High Organic Loading Rate Research Articles

Feasibility of successive hydrogen and methane production: Effects of temperature and organic loads on energy potential and microbial dynamics

This study aims to assess the co-digestion of Cassava Wastewater (CW) and glycerol in a two-stage process using fluidized bed reactors (AFBR), verifying the effect of organic loading rate (OLR) and temperature (mesophilic [SMR] and thermophilic [STR]) in sequential reactors on CH4 production. The OLR ranged from 1.2 to 15 g COD.L−1.d−1 and the hydraulic retention time (HRT) was set at 20 h. The mesophilic sequential reactor (MSR) (average temperature of 30 °C) showed greater tolerance to high OLR and its best MPR was 101.12 mL of CH4.d−1.L−1 h−1, obtained at a OLR of 15 g COD.L−1.d−1). The maximum yield was 341.10 mL of CH4.g−1CODcons, found at the OLR of 1.2 g COD.L−1.d−1. The sequential thermophilic reactor (STR) showed the maximum yield and MPR of 333.03 mL of CH4.g−1CODcons (1.2 g COD.L−1.d−1) and 58.84 mL of CH4.d− 1.L−1 h−1 (12 g COD.L−1.d−1), respectively. Through the massive sequencing analysis of the 16S rRNA gene, it was possible to observe a greater diversity of microorganisms in the TSR than in the MSR. A predominance of acetoclastic microorganisms was observed, with the genera Methanobacterium, Methanosarcina and Methanobrevibacter being the most abundant in both reactors. The two-stage system composed of mesophilic acidogenic reactor + MSR was more suitable for the co-digestion of CW and glycerol than the acidogenic reactor + TSR. These results support the notion of standard operating conditions at the industrial plant, where the cassava processing process is carried out at room temperature (25–30 °C).

Enrichment of Methanothrix species via riboflavin-loaded granular activated carbon in anaerobic digestion of high-concentration brewery wastewater amidst continuous inoculation of Methanosarcina barkeri

Effective treatment of high-concentration brewery wastewater through anaerobic digestion (AD) has always been a challenging issue. Enhancing direct interspecies electron transfer (DIET) was demonstrated to increase methane production during AD under high organic loading rate (OLR). Herein, the feasibility of enhancing DIET with the addition of riboflavin-loaded granular activated carbon (RF-GAC) as well as co-addition with Methanosarcina barkeri (Rf-GAC+M.barkeri) was investigated (M.barkeri is well-known to be capable of DIET with electroactive bacteria). During the whole process, the Rf-GAC and the Rf-GAC+M.barkeri group both achieved average COD removal rates above 97 %, which was 14 % higher than that of the control. The average methane production in the Rf-GAC group and the Rf-GAC+M.barkeri group respectively reached 0.334 ± 0.02 L(stp)/g COD and 0.345 ± 0.02 L(stp)/g COD, 1.35 and 1.39 times higher than the 0.247 ± 0.03 L(stp)/g COD reached by the control. The control reactor deteriorated at an OLR of 12 kg COD/(m3·d), whereas the Rf-GAC and the Rf-GAC+M.barkeri group maintained stable as the OLR reached as high as 17.5 kg COD/(m3·d) and the volatile fatty acids concentration was consistently below 10 mM. The RF-GAC performed better than Rf-GAC+M.barkeri in enriching Methanothrix, whose relative abundance was 60.6 % in the former group. Metabolic pathway analysis revealed the addition of RF-GAC upregulated genes related to DIET in Methanothrix species, including hdrA and fpoD. Furthermore, Methanothrix remained the dominant archaea even continuously inoculating pure strains of M.barkeri during the entire operational period. Pure culture experiments proved that GAC inhibited M.barkeri growth. The results of this study can be optimized for practical application of AD treating high-concentration brewery wastewater.

Metabolic cross-feeding interactions modulate the dynamic community structure in microbial fuel cell under variable organic loading wastewaters.

The efficiency of microbial fuel cells (MFCs) in industrial wastewater treatment is profoundly influenced by the microbial community, which can be disrupted by variable industrial operations. Although microbial guilds linked to MFC performance under specific conditions have been identified, comprehensive knowledge of the convergent community structure and pathways of adaptation is lacking. Here, we developed a microbe-microbe interaction genome-scale metabolic model (mmGEM) based on metabolic cross-feeding to study the adaptation of microbial communities in MFCs treating sulfide-containing wastewater from a canned-pineapple factory. The metabolic model encompassed three major microbial guilds: sulfate-reducing bacteria (SRB), methanogens (MET), and sulfide-oxidizing bacteria (SOB). Our findings revealed a shift from an SOB-dominant to MET-dominant community as organic loading rates (OLRs) increased, along with a decline in MFC performance. The mmGEM accurately predicted microbial relative abundance at low OLRs (L-OLRs) and adaptation to high OLRs (H-OLRs). The simulations revealed constraints on SOB growth under H-OLRs due to reduced sulfate-sulfide (S) cycling and acetate cross-feeding with SRB. More cross-fed metabolites from SRB were diverted to MET, facilitating their competitive dominance. Assessing cross-feeding dynamics under varying OLRs enabled the execution of practical scenario-based simulations to explore the potential impact of elevated acidity levels on SOB growth and MFC performance. This work highlights the role of metabolic cross-feeding in shaping microbial community structure in response to high OLRs. The insights gained will inform the development of effective strategies for implementing MFC technology in real-world industrial environments.

Different aspects of biochar addition on semi-dry anaerobic digestion of organic fraction of municipal solid waste in continuous mode

This study investigated the use of biochar, derived from a wood gasifier, in semi-dry anaerobic digestion of organic fraction of municipal solid waste (OFMSW). The experiment was conducted in three phases, without biochar and changing the hydraulic retention time (HRT) from 50 to 15 days until first acidification condition (on pH = 6.5), with biochar at an optimal concentration of 30 g/L in HRT= 20–10 day. Also, countermeasures for the acidified reactor with biochar during the dormancy period were investigated. The results demonstrated that adding biochar led to a rapid recovery of the acidified reactor, improved stability parameters, and removed foaming as a disturbance. Biochar addition (30 g/L) enhanced the organic loading rate (OLR) up to 11 kgVS/m3.day with an HRT of 20 days leading to specific methane production of 383 L/kgVS and a volumetric production increase of biomethane by 85 %. However, at higher OLRs with HRT of 10 days, acidification condition resurfaced leading to homogeneous foaming. Excess adding of biochar did not have significant treatment effects but necessitated a no-feeding period (about 45 days) and gentle stirring with long intervals for stable conditions. Overall, the use of biochar along with the OFMSW biogas plant was demonstrated to enhance production efficiency.

Fixing collapsed dry anaerobic digestion system of kitchen waste caused by severe VFAs accumulation

Severe inhibition by volatile fatty acids (VFAs) frequently occurs during the dry anaerobic digestion of kitchen waste at high organic load rate (OLR), leading to decreased biogas production and even the collapse of the anaerobic digestion system. In this study, VFAs accumulation was significantly reduced by adding powder-activated carbon (PAC) or alkaline additives (AA), thus restoring the collapsed system to a stable operating state. With the addition of PAC or AA, the maximal OLR reached 11.14 g VS/(L·d), which was 30 % higher than observed without any additives (control group), while the VFAs concentration was maintained below 3000 mg/L. At this OLR, the volumetric biogas production rate stabilized at 5.1 L/(L·d) in the presence of PAC and AA, while that of the control group gradually decreased to zero. The VFAs concentration with PAC addition was 86 % lower than that of the control group, possibly because PAC might stimulate the formation of direct interspecies electron transfer between syntrophic bacteria and methanogens (Methanosarcina), thereby promoting VFAs degradation. The addition of AA, which resulted in a 95 % decrease in ammonia-nitrogen concentration, can provide a good growth environment for the microorganisms involved in acidogenesis and hydrogenotrophic methanogenesis.

Optimization of two-stage thermophilic anaerobic digestion of dairy wastewater: Effect of carrier material on process performance and microbial community

Although two-stage anaerobic digestion (TSAD) is not a novel process, information of the process stability, microbial community and effective operating conditions is limited and often contradictory. In this work, the influence of different carrier materials (polyurethane foam, carbon felt, Raschig rings and a combination of carbon felt and Raschig rings) on the performance of the thermophilic TSAD of dairy wastewater was studied. The organic loading rate (OLR) in the acidogenic reactor (RH) was gradually increased from 13.74 to 32.56 g COD/(L d), and from 0.64 to 11.46 g COD/(L d) in the methanogenic reactors by correspondingly reducing the hydraulic retention time (HRT). The highest hydrogen production rate of 1280.3 mL/(L·d) and hydrogen yield of 93.2 mL/g COD were achieved at OLR of 13.74 g COD/(L·d), but hydrogen production stopped at higher OLR. The main soluble metabolites in RH were butyrate and lactate, and the microbial community was dominated by Streptococcus, Thermoanaerobacterium, Veillonellales-Selenomonadales and Pseudomonas. The highest methane production rate (2674 mL/(L·d) at HRT of 0.5 days) was observed in the reactor with polyurethane foam, while the highest methane yield (305.5 mL/g COD at HRT of 1.5 days) was obtained in a reactor containing carbon felt. Spirochaetaceae, Desulfomicrobium, Anaerolineaceae, Candidatus Caldatribacterium and Cloacimonadaceae W5 were linked to these materials and explained the highest methanogenic performance.

Response of food waste anaerobic digestion to the dimensions of micron-biochar under 30 g VS/L organic loading rate: Focus on gas production and microbial community structure

Biochar modification is an effective approach to enhance its ability to promote anaerobic digestion (AD). Focusing on the physical properties of biochar, the impact of different particle sizes of biochar on AD of food waste (FW) at high organic loading rate (OLR) was investigated. Four biochar with different sizes (40–200 mesh) were prepared and used in AD systems at OLR 30 g VS/L. The research results found that biochar with a volume particle size of 102 μm (RBC-P140) had top-performance in promoting cumulative methane production, increasing by 13.20% compared to the control group. The analysis results of the variety in volatile acids and alkalinity in the system did not show a correlation with the size of biochar, but small size has the potential to improve the environmental tolerance of the system to high acidity. Microbial community analysis showed that the abundance of aceticlastic methanogen and the composition of zoogloea were optimized through relatively small-sized biochar. Through revealing the effect of biochar particle size on AD system at high OLR, this work provided theoretical guidance for regulating fermentation systems using biochar.

Long-term and multiscale assessment of methanogenesis enhancement mechanisms in magnetite nanoparticle-mediated anaerobic digestion reactor

Magnetite nanoparticles (Fe3O4-NPs) have been demonstrated to be involved in direct interspecies electron transfer between syntrophic bacteria, yet a comprehensive assessment of the ability of Fe3O4-NPs to cope with process instability and volatile fatty acids (VFAs) accumulation in scaled-up anaerobic reactors is still lacking. Here, we investigated the start-up characteristics of an expanded granular sludge bed (EGSB) with Fe3O4-NPs as an adjuvant at high organic loading rate (OLR). The results showed that the methane production rate of R1 (with Fe3O4-NPs) was approximately 1.65 folds of R0 (control), and effluent COD removal efficiency was maintained at approximately 98.32% upon 20 kg COD/(m3·d) OLR. The components of volatile fatty acids are acetate and propionate, and the rapid scavenging of propionate accumulation was the difference between R1 and the control. The INT-ETS activity of R1 was consistently higher than that of R0 and R2, and the electron transfer efficiencies increased by 68.78% and 131.44%, respectively. Meanwhile, the CV curve analysis showed that the current of R1 was 40% higher than R3 (temporary addition of Fe3O4-NPs), indicating that multiple electron transfer modes might coexist. High-throughput analysis further revealed that it was difficult to reverse the progressive deterioration of system performance with increasing OLR by simply reconfiguring bacterial community structure and abundance, demonstrating that the Fe3O4-NPs-mediated DIET pathway is a prerequisite for establishing multiple electron transfer systems. This study provides a long-term and multi-scale assessment of the gaining effect of Fe3O4-NPs in anaerobic digestion scale-up devices, and provides technical support for their practical engineering applications.

The Valorization of Fruit and Vegetable Wastes Using an Anaerobic Fixed Biofilm Reactor: A Case of Discarded Tomatoes from a Traditional Market

Tomato waste, characterized by high organic matter and moisture content, offers a promising substrate for anaerobic digestion, though rapid acidification can inhibit methanogenic activity. This study investigated the performance of a 10.25 L anaerobic fixed biofilm reactor for biogas production using liquid tomato waste, processed through grinding and filtration, at high organic loading rates, without external pH control or co-digestion. Four scouring pads were vertically installed as a fixed bed within a fiberglass structure. Reactor performance and buffering capacity were assessed over three stages with progressively increasing organic loading rates (3.2, 4.35, and 6.26 gCOD/L·d). Methane yields of 0.419 LCH4/gCOD and 0.563 LCH4/g VS were achieved during the kinetic study following stabilization. Biogas production rates reached 1586 mL/h, 1804 mL/h, and 4117 mL/h across the three stages, with methane contents of 69%, 65%, and 72.3%, respectively. Partial alkalinity fluctuated, starting above 1500 mg CaCO3/L in Stage 1, dropping below 500 mg CaCO3/L in Stage 2, and surpassing 3000 mg CaCO3/L in Stage 3. Despite periods of forced acidification, the system demonstrated significant resilience and high buffering capacity, maintaining stability through hydraulic retention time adjustments without the need for external pH regulation. The key stability indicators identified include partial alkalinity, effluent chemical oxygen demand, pH, and one-day cumulative biogas. This study highlights the effectiveness of anaerobic fixed biofilm reactors in treating tomato waste and similar fruit and vegetable residues for sustainable biogas production.

Cascading valorization of defatted rice bran for lactic acid fermentation and biogas production

This study investigated the integrated valorization of defatted rice bran (DRB) by converting it into lactic acid (LA) and subsequently utilizing the residues from LA production for biomethane generation through anaerobic digestion (AD). Processing 480 kg of DRB resulted in the production of 70 L of pure LA and generated significant waste streams, primarily consisting of 572 kg of decanted hydrolysate pellet (Pellet DEC) and 220 kg of microfiltration retentate (Retentate MF). Exceptionally high methane yields of 374‒434 LN kgVS-1 were observed for residues from LA fermentation in biochemical methane potential tests, indicating their high potential for biogas production. During long-term semi-continuous AD, varying organic loading rates (OLRs) from 0.5‒2.5 kgVS m-3 d⁻¹ demonstrated feedstock- and OLR-dependent methane production. Reactor failure at higher OLRs was attributed to the accumulation of total ammoniacal nitrogen (TAN). The co-digestion of Pellet DEC and Retentate MF proved to be more resilient, with OLRs up to 2 kgVS m-3 d-1, mitigating TAN inhibition. Methane yields, ranging from 265‒334 LN kgVS-1 before reaching inhibitory OLR levels, were higher than those found in the literature. Process integration has emerged as a promising approach because the biogas generated from residues could effectively offset the energy demands of LA production. Supported by life cycle assessment, the integrated processes showed a 67% lower environmental impact at the midpoint and a 71% lower environmental impact at the endpoint, along with an 80% reduction in energy costs compared to the standalone LA production. Results proved a significant enhancement of the sustainability and economic viability of this integrated biorefinery approach.

Microbial community dynamics and volatile fatty acid production during anaerobic digestion of microaerated food waste under different organic loadings

This study examined the effects of microaeration pretreatment and varying organic loading rates (OLRs) on volatile fatty acids (VFAs) production from food waste (FW) via anaerobic digestion process. FW was pretreated with 6 mL O2/g VS and digested at OLRs ranging from 10 to 25 g/L VS. Microaeration pretreatment increased the VFA yield to 7.4 g/L, whereas without pretreatment, it was 5.19 g/L, with butyrate as the dominant VFA (92 % at 25 g/L OLR) due to Firmicutes enrichment. A relatively high OLR initially increased VFA production but later favored propionate without pretreatment. Microbial analysis revealed that aerobic conditions promoted lactic acid bacteria such as Lactobacillus, increasing lactic acid production, whereas anaerobic conditions promoted Prevotella and butyrate producers such as Clostridium, which specialize in acetate and butyrate production. This study highlights that microaeration and an optimal OLR can increase and stabilize VFA yields, modulating the microbial community for tailored VFA profiles that are useful in biofuel and biochemical applications.

Evolution of quorum sensing process and their regulatory role on biochemical metabolism during the organic loading rate increase in dry anaerobic digestion

The organic loading rate (OLR) is a critical parameter affecting the stability of dry anaerobic digestion (AD) of kitchen waste (KW), and significantly impacting the variations in physicochemical parameters and microbial communities. However, the evolution of quorum sensing (QS) and their role on anaerobic biochemical metabolism during the increase in OLR in dry AD remain unknown. Therefore, this study systematically elucidated the matter through multi-omics analysis based on a pilot-scale dry AD of KW. The results demonstrated that fluctuations in the OLR significantly influenced the microbial QS in dry AD. When the OLR ≤4.0 g·VS/L·d, the system operated stably, and methane production increased. The enrichment of Proteobacteria was crucial for sustaining high levels of functional genes associated with various types of QS, including acyl-homoserine lactones (AI-1), autoinducer-2 (AI-2), autoinducer-3 (AI-3), and gamma-aminobutyric acid (GABA). This enabled cooperative communication among microbes under low OLR. Furthermore, most genes associated with these QS processes positively affected hydrolysis, acidogenesis, and methanogenesis. When the OLR increased to 6.0 g·VS/L·d, the fatty acids and hydrogen partial pressure increased significantly. The autoinducing peptides (AIP)-type became the predominant QS and was positively correlated with fatty acids abundance. Syntrophaceticus and Syntrophomonas may promote syntrophic oxidation of acetate at high OLR through AIP-type QS. These findings provided new insights into the QS processes of microbes during dry AD of KW and a theoretical foundation for optimizing biochemical metabolic processes in dry AD through QS.

Performance of anaerobic baffled reactor (ABR) and multi-staged UASB in anaerobic digestion process for treating leachate from refuse transfer stations under loading shocks

ABSTRACT This study compared an anaerobic baffled reactor (ABR) with a multi-staged UASB (MS-UASB) to investigate the effects of loading shocks on the anaerobic digestion in both reactors. Both reactors were subjected to five hydraulic/organic loading shocks, each lasting 3 days. During the hydraulic shock with the highest organic loading rate (OLR) (OLR of 24 g COD L−1 d−1), MS-UASB and ABR exhibited minimum effluent COD removal efficiency of 90.9 and 73.0%, with average methane concentrations decreasing to 62.4 ± 0.9% and 59.8 ± 3.0%. Under the highest organic shock (OLR of 12 g COD L−1 d−1), the minimum effluent COD removal efficiency of MS-UASB and ABR was 81.5 and 73.4%, with average methane concentrations decreasing to 60.4 ± 1.1% and 58.6 ± 0.8%. After the hydraulic and organic shock phase, the biomass concentration in the MS-UASB reached 159 and 130% of the ABR, respectively. The reason for the improved operational stability of the MS-UASB is due to the presence of the solid/liquid/gas separator, which promotes the formation of granular sludge and reduces biomass washout. In addition, MS-UASB exhibited a higher abundance of the syntrophic bacterium Candidatus cloacamonas, which plays a crucial role in maintaining the stability of anaerobic digestion systems.

Interspecies electron transfer regulation in a stage-separated anaerobic digestion system: The role of inoculum strategies, activated carbon integration, and organic loading rate adjustment

Understanding and regulation of interspecies electron transfer, including hydrogen-mediated interspecies electron transfer (MIET) and direct interspecies electron transfer (DIET), is crucial for enhancing anaerobic digestion (AD). In this study, MIET and DIET were regulated in a multistage anaerobic hythane reactor (MAHR) via inoculum management, activated carbon (AC) addition, and organic loading rate (OLR) adjustment. The operational stability, organic removal, and methane production can be enhanced, with AC addition in the methane production zone (Mm) (RpCMm and RnpCMm). The reactors of RpCMm and RnpCMm achieved start-up within 20 days and showed a 20 % and 104 % increase in maximum methane production rates, respectively. Microbial community characteristics confirmed that AC in Mm could facilitate DIET establishment, but its addition in Mh could not. This assertion was supported by the observed tighter and more positive correlation between biofilm conductivity and DIET-related microbes in RpCMm and RnpCMm, as opposed to RpCMh and RnpCMh. Further, the dual drive of DIET and MIET was revealed through the comprehensive analysis of operational performances and microbial community with the structure equation model. The electron fluxes analysis highlighted a dynamic shift of DIET and MIET in the methanogenic stage with AC addition and OLR variation. The DIET was proved more efficient than MIET in Mm with AC addition and OLR below 40 g COD/L/d, while high OLR (80 g COD/L/d) favored MIET with lower methane production efficiency. The results of this study shed light on the nuanced characteristics of interspecies electron transfer, providing valuable insights for the further design and operation of stage-separated AD.

Unlocking potential: Exploring the methane production potential in anaerobic co-digestion of cassava wastewater and glycerol

This study aimed at evaluating the feasibility of CH4 production in a mesophilic (30 °C) single-stage anaerobic co-digestion of cassava wastewater (CW) and glycerol in an anaerobic fluidized bed reactor (AFBR) with mixing ratio of 50 % / 50 % CW/Gly on a (chemical oxygen demand)COD basis. Thus, the following strategy was used: increasing the organic loading rate (OLR) (1–20 g COD.L−1.d−1) by increasing the substrate concentration (1–20 g COD.L−1) and applying a hydraulic detention time (HRT) of 24 h. The AFBR presented a maximum methane (CH4) content of 88.38±1.94 % and a CH4 yield (MY) of 293.48±18.37 mL of CH4.mL−1CODrem in the OLR of 1 g COD.L−1.d−1 and the maximum methane production rate (MPR) of 62.09±2.75 mL of CH4.L−1.h−1 in the OLR of 20 g COD.L−1.d−1. The conversion of glycerol and carbohydrates remained above 91±0.54 % and 82±2.79 %, respectively, and the COD conversion was above 80±0.65 %. The main metabolites were HAc, HPr, HBu, EtOH and HVa. The most abundant genera identified in the AFBR were Izemoplasmatales and Enterobacteriaceae for the bacteria domain, and Methanosaeta, Methanobacterium for the archaea domain. The single-stage system is robust and can operate with higher OLR without reducing the efficiency of wastewater treatment and CH4 production.

Enhanced anaerobic co-digestion of cattle manure with food waste and pig manure: Statistical optimization of pretreatment condition and substrate mixture ratio

This study investigated the optimal pretreatment condition and mixture ratio of cattle manure (CM) for its efficient anaerobic co-digestion (AcoD) with food waste (FW) and pig manure (PM). The pretreatment performances of thermal (TM), microwave (MW), and ultrasound (US) technologies and the AcoD performance were statistically and experimentally evaluated at various mixture ratios of CM, FW, and PM. The results revealed that the most effective pretreatment condition with the TM, MW, and US pretreatments was 129.3 °C for 49.6 min, 824.2 W for 7.3 min, and 418.0 W for 36.3 min, respectively. The best AcoD performance of optimally pretreated CM (PCM) was achieved when 30.5 % PCM was mixed with 42.5 % FW and 27.0 % PM. A long-term evaluation showed that the start-up rate for the anaerobic mono-digestion of PCM was 2.3 times faster than that of CM and the amount of methane produced was 4.7 times higher; process stability was thus preferentially maintained under a higher organic loading rate (OLR) (2.0 kg-VS/m3∙d). The start-up rate for the AcoD of PCM with FW and PM was 1.2 times higher than that of the AcoD of CM with FW and PM. Although the performance gap between the AcoD reactors after steady state was not significantly different, the PCM AcoD reactor provided a more stable operation under a higher OLR (5.0 kg-VS/m3∙d). This study demonstrates that the pretreatment and co-digestion of CM could significantly enhance the production of biogas and improve process stability.

Correlations and impact of anaerobic digestion operating parameters on the start-up duration: Database construction for robust start-up guidelines

Anaerobic digestion (AD) has become a popular technique for organic waste management while offering economic and environmental advantages. As AD becomes increasingly prevalent worldwide, research efforts are primarily focused on optimizing its processes. During the operation of AD systems, the occurrence of unstable events is inevitable. So far, numerous conclusions have been drawn from full and lab-scale studies regarding the driving factors of start-up perturbations. However, the lack of standardized practices reported in start-up studies raises concerns about the comparability and reliability of obtained data. This study aims to develop a knowledge database and investigate the possibility of applying machine learning techniques on experimentation-extracted data to assist start-up planning and monitoring. Thus, a standardized database referencing 75 cases of start-up of one-stage wet continuously-stirred tank reactors (CSTR) processing agricultural, industrial, or municipal organic effluent in mono-digestion from 31 studies was constructed. 10 % of the total observations included in this database concern failed start-up experiments. Then, correlations between the parameters and their impacts on the start-up duration were studied using multivariate analysis and a model-based ranking methodology. Insights into trends of choices were highlighted through the correlation analysis of the database. As such, scenarios favoring short start-up duration were found to involve relatively low retention times (average initial and final hydraulic retention times, (HRTi) and (HRTf) of 26.25 and 20.6 days, respectively), high mean organic loading rates (average OLRmean of 5.24 g VS·d−1·L −1) and the processing of highly fermentable substrates (average feed volatile solids (VSfeed) of 81.35 g L−1). The model-based ranking of AD parameters demonstrated that the HRTf, the VSfeed, and the target temperature (Tf) have the strongest impact on the start-up duration, receiving the highest relative scores among the evaluated AD parameters. The database could serve as a reference for comparison purposes of future start-up studies allowing the identification of factors that should be closely controlled.

Simulation models of microbial community, pH, and volatile fatty acids of anaerobic digestion developed by machine learning

Anaerobic digestion (AD) is a promising method for treating high-content organic waste via the microbiome in an anaerobic environment. However, because AD is a complex process involving microbial community (MC), it is essential to determine the relationship between the MC, pH, and volatile fatty acids (VFAs). This study employed five machine learning models: artificial neural network, convolutional neural network, extra trees, extreme gradient boost, and support vector machine to estimate the relationship between MC, pH, and VFAs. Methanoregulaceae, Methanomicrobiaceae, Spirochaetes, pH, acetic acid, isobutyric acid, and isovaleric acid were estimated with high accuracy, with R2 >0.800. The variable importance of the models with the highest accuracy was calculated based on the shapley additive explanations analysis. As a result, the MC estimation models suggested syntrophic acetate oxidation by Spirochaetes, and VFAs-pH estimation models suggested that the high organic loading rate during the start-up phase can lead to isovaleric acid accumulation. In an in-silico test using the machine learning simulator, the change of MC resulted in the critical effects on the acetate accumulation from 500 to 1500 mg/L. The optimal relative abundance of Mehtanoregulaceae and Methanomicrobiaceae for the stable operation was suggested as 15–37 % and 3–15 %, respectively, resulting in the acetic acid concentration below 1000 mg/L.

Synergistic effects of biochar addition and filtration mode optimization on mitigating membrane fouling in high-solid anaerobic membrane bioreactors

In this study, a high-solid anaerobic membrane bioreactor was established for treating food waste, and membrane fouling rates were regulated through multivariate modulation. The anaerobic membrane bioreactor operated stably at a high organic loading rate of 28.75 gCOD/L/d achieved a methane production rate of 8.03 ± 0.61 L/L/d. Experimental findings revealed that the most effective control of membrane fouling was achieved at a filtration– relaxation ratio (F/R) of 10/90 s. This indicates that a higher relaxation frequency provided improved the mitigation of membrane fouling. Compared with single F/R modulation, the combined modulation of biochar and F/R provided enhanced control over membrane fouling. Moreover, the addition of biochar altered the sludge properties of the reactor, thereby preventing the formation of a dense cake layer. Additionally, biochar enhanced the sheer force of the fluid on the membrane surface and facilitated the separation of pollutants during the relaxation stage, thereby contributing to improved control of membrane fouling.

Evaluation of the effect of increasing the organic load in the thermophilic co-fermentation of sugarcane industry by-products on hydrogen, ethanol and lactic acid generation

The effects of increased organic loading rate (OLR) on H2 production and soluble metabolites in a thermophilic anaerobic fluidized bed reactor (AFBR-AT) at 55 °C were evaluated by the fermentation of vinasse and molasses from sugarcane (at a ratio of 1:1). The OLR varied between 30 and 135 kg COD.m−3.d−1, while the hydraulic retention time (HRT) was fixed at 4 h, and the initial co-substrate concentration varied between 5 and 22.5 g COD.L−1. The thermophilic acidogenic reactor showed a maximum hydrogen yield (HY) (2.49 mmol H2.g−1CODapp) at an OLR of 30 kg COD.m−3.d−1 and a higher hydrogen production rate (HPR) (2.8 L H2.L−1.d−1) at an OLR of 45 kg COD.m−3.d−1. Increasing the OLR negatively influenced H2 production. The results showed an increase in ethanol (EtOH) concentrations of up to 47.3 times and lactic acid (HLa) concentrations of up to 15.2 times when comparing OLRs of 30 and 135 kg COD.m−3.d−1, indicating changes in metabolic pathways for acid formation at organic loads ≥90 kg COD.m−3.d−1. Thermoanaerobacterium was the bacteria with higher relative abundance and was directly involved in H2 production, while Lactobacillus was mainly involved in HLa production in higher OLR, decreasing H2 production instead. Besides that, higher OLRs were favorable for sulfidogenesis and the development of sulfate-reducing bacteria, such as Desulfobacterota, due to the higher COD/sulfate ratio in OLR 135 kg COD.m−3.d−1. According to the metabolic inference in this condition, the enzyme activity of sulfite reductase and sulfite dehydrogenase may have occurred, consuming H+, thereby reducing H2 production.