Enhancement of volatile fatty acid production by co-fermentation of food waste and excess sludge without pH control: The mechanism and microbial community analyses

Upscaling volatile fatty acids production: Demonstrating the reliability of anaerobic fermentation of food wastes from the lab towards industrial implementation

In recent years, the anaerobic fermentation (AF) of food waste (FW) has gained significant attention as a sustainable solution for waste valorization. However, the challenge of scaling up biotechnological processes for industrial applications remains a key barrier to commercialization. This investigation addressed this challenge by scaling up an auto-AF process from laboratory scale (4 L) to pilot (50 L) and demonstration scale in an industrial environment (250 L), using a lipid-rich FW (46.6 %, w/w) as the feedstock and endogenous microbiota as the inoculum. The applied operating conditions promoted the hydrolysis (>35 % volatile solids (VS) removal) and acidogenesis (>58 % of soluble chemical oxygen demand (sCOD) acidified) steps. As the reactor size for technology testing was increased, efficient mixing was crucial to ensure a proper homogenization of the fermentation broth. Lactic acid bacteria (LAB) prevailed in the endogenous microbiota, contributing to the enhanced hydrolysis and acidification efficiencies determined at all the scales. The minimal performance variations determined at different reactors' scales, along with the stability of the metabolite profiles, demonstrated the robustness and reliability of AF, opening the door to continue further industrialization.

Anaerobic fermentation integrated with pyrolysis for carbon resource recovery from food waste and biogas sludge: Effects of inoculation ratio and pyrolysis temperature

In view of the food waste (FW) as well as its digestate are both the organic sources of municipal solid waste, this study explored the anaerobic fermentation (AF) and following pyrolysis carbonization to co-disposal the two wastes for carbon resource recovery, including short chain organic acid (SCOAs), pyrolysis gas and biochar. Results indicated that both the rate and yield of SCOAs production both increase with the rising ratio of biogas sludge (BS) to FW, enhancing the soluble carbon recovery. The highest SCOAs production of 474.33 mg/g-VS was achieved at the ratio of 2:1 in 72 h. To further utilize the carbon source, the solids from the fermented residue (FR) was pyrolyzed at 400, 600 and 800 °C, respectively. Findings showed that the carbon content in biochar decreases with the increasing pyrolysis temperature, while the carbon in pyrolysis gas exhibits the opposite trend. Integrating the AF and pyrolysis contributed to a carbon recovery about 56.39% when the FW and BS were co-fermented at a 2:1 ratio, followed by its FR was pyrolyzed at 600 °C. Additionally, the biochar prepared under these conditions displayed a specific surface area (SSA) of 313.10 m2/g, along with abundant pore structures and functional groups, indicating its potential applications as pollutant adsorbents and soil amendments. This research offers a new perspective on efficiently recovering high-value carbon sources through the co-treatment of FW and its digestate via AF integrated with pyrolysis.

Caproic acid production from short-chained carboxylic acids produced by arrested anaerobic digestion of green waste: Development and optimization of a mixed caproic acid producing culture

Caproic acid is an important compound for producing a variety of chemicals and a potential precursor for Sustainable Aviation Fuels (SAF). This study aimed at developing a stable mixed culture producing caproic acid using short-chain carboxylic acids (SCCAs) derived from arrested anaerobic digestion (AAD) of green waste and optimizing chain elongation (CE) conditions. The results showed that the mixed culture was dominated by Clostridium sensu stricto 12 (53.09 %), optimally producing caproic acid at 37 °C, pH 7.20, with a 15 % inoculum concentration. Under these conditions, 75.59 % of the consumed electrons contributed to caproic acid formation. Gradual ethanol feeding in bioreactor experiments resulted in caproic acid titer of 13.36 g/L, with carbon conversion efficiency (CCE) of 81.91 % and electron transfer efficiency (ETE) of 76.39 %, surpassing batch experiments without gradual ethanol addition. These findings demonstrate that the effectiveness of a mixed culture can be improved, resulting in increased caproic acid production.

Conversion of wine lees and waste activated sludge into caproate and heptanoate: Thermodynamic and microbiological insights

In this study, wine lees and waste activated sludge (WAS) were co-fermented for the first time in a 4:1 ratio (COD basis) at 20, 40, 70 and 100 gCOD/L, in batch at 37 °C and pH 7.0. The substrates were successfully converted to caproate (C6) and heptanoate (C7) with a high selectivity (40.2 % at 40 gCOD/L). The rapidly-growing chain-elongating microbiome was enriched inClostridiaceaeandOscillospiraceae, representing together 3.4-8.8 % of the community. Substrate concentrations higher than 40 gCOD/L negatively affected C6 and C7 selectivities and yields, probably due to microbial inhibition by high ethanol concentrations (15.82-22.93 g/L). At 70 and 100 gCOD/L, chain elongation shifted from ethanol-based to lactate-based, with a microbiome enriched in the lactic acid bacteriaRoseburia intestinalis(27.3 %) andEnterococcus hirae(13.8 %). The partial pressure of H2(pH2) was identified by thermodynamic analysis as a fundamental parameter for controlling ethanol oxidation and improving C6 and C7 selectivities.

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.

Continuous chain elongation process for carbon resource recovery from excess sludge: Enhanced n-caprylate production and specific microbial functionalities

Thermal hydrolyzed sludge (THS) exhibits considerable promise in generating medium-chain fatty acids (MCFAs) through chain elongation (CE) technology. This study developed a novel continuous CE process using THS as the substrate, achieving an optimal ethanol loading rate (5.8 g COD/L/d) and stable MCFA production at 10.9 g COD/L, with a rate of 3.6 g COD/L/d. The MCFAs primarily comprised n-caproate and n-caprylate, representing 41.5 % and 54.3 % of the total MCFAs, respectively. Utilization efficiencies for ethanol and acetate were nearly complete at 100 % and 92.8 %, respectively. Key microbial taxa identified under these optimal conditions included Alcaligenes, SRB2, Sporanaerobacter, and Kurthia, which were instrumental in critical pathways such as the generation of acetyl-CoA, the initial carboxylation of acetyl-CoA, the fatty acid biosynthesis cycle, and energy metabolism. This research provides a theoretical and technical blueprint for converting waste sludge into valuable MCFAs, promoting sustainable waste-to-resource strategies.

Upgrading soybean dreg to caproate via intermediate of lactate and mediator of biochar

To address the environmental hazards posed by high-yield soybean dreg (SD), a high-value strategy is firstly proposed by synthesizing caproate through chain elongation (CE). Optimized conditions for lactate-rich broth as intermediate, utilizing 50 % inoculum ratio, 40 g/L substrate concentration, and pH 5, resulting in 2.05 g/L caproate from direct fermentation. Leveraging lactate-rich broth supplemented with ethanol, caproate was optimized to 2.76 g/L under a refined electron donor to acceptor of 2:1. Furthermore, incorporating 20 g/L biochar elevated caproate production to 3.05 g/L and significantly shortened the lag phase. Mechanistic insights revealed that biochar's surface-existed quinone and hydroquinone groups exhibit potent redox characteristics, thereby facilitating electron transfer. Moreover, biochar up-regulated the abundance of key genes involved in CE process (especially fatty acids biosynthesis pathway), also enriching Lysinibacillus and Pseudomonas as an unrecognized cooperation to CE. This study paves a way for sustainable development of SD by upgrading to caproate.

Impacts of seasonal variation on volatile fatty acids production of food waste anaerobic fermentation

This study aims to investigate the influence of seasonal variations on Volatile fatty acids (VFAs) production from food waste (FW) and to quantify their impact. Results of batch experiments with external pH control indicated that the properties of FW exhibited significant seasonal variations and were markedly different from kitchen waste (KW). The spring group demonstrated the highest VFA concentration and VFA/SCOD, at 31.24 g COD/L and 92.20 % respectively, which were 1.22 and 1.27 times higher than those observed in the summer season. The combined proportion of acetic acid and butyric acid accounted for 81.10 % of the total VFAs in spring, suggesting the highest applicability to the carbon source. The VFA content of all seasonal groups in descending order was butyric acid, propionic acid and acetic acid. Carbohydrates, along with spicy and citrusy substances, promoted the conversion of total VFA and butyric acid, while proteins and lipids favored the production of acetic acid and propionic acid.

Innovative cation exchange-driven carbon migration and recovery patterns in anaerobic fermentation of waste activated sludge

Despite numerous treatments have been developed to enhance anaerobic fermentation of waste activated sludge, the innovative cation exchange (CE) approach has been rarely reported, little attempt was conducted to revealcarbon source fate. The interphase carbon balance was illustrated to clarify endogenous carbon dissolution, biotransformation,and recovery patterns. By CE-mediated divalent cation removal, almost 34.72 % of particulate carbon sources were dissolved in 2-day treatment, corresponding to soluble carbon content of 1165.58 mg C/L. Most of the originally dissolved carbon sources (58.01-66.81 %) were bio-transformed to volatile fatty acids with high bioavailability, while the further transformation to biogas was inhibited, contributing to recoverable carbon source accumulation. Overall, 21.38 % of total solid carbon sources were recovered through 8-day fermentation, the carbon extraction was implemented by solid-liquid separation with carbon loss of 14.21-22.91 %, manifesting the valid carbon recovery of 85.05-87.96 mg C/g VSS. Such CE-driven carbon recovery provided negentropy benefits in sustainable cycle economy.

Cation exchange resins enhance anaerobic digestion of sewage sludge: Roles in sequential recovery of hydrogen and methane

The recovery of renewable bioenergy from anaerobic digestion (AD) of sludge is a promising method to alleviate the energy problem. Although methane can be effectively recovered through sludge pretreatment by cation exchange resin (CER), the simultaneous enhancement of hydrogen and methane generation from AD using CER has not been extensively investigated. Herein, the effect of CER on the sequential recovery of hydrogen and methane and the corresponding mechanisms were investigated. When CER is introduced, the maximum increases for the hydrogen and methane production are 104.7 % and 35.3 %, respectively, confirming the sequential enhancement effects of CER on the hydrogen and methane production. Analyses of the variations in the main biochemical components with and without the effect of CER demonstrate that CER promotes sludge organic solubilisation, hydrolysis, and acidification in both hydrogen- and methane-production stages. Moreover, investigations of variations in the solid-liquid interfacial thermodynamics and removal rates of main multivalent metals of sludge reveal that the ion exchange reactions between the CER and sludge in the hydrogen-production stage provide the direct driving force of effective contact between bacteria and organic particulates. Additionally, the residual effect of the CER during methane production reduces the energy barrier for mass transfer and provides a driving force for this transfer. Further analyses of the microbial community structure and metagenomics indicate that CER directly drives the enrichment of hydrogen-producing bacteria (+ 15.1 %) and key genes encoding enzymes in the hydrogen-production stage. Moreover, CER indirectly induces the enrichment of methane-producing anaerobes (e.g. Methanosaeta: + 16.7 %, Methanosarcina: + 316.5 %); enhances the bioconversion of different substrates into methyl-coenzyme M; and promotes the metabolism pathway of acetoclastic process and CO2 reduction in the methane-production stage. This study can provide valuable insights for simultaneously enhancing the production of hydrogen and methane from AD through sequential recovery.

Anaerobic co-digestion of sewage sludge pretreated by thermal hydrolysis and food waste: gas production, dewatering performance, and community structure

Anaerobic co-digestion can effectively break the limitations of mono-digestion. However, there are still some problems such as long residence time, unsatisfactory methane yield, and unstable performance for co-digestion of sewage sludge (SS) and food waste (FW). Therefore, the SS in the reactor treating co-digestion of SS and FW is considered to be pretreated by thermal hydrolysis. In this work, the anaerobic co-digestion of SS of thermal hydrolysis pretreatment (THP) and FW significantly improved the stability, methane production of the digestive reactor, and dewaterability of the digested sludge. The R6 obtained the most cumulative methane production (315.76 mL/g VS). In addition, compared to R3, the cumulative methane production and maximum methane production rate of R5 increased by 9.93% and 14.56%, respectively. The dewaterability of R4, R5, and R6 was improved, while the dewatering performance of the R3 decreased to a greater extent. The results of the kinetic model fitting were consistent with the experimental results. Among them, the hydrolysis constants (Kh) of anaerobic co-digestion of THP-SS and FW were 0.121, 0.130, and 0.114 d-1, respectively, which were higher than those of other groups. And the estimated lag time (λ) of co-digestion was also lower than that of mono-digestion groups. Microbial community analysis indicated that the bacterial diversity and richness of anaerobic co-digested groups of THP-SS and FW were enhanced, while the methanogens with acetoclastic pathway became the main methanogenic microorganisms. This work provides essential information on anaerobic co-digestion containing different THP-SS contents.

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.

Synthesis of isobutanol using acetate as sole carbon source in Escherichia coli

BACKGROUND

With concerns about depletion of fossil fuel and environmental pollution, synthesis of biofuels such as isobutanol from low-cost substrate by microbial cell factories has attracted more and more attention. As one of the most promising carbon sources instead of food resources, acetate can be utilized by versatile microbes and converted into numerous valuable chemicals.

RESULTS

An isobutanol synthetic pathway using acetate as sole carbon source was constructed in E. coli. Pyruvate was designed to be generated via acetyl-CoA by pyruvate-ferredoxin oxidoreductase YdbK or anaplerotic pathway. Overexpression of transhydrogenase and NAD kinase increased the isobutanol titer of recombinant E. coli from 121.21 mg/L to 131.5 mg/L under batch cultivation. Further optimization of acetate supplement concentration achieved 157.05 mg/L isobutanol accumulation in WY002, representing the highest isobutanol titer by using acetate as sole carbon source.

CONCLUSIONS

The utilization of acetate as carbon source for microbial production of valuable chemicals such as isobutanol could reduce the consumption of food-based substrates and save production cost. Engineering strategies applied in this study will provide a useful reference for microbial production of pyruvate derived chemical compounds from acetate.

Insights into thermal hydrolysis pretreatment temperature for enhancing volatile fatty acids production from sludge fermentation: Performance and mechanism

To reveal the impact of thermal hydrolysis pretreatment (THP) temperature on the unclear mechanisms of volatile fatty acids (VFAs) production, four groups were established with different temperatures (100, 120, 140 and 160 °C), and high throughput sequencing technology was utilized. The results indicated that the optimal VFAs production occurred at 140 °C. Moreover, as the THP temperature increased, the proportion of acetic acid also increased, accounting for 10.8% to 26.7% of the VFAs, compared to only 4.9% in the control group. Mechanism investigations revealed that THP facilitated the hydrolysis and release of biodegradable organic matter. Moreover, the abundance of VFAs production and hydrolytic microorganisms and related metabolic functional genes expression were evidently improved by THP. Overall, this study deepens the understanding of the mechanisms through which different THP temperatures stimulate the production of VFAs through acidogenic fermentation, providing technical support for future THP application in sludge treatment.

Volatile fatty acid production in anaerobic fermentation of food waste saccharified residue: Effect of substrate concentration

In this study, food waste saccharified residue was used to produce volatile fatty acids (VFAs), and the effects of substrate concentration on VFA production, VFA composition, acidogenic efficiency, microbial community, and carbon transfer were investigated. Interestingly, chain elongation from acetate to n-butyrate played an important role with a substrate concentration of 200 g/L in the acidogenesis process. Results showed that 200 g/L was a suitable substrate concentration for both VFA and n-butyrate production, the highest VFA production, and n-butyrate composition were 280.87 mg COD/g vS and more than 90.00 %, respectively, and VFA/SCOD reached 82.39 %. Microbial analysis showed that Clostridium_Sensu_Stricto_12 promoted n-butyrate production by chain elongation. Carbon transfer analysis indicated that chain elongation made a contribution of 43.93 % to n-butyrate production. Totally 38.47 % of organic matter in food waste saccharified residue was further utilized. This study provides a new way for n-butyrate production with waste recycling and low cost.

Dose-response and type-dependent effects of antiviral drugs in anaerobic digestion of waste-activated sludge for biogas production

In the context of the COVID-19 pandemic, antiviral drugs (AVDs) were heavily excreted into wastewater and subsequently enriched in sewage sludge due to their widespread use. The potential ecological risks of AVDs have attracted increasing attention, but information on the effects of AVDs on sludge anaerobic digestion (AD) is limited. In this study, two typical AVDs (lamivudine and ritonavir) were selected to investigate the responses of AD to AVDs by biochemical methane potential tests. The results indicated that the effects of AVDs on methane production from sludge AD were dose- and type-dependent. The increased ritonavir concentration (0.05-50 mg/kg TS) contributed to an 11.27-49.43% increase in methane production compared with the control. However, methane production was significantly decreased at high lamivudine doses (50 mg/kg TS). Correspondingly, bacteria related to acidification were affected when exposed to lamivudine and ritonavir. Acetoclastic and hydrotropic methanogens were inhibited at a high lamivudine dose, while ritonavir enriched methylotrophic and hydrotropic methanogens. Based on the analysis of intermediate metabolites, the inhibition of lamivudine and the promotion of ritonavir on acidification and methanation were confirmed. In addition, the existence of AVDs could affect sludge properties. Sludge solubilization was inhibited when exposed to lamivudine and enhanced by ritonavir, perhaps caused by their different structures and physicochemical properties. Moreover, lamivudine and ritonavir could be partially degraded by AD, but 50.2-68.8% of AVDs remained in digested sludge, implying environmental risks.

Conceptual system for sustainable and next-generation wastewater resource recovery facilities

Shifting the concept of municipal wastewater treatment to recover resources is one of the key factors contributing to a sustainable society. A novel concept based on research is proposed to recover four main bio-based products from municipal wastewater while reaching the necessary regulatory standards. The main resource recovery units of the proposed system include upflow anaerobic sludge blanket reactor for the recovery of biogas (as product 1) from mainstream municipal wastewater after primary sedimentation. Sewage sludge is co-fermented with external organic waste such as food waste for volatile fatty acids (VFAs) production as precursors for other bio-based production. A portion of the VFA mixture (product 2) is used as carbon sources in the denitrification step of the nitrification/denitrification process as an alternative for nitrogen removal. The other alternative for nitrogen removal is the partial nitrification/anammx process. The VFA mixture is separated with nanofiltration/reverse osmosis membrane technology into low-carbon VFAs and high-carbon VFAs. Polyhydroxyalkanoate (as product 3) is produced from the low-carbon VFAs. Using membrane contactor-based processes and ion-exchange techniques, high-carbon VFAs are recovered as one-type VFA (pure VFA) and in ester forms (product 4). The nutrient-rich fermented and dewatered biosolid is applied as a fertilizer. The proposed units are seen as individual resource recovery systems as well as a concept of an integrated system. A qualitative environmental assessment of the proposed resource recovery units confirms the positive environmental impacts of the proposed system.

The coupling system of magnetite-enhanced thermophilic hydrolysis-acidification and denitrification for refractory organics removal from anaerobic digestate food waste effluent (ADFE)

The aim of this study was to remove the refractory organics from high-temperature anaerobic digestate food waste effluent by the coupling system of hydrolysis-acidification and denitrification. Iron-based materials (magnetite, zero-valent iron, and iron-carbon) were used to enhance the performance of thermophilic hydrolysis-acidification. Compared with the control group, magnetite had the best strengthening effect, increasing volatile fatty acids concentration and fluorescence intensity of easily biodegradable organics in the effluent by 47.6 % and 108.4 %, respectively. The coupling system of magnetite-enhanced thermophilic hydrolysis-acidification and denitrification achieved a nitrate removal efficiency of 91.2 % (influent NO₃^--N was 150 mg L^-1), and reduced the fluorescence intensity of refractory organics by 33.8 %, compared with influent. Microbiological analysis indicated that magnetite increased the relative abundance of thermophilic hydrolytic acidifying bacteria, and coupling system enriched some genera simultaneously removing nitrate and refractory organics. This study provided fresh information on refractory organics and nitrogen removal of thermophilic wastewater biologically.

Enhancement of methane production in anaerobic digestion of high salinity organic wastewater: The synergistic effect of nano-magnetite and potassium ions

During high salinity organic wastewater (HSOW) anaerobic digestion treatment, the process of methanogenesis can be severely inhibited in the high salinity environment, and the accumulation of volatile organic acids (VFAs) leads to failure of the anaerobic reaction. In this study, nano-magnetite and KCl were adopted to alleviate the inhibitory effect of high salinity and enhance the HSOW anaerobic digestion performance. The result showed that, under the optimal dosage of 200 mg/L, nano-magnetite addition promoted the anaerobic digestion performance, and the methane production increased by 11.06%. When KCl was added with a dosage of 0.174%, the methane production increased by 98.37%. The simultaneous addition of nano-magnetite (200 mg/L) and KCl showed a synergistic effect on enhancing HSOW anaerobic digestion performance, and the methane production increased by 124.85%. The addition of nano-magnetite and KCl promoted the conversion of VFAs, especially accelerated the degradation of propionic acid and butyric acid, also it promoted the activity of acetate kinase, dehydrogenase and F₄₂₀, and thereby enhanced the methanogenesis process. This study could provide a new method for enhancing the anaerobic digestion of HSOW.

A novel strategy and mechanism for high-quality volatile fatty acids production from primary sludge: Peroxymonosulfate pretreatment combined with alkaline fermentation

To obtain high-quality VFAs production from primary sludge, a novel strategy that combined peroxymonosulfate (PMS) pretreatment and alkaline fermentation (i.e., PMS & pH9) was proposed in the study. The results showed that PMS & pH9 was efficient in sludge solubilization and hydrolysis, resulting in a maximal VFAs yield of 401.2 mg COD/g VSS, which was 7.3-, 2.1-, and 8.8-fold higher than the sole PMS, sole pH9, and control, respectively. Acetate comprised 87.6% of VFAs in this integration system. Mechanism investigations revealed that sulfate and free radicals produced by PMS play roles in improving VFAs yield under alkaline conditions. Besides, sulfate also aided in C3∼C5 VFAs converting to acetate under alkaline conditions depending on the increase of incomplete-oxidative sulfate-reducing bacteria (iso-SRB) (i.e., Desulfobulbus and Desulfobotulus). Moreover, the relative abundances of acid-forming characteristic genera (i.e., Proteiniborus, Proteinilcasticum, and Acetoanaerobium) were higher in PMS & pH9.

Fermentative bioconversion of food waste into biopolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) using Cupriavidus necator

Dependency on plastic commodities has led to a recurrent increase in their global production every year. Conventionally, plastic products are derived from fossil fuels, leading to severe environmental concerns. The recent coronavirus disease 2019 pandemic has triggered an increase in medical waste. Conversely, it has disrupted the supply chain of personal protective equipment (PPE). Valorisation of food waste was performed to cultivate C. necator for fermentative production of biopolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). The increase in biomass, PHBV yield and molar 3-hydroxy valerate (3HV) content was estimated after feeding volatile fatty acids. The fed-batch fermentation strategy reported in this study produced 15.65 ± 0.14 g/L of biomass with 5.32 g/L of PHBV with 50% molar 3HV content. This is a crucial finding, as molar concentration of 3HV can be modulated to suit the specification of biopolymer (film or fabric). The strategy applied in this study addresses the issue of global food waste burden and subsequently generates biopolymer PHBV, turning waste to wealth.

Enhancing methane production in anaerobic co-digestion of sewage sludge and food waste by regulating organic loading rate

This study presented mechanistic insights into the long-term effects of stepwise-increasing organic loading rates (OLRs) on anaerobic co-digestion (AcoD) of sewage sludge and food waste. The maximum methane (CH₄) yield of 500.0 ± 10.5 mL CH₄/g VS_fed was achieved at medium OLR of 3.5 g VS/L/d. This excellent performance was associated with the high hydrolysis efficiency (78.4%), three-fold enhancement in the acidogenesis enzyme activity, and 87.0% enhanced methanogen activity. Soluble intermediates (carbohydrates and proteins) were largely degraded (>98.5%), especially tyrosine-like and tryptophan-like aromatic proteins. The particulates were effectively decomposed from macromolecules to micromolecules, and the crystallinity of cellulosic substances decreased by 24.5%. The newly-shaped combined syntrophic acetate oxidation-hydrogenotrophic methanogenesis pathway dominated enhanced CH₄ production. Energy balance analysis based on medium OLR demonstrated the high energy recovery potential in full-scale AcoD. These findings suggest the optimal medium OLR can facilitate the bioconversion of organics to CH₄ through a new metabolic pathway.

Dark fermentative volatile fatty acids production from food waste: A review of the potential central role in waste biorefineries

Volatile fatty acids (VFAs) are high-value chemicals that are increasingly demanded worldwide. Biological production via food waste (FW) dark fermentation (DF) is a promising option to achieve the sustainability and environmental benefits typical of biobased chemicals and concurrently manage large amounts of residues. DF has a great potential to play a central role in waste biorefineries due to its ability to hydrolyze and convert complex organic substrates into VFAs that can be used as building blocks for bioproducts, chemicals and fuels. Several challenges must be faced for full-scale implementation, including process optimization to achieve high and stable yields, the development of efficient techniques for selective recovery and the cost-effectiveness of the whole process. This review aims to critically discuss and statistically analyze the existing relationships between process performance and the main variables of concern. Moreover, opportunities, current challenges and perspectives of a FW-based and fermentation-centred biorefinery layout are discussed.

Application of persulfate-based oxidation processes to address diverse sustainability challenges: A critical review

Over the past years, persulfate (PS) is widely applied due to their high versatility and efficacy in decontamination and sterilization. While treatment of organic chemicals, remediation of soil and groundwater, sludge treatment, disinfection on pathogen microorganisms have been covered by most published reviews, there are no comprehensive and specific reviews on its application to address diverse sustainability challenges, including solid waste treatment, resources recovery and regeneration of ecomaterials. PS applications mainly rely on direct oxidation by PS itself or the reactive sulfate radical (SO₄^•-) or hydroxyl radical (^•OH) from the activation of peroxodisulfate (PDS, S₂O₈^2-) or peroxymonosulfate (PMS, HSO₅^-) in SO₄^•--based advanced oxidation processes (SO₄^•--AOPs). From a broader perspective of environmental cleanup and sustainability, this review summarizes the various applications of PS except pollutant decontamination and elaborates the possible reaction mechanisms. Additionally, the differences between PS treatment and conventional technologies are highlighted. Challenges, research needs and future prospect are thus discussed to promote the development of the applications of PS-based oxidation processes in niche environmental fields. In all, this review is a call to pay more attention to the possibilities of PS application in practical resource reutilization and environmental protection except widely reported pollutant degradation.

Electrochemical pretreatment enhancing co-fermentation of waste activated sludge and food waste into volatile fatty acids: Performance, microbial community dynamics and metabolism

Waste activated sludge (WAS) has low biodegradability that restricts acidogenic fermentation (AF), thereby limiting the high-value volatile fatty acids (VFAs) production. This study investigated an alternative electrochemical pretreatment (EPT) approach that can facilitate AF of WAS and food waste (FW) and therefore enhance VFAs production. The results showed through introducing 50 % volatile solid basis of FW (containing massive chloride) into WAS, a 60-min EPT produced reactive chlorine species (RCS), which diffused into WAS-FW inner layers resulting in cell lysis, therefore significantly promoted and accelerated WAS-FW disintegration, contributing to more soluble and biodegradable dissolved organic matter (DOM). Then during the subsequent 15-day acidogenic co-fermentation (Co-AF), the residual RCS (approximate 5 mg Cl₂/L) also caused acidogenic bacteria (including Prevotella_7, Lactobacillus and Veillonella) gradually outcompeted methanogens due to their different tolerance to residual RCS. Consequently, the maximum VFAs yield of the WAS-FW Co-AF with EPT was 40.8 % higher than WAS-AF without EPT.

Current Trends in Biological Valorization of Waste-Derived Biomass: The Critical Role of VFAs to Fuel A Biorefinery

The looming climate and energy crises, exacerbated by increased waste generation, are driving research and development of sustainable resource management systems. Research suggests that organic materials, such as food waste, grass, and manure, have potential for biotransformation into a range of products, including: high-value volatile fatty acids (VFAs); various carboxylic acids; bioenergy; and bioplastics. Valorizing these organic residues would additionally reduce the increasing burden on waste management systems. Here, we review the valorization potential of various sustainably sourced feedstocks, particularly food wastes and agricultural and animal residues. Such feedstocks are often micro-organism-rich and well-suited to mixed culture fermentations. Additionally, we touch on the technologies, mainly biological systems including anaerobic digestion, that are being developed for this purpose. In particular, we provide a synthesis of VFA recovery techniques, which remain a significant technological barrier. Furthermore, we highlight a range of challenges and opportunities which will continue to drive research and discovery within the field. Analysis of the literature reveals growing interest in the development of a circular bioeconomy, built upon a biorefinery framework, which utilizes biogenic VFAs for chemical, material, and energy applications.

Performance of pilot scale two-stage anaerobic co-digestion of waste activated sludge and greasy sludge under uncontrolled mesophilic temperature

Waste-activated sludge (WAS) and greasy sludge (GS) discharged from the canned tuna industry are considerably characterized as harsh organic wastes to be individually treated by using traditional anaerobic digestion. This study was attempted to anaerobically co-digest WAS and GS in continuous pilot scale two-stage process, comprising the first 50 L continuous stir tank reactor (CSTR₁) and the second 250 L continuous stir tank reactor (CSTR₂). The two-stage co-digesting operation of dewatered WAS:GS ratio of 0.4:1 (g-VS) at ambient temperature with the organic loading rate (OLR) of 12.6 ± 0.75 g-VS/L·d and 2.26 ± 0.13 g-VS/L·d, corresponding to 3-day and 17-day hydraulic retention time (HRT) for the first and second stage, respectively generated highest methane production rate of 957 ± 86 mL-CH₄/L·d, corresponding to methane yield of 423.4 ± 36 mL-CH₄/g-VS. Organic removal efficiency obtained was around 67.5% on COD basis. The microbial diversity was depended on the process's activity. Bacteria were mostly detected in the CSTR₁, dominating with the phylum Firmicutes and Proteobacteria, whereas genus Methanosaeta archaea were found dominantly in the CSTR₂. The economic analysis of process shows payback period (PBP), internal rate of return (IRR), and net present value (NPV) of 3 years, 30%, and 250,177 USD, respectively. This study demonstrated the potential approach to applying the two-stage anaerobic co-digestion process to stabilize both WAS and GS along with generating valuable bioenergy carriers.

Advanced organic recovery from municipal wastewater with an enhanced magnetic separation (EMS) system: Pilot-scale verification

The up-concentration process has been demonstrated as an attractive approach to carbon-neutral wastewater treatment. Innovation in the separation processes can help eliminate the current heavy dependence on gravity, and credible pilot-scale verification is crucial for application promotion. We hereby proposed a pilot-scale enhanced magnetic separation (EMS) system as an up-concentration step to maximize energy recovery from municipal wastewater. The design of EMS was based on the hypothesis that magnetic-driven separation could be a breakthrough in separation speed, and adsorption could further enhance the separation efficiency by capturing soluble substances. Jar tests confirmed the feasibility of activated carbon adsorption, which could also roughen the surface of aggregates. Further, over one-year operation of a 300 m³/d EMS equipment provided optimum operation strategies and evidence of system effectiveness. More than 80% of particulate organics and 60% of soluble organics were removed within 10 min at an energy consumption of only 0.036 kWh/m³. The characteristics of sludge were clarified in terms of organic concentration, extracellular polymeric substances composition, and micro-community analysis. The anaerobic experiments further demonstrated the potential value of the concentrated products. Surprisingly, the developed EMS system exhibited significant advantages in time consumption and space occupation, with competitive operating cost and energy consumption. Overall, the results of this study posed the EMS process for up-concentration as a potential approach to organics recovery from municipal wastewater.

Deeper insights into the effects of substrate to inoculum ratio selection on the relationship of kinetic parameters, microbial communities, and key metabolic pathways during the anaerobic digestion of food waste

The substrate to inoculum ratio (S/I) is a crucial factor that affects not only the stability of the anaerobic digestion (AD) of food waste (FW) but also the methanogenic capacity of the substrate. This is of great significance for the start-up of small-scale batch reactors and the directional regulation of methanogenesi and organic acid production. Most studies have merely clarified the optimal S/I ratio for methane production and revealed the basic composition of microbial communities. However, the mechanism of microbial interactions and the metabolic pathways behind the optimal S/I ratio still remain unclear. Herein, the effects of different S/I ratios (VS basis) on the relationship of kinetic parameters, microbial communities, and metabolic pathways during the AD process of FW were holistically explored. The results revealed that high S/I ratios (4:1, 3:1, 2:1, and 1:1) were prone to irreversible acidification, while low S/I ratios (1:2, 1:3, and 1:4) were favorable for methanogenesis. Moreover, a kinetic analysis demonstrated that the methane yield of S/I = 1:3 were the highest. A bioinformatics analysis found that the diversity of bacteria and archaea of S/I = 1:3 were the most abundant, and the enrichment of Bacteroides and Synergistetes could help to establish a syntrophic relationship with hydrogenotrophic methanogens, which could aid in the fulfillment of a unique niche in the system. In contrast to the findings with the other S/I ratios, the cooperation among microbes in S/I = 1:3 was more apparent. Notably, the abundances of genes encoding key enzymes involved in the methanogenesis pathway under S/I = 1:3 were all the highest. This knowledge will be helpful for revealing the influence mechanism of the ratio relationship between microorganisms and substrates on the biochemical metabolic process of anaerobic digestion, thereby providing effective guidance for the directional regulation of FW batch anaerobic reactors.

Cation exchange resin pretreatment enhancing methane production from anaerobic digestion of waste activated sludge

The application of anaerobic digestion (AD) to treat waste activated sludge (WAS) still exhibits some limitations, such as low methane production. In this study, cation exchange resin (CER) pretreatment was explored to enhance the efficiency of the AD of WAS. Based on the response surface methodology, the optimal conditions for CER pretreatment were reaction time of 7.4 h, 33.8 g CER (wet weight) /g volatile solids and sludge total solids of 2.4%. Under these optimal CER pretreatment conditions, approximately 30% of metals were removed from the WAS, particularly organic-binding metals. This metal removal disrupted the structures of extracellular polymer substances and led to sludge deflocculation, thereby releasing large amounts of organic substances from the sludge solids. Batch AD experiments showed that CER pretreatment increased the maximal production of volatile fatty acids and methane by 565.7% and 80.5%, respectively. Additionally, CER pretreatment promoted each stage of AD (i.e. solubilisation, hydrolysis, acidification and methanation) and the corresponding activities of key enzymes. Experimental results for semi-continuous AD further confirmed that CER pretreatment enhanced the proportion of methane in the biogas (from 62.75 ± 2.14% to 73.96 ± 0.99%) and the production of methane. An analysis of changes in the microbial communities demonstrated that CER pretreatment enhanced the abundance of microorganisms involved in hydrolysis, acidification and acetification and changed the major methanogenic pathway from acetoclastic methanogens to methylotrophic methanogens. These findings are expected to provide a reference for developing new pretreatment methods for enhancing anaerobic biodegradability of organic matters.

A comprehensive review on current status and future perspectives of microbial volatile fatty acids production as platform chemicals

Volatile fatty acids (VFA), the secondary metabolite of microbial fermentation, are used in a wide range of industries for production of commercially valuable chemicals. In this review, the fermentative production of VFAs by both pure as well mixed microbial cultures is highlighted along with the strategies for enhancing the VFA production through innovations in existing approaches. Role of conventionally applied tools for the optimization of operational parameters such as pH, temperature, retention time, organic loading rate, and headspace pressure has been discussed. Furthermore, a comparative assessment of above strategies on VFA production has been done with alternate developments such as co-fermentation, substrate pre-treatment, and in situ removal from fermented broth. The review also highlights the applications of different bioreactor geometries in the optimum production of VFAs and how metagenomic tools could provide a detailed insight into the microbial communities and their functional attributes that could be subjected to metabolic engineering for the efficient production of VFAs.

Potential of anaerobic co-fermentation in wastewater treatments plants: A review

Fermentation (not anaerobic digestion) is an emerging biotechnology to transform waste into easily assimilable organic compounds such as volatile fatty acids, lactic acid and alcohols. Co-fermentation, the simultaneous fermentation of two or more waste, is an opportunity for wastewater treatment plants (WWTPs) to increase the yields of sludge mono-fermentation. Most publications have studied waste activated sludge co-fermentation with food waste or agri-industrial waste. Mixing ratio, pH and temperature are the most studied variables. The highest fermentation yields have been generally achieved in mixtures dominated by the most biodegradable substrate at circumneutral pH and mesophilic conditions. Nonetheless, most experiments have been performed in batch assays which results are driven by the capabilities of the starting microbial community and do not allow evaluating the microbial acclimation that occurs under continuous conditions. Temperature, pH, hydraulic retention time and organic load are variables that can be controlled to optimise the performance of continuous co-fermenters (i.e., favour waste hydrolysis and fermentation and limit the proliferation of methanogens). This review also discusses the integration of co-fermentation with other biotechnologies in WWTPs. Overall, this review presents a comprehensive and critical review of the achievements on co-fermentation research and lays the foundation for future research.

Polysorbate-80 pretreatment contributing to volatile fatty acids production associated microbial interactions via acidogenic fermentation of waste activated sludge

Polyoxyethylene dehydration sorbitol monooleate (polysorbate-80) pretreatment enhanced volatile fatty acids (VFAs) production of waste activated sludge (WAS) in acidogenic fermentation. The results showed that polysorbate-80 ameliorated WAS solubilization obviously with a soluble chemical oxygen demand (SCOD) increasing to 1536 mg/L within 4 h. Within 2 days of acidogenic fermentation, the maximal VFAs arrived to 2958.35 mg COD/L via polysorbate-80-pretreatment. The polysorbate-80 pretreatment boosted microbial diversity and richness in fermentation process. The Clostridium, Macellibacteroides and Acidocella strengthened microbial cooperation for the metabolic functions enhancement (e.g. amino acid metabolism and carbohydrate metabolism) for VFAs generation from WAS organics. Overall, the polysorbate-80 could play positive roles on the transformation of organic matter from sludge solid matters to VFAs, which was turned out to become an effective enhancing strategy for future WAS treatment / bioresource recovery with relatively low cost.

Long-term alkaline volatile fatty acids production from waste streams: Impact of pH and dominance of Dysgonomonadaceae

Alkaline co-fermentation of primary sludge and external organic waste (OW) was studied to elucidate the influence of substrate ratios and long-term system robustness and microbial community dynamics using batch and semi-continuous reactors. Volatile fatty acid (VFA) production increased with increasing OW fraction in the substrate due to synergistic effects of co-degradation. VFA production at pH 10 increased up to 30,300 mgCOD/L (yield of 630 mg COD/gVS_fed) but reduced over time to ≈10,000 mgCOD/L. Lowering pH to 9 led to the restoration of VFA production with a maximum of 32,000 mg COD/L (676 mg COD/g VS_fed) due to changes in microbial structure. VFA was composed mainly of acetic acid, but propionic acid increased at pH 9. The microbial community was dominated by Bacillaceae (34 ± 10%) and Proteinivoracales_uncultured (16 ± 11%) at pH 10, while Dysgonomonadaceae (52 ± 8%) was enriched at pH 9. The study demonstrated a zero-waste strategy that turns organic wastes into bio-based products.

A Glimpse of the World of Volatile Fatty Acids Production and Application: A review

Sustainable provision of chemicals and materials is undoubtedly a defining factor in guaranteeing economic, environmental, and social stability of future societies. Among the most sought-after chemical building blocks are volatile fatty acids (VFAs). VFAs such as acetic, propionic, and butyric acids have numerous industrial applications supporting from food and pharmaceuticals industries to wastewater treatment. The fact that VFAs can be produced synthetically from petrochemical derivatives and also through biological routes, for example, anaerobic digestion of organic mixed waste highlights their provision flexibility and sustainability. In this regard, this review presents a detailed overview of the applications associated with petrochemically and biologically generated VFAs, individually or in mixture, in industrial and laboratory scale, conventional and novel applications.

Production of n-caproate using food waste through thermophilic fermentation without addition of external electron donors

The effectiveness of producing n-caproate from food waste without external electron donors (EDs) was investigated through batch and semi-continuous fermentation. The maximum concentration of n-caproate reached 10,226.28 mg COD/L during semi-continuous fermentation. The specificity for n-caproate was the highest at 40.19 ± 3.95%, and the soluble COD conversion rate of n-caproate reached up to 22.50 ± 1.09% at the end of batch fermentation. The production of n-caproate was coupled with the generation of lactate as an ED to facilitate chain elongation reactions. When lactate was used as the only substrate, n-butyrate (64.12 ± 20.11%) markedly dominated the products, instead of n-caproate (0.63 ± 0.07%). Microbial community analysis revealed that Caproiciproducens, Rummeliibacillus, and Clostridium_sensu_stricto_12 were the key genera related to n-caproate production. In addition to n-caproate, n-butyrate dominated the products in batch and semi-continuous fermentation with a maximum specificity of 47.59 ± 3.39%. Clostridium_sensu_stricto_7 was committed to producing n-butyrate from lactate.

Volatile Fatty Acids (VFA) Production from Wastewaters with High Salinity—Influence of pH, Salinity and Reactor Configuration

The hydrocarbon-based economy is moving at a large pace to a decarbonized sustainable bioeconomy based on biorefining all types of secondary carbohydrate-based raw materials. In this work, 50 g L−1 in COD of a mixture of food waste, brine and wastewater derived from a biodiesel production facility were used to produce organic acids, important building-blocks for a biobased industry. High salinity (12–18 g L−1), different reactors configuration operated in batch mode, and different initial pH were tested. In experiment I, a batch stirred reactor (BSR) at atmospheric pressure and a granular sludge bed column (GSBC) were tested with an initial pH of 5. In the end of the experiment, the acidification yield (ηa) was similar in both reactors (22–24%, w/w); nevertheless, lactic acid was in lower concentrations in BSR (6.3 g L−1 in COD), when compared to GSBC (8.0 g L−1 in COD), and valeric was the dominant acid, reaching 17.3% (w/w) in the BSR. In experiment II, the BSR and a pressurized batch stirred reactor (PBSR, operated at 6 bar) were tested with initial pH 7. The ηa and the VFA concentration were higher in the BSR (46%, 22.8 g L−1 in COD) than in the PBSR (41%, 20.3 g/L in COD), and longer chain acids were more predominant in BSR (24.4% butyric, 6.7% valeric, and 6.2% caproic acids) than in PBSR (23.2%, 6.2%, and 4.2%, respectively). The results show that initial pH of 7 allows achieving higher ηa, and the BSR presents the most suitable reactor among tested configurations to produce VFA from wastes/wastewaters with high salinity.

Enhancing phosphorus recovery from sewage sludge using anaerobic-based processes: Current status and perspectives

Anaerobic-based processes are green and sustainable technologies for phosphorus (P) recovery from sewage sludges economically and are promising in practical application. However, the P release efficiency is always not satisfied. In this paper, the P release mechanisms (regarding to different P species) from sewage sludge using anaerobic-based processes are systematically summarized. The obstacles of P release and the updated achievements of enhancing P release from sewage sludges are analyzed and discussed. It can be concluded that different P species can release from sewage sludge via different anaerobic-based processes. Extracellular polymeric substances and excessive metal ions are the two main limiting factors to P release. Acid fermentation and anaerobic fermentation with sulfate reduction could be two promising ways, with P release efficiencies of up to 64% and 63%. Based on the summarization and discussion, perspectives on practical application of P recovery from sewage sludge using anaerobic-based processes are proposed.

The role of available phosphorous in vanadate decontamination by soil indigenous microbial consortia

Indigenous microbial consortia are closely associated with soil inherent components including nutrients and minerals. Although indigenous microbial consortia present great prospects for bioremediation of vanadate [V(V)] contaminated soil, influences of some key components, such as available phosphorus (AP), on V(V) biodetoxification are poorly understood. In this study, surface soils sampled from five representative vanadium smelter sites were employed as inocula without pretreatment. V(V) removal efficiency ranged from 81.7 ± 1.4% to 99.5 ± 0.2% in batch experiment, and the maximum V(V) removal rates were positively correlated with AP contents. Long-term V(V) removal was achieved under fluctuant hydrodynamic and hydrochemical conditions in column experiment. Geobacter and Bacillus, which were found in both original soils and bioreactors, catalytically reduced V(V) to insoluble tetravalent vanadium. Phosphate-solubilizing bacterium affiliated to Gemmatimonadaceae were also identified abundantly. Microbial functional characterization indicated the enrichment of phosphate ABC transporter, which could accelerate V(V) transfer into intercellular space for efficient reduction due to the structural similarity of V(V) and phosphate. This study reveals the critical role of AP in microbial V(V) decontamination and provides promising strategy for in situ bioremediation of V(V) polluted soil.

Phase Separation in Anaerobic Digestion: A Potential for Easier Process Combination?

The flexibilization of bioenergy production has the potential to counteract partly other fluctuating renewable energy sources (such as wind and solar power). As a weather-independent energy source, anaerobic digestion (AD) can offer on-demand energy supply through biogas production. Separation of the stages in anaerobic digestion represents a promising strategy for the flexibilization of the fermentative part of biogas production. Segregation in two reactor systems facilitates monitoring and control of the provision of educts to the second methanogenic stage, thus controlling biogas production. Two-stage operation has proven to reach similar or even higher methane yields and biogas purities than single-stage operation in many different fields of application. It furthermore allows methanation of green hydrogen and an easier combination of material and energy use of many biogenic raw and residual biomass sources. A lot of research has been conducted in recent years regarding the process phase separation in multi-stage AD operation, which includes more than two stages. Reliable monitoring tools, coupled with effluent recirculation, bioaugmentation and simulation have the potential to overcome the current drawbacks of a sophisticated and unstable operation. This review aims to summarize recent developments, new perspectives for coupling processes for energy and material use and a system integration of AD for power-to-gas applications. Thereby, cell physiological and engineering aspects as well as the basic economic feasibility are discussed. As conclusion, monitoring and control concepts as well as suitable separation technologies and finally the data basis for techno-economic and ecologic assessments have to be improved.

Influence of different household Food Wastes Fractions on Volatile Fatty Acids production by anaerobic fermentation

This research investigated for the first time the influence of the single fractions (proteins, lipids, starch, cellulose, fibers and sugars) composing Household Food Wastes on Volatile Fatty Acids (VFA). A production at different pH (uncontrolled, 5.5 and 7.0): both the amount and profile of VFA were investigated. It was found that fractions rich in proteins and starch led to the greatest VFA productions (12-15 g/L), especially at neutral pH condition. On the contrary, fractions rich in cellulose, fibers, and sugars showed a very low VFA production (<2 g/L). The chemical nature of HFW influenced the speciation of the microbial communities too. Lactobacillaceae family was highly represented in proteins-, starch-, fibers and sugars-rich substrates and Atopobiaceae, Eggerthellaceae, Acidaminococcaceae and Veillonellaceae displayed positive correlation to VFAs production. Instead, Comamonadaceae showed high relative abundance in lipids- and cellulose-rich fraction and was negatively correlated to the VFAs generation.

Fate of antibiotics and antibiotic resistance genes during aerobic co-composting of food waste with sewage sludge

Aerobic composting is widely used on transforming organic solid waste into proliferating products. However, the removal of antibiotics and antibiotic resistance genes (ARGs) in the process of co-composting of food waste with sewage sludge has been rarely reported to date. Therefore, we investigated a laboratory-scale composting using food waste and sewage sludge as substrates to study changes in antibiotics and ARGs during composting. Varying dose of antibiotics were added to allow the evaluation of changes in antibiotics, the microbial community and ARGs. The results revealed that composting effectively removed fluoroquinolones and macrolides, while showed poor efficiency in removing sulfonamides. Results from the 16S rRNA sequencing revealed that Firmicutes dominated on D0, while Proteobacteria and Actinomycetes dominated on D28, and a high concentration of antibiotics affected the microbial succession. The quantitative PCR demonstrated that the abundance of sul3, sulA, qnrB, qnrS, and ermB was reduced after 28 days composting, while an increase in the abundance of sul1, sul2, qnrD, ermC, and ermF was induced by high concentrations of antibiotics. Redundancy analysis revealed that total organic matter was the most important factor for the variation in the ARGs abundance. Overall, our findings indicated that the aerobic co-composting of food waste with sewage sludge can effectively remove antibiotics and ARGs. Our study sheds a new idea light on the strategy for the removal of antibiotics and ARGs from organic solid waste.

Volatile Fatty Acid Production from Organic Waste with the Emphasis on Membrane-Based Recovery

In recent years, interest in the biorefinery concept has emerged in the utilization of volatile fatty acids (VFAs) produced by acidogenic fermentation as precursors for various biotechnological processes. This has attracted substantial attention to VFA production from low-cost substrates such as organic waste and membrane based VFA recovery techniques to achieve cost-effective and environmentally friendly processes. However, there are few reviews which emphasize the acidogenic fermentation of organic waste into VFAs, and VFA recovery. Therefore, this article comprehensively summarizes VFA production, the factors affecting VFA production, and VFA recovery strategies using membrane-based techniques. Additionally, the outlook for future research on VFA production is discussed.

An innovative cation regulation-based anaerobic fermentation strategy for enhancing short-chain fatty acids production from waste activated sludge: Metal ion removal coupled with Na+-regulation

This study proposed a cation-regulation strategy based on metal ion removal coupled Na⁺-regulation for enhancing anaerobic fermentation of waste activated sludge. The optimal treatment condition was: cation-exchange resin dosage of 1.75 g/g SS for 1-day treatment, followed by Na⁺-enhanced anaerobic fermentation at NaCl concentration of 20 g/L. The CER induced sludge solubilization and the Na⁺-regulation treatment triggered secondary hydrolysis of CER-solubilized sludge, causing remarkable sludge disintegration and extracellular polymeric substance (EPS) disruption. Numerous SCOD of 6588 mg/L (SCOD/TCOD = 40.6%) was released within 2 days, and the short-chain fatty acids (SCFAs) of 439.9 mg COD/g VSS was produced through 4-day anaerobic fermentation. More than 59% of the SCFAs was composed of acetate and propionate. Nitrogen-free organic matters (i.e. SCFAs and carbohydrates) accounted for 77.9% of SCOD, while considerable sludge solid reduction (51.6% of total VSS) was achievable, which was beneficial for fermentative liquid utilization and sludge disposal.

Pilot-scale fermentation of urban food waste for volatile fatty acids production: The importance of pH

Here, a pilot-scale volatile fatty acids (VFAs) production system was established using food waste (FW) as feedstock under acidic conditions. The effects of pH (uncontrolled, 4.5, 5.5, and 6.5) on the FW acidification system were investigated. The results showed that VFAs concentration increased from 8419 to 15048 mg COD/L with pH level increasing from 4.5 to 6.5, and the highest VFA production yield (0.79 mgCOD/mgCOD) was obtained at a pH of 6.5. A larger proportion of butyric acid (52.9%) was observed, accompanied by a 23% decrease of acetic acid when pH was elevated to 6.5. Microbial analysis showed that Clostridium sensu stricto 1, Sporanaerobacter, and Proteiniphilum were dominant, which not only positively influence the hydrolysis and acidogenesis processes but also play an essential role in the conversion of acetic acid to butyric acid. In summary, this study provides a valuable reference for large-scale FW treatment to recover valuable resources.

Microbial mechanism of enhancing methane production from anaerobic digestion of food waste via phase separation and pH control

Phase separation and pH control are commonly used to improve methane production during anaerobic digestion (AD) of food waste, but their influencing mechanisms have not been fully discovered through microbial analysis. In this study, single-phase AD (SPAD), two-phase AD without pH control (TPAD-pHUC), and TPAD with fermentation pH controlled at 6.0 and 4.5 were conducted. The results showed that phase separation decreased the ratio of total bacteria to total archaea in the methanogenic phase. At the organic loading rate (OLR) of 1.9 g/(L·d), methanogenesis was dominated by acetoclastic Methanosaeta in both SPAD and TPAD-pHUC, while elevated Methanoculleus and active hydrogen production initiated a shift from the acetoclastic to hydrogenotrophic pathway in SPAD as OLR increased, eventually resulting in excessive acidification at OLR 3.2 g/(L·d). TPAD-pHUC was dominated by Methanosaeta with scarce hydrogen production genes, and thus maintained a delicate balance between fewer acidogens and methanogens at OLR 3.2-3.7 g/(L·d). TPAD with pH control exhibited higher methane yield (460-482 ml/g) at OLR 1.9 g/(L·d) due to the enhancement of protein degradation and the conversion from methylated compounds to methane by Methanosarcina. High Na⁺ concentration facilitated the proliferation of hydrogen production bacteria, but inhibited acetoclastic methanogenesis at OLR 2.4 g/(L·d). In comparison with SPAD and pH control, TPAD without pH control, integrating 4 d acidogenesis and 22 d methanogenesis, exhibited the best and steady performance at OLR 3.7 g/(L·d) with methane production exceeding 370 ml/g.

Bisphenol A alters volatile fatty acids accumulation during sludge anaerobic fermentation by affecting amino acid metabolism, material transport and carbohydrate-active enzymes

Bisphenol A (BPA), a typical persistent organic pollutant in waste activated sludge, was chosen to explore its influence on the accumulation of volatile fatty acids (VFAs), which is an important raw material, during anaerobic fermentation. BPA in the range of 0-200 mg/kg dry sludge was beneficial to VFAs production, from 1564 mg chemical oxygen demand (COD)/L in the control to 2095 mg COD/L with 50 mg/kg BPA; the acetic acid yield was 563 and 1010 mg COD/L with 0 and 50 mg/kg BPA, respectively. The abundance of microorganisms that can consume VFAs was reduced and those responsible for producing VFAs was increased by BPA. Homologous genes of related enzymes in the pathways for amino acid metabolism, fatty acid biosynthesis, ABC transporters and quorum sensing were enhanced in the presence of BPA. The abundance of carbohydrate-active enzymes increased with BPA when compared with the control, benefitting VFAs production.

Assessing the potential of waste activated sludge and food waste co-fermentation for carboxylic acids production

This study investigated waste activated sludge (WAS) and food waste (FW) co-fermentation in batch assays to produce carboxylic acids. Three mixtures (50%, 70% and 90% WAS in VS basis) were studied under different conditions: with and without extra alkalinity, and with and without WAS auto-hydrolysis pre-treatment. All tests were carried out at 35 °C, without pH adjustment and without external inoculum. Experimental results showed that co-fermentation yields, including volatile fatty acids and lactic acid, were always higher than WAS and FW mono-fermentation yields (ca. 100 and 80 mgCOD/gVS, respectively). Co-fermentation yields increased as the proportion of FW in the mixture increased, indicating that the improvement was primarily due to a higher FW degradation under co-fermentation conditions. The maximum co-fermentation yield was on average 480 mgCOD/gVS for the WAS/FW_50/50 mixture. The importance of pH on co-fermentation performance was evident in the experiments carried out with extra alkalinity, which showed that the proportion of WAS in the mixture should be high enough to keep the pH above 5.0. However, fermenters operational conditions should also prevent the enrichment of acetic acid consuming microorganisms. WAS auto-hydrolysis pre-treatment did not enhance co-fermentation yields but showed minor kinetic improvements. Regarding the product profile, butyric acid was enriched as the proportion of FW in the mixture increased and the concomitant pH decreased to the detriment of propionic acid. Propionic acid prevailed under neutral pH in the WAS mono-fermentation and the WAS/FW_90/10 mixture.

Microbial biodiesel production from industrial organic wastes by oleaginous microorganisms: Current status and prospects

This review aims to encourage the technical development of microbial biodiesel production from industrial-organic-wastes-derived volatile fatty acids (VFAs). To this end, this article summarizes the current status of several key technical steps during microbial biodiesel production, including (1) acidogenic fermentation of bio-wastes for VFA collection, (2) lipid accumulation in oleaginous microorganisms, (3) microbial lipid extraction, (4) transesterification of microbial lipids into crude biodiesel, and (5) crude biodiesel purification. The emerging membrane-based bioprocesses such as electrodialysis, forward osmosis and membrane distillation, are promising approaches as they could help tackle technical challenges related to the separation and recovery of VFAs from the fermentation broth. The genetic engineering and metabolic engineering approaches could be applied to design microbial species with higher lipid productivity and rapid growth rate for enhanced fatty acids synthesis. The enhanced in situ transesterification technologies aided by microwave, ultrasound and supercritical solvents are also recommended for future research. Technical limitations and cost-effectiveness of microbial biodiesel production from bio-wastes are also discussed, in regard to its potential industrial development. Based on the overview on microbial biodiesel technologies, an integrated biodiesel production line incorporating all the critical technical steps is proposed for unified management and continuous optimization for highly efficient biodiesel production.