Food waste-derived volatile fatty acids platform using an immersed membrane bioreactor

Recyclable and (Bio)degradable Polyesters in a Circular Plastics Economy

Polyesters carrying polar main-chain ester linkages exhibit distinct material properties for diverse applications and thus play an important role in today's plastics economy. It is anticipated that they will play an even greater role in tomorrow's circular plastics economy that focuses on sustainability, thanks to the abundant availability of their biosourced building blocks and the presence of the main-chain ester bonds that can be chemically or biologically cleaved on demand by multiple methods and thus bring about more desired end-of-life plastic waste management options. Because of this potential and promise, there have been intense research activities directed at addressing recycling, upcycling or biodegradation of existing legacy polyesters, designing their biorenewable alternatives, and redesigning future polyesters with intrinsic chemical recyclability and tailored performance that can rival today's commodity plastics that are either petroleum based and/or hard to recycle. This review captures these exciting recent developments and outlines future challenges and opportunities. Case studies on the legacy polyesters, poly(lactic acid), poly(3-hydroxyalkanoate)s, poly(ethylene terephthalate), poly(butylene succinate), and poly(butylene-adipate terephthalate), are presented, and emerging chemically recyclable polyesters are comprehensively reviewed.

Genome-centric metagenomics revealed the effect of pH on the microbiome involved in short-chain fatty acids and ethanol production

Added-value chemicals production via food waste (FWs) valorization using open-mixed cultures is an emerging approach to replace petrochemical-based compounds. Nevertheless, the effects of operational parameters on the product spectrum remain uncertain given the wide number of co-occurring species and metabolisms. In this study, the identification of 58 metagenome-assembled genomes and their investigation assessed the effect of slight pH variations on microbial dynamics and the corresponding functions when FWs were subjected to anaerobic fermentation (AF) in 1-L continuous stirred tank reactors at 25 °C. The initial pH of 6.5 promoted a microbial community involved in acetate, butyrate and ethanol production, mediated by Bifidobacterium subtile IE007 and Eubacteriaceae IE027 as main species. A slight pH decrease to 6.1 shaped microbial functions that resulted in caproate and H₂ production, increasing the relevance of Eubacteriaceae IE037 role. This study elucidated the strong pH effect on product outputs when minimal variations take place in AF.

Application of Immersed Membrane Bioreactor for Semi-Continuous Production of Polyhydroxyalkanoates from Organic Waste-Based Volatile Fatty Acids

Volatile fatty acids (VFAs) appear to be an economical carbon feedstock for the cost-effective production of polyhydroxyalkanoates (PHAs). The use of VFAs, however, could impose a drawback of substrate inhibition at high concentrations, resulting in low microbial PHA productivity in batch cultivations. In this regard, retaining high cell density using immersed membrane bioreactor (iMBR) in a (semi-) continuous process could enhance production yields. In this study, an iMBR with a flat-sheet membrane was applied for semi-continuous cultivation and recovery of Cupriavidus necator in a bench-scale bioreactor using VFAs as the sole carbon source. The cultivation was prolonged up to 128 h under an interval feed of 5 g/L VFAs at a dilution rate of 0.15 (d^-1), yielding a maximum biomass and PHA production of 6.6 and 2.8 g/L, respectively. Potato liquor and apple pomace-based VFAs with a total concentration of 8.8 g/L were also successfully used in the iMBR, rendering the highest PHA content of 1.3 g/L after 128 h of cultivation. The PHAs obtained from both synthetic and real VFA effluents were affirmed to be poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with a crystallinity degree of 23.8 and 9.6%, respectively. The application of iMBR could open an opportunity for semi-continuous production of PHA, increasing the feasibility of upscaling PHA production using waste-based VFAs.

Current Status and Prospects of Valorizing Organic Waste via Arrested Anaerobic Digestion: Production and Separation of Volatile Fatty Acids

Volatile fatty acids (VFA) are intermediary degradation products during anaerobic digestion (AD) that are subsequently converted to methanogenic substrates, such as hydrogen (H2), carbon dioxide (CO2), and acetic acid (CH3COOH). The final step of AD is the conversion of these methanogenic substrates into biogas, a mixture of methane (CH4) and CO2. In arrested AD (AAD), the methanogenic step is suppressed to inhibit VFA conversion to biogas, making VFA the main product of AAD, with CO2 and H2. VFA recovered from the AAD fermentation can be further converted to sustainable biofuels and bioproducts. Although this concept is known, commercialization of the AAD concept has been hindered by low VFA titers and productivity and lack of cost-effective separation methods for recovering VFA. This article reviews the different techniques used to rewire AD to AAD and the current state of the art of VFA production with AAD, emphasizing recent developments made for increasing the production and separation of VFA from complex organic materials. Finally, this paper discusses VFA production by AAD could play a pivotal role in producing sustainable jet fuels from agricultural biomass and wet organic waste materials.

Enhanced chloride-free snow-melting agent generation from organic wastewater by integrating bioconversion and synthesis

In this study, a new process for producing chloride-free snow-melting agents (CSAs) was proposed. Organic wastewater was converted to total volatile fatty acids (TVFA) by anaerobic acidogenic fermentation. The experiments for acid generation showed that the maximum TVFA concentration of 45.9 g/L was obtained at an organic loading rate of 5 g chemical oxygen demand /(L·d), and the proportion of acetic acid reached 78.8 %. Forward osmosis was used for concentrating the TVFA solution. The obtained CSAs, after evaporation and crystallization, had a better ice-melting capacity and less corrosion on metal and concrete than NaCl and CaCl₂. Additionally, the damage caused by CSAs to the germination of plant seeds was significantly lesser than that caused by chloride salts. This study proposed a feasible method for the high-value conversion of organic wastewater, providing a new direction for the reuse of organic wastewater.

Effect of pH dynamic control on ethanol-lactic type fermentation (ELTF) performance of glucose

This study proposed a new ethanol-lactic type fermentation (ELTF) and explored the optimal control strategy. Using batch experiments, the effects of pH, temperature and organic loading (OL) on ELTF were investigated. The sum of ethanol and lactic acid yield was highest at whole-control pH value of 4.0, 35°C temperature and OL of 33 gCOD/L. To improve ELTF, the dynamic pH control in the long-term CSTR was adjusted at 4.0 (1-28 days), 5.0 (29-44 days) and 4.0 (46-62 days) successively. The high concentration of ethanol and lactic acid was 8190.5 mg/L at 16th day of pH 4.0. At pH of 5.0, the average acidogenesis rate and total concentration of fermentation products increased 111.0% and 128.0%, respectively. Organisms of Lactobacillus and Bifidobacterium were the predominant bacteria in reactor. It can achieve the directional regulation of ELTF and provides parameter support for the application of two-phase anaerobic digestion.

Optimization and Enhancement of Volatile Fatty Acids Biosynthesis via Anaerobic Mesophilic Digestion of Biomass Waste Mixtures

Anaerobic mesophilic batch digestion of lime-pretreated rice straw/chicken manure/ rumen fiber mix was investigated to determine their feasibility as a feedstock to produce volatile fatty acids (VFAs). The influence of solid content (10, 15 & 20%) on the bioreactor performance was studied in batch system. The increase in solid content gave a slight increase in VFAs concentration but higher productivity of 12.1 gL−1d−1. Response surface methodology (RSM) was utilized to optimize the process responses (acid concentration and productivity) using central composite design (face centered) with three coded levels and independent process variables (solid content, and time). Based on the significant predicted mathematical models, the optimum operating parameters for volatile fatty acids production were identified to maximize the acid concentration and the productivity. The optimum values were 20% for solid content at 24 h contact time. The acid concentration and productivity values achieved at this condition were 11.64 g/L and 10.87 gL−1d−1, respectively. So, the two developed mathematical models for both acid concentration and productivity responses have been confirmed the experimental results and can be used to navigate the design space.

Novel strategies towards efficient molecular biohydrogen production by dark fermentative mechanism: present progress and future perspective

In the scenario of alarming increase in greenhouse and toxic gas emissions from the burning of conventional fuels, it is high time that the population drifts towards alternative fuel usage to obviate pollution. Hydrogen is an environment-friendly biofuel with high energy content. Several production methods exist to produce hydrogen, but the least energy intensive processes are the fermentative biohydrogen techniques. Dark fermentative biohydrogen production (DFBHP) is a value-added, less energy-consuming process to generate biohydrogen. In this process, biohydrogen can be produced from sugars as well as complex substrates that are generally considered as organic waste. Yet, the process is constrained by many factors such as low hydrogen yield, incomplete conversion of substrates, accumulation of volatile fatty acids which lead to the drop of the system pH resulting in hindered growth and hydrogen production by the bacteria. To circumvent these drawbacks, researchers have come up with several strategies that improve the yield of DFBHP process. These strategies can be classified as preliminary methodologies concerned with the process optimization and the latter that deals with pretreatment of substrate and seed sludge, bioaugmentation, co-culture of bacteria, supplementation of additives, bioreactor design considerations, metabolic engineering, nanotechnology, immobilization of bacteria, etc. This review sums up some of the improvement techniques that profoundly enhance the biohydrogen productivity in a DFBHP process.

Double-stage membrane-assisted anaerobic digestion process intensification for production and recovery of volatile fatty acids from food waste

The potential of organic waste streams (i.e., food waste) for the sustainable production of precursor chemicals such as volatile fatty acids (VFAs) using anaerobic digestion (AD) has received significant attention in the present days. AD-derived VFAs have great market appeal if the challenges with their recovery and purification from the complex AD effluent is overcome. In this study, a microfiltration immersed membrane bioreactor (MBR) was used for the production of VFAs from food waste and simultaneously in-situ recovery of VFAs. The MBR set-up was applied for 98 days, with a maximum yield of 0.2 gVFA/gVS_added at an organic loading rate (OLR) of 4 g VS/L/d. The recovered permeate was then subjected to further purification using a side stream ultrafiltration unit. It was found that the removal rates of total solids (TS), total suspended solids (TSS), dissolved solids (DS), volatile solids (VS) and volatile suspended solids (VSS) were above 70-80% in both membranes (10 kDa and 50 kDa), and Phosphorus (P), Total Kjeldahl Nitrogen (TKN), chemical oxygen demand (COD), and NH₄₊-N were also removed partially. Particularly, VFAs concentration (above 6 g/L) was higher for 10 kDa at pH 5.4 in ultrafiltered solution and permeate flux decline was higher for 10 kDa at pH 5.4. These results are also supported by the measurement of UV-Vis spectra of the solution and visual appearance, providing a promising approach towards building a VFAs-based platform.

Low-temperature anaerobic digestion of chicken manure at high organic and nitrogen loads - strategies for controlling short chain fatty acids

Objective of this work was to investigate the technical feasibility of low-temperature, closed-loop two-stage (liquid-solid) anaerobic digesters to treat chicken-manure (TS:68%; NH₃:8 g/L) as a sole-feedstock. Effect of pH, temperature, treatment-duration, organic loading rate (OLR) and inoculum-recirculation ratio on short chain fatty acids (SCFA) production was studied. Digesters were operated at 20 ± 1 °C for 282-d over 4 batch-runs (∼70-d/batch) at an OLR of 8.78-4.3 gVS/L/d. Results showed that specific methane yield above 0.6 LCH₄/gVS was feasible with a methane concentration > 60%. SCFA speciation of the entire system was monitored through the liquid-digester. Among SCFA indicators, the ratios of propionic-to-acetic acids, (butyric + valeric)-to-acetic acids, and total SCFA-to-alkalinity were observed within the limit, i.e., below 1.4, 0.3 and 0.8, respectively, indicating high-digester stability. This strategy allowed early detection, diagnosis of process failures in high-solids digester in fed-batch mode, and re-evaluation of operating protocol to enrich performance with economic-benefits.

Factors influencing pressure-driven membrane-assisted volatile fatty acids recovery and purification-A review

Volatile fatty acids (VFAs) are building block chemicals that can be produced through bioconversion of organic waste streams via anaerobic digestion as intermediate products. Purified VFAs are applicable in a wide range of industrial applications such as food, textiles, cosmetics, pharmaceuticals etc. production. The present review focuses on VFAs recovery methods and technologies such as adsorption, distillation, extraction, gas stripping, esterification and membrane based techniques etc., while presenting a discussion of their pros and cons. Moreover, a great attention has been given to the recovery of VFAs through membrane filtration as a promising sustainable clarification, fractionation and concentration approach. In this regard, a thorough overview of factors affecting membrane filtration performance for VFAs recovery has been presented. Filtration techniques such as nanofiltration and reverse osmosis have shown to be capable of recovering over 90% of VFAs content from organic effluent steams, proving the direct effect of membrane materials/surface chemistry, pore size and solution pH in recovery success level. Overall, this review presents a new insight into challenges and potentials of membrane filtration for VFAs recovery based on the effects of factors such as operational parameters, membrane properties and effluent characteristics.

Microbiological insights into anaerobic digestion for biogas, hydrogen or volatile fatty acids (VFAs): a review

In the past decades, considerable attention has been directed toward anaerobic digestion (AD), which is an effective biological process for converting diverse organic wastes into biogas, volatile fatty acids (VFAs), biohydrogen, etc. The microbial bioprocessing takes part during AD is of substantial significance, and one of the crucial approaches for the deep and adequate understanding and manipulating it toward different products is process microbiology. Due to highly complexity of AD microbiome, it is critically important to study the involved microorganisms in AD. In recent years, in addition to traditional methods, novel molecular techniques and meta-omics approaches have been developed which provide accurate details about microbial communities involved AD. Better understanding of process microbiomes could guide us in identifying and controlling various factors in both improving the AD process and diverting metabolic pathway toward production of selective bio-products. This review covers various platforms of AD process that results in different final products from microbiological point of view. The review also highlights distinctive interactions occurring among microbial communities. Furthermore, assessment of these communities existing in the anaerobic digesters is discussed to provide more insights into their structure, dynamics, and metabolic pathways. Moreover, the important factors affecting microbial communities in each platform of AD are highlighted. Finally, the review provides some recent applications of AD for the production of novel bio-products and deals with challenges and future perspectives of AD.

Membrane bioreactors for the production of value-added products: Recent developments, challenges and perspectives

The potential of membrane bioreactors to produce value-added products such as biofuels, biopolymers, proteins, organic acids and lipids at high productivities is emerging. Despite the promising results at laboratory scale, industrial deployment of this technology is hindered due to challenges associated with scale-up. This review aims to address these challenges and create a framework to encourage further research directed towards industrial application of membrane bioreactors to produce value-added products. This review describes the current state-of-the art in such bioreactor systems by exploiting membranes to increase the mass transfer rate of the limiting substrates, reach high cell concentrations and separate the inhibitory substances that may inhibit the bioconversion reaction. It also covers the current trends in commercialization, challenges linked with membrane usage, such as high costs and membrane fouling, and proposes possible future directions for the wider application of membrane bioreactors.

A novel cascade biorefinery approach to transform food waste into valuable chemicals and biogas through thermal pretreatment integration

A novel biorefinery platform integrating thermal pretreatment and solid-liquid separation unit is here proposed to fully exploit food waste (FW) potential for production of valuable chemicals and energy through semi-continuous anaerobic bioconversion. The liquid fraction deriving from raw or pretreated FW, was fermented into volatile fatty acids (VFAs, from acetic to caproic acid) while the residual fraction was converted into biomethane. Thermal pretreatment effectively extracted a portion of the macromolecular organics, especially starch, to the liquid phase, promoting acidogenic fermentation and chain elongation pathways (0.43 gVFA g^-1VS_fed and 0.58 gVFA g^-1VS_fed with raw and pretreated extract, respectively). In parallel, anaerobic digestion of solid residue in 10 L reactors showed process stability and higher conversion rate for the pretreated residue (0.31 against 0.26 Nm³CH₄ kg^-1VS_fed). The mass-transfer balance coupled with the economic assessment, calculated in terms of direct gross added value, indicated promising revenues by integrating the thermal upstream treatment.

Effects of plastics on reactor performance and microbial communities during acidogenic fermentation of food waste for production of volatile fatty acids

The aim of this work was to study the effects of plastics (high-density polyethylene (HDPE), polystyrene (PS), polypropylene (PP), and polyethylene terephthalate (PET)) on reactor performance and microbial communities during acidogenic fermentation of food waste for the production of volatile fatty acids (VFA). The addition of HDPE and PS increased total VFA yields by 28% and 47%, respectively, whereas the addition of PP and PET decreased total VFA yields by 6% and 2%, respectively. The highest enhancing performance of PS could be ascribed to its highly porous structure that could provide immobilization effects for microbial growth. Degradation of various plastics was confirmed by FESEM results, but the degrees were limited (i.e., 3.9-8.7%). Bacterial analysis showed that the addition of various plastics altered the community diversity. Phylum Thermotogae and genus Defluviitoga dominated all the reactors. Potential HDPE- and PS-degrading microbes could belong to genus Clostridium_sensu_stricto_8, while Tepidanaerobacter_syntrophicus could be PET-degrading microbes.

Bioproducts generation from carboxylate platforms by the non-conventional yeast Yarrowia lipolytica

In recent years, there has been a growing interest in the use of renewable sources for bio-based production aiming at developing sustainable and feasible approaches towards a circular economy. Among these renewable sources, organic wastes (OWs) can be anaerobically digested to generate carboxylates like volatile fatty acids (VFAs), lactic acid, and longer-chain fatty acids that are regarded as novel building blocks for the synthesis of value-added compounds by yeasts. This review discusses on the processes that can be used to create valuable molecules from OW-derived VFAs; the pathways employed by the oleaginous yeast Yarrowia lipolytica to directly metabolize such molecules; and the relationship between OW composition, anaerobic digestion, and VFA profiles. The review also summarizes the current knowledge about VFA toxicity, the pathways by which VFAs are metabolized and the metabolic engineering strategies that can be employed in Y. lipolytica to produce value-added biobased compounds from VFAs.

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.

Review and perspectives of enhanced volatile fatty acids production from acidogenic fermentation of lignocellulosic biomass wastes

Lignocellulosic biomass wastes are abundant resources that are usually valorized for methane-rich biogas via anaerobic digestion. Conversion of lignocellulose into volatile fatty acids (VFA) rather than biogas is attracting attention due to the higher value-added products that come with VFA utilization. This review consolidated the latest studies associated with characteristics of lignocellulosic biomass, the effects of process parameters during acidogenic fermentation, and the intensification strategies to accumulate more VFA. The differences between anaerobic digestion technology and acidogenic fermentation technology were discussed. Performance-enhancing strategies surveyed included (1) alkaline fermentation; (2) co-digestion and high solid-state fermentation; (3) pretreatments; (4) use of high loading rate and short retention time; (5) integration with electrochemical technology, and (6) adoption of membrane bioreactors. The recommended operations include: mesophilic temperature (thermophilic for high loading rate fermentation), C/N ratio (20-40), OLR (< 12 g volatile solids (VS)/(L·d)), and the maximum HRT (8-12 days), alkaline fermentation, membrane technology or electrodialysis recovery. Lastly, perspectives were put into place based on critical analysis on status of acidogenic fermentation of lignocellulosic biomass wastes for VFA production.

Enhancement of enzyme activities and VFA conversion by adding Fe/C in two-phase high-solid digestion of food waste: Performance and microbial community structure

Two-phase high-solid digestion is conducive to the degradation of food waste. In this study, Fe/C was added in high-solid digestion in different acidification and/or methanogenic phase. The experimental results indicated that it significantly increased the cumulative yield of biomethane. When Fe/C was added to the acidification phase only and both the acidification and methanogenic phases, the biomethane yield reached 474.07 ± 7.03 and 475.47 ± 4.68 mL·g VS ^-1, respectively, and the biodegradation rate reached 87.30% and 87.58%, respectively, indicating that Fe/C had mainly effect on the performance of acidification phase. In a two-phase anaerobic fermentation system, the activity of dehydrogenases and the concentration of coenzyme F₄₂₀ were 2.23-2.95 mg·g^-1·h^-1 and 0.0063-0.0294 mol·g^-1 volatile solids, respectively. Additionally, the archaeal communities production pathway of methane from using acetic acid to using hydrogen as the reactant.

Production of polyhydroxyalkanoates (PHAs) by Bacillus megaterium using food waste acidogenic fermentation-derived volatile fatty acids

High production costs still hamper fast expansion of commercial production of polyhydroxyalkanoates (PHAs). This problem is greatly related to the cultivation medium which accounts for up to 50% of the whole process costs. The aim of this research work was to evaluate the potential of using volatile fatty acids (VFAs), derived from acidogenic fermentation of food waste, as inexpensive carbon sources for the production of PHAs through bacterial cultivation. Bacillus megaterium could assimilate glucose, acetic acid, butyric acid, and caproic acid as single carbon sources in synthetic medium with maximum PHAs production yields of 9-11%, on a cell dry weight basis. Single carbon sources were then replaced by a mixture of synthetic VFAs and by a VFAs-rich stream from the acidogenic fermentation of food waste. After 72 h of cultivation, the VFAs were almost fully consumed by the bacterium in both media and PHAs production yields of 9-10%, on cell dry weight basis, were obtained. The usage of VFAs mixture was found to be beneficial for the bacterial growth that tackled the inhibition of propionic acid, iso-butyric acid, and valeric acid when these volatile fatty acids were used as single carbon sources. The extracted PHAs were revealed to be polyhydroxybutyrate (PHB) by characterization methods of Fourier-transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The obtained results proved the possibility of using VFAs from acidogenic fermentation of food waste as a cheap substrate to reduce the cost of PHAs production.

Food Waste Biorefinery: Pathway towards Circular Bioeconomy

Food waste biorefineries for the production of biofuels, platform chemicals and other bio-based materials can significantly reduce a huge environmental burden and provide sustainable resources for the production of chemicals and materials. This will significantly contribute to the transition of the linear based economy to a more circular economy. A variety of chemicals, biofuels and materials can be produced from food waste by the integrated biorefinery approach. This enhances the bioeconomy and helps toward the design of more green, ecofriendly, and sustainable methods of material productions that contribute to sustainable development goals. The waste biorefinery is a tool to achieve a value-added product that can provide a better utilization of materials and resources while minimizing and/or eliminating environmental impacts. Recently, food waste biorefineries have gained momentum for the production of biofuels, chemicals, and bio-based materials due to the shifting of regulations and policies towards sustainable development. This review attempts to explore the state of the art of food waste biorefinery and the products associated with it.

Selective Extraction of Medium-Chain Carboxylic Acids by Electrodialysis and Phase Separation

Carboxylic acids obtained via the microbial electrochemical conversion of waste gases containing carbon dioxide (i.e., microbial electrosynthesis) can be used in lieu of nonrenewable building-block chemicals in the manufacture of a variety of products. When targeting valuable medium-chain carboxylic acids such as caproic acid, electricity-driven fermentations can be limited by the accumulation of fermentation products in the culturing media, often resulting in low volumetric productivities and titers due to direct toxicity or inhibition of the biocatalyst. In this study, we tested the effectiveness of a simple electrodialysis system in upconcentrating carboxylic acids from a model solution mimicking the effluent of a microbial electrochemical system producing short- and medium-chain carboxylic acids. Under batch extraction conditions, the electrodialysis scheme enabled the recovery of 60% (mol mol-1) of the total carboxylic acids present in the model fermentation broth. The particular arrangement of conventional monopolar ion exchange membranes and hydraulic recirculation loops allowed the progressive acidification of the extraction solution, enabling phase separation of caproic acid as an immiscible oil with 76% purity.

Membrane bioreactor-assisted volatile fatty acids production and in situ recovery from cow manure

Cow manure (CM) generation in large volumes has for long been considered a waste management challenge. However, the organic content of CM signals opportunities for the production of value-added bioproducts such as volatile fatty acids (VFAs) through anaerobic digestion (AD). However, a robust VFAs fermentation process requires effective methane formation inhibition and enhance VFAs recovery. In this study, thermal pretreatment was applied to inhibit methanogens for enhanced VFAs production and an immersed membrane bioreactor (iMBR) for in situ recovery of VFAs in a semi-continuous AD. Maximal VFAs yield of 0.41 g VFAs/g volatile solids (VS) was obtained from thermally-treated CM without inoculum addition. The CM was further fed to the iMBR operating at organic loading rates of 0.8-4.7 gVS/L.d. The VFAs concentration increased to 6.93 g/L by rising substrate loading to 4.7 g VS/L.d. The applied iMBR set-up was successfully used for stable long-term (114 days) VFAs production and recovery.

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.

Metagenomics for taxonomy profiling: tools and approaches

The study of metagenomics is an emerging field that identifies the total genetic materials in an organism along with the set of all genetic materials like deoxyribonucleic acid and ribose nucleic acid, which play a key role with the maintenance of cellular functions. The best part of this technology is that it gives more flexibility to environmental microbiologists to instantly pioneer the immense genetic variability of microbial communities. However, it is intensively complex to identify the suitable sequencing measures of any specific gene that can exclusively indicate the involvement of microbial metagenomes and be able to advance valuable results about these communities. This review provides an overview of the metagenomic advancement that has been advantageous for aggregation of more knowledge about specific genes, microbial communities and its metabolic pathways. More specific drawbacks of metagenomes technology mainly depend on sequence-based analysis. Therefore, this 'targeted based metagenomics' approach will give comprehensive knowledge about the ecological, evolutionary and functional sequence of significantly important genes that naturally exist in living beings either human, animal and microorganisms from distinctive ecosystems.

Characterization of volatile fatty-acid utilization in Escherichia coli aiming for robust valorisation of food residues

Valorisation of food residues would greatly benefit from development of robust processes that create added value compared to current feed- and biogas applications. Recent advances in membrane-bioreactor-based open mixed microbial cultures, enable robust conversion of fluctuating streams of food residues to a mixture of volatile fatty acids (VFAs). In this study, such a mixed stream of VFAs was investigated as a substrate for Escherichia coli, a well-studied organism suitable for application in further conversion of the acids into compounds of higher value, and/or that are easier to separate from the aqueous medium. E. coli was cultured in batch on a VFA-rich anaerobic digest of food residues, tolerating up to 40 mM of total VFAs without any reduction in growth rate. In carbon-limited chemostats of E. coli W3110 ΔFadR on a simulated VFA mixture, the straight-chain VFAs (C₂-C₆) in the mixture were readily consumed simultaneously. At a dilution rate of 0.1 h^-1, mainly acetic-, propionic- and caproic acid were consumed, while consumption of all the provided acids were observed at 0.05 h^-1. Interestingly, also the branched isovaleric acid was consumed through a hitherto unknown mechanism. In total, up to 80% of the carbon from the supplied VFAs was consumed by the cells, and approximately 2.7% was excreted as nucleotide precursors in the medium. These results suggest that VFAs derived from food residues are a promising substrate for E. coli.

Utilization of food waste-derived volatile fatty acids for production of edible Rhizopus oligosporus fungal biomass

Rhizopus oligosporus is an edible filamentous fungus that can contribute to meet the growing demand for single-cell protein. Volatile fatty acids (VFAs) are favorable potential substrates for producing R. oligosporus biomass due to their capacity to be synthesized from a wide range of low-value organic solid wastes via anaerobic digestion. The goal of this work was to cultivate R. oligosporus using food waste-derived VFAs as the sole carbon source. To maintain the requisite low substrate concentrations, the fed-batch cultivation technique was applied. This resulted in a four-fold improvement in biomass production relative to standard batch cultivation. Maximum biomass yield of 0.21 ± 0.01 g dry biomass/g VFAs _{COD eq. consumed}, containing 39.28 ± 1.54% crude protein, was obtained. In the bubble-column bioreactors, the complete uptake of acetic acid was observed, while the consumptions of caproic and butyric acids reached up to 97.64% and 26.13%, respectively.

Acidogenic fermentation of food waste for production of volatile fatty acids: Bacterial community analysis and semi-continuous operation

Acidogenic fermentation of food waste for production of volatile fatty acids (VFAs) contributes to both food waste minimization and resource recovery. To gain knowledge on functional bacterial communities and facilitate continuous production of VFAs, this research firstly studied the effects of initial pH values (i.e. 5, 6 and 7) and temperatures (i.e. 35 °C and 55 °C) on VFAs production, distribution, and bacterial communities during acidogenic fermentation of food waste. The optimal conditions were determined as pH 7 and 35 °C, corresponding to the highest total VFAs yield of 11.8 g COD/L with major components of acetic, propionic and butyric acid. Bioinformatic analysis showed that the relative abundance of the dominant bacterial classes (e.g. Clostridia, Bacteroidia and Bacilli) were changed by the initial pH values in both mesophilic and thermophilic reactors. NMDS analysis confirmed a significant difference between mesophilic and thermophilic communities. Finally, the feasibility of continuous production and recovery of VFAs was validated using a two-phase leachate bed bioreactor at the optimal conditions. Average concentration and yield of the total VFAs in the continuous operation were 6.3 g COD/L and 0.29 g VFA/g VS_added, respectively. The findings in this study could provide pivotal technical supports for potential pilot- and commercial-scale biorefinery plants for VFAs production from food waste.

MBR-Assisted VFAs Production from Excess Sewage Sludge and Food Waste Slurry for Sustainable Wastewater Treatment

The significant amount of excess sewage sludge (ESS) generated on a daily basis by wastewater treatment plants (WWTPs) is mainly subjected to biogas production, as for other organic waste streams such as food waste slurry (FWS). However, these organic wastes can be further valorized by production of volatile fatty acids (VFAs) that have various applications such as the application as an external carbon source for the denitrification stage at a WWTP. In this study, an immersed membrane bioreactor set-up was proposed for the stable production and in situ recovery of clarified VFAs from ESS and FWS. The VFAs yields from ESS and FWS reached 0.38 and 0.34 gVFA/gVSadded, respectively, during a three-month operation period without pH control. The average flux during the stable VFAs production phase with the ESS was 5.53 L/m2/h while 16.18 L/m2/h was attained with FWS. Moreover, minimal flux deterioration was observed even during operation at maximum suspended solids concentration of 32 g/L, implying that the membrane bioreactors could potentially guarantee the required volumetric productivities. In addition, the techno-economic assessment of retrofitting the membrane-assisted VFAs production process in an actual WWTP estimated savings of up to 140 €/h for replacing 300 kg/h of methanol with VFAs.

A Wake-Up Call for the Efficient Use of the Bacterial Resting Cell Process, with Focus on Low Solubility Products

Micro-organisms are often subjected to stressful conditions. Owing to their capacity to adapt, they try to rapidly cope with the unfavorable conditions by lowering their growth rate, changing their morphology, and developing altered metabolite production and other stress-related metabolism. The stress-related metabolism of the cells which interrupted their growth is often referred to as resting metabolism and can be exploit for specific and high rate production of secondary metabolites. Although the bacterial resting cell process has been described decades ago, we find it worthwhile to bring the process under renewed attention and refer to this type of processes as non-growing metabolically active (NGMA) cell processes. Despite their use may sound counterproductive, NGMA cells can be of interest to increase substrate conversion rates or enable conversion of certain substrates, not accessible to growing cells due to their bacteriostatic nature or requirement of resistance to a multitude of different stress mechanisms. Biomass reuse is an interesting feature to improve the economics of NGMA cell processes. Yet, for lipophilic compounds or compounds with low solubility, biomass separation can be delicate. This review draws the attention on existing examples of NGMA cell processes, summarizing some developmental tools and highlighting drawbacks and opportunities, to answer the research question if NGMA cells can have a distinct added value in industry. Particular elaboration is made on a novel and more broadly applicable strategy to enable biomass reuse for conversions of compounds with low solubility.

Resource recovery and circular economy from organic solid waste using aerobic and anaerobic digestion technologies

With the inevitable rise in human population, resource recovery from waste stream is becoming important for a sustainable economy, conservation of the ecosystem as well as for reducing the dependence on the finite natural resources. In this regard, a bio-based circular economy considers organic wastes and residues as potential resources that can be utilized to supply chemicals, nutrients, and fuels needed by mankind. This review explored the role of aerobic and anaerobic digestion technologies for the advancement of a bio-based circular society. The developed routes within the anaerobic digestion domain, such as the production of biogas and other high-value chemicals (volatile fatty acids) were discussed. The potential to recover important nutrients, such as nitrogen through composting, was also addressed. An emphasis was made on the innovative models for improved economics and process performance, which include co-digestion of various organic solid wastes, recovery of multiple bio-products, and integrated bioprocesses.

Recycling strategies for polyhydroxyalkanoate-based waste materials: An overview

The plastics market is dominated by fossil-based polymers, but their gradual replacement by bioplastics (e.g., polyhydroxyalkanoates) is occurring. However, recycling strategies need to be developed to truly unveil the impact of bioplastics on waste accumulation. This review provides a state of the art of recycling strategies investigated for polyhydroxyalkanoate-based polymers and proposes future research avenues. Research on mechanical and chemical recycling is dominated by the use of extrusion and pyrolysis, respectively, while that on biodegradation of polyhydroxyalkanoates is related to soil and aquatic samples, and to anaerobic digestion towards biogas production. Research gaps exist in the relationships between polymer composition and ease of use of all recycling strategies investigated. This is of utmost importance since it will influence the need for separation at the source. Therefore, research emphasis needs to be given to the area to follow the continuous improvement of the process economics towards widespread commercial production of polyhydroxyalkanoates.

Bioengineering of anaerobic digestion for volatile fatty acids, hydrogen or methane production: A critical review

Anaerobic digestion (AD) is a well-established technology used for producing biogas or biomethane alongside the slurry used as biofertilizer. However, using a variety of wastes and residuals as substrate and mixed cultures in the bioreactor makes AD as one of the most complicated biochemical processes employing hydrolytic, acidogenic, hydrogen-producing, acetate-forming bacteria as well as acetoclastic and hydrogenoclastic methanogens. Hydrogen and volatile fatty acids (VFAs) including acetic, propionic, isobutyric, butyric, isovaleric, valeric and caproic acid and other carboxylic acids such as succinic and lactic acids are formed as intermediate products. As these acids are important precursors for various industries as mixed or purified chemicals, the AD process can be bioengineered to produce VFAs alongside hydrogen and therefore biogas plants can become biorefineries. The current review paper provides the theory and means to produce and accumulate VFAs and hydrogen, inhibit their conversion to methane and to extract them as the final products. The effects of pretreatment, pH, temperature, hydraulic retention time (HRT), organic loading rate (OLR), chemical methane inhibitions, and heat shocking of the inoculum on VFAs accumulation, hydrogen production, VFAs composition, and the microbial community were discussed. Furthermore, this paper highlights the possible techniques for recovery of VFAs from the fermentation media in order to minimize product inhibition as well as to supply the carboxylates for downstream procedures.

High rejection rate of polysaccharides by microfiltration benefits Christensenella minuta and acetic acid production in an anaerobic membrane bioreactor for sludge fermentation

Substrates, the microbial community, and volatile fatty acids (VFAs) are interacting factors in anaerobic fermentation. In this paper, a microfiltration membrane was installed in a conventional reactor for sludge anaerobic fermentation, and changes in the VFA composition and the microbial community structure after the application were monitored. The membrane rejected much more polysaccharide than protein. VFA production, mainly acetic acid, was greatly enhanced by the membrane separation. The glycolytic species Christensenella minuta was significantly enriched. The explanation was inferred to be the higher retention rate of polysaccharide compared with proteins by the microfiltration. However, surprisingly, another polysaccharide using genus, Parabacteroides, did not benefit from the application of the membrane separation, suggesting more complicated mechanisms might be involved.