Anaerobic co-digestion of food waste/excess sludge: substrates - products transformation and role of NADH as an indicator

Inside the microbial black box: a redox-centric framework for deciphering microbial metabolism

Microbial metabolism influences the global climate and human health and is governed by the balance between NADH and NAD+ through redox reactions. Historically, oxidative (i.e., catabolism) and reductive (i.e., fermentation) pathways have been studied in isolation, obscuring the complete metabolic picture. However, new omics technologies and biotechnological tools now allow an integrated system-level understanding of the drivers of microbial metabolism through observation and manipulation of redox reactions. Here we present perspectives on the importance of viewing microbial metabolism as the dynamic interplay between oxidative and reductive processes and apply this framework to diverse microbial systems. Additionally, we highlight novel biotechnologies to monitor and manipulate microbial redox status to control metabolism in unprecedented ways. This redox-focused systems biology framework enables a more mechanistic understanding of microbial metabolism.

Harnessing the potential of ginkgo biloba extract: Boosting denitrification performance through accelerated electron transfer

Ginkgo biloba extract (GBE) had several effects on the human body as one of the widely used phytopharmaceuticals, but it had no application in microbial enhancement in the environmental field. The study focused on the impact of GBE on denitrification specifically under neutral conditions. At the identified optimal addition ratio of 2% (v/v), the system exhibited a noteworthy increase in nitrate reduction rate (NRR) by 56.34%, elevating from 0.71 to 1.11 mg-N/(L·h). Moreover, the extraction of microbial extracellular polymeric substance (EPS) at this ratio revealed changes in the composition of EPS, the electron exchange capacity (EEC) was enhanced from 87.16 to 140.4 μmol/(g C), and the transfer impedance was reduced within the EPS. The flavin, fulvic acid (FA), and humic acid (HA) provided a π-electron conjugated structure for the denitrification system, enhancing extracellular electron transfer (EET) by stimulating carbon source metabolism. GBE also improved electron transfer system activity (ETSA) from 0.025 to 0.071 μL O2/(g·min·prot) and the content of NADH enhanced by 22.90% while significantly reducing the activation energy (Ea) by 85.6% in the denitrification process. The synergy of improving both intracellular and extracellular electron transfer, along with the reduction of Ea, notably amplified the initiation and reduction rates of the denitrification process. Additionally, GBE demonstrated suitability for denitrification across various pH levels, enhancing microbial resilience in alkaline conditions and promoting survival and proliferation. Overall, these findings open the door to potential applications of GBE as a natural additive in the environmental field to improve the efficiency of denitrification processes, which are essential for nitrogen removal in various environmental contexts.

Effect of 2,4-dichlorophenol on the production of methane from anaerobic granular sludge during anaerobic digestion through spectroscopy analysis

Chlorophenols in urban high organic wastewater increases, which plays an inhibitory role in the process of anaerobic fermentation and methanogenesis. The release rules of extracellular polymers (EPS) and soluble microbial products (SMP) and the production of methane from anaerobic granular sludge were evaluated by spectroscopic analysis. The methane production was reduced by 21.6%, 41.4% and 50.5% respectively by adding 2,4-DCP of different concentrations (25, 50 and 100 mg/L). Activity tests of methanogenic functional enzymes indicated that F420 was more susceptible to the toxic of 2, 4-DCP than Acetyl-CoA and NADH. The decrease in methane production was due to the reduction in the activity of conversion enzymes rather than the loss of crucial precursors for methanogenesis. 2,4-DCP disintegrated the protein 'shell' of anaerobic granular sludge by destroying α-helix and β-sheet structures. After the protein 'shell' in EPS was destroyed, 2, 4-DCP entered the interior of granular, which inhibited the activity of functional enzymes and affected the process of acidogenesis and methanogenesis. At the same time, due to the partial rupture of the cells after being affected by the toxicity of 2,4-DCP, the protein material could be dissolved into the aqueous phase and complexed with 2,4-DCP to reduce the toxic effect of 2,4-DCP.

Enhanced volatile fatty acid production from food waste via anaerobic fermentation: effect of irons with different sizes

ABSTRACTFood waste is an excellent organic matter for anaerobic fermentation. This study provided a cost-effective and highly efficient volatile fatty acid (VFA) production strategy by the addition of zero-valent iron (ZVI). Results showed that VFA concentration of 44.6 g/L was obtained with the optimized conditions of 200-mesh iron powder at a dosage of 20.0 g, fermentation time of 11 d, total solids (TS) of 10 wt.%, and fermentation temperature of 37 ℃. Further, the iron of different particle sizes (iron scraps, 200-mesh iron powder, and 800-mesh iron powder) had a differential influence on total organic carbon (TOC), total nitrogen (TN), and VFA concentrations. For the reactor containing 200-mesh iron powder, the conversion rate of organic compound into VFA increased with the increase of dosage, which reached 58.4% at the 40.0 g dosage. The mechanism revealed that the VFA production was enhanced by micro-electrolysis, which can rapidly inactivate bacteria and increase the conversion of macromolecular organics into micromolecular organics.

Contribution identification of hydrolyzed products of potassium ferrate on promoting short-chain fatty acids production from waste activated sludge

Potassium ferrate (K2FeO4) has been extensively employed to promote short-chain fatty acids (SCFAs) production from anaerobic fermentation of waste activated sludge (WAS) because of its potent oxidizing property and formation of alkaline hydrolyzed products (potassium hydroxide, KOH and ferric hydroxide, Fe(OH)3). However, whether K2FeO4 actually works as dual functions of both an oxidizing agent and an alkalinity enhancer during the anaerobic fermentation process remains uncertain. This study aims to identify the contributions of hydrolyzed products of K2FeO4 on SCFAs production. The results showed that K2FeO4 did not execute dual functions of oxidization and alkalinity in promoting SCFAs production. The accumulation of SCFAs using K2FeO4 treatment (183 mg COD/g volatile suspended solids, VSS) was less than that using either KOH (192 mg COD/g VSS) or KOH & Fe(OH)3 (210 mg COD/g VSS). The mechanism analysis indicated that the synergistic effects caused by oxidization and alkalinity properties of K2FeO4 did not happen on solubilization, hydrolysis, and acidogenesis stages, and the inhibition effect caused by K2FeO4 on methanogenesis stage at the initial phase was more severe than that of its hydrolyzed products. It was also noted that the inhibition effects of K2FeO4 and its hydrolyzed products on the methanogenesis stage could be relieved during a longer sludge retention time, and the final methane yields using KOH or KOH & Fe(OH)3 treatment were higher than that using K2FeO4, further confirming that dual functions of K2FeO4 were not obtained. Therefore, K2FeO4 may not be an alternative strategy for enhancing the production of SCFAs from WAS compared to its alkaline hydrolyzed products. Regarding the strong oxidization property of K2FeO4, more attention could be turned to the fates of refractory organics in the anaerobic fermentation of WAS.

Comparison of Black Tea Waste and Legume Roughages: Methane Mitigation and Rumen Fermentation Parameters

The chemical composition, in vitro total gas and CH₄ production and performance of cattle fed on factory black tea waste (Camellia sinensis) (BTW), alfalfa (Medicago Sativa), sainfoin (Onobrychis sativa) and white clover (Trifolium repens) was investigated. The gas production was quantified at the 24th hour of the incubation process. BTW was found to vary from roughages in chemical composition (p < 0.05). In addition, the roughages differed in terms of nutrient composition and gas production (p < 0.05). In legume roughages, acetic acid (AA), propionic acid (PA), butyric acid (BA), and total volatile fatty acids (TVFA) values ranged from 52.36-57.00 mmol/L, 13.46-17.20 mmol/L, 9.79-12.43 mmol/L, and 79.71-89.05 mmol/L, respectively. In comparison with black tea waste, legume roughages had higher values of AA, PA, BA, and TVFA. Black tea waste contained a higher acetic acid ratio than legume roughages when compared as a percentage. There was a similar ratio of propionic acid to the rate calculated for sainfoin (Onobrychis sativa) and clover (Trifolium repens), and a similar ratio of butyric acid to the ratio determined for alfalfa (Medicago Sativa). The current study shows that the 5.7-6.3% tannin content of black tea waste can be used in ruminant rations with high-quality roughages. Due to the fact that BTW reduces methane emissions from ruminants and eliminates energy waste from them, the environment can be improved. To obtain more reliable results, further animal feeding experiments on legume roughages and BTW are required.

Development of a Statistical Model to Predict Methane Production from Waste Activated Sludge Co-Digested with Olive Mill Wastewater and Cattle Dung by Response Surface Methodology

Nowadays, population growth is likely to lead to a wide variety of biomass wastes generation from the diversified human, industrial, and agricultural activities. Anaerobic digestion is mostly applied to manage biomass wastes and mitigate a huge spectrum of environmental damages. This paper aims to enhance the anaerobic digestion efficiency of multicomponent substrates, using a mixture of waste activated sludge (WAS), olive mill wastewater (OMW), and cattle manure (CM). A Response Surface Methodology is employed in experimental design to determine individual and interactive effects on methane yield and chemical oxygen demand reduction. After numerical optimization using Design Expert®, the optimum values of the test factors in actual were as follows: initial pH = 8, COD/N ratio = 47, 42, CM/WAS-OMW ratio = 0.352, TS = 42.94 g/L. The obtained results indicate that anaerobic co-digestion performance could be achieved by optimising substrate composition to assure a larger microbial synergistic effect.

Dioctyl phthalate enhances volatile fatty acids production from sludge anaerobic fermentation: Insights of electron transport and metabolic functions

As one of the most widely used phthalate plasticizers, dioctyl phthalate (DOP) has been detected in wastewater and accumulates in sludge through wastewater treatment, which may adversely affect further sludge treatment. However, the role of DOP on sludge anaerobic fermentation and its mechanism are not yet clear. Therefore, this study focused on the effect of DOP on the volatile fatty acids (VFAs) generation via the anaerobic fermentation of sludge. The results demonstrated that the presence of DOP had a considerable contribution to the generation of VFAs, and the maximum production of VFAs reached 4769 mg COD/L at 500 mg/kg DOP, which was 1.57 folds that of the control. Mechanistic investigation showed that DOP mainly enhanced the hydrolysis, acidification and related enzymes activities of sludge. VFAs-producing microorganisms (e.g., Clostridium and Conexibacter) were also enriched under DOP exposure. Importantly, the presence of DOP increased the electron transfer activity by 26 %, consequently facilitating the organics conversion and fermentation process. Notably, the functional gene expressions involved in substrate metabolism and VFAs biosynthesis were enhanced with DOP, resulting in increased VFAs production from sludge. The results obtained in this study offered a new strategy for the control of pollutants and the recycling of valuable products from sludge.

Effects of ferroferric oxide on propionate methanogenesis in sequencing batch reactors: Microbial community structure and metagenomic analysis

This study investigated the effects of ferroferric oxide (Fe₃O₄) on propionate methanogenesis in anaerobic sequencing batch reactor (ASBR). Compared to ASBR_C (without Fe₃O₄ addition), the addition of 10 g/L Fe₃O₄ (ASBR_Fe) decreased the maximum methane production rate by 69.6 % when propionate was used as the sole substrate. The addition of Fe₃O₄ reduced the contents of humic substances, riboflavin and nicotinamide adenine dinucleotide in extracellular polymeric substances. Therefore, Fe₃O₄ inhibited interspecies electron transfer of microorganisms through electronic mediators. Microbial community analysis revealed that Fe₃O₄ addition increased the relative abundance of acetate oxidizing bacterium (Mesotoga), but decreased the abundance of hydrogenotrophic methanogen (Methanobacterium). Further metagenomics analysis indicated that Fe₃O₄ increased the abundance of acetate oxidation genes and decreased that of hydrogenotrophic methanogenesis, quorum sensing and V/A-type ATPase genes. Thus, Fe₃O₄ reduced propionate methanogenesis during anaerobic digestion. The overall results indicate that Fe₃O₄ addition inhibits methanogenesis for treatment of propionate-contaminated wastewater in ASBR.

Amino Acid Configuration Affects Volatile Fatty Acid Production during Proteinaceous Waste Valorization: Chemotaxis, Quorum Sensing, and Metabolism

During proteinaceous waste valorization to produce volatile fatty acids (VFAs), protein needs to be hydrolyzed to amino acids (AAs), but the effects of the configuration of AAs on their biotransformation and VFA production have not been investigated. In this study, more residual d-AAs than their corresponding l-AAs were observed after VFAs were produced from kitchen waste in a pilot-scale bioreactor. For all AAs investigated, the VFA production from d-AAs was lower than that from corresponding l-AAs. The metagenomics and metaproteomics analyses revealed that the l-AA fermentation system exhibited greater bacterial chemotaxis and quorum sensing (QS) than d-AAs, which benefited the establishment of functional microorganisms (such as Clostridium, Sedimentibacter, and Peptoclostridium) and expression of functional proteins (e.g., substrate transportation cofactors, l-AA dehydrogenase, and acidogenic proteins). In addition, d-AAs need to be racemized to l-AAs before being metabolized, and the difference of VFA production between d-AAs and l-AAs decreased with the increase of racemization activity. The findings of the AA configuration affecting bacterial chemotaxis and QS, which altered microorganism communities and functional protein expression, provided a new insight into the reasons for higher l-AA metabolism than d-AAs and more d-AAs left during VFA production from proteinaceous wastes.

Deciphering the key operational factors and microbial features associated with volatile fatty acids production during paper wastes and sewage sludge co-fermentation

This work explored the feasibility of paper waste (PW)/sewage sludge (SS) co-fermentation for volatile fatty acids (VFAs) production, and disclosed its correlation with the key operational parameters (i.e., pH and PW/SS ratio). The results indicated that the maximal VFAs was 251.55 mg COD/g TSS at optimal conditions, which was approximately 10-folds of sole SS fermentation. PW feeding contributed to the bioavailable substrates and C/N balance during co-fermentation process. The pH exhibited evident impacts on organics solubilization/hydrolysis, in which acidic pH was more beneficial for carbohydrates metabolism while alkaline pH was better for proteins. Under optimal operational conditions, the metabolic functions associated with VFAs production (i.e., substrate membrane transport, intracellular metabolism and VFAs biosynthesis) were up-regulated. Moreover, functional microorganisms (i.e., Saccharofermentans and Bacteroides) responsible for VFAs generation were enriched. This work provided an innovative approach to recovery valuable products from biowastes, and in-depth understandings of microbial features in PW/SS co-fermentation systems.

Applying EEM- PARAFAC Analysis With Quantitative Real-Time PCR to Monitor Methanogenic Activity of High-Solid Anaerobic Digestion of Rice Straw

The methanogenic activity is an important indicator to assess the efficiency of high-solid anaerobic digestion. However, it is not yet elucidated clearly how to detect the parameter rapidly and reliably in the rice straw feeding reactor. Co-inoculated with ruminal digesta and anaerobic sludge, the digestion performance was studied at three different organic loading rates (OLRs). The excitation emission matrix–parallel factor analysis (EEM–PARAFAC) was used to detect dynamic changes in the characteristic of fluorescence components. Our results revealed that CH₄ productivity reached 280.90 mL/g volatile solid (VS) with a 54.39% CH₄ content under the OLR of 2.26 g/(L⋅d), which amount to 80.29% of its theoretical value. At the OLR of 2.47 g/(L⋅d), the average accumulated NH₄⁺ concentration was 1082.63 mg/L, which resulted in the hydrogenotrophic Methanobacteriales decreasing from 1.70 × 10⁹ to 1.04 × 10⁶ copies/g in the solid residues, whereas the acetotrophic Methanosarcinales increased from 7.89 × 10⁶ to 9.44 × 10⁶ copies/g. The dynamics of the methanogenic community consequently influenced the bioconversion efficiency of rice straw, and CH₄ productivity was reduced to 256.54 mL/g VS. The three fluorescent components, at the excitation/emission wavelength of 420 nm/470 nm, 340 nm/430 nm, and 280 nm/340 nm, were decomposed by PARAFAC model in the digestate. Fluorescence intensities of coenzyme F420 and NADH reflected the dynamic changes of CH₄-producing activity and anaerobic digestion efficiency, respectively. The coenzyme F420, unique to hydrogenotrophic methanogens, was correlated with methane yield, suggesting they played a dominant role in the anaerobic reactor. This study demonstrates that the EEM–PARAFAC combined with Q-PCR can be used to characterize methanogenic activity variation during the high-solid anaerobic digestion of rice straw with 15% total solid (TS).

Effect of pH on volatile fatty acid production from anaerobic digestion of potato peel waste

In this study, potato peel waste was used as feedstock to produce volatile fatty acids (VFAs) by anaerobic digestion. The effects of different pH levels (pH 5.0, pH 7.0, pH 11.0, and uncontrolled pH) on VFA concentration and composition, intermediate products, and metabolic state were evaluated. The results showed that the highest total VFA production was achieved with pH 7.0 (41.9 g COD/L and 632.2 mg COD/g VS_fed), followed by that with uncontrolled pH. Butyric acid was the dominant product under acidic pH, whereas acetic acid dominated under alkaline pH. The type of acidogenic fermentation at pH 7.0 was the mixed-acid type. The change in NADH level in the mixed-acid type of fermentation consisted of small fluctuations, enhancing the stability and efficiency of fermentation. The enzymatic activities of acetate kinase and butyrate kinase were slightly inhibited at pH 5.0 and 11.0, resulting in relatively low VFAs production.

From waste to fuel: Energy recovery from household food waste via its bioconversion to energy carriers based on microbiological processes

In the present study the bioconversion of dried household food waste (FORBI) to energy carriers was investigated aiming to its sustainable management and valorization. FORBI was either directly fermented towards ethanol and hydrogen or was previously subjected to extraction with water resulting to a liquid fraction (extract) rich in sugars and a solid residue, which were then fermented separately. Subsequently, the effluents were assessed as substrates for methane production via anaerobic digestion (AD). Mono-cultures and co-cultures of C5 and C6 yeasts were used for the alcoholic fermentation whereas for the production of hydrogen, mixed acidogenic consortia were used. Taking into account the optimum yields of biofuels, the amount of recoverable energy was estimated based for each different approach. The maximum ethanol yield was 0.16 g ethanol per kg of FORBI and it was achieved for separate fermentation of liquid and solid fractions of the waste. The highest hydrogen yield that was observed was 210.44 L ± 4.02 H₂/kg TS FORBI for 1% solids loading and supplementation with cellulolytic enzymes. Direct AD of either the whole FORBI or its individual fractions led to lower overall energy recovery, compared to that obtained when fermentation and subsequent AD were applied. The recoverable energy was estimated for the different exploitation approaches of the waste. The maximum achieved recoverable energy was 21.49 ± 0.57 MJ/kg.

The effects of thiosulfinates on methane production from anaerobic co-digestion of waste activated sludge and food waste and mitigate method

Thiosulfinates, a natural antibiotic, existed in all parts of Allium, therefore might be accumulated in large amounts in food waste (FW). FW was often added into waste activated sludge (WAS) anaerobic digestion process as a kind of supplement for nutrition balance. However, the impact of thiosulfinates on methane production and the possible approach to mitigate its inhibition on the co-digestion process could be available in few literatures. This work was carried out in a series of batch experiment at pH 7.0 ± 0.2 and 35 ± 1.0 ℃ to promote the further understanding of this process. The experimental results showed that the methane accumulation decreased from 270.6 ± 13.4 to 16.7 ± 7.0 mL/g VSS (volatile suspended solids) when the initial concentration of thiosulfinates increased from 0 to 2.5 μg/g VSS. The activities of functional enzymes (F₄₂₀ and CoM) were inhibited by 99.06% and 99.82% compared with control group when reactor contained 2.5 μg/g VSS thiosulfinates. Furthermore, different temperature, pH, and combination pretreat were applied to impair the inhibition of thiosulfinate. Compared with no pretreatment group, methane yield was increased by 2.26, 32.18 and 42.2-fold, respectively which group was under pretreatment method of heat (100 ℃), alkali (pH 9) and combination.

The inhibitory effect of thiosulfinate on volatile fatty acid and hydrogen production from anaerobic co-fermentation of food waste and waste activated sludge

Thiosulfinate, a nature antibiotic, existed in all parts of Allium thereby accumulating in kitchen waste vastly. However, few literatures were available related to its influence on volatile fatty acids (VFA) and hydrogen production when kitchen waste digestion technology was applied. This study aimed to explore the inhibitory effect and the relevant mechanism. Experimental results showed that the hydrogen accumulation decreased from 23.2 ± 0.8 to 8.2 ± 0.1 mL/g VSS (volatile suspended solid) and maximal total VFA yield decreased from 765.7 ± 21.2 to 376.4 ± 21.7 mg COD (chemical oxygen demand)/g VSS when the dosage of thiosulfinate increased from 0 to 12.5 µg/g VSS. The mechanism study indicated, compared with control group, that the butyric acid decreased from 59% to 20.1% of total VFA yield when reactor in present of 12.5 µg/g VSS thiosulfinate. Moreover, the relative activities of functional enzymes were inhibited 73.4% (butyryl-CoA) and 72.7% (NADH), respectively.

Inhibition of urease activity by humic acid extracted from sludge fermentation liquid

This study achieved effective extraction of humic acid from sludge fermentation liquid, and the inhibition of urease activity by the extract were investigated in the urea decomposition. The addition of extract could remarkably inhibit urease activity and extend the releasing time of ammonia nitrogen. The interaction between the extract and urease took times, and the inhibition was irreversible. The results of fluorescence analysis revealed that the inhibition of urease activity was correlated to the amount of humic acid extracted. The mechanisms of inhibition were proposed that the functional groups of humic acid might interact with the thiol group of urease and formed a larger particle size of complex to inhibit the activity of urease. The extraction of humic acid from sludge fermentation liquid can not only recover the resource from the fermentation liquid, but also provide a potential urease inhibitor for the sustained-release effect of the soil organic nitrogen fertilizer.

Impacts of mixing on foaming, methane production, stratification and microbial community in full-scale anaerobic co-digestion process

This study investigated the impact of mixing on key factors including foaming, substrate stratification, methane production and microbial community in three full scale anaerobic digesters. Digester foaming was observed at one plant that co-digested sewage sludge and food waste, and was operated without mixing. The lack of mixing led to uneven distribution of total chemical oxygen demand (tCOD) and volatile solid (VS) as well as methane production within the digester. 16S rRNA gene-based community analysis clearly differentiated the microbial community from the top and bottom. By contrast, foaming and substrate stratification were not observed at the other two plants with internal circulation mixing. The abundance of methanogens (Methanomicrobia) at the top was about four times higher than at the bottom, correlating to much higher methane production from the top verified by ex-situ biomethane assay, causing foaming. This result is consistent with foaming potential assessment of digestate samples from the digester.