The mechanisms of pH regulation on promoting volatile fatty acids production from kitchen waste.

The anaerobic acid production experiments were conducted with the pretreated kitchen waste under pH adjustment. The results showed that pH 8 was considered to be the most suitable condition for acid production, especially for the formation of acetic acid and propionic acid. The average value of total volatile fatty acid at pH 8 was 8814 mg COD/L, 1.5 times of that under blank condition. The average yield of acetic acid and propionic acid was 3302 mg COD/L and 2891 mg COD/L, respectively. The activities of key functional enzymes such as phosphotransacetylase, acetokinase, oxaloacetate transcarboxylase and succinyl-coA transferase were all enhanced. To further explore the regulatory mechanisms within the system, the distribution of microorganisms at different levels in the fermentation system was obtained by microbial sequencing, results indicating that the relative abundances of Clostridiales, Bacteroidales, Chloroflexi, Clostridium, Bacteroidetes and Propionibacteriales, which were great contributors for the hydrolysis and acidification, increased rapidly at pH 8 compared with the blank group. Besides, the proportion of genes encoding key enzymes was generally increased, which further verified the mechanism of hydrolytic acidification and acetic acid production of organic matter under pH regulation.

Temperature-dependent microbial mechanism and accumulation of volatile fatty acids in primary sludge pretreated with peroxymonosulfate.

For revealing the influence of temperature on volatile fatty acids (VFAs) generation from primary sludge (PS) during the anaerobic fermentation process facilitated by peroxymonosulfate (PMS), five fermentation groups (15, 25, 35, 45, and 55 °C) were designed. The results indicated that the production of VFAs (5148 mg COD/L) and acetic acid (2019 mg COD/L) reached their peaks at 45 °C. High-throughput sequencing technology disclosed that Firmicutes, Proteobacteria, and Actinobacteria was the dominant phyla, carbohydrate metabolism and membrane transport were the most vigorous at 45 °C. Additionally, higher temperature and PMS exhibit synergistic effects in promoting VFAs accumulation. This study unveiled the mechanism of the effect of the pretreatment of PS with PMS on the VFAs production, which established a theoretical foundation for the production of VFAs.

Effects of Steviol Glycosides on Growth Performance, Ruminal Fermentation and Microbial Diversity of Hu Sheep.

The experiment was conducted to investigate the potential effects of steviol glycosides on growth performance, rumen fermentation processes, and microbial diversity in Hu sheep. A single-factor design was used for the trial. Twenty healthy weaned Hu lambs, possessing comparable body weights averaging 18.31 ± 1.24 kg, were randomly allocated into two distinct groups: the control group (CON) and the experimental group (STE), with each comprising 10 lambs. The CON was fed the basal diet, and the STE was supplemented with 0.07% steviol glycosides based on the basal diet. During the experimental period, variations in body weight and feed intake were closely monitored and recorded. After feeding for 90 d, blood was collected to determine blood biochemical indices, and rumen fluid samples were gathered for an in-depth analysis of rumen fermentation parameters and microbial diversity. The outcomes revealed no statistically significant differences in growth performance or serum biochemical indices between the two groups (p > 0.05). Rumen pH in STE and CON was within the normal range. The rumen ammonia nitrogen (NH3-N) and acetic acid (AA) content of STE decreased significantly compared with CON (p < 0.05). No significant variations were observed in the levels of other volatile fatty acids (VFAs) between the two groups (p > 0.05). The rumen microbial OTUs count, as well as the Shannon, Simpson, Chao1, and Ace indices, were notably lower in the STE group compared to the CON group (p < 0.05). Additionally, at the phylum level, the relative abundance of Firmicutes, Bacteroidetes, and Proteobacteria collectively accounted for over 97% of the total phylum composition. In comparison to the CON group, the STE group exhibited an increase in the relative abundance of Proteobacteria (p < 0.05), accompanied by a significant reduction in the relative abundance of Patescibacteria and Desulfobacteria (p < 0.05). At the genus level, there was a notable increase in the relative abundance of Prevotella_7 and Succinivibrionaceae_UCG_001 in the STE group, whereas the relative abundance of Rikenellaceae_RC9_gut_group significantly decreased (p < 0.05). According to the correlation analysis between rumen microflora and VFAs, the relative abundance of Succinivibrionaceae_UCG_001 displayed a significant negative correlation with AA (p < 0.05), whereas Lactobacillus exhibited a notable positive correlation with isobutyric acid (IBA) (p < 0.05). In summary, steviol glycosides had no significant effect on the production performance and blood biochemical indexes of Hu sheep. Steviol glycosides can improve rumen fermentation parameters and rumen microflora structure of Hu sheep and have a certain effect on rumen microbial diversity and composition.

Perspectives on carboxylates generation from Ecuadorian agro-wastes.

Carboxylates generation from banana (peel and pulp), coffee, and cacao fermentation agro-waste, upon uncontrolled and controlled pHs of 6.6 (heat-driven methanogens inactivation) and 5.2 (pH inactivation), was studied. Regarding volatile fatty acids (VFAs), acetic was the highest for cocoa (96.2 g kg-1TVS) at pH 4.5. However, butyric was relevant for banana pulp (90.7 g kg-1TVS), at controlled pH 6.6. The highest medium chain fatty acid (MCFAs) level was hexanoic (cocoa, 3.5 g kg-1TVS), while octanoic reached a maximum of 2.8 g kg-1TVS for coffee at pH 6.6. At pH 5.2 MCFAs yield was relatively low. Uncontrolled pH conditions, using banana resulted in superior VFAs production compared to controlled conditions. Thus, pH became a determining variable when deciding the time and kind of carboxylic acid to be recovered. The bacterial community at the end of the chain elongation process was dominated by phyla Firmicutes, and Clostridium as the most common genera.

Corrigendum: Effects of Caragana korshinskii tannin on fermentation, methane emission, community of methanogens, and metabolome of rumen in sheep.

[This corrects the article DOI: 10.3389/fmicb.2024.1334045.].

Energy recovery from syngas and pyrolysis wastewaters with anaerobic mixed cultures.

The anaerobic digestion of aqueous condensate from fast pyrolysis is a promising technology for enhancing carbon and energy recovery from waste. Syngas, another pyrolysis product, could be integrated as a co-substrate to improve process efficiency. However, limited knowledge exists on the co-fermentation of pyrolysis syngas and aqueous condensate by anaerobic cultures and the effects of substrate toxicity. This work investigates the ability of mesophilic and thermophilic anaerobic mixed cultures to co-ferment syngas and the aqueous condensate from either sewage sludge or polyethylene plastics pyrolysis in semi-batch bottle fermentations. It identifies inhibitory concentrations for carboxydotrophic and methanogenic reactions, examines specific component removal and assesses energy recovery potential. The results show successful co-fermentation of syngas and aqueous condensate components like phenols and N-heterocycles. However, the characteristics and load of the aqueous condensates affected process performance and product formation. The toxicity, likely resulting from the synergistic effect of multiple toxicants, depended on the PACs' composition. At 37 °C, concentrations of 15.6 gCOD/gVSS and 7.8 gCOD/gVSS of sewage sludge-derived aqueous condensate inhibited by 50% carboxydotrophic and methanogenic activity, respectively. At 55 °C, loads between 3.9 and 6.8 gCOD/gVSS inhibited by 50% both reactions. Polyethylene plastics condensate showed higher toxicity, with 2.8 gCOD/gVSS and 0.3 gCOD/gVSS at 37 °C decreasing carboxydotrophic and methanogenic rates by 50%. At 55 °C, 0.3 gCOD/gVSS inhibited by 50% CO uptake rates and methanogenesis. Increasing PAC loads reduced methane production and promoted short-chain carboxylates formation. The recalcitrant components in sewage sludge condensate hindered e-mol recovery, while plastics condensate showed high e-mol recoveries despite the stronger toxicity. Even with challenges posed by substrate toxicity and composition variations, the successful conversion of syngas and aqueous condensates highlights the potential of this technology in advancing carbon and energy recovery from anthropogenic waste streams.

Valorisation of industrial hemp (Cannabis sativa L.) residues and cheese whey into volatile fatty acids for single cell protein production.

The production of single cell protein (SCP) using lignocellulosic materials stands out as a promising route in the circular bioeconomy transition. However, multiple steps are necessary for lignocellulosics-to-SCP processes, involving chemical pretreatments and specific aerobic cultures. Whereas there are no studies that investigated the SCP production from lignocellulosics by using only biological processes and microbial biomass able to work both anaerobically and aerobically. In this view, the valorisation of industrial hemp (Cannabis sativa L.) biomass residues (HBRs), specifically hurds and a mix of leaves and inflorescences, combined with cheese whey (CW) was investigated through a semi-continuous acidogenic co-fermentation process (co-AF). The aim of this study was to maximise HBRs conversion into VFAs to be further used as carbon-rich substrates for SCP production. Different process conditions were tested by either removing CW or increasing the amount of HBRs in terms of VS (i.e., two and four times) to evaluate the performance of the co-AF process. Increasing HBRs resulted in a proportional increase in VFA production up to 3115 mg HAc L-1, with experimental production nearly 40% higher than theoretical predictions. The synergy between HBRs and CW was demonstrated, proving the latter as essential to improve the biodegradability of the former. The produced VFAs were subsequently tested as substrates for SCP synthesis in batch aerobic tests. A biomass concentration of 2.43 g TSS L-1 was achieved with a C/N ratio of 5.0 and a pH of 9.0 after two days of aerobic fermentation, reaching a protein content of 42% (g protein per g TSS). These results demonstrate the overall feasibility of the VFA-mediated HBR-to-SCP valorisation process.

Comparison of the Effects of Essential Oils from Cannabis sativa and Cannabis indica on Selected Bacteria, Rumen Fermentation, and Methane Production-In Vitro Study.

This study aimed to evaluate the effects of essential oils (EOs) extracted from Cannabis sativa L. and Cannabis indica Lam. on in vitro ruminal fermentation characteristics, selected rumen microbial populations, and methane production. GC-MS analyses allowed us to identify 89 compounds in both EOs. It was found that E-ß-caryophyllene predominated in C. sativa (18.4%) and C. indica (24.1%). An in vitro (Ankom) test was performed to analyse the control and monensin groups, as well as the 50 µL or 100 µL EOs. The samples for volatile fatty acids (VFAs), lactate, and microbiological analysis were taken before incubation and after 6 and 24 h. The application of EOs of C. indica resulted in an increase in the total VFAs of acetate and propionate after 6 h of incubation. The applied EOs had a greater impact on the reduction in methane production after 6 h, but no apparent effect was noted after 24 h. Lower concentrations of C. sativa and C. indica had a more pronounced effect on Lactobacillus spp. and Buryrivibrio spp. than monensin. The presented findings suggest that C. sativa and C. indica supplementation can modify ruminal fermentation, the concentrations of specific volatile fatty acids, and methane production.

Enhancing corn stalk-based anaerobic digestion with different types of zero-valent iron added during the acidification stage: Performance and mechanism.

Anaerobic digestion has been defined as a competitive approach to facilitate the recycling of corn stalks. However, few studies have focused on the role of direct interspecies electron transfer (DIET) pathway in the acidification stage under the addition of different particle sizes of zero-valent iron (ZVI). In this study, three types of ZVI, namely iron filings, iron powder and nanoscale iron, were investigated, respectively, to enhance its high-value conversion. Variations in volatile fatty acids (VFAs) and methane (CH4) production associated with the underlying mechanisms were emphatically determined. Results indicated that the addition of ZVI could increase the concentration of VFAs, with the most outstanding performance observed with the use of nanoscale iron. Importantly, the conversion of propionic acid to acetic acid was driven by adding ZVI with no between-group differences in acidizing phase. Conversely, the substrate was more fully utilized when supplied with iron powder compared with other groups in methanogenic phase, thereby displaying the maximum CH4 yield with a value of 263.1 mL/(g total solids (TS)). However, adding nanoscale iron could signally shorten the digestion time (T80), saving 7 days in comparison to the group of iron powder.

Effects of bulk tank milk, waste milk, and pasteurized waste milk on the nutrient utilization, gastrointestinal tract development, and antimicrobial resistance to Escherichia coli in pre-weaned dairy calves.

This study aimed to assess the impact of bulk tank milk (BTM), waste milk (WM), and pasteurized waste milk (PWM) on nutrient digestibility, ruminal and cecal fermentation, gastrointestinal tract (GIT) development, and antimicrobial resistance of fecal Escherichia coli from dairy calves at 2 periods (30 and 60 d of age). Calves were grouped according to body weight, serum protein levels, and breed composition. Three treatments were included: BTM (n = 21), WM from cows under antibiotic treatment (n = 21), and PWM (waste milk submitted to high-temperature, short-time pasteurization; n = 21). A total of 63 calves were used, of which: 18 animals (n = 6 per treatment) evaluated in the period of 4 - 30 d and 45 (n = 15 per treatment) from 4 - 60 d. During the experimental period, a daily intake of 6 L of milk was divided into 2 equal meals, with ad libitum access to water and starter. Milk and feed intakes were recorded daily. Apparent total-tract digestibility and nitrogen balance were conducted from 25 to 29 d of age (n = 6) and from 53 to 57 d of age (n = 15). Animals were euthanized at 30 ± 1 and 60 ± 1 d of age for the assessment of ruminal and cecal fermentation and GIT development. Antimicrobial susceptibility testing was conducted at 1, 30, and 60 d of age (n = 15/treatment). Statistical analysis utilized a linear mixed-effects model for continuous outcomes and generalized linear models for single measurements (R software). Treatments WM and PWM had lower rumen pH, higher ruminal acetate concentration, larger reticulorumen and liver, and a higher prevalence of fecal-resistant E. coli compared with BTM at both 30 and 60 d. Up to 60 d, both BTM and WM treatments exhibited higher digestibility of ether extract and gross energy compared with the PWM, whereas WM and PWM treatments showed increased nitrogen intake and retention compared with the BTM. These findings suggest that pasteurization of waste milk negatively affects nutrient digestibility and calf performance, while also impacting rumen development. Additionally, the use of milk containing antibiotic residue leads to the selection of resistant E. coli in the GIT over time.

Higher thermal remediation temperature facilitates the sequential bioaugmented reductive dechlorination.

The coupling of thermal remediation with microbial reductive dechlorination (MRD) has shown promising potential for the cleanup of chlorinated solvent contaminated sites. In this study, thermal treatment and bioaugmentation were applied in series, where prior higher thermal remediation temperature led to improved TCE dechlorination performance with both better organohalide-respiring bacteria (OHRB) colonization and electron donor availability. The 60 °C was found to be a key temperature point where the promotion effect became obvious. Amplicon sequencing and co-occurrence network analysis demonstrated that temperature was a more dominating factor than bioaugmentation that impacted microbial community structure. Higher temperature of prior thermal treatment resulted in the decrease of richness, diversity of indigenous microbial communities, and simplified the network structure, which benefited the build-up of newcoming microorganisms during bioaugmentation. Thus, the abundance of Desulfitobacterium increased from 0.11 % (25 °C) to 3.10 % (90 °C). Meanwhile, released volatile fatty acids (VFAs) during thermal remediation functioned as electron donors and boosted MRD. Our results provided temperature-specific information on synergistic effect of sequential thermal remediation and bioaugmentation, which contributed to better implementation of the coupled technologies in chloroethene-impacted sites.

Time-dependent interference of surfactants and CeO2/Fe2O3 nanoparticles co-occurrence on the volatile fatty acids biosynthesis during semi-continuous sludge fermentation.

Various exogenous contaminants typically coexist in waste activated sludge (WAS), and the long-term impacts of these co-occurring contaminants on WAS anaerobic fermentation and associated mechanisms remain largely unknown. This study reveals that the co-occurrence of surfactants and nanoparticles (NPs, i.e., Fe2O3 and CeO2, frequently detected in sludge) exhibited time-dependent impacts on the volatile fatty acids (VFAs) biosynthesis. Surfactants triggered WAS decomposition and enhanced NPs dispersion, leading to increased exposure of functional anaerobes to NPs toxicity, negatively affecting them. Consequently, key fermentation processes, acidogenic bacterial abundance, and metabolic functions were inhibited in co-occurrence reactors compared to those containing only surfactants in the early stage (before 56 d). Surprisingly, the fermentation systems containing surfactants collapsed subsequently, with VFAs yield at 72 d decreasing by 48.59-71.27 % compared to 56 d. The keystone microbes (i.e., Acidobacteria (16 d) vs Patescibacteria (56 d)) were reshaped, and metabolic traits (i.e., proB involved in intracellular metabolism) were downregulated by 0.05-78.02 % due to reduced microbial adaptive capacity (i.e., quorum sensing (QS)). Partial least squares path modeling (PLS-PM) analysis suggests that the microbial community was the predominant factor influencing VFAs generation. This study provides new insights into the long-term effects of co-contaminants on the biological treatment of WAS.

Integrated Biorefinery for a Next-Generation Methanization Process Focusing on Volatile Fatty Acid Valorization: A Critical Review.

This review addresses the critical issue of a rapidly increasing worldwide waste stream and the need for sustainable management. The paper proposes an integrated transformation toward a next-generation methanization process, which leads not only to treating waste but also to converting it into higher value compounds and greener energy. Although the current and commonly used anaerobic digestion process is useful for biogas production, it presents limitations of resource exploitation and some negative environmental impacts. Focusing on the acidogenic stage in waste stream processing, the paper discusses the recent strategies to enhance the recovery of volatile fatty acids (VFAs). These acids serve as precursors for synthesizing a variety of biochemicals and biofuels, offering higher value products than solely energy recovery and soil fertilizers. Additionally, the importance of recycling the fermentation residues back into the biorefinery process is highlighted. This recycling not only generates additional VFAs but also contributes to generating clean energy, thereby enhancing the overall sustainability and efficiency of the waste management system. Moreover, the review discusses the necessity to integrate life cycle assessment (LCA) and techno-economic analysis (TEA) to evaluate the environmental impacts, sustainability, and processing costs of the proposed biorefinery.

Role of polyferric sulfate and ferric chloride on anaerobic fermentation of sludge from novel two-stage process for resource recovery.

Polyferric sulfate (PFS) and ferric chloride (FC) were compared for their efficiencies in capturing organic carbon and phosphorus, and their effects on the anaerobic fermentation process of sludge from a pilot-scale two-stage reactor were studied. Both PFS and FC promoted organic carbon and phosphorus capture. Further study revealed that PFS-based sludge with a dosage of 18 mg Fe/Lsewage showed a better volatile fatty acids (VFAs) production performance (202.97 ± 2.38 mg chemical oxygen demand (COD)/g volatile solids (VS)) than that of FC-based sludge (169.25 ± 1.56 mg COD/g VS). Besides, the high dosage of PFS effectively promoted the activities of the α-glucosidase and proteases. The dissimilatory iron reduction process enhanced sludge flocs disintegration and the conversion of carbohydrates and proteins to VFAs. Non-hydroxyapatite phosphorus predominated in the total phosphorus of all samples. This study contributes to developing strategies for optimizing iron-based sludge management and high-value product recovery.

Resilience towards organic load and activated sludge variations in co-fermentation for carboxylic acid production.

Two perturbations were investigated in acidogenic co-fermentation of waste activated sludge (WAS) and food waste in continuous mesophilic fermenters: increasing the organic loading rate (OLR) and changing the WAS. A control reactor maintained an OLR of 11 gVS/(L·d), while a test reactor had a prolonged OLR change to 18 gVS/(L·d). For each OLR, two WAS were studied. The change in OLR led to differentiated fermentation product profile without compromising the fermentation yields (∼300 mgCOD/gVS). At 11 gVS/(L·d), the product profile was dominated by acetic, butyric, and propionic acids while at 18 gVS/(L·d) it shifted to acetic acid, ethanol, and caproic acid. Reverting the OLR also reverted the fermentation profile. The biomass immigration with the WAS changed the fermentation microbial structure and introduced acetic acid-consuming methanogens, which growth was only delayed by the OLR increase. Microbial monitoring and post-fermentation tests can be used for early detection of acetic acid-consuming events.

Dietary isoacids effects on growth, nitrogen, and energy digestibility, and fecal volatile fatty acids and microbial ecology in finishing pigs.

Isoacids are branched ketoacids which when fed to ruminants have been shown to enhance the growth of fiber-digesting organisms. Ninety finishing gilts were individually fed dietary treatments consisting of diet type: corn-soybean meal (CSBM), a diet containing 40% distillers dried grains with solubles (DDGS), or a diet containing 40% sugar beet pulp (SBP); in combination with either no feed additive (CNT), the addition of 0.50% isobutyrate (IB), or the addition of a 0.88% mix of isobutyrate, isovalerate, and 2-methylbutyrate (MX). Gilts consumed an average of 2.171 kg/d over the 28-d trial. On d 26, fresh fecal samples were collected for determination of apparent total tract digestibility (ATTD) of gross energy (GE) and nitrogen (N), determination of fecal volatile fatty acids (VFA), and evaluation of microbial ecology. There was no interaction between diet type and isoacid addition, and no main effect of isoacid or diet type on alpha or Shannon microbial diversity measures (P > 0.05). There was no interaction between isoacid addition and diet type, and no main effect of isoacid addition on microbial beta diversity (P > 0.05), but differences were observed in microbial beta diversity due to diet type (P ≤ 0.05). There was no interaction between diet type and isoacid addition observed in fecal VFA concentrations (P > 0.05), with only minor differences in fecal VFA concentrations noted due to isoacid addition (P ≤ 0.05). The interaction between diet type and isoacid addition on ATTD of dietary GE and N (P ≤ 0.01) was large because the addition of IB did not affect the ATTD of GE or N in pigs fed the CSBM diet, but increased ATTD of GE and N in pigs fed diets containing DDGS and decreased the ATTD of GE and N in pigs fed diets containing SBP. In contrast, adding a blend of isoacids (i.e., MX) reduced the ATTD of GE and N, regardless of diet type. There was no interaction between diet type and isoacid addition, and no effect of isoacid addition was observed on pig performance (P > 0.05). Diet type did not affect average daily gain (P > 0.05), but pigs fed diets containing DDGS or SBP consumed less feed (P = 0.01) and exhibited greater GF ratios compared to pigs fed the low-fiber CSBM diet (P ≤ 0.05). In conclusion, there was little to no effect of isoacid addition on microbial ecology, fecal VFA concentrations, ATTD of GE or N, or pig performance, but the improvement in ATTD of GE and N in pigs fed diets containing DDGS when IB was added warrants further investigation.

Legislative restrictions on the use of most antibiotics for growth promotion have resulted in research needed to evaluate in-feed additives to improve diet digestibility to promote growth and feed efficiency. Thus, strategies to enhance energy and nutrient digestibility, gastrointestinal volatile fatty acid concentrations, and microbial ecology in finishing pigs are desirable targets. The results of the experiment conducted herein do not indicate that supplementing diets with isoacids has a consistent positive or negative effect on microbial ecology, fecal volatile fatty acids, digestibility of energy and nutrients, or pig performance.

Effect of a Multi-Species Direct-Fed Microbial on Growth Performance, Nutrient Digestibility, Intestinal Morphology and Colonic Volatile Fatty Acids in Weanling Pigs.

The potentials of ABO replacer of ENZ and DFM on growth performance, AID, colonic VFAs, gut morphology, fecal score and diarrhea incidence were evaluated. We randomly assigned 120 piglets to four experimental diets that included: (1) control diet (CON), fed the basal ration; (2) ABO was added at 250 ppm of in-feed ABO; (3) ENZ was added at a rate of 3 kg/ton feed; (4) DFM was added with 50 × 106 cfu/g of Bacillus subtilis and 2 × 106 cfu/g of Lactobacillus spp. at a rate of 1.2 kg/ton feed. A complete randomized design used six pens per treatment with five pigs per pen. Pigs had ad libitum access to feed and water throughout the 6-week trial. Feed intake and BW were recorded on weeks 0, 2, 4 and 6, as well as fecal scores and diarrhea incidences (visually recorded and calculated). At weeks 2 and 4, a sub-sample of pigs (n = 6) was sacrificed for intestinal morphology, enzyme activity and VFAs. The results of the study demonstrated that DFM piglets showed increased final BW (3 kg) (p < 0.001) vs. CON. Likewise, ADG was positively affected by the incorporation of ABO, ENZ and DFM in the diets, with an average increase of 8 to 17% on ADG compared with CON (p < 0.001). The AID of gross energy, organic matter, CP and EAAs in piglets fed ENZ and DFM were significantly higher (p < 0.05) than those of CON and ABO at weeks 2 and 4. Inclusion of DFM increased intestinal morphology, enzymatic activities and propionic and butyric acid more than in pigs fed CON, ABO and ENZ (p < 0.05). The fecal score and diarrhea incidence generally decreased over time in pigs fed DFM (p < 0.05). These findings indicate that dietary supplementation with DFM has better effects at any period on growth performance, CP and AA digestibility and beneficially altered the intestinal health in weanling piglets.

Boron induced multiple quorum-sensing circuits in parallel to assist in anaerobic digestion recovery from volatile fatty acids accumulation.

Exogenous quorum sensing (QS) molecular can regulate the activity and granulation process of anaerobic sludge in anaerobic digestion process, but would be impractical as a standalone operation. Here we demonstrated that application of 1 mg L-1 boric acid assisted in an upflow anaerobic sludge blanket (UASB) reactor recovery from volatile fatty acids (VFAs) accumulation. After VFAs accumulation, the chemical oxygen demand (COD) removal suddenly reduced from 78.98% to 55.86%. The relative abundance of acetoclastic methanogens decreased from 55.79% to 68.28%-23.14%∼25.41%, and lead to the acetate accumulate as high as 1317.03 mg L-1. Granular sludge disintegrated and the average size of sludge decreased to 586.38 ± 42.45 µm. Application of 1 mg L-1 boric acid activated the interspecies QS signal (AI-2) and then induced the secretion of intraspecies QS signal (N-acyl-homoserine lactones, AHLs). AHLs were then stimulated the growth of syntrophic acetate oxidizing bacteria and hydrogenotrophic methanogen. Moreover, the concentration of acetate decreased to 224.50 mg‧L-1, and the COD removal increased to 75.10% after application of 1 mg L-1 boric acid. The activated AI-2 may induce multiple quorum-sensing circuits enhance the level of AI-2 and AHLs in parallel, and in turn assisted in anaerobic digestion recovery from VFAs accumulation.

Electrogenerated singlet oxygen and reactive chlorine species enhancing volatile fatty acids production from co-fermentation of waste activated sludge and food waste: The key role of metal oxide coated electrodes.

Electrochemical pretreatment (EPT) has shown to be superior in improving acidogenic co-fermentation (Co-AF) of waste activated sludge (WAS) and food waste (FW) for volatile fatty acids (VFAs). However, the influence of EPT electrode materials on the production of electrogenerated oxidants (such as singlet oxygen (1O2) and reactive chlorine species (RCS)), as well as their effects on properties of electrodes, the microbial community structure and functional enzymes remain unclear. Therefore, this study investigated the effects of various metal oxide coated electrodes (i.e., Ti/PbO2, Ti/Ta2O5-IrO2, Ti/SnO2-RuO2, and Ti/IrO2-RuO2) on EPT and subsequent Co-AF of WAS-FW. The results showed that EPT with Ti/PbO2, Ti/Ta2O5-IrO2, Ti/SnO2-RuO2 and Ti/IrO2-RuO2 electrodes generated 165.3-848.2 mg Cl2/L of RCS and 5.643 × 1011-3.311 × 1012 spins/mm3 of 1O2, which significantly enhanced the solubilization and biodegradability of WAS-FW by 106.4 %-233.6 % and 177.3 %-481.8 %, respectively. Especially with Ti/Ta2O5-IrO2 as the electrode material, an appropriate residual RCS (2.0-10.4 mg Cl2/L) remained in Co-AF step, resulted in hydrolytic and acidogenic bacteria (e.g., Prevotella_7, accounting for 78.9 %) gradually become dominant rather than methanogens (e.g., Methanolinea and Methanothrix) due to their different tolerance to residual RCS. Meanwhile, the functional gene abundances of hydrolytic and acidogenic enzymes increased, while the methanogenic enzymes deceased. Consequently, this reactor produced the highest VFAs up to 545.5 ± 36.0 mg COD/g VS, which was 101.8 % higher than that of the Control (without EPT). Finally, the economic analysis and confirmatory experiments further proved the benefits of WAS-FW Co-AF with EPT.

Phytotoxicity and hormesis in common mobile organic compounds in leachates of wood-derived biochars.

Although addition of pyrolyzed organic materials (biochars) to soil generally results in increased growth and physiological performance of plants, neutral and negative responses have also commonly been detected. Toxicity of organic compounds generated during pyrolysis, sorbed by biochars, and then released into the soil solution, has been implicated as a possible mechanism for such negative effects. Conversely, water-soluble biochar constituents have also been suggested to have "hormetic" effects (positive effects on plants at low concentrations); however, no specific compounds responsible have been identified. We investigated the relative phytotoxicity-and possible hormetic effects-of 14 organic compounds common in aqueous extracts of freshly produced lignocellulosic biochars, using seed germination bioassays. Of the compounds examined, volatile fatty acids (VFAs: acetic, propionic, butyric, valeric, caproic, and 2-ethylbutyric acids) and phenol, showed acute phytotoxicity, with germination-based ED50 values of 1-30 mmol L-1, and 2-ethylbutyric acid showed ED50 values of 0.1-1.0 mmol L-1. Other compounds (benzene, benzoic acid, butanone, methyl salicylate, toluene, and 2,4-di-tert-butylphenol) showed toxic effects only at high concentrations close to solubility limits. Although phytotoxic at high concentrations, valeric and caproic acid also showed detectable hormetic effects on seedlings, increasing radicle extension by 5-15% at concentrations of ~ 0.01-0.1 mmol L-1. These data support the hypothesis that VFAs are the main agents responsible for phytotoxic effects of lignocellulosic biochar leachates, but that certain VFAs also have hormetic effects at low concentrations and may contribute to positive effects of biochar leachates on early plant development in some cases. Supplementary Information: The online version contains supplementary material available at 10.1007/s42773-024-00339-w.