Anaerobic Process Research Articles

Elucidating the synergic mechanism of sodium pyrophosphate assisted enzymolysis on the waste activated sludge fermentation enhancement: Insights from organic conversion and metagenomic analysis

Lysozyme has been identified as a promising approach for enhancing waste activated sludge (WAS) anaerobic fermentation. However, the extracellular polymeric substance (EPS), functioning as a ‘protective shell’ for microbial cells, hindered the lysozyme transfer and subsequent interaction with cell wall, resulting in excessive dosage and limited efficiency in practical engineering applications. This study innovatively proposed a sodium pyrophosphate (SP) assisted lysozyme treatment, aiming to overcome such limitations and enhance the solubilization and hydrolysis efficiency. As a result, 3583.5 mg COD/L short-chain fatty acids (SCFAs) and 23.1 % acidification rate were achieved after 2-day anaerobic fermentation within SP-lysozyme treatment. Mechanism investigation revealed that SP facilitated EPS disruption by inducing a loosening of the protein structure, leading to the enhanced solubilization of organic matter within the EPS and microbial cells. Consequently, the release of substantial substrates contributed to the enrichment of functional microbes associated with hydrolysis and acidogenesis, accompanied by the augmentation of corresponding metabolic functions. Besides, the key functional genes analysis demonstrated the amino acid metabolism, glycolysis, pyruvate metabolism and SCFAs transport were facilitated and the methanogenesis was inhibited on the contrary with SP-lysozyme treatment, which was undoubtedly responsible for the superior SCFAs accumulation in anaerobic fermentation process. Considering the carbon resource recovery in SCFAs form, such novel SP-lysozyme treatment exhibited 325.1 CNY /ton SS (45.5 USD / ton SS) economic benefit and avoided 0.12 CO2/ton SS carbon emission, suggesting its promising application potential in future WAS management.

Multi-omics analysis reveals the core microbiome and biomarker for nutrition degradation in alfalfa silage fermentation.

Silage fermentation is a microbial-driven anaerobic process that efficiently converts various substrates into nutrients readily absorbable and metabolizable by ruminant animals. This study, integrating culturomics and metagenomics, has successfully identified core microorganisms involved in silage fermentation, including those at low abundance. This discovery is crucial for the targeted cultivation of specific microorganisms to optimize fermentation processes. Furthermore, our research has uncovered signature microorganisms that play pivotal roles in nutrient metabolism, significantly advancing our understanding of the intricate relationships between microbial communities and nutrient degradation during silage fermentation.

Mechanisms of Irrigation Water Levels on Nitrogen Transformation and Microbial Activity in Paddy Fields

Nitrogen is a vital nutrient for rice growth; however, its inefficient use often results in nutrient loss, environmental degradation, and the emission of greenhouse gases. In this study, a rice paddy simulation was conducted under different water levels (1–4 cm), incorporating a comprehensive analysis of nitrogen dynamics, environmental factors, and microbial communities to evaluate the impact of water levels on nitrogen concentrations and microbial composition. The results indicated that the water level had a greater impact on nitrogen concentrations in surface water than in soil water. Compared to low water level conditions (1 cm), the average concentrations of ammonium nitrogen, nitrate nitrogen, and nitrite nitrogen in surface water under 2–4 cm water levels decreased by approximately 53.8%, 36.7%, and 78.9%, respectively. Water levels also influenced the microbial composition and nitrogen cycling in paddy soil, with lower water levels promoting aerobic processes such as nitrification, while higher water levels facilitated anaerobic processes such as denitrification and dissimilatory nitrate reduction to ammonium. Correspondingly, microbial composition shifted, with aerobic bacteria predominating in shallow water conditions and anaerobic bacteria flourishing under deeper water. These findings suggest that optimized water management, particularly through shallow irrigation, may mitigate nitrogen loss and improve nitrogen use efficiency. Nevertheless, additional field studies are necessary to validate these results and explore their interaction with other agricultural practices.

Progress and Trends in Forage Cactus Silage Research: A Bibliometric Perspective

Opuntia spp. (forage cactus or spineless cactus) is a plant native to Mexico that is commonly used as alternative nutrient-rich fodder in semi-arid regions. Due to its resistance to drought, forage cactus has become an important least-cost ingredient for formulating balanced rations for ruminants during times of scarcity. In addition, ensiling, an anaerobic fermentation process, is also a strategy used to allow a supply of bulky food all year round, since it conserves forage and maintains its nutritional value. In this sense, using the Scopus database and the visualization tool VOSviewer, the present work proposes a bibliometric analysis of forage cactus silage to track and map the evolution and main issues in the research field, current trends, and future directions. The results revealed that the first publication was in 2013; and since 2020, the number of publications has been growing. Brazil was highlighted, by far, as the most relevant country on the topic, and the top institutions were from northeast Brazil, which has been working on co-authored articles. The current hot research topics are focusing on the mixed silage of forage cactus and other forages such as gliricidia, maniçoba, and sorghum biomass, as well as evaluating the fermentative performance and chemical characteristics for improving ruminal diets, especially for goats and sheep. This study provides important information for researchers to identify gaps and direct their studies to better use the whole potential of forage cactus as an alternative roughage source.

A Novel First-Order Kinetic Model for Simultaneous Anaerobic–Aerobic Degradation of Municipal Solid Waste in Landfills

A first-order kinetic model for the simultaneous anaerobic–aerobic degradation of municipal solid waste (MSW) is presented in the study. The model incorporates the effect of oxygen concentration on anaerobic degradation, enabling the coexistence of anaerobic and aerobic processes within specific oxygen ranges. The model can thoroughly consider the impacts of temperature, moisture content, oxygen concentration, and free air space (FAS) on the degradation rates of five substrates, i.e., holocellulose, non-cellulosic sugars, proteins, lipids, and lignin. The model was successfully verified against two experimental results. The sequential model underestimates both compression strain and degradation ratio, with peak underestimation ratios of 8.7% and 9.2%, respectively. Using the simultaneous model, the effects of anaerobic age, temperature, and aeration rate on landfill aerobic remediation efficacy are quantitatively assessed. Two evaluation criteria, namely the advance rate of aerobic remediation stabilization time (Rt) and the degradation rate after 100 days of aerobic remediation (λ100a), are adopted. The results indicate the following: (1) Rt is more sensitive to anaerobic age and temperature, while λ100a is more affected by anaerobic age and aeration rate; (2) under optimal conditions, Rt and λ100a can reach 86.3% and 70.9%, respectively. The present model provides a crucial theoretical framework for evaluating aerobic remediation effectiveness in both anaerobic sanitary landfills and informal landfills, offering valuable insights for practical implementation and management.

Effect of nutrient imbalance on performance efficiency of an anaerobic baffled reactor and the production of membrane fouling compounds

ABSTRACT Anaerobic processes effectively treat high-strength wastewater, while membrane-based technologies are attractive post-treatment options for improving effluent quality. A balanced nutrient level (i.e., chemical oxygen demand (COD):N:P = 300:5:1) is required for optimal anaerobic reactor performance. This research investigated the performance efficiency of an anaerobic baffled reactor (ABR) treating low COD:N:P wastewater and its effluent membrane fouling potential. An unbaffled reactor was used as a control. Initially, both reactors showed ≥80% soluble COD (SCOD) removal even at a low COD:N:P of 27:5:3.5. However, after 134 days, the SCOD removal in the unbaffled reactor decreased from 80.7 ± 3 to 48.7 ± 4.6%. In comparison, it took an extra 46 days for the SCOD removal in the ABR to decrease from 87.7 ± 1.7 to 57.8 ± 2.4%. Once an optimal COD:N:P of 300:5:1 was established, the SCOD removal recovered to 88 ± 2.9% in the control and 93 ± 1.2% in the ABR. Concurrently, both reactors’ effluent membrane fouling potential increased with time at a low COD:N:P ratio and decreased at the optimal nutrient ratio. The ABR's volume compartmentation effectively improved the anaerobic biomass's ability to deal with unbalanced nutrient levels during wastewater treatment.

Insides into molecular structural elucidation on the pesticidal and herbicidal potency of AD biogas slurry

Biogas slurry is the residue liquid after anaerobic fermentation of biomass. The bio-slurry has the potential to promote plant growth and control pests. This study explores the pesticidal potency of anaerobic digestate focusing on structural elucidation through gas chromatography–mass spectrometry analysis. Samples were collected from three anaerobic digesters processing cow dung, mixed substrates, and pig waste. Physicochemical parameters such as pH, electrical conductivity, total dissolved solids, and nitrogen content were measured. The slurries exhibited slightly basic pH values (7.5–7.8) and electrical conductivity (13.8–21.9 mS/m). GC–MS analysis revealed diverse phytochemical compounds in the bio-slurries, such as gramine, 2,2-dimethoxybutane, and 4-methyl-3-penten-2-one, with potential pesticidal properties. Gramine exhibited insecticidal, herbicidal, and algaecidal effects, while 2,2-dimethoxyethane demonstrated fungicidal and herbicidal properties. Gramine was identified and possesses its potential as a natural biopesticide. The identified compounds offer promising alternatives to synthetic pesticides, emphasizing the potential of biogas slurry as a sustainable biopesticide resource.

Response of denitrifying anaerobic methane oxidation processes in freshwater and marine sediments to polyvinyl chloride microplastics.

Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) plays a crucial role in mitigating methane (CH4) in natural environments. The increasing presence of microplastics (MPs) in these environments due to human activities is a growing concern. However, the impact of MPs on n-DAMO microorganisms and their role in greenhouse gas regulation, particularly CH4 reduction, remains unclear. This study investigates the effects of polyvinyl chloride (PVC) MPs on n-DAMO activity and the associated microbial communities in freshwater and marine sediments at varying concentrations of (R0/M0-no addition, R1/M1-0.5 %, R2/M2-2%). The results showed that the presence of MPs significantly increased the n-DAMO rate (2.89-3.58 nmol 13CO2 g-1 d-1) compared to the control groups (R0: 1.29 nmol 13CO2 g-1 d-1, M0: 0.11 nmol 13CO2 g-1 d-1), with marine sediments showing a more pronounced response. Additionally, the proportional contribution of nitrate-DAMO processes increased following MP exposure. The presence of PVC MPs also altered the microbial diversity of n-DAMO. Upon the addition of MPs, the microbial community composition of n-DAMO in marine sediments changed more significantly. This study provides the first evidence of a positive impact of PVC MPs on n-DAMO processes, suggesting that the presence of PVC MPs in sediments could potentially contribute to the reduction of CH4 emissions.

Comparison of behavioral responses, respiratory metabolism-related enzyme activities, and metabolomics of the juvenile Chinese mitten crab Eriocheir sinensis with different tolerance to air exposure

Air exposure is a common stressor for Chinese mitten crab (Eriocheir sinensis) during rearing and transport, and air exposure tolerance can serve as a crucial indicator for assessing the quality of juvenile E. sinensis. In this study, juvenile E. sinensis were divided into two groups based on their behavioral responses: Group S, which exhibited strong tolerance to air exposure, and Group W, which exhibited weak tolerance. Immersed crabs, not exposed to air, served as a control group (Group C). Whole body morphological characteristics and enzyme activities related to respiratory metabolism in the hemolymph and anterior gills were compared among the three groups. Non-targeted LC-MS metabolomic analysis was conducted on anterior gills. The results showed that, independent of developmental stage, crabs that were larger and had higher condition factor were more tolerant to air exposure. Additionally, compared to Group W, air exposure had a relatively small effect on glycolysis and anaerobic respiratory metabolic processes in the hemolymph and anterior gills of Group S. In response to air exposure, E. sinensis experienced increased energy demand, and switched from aerobic to anaerobic respiration to increase energy supply. Simultaneously, air exposure induced oxidative stress in the hemolymph and anterior gills. This study enhances our understanding of the response mechanism of E. sinensis to air exposure and provides a theoretical reference for the identification of high-quality juvenile E. sinensis.

Life cycle assessment (LCA) and economic analysis of two-stage anaerobic process of co-digesting palm oil mill effluent (POME) with concentrated latex wastewater (CLW) for biogas production

This study outlines the results of a process-based life cycle assessment (LCA) of the environmental performance of biogas production from co-digestion of palm oil mill effluent (POME) and concentrated latex wastewater (CLW) in the two-stage anaerobic digestion (AD) process. Biogas is utilized global across several sectors, including electricity generation, heating, and transportation. The Open LCA 2.0.2 software was used to evaluate the LCA at the midpoint level by using 18 different impact categories. This study reveals the three main contributions of the AD process, which were global warming (8.65 × 102 kg CO2eq), water consumption (5.32 m3), and land use change (3.39 × 102 m2 a crop eq), this result corresponded with the previous study in Malaysia that examined biogas production from the AD of POME In addition, alternative scenarios, in which a single substrate, have been established to quantify the impacts of potential improvements of biogas production to compare with base scenario of co-digestion. The results show that the environmental impacts of the base scenario released were lower than those of the single POME substrate of alternative scenario 1 in most impact categories. Furthermore, economic analysis is a crucial factor in the practical implementation of the AD process. Linking LCA with economic analysis offers a comprehensive perspective on sustainability, balancing environmental impacts with financial performance to optimize the potential of the AD co-digestion process. This is the first study to apply LCA and economic analysis to this specific co-digestion process in a two-stage AD system. The economic analysis of the two-stage AD of co-digesting POME and CLW was 1200 m3/d production of wastewater (70 % of POME and 30 % of CLW) based on the factories' data. The result indicates net present value, internal rate of return, and payback period of 515,813.94 USD, 9.13 %, and 8.87 years, respectively. This work proposed the potential approach for implementing the two-stage AD co-digestion of POME and CLW while concurrently producing valuable bioenergy carriers.

Uncovering the mechanisms of ethanol stimulation on magnetite-enhanced anaerobic process treating oxytetracycline contained wastewater

Magnetite has been proved to facilitate direct interspecies electron transfer (DIET)-based syntrophys and might alleviate inhibitory effects of antibiotics in anaerobic digestion (AD), while feeding ethanol was an effective approach to enrich the DIET partners. However, most of the existing studies were conducted at fixed ethanol concentration, few attentions were paid on the effects of differential ethanol proportion on AD, the underlying roles and mechanisms of ethanol stimulation remains unclear. This study systematically investigated the impact of ethanol stimulation on anaerobic processes treating oxytetracycline (OTC)-contaminated wastewater at varying proportions (20%, 50%, and 80%, based on equivalent COD value). In the presence of magnetite, ethanol stimulation promoted the methane production from 244.9 mL/g COD to a maximum 434.2 mL/g COD, with the most pronounced enhancement observed at high ethanol proportions. In particular, the average methane production obtained at 50% and 80% ethanol was 328.5 and 297.7 mL/g COD, respectively, whereas the enhancement of 20% ethanol stimulation was relatively limited. Concurrently, more stable COD removal and OTC reduction was noted in the existence of both magnetite and high ethanol proportions. Microbial analysis revealed the pivotal roles of Methanosaeta, alongside the predominance of Methanobacterium, in regulating COD conversion and driving methanogenesis through the CO2 reduction pathway. Notably, high ethanol proportions fostered the enrichment of exoelectrogens (Geobacter, Desulfovibrio) in the magnetite-amended system, accompanied by the up-regulation of genes involved in organic metabolism pathways. Further investigation of functional genes highlighted the prevalence of pilA enrichment in the magnetite-amended system at low ethanol proportions, whereas omcS became more abundant at high ethanol proportions.

Adaptive vertical Plug-flow transition metal catalyzed Bioelectric membrane reactor for enhancing the efficient treatment of rare earth wastewater

The presence of rare earth elements (REEs) has a certain inhibitory effect on microorganisms, making the biological treatment of REEs wastewater an area of ongoing research and exploration. In this study, a vertical architecture anaerobic process and transition metal catalyzed bioelectromembrane reactor (IVABEMR) were integrated to continuously treat REEs, enhancing the potential for efficient treatment and recovery analysis. The IVABEMR demonstrated exceptional performance in removing La(III), Ce(III), Y(III), Eu(III) and Tb(III), the highest removal efficiency can reach 99.9%, 99.8%, 99.9%, 99.9%, 100%. Removal efficiency was enhanced by an electroactive electrode membrane driven by autogenous bioelectricity. Feature analysis revealed that self-released electroflocculated nucleus, recycling electromembrane reduction, and activated sludge had significant impacts on the decontamination outcomes of REEs. Functional microorganisms including Proteobacteria, Bacteroidetes, Firmicutes at phylum level survived at high abundance in the process, establishing an adaptive and stable relationship. This study provides technological optimization for efficient biological treatment, has the advantages of environmental protection and economy, and provides a feasible idea for the treatment of REEs wastewater.

Assessing the potential of olive mill solid waste as feedstock for methane and volatile fatty acids production via anaerobic bioprocesses

The extensive production of olive mill solid waste (OMSW) from olive oil industry in the Mediterranean basin claims effective treatments and valorization strategies. This study aims to elucidate the potential of anaerobic digestion (AD) and anaerobic fermentation (AF) to convert pre-treated OMSW into biogas (CH4) and volatile fatty acids (VFA), respectively. The two thermal treatment conditions (65 °C and 180 °C) that are being implemented in the industry that manages the OMSW were tested. Comparing the two treatments aims to demonstrate the influence on the AD process of the degree of solubilization and degradation of the metabolites produced from the same substrate. AD of OMSW treated at low-temperature (65 °C) exhibited similar methane yields (195 ± 8 mL CH4/g volatile solid (VS)) to raw OMSW. AD of the solid phase (SP) after high-temperature treatment with acid addition at 180 °C resulted in methane yields comparable to raw OMSW while the liquid phase (LP) exhibited low methane yields (85 ± 10 mL CH4/g VS). Nevertheless, LP/180 °C exhibited the highest VFA bioconversion at 27.6 %, compared to less than 10 % for SP/180 ºC, SP/65 °C, and raw OMSW. The VFA profile showed notable variations with thermal treatment temperatures. Propionic acid dominated at SP/65 °C, while acetic acid became the primary VFA at 180 °C. Furthermore, significant degradation rates of phenolic compounds and furans were observed during the final day of both anaerobic processes. Overall, these findings suggest that AD is more suitable for raw OMSW, treated at low temperature and SP at high temperature, while AF offers a promising alternative for high-temperature-treated LP.

Pengurangan Nilai COD pada Effluent Fermentor Biogas UASB-HCPB dengan Memanfaatkan Mikroalga Chlorella vulgaris

Palm oil mill effluent (POME) is the main waste generated from the palm oil industry. LCPKS has a COD level of 57,000-60,400 mg/L. High levels of COD can harm the environment. LCPKS treatment techniques generally use a combination of anaerobic and aerobic waste treatment methods. The anaerobic process has been conducted using a UASB-HCPB reactor. But, the COD levels of anaerobic results have not reached the quality standards so that further processing is needed. Microalgae have the potential to reduce contaminant levels in wastewater. Chlorella vulgaris microalgae is a microalgae that is widely used in wastewater treatment. This study utilized Chorella vulgaris in reducing the COD levels of the effluent of the UASB-HCPB biogas fermentor with variations in lighting intensity and variations in the ratio of effluent to microalgae. It was found that the lighting intensity of light:dark (24:0) hours provided optimal microalgae growth so as to provide the best COD reduction with a COD reduction efficiency of 87.5%. The ratio of microalgae to effluent of 1000 mL: 500 mL showed the best COD reduction with an efficiency of 93.75%.

Nutrient recovery from source-separated urine via sorption and a comparative investigation on the improvement of the residual liquid quality

Human urine, a highly saline solution rich in plant-available nutrients, leaves behind significant organic matter after nutrient recovery, necessitating additional treatment for environmental protection. While nutrient recovery from human urine is well-documented in the literature, research on the safe handling of the residual liquid phase is notably lacking. This study investigates nutrient recovery from source-separated human urine using clinoptilolite for the ion exchange/adsorption process and evaluates the safe management of the residual liquid through anaerobic granular sludge and a second-stage of sorption. The results indicated that the ion exchange/adsorption process, using an ammonium loading of 15 mg NH4+/g clinoptilolite, removed the majority of nutrients, achieving 82% ammonium removal and 100% phosphorus removal, along with 30% removal of organic matter. The residual liquid phase from the nutrient removal stage was treated separately with anaerobic digestion and a second-stage of sorption for further processing. Results showed that anaerobic processing removed 68%–84% of organic matter, with no additional nitrogen removal observed as expected, and produced 0.20–0.46 L CH4/L urine. The second-stage of sorption removed 59%–62% of organic matter and nearly all nitrogen. Both processes effectively removed organic matter, with sorption also eliminating nitrogen and anaerobic processing potentially generating biogas, making them recommended for improving the quality of the residual liquid phase before final disposal.

Effect of plant species on wastewater treatment performance of a subsurface vertical-flow constructed wetland with step-feeding at low temperature

To improve the treatment performance of constructed wetlands under low-temperature conditions, this study investigated the effects of plant species on wastewater treatment performance at low temperature and the associated microbiological characteristics in a subsurface vertical-flow constructed wetland (VFCW) with step-feeding. The results showed that the redox microenvironment in the VFCW filter with step-feeding could be restored and optimized by planting appropriate species that can tolerate low temperature, ensuring a high nitrification performance for the system. Correspondingly, the abundance and activity of three functional microbes (namely nitrifiers, denitrifiers, and anammox bacteria) increased to different degrees in the system, eventually ensuring ideal nitrogen removal by the VFCW. Compared with the VFCW planted with Phragmites australis and Acorus gramineus, the operation performance of the VFCW planted with Iris wilsonii could be recovered at low temperature, and its chemical oxygen demand, total phosphorus, total nitrogen, and ammonium nitrate removal rates could respectively reach 95.7%, 99.2%, 93.0%, and 94.4%, respectively. Moreover, nitrogen removal in the system relied on the nitrification/denitrification and partial denitrification - anaerobic ammonium oxidation processes. Nitrosomonas, Nitrospira, Thauera, and Candidatus Brocadia were the four dominant bacterial genera in the filter layer.

Promotion of polyhydroxyalkanoates-producing granular sludge formation by lactic acid using anaerobic dynamic feeding process

To promote the formation of granular sludge with high polyhydroxyalkanoates (PHAs) synthesis ability, an anaerobic dynamic feeding process (AnDF) was proposed. This process combines the feast-famine mode with an anaerobic plug flow feeding process and involving variations in cycle length and settling time. The effects of lactic acid (LA) content (0 %, 20 %, and 40 % COD) on sludge granulation and PHAs production were investigated using three AnDF reactors (R1, R2, and R3). The results showed that the AnDF process feeding with LA not only effectively promoted sludge granulation but also improved its PHAs synthesis ability. The granules were quickly observed in R3 after 50 days of cultivation, with an average diameter of 0.69 mm. The maximum PHAs content reached 47.0 wt% in R3, representing a 30.09 % increase compared to R1. Additionally, extracellular polymeric substances (EPS)-producing bacteria observed in granular sludge may be the prime drivers of the formation of PHAs-producing granular sludge (PHAGS), which was defined as granular sludge with an average particle size larger than 0.30 mm and PHAs content above 40 % cell dry weight (CDW) of sludge samples.

Improving the anaerobic digestion of sewage sludge by adding cobalt nanoparticles

ABSTRACT This work evaluated the effects of cobalt nanoparticles (CoNPs) (0.025–7 mg/gVS) on the intensification of sewage sludge anaerobic digestion (AD) using biochemical methane potential (BMP) tests. This study was motivated by the need to improve the efficiency and stability of anaerobic digestion of sewage sludge, a critical process in waste management and renewable energy production. The effects at doses less than 2 mg/gVS were not substantial, but 3–7 mg/gVS improved the performance. The maximum biogas yield was 232 mL/gVS (at a dose of 7 mg/gVS), whereas it was 132 mL/gVS in the control (zero dose). Similarly, the reductions in the volatile solids and methane contents reached maxima of 16 and 74.3%, respectively. The analyses of volatile fatty acids, redox potential, and electron transfer system activity indicated that the addition of CoNPs stimulated the early stages of AD. Finally, acetate consumption and the increase in CH4 content suggested that CoNPs positively affected system stability and acetoclastic methanogenesis. That is, CoNPs effectively intensified the behaviour and stability of the anaerobic process. The novelty of this research lies in the comprehensive evaluation of the effects of CoNPs across a wide range of doses on sewage sludge AD, providing new insights into the optimisation of this process for increased biogas production and organic matter reduction.

Mechanistic insights into Fe3O4-mediated inhibition of H2S gas production in sludge anaerobic digestion

The addition of iron-based conductive materials has been extensively validated as a highly effective approach to augment methane generation from anaerobic digestion (AD) process. In this work, it was additionally discovered that Fe3O4 notably suppressed the production of hazardous H2S gas during sludge AD. As the addition of Fe3O4 increased from 0 to 20 g/L, the accumulative H2S yields decreased by 89.2 % while the content of element sulfur and acid volatile sulfide (AVS) respectively increased by 55.0 % and 30.4 %. Mechanism analyses showed that the added Fe3O4 facilitated sludge conductive capacity, and boosted the efficiency of extracellular electron transfer, which accelerated the bioprocess of sulfide oxidation. Although Fe3O4 can chemically oxidize sulfide to elemental sulfur, microbial oxidation plays a major role in reducing H2S accumulation. Moreover, the released iron ions reacted with soluble sulfide, which promoted the chemical equilibrium of sulfide species from H2S to metal sulfide. Microbial analysis showed that some SRBs (i.e., Desulfomicrobium and Defluviicoccus) and SOB (i.e., Sulfuritalea) changed into keystone taxa (i.e., connectors and module hubs) in the reactor with Fe3O4 addition, showing that the functions of sulfate reduction and sulfur oxidation may play important roles in Fe3O4-present system. Fe3O4 presence also increased the content of functional genes encoding sulfide quinone reductase and flavocytochrome c sulfidedehydrogenase (e.g., Sqr and Fcc) that could oxidize sulfide to sulfur. The impact of other iron-based conductive material (i.e., zero-valent iron) was also verified, and the results showed that it could also significantly reduce H2S production. These findings provide new insights into the effect of iron-based conductive materials on anaerobic process, especially sulfur conversion.

Optimizing operation strategy to improve storage of intracellular carbon sources in anaerobic/oxic/anoxic system: Enhanced nitrogen removal by endogenous denitrification

Endogenous denitrification (ED) can make full use of the carbon sources and avoid replenishment of it. However, strengthening the storage of intracellular carbon sources is an important factor in improving ED efficiency. In this study, employed batch experiments using real domestic wastewater in the anaerobic/oxic (A/O) process. The anaerobic and oxic processes were run for 4 h under ambient conditions with the dissolved oxygen (DO) concentrations in the oxic stage controlled at 0.5, 1.0, 1.5, and 3.0 mg/L, respectively. The results showed that the content of poly-β-hydroxyalkanoates (PHA) reached its peak at 60 min (1.25 mmolC/L). And with DO concentrations of 1.5 mg/L, the contents of glycogen (Gly) were 27.74 mmolC/L. Subsequently, the AOA-SBR was established to investigate its effect on the long-term nitrogen removal performance of domestic wastewater by optimizing the anaerobic time and DO concentrations. The results showed that at an anaerobic time of 60 min and DO concentration of 1.5 mg/L, the storage of the intracellular carbon sources was highest and the total nitrogen (TN) removal efficiency increased to 82.12%. In addition, Candidatus Competibacter dominated gradually in the system as the strategy was optimized.