Response of morphology and microbial community structure of granules to influent COD/SO42 - ratios in an upflow anaerobic sludge blanket (UASB) reactor treating starch wastewater

Lighting promotes sulfate removal and improves microbial community stability in upflow anaerobic sludge bed reactors under low ratio of chemical oxygen demand to sulfate

The anaerobic treatment of sulfur-laden organic wastewater is common; however, competition between sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) can result in low removal efficiencies and unstable systems. Photosynthetic bacteria, capable of oxidizing reduced sulfides, can alleviate sulfide toxicity to microorganisms, thereby enhancing sulfate removal. This study compared the performance of anaerobic reactors under identical organic loads but with varying light conditions and different carbon-to-sulfur (C/S) ratios. The illuminated reactors outperformed the non-illuminated ones, achieving sulfate removal rates exceeding 85% when the light wavelength was optimized. Sludge analysis revealed that the illuminated group had larger particle sizes and higher protein and polysaccharide contents compared to the non-illuminated group. These findings suggest that light exposure enhances the removal of sulfate and organic matter, mitigates competitive inhibition, and promotes synergistic interactions among microbial populations, offering valuable insights for treating sulfate-rich wastewater using photosynthetic bacteria.

From waste to wealth: Exploring the effect of particle size on biopolymer harvesting from aerobic granular sludge

This study aimed to examine the impact of aerobic granular sludge (AGS) sizes on its properties and alginate-like exopolymers (ALE) recovery potential. The AGS was cultivated in a lab-scale bioreactor and categorized into six size classes with 200 μm intervals. There appeared a critical size (400-800 μm) for developing stable AGS structure and excellent ALE recovery. A higher hydrophobicity (74.36 %) and density (1,037 g/L) was observed in AGS400-600μm than other sizes. Moreover, the highest ALE yield was obtained in ALE600-800μm (388 mg/g VSS) for its higher abundance of EPS-producers (35.1 %), while the PN content of ALE400-600μm was higher than other samples. Meanwhile, the concentrations of metal elements within the ALE and AGS identified that there was no bio-accumulation of metal elements in the ALE. This study offers an in-depth understanding of biopolymer recovery from AGS, paving the way for a novel resource recovery strategy through the regulation of AGS sizes.

Sludge bound-EPS solubilization enhance CH4 bioconversion and membrane fouling mitigation in electrochemical anaerobic membrane bioreactor: Insights from continuous operation and interpretable machine learning algorithms

Bound extracellular polymeric substances (EPS) are complex, high-molecular-weight polymer mixtures that play a critical role in pore clogging, foulants adhesion, and fouling layer formation during membrane filtration, owing to their adhesive properties and gelation tendencies. In this study, a novel electrochemical anaerobic membrane bioreactor (EC-AnMBR) was constructed to investigate the effect of sludge bound-EPS solubilization on methane bioconversion and membrane fouling mitigation. During the 150-days' operation, the EC-AnMBR demonstrated remarkable performance, characterized by an exceptionally low fouling rate (transmembrane pressure (TMP) < 4.0 kPa) and high-quality effluent (COD removal > 98.2 %, protein removal > 97.7 %, and polysaccharide removal > 98.5 %). The highest methane productivity was up to 38.0 ± 3.1 mL/Lreactor/d at the applied voltage of 0.8 V with bound-EPS solubilization, 107.6 % higher than that of the control stage (18.3 ± 2.4 mL/Lreactor/d). Morphological and multiplex fluorescence labeling analyses revealed higher fluorescence intensities of proteins, polysaccharides, total cells and lipids on the surface of the fouling layer. In contrast, the interior exhibited increased compression density and reduced activity, likely attributable to compression effect. Under the synergistic influence of the electric field and bound-EPS solubilization, biomass characteristics exhibited a reduced propensity for membrane fouling. Furthermore, the bio-electrochemical regulation enhanced the electroactivity of microbial aggregates and enriched functional microorganisms, thereby promoting biofilm growth and direct interspecies electron transfer. Additionally, the potential hydrogenotrophic and methylotrophic methanogenesis pathways were enhanced at the cathode and anode surfaces, thereby increasing CH₄ productivity. The random forest-based machine learning model analyzed the nonlinear contributions of EPS characteristics on methane productivity and TMP values, achieving R² values of 0.879 and 0.848, respectively. Shapley additive explanations (SHAP) analysis indicated that S-EPSPS and S-EPSPN were the most critical factors affecting CH₄ productivity and membrane fouling, respectively. Partial dependence plot analysis further verified the marginal and interaction effects of different EPS layers on these outcomes. By combining continuous operation with interpretable machine learning algorithms, this study unveils the intricate impacts of EPS characteristics on methane productivity and membrane fouling behaviors, and provides new insights into sludge bound-EPS solubilization in EC-AnMBR.

Hydrolytic enzyme activity in high-rate anaerobic reactors treating municipal wastewater in temperate climates

Particulate matter hydrolysis is the bottleneck in anaerobic treatment of municipal wastewater in temperate climates. Low temperatures theoretically slow enzyme-substrate interactions, hindering utilization kinetics, but this remains poorly understood. β-glucosidase, protease, and lipase activities were evaluated in two pilot-scale upflow anaerobic sludge blanket (UASB) reactors, inoculated with different sludges and later converted to anaerobic membrane bioreactors (AnMBRs). Despite similar methane production and solids hydrolysis rates, significant differences emerged. Specific activity peaked at 37 °C, excluding the predominance of psychrophilic enzymes. Nevertheless, the Michaelis-Menten constant (Km) indicated high enzyme-substrate affinity at the operational temperature of 15-20 °C, notably greater in AnMBRs. It is shown, for the first time, that different seed sludges can equally adapt, as hydrolytic enzymatic affinity to the substrate reached similar values in the two reactors at the operational temperature and identified that membrane ultrafiltration impacted hydrolysis by a favourable enzyme Michaelis-Menten constant.

Optimizing bioelectromethanosynthesis of CO2 and membrane fouling mitigation in MECs via in-situ biogas recirculation

The CO2 bioelectromethanosynthesis via two-chamber microbial electrolysis cell (MEC) holds tremendous potential to solve the energy crisis and mitigate the greenhouse gas emissions. However, the membrane fouling is still a big challenge for CO2 bioelectromethanosynthesis owing to the poor proton diffusion across membrane and high inter-resistance. In this study, a new MEC bioreactor with biogas recirculation unit was designed in the cathode chamber to enhance secondary-dissolution of CO2 while mitigating the contaminant adhesion on membrane surface. Biogas recirculation improved CO2 re-dissolution, reduced concentration polarization, and facilitated the proton transmembrane diffusion. This resulted in a remarkable increase in the cathodic methane production rate from 0.4 mL/L·d to 8.5 mL/L·d. A robust syntrophic relationship between anodic organic-degrading bacteria (Firmicutes 5.29%, Bacteroidetes 25.90%, and Proteobacteria 6.08%) and cathodic methane-producing archaea (Methanobacterium 65.58%) enabled simultaneous organic degradation, high CO2 bioelectromethanosynthesis, and renewable energy storage.

Artificial Sweetener Enhances the Spread of Antibiotic Resistance Genes During Anaerobic Digestion

Artificial sweeteners have been frequently detected in the feedstocks of anaerobic digestion. As these sweeteners can lead to the shift of anaerobic microbiota in the gut similar to that caused by antibiotics, we hypothesize that they may have an antibiotic-like impact on antibiotic resistance genes (ARGs) in anaerobic digestion. However, current understanding on this topic is scarce. This investigation aimed to examine the potential impact of acesulfame, a typical artificial sweetener, on ARGs in anaerobic digestion by using metagenomics sequencing and qPCR. It was found that acesulfame increased the number of detected ARG classes and the abundance of ARGs during anaerobic digestion. The abundance of typical mobile genetic elements (MGEs) and the number of potential hosts of ARGs also increased under acesulfame exposure, suggesting the enhanced potential of horizontal gene transfer of ARGs, which was further confirmed by the correlation analysis between absolute abundances of the targeted ARGs and MGEs. The increased horizontal dissemination of ARGs may be associated with the SOS response induced by the increased ROS production, and the increased cellular membrane permeability. These findings indicate that artificial sweeteners may accelerate ARG spread through digestate disposal, thus corresponding strategies should be considered to prevent potential risks in practice.

Emerging Strategies for Enhancing Propionate Conversion in Anaerobic Digestion: A Review

Anaerobic digestion (AD) is a triple-benefit biotechnology for organic waste treatment, renewable production, and carbon emission reduction. In the process of anaerobic digestion, pH, temperature, organic load, ammonia nitrogen, VFAs, and other factors affect fermentation efficiency and stability. The balance between the generation and consumption of volatile fatty acids (VFAs) in the anaerobic digestion process is the key to stable AD operation. However, the accumulation of VFAs frequently occurs, especially propionate, because its oxidation has the highest Gibbs free energy when compared to other VFAs. In order to solve this problem, some strategies, including buffering addition, suspension of feeding, decreased organic loading rate, and so on, have been proposed. Emerging methods, such as bioaugmentation, supplementary trace elements, the addition of electronic receptors, conductive materials, and the degasification of dissolved hydrogen, have been recently researched, presenting promising results. But the efficacy of these methods still requires further studies and tests regarding full-scale application. The main objective of this paper is to provide a comprehensive review of the mechanisms of propionate generation, the metabolic pathways and the influencing factors during the AD process, and the recent literature regarding the experimental research related to the efficacy of various strategies for enhancing propionate biodegradation. In addition, the issues that must be addressed in the future and the focus of future research are identified, and the potential directions for future development are predicted.

Upflow anaerobic sludge blanket reactor operation under high pressure for energy-rich biogas production

Autogenerative high-pressure digestion has an advantage of producing CH₄-rich biogas directly from the reactor. However, its continuous operation has rarely been reported, and has never been attempted in an upflow anaerobic sludge blanket reactor (UASB). Here, UASB was continuously operated at 10 g COD/L/d with increasing pressure from 1 to 8 bar. As the pressure increased, the CH₄ content in the biogas increased gradually, reaching 96.7 ± 0.8% at 8 bar (309 MJ/m³ biogas). The pH was dropped from 8.2 to 7.2 with pressure increase, but COD removal efficiency was maintained > 90%. The high pressure up to 8 bar did not adversely impact the physicochemical properties of granules, which was due to the increased production of extracellular polymeric substances (EPS), particularly, tightly bound EPS (34% increase). With pressure increase, there was no changes in the microbial community and ATPase gene expression, but 41% increase in carbonic anhydrase gene expression was observed.

Effects of submicron magnetite particles on granulation of flocculent sludge and process stability in upflow anaerobic sludge blanket reactor

Promoting direct interspecies electron transfer (DIET) with conductive additives is considered a promising approach to enhance methanogenesis. This study investigated the effects of adding submicron magnetite particles on sludge granulation and methanogenic performance in upflow anaerobic sludge blanket reactors inoculated with flocculent sludge. The reactor supplemented with magnetite was more stable and resilient than the no-magnetite control, with higher degree of granulation (up to 26.6-fold) and biomass retention. Magnetite addition to unstable reactors improved the methane yield in both reactors (1.2-1.3-fold). Electroactive Deltaproteobacteria bacteria, including Geobacter and Syntrophobacter, were enriched in the presence of magnetite. Methanogenic functional genes involved in DIET-based syntrophy were more abundant under magnetite-supplemented conditions. However, the improvement of methanogenic performance and granulation was limited, and inducing the self-embedment of magnetite into mature sludge granules rather than granulating flocculent sludge with magnetite appears to be a better strategy for engineering DIET in anaerobic granular sludge systems.

Simultaneous recovery of bio-sulfur and bio-methane from sulfate-rich wastewater by a bioelectrocatalysis coupled two-phase anaerobic reactor

The microbial electrolysis cell coupled the two-phase anaerobic digestion (MEC-TPAD) was developed for simultaneous recovery of bio-sulfur and bio-methane from sulfate-rich wastewater. In acidogenic phase, the produced sulfides were efficiently converted into bio-sulfur via anodic bio-oxidation, with a maximum recovery of 59 ± 5.5 %. The anode coupled acidogenesis produced more volatile fatty acids which were benefit for the subsequent methanogenesis. The cathode in methanogenic phase created a suitable pH condition and enhanced the methanogenesis. Correspondingly, the maximum bio-methane yield in MEC-TPAD was 2 times higher than that in TPAD. Microbial communities revealed that major functional consortia capable of sulfides oxidation (e.g. Alcaligenes) in anode biofilm, hydrogenotrophic methanogenesis (e.g. Methanobacterium) in cathode biofilm, and acetotrophic methanogenesis (e.g. Methanosaeta) in methanogenic sludge were enriched. Economic benefit could totally cover the cost of input electric energy. This work opens an appealing avenue for recovering nutrient and energy from wastewater.

Effects of sulfate concentration on anaerobic treatment of wastewater containing monoethanolamine using an up-flow anaerobic sludge blanket reactor

Monoethanolamine (MEA), a toxic organic chemical, is widely used in industries and is found in their wastewater. Anaerobic MEA degradation is an appropriate strategy to reduce energy and cost for treatment. Industry wastewaters also contain sulfate, but information on the effects of sulfate on MEA degradation is limited. Here, an up-flow anaerobic sludge blanket (UASB) for MEA-containing wastewater treatment was operated under psychrophilic conditions (18-20 ºC) to investigate the effects of sulfate on the microbial characteristics of the retained sludge. To acclimatize the sludge, the proportion of MEA in the influent (containing sucrose, acetate, and propionate) was increased from 15% to 100% of total COD (1500 mg L^-1); sulfate was then added to the influent. The COD removal efficiency remained above 95% despite the increase in MEA and sulfate. However, granular sludge disintegration was observed when sulfate was increased from 20 to 330 mg L^-1. Batch tests revealed that propionate and acetate were produced as the metabolites of MEA degradation. In response to sulfate acclimation, methane-producing activities for propionate and hydrogen declined, while sulfate-reducing activities for MEA, propionate, and hydrogen increased. Accordingly, acclimation and changes in dominant microbial groups promoted the acetogenic reaction of propionate by sulfate reduction.

The influences of soil sulfate content on the transformations of nitrate and sulfate during the reductive soil disinfestation (RSD) process

The transformations and products of sulfate (SO₄^2-) and nitrate (NO₃^-), especially the influences of SO₄^2- content on the transformations during RSD process, are unclear. In this study, a series of soil SO₄^2- contents (from 333 to 3000 mg S kg^-1) were prepared before RSD treatment. The results indicated that nearly all the cumulative NO₃^- (>98.6%) was removed and not affected by the soil SO₄^2- content. The ¹⁵N recovery results showed that 0.57-1.24% and 2.94-4.59% of NO₃^- translated into ammonium (NH₄⁺) and organic N, respectively, and high SO₄^2- contents stimulated the processes of NO₃^- dissimilatory reduction and NO₃^- immobilization. The soluble SO₄^2- contents decreased by 397-922 mg S kg^-1, but the contents of total sulfur, sulfide, and sulfate precipitation varied slightly after RSD, indicating that the decreased SO₄^2- was mainly immobilized into organic sulfur in all soils. In addition, a fraction of decreased SO₄^2- was adsorbed to the soil with a relatively high SO₄^2- content. The leaching of SO₄^2- was high (42.9-602 mg S kg^-1) during the RSD process, and the leaching amounts increased with increasing soil SO₄^2- content. In terms of the gases emitted from the transformations of NO₃^- and SO₄^2-, the cumulative emissions of nitrous oxide (N₂O) and six sulfurous gases (hydrogen sulfide, carbonyl sulfide, carbon disulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide) were in the ranges of 17.1-21.2 mg N kg^-1 and 7.78-23.5 μg S kg^-1, respectively, during the whole RSD process. The emissions of sulfurous gases were inhibited by high soil SO₄^2- content, but the N₂O emissions were unaffected. In conclusion, the soil SO₄^2- content influenced the transformations of NO₃^- and SO₄^2- during RSD process, and the SO₄^2- leaching and N₂O emissions might threaten the environment which should be concerned.

Bioelectrochemical regulation accelerates biomethane production from waste activated sludge: Focusing on operational performance and microbial community

Bioelectrochemical regulation represents a newly emerging strategy to enhance anaerobic digestion (AD) of biowastes. Herein, a novel microbial electrolysis cell (MEC) system, equipped with a pair of carbon brush anode and hybrid Ti/RuO₂-graphite felt cathode, was developed to explore the role of bioelectrochemical regulation in the proliferation/enrichment of functional microbes and methanation of waste activated sludge. The methane production was significantly improved by applying bioelectrochemical regulation. The maximum methane yield was 16.4 mL/L_-reactor at the applied external voltage 1.2 V and solids retention time 15 d, 8.6-time higher than that of a single AD. Further analysis demonstrated that bioelectrochemical regulation selectively enriched electroactive fermentative partners and methanogens (especially Thermincola, Methanobacterium) in the MEC-AD system and built up a robust syntrophic interaction. This drove the decomposition of complex organics and concurrent bioelectroreduction of CO₂ in biogas and subsequently enhanced methane generation. Besides, bioelectrochemical simulation attenuated N₂O emissions and enhanced the dewaterability of digested sludge.

Long-term performance, membrane fouling behaviors and microbial community in a hollow fiber anaerobic membrane bioreactor (HF-AnMBR) treating synthetic terephthalic acid-containing wastewater

Purified terephthalic acid (PTA) wastewater with properties of poor biodegradation and high toxicity is produced from refining and synthesis of petrochemical products. In this study, a lab-scale hollow fiber membrane bioreactor (HF-AnMBR) fed with synthetic PTA wastewater was operated over 200 days with stepwise decreased hydraulic retention time (HRT) to investigate the long-term performance, membrane fouling mechanism and microbial community evolution. Results showed that a stable chemical oxygen demand (COD) removal rate of 65.8 ± 4.1% was achieved at organic loading rate of 3.1 ± 0.3 g-COD/L-reactor/d and HRT 24 h, under which the methane production rate reached 0.33 ± 0.02 L/L-reactor/d. Further shortening HRT, however, led to the decreased COD removal efficiency and low methane bioconversion. A mild membrane fouling occurred due to the production of colloidal biopolymers and the interaction between increased colloidal substances secreted/cracked by microorganisms and membrane interface. Further 16S rRNA analysis indicated that microbial diversity and richness had changed with the variation of HRT while Methanosaeta, and Methanolinea species were always the dominant methanogens responsible for methane production. The results verify that HF-AnMBR is an alternative technology for PTA wastewater treatment along with energy harvesting, and provide a new avenue toward sustainable petrochemical wastewater management.

Formation and characterization of conductive magnetite-embedded granules in upflow anaerobic sludge blanket reactor treating dairy wastewater

Promoting direct interspecies electron transfer (DIET) with conductive additives has proved effective in improving anaerobic digestion performance and stability. However, its application is limited by the need to replenish the washout loss of conductive materials. This study reports the formation of conductive magnetite-embedded granular sludge and its long-term influence on the performance of upflow anaerobic sludge blanket reactors treating dairy wastewater. The magnetite-supplemented reactor maintained better performance than the no-magnetite control, with greater sludge settling and electron transport activity, throughout the 192-d experiment at increasing organic loading rates (1.2-8.5 g chemical oxygen demand/L·d). The abundance of electroactive microbes also remained higher in the magnetite-supplemented reactor. The results suggest that DIET-based electric syntrophy was promoted in the magnetite-embedded granules. This study is the first to demonstrate the self-embedment of submicron conductive material into granular sludge and its benefits. These findings offer a new approach to enhancing anaerobic granular sludge systems.

Hydrogenotrophic methanogenic granular sludge formation for highly efficient transforming hydrogen to CH4

This paper presents a potential process that can enhance H₂ transformation to CH₄ and simultaneously upgrading biogas by using hydrogenotrophic methanogens. For the first time, anaerobic granules were developed in upflow anaerobic sludge blanket (UASB) reactor feeding H₂/CO₂ syngas as the sole substrate and the granule characterization was thoroughly investigated. The results from experiment revealed that the H₂ consumption rates of UASB reactor increased from 32.2 mmol L^-1·d^-1 at H₂ feeding rate 0.08 g L^-1·d^-1 to 132.0 mmol L^-1·d^-1 at 0.37 g L^-1·d^-1, indicating that the hydrogenotrophic methanogenesis pathway was stimulated by injection of H₂. Abundant cavities and cracks were observed on the surface and cross-section of granules, which greatly facilitated internally transferring H₂/CO₂ synthesis gas and biogas escape. The abundance of hydrogenotrophic Methanobacterium increased, while Methanosaeta, Methanosarcina, and Methanomassiliicoccus decreased with increasing H₂ feeding rate. In general, this paper offers a feasible solution in terms of energy transformation and connecting power to fuel.

The potential of electrotrophic denitrification coupled with sulfur recycle in MFC and its responses to COD/SO42- ratios

Electrotrophic denitrification is a promising novel nitrogen removal technique. In this study, the performance and the mechanism of electrotrophic denitrification coupled with sulfate-sulfide cycle were investigated under different anodic influent COD/SO₄^2- ratios. The results showed that electrotrophic denitrification contributed to more than 22% total nitrogen removal in cathode chamber. Higher COD/SO₄^2- ratios would deteriorate the sulfate reduction but enhance methane production. Further mass balance indicated that the electron flow utilized by methanogenic archaea (MA) increased while that utilized by sulfate-reducing bacteria (SRB) decreased as the COD/SO₄^2- ratio increased from 0.44 to 1.11. However, higher COD/SO₄^2- ratios would produce more electrons to strengthen electrotrophic denitrification. Microbial community analysis showed that the biocathode was predominantly covered by Thiobacillus that encoded with narG gene. These findings collectively suggest that electrotrophic denitrification could be a sustainable approach to simultaneously remove COD and nitrogen under suitable COD/SO₄^2- ratio based on sulfur cycle in wastewater.

Valorisation of Starch Wastewater by Anaerobic Fermentation

Starch production is mainly focused on feedstocks such as corn, wheat and potato in the EU, whereas cassava, rice, and other feedstocks are utilised worldwide. In starch production, a high amount of wastewater is generated, which accumulates from different process steps such as washing, steeping, starch refining, saccharification and derivatisation. Valorisation of these wastewaters can help to improve the environmental impact as well as the economics of starch production. Anaerobic fermentation is a promising approach, and this review gives an overview of the different utilisation concepts outlined in the literature and the state of the technology. Among bioenergy recovery processes, biogas technology is widely applied at the industrial scale, whereas biohydrogen production is used at the research stage. Starch wastewater can also be used for the production of bulk chemicals such as acetone, ethanol, butanol or lactic acids by anaerobic microbes.

Anaerobic co-digestion of landfill leachate and acid mine drainage using up-flow anaerobic sludge blanket reactor

A laboratory-scale up-flow anaerobic sludge blanket (UASB) reactor was developed and constructed for the treatment of landfill leachate and acid mine drainage (AMD). The removal of chemical oxygen demand (COD), sulfate, and metal ions was studied. The maximum COD and sulfate removal efficiency reached 75% and 69%, respectively, during the start-up phase of the UASB. The hydraulic retention time (HRT) had a significant influence on the system. The maximum removal efficiency for COD and sulfate reached 83% and 78%, respectively, at an HRT of 20 h. The methane production process competed with the sulfate reduction process in the UASB. The fractionation of metals in the sludge was analyzed to facilitate metal recovery in a later processing stage. The most abundant sulfate-reducing bacteria was Desulfobulbus, and the methanogen archaeal community in the reactor was mainly composed of Methanobacterium.

On-Site Blackwater Treatment Fosters Microbial Groups and Functions to Efficiently and Robustly Recover Carbon and Nutrients

Wastewater is considered a renewable resource water and energy. An advantage of decentralized sanitation systems is the separation of the blackwater (BW) stream, contaminated with human pathogens, from the remaining household water. However, the composition and functions of the microbial community in BW are not known. In this study, we used shotgun metagenomics to assess the dynamics of microbial community structure and function throughout a new BW anaerobic digestion system installed at The Netherlands Institute of Ecology. Samples from the influent (BW), primary effluent (anaerobic digested BW), sludge and final effluent of the pilot upflow anaerobic sludge blanket (UASB) reactor and microalgae pilot tubular photobioreactor (PBR) were analyzed. Our results showed a decrease in microbial richness and diversity followed by a decrease in functional complexity and co-occurrence along the different modules of the bioreactor. The microbial diversity and function decrease were reflected both changes in substrate composition and wash conditions. Our wastewater treatment system also decreased microbial functions related to pathogenesis. In summary, the new sanitation system studied here fosters microbial groups and functions that allow the system to efficiently and robustly recover carbon and nutrients while reducing pathogenic groups, ultimately generating a final effluent safe for discharge and reuse.

Rational design of sulfidogenic granular sludge reactor with clostridia as dominant bacteria for energy-efficient sulfate-laden wastewater treatment

The sludge flotation and washout are frequently observed in anaerobic sulfidogenic reactor. This challenge raised the interests of re-thinking/re-designing of a compact and low-flotation bioreactor. The present study investigated to understand the temporal dynamics of microbial community and granular sludge properties in a pneumatic-mixing reactor treating sulfate-laden wastewater. The findings revealed that the reactor performance and sludge properties were dynamically changed and correlated over long-term run. In the bioreactor, a rarer type of sulfate reducing bacteria (genus Clostridium XVIII) was remarkably enriched (~30% abundance). The Clostridium XVIII-mediated COD removal (92.7 ± 3.9%) was further confirmed via mass balance which demonstrated the growth rate of total active biomass and sulfate-reducing active biomass were 19.95 and 6.0 mg-COD/L_influent respectively. The PICRUSt data suggested that i) high abundance of carbohydrate metabolism and S-reductase enzymes enriched, and ii) energy metabolism enzymes decreased which implies that the new SRB communities are more energy-efficient than conventional ones.

Multiple strategies for maintaining stable partial nitritation of low-strength ammonia wastewater

Stable production of nitrite is an essential technical challenge for mainstream anaerobic ammonia oxidation (Anammox). Due to difficulties in the stable inhibition of nitrite oxidizing bacteria (NOB) and maintenance of long-term partial nitritation (PN), integrated multiple, rather than a single, controlling strategies were preferred especially in a continuous-flow treatment system. A mathematically model was developed to evaluate effects of integrated multiple-strategies on ammonia oxidizing bacteria (AOB) and NOB. Through experimental study and model simulation, intermittent aeration and low SRT (3.5 d) resulted in unstable nitrite accumulation. Integrated multiple-strategies of intermittent aeration, low SRT (3.5 d) and bioaugmentation achieved nitrite accumulation rate of 81% and NO₂^--N/NH₄⁺-N ratio in effluent of 1.29, which was preferable for further anammox process. Meanwhile, the richness and diversity of microbial community increased due to the bioaugmentation. The AOB/NOB ratio increased from 13.8 to 34.1 which facilitated nitrite accumulation. In combination with bioaugmentation, the observed growth rates of AOB and NOB increased from -0.0835 and -0.0282 to 0.0434 and 0.0127 d^-1, respectively, which promoted AOB outcompeting NOB in the mixed liquid.

Anaerobic digestion of spectinomycin mycelial residues pretreated by thermal hydrolysis: removal of spectinomycin and enhancement of biogas production

Anaerobic digestion (AD) is an effective technology to dispose antibiotic mycelial residues, but biogas production is influenced by hydrolysis rates and antibiotic residue. Herein, the effects of thermal hydrolysis pretreatment for AD of spectinomycin mycelial residues (SMRs) were investigated. The results showed that the removal ratio of spectinomycin was increased while the temperature of pretreatment was escalating. Meanwhile, thermal hydrolysis facilitated the dissolution of organic matters. However, non-biodegradable substances measured by fluorescence excitation-emission matrix accumulated and thus had an adverse influence on biogas production. Based on batch assays, the optimal pretreatment temperature for SMRs was 120 °C. The removal of spectinomycin was benefit for biogas production (increasing by 7.6%), and the overall biogas production increased by 27.6% compared with 289.90 mL gVS^-1 of untreated SMRs. The microbial community analysis revealed that spectinomycin (265 mg L^-1) might influence bacteria in the early stage of AD (first 5 days), while redundancy analysis showed that spectinomycin had a non-significant influence on community succession over the 32 days of fermentation.

Performance of HABR + MSABP system for the treatment of dairy wastewater and analyses of microbial community structure and low excess sludge production

The potential of the system, a hybrid anaerobic baffled reactor (HABR) coupled with a multi-stage active biological process (MSABP) reactor, for simulated dairy wastewater at various temperature, hydraulic retention time (HRT), and pH was investigated. Percentage removals of chemical oxygen demand (COD) and NH₄⁺ were optimized using response surface methodology. Under optimized conditions (temperature, 33 °C; HRT, 24 h; pH, 7.35), the removal efficiencies of COD and NH₄⁺ were 99.89% and 97.83%, respectively. Miseq sequencing analysis exhibited that the anaerobic segment of the system was dominated by fermentation and acetogenic bacteria, and in the aerobic segment, microorganisms involved in the nitrogen cycle were significantly enriched. Meanwhile, it could be found that the excess sludge production of the process was much lower than that of other bio-processes. The average excess sludge production rate was 0.025-0.05 g SS/g COD removed under different organic loadings.

Zero-valent iron enhanced anaerobic digestion of pre-concentrated domestic wastewater for bioenergy recovery: Characteristics and mechanisms

Pre-concentrated domestic wastewater (PDWW) rich in organic matters can be a suitable substrate for anaerobic digestion (AD) towards holistic resource and bioenergy recovery. Micron zero-valent iron (ZVI) was applied in designed batch experiments during anaerobic treatment of PDWW to verify its roles in performance enhancement and associated mechanisms. In the selected range of food to microorganism (F/M) ratio, 0.5 gCOD/gMLVSS was most appropriate as biomethane production potential (BMP) of 0.275 L CH₄/gCOD was obtained. The optimal ZVI dosage at fixed F/M of 0.5 was 6 g/L, further enhancing the BMP by 15.2%. Furthermore, ZVI improved the hydrolysis process (producing more soluble organics) and regulated acidification process (affecting volatile fatty acids distribution). No obvious impact on acetoclastic and hydrogenotrophic methanogenesis processes was noted with ZVI addition. ZVI based AD of the PDWW is promising for promoting the practical application of advanced domestic wastewater treatment strategy (pre-concentration plus anaerobic digestion).

Does the combined free nitrous acid and electrochemical pretreatment increase methane productivity by provoking sludge solubilization and hydrolysis?

Free nitrous acid based pretreatments are novel and effective chemical strategies for enhancing waste activated sludge solubilization. In this study, the synergetic effects of the combined free nitrous acid and electrochemical pretreatment on sludge solubilization and subsequent methane productivity were evaluated. The results indicated that pretreatment with 10 V plus 14.17 mg N/L substantially enhanced sludge solubilization, with the highest soluble chemical oxygen demand concentration of 3296.7 mg/L, 25.6-time higher than that without pretreatment (128.9 mg/L). Due to the potential toxicity of NO₂^- and NO₃^- to microorganisms and its bioprocesses, the methane production of sludge pretreated by free nitrous acid was significantly deteriorated. The maximum methane yield (152.0 ± 9.6 mL/g-VS_added) was observed at 10 V pretreatment alone, only 1.7% higher than that of the control (149.4 ± 1.6 mL/g-VS_added). Combined pretreatment indeed enhances the sludge solubilization and hydrolysis, but does not always induce an improved anaerobic digestion efficiency.

Enhanced methane production by alleviating sulfide inhibition with a microbial electrolysis coupled anaerobic digestion reactor

Anaerobic digestion (AD) of organics is a challenging task under high-strength sulfate (SO₄^2-) conditions. The generation of toxic sulfides by SO₄^2--reducing bacteria (SRB) causes low methane (CH₄) production. This study investigated the feasibility of alleviating sulfide inhibition and enhancing CH₄ production by using an anaerobic reactor with built-in microbial electrolysis cell (MEC), namely ME-AD reactor. Compared to AD reactor, unionized H₂S in the ME-AD reactor was sufficiently converted into ionized HS^- due to the weak alkaline condition created via cathodic H₂ production, which relieved the toxicity of unionized H₂S to methanogenesis. Correspondingly, the CH₄ production in the ME-AD system was 1.56 times higher than that in the AD reactor with alkaline-pH control and 3.03 times higher than that in the AD reactors (no external voltage and no electrodes) without alkaline-pH control. MEC increased the amount of substrates available for CH₄-producing bacteria (MPB) to generate more CH₄. Microbial community analysis indicated that hydrogentrophic MPB (e.g. Methanosphaera) and acetotrophic MPB (e.g. Methanosaeta) participated in the two major pathways of CH₄ formation were successfully enriched in the cathode biofilm and suspended sludge of the ME-AD system. Economic revenue from increased CH₄ production totally covered the cost of input electricity. Integration of MEC with AD could be an attractive technology to alleviate sulfide inhibition and enhance CH₄ production from AD of organics under SO₄^2--rich condition.

Effect of Hydraulic Retention Time on the Treatment of Real Cattle Slaughterhouse Wastewater and Biogas Production from HUASB Reactor

Anaerobic digestion technology provides an alternative route for sustainable management of organic waste. In this study, the performance of the hybrid upflow anaerobic sludge blanket (HUASB) reactor consisting of synthetic grass media as attached growth surface was investigated for the treatment of cattle slaughterhouse wastewater under mesophilic (35 ± 1 °C) condition. After acclimatization with synthetic wastewater, the reactor was loaded up to OLR 10 g L−1d−1, corresponding to 20 g COD/L at a varying hydraulic retention time (HRT) of 24, 30, 36, 42, and 48 h. The system attained a maximum COD removal efficiency of 97% total suspended solids (TSS), volatile suspended solids (VSS), fats, oil, and grease (FOG), color removal, and turbidity were found as 97%, 284 mg/L, 79%, 78%, and 91% respectively. The biogas production after 48 h was found as 38 L/d, with about 85% methane and specific methane production of 0.24 LCH4/gCODadded. The ratio of alkalinity was 0.22, while ammonia nitrogen concentration reached a maximum of 839 mg/L at a steady state. Scanning electron microscopic (SEM) analysis revealed a predominance of Methanosarcina bacteria with the coccoidal shape at the end of the performance study. Therefore, the results of the experiment showed that increasing HRT significantly affects the performance of the system.

Elucidating the effect of mixing technologies on dynamics of microbial communities and their correlations with granular sludge properties in a high-rate sulfidogenic anaerobic bioreactor for saline wastewater treatment

In this study, three lab-scale anaerobic sulfidogenic bioreactors were operated independently using three different mixing modes (hydraulic, mechanical and pneumatic). One-way ANOVA test indicated various performance parameters (e.g. sulfate reduction and sulfide production) and granular sludge properties (e.g. EPS and particle size) statistically different in three mixing modes. Principal component analysis (PCA) and OTUs-based network demonstrated bacterial composition greatly varied among the three mixing modes. The phylum Proteobacteria was predominant among the bacterial communities, and the genus Desulfobacter (35.1% in hydraulic, 31.1% in mechanical and 27.4% in pneumatic sample) was the most dominant SRB. The PCA/Pearson's correlation analysis confirmed SRB had significant positive relationship with sludge properties (e.g. particle size). PICRUSt data highlighted that bacterial communities contained diverse predicted functions including sulfur metabolism enzymes (sulfite reductase and adenylylsulfate reductase). The findings of this research could be helpful for selection of an appropriate mixing technology for anaerobic sulfidogenic or similar bioprocess.

A Comparative Study of Biogas Production from Cattle Slaughterhouse Wastewater Using Conventional and Modified Upflow Anaerobic Sludge Blanket (UASB) Reactors

Cattle slaughterhouses generate wastewater that is rich in organic contaminant and nutrients, which is considered as high strength wastewater with a high potential for energy recovery. Work was undertaken to evaluate the efficiency of the 12 L laboratory scale conventional and a modified upflow anaerobic sludge blanket (UASB) reactors (conventional, R1 and modified, R2), for treatment of cattle slaughterhouse wastewater (CSWW) under mesophilic condition (35 ± 1 °C). Both reactors were acclimated with synthetic wastewater for 30 days, then continuous study with real CSWW proceeds. The reactors were subjected to the same loading condition of OLR, starting from 1.75, 3, 5 10, 14, and 16 g L-1d-1, corresponding to 3.5, 6, 10, 20, 28, and 32 g COD/L at constant hydraulic retention time (HRT) of 24 h. The performance of the R1 reactor drastically dropped at OLR 10 g L-1d-1, and this significantly affected the subsequent stages. The steady-state performance of the R2 reactor under the same loading condition as the R1 reactor revealed a high COD removal efficiency of 94% and biogas and methane productions were 27 L/d and 89%. The SMP was 0.21 LCH4/gCOD added, whereas the NH3-N alkalinity ratio stood at 651 mg/L and 0.2. SEM showed that the R2 reactor was dominated by Methanosarcina bacterial species, while the R1 reactor revealed a disturb sludge with insufficient microbial biomass.

The Influence of Waste Composition on Landfill Gas Generation in a Pilot-Scale Lysimeter

The Sudokwon landfill site in Korea, is one of the largest landfill sites in the world, and consists of a first landfill site and second landfill site. The second landfill site generates 3–30 times more H2S than that of the first landfill site. However, the cause of the increase in H2S has not been identified. In this study, the main causes of H2S concentration increase were investigated in the second landfill site in the Sudokwon landfill site. We classified wastes at the Sudokwon landfill site into seven types including Construction and demolition (C&D) debris waste. A lysimeter reactor was designed as a similar environment to the Sudokwon landfill site for simulation. In addition, the experiment was conducted under the same conditions. Three components and elements were analyzed to identify the composition of waste in the landfill site. Leachate was analyzed through a chemical oxygen demand and SO42− standard method. For landfill gas, a gas analyzer was used. The trend in the generation of leachate and landfill gas depending on waste composition at the landfill site was observed and the cause of the increase in H2S was examined. As a result, landfilling of C&D debris waste is recommended as a single landfill.

Electrically regulating co-fermentation of sewage sludge and food waste towards promoting biomethane production and mass reduction

Microbial electrolysis cell (MEC) was integrated into conventional anaerobic digestion (AD) system (i.e. MEC-AD) to electrochemically regulate the co-fermentation of food waste (FW) and sewage sludge (SS). Two anaerobic systems (i.e. MEC-AD, and single AD) were operated in parallel to explore the potential stimulation of electrical regulation in metabolic behaviors of FW and SS and subsequent biomethane production. The highest accumulative methane yield was achieved at an applied voltage of 0.4 V and the FW and SS ratio of 0.2:0.8, increasing by 2.8-fold than those in AD. The combined MEC-AD system mitigated N₂O emission and considerably improved ammonia removal and the dewaterability of digestate, in contrast to AD. Scanning electron microscope (SEM) visualized the presence of a large number of rod-like and cocci-like electroactive microbes on the electrode surface. Electrical regulation stimulated the self-growth and proliferation of typical Methanobacterium and Methanosaeta, accordingly contributing to biomethane production greatly.

Performance and microbial protein expression during anaerobic treatment of alkali-decrement wastewater using a strengthened circulation anaerobic reactor

Herein, a strengthened circulation anaerobic (SCA) reactor was employed for the treatment of actual alkali-decrement wastewater. The degradation mechanism of polyester oligomers and the relationship between the treatment performance and microbial community structure were systematically investigated using various advanced techniques. Results suggest that the accumulation of volatile fatty acids has an inhibitory effect on methanogenic activity. Molecular weight distributions suggest that only incomplete degradation of oligomers was achieved, due to acetogenic inhibition in the lower part of the SCA reactor. Meta-proteomic approach analysis revealed that the methanogens containing heterodisulfide reductase were the primary species involved in methane metabolism. Based on these findings, a possible degradation mechanism for alkali-decrement wastewater in the SCA reactor is proposed. This high-performance anaerobic reactor could be further scaled-up and optimized to serve as a promising and effective unit for the treatment of other refractory industrial wastewaters.

Treatment of industrial dyeing wastewater with a pilot-scale strengthened circulation anaerobic reactor

We developed a pilot-scale strengthened circulation anaerobic (SCA) reactor (with an effective volume of 27 m³) and applied to the treatment of industrial textile wastewater. The treatment performance and the working mechanism were studied systematically and the key operational parameters were identified. The results demonstrated that a stable and excellent chemical oxygen demand removal efficiency of 62.7% and a maximum chromaticity removal efficiency of 73.5% were obtained at an optimal reflux ratio of 4. Interestingly, the bio-degradability was evidently improved after the SCA reactor treatment. The high throughput sequencing analysis indicated that the diversity of the bacteria or archaebacteria before the treatment was slightly higher than that after the treatment, which may be attributed to the production of certain toxic intermediates and/or characteristic pollutants during the treatment. Enzyme activity test and COD removal show that numerous microorganisms still maintained active in the anaerobic granular sludge even in a severe environment.