation (IC) reactors (A and B, respectively). The reactors showed different performance in the anaerobic digestion process. The maximum difference, was observed for the soluble chemical oxygen demand (SCOD) removal rate and the biogas production, which were 23.2% and 95.7 L/2 d higher in reactor B than reactor A, respectively. The overall VFAs level of reactor A was higher than that of reactor B. Microbial community analyses indicated that the abundances of dominant bacteria and methanogens became higher in the reactor B than in reactor A as the digestion process progressed. Hence, semi-continuous feeding showed superior performance than continuous feeding for SCOD removal rate, biogas production, and the relative abundances of methanogens in the case of high OLR.

Correlating microbial community structure with operational conditions in biological aerated filter reactor for efficient nitrogen removal of municipal wastewater

In this study, the combination of strengthen circulation anaerobic (SCA) and biological aerated filter (BAF) reactor was employed to treat municipal wastewater. Different reflux percentages or gas/water ratios were selected for evaluating the removal performance of contaminants in SCA-BAF system and sequential nitrification and denitrification process in BAF reactor. In general, reflux percentage (200%) and gas/water ratio (3:1) were a relatively suitable operational condition for BAF reactor. The COD, NH₃-N, TN concentrations of effluents collected from BAF reactor varied in the ranges of 18-80, 0.2-7.2, 9.1-33.0 mg L^-1, respectively. A higher NO₃-N concentration in effluents of BAF reactor resulted from the lack of organic carbon resource in wastewater. High throughput sequencing analysis indicated that different nitrification and denitrification bacteria thrived in the BAF reactor. The DO, NO₂-N and NO₃-N concentrations showed a strong correlation with Nitrospira and Nitrosomonas in bacterial samples outlet (c and e) under gas/water ratio of 3:1.

Insights into sludge granulation during anaerobic treatment of high-strength leachate via a full-scale IC reactor with external circulation system

In this study, a full-scale internal circulation (IC) reactor coupled with an external circulation system was developed to treat high-strength leachate from a municipal solid waste (MSW) incineration plant, in which anaerobic sludge granulation was intensively investigated. Results showed that the IC reactor achieved excellent treatment performance under high organic loading rates (OLR) of 21.06-25.16kg chemical oxygen demand (COD)/(m³∙day). The COD removal efficiency and biogas yield respectively reached 89.4%-93.4% and 0.42-0.50m³/kgCOD. The formation of extracellular polymeric substances (EPS) was closely associated with sludge granulation. Protein was the dominant component in sludge EPS, and its content was remarkably increased from 21.6 to 99.7mg/g Volatile Suspended Solid (VSS) during the reactor operation. The sludge Zeta potential and hydrophobicity positively correlated with the protein/polysaccharide ratio in EPS, and they were respectively increased from -26.2mV and 30.35% to -10.6mV and 78.67%, which was beneficial to microbial aggregation. Three-dimensional fluorescence spectroscopy (3D-EEM) and Fourier transform infrared spectroscopy (FT-IR) analysis further indicated the importance of protein-like EPS substances in the sludge granulation. Moreover, it was also found that the secondary structures of EPS proteins varied during the reactor operation.

Bacterial and archaeal community distribution and stabilization of anaerobic sludge in a strengthen circulation anaerobic (SCA) reactor for municipal wastewater treatment

In this study, a SCA reactor was employed for municipal wastewater treatment at a mesophilic temperature (30°C) under different hydraulic retention times (HRT) and upflow velocities (V_up) to investigate granule sludge stability and spatial microbial distribution. The stable COD removal efficiency readied at HRT of 15, 12, 9 and 6h, and V_up ranging from 0.6 to 5.9mh^-1. EPS fraction analysis of granule sludge shows that municipal wastewater was mainly attributed to the enrichment influence of polysaccharide and tightly bound-EPS. SEM images exhibited that the stability and floating of anaerobic granular sludge may be promoted in the primary three-phase separator area because the channels of the granules was clogged by EPS. The SMA and high-throughput sequencing analysis indicated acetoclastic methanogens and hydrogenotrophic methanogens played an important role in formation and maintenance of the anaerobic granule sludge in low and high organic load rate operation conditions.

Performance, kinetics behaviors and microbial community of internal circulation anaerobic reactor treating wastewater with high organic loading rate: Role of external hydraulic circulation

Performance of internal circulation anaerobic reactor (IC) treating wastewater at high organic loading rate (OLR) and role of external hydraulic circulation were evaluated. When the OLR was increased from 2.50 to 18.94kgCOD/m³/d, COD removal decreased to 85% slightly and methane production increased to 4.49L/L/d with the upflow velocity of 1.0m/h resulted from the additional hydraulic circulation. Withdrawal of external hydraulic circulation led to decrease of COD removal to lower than 60% drastically and methane production by 81%. Accumulation of volatile fatty acids caused decline of pH to below 6.0 and the shift of substrate metabolic pathway to the hybrid fermentation. In addition, both maximum methane production rate and maximum substrate degradation rate obtained from mathematical models decreased significantly. Hydrogenotrophic methanogens including Methanobacterium and Methanocorpusculum predominated in the anaerobic sludge and the shift of microbial community was also observed.