Carbon cloth facilitates semi-continuous anaerobic digestion of organic wastewater rich in volatile fatty acids from dark fermentation

Urea promotes alkaline anaerobic fermentation of waste activated sludge for hydrogen production

Hydrogen production from waste activated sludge (WAS) represents a promising pathway for sustainable energy generation. This study explores the impact of urea on enhancing hydrogen production during alkaline fermentation of WAS, with the aim of reducing alkali use. Experimental results revealed that treating WAS with 90 mg/g VSS urea at a constant pH of 9.5, followed by anaerobic fermentation for 10 days, yielded 24.57 mL/g VSS of hydrogen, which is 1.42 times higher than the fermentation at constant pH 9.5 without urea. Additionally, urea exposure reduced NaOH consumption by 40.74 % and 15.79 % at constant pH 10 and 9.5, respectively, achieving a cost-effective hydrogen production at 9.16 USD/m3 H2. The observed reduction in NaOH consumption is attributed to free ammonia from urea decomposition, which acts as an NH3/NH4+ buffer. Mechanistic analysis suggests that urea disrupts hydrogen bonds within proteins, enriching hydrogen-producing microbes while inhibiting hydrogen-consuming ones, thereby promoting hydrogen production.

Effects of different quorum sensing signal molecules on alleviation of ammonia inhibition during biomethanation

Anaerobic digestion (AD) is a promising technology for achieving both organic wastes treatment and energy recovery. However, challenges such as ammonia inhibition still remain. Quorum sensing (QS) system is relevant with the regulation of microbial community behaviors by releasing and sensing signal molecules, which could improve methane production during AD process. Therefore, the current study explored the effects of different quorum sensing signal molecules on alleviation of ammonia inhibition. The results showed that both secretion of N-butyryl-DL-homoserine lactone (C4-HSL) and N-(β-ketocaproyl)-DL-homoserine lactone (3OC6-HSL) could be inhibited by high ammonia stress while stimulation of N-hexanoyl-L-homoserine lactone (C6-HSL) and N-octanoyl-DL-homoserine lactone (C8-HSL) secretion might be triggered by ammonia toxicity. Moreover, the alleviation of ammonia inhibition could be achieved by both introducing 3OC6-HSL (0.5 μM) and combination of 3OC6-HSL (0.1 μM) and biochar (4 g/L). Exogenous 3OC6-HSL could regulate microbial social behaviors and enhance the secretion of extracellular polymeric substances (EPS) to promote anaerobic digestion. In addition, the mitigation of ammonia inhibition through exogenous 3OC6-HSL and biochar were confirmed by microbial community changes (Methanobacterium, Propionicicella and Petrimonas). Critical enzymes involved in both acidification and methanogenic steps were enhanced after adding the combination of 3OC6-HSL and biochar. The combination of low levels of 3OC6-HSL and biochar could promote both direct interspecies electron transfer (DIET) process and communication between different anaerobic microorganisms to mitigate ammonia inhibition. The current study will provide primary insights for conquering ammonia inhibition during biomethanation.

Insights into the role of electrochemical stimulation on sulfur-driven biodegradation of antibiotics in wastewater treatment

The presence of antibiotics in wastewater poses significant threat to our ecosystems and health. Traditional biological wastewater treatment technologies have several limitations in treating antibiotic-contaminated wastewaters, such as low removal efficiency and poor process resilience. Here, a novel electrochemical-coupled sulfur-mediated biological system was developed for treating wastewater co-contaminated with several antibiotics (e.g., ciprofloxacin (CIP), sulfamethoxazole (SMX), chloramphenicol (CAP)). Superior removal of CIP, SMX, and CAP with efficiencies ranging from 40.6 ± 2.6 % to 98.4 ± 1.6 % was achieved at high concentrations of 1000 μg/L in the electrochemical-coupled sulfur-mediated biological system, whereas the efficiencies ranged from 30.4 ± 2.3 % to 98.2 ± 1.4 % in the control system (without electrochemical stimulation). The biodegradation rates of CIP, SMX, and CAP increased by 1.5∼1.9-folds under electrochemical stimulation compared to the control. The insights into the role of electrochemical stimulation for multiple antibiotics biodegradation enhancement was elucidated through a combination of metagenomic and electrochemical analyses. Results showed that sustained electrochemical stimulation significantly enriched the sulfate-reducing and electroactive bacteria (e.g., Desulfobulbus, Longilinea, and Lentimicrobiumin on biocathode and Geobactor on bioanode), and boosted the secretion of electron transport mediators (e.g., cytochrome c and extracellular polymeric substances), which facilitated the microbial extracellular electron transfer processes and subsequent antibiotics removal in the sulfur-mediated biological system. Furthermore, under electrochemical stimulation, functional genes associated with sulfur and carbon metabolism and electron transfer were more abundant, and the microbial metabolic processes were enhanced, contributing to antibiotics biodegradation. Our study for the first time demonstrated that the synergistic effects of electrochemical-coupled sulfur-mediated biological system was capable of overcoming the limitations of conventional biological treatment processes. This study shed light on the mechanism of enhanced antibiotics biodegradation via electrochemical stimulation, which could be employed in sulfur-mediated bioprocess for treating antibiotic-contaminated wastewaters.

Metagenomic insights into the mechanism of sophorolipid in facilitating co-anaerobic digestion of mushroom residues and cattle manure: Functional microorganisms and metabolic pathway analysis

The present study aimed to enhance the co-anaerobic digestion system of mushroom residues and cattle manure by incorporating biosurfactant sophorolipid. Results demonstrated that the addition of 75 mg/L sophorolipid increased cumulative methane production by 33.68%, acetate content by 9-10 times, and the abundance of Methanosarcina by 69.22%. The electroactive microorganisms (Bacteroides, Petrimonas, etc.) were enriched, while the up-regulation of functional genes associated with carbohydrate metabolism and methane metabolism was observed. The metagenomic analysis revealed the significant involvement of inter-microbial communication and extracellular electron transfer in anaerobic digestion. Petrimonas was identified as the predominant host involved in cellular processes and environmental information processing. The supplementation of sophorolipid significantly enhanced its abundance during the late anaerobic digestion period (by 12.30%-64.84%). The results emphasize the crucial function of sophorolipid as biosurfactant in enhancing the efficiency of anaerobic digestion.

Operational mode and powdered activated carbon promoting syntrophic propionate oxidation during anaerobic digestion of complex organic substances

Operational mode and powdered activated carbon (PAC) are key factors facilitating microbial syntrophy and interspecies electron transfer during anaerobic digestion, consequently benefiting process stability and efficient methanogenesis. In this study, continuous-flow reactor (CFR) and sequencing batch reactor (SBR), with and without the addition of PAC, respectively, were operated to examine their effects on system performance and methanogenic activity. Based on the cycle-test result, the PAC-amended CFR (CFRPAC) recorded both the highest methane yield (690.1 mL/L) and the maximum CH4 production rate (28.8 mL/(L·h)), while SBRs exhibited slow methanogenic rates. However, activity assays indicated that SBRs were beneficial for organics removal in batch experiments fed with peptone. Taxonomic and functional analysis confirmed that CFRs were optimal for proliferating oligotrophs (e.g., Geobacter) and SBRs were more suitable for copiotrophs (e.g., Desulfobulbus). Metagenomic analysis revealed that CFRs had efficient acetate metabolic pathways from propionate and ethanol, whereas SBRs did not, resulting in the buildup of propionate. Furthermore, Methanobacterium and Methanothrix were acclimated to the different operational conditions, while acetoclastic Methanosarcina and hydrogenotrophic Methanolinea were acclimated in SBRs (5.1-13.4%) and CFRs (0.3-1.7%), respectively. This study confirmed the enhancement of microbial syntrophy by the addition of PAC as well as the acclimation of electroactive bacteria (e.g., Geobacter) with complex organic substances.

Bioelectrochemical stability improvement by Ce-N modified carbon-based cathode in high-salt stress and mechanism research

Although microbial fuel cells (MFCs) have potential for high-salt wastewater treatment, their application is limited by poor salt tolerance, deactivation and unstable catalytic performance. This study designed Ce-C, N-C, and Ce-N modified activated carbon (Ce-N-C) based on the catalytic mechanism and salt tolerance performance of Ce and N elements to address these limitations. With activated carbon (AC) as the control, this study analyzed the stability of the four cathodes under different salinity environments using norfloxacin (NOR) as a probe to assess the effect of cathodes and salinity on MFC degradation performance. After three months, comparing with other three cathodes, the Ce-N-C cathode demonstrated superior and stable electrochemical and power generation performance. In particular, the advantages of Ce-N-C in high-salt (600 mM NaCl) environment is more significant than no-salt or low-salt. The potential of Ce-N-C-End at current density of 0 was 14.0% higher than AC-End, and the power density of the MFC with Ce-N-C cathode was 105.7 mW/m², which was 3.1 times higher than AC. Also, the stability of NOR removal under the function of Ce-N-C improved with the increase of NaCl concentration or operation time. The CeO₂(111) crystal form, N-Ce-O bond and pyridine N might be the key factors in improving the catalytic performance and salt tolerance of the Ce-N modified carbon-based cathode using XPS and XRD analysis.

Mechanisms, performance, and the impact on microbial structure of direct interspecies electron transfer for enhancing anaerobic digestion-A review

Direct interspecies electron transfer (DIET) has been received tremendous attention, recently, due to the advantages of accelerating methane production via organics reduction during anaerobic digestion (AD) process. DIET-based syntrophic relationships not only occurred with the existence of pili and some proteins in the microorganism, but also can be conducted by conductive materials. Therefore, more researches into understanding and strengthening DIET-based syntrophy have been conducted with the aim of improving methanogenesis kinetics and further enhance methane productivity in AD systems. This study summarized the mechanisms, application and microbial structures of typical conductive materials (carbon-based materials and iron-based materials) during AD reactors operation. Meanwhile, detail analysis of studies on DIET (from substrates, dosage and effectiveness) via conductive materials was also presented in the study. Moreover, the challenges of applying conductive materials in boosting methane production were also proposed, which was supposed to provide a deep insight in DIET for full scale application.

Enhancing two-phase anaerobic digestion of mixture of primary and secondary sludge by adding granular activated carbon (GAC): Evaluating acidogenic and methanogenic efficiency

Although the granular activated carbon (GAC) has been proved to enhance conventional single-phase anaerobic digestion (AD), how it impacts on acidogenic and methanogenic fermentation is still unknown. In this study, GAC was introduced to elevate the efficiency of two-phase AD, with mixture of primary and secondary sludge as substrate. Five dosages: 0, 0.1, 0.3, 0.5 and 0.7 g GAC/g TSS (Total Suspended Solids) were investigated to determine influences of GAC. The variations of biogas (hydrogen and methane), volatile fatty acids (VFAs), organics degradation and transformation in extracellular polymeric substances (EPS) and dissolved organic matters (DOM) were analyzed. Modified Gompertz model and first-order reaction equation was applied to analyze the kinetics of biogas yield and VFAs utilization, respectively. Sludge reduction, electrical conductance and pH were also quantified to evaluate the system performance. The results showed that GAC could improve two-phase AD performance by enhancing methane production and organics conversion.

Removal of oxytetracycline and ofloxacin in wastewater by microalgae-bacteria symbiosis for bioenergy production

The development of microalgae-bacteria symbiosis for treating wastewater is flourishing owing to its high biomass productivity and exceptional ability to purify contaminants. A nature-selected microalgae-bacteria symbiosis, mainly consisting of Dictyosphaerium and Pseudomonas, was used to treat oxytetracycline (OTC), ofloxacin (OFLX), and antibiotic-containing swine wastewater. Increased antibiotic concentration gradually reduced biomass productivity and intricately changed symbiosis composition, while 1 mg/L OTC accelerated the growth of symbiosis. The symbiosis biomass productivity reached 3.4-3.5 g/L (5.7-15.3 % protein, 18.4-39.3 % carbohydrate, and 2.1-3.9 % chlorophyll) when cultured in antibiotic-containing swine wastewater. The symbiosis displayed an excellent capacity to remove 76.3-83.4 % chemical oxygen demand, 53.5-62.4 % total ammonia nitrogen, 97.5-100.0 % total phosphorus, 96.3-100.0 % OTC, and 32.8-60.1 % OFLX in swine wastewater. The microbial community analysis revealed that the existence of OTC/OFLX increased the richness and evenness of microalgae but reduced bacteria species in microalgae-bacteria, and the toxicity of OFLX to bacteria was stronger than that of OTC.

Microalgae cultivation for antibiotic oxytetracycline wastewater treatment

Microalgae-based technology provides a potential approach to biologically treating oxytetracycline (OTC) wastewater due to its environmental friendliness, low cost, and high efficiency. However, the OTC degradation and transformation characteristics by microalgae are still unclear and need further exploration. This study used microalgae Chlorella sorokiniana MB-1 for OTC wastewater treatment. The OTC with an initial concentration less than 50 mg L^-1 promoted microalgae growth, while OTC with a concentration higher than 100 mg L^-1 inhibited microalgae growth significantly. More than 99% OTC was removed with the biomass productivity up to 1.8 g L^-1 when treated OTC with 10 mg L^-1 initial concentration for 7 days. Chlorophyll and total sugar contents decreased, while protein and lipid contents increased compared to the control without OTC. The malondialdehyde content firstly reduced but subsequently enhanced when increased OTC concentration, while superoxide dismutase content gradually enhanced, manifesting that traces of OTC stimulate microalgae antioxidant capacity, while the increasing OTC caused further oxidative damage to microalgae cells. The removal pathways of OTC mainly include photolysis (75.8%), biodegradation (17.8%), biosorption (3.6%), and hydrolysis (2.7%). Overall, removing OTC by microalgae was confirmed to be an excellent technology for treating antibiotics wastewater whilst accumulating microalgae biomass.

Biochemical Process and Microbial Evolution in the Conversion of Corn Straw Combined with Coal to Biogas

The combined anaerobic fermentation of coal and straw can increase the production of biogas. To explore the mechanism of adding corn straw to increase methane production, coal with different metamorphic degrees and corn straw were collected for biogas production simulation experiments under different substrate ratios. The changes in liquid products, the structure of lignocellulose in corn straw, and microbial evolution were monitored. The results showed that the combined fermentation of bituminous coal A with corn straw and bituminous coal C with corn straw at a mass ratio of 2:1 each ((AC-2) and (CC-2)) and that of bituminous coal B and corn straw at a mass ratio of 3:1 (BC-3) had the best gas production, and methane yields reached 17.28, 12.51, and 14.88 mL/g, respectively. The fermentation liquid had organic matter with more types and higher contents during the early and peak stages of gas production, and fewer types of organic matter were detected in the terminal stage. The degradation of lignocelluloses in the corn straw of AC-2 was higher. With the increase in fermentation time, the carbohydrates in the fermentation system increased and the degradation rate of cellulose decreased gradually. The abundance of genes related to nitrate reduction gradually increased, while that of sulfate reduction was on the contrary. Bacteria in the cofermentation system mainly metabolized carbohydrates. During cofermentation with high metamorphic coal, corn straw would be preferentially degraded. The structure of the archaea community changed from Methanosarcina and Methanothrix to Methanobacterium.

Microbial Electrosynthesis Inoculated with Anaerobic Granular Sludge and Carbon Cloth Electrodes Functionalized with Copper Nanoparticles for Conversion of CO2 to CH4

Microbial electrosynthesis (MES) can sustainably convert CO₂ to products and significant research is currently being conducted towards this end, mainly in laboratory-scale studies. The high-cost ion exchange membrane, however, is one of the main reasons hindering the industrialization of MES. This study investigates the conversion of CO₂ (as a sole external carbon source) to CH₄ using membraneless MES inoculated with anaerobic granular sludge. Three types of electrodes were tested: carbon cloth (CC) and CC functionalized with Cu NPs, where Cu NPs were deposited for 15 and 45 min, respectively. During the MES experiment, which lasted for 144 days (six cycles), methane was consistently higher in the serum bottles with CC electrodes and applied voltage. The highest CH₄ (around 46%) was found in the second cycle after 16 days. The system’s performance declined during the following cycles; nevertheless, the CH₄ composition was twice as high compared to the serum bottles without voltage. The MES with Cu NPs functionalized CC electrodes had a higher performance than the MES with plain CC electrodes. Microbial profile analysis showed that the Methanobacterium was the most dominant genus in all samples and it was found in higher abundance on the cathodes, followed by the anodes, and then in the suspended biomass. The genus Geobacter was identified only on the anodes regarding relative bacterial abundance at around 6–10%. Desulfovibrio was the most dominant genus in the cathodes; however, its relative abundance was significantly higher for the cathodes with Cu NPs.

Biomethane enhancement using reduced graphene oxide in anaerobic digestion of municipal solid waste

The present research investigated the impact of reduced graphene oxide (rGO) addition on the semi-continuous anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) in the range of 0.5-10 gVolatileSolid(VS)/L_reactorday organic loading rates (OLR). Adding rGO enhanced the rate and yield of biomethane production, and the maximum biomethane increment rate was obtained as 110% at an OLR of 4.0 gVS/L_reactorday. However, after increasing the OLR to 6 gVS/L_reactorday, there was a dramatic decrease in biomethane production because of volatile fatty acid (VFA) accumulation. Methanotrix is the predominant archaeal genus at OLRs lower than 6 gVS/L_reactorday in reactors (89-97%). An increment in biomethane production was associated with the higher abundance of the Methanothrix genus in the rGO-supported reactor (rG) than in the control reactor (rC).

Enhancing the resistance to H2S toxicity during anaerobic digestion of low-strength wastewater through granular activated carbon (GAC) addition

Low-strength wastewater was treated using two laboratory-scale up-flow anaerobic sludge blankets (UASB) for 130 days under sulfate-reducing conditions. Granular activated carbon (GAC) was added to one of the reactors. The GAC addition increased the total chemical oxygen demand removal by 21-28% and total methane production by 32-78%. The sludge from the GAC-amended UASB showed higher specific methanogenic activities (SMA) and higher activities in the presence of H₂S, indicating that the GAC addition enhanced the resistance of methanogens to H₂S toxicity. Further, the microbial communities showed that the GAC addition shifted microbial communities. A robust syntrophic partnership between bacteria (i.e., Bacteroidetes_vadinHA17 and Trichococcus) and methanogens was established in the GAC-amended UASB. Sulfate-reducing bacteria (SRB) were enriched in the GAC biofilm, indicating the coexistence of competition and cooperation between SRB and methanogens. These findings provide significant insights regarding microbial community dynamics, especially SRB and methanogens, in a GAC-amended anaerobic digestion process under sulfate-reducing conditions.

Freezing-low temperature treatment facilitates short-chain fatty acids production from waste activated sludge with short-term fermentation

This study proposed a novel and high-efficiency strategy, i.e., freezing followed by low-temperature thermal treatment, to significantly promote short-chain fatty acids (SCFAs) production from waste activated sludge compared to traditional freezing/thawing treatment. The maximal production of SCFAs was 212 mg COD/g VSS with a shortened retention time of five days, and the potentially recovered carbon source, including SCFAs, soluble polysaccharides and proteins, reached 321 mg COD/g VSS, increased by 92.1 and 28.3% compared to sole freezing and thermal treatment. Both the solubilization and hydrolysis steps of WAS were accelerated, and the acid-producing microorganisms, such as Macellibacteroides, Romboutsia and Paraclostridium, were greatly enriched, with a total abundance of 13.9%, which was only 0.54% in control. Interestingly, the methane production was inhibited at a shortened retention time, resulting in SCFAs accumulation, whereas it was increased by 32.0% at a longer sludge retention time, providing a potential solution for energy recovery from WAS.

Carbon-based conductive materials enhance biomethane recovery from organic wastes: A review of the impacts on anaerobic treatment

Amongst the most important sustainable waste management strategies, anaerobic biotechnology has had a central role over the past century in the management of high-pollution load sources, such as food, agricultural and municipal wastes. During anaerobic digestion (AD), valuable by-products such as digestate and biogas are produced. Biogas (mainly composed of methane) is generated through a series of reactions between bacteria and archaea. Enhancement of AD process with higher methane yield, accelerated methane production rate, and shorter start-up time is possible via tapping into a novel methanogenic pathway discovered a decade ago. This fundamentally new concept that is a substitute to interspecies hydrogen transfer is called direct interspecies electron transfer (DIET). DIET, a thermodynamically more feasible way of electron transfer, has been proven to occur between bacteria and methanogens. It is well-documented that amendment of carbon-based conductive materials (CCMs) can stimulate DIET via serving as an electrical conduit between microorganisms. Therefore, different types of CCMs such as biochar and activated carbon have been amended to a variety of AD reactors and enhancement of process performance was reported. In this review, a comparative analysis is presented for enhancement of AD performance in relation to major CCM related factors; electrical conductivity, redox properties, particle size and dosage. Additionally, the impacts of AD operational conditions such as organic loading rate and temperature on CCM amended reactors were discussed. Further, the changes in microbial communities of CCM amended reactors were reviewed and future perspectives along with challenges for CCM application in AD have been provided.

Effects of carbon cloth on anaerobic digestion of high concentration organic wastewater under various mixing conditions

Anaerobic digestion (AD) has been considered an energy efficient strategy in treating high concentration organic wastewater rich in volatile fatty acids (VFAs). Continuous stirred tank reactors (CSTRs) have been widely applied in the AD process; however, they may suffer from low efficiency with a relatively short hydraulic retention time (HRT) in wastewater treatment. In this study, carbon cloth was supplemented to investigate the effects on syntrophic degradation of VFA wastewater by increasing organic loading rates (OLRs) under various mixing conditions in CSTRs operating at an HRT of 10 days. The results demonstrated that the methane production rate could be increased by 10.1-23.0% and the chemical oxygen demand (COD) removal efficiency was enhanced up to 14.6% with carbon cloth addition in the unmixed reactor at OLRs between 2.1 and 4.2 g COD/L-d. In contrast, the enhancement effect was only observed under a high OLR of 4.2 g COD/L-d in well-mixed anaerobic digester. Cyclic voltammetry results indicated that an electroactive biofilm was formed on the surface of carbon cloth. The microbial communities revealed that the electroactive biofilms had the highest abundances of exoelectrogen Sedimentibacter and electrotrophic methanogen Methanosaeta species, which were 5.5 and 4.2 times higher than the suspension, respectively.

Evaluations of biochar amendment on anaerobic co-digestion of pig manure and sewage sludge: waste-to-methane conversion, microbial community, and antibiotic resistance genes

Effects of biochar on co-digestion of pig manure and dewatered sewage sludge under different total solids (TS) were investigated. Biochar could accelerate the start-up of methanogenesis and shorten the adaptation phase. At TS5%, the methane daily production in biochar group was 60.6% higher than the control; nevertheless, when TS increased, the gap between two groups gradually narrowed. Additionally, the change on antibiotics resistance genes (ARGs) was also affected by TS and the biochar addition. Moreover, biochar was beneficial to reduce ARGs in liquid phase. At TS14%, the total ARGs abundance in the liquid phase of biochar group was 41.4% lower than the control, among which the reduction rates of etB(P), sul1, rpoB2, macA, mupA and mupB were more prominent. These findings could provide useful guidance for developing ARGs elimination strategy before their release into the environment.

How can ethanol enhance direct interspecies electron transfer in anaerobic digestion?

Anaerobic digestion (AD) of organic waste to produce biogas is a mature biotechnology commercialised for decades. However, the relatively recent discovery of direct interspecies electron transfer (DIET) brings a new opportunity to improve the efficiency of biogas technology. DIET may replace mediated interspecies electron transfer (MIET) by efficient electron transfer between exoelectrogens and electrotrophic methanogens, thereby enhancing yields and rates of biogas production. Ethanol, as the initial electron donor in the discovery of the DIET pathway, is now a "hot topic" in the literature. Recent studies have indicated that ethanol in AD functions not only as the substrate, but also as the precursor to stimulate DIET by enriching exoelectrogens and electrotrophic methanogens for co-digesting complex organic wastes. This review aims to highlight the state of the art and recent advances in ethanol-based DIET in AD. The DIET associated reactions of ethanol oxidation and carbon dioxide reduction are assessed by thermodynamic analysis to reveal the extent of the potential for improvement of the AD processes that utilizes DIET pathways. Three ethanol-based DIET strategies are discussed: (1) ethanol as the sole substrate supplemented with conductive materials in AD, (2) ethanol co-digestion with complex substrates and (3) ethanol-type fermentation prior to AD. This review aims to chart the pathways for improved AD performance by utilizing ethanol-based DIET in specific treatments of biological wastes.