Waste-to-energy: Cellulase induced waste activated sludge and paper waste co-fermentation for efficient volatile fatty acids production and underlying mechanisms

A review of volatile fatty acids production from organic wastes: Intensification techniques and separation methods

High value-added products from organic waste fermentation have garnered increasing concern in modern society. VFAs are short-chain fatty acids, produced as intermediate products during the anaerobic fermentation of organic matter. VFAs can serve as an essential organic carbon source to produce substitutable fuels, microbial fats and oils, and synthetic biodegradable plastics et al. Extracting VFAs from the fermentation broths is a challenging task as the composition of suspensions is rather complex. In this paper, a comprehensive review of methods for VFAs production, extraction and separation are provided. Firstly, the methods to enhance VFAs production and significant operating parameters are briefly reviewed. Secondly, the evaluation and detailed discussion of various VFAs extraction and separation technologies, including membrane separation, complex extraction, and adsorption methods, are presented, highlighting their specific advantages and limitations. Finally, the challenges encountered by different separation technologies and novel approaches to enhance process performance are highlighted, providing theoretical guidance for recycling VFAs from organic wastes efficiently.

Vitamin B12 and iron-rich sludge-derived biochar enhanced PFOA biodegradation: Importance of direct inter-species electron transfer and functional microbes

Owing to the strong C-F bond in nature and the rigidity of the poly-fluoroalkyl chain, perfluorooctanoic acid (PFOA) is difficult to be eliminated by reactive species and microbes in environments, thus posing a serious threat to ecosystems. Vitamin B12 as a cofactor for enzymes, and biochar as the electron providers and conductors, were integrated to enhance PFOA biodegradation. The raw material of biochar was the sludge after dewatering by adding 50 mg/g DS of Fe(III). After pyrolysis under high temperature (800 °C), biochar (SC800) detected high content of Fe(II) (197.64 mg/g) and abundant oxygen-containing functional groups, thus boosting PFOA biodegradation via donating electrons. 99.9% of PFOA could be removed within 60 d as 0.1 g/L SC800 was presented in the microbial systems containing vitamin B12. Moreover, vitamin B12 facilitated the evolution of Sporomusa which behaved the deflorination. Via providing reactive sites and mediating direct inter-species electron transfer (DIET), SC800 boosted PFOA biodegradation. Corresponding novel results in the present study could guide the development of bioremediation technologies for PFOA-polluted sites.

Co-production of lactate and volatile fatty acids through repeated-batch fermentation of fruit and vegetable waste: Effect of cycle time and replacement ratio

In this study, repeated-batch fermentation was used to convert fruit and vegetable waste to lactate and volatile fatty acids (VFAs), which are essential carbon sources for medium-chain fatty acids (MCFAs) production. The effect of cycle time and replacement ratio on acidification in long-term fermentation was investigated. The results showed that they had a significant impact on product yield, productivity, and type of products. Considering the yield, productivity, and lactate/VFAs ratio, a replacement ratio of 30% and a cycle time of 2 d may be more suitable for further production of MCFAs. Its productivity and lactate/VFAs ratio were 4.07 ± 0.24 g/(L·d) and 5 ± 0.6, respectively. The lactic acid bacteria, such as Enterococcus (63%) and Lactobacillus (33%), stabilized in the reactor, resulting in the generation of both lactate and VFAs by heterolactic fermentation. The present study demonstrated a new strategy with the potential to recover high-value products from organic waste streams.

Response characteristics of indigenous microbial community in polycyclic aromatic hydrocarbons (PAHs) contaminated aquifers under polyethylene microplastics stress: A microcosmic experimental study

To understand the response characteristics of indigenous microbial community in PAH-contaminated aquifers to the coexistence of microplastics. In this paper, we constructed a groundwater microecosystem using lithologic media collected from the field and subjected it to the stress of a polyethylene microplastics (PE-MPs) concentration gradient. By conducting adsorption experiments and 16S rRNA sequencing, we revealed the growth, structure, metabolism, and resistance mechanisms of the indigenous microbial community in the aquifer lithologic media exposed to varying levels of co-stress from PE-MPs and phenanthrene. Our findings suggest that the adsorption capacity of aquifer lithologic media for phenanthrene is significantly weaker than that of PE-MPs. Additionally, our observations indicated that small particle lithologic media had a greater adsorption capacity for phenanthrene than large particle lithologic media. The presence of PE-MPs was found to increase both the abundance and diversity of microbial communities, although the relationship was not linear with the content of PE-MPs. When exposed to the combined stress of PE-MPs and phenanthrene, the relative abundance of Proteobacteria decreased while that of Bacteroidetes increased. Several genera belonging to Proteobacteria (Aeromonas, Desulfovibrio, Klebsiella, Pantoea, and Microvirgula) and Bacteroidetes (Macellibacteroides and Bacteroides) occupied a central position in the microbial community interaction network and showed significant correlations with other genera. Furthermore, an increase in the proportion of genera capable of degrading various refractory organics was observed. The presence of PE-MPs increased the phenanthrene content in the aquifer lithologic media, thereby intensifying the inhibitory effect on indigenous microbial community in this environment. Despite an increase in the phenanthrene content of aquifer lithologic media due to the presence of PE-MPs, indigenous microbial community in this environment exhibited resistance to the combined inhibition of PE-MPs and phenanthrene through a series of resistance mechanisms. These mechanisms included strengthening the N-cycle process, enhancing metabolic capacity for phenanthrene, improving perception, response, and adaptation to changes in the external environment or intracellular state, modifying the transmembrane transport of the cell membrane to the substrate, and regulating life processes.

Phenanthrene regulates metabolic pathways for hydrogen accumulation in sludge alkaline dark fermentation

The influence of phenanthrene (PHE), a general polycyclic aromatic hydrocarbon in waste activated sludge, on sludge alkaline dark fermentation for hydrogen accumulation was investigated prospectively. The yield of hydrogen was 16.2 mL/g TSS with 50 mg/kg TSS PHE, which was 1.3-fold greater than that of the control. Mechanism research demonstrated that hydrogen production and the abundance of functional microorganisms were facilitated, whereas those of homoacetogenesis were reduced. The activity of pyruvate ferredoxin oxidoreductase in the conversion of pyruvate to reduced ferredoxin for hydrogen production was promoted by 57.2%, and that of carbon monoxide dehydrogenase and formyltetrahydrofolate synthetase, closely associated with hydrogen consumption, was suppressed by 60.5% and 55.9%, respectively. Moreover, the encoding genes involved in pyruvate metabolism were significantly up-regulated, while genes related to consuming hydrogen to reduce carbon dioxide and produce 5-methyltetrahydrofolate were down-regulated. This study notably illustrates the effect of PHE on hydrogen accumulation from metabolic pathways.

Exploration of the mechanism of 2-CP degradation by Acinetobacter sp. stimulated by Lactobacillus plantarum fermentation waste: A bio-waste reuse

Green and economical pollution management methods which reusing bio-waste as biostimulant to effectively improve the removal of target pollutants are receiving more and more attention. In this study, Lactobacillus plantarum fermentation waste solution (LPS) was used to investigate its facilitative effect and the stimulation mechanisms on the degradation of 2-chlorophenol (2-CP) by strain Acinetobacter sp. Strain ZY1 in terms of both cell physiology and transcriptomics. The degradation efficiency of 2-CP was improved from 60% to >80% under LPS treatment. The biostimulant maintained the morphology of strain, reduced the level of reactive oxygen species, and recovered the cell membrane permeability from 39% to 22%. It also significantly increased the level of electron transfer activity and extracellular polymeric substances secretion and improved the metabolic activity of the strain. The transcriptome results revealed the stimulation of LPS to promote biological processes such as bacterial proliferation, metabolism, membrane structure composition, and energy conversion. This study provided new insights and references for the reuse of fermentation waste streams in biostimulation methods.

Enhanced medium-chain fatty acids production from Cephalosporin C antibiotic fermentation residues by ionizing radiation pretreatment

Antibiotic fermentation residues (AFRs) have been classified as hazardous waste in China. Anaerobic fermentation may be a good approach for AFRs treatment, through which value-added chemicals could be obtained simultaneously. This study firstly explored medium-chain fatty acids (MCFAs) production from AFRs through two-stage anaerobic fermentation, and gamma radiation was adopted for AFRs pretreatment. The results showed that both antibiotics removal and MCFAs production from AFRs were significantly promoted by gamma radiation pretreatment. No residual Cephalosporin C (CEP-C) was detected in gamma radiation treated groups after fermentation. Highest MCFAs concentration of 90.55 mmol C/L was obtained in 50 kGy treated group, which was 2.22 times of the control group. Genera that were positively correlated with MCFAs production were enriched in gamma radiation treated groups, like genus Paraclostridium, Terrisporobacter, Caproiciproducens and Sporanaerobacter, while genera that were negatively correlated with MCFAs production were diminished during the chain elongation process, like genus Bacteroides and NK4A214_group. Enzymes analysis suggested that the promoted MCFAs production was induced by the enrichment of functional enzymes involved in Acetyl-CoA formation and RBO pathway. This work suggested that gamma radiation pretreatment and two-stage anaerobic fermentation could achieve the dual benefits of AFRs treatment and value-added chemicals recovery.

How does the polyhexamethylene guanidine interact with waste activated sludge and affect the metabolic functions in anaerobic fermentation for volatile fatty acids production

Antibacterial agents are frequently used to ensure public hygiene. Most of the massively consumed chemicals are discarded and accumulated in waste activated sludge (WAS), which might influence the subsequent anaerobic fermentation process for WAS treatment. This study mainly investigated the impacts of polyhexamethylene guanidine (PHMG, considered as a safe and efficient broad-spectrum antibacterial agent) on the volatile fatty acids (VFAs) production derived from WAS anaerobic fermentation and disclosed the key mechanisms. Results demonstrated that low level of PHMG evidently increased the VFAs accumulation as well as the acetic acid proportion, while the excessive dose posed evident negative effects. Further analysis found that appropriate PHMG synchronously stimulated the solubilization/hydrolysis and acidification processes but inhibited methanogenesis. Mechanistic exploration revealed that PHMG firstly absorbed on WAS due to electric attraction but then interacted with WAS to promote extracellular polymeric substance (EPS) disintegration and organics release (especially proteinaceous matter). Moreover, PHMG affected the microbial community structure and metabolic functions. The low level of PHMG evidently enriched functional VFAs producers (i.e., Desulfobulbus, Macellibacteroides and Sporanaerobacter) and upregulated the critical genes expression responsible for substrates metabolism (particularly the proteins) and VFAs biosynthesis (i.e., aldehyde dehydrogenase (NAD+) (K00128) and molybdopterin oxidoreductase (K00184)). This study provides an in-depth understanding of emerging pollutant impacts on WAS fermentation and provides insightful guidance on WAS disposal.

Sulfadiazine inhibits hydrogen production during sludge anaerobic fermentation by affecting pyruvate decarboxylation

The overuse and random discharge of antibiotics can cause serious environmental pollution. Sludge acts as a repository for antibiotics, its anaerobic fermentation process will inevitably be affected. This study investigated the effects of a typical antibiotic contaminant, sulfadiazine (SDZ), on the anaerobic fermentation of sludge for hydrogen production. Results demonstrated that the production of hydrogen was significantly inhibited by SDZ, and the inhibition was enhanced with increasing SDZ content. Within 5 days, the cumulative amount of hydrogen with 500 mg SDZ/kg dry sludge was 8.5 mL, which was only 32.2% of that in the control (26.4 mL). Mechanistic investigation showed that the reduced hydrogen production when SDZ existed was mainly attributed to the suppression of pyruvate decarboxylation during the hydrogen production stage, and the diversity of microorganisms, especially the abundance of microorganisms and the activities of key enzymes closely related to hydrogen production were inhibited with SDZ, resulting in less hydrogen accumulation.

Volatile fatty acids production from waste streams by anaerobic digestion: A critical review of the roles and application of enzymes

Volatile fatty acids (VFAs) produced from organic-rich wastewater by anaerobic digestion attract attention due to the increasing volatile fatty acids market, sustainability and environmentally friendly characteristics. This review aims to give an overview of the roles and applications of enzymes, a biocatalyst which plays a significant role in anaerobic digestion, to enhance volatile fatty acids production. This paper systematically overviewed: (i) the enzymatic pathways of VFAs formation, competition, and consumption; (ii) the applications of enzymes in VFAs production; and (iii) feasible measures to boost the enzymatic processes. Furthermore, this review presents a critical evaluation on the major obstacles and feasible future research directions for the better applications of enzymatic processes to promote VFAs production from wastewater.

Production of medium-chain fatty acids by co-fermentation of antibiotic fermentation residue with fallen Ginkgo leaves

The co-fermentation of antibiotic fermentation residues (AFRs) and fallen Ginkgo leaves at C/N ratios of 10-60 was conducted for medium-chain fatty acids (MCFA) production. It was found that a proper C/N ratio could largely promote the MCFA accumulation. Group with C/N ratio of 50 exhibited highest MCFA production of 133.14 mmol C/L, which was 42 %-121 % higher than the other groups. Through the co-fermentation, substrate condition was optimized with rich micro-nutrients in AFRs and abundant polysaccharides in Ginkgo leaves, the hydrolysis of leaves was promoted by the active microbes in AFRs, and the predominance of CE microbes was also stimulated with the dilution of AFRs. The increased C/N ratio significantly affected the SCFA producers like genus Escherichia Shigella and Proteiniphilum, and enriched CE microbes like genus Romboutsia, Eubacterium and Clostridium_sensu_stricto_12. Functional enzymes analysis showed that both reverse β oxidation and fatty acid biosynthesis pathways were strengthened with the increased C/N ratio.

Long-term effects of copper nanoparticles on volatile fatty acids production from sludge fermentation: Roles of copper species and bacterial community structure

The long-term effects of copper nanoparticles (Cu NPs) on volatile fatty acids (VFAs) production during the waste activated sludge (WAS) fermentation, and the underlying mechanisms regarding copper species distribution and bacterial community evolution were explored. The yield of VFAs in the control was 1086 mg COD/L, whereas those were inhibited by 11.1%, 56.0% and 83.1%, with 25, 50, and 100 mg/g-TSS Cu NPs, respectively. Further investigation indicated that Cu NPs severely affected hydrolysis and acidification of WAS in a dose-dependent manner, while had little impact on solubilization. Besides, Cu NPs enriched the acid-consuming anaerobe while reducing the acid-forming bacteria. The metabolic pathways, microbial function, and enzymatic activities involved were inhibited at all tested dosages. Moreover, soluble and acid-extractable fractions dominated the copper speciation, which were also the main factors inhibiting the VFA production. This study provides a new perspective to interpret the long-term impacts of Cu NPs on WAS fermentation.

Medium-chain carboxylates production by co-fermentation of sewage sludge and macroalgae

The co-fermentation of sewage sludge and macroalgae at different mixing ratios was performed for medium-chain carboxylates (MCCs) production. The results showed that MCCs production was enhanced in co-fermentation groups due to the abundant readily available organics supplied by macroalgae and the alkaline buffer capacity provided by sewage sludge. Highest MCCs concentration of 112.7 mmol C/L (25.5 mmol C/g VS_added) was obtained in the co-fermentation group with sludge/macroalgae ratio of 4:6, which was higher than MCCs produced from the mono-fermentation of sewage sludge (41.7 mmol C/L, 9.4 mmol C/g VS_added) or macroalgae (79.9 mmol C/L, 18.1 mmol C/g VS_added). Microbial analysis showed that species from genus Romboutsia, Terrisporobacter, Clostridium_sensu_stricto_12, Paraclostridium, unclassified_f_Peptostreptococcaceae and Caproiciproducens were significantly positively correlated with MCCs production. Metabolic pathways analysis demonstrated that the co-fermentation promoted the chain elongation process by stimulating the rate-limiting steps involved in the conversion of ethanol to Acetyl-CoA and circular fatty acid biosynthesis pathway.

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.