Enhancement of acidogenic fermentation for volatile fatty acid production from food waste: Effect of redox potential and inoculum

New insights of polyacrylamide on volatile fatty acid accumulation: Sludge crushing mechanism and microbial community function

To investigate the impact of polyacrylamide (PAM) on hydrolysis, acidification, and microbial properties during anaerobic fermentation of waste activated sludge (WAS), the fermentation system was supplemented with four different concentrations of PAM. Results showed that PAM inhibited WAS cell disruption, which reduced volatile fatty acid (VFA) production but increased the proportions of propionic acid. The addition of PAM reduced protease and α-glucosidase activities by 41.52-75.58% and 21.7-41.63%, respectively, thus inhibiting the breakdown of organic substrates. Microbiologically, PAM reduced microbial richness and diversity, but increased the relative abundances of Bacteroidota and Actinobacteriota, which promoted the production of propionic acid. Additionally, PAM lowered the levels of key hydrolytic and acidogenic enzymes. This study offers further theoretical insights into how PAM influences VFA production during anaerobic sludge fermentation.

Anaerobic Volatile Fatty Acid Production Performance and Microbial Community Characteristics from Solid Fraction of Alkali-Thermal Treated Waste-Activated Sludge: Focusing on the Effects of Different pH Conditions

The waste-activated sludge (WAS) is rich in organic matter and various nutrients. Alkali-thermal hydrolysis of WAS can be employed to produce a liquid fertilizer with high plant-promoting nutrient content. However, the solid fraction (abbreviated as SF) generated from this process requires further treatment. Although there have been studies on the recovery of plant nutrients from WAS via alkali-thermal hydrolysis, researches on the safe treatment of the SF are limited. This study aims to explore the potential and the microbiological mechanisms on anaerobic volatile fatty acid (VFA) production from the SF under different pH conditions (i.e., 6, 7, 8, 9, and 10). The results showed that the VFA yield was highest at pH 6, reaching 4095.84 mg COD/L (i.e., 0.16 g-COD/g-volatile solids), followed by pH 10, 8, 7, and 9, with acetate being the main component (> 56%). Microbial community analysis revealed that members in phyla Firmicutes and Bacteroidota constituted the main acid-producing microbial community during the anaerobic fermentation of SF. Furthermore, different pH conditions influenced the yield and composition of VFAs by altering the structure and functions of microbial community. This research provides a new direction for the fully resourceful utilization of sludge by producing both liquid fertilizer and VFAs from WAS.

Graph-based deep learning for predictions on changes in microbiomes and biogas production in anaerobic digestion systems

Anaerobic digestion (AD), which relies on a complex microbial consortium for efficient biogas generation, is a promising avenue for renewable energy production and organic waste treatment. However, understanding and optimising AD processes are challenging because of the intricate interactions within microbial communities and the impact of volatile fatty acids (VFAs) on biogas production. To address these challenges, this study proposes the application of graph convolutional networks (GCNs) to comprehensively model AD processes. GCN models were developed to predict microbial dynamics and biogas production by integrating network analyses of high-throughput sequencing data and VFA inhibition effects. The models were trained based on the responses of anaerobic digesters to organic loading rate shock, starvation, and bioaugmentation for 281 d under various feeding conditions. Shifts in microbial community composition during AD stages and feeding conditions were successfully identified using next-generation sequencing tools. Graph topological features indicated a significant coupling between VFAs and microbial families, and the hydrogenotrophic archaeal families were most frequently connected to other families or residual acids. The GCN accurately predicted microbial abundances and gas production rates, achieving a mean squared error of 0.11 and 0.01 and a coefficient of determination of 0.72 and 0.87 for the testing dataset. These results provide valuable insights into the effects of starvation and bioaugmentation on the microbiome by utilising GCNs to model anaerobic treatment processes, predict microbial dynamics, and assess reactor productivity. Our study suggests a new modelling framework for understanding and improving AD systems by considering microbial interaction networks in relation to chemical parameter information at relevant operating scales.

Effect of variation in temperature on malodor generation from different units of a wastewater treatment plant

This study investigates the effect of temperature variation on malodor generation across different units of a wastewater treatment plant (WWTP). The results demonstrate that higher temperatures exacerbated odor emission due to increased microbial activity with all the different units showing maximum odorous gas production at the highest temperatures used (35 °C and 45 °C) in this study. The maximum total odor activity value (OAV) of 353106 was obtained for anoxic and anaerobic unit at 45 °C. The variation in composition of odor-causing gases was also dependent on wastewater characteristics than temperature alone. Volatile reduced sulfur compounds, including hydrogen sulfide and methyl mercaptan, were dominant in most wastewater samples, while units with higher dissolved oxygen (DO) content, such as aeration and sedimentation units, exhibited elevated levels of phenol and dimethyl disulfide and reduced H2S concentration. Analysis of the liquid composition following incubations revealed presence of mainly aldehydes (> 75%) which are produced due to incomplete organic matter degradation, particularly at lower temperatures. Statistical analysis showed positive correlation between temperature and odor generation. DO had negative correlation with H2S (r =  - 0.78, - 0.93) along with total gas concentration and total OAV, but positively correlated with other gases, namely methyl mercaptan (r = 0.22, 0.97), dimethyl disulfide (r = 0.93, 0.98), phenol (r = 0.99, 0.97), and ammonia (r = 0.99, 0.98). Solids concentration and volatile solids to total solids (VS/TS) ratio had positive correlation with H2S, total gas concentration, and total OAV (r = 0.68, 0.54, and 0.90). These findings highlight the need for tailored odor management strategies based on temperature fluctuations and unit-specific conditions to optimize WWTP operations and reduce odor emissions effectively.

Targeted volatile fatty acid production based on lactate platform in mixed culture fermentation: Insights into carbon conversion and microbial metabolic traits

In this study, the effects of fermentation pH and redox potential on the performance of the lactate platform were comprehensively evaluated. The results indicated that the type of acidogenic fermentation was influenced by redox potential, while pH was correlated with volatile fatty acid yield. The highest propionate yield was achieved under anaerobic conditions at a pH of9, with the dominant genus Serpentinicella producing propionate through the acrylate pathway. The highest acetate yield was produced under facultative conditions at a pH of 6. This production was primarily facilitated by the dominant genera unclassified_f__Enterobacteriaceae and Desulfovibrio, which exhibited significant upregulation of the expression of related genes. Furthermore, ecological processes were employed to establish the relationship between environmental factors and microbial communities. This study emphasized the process of converting lactate into volatile fatty acid, providing a theoretical basis for future strategies aimed at regulating targeted acid production.

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

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

Effect of plant species on wastewater treatment performance of a subsurface vertical-flow constructed wetland with step-feeding at low temperature

To improve the treatment performance of constructed wetlands under low-temperature conditions, this study investigated the effects of plant species on wastewater treatment performance at low temperature and the associated microbiological characteristics in a subsurface vertical-flow constructed wetland (VFCW) with step-feeding. The results showed that the redox microenvironment in the VFCW filter with step-feeding could be restored and optimized by planting appropriate species that can tolerate low temperature, ensuring a high nitrification performance for the system. Correspondingly, the abundance and activity of three functional microbes (namely nitrifiers, denitrifiers, and anammox bacteria) increased to different degrees in the system, eventually ensuring ideal nitrogen removal by the VFCW. Compared with the VFCW planted with Phragmites australis and Acorus gramineus, the operation performance of the VFCW planted with Iris wilsonii could be recovered at low temperature, and its chemical oxygen demand, total phosphorus, total nitrogen, and ammonium nitrate removal rates could respectively reach 95.7%, 99.2%, 93.0%, and 94.4%, respectively. Moreover, nitrogen removal in the system relied on the nitrification/denitrification and partial denitrification - anaerobic ammonium oxidation processes. Nitrosomonas, Nitrospira, Thauera, and Candidatus Brocadia were the four dominant bacterial genera in the filter layer.

Assessing the potential of olive mill solid waste as feedstock for methane and volatile fatty acids production via anaerobic bioprocesses

The extensive production of olive mill solid waste (OMSW) from olive oil industry in the Mediterranean basin claims effective treatments and valorization strategies. This study aims to elucidate the potential of anaerobic digestion (AD) and anaerobic fermentation (AF) to convert pre-treated OMSW into biogas (CH4) and volatile fatty acids (VFA), respectively. The two thermal treatment conditions (65 °C and 180 °C) that are being implemented in the industry that manages the OMSW were tested. Comparing the two treatments aims to demonstrate the influence on the AD process of the degree of solubilization and degradation of the metabolites produced from the same substrate. AD of OMSW treated at low-temperature (65 °C) exhibited similar methane yields (195 ± 8 mL CH4/g volatile solid (VS)) to raw OMSW. AD of the solid phase (SP) after high-temperature treatment with acid addition at 180 °C resulted in methane yields comparable to raw OMSW while the liquid phase (LP) exhibited low methane yields (85 ± 10 mL CH4/g VS). Nevertheless, LP/180 °C exhibited the highest VFA bioconversion at 27.6 %, compared to less than 10 % for SP/180 ºC, SP/65 °C, and raw OMSW. The VFA profile showed notable variations with thermal treatment temperatures. Propionic acid dominated at SP/65 °C, while acetic acid became the primary VFA at 180 °C. Furthermore, significant degradation rates of phenolic compounds and furans were observed during the final day of both anaerobic processes. Overall, these findings suggest that AD is more suitable for raw OMSW, treated at low temperature and SP at high temperature, while AF offers a promising alternative for high-temperature-treated LP.

Bulking agent in dry anaerobic digestion as a key factor for the enhancement of biogas production

Dry anaerobic digestion (dry-AD) is an attractive process for solid wastes such as agri-food waste. However, some limitations mainly associated to lack of effective mixing, can hinder the methane production capacity of the systems. Bulking agent (BA) has been proposed as a solution to the compaction issues in systems without mechanical agitation, such as leaching bed reactors. However, effects of BA are still not clear, and, thus, the factors to consider for its dose has not been optimized yet. This work studies the effect of BA in dry-AD. Two substrates with different characteristics were proposed as models, bean peel as a lignocellulosic substrate and a mixture of food waste as a readily biodegradable substrate. Inert plastic rings were used as BA at different BA:S ratios. Assessed BA:S ratio did not affect the performance of methane production for the lignocellulosic waste, but it did significantly affect to the easily biodegradable substrate, showing up to a 28% of methane production increase. This result could be due to the presence of lignocellulosic compounds in the bean peel, behaving like a natural BA. In assays with an increased bed height, the compaction of the system was more severe, resulting in the rapid acidification of the processes. At these conditions, the positive effect of BA addition was more marked, allowing methane production and no acidification of the system. Thus, the addition of BA is a suitable strategy for improving methane production or stability in dry-AD systems without requiring the stirring of the systems.

Statistical modeling and optimization of volatile fatty acids production by anaerobic digestion of municipal wastewater sludge

Obtaining value-added products from renewable resources is limited by the lack of specific operating conditions optimized for the physico-chemical characteristics of the biomass and the desired end product. A mathematical model and statistical optimization were developed for the production of volatile fatty acids (VFAs) by anaerobic digestion of municipal sewage sludge. The experimental tests were carried out in triplicate and investigated a wide range of conditions: pH 9.5, 10.5, and 11.5; temperatures 25 °C, 35 °C, 45 °C, and 55 °C; primary sludge with organic loading (OL) of 10 and 14 g VS (volatile solids); and digested sludge with 4 and 6 g VS. Subsequently, a statistical search was performed to obtain optimal production conditions, then a statistical model of VFA production was developed and the optimal conditions were validated at pilot plant scale. The maximum VFA concentration predicted was 6975 mg COD (chemical oxygen demand)/L using primary sludge at 25 °C, initial OL of 14 g VS, and pH 10.5. The obtained third-degree model (r2 = 0.83) is a powerful tool for bioprocess scale-up, offering a promising avenue for sustainable waste management and biorefinery development.

Selective separation of nutrients and volatile fatty acids from food wastes using electrodialysis and membrane contactor for resource valorization

Transport and selectivity parameters describe the quantity and purity of nutrients and volatile fatty acids (VFAs) separated from fermentation media. However, the complexity of fermentation media and low nutrient concentrations hinder the optimal conditions of such parameters. Exploring technologies to overcome such limitations is crucial for selectively separating VFAs from nutrients in fermented media. The objectives of this study were to investigate the: (1) flux, (2) recovery, (3) concentration factor, and (4) specific energy consumption of nutrients (NH4+, K+, NO3-, and PO43-) and VFAs (acetic, propionic, and butyric acid) via electrodialysis (ED), and (5) selectively separate the VFAs from the nutrients in the ED concentrate using a hydrophobic membrane contactor (HMC). Synthetic feed and real industrial fermented food wastes were used for ED and HMC experiments. The ED consumed 0.395 kWh/kg, recovering 64-95% of the nutrients and VFAs, corresponding to 4.1-9.4 and 0.6-22.1 g/L nutrients and VFAs, respectively. The HMC selectively separated over 94% of VFAs after ED, with <2% nutrients contamination in the final VFA stream. The results suggest that applying HMC after ED can concentrate and selectively separate VFAs from nutrients in fermented food wastes, which can be valorized for bio-based fertilizers and chemical platforms.

Biogas slurry recirculation regulates food waste fermentation: Effects and mechanisms

Regularly adding biogas slurry into fermentation reactors is an effective way to enhance hydrogen or methane production. However, how this method affects the production of valuable organic acids and alcohols is still being determined. This study investigated the effects of different addition ratios on semi-continuous fermentation reactors using food waste as a substrate. The results showed that an addition ratio of 0.2 increased lactic acid production by 30% with a yield of 0.38 ± 0.01 g/g VS, while a ratio of 0.4 resulted in mixed acid fermentation dominated by n-butyric acid (0.07 ± 0.01 g/g VS) and n-caproic acid (0.06 ± 0.00 g/g VS). The introduction of Bifidobacteriaceae by biogas slurry played a crucial role in increasing lactic acid production. In contrast, exclusive medium-chain fatty acid producers enhanced the synthesis of caproic acid and heptanoic acid via the reverse β-oxidation pathway. Mechanism analyses suggested that microbial community structure and activity, substrate hydrolysis, and cell membrane transport system and structure changed to varying degrees after adding biogas slurry.

Disinfectant sodium dichloroisocyanurate synergistically strengthened sludge acidogenic process and pathogens inactivation: Targeted upregulation of functional microorganisms and metabolic traits via self-adaptation

Harmless and resourceful treatment of waste activated sludge (WAS) have been the crucial goal for building environmental-friendly and sustainable society, while the synergistic realization approach is currently limited. This work skillfully utilized the disinfectant sodium dichloroisocyanurate (NaDCC) to simultaneously achieve the pathogenic potential inactivation (decreased by 60.1 %) and efficient volatile fatty acids (VFAs) recovery (increased by 221.9 %) during WAS anaerobic fermentation in rather cost-effective way (Chemicals costs:0.4 USD/kg VFAs versus products benefits: 2.68 USD/kg chemical). Mechanistic analysis revealed that the C=O and NCl bonds in NaDCC could spontaneously absorb sludge (binding energy -4.9 kJ/mol), and then caused the sludge disintegration and organic substrates release for microbial utilization due to the oxidizability of NaDCC. The disruption of sludge structure along with the increase of bioavailable fermentation substrates contributed to the selectively regulation of microbial community via enriching VFAs-forming microorganisms (e.g., Pseudomonas and Streptomyces) and reducing VFAs-consuming microorganisms, especially aceticlastic methanogens (e.g., Methanothrix and Methanospirillum). Correspondingly, the metabolic functions of membrane transport, substrate metabolism, pyruvate metabolism, and fatty acid biosynthesis locating in the central pathway of VFAs production were all upregulated while the methanogenic step was inhibited (especially acetate-type methanogenic pathway). Further exploration unveiled that for those enriched functional anaerobes were capable to activate the self-adaptive systems of DNA replication, SOS response, oxidative stress defense, efflux pump, and energy metabolism to counteract the unfavorable NaDCC stress and maintain high microbial activities for efficient VFAs yields. This study would provide a novel strategy for synergistic realization of harmless and resourceful treatment of WAS, and identify the interrelations between microbial metabolic regulations and adaptive responses.

Promoting short-chain fatty acids production from sewage sludge via acidogenic fermentation: Optimized operation factors and iron-based persulfate activation system

Promoting short-chain fatty acids (SCFAs) production and ensuring the stability of SCFAs-producing process are becoming the two major issues for popularizing the acidogenic fermentation (AF). The key controlling operating and influencing factors during anaerobic fermentation process were thoroughly reviewed to facilitate better process performance prediction and to optimize the process control of SCFAs promotion. The wide utilization of iron salt flocculants during wastewater treatment could result in iron accumulating in sewage sludge which influenced AF performance. Additionally, appropriate ferric chloride (FC) could promote the SCFAs accumulation, while poly ferric sulfate (PFS) inhibited the bioprocess. Iron/persulfate (PS) system was proved to effectively enhance the SCFAs production while mechanism analysis revealed that the strong oxidizing radicals remarkably enhanced the solubilization and hydrolysis. Moreover, the changes of oxidation-reduction potential (ORP) and pH caused by iron/PS system exhibited more negative effects on the methanogens, comparing to the acidogenic bacteria. Furthermore, performance and mechanisms of different iron species-activating PS, organic chelating agents and iron-rich biochar derived from sewage sludge were also elucidated to extend and strengthen understanding of the iron/PS system for enhancing SCFAs production. Considering the large amount of generated Fe-sludge and the multiple benefits of iron activating PS system, carbon neutral wastewater treatment plants (WWTPs) were proposed with Fe-sludge as a promising recycling composite to improve AF performance. It is expected that this review can deepen the knowledge of optimizing AF process and improving the iron/PS system for enhancing SCFAs production and provide useful insights to researchers in this field.

Simultaneous sludge minimization and membrane fouling mitigation in membrane bioreactors by using a microaerobic - Settling pretreatment module

Membrane fouling is the major obstacle for membrane bioreactors operated at a long sludge retention time to reduce sludge production. In this study, a sludge process reduction (SPR) module, consisting of a microaerobic tank and a settler, was inserted before an anoxic/oxic MBR (AO-MBR) to achieve dual objectives of fouling alleviation and sludge reduction. Three SPR-MBRs were operated to investigate influences of sludge recirculation ratios from the SPR settler to the microaerobic tank on process performance. Compared to AO-MBR, the SPR-MBRs reduced sludge production by 43.1-56.4% by maintaining sludge retention times above 175 d, and decreased foulant layer resistance and pore clogging resistance. Inserting SPR reduced the accumulation of dissolved organic matters and extracellular polymeric substances, enlarged sludge flocs, and decreased sludge viscoelasticity. However, increasing R_SPR stimulated outward diffusion of extracellular polymeric substances and increased sludge viscosity. SPR-MBRs achieved effective sludge reduction by enriching hydrolytic (Trichococcus and Aeromonas) and fermentative genera (Lactococcus, Paludibacter, Macellibacteroides, and Acinetobacter) in the SPR, and alleviated membrane fouling by prohibiting the growth of extracellular polymeric substance-secreting bacteria and enriching filamentous bacteria to enlarge particle size. The results revealed that the SPR-MBR maximized sludge reduction with a very long sludge retention time, and alleviated membrane fouling synchronously.

Impact of N-butyryl-l-homoserine lactone-mediated quorum sensing on acidogenic fermentation under saline conditions: Insights into volatile fatty acids production and microbial community

Anaerobic fermentation is often inhibited under high salinity conditions. This study discovered a strong, positive association between N-butyryl-l-homoserine lactone (C4-HSL)-mediated quorum sensing (QS) and the production of volatile fatty acids (VFAs) under saline conditions. N-acyl-homoserine lactones were identified during acidogenic fermentation for VFA production. Only C4-HSL was detected at all salt concentrations, and a maximum C4-HSL concentration of 0.49 μg/L was observed at a salt concentration of 15 g/L. C4-HSL secretion was closely related to salinity, and a strong correlation was observed between C4-HSL and VFAs (p < 0.01), especially butyrate. Further experiments with C4-HSL addition indicated that exogenous C4-HSL promoted substrate hydrolysis and increased butyrate production by 1.5 times at 15 g/L NaCl. Microbial community analysis indicated that unclassified_f__Enterobacteriaceae and Clostridium_sensu_stricto_1, associated with QS genes and butyrate production, were positively associated with C4-HSL. This study demonstrates the positive effect of C4-HSL-mediated QS on acidogenic fermentation.

Volatile fatty acid-treated mixotrophic cultivation of lipid/carbohydrate-rich cyanobacterial species, Pseudanabaena mucicola GO0704, for the enhancement of biofuel production

Cyanobacteria-derived biofuels can be helpful in achieving a circular bioeconomy. To increase the production of biodiesel/bioethanol from cyanobacterium, Pseudanabaena mucicola GO0704, mixotrophic cultivation using volatile fatty acid (VFA), a cheap organic carbon source, was performed. The treatment of butyric acid or acetic acid enhanced the cell growth, particularly, the dry weight of the butyric acid-treated cells was 2.30-fold higher than the control. The enhancement of the growth led to the increase of metabolite (i.e., lipid and carbohydrate) productions, resulting in high amount of biodiesel and bioethanol to be produced. Butyric acid was more effective compared to acetic acid and the productions of biodiesel (52.2 mg/L) and bioethanol (132.6 mg/L) from the butyric acid-treated P. mucicola GO0704 were 2.34- and 2.17-fold higher compared to the control, respectively. This study will provide a foundation to commercialize the cyanobacteria-based carbon-neutral fuels, and ultimately, achieve a circular bioeconomy.

A critical review of wastewater quality variation and in-sewer processes during conveyance in sewer systems

In-sewer physio-biochemical processes cause significant variations of wastewater quality during conveyance, which affects the influent to a wastewater treatment plant (WWTP) and arguably the microbial community of biological treatment units in a WWTP. In wet weather, contaminants stored in sewer deposits can be resuspended and migrate downstream or be released during combined sewer overflows to the urban water bodies, posing challenges to the treatment facilities or endangering urban water quality. Therefore, in-sewer transformation and migration of contaminants have been extensively studied. The compiled results from representative research in the past few decades showed that biochemical reactions are both cross-sectionally and longitudinally organized in the deposits and the sewage, following the redox potential as well as the sequence of macromolecule/contaminant degradation. The sewage organic contents and sewer biofilm microorganisms were found to covary but more systematic studies are required to examine the temporal stability of the feature. Besides, unique communities can be developed in the sewage phase. The enrichment of the major sewage-associated microorganisms can be explained by the availability of biodegradable organic contents in sewers. The sewer deposits, including biofilms, harbor both microorganisms and contaminants and usually can provide longer residence time for in-sewer transformation than wastewater. However, the interrelationships among contaminant transformation, microorganisms in the deposits/biofilms, and those in the sewage are largely unclear. Specifically, the formation and migration of FOG (fat, oil, and grease) deposits, generation and transport of contaminants in the sewer atmosphere (e.g., H₂S, CH₄, volatile organic compounds, bioaerosols), transport and transformation of nonconventional contaminants, such as pharmaceuticals and personal care products, and wastewater quality variation during the biofilm rehabilitation period after damages caused by rains/storms are some topics for future research. Moreover, systematic and standardized field analysis of real sewers under dynamic wastewater discharge conditions is necessary. We believe that an improved understanding of these processes would assist in sewer management and better prepare us for the challenges brought about by climate change and water shortage.

Recent achievements in platform chemical production from food waste

Food waste conversion/valorization to produce bio-based chemicals plays a key role toward achieving carbon neutrality by 2050. Food waste valorization to renewable chemicals is thus an attractive and eco-friendly approach to handling food waste. The production of platform chemicals from food waste is crucial for making highly value-added renewable chemicals. However, earlier reviews dealing with food waste valorization to produce value-added chemicals have emphasized the enhancement of methane, hydrogen, and ethanol production. Along these lines, the existing methods of food waste to produce platform chemicals (e.g., volatile fatty acids, glucose, hydroxymethylfurfural, levulinic acid, lactic acid, and succinic acid) through physical, chemical, and enzymatic pretreatments, hydrolysis, fermentation, and hydrothermal conversion are extensively reviewed. Finally, the challenges faced under these methods are discussed, along with suggestions for future research on platform chemical production from food waste.

Highly selective butyric acid production by coupled acidogenesis and ion substitution electrodialysis

Selective production of carboxylic acids (CAs) from mixed culture fermentation remains a difficult task in organic waste valorization. Herein, we developed a facile and sustainable carbon loop strategy to regulate the fermentation micro-environment and steer acidogenesis towards selective butyric acid production. This new ion substitution electrodialysis-anaerobic membrane bioreactor (ISED-AnMBR) integrated system demonstrated a high butyric acid production at 11.19 g/L with a mass fraction of 76.05%. In comparison, only 1.04 g/L with a mass fraction of 30.56% was observed in the uncoupled control reactor. The carbon recovery reached a maximum of 96.09% with the assistance of ISED. Inorganic carbon assimilation was believed to be an important contributor, which was verified by ¹³C isotopic tracing. Microbial community structure shows the dominance of Clostridia (80.16%) in the unique micro-environment (e.g., pH 4.80-5.50) controlled by ISED, which is believed beneficial to the growth of such fermentative bacteria with main products of butyric acid and acetic acid. In addition, the emergence of chain elongators such as Clostridium sensu stricto 12 was observed to have a great influence on butyric acid production. This work provides a new approach to generate tailored longer chain carboxylic acids from organic waste with high titer thus contributing to a circular economy.

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.

High salinity slowed organic acid production from acidogenic fermentation of kitchen wastewater by shaping functional bacterial community

The high salinity of kitchen wastewater might have adverse effects on the production of short-chain fatty acids (SCFAs) in anaerobic fermentation. The effects and mechanisms of salinity on SCFA production in the anaerobic fermentation of kitchen wastewater were studied by varying the salt concentration, as follows: 0 g/L (S0), 2 g/L (S2), 6 g/L (S6), 10 g/L (S10), 15 g/L (S15), and 20 g/L (S20). Experimental results showed that hypersaline conditions (>10 g NaCl/L) accelerated the release of soluble proteins at the initial stage of anaerobic fermentation. They also significantly prohibited the hydrolysis and degradation of soluble proteins and carbohydrates. Compared with low salinity tests, the SCFA concentrations under hypersaline conditions (>10 g NaCl/L) only reached approximately 43% of the highest concentration on day 10, although the SCFA concentrations in all tests were very close on day 10 (14 g COD/L). High salinity delayed the production of n-butyric acid but did not change the composition of the total SCFAs. High salinity enriched Enterococcus and Bifidobacterium, the relative abundance levels of which reached 27.57% and 49.71%, respectively, before the depletion of substrate. High salinity showed a negative correlation with the relative abundance of the genera Clostridium_sensu_stricto_1, Prevotella and unclassified_f_Oscillospiraceae which are responsible for SCFA production. This study provided a theoretical basis for the fficient utilization of kitchen wastewater.

Dark fermentation: Production and utilization of volatile fatty acid from different wastes- A review

Volatile fatty acids (VFAs) are the building blocks of the chemical industry, and they are the primary contributors to the planet's organic carbon cycle. VFA production from fossil fuels (mostly petroleum) is unsustainable, pollutes the environment, and generates greenhouse gases. As a result of these issues, there is a pressing need to develop alternate sources for the long-term generation of VFAs via anaerobic digestion. The accessible feedstocks for its sustainable production, as well as the influencing parameters, are discussed in this review. The use of VFAs as a raw material to make a variety of consumer products is reviewed in order to find a solution. It also bridges the gap between traditional and advanced VFA production and utilization methods from a variety of solid and liquid waste sources for economical stability.

Production of volatile fatty acid from fruit waste by anaerobic digestion at high organic loading rates: Performance and microbial community characteristics

This study examined the performance and microbial community dynamics of an anaerobic volatile fatty acid (VFA) production reactor for treating fruit waste by stepwise increasing organic loading rates (OLRs) from 8 to 24 g volatile total solids (VTS)/(L·d). Results showed that higher VFA concentrations of 52.25-61.90 g chemical oxygen demand (COD)/L can be maintained at each OLR, thereby resulting to a production of 0.70-0.76 g chemical oxygen demand (COD)_VFA/g VTS. Notably, an increase in OLR from 8 to 14 g VTS/(L·d) was beneficial for achieving higher VFA concentrations and yields. Moreover, an increase in OLR affected the VFA distribution significantly; acetate and butyrate became dominant in the fermentation liquid at OLRs ≥ 14 g VTS/(L·d). Microbial community dynamics analysis revealed that phyla Firmicutes and Actinobacteriota were predominant at each OLR, and the genera Lactobacillus, Clostridium_sensu_stricto_12, and Caproiciproducens were closely related to anaerobic VFA production.

Effects of Turning Frequency on Ammonia Emission during the Composting of Chicken Manure and Soybean Straw

Here, we investigated the impact of different turning frequency (TF) on dynamic changes of N fractions, NH3 emission and bacterial/archaeal community during chicken manure composting. Compared to higher TF (i.e., turning every 1 or 3 days in CMS1 or CMS3 treatments, respectively), lower TF (i.e., turning every 5 or 7 days in CMS5 or CMS7 treatments, respectively) decreased NH3 emission by 11.42-18.95%. Compared with CMS1, CMS3 and CMS7 treatments, the total nitrogen loss of CMS5 decreased by 38.03%, 17.06% and 24.76%, respectively. Ammonia oxidizing bacterial/archaeal (AOB/AOA) communities analysis revealed that the relative abundance of Nitrosospira and Nitrososphaera was higher in lower TF treatment during the thermophilic and cooling stages, which could contribute to the reduction of NH3 emission. Thus, different TF had a great influence on NH3 emission and microbial community during composting. It is practically feasible to increase the abundance of AOB/AOA through adjusting TF and reduce NH3 emission the loss of nitrogen during chicken manure composting.

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.

Valorization of municipal organic waste into purified lactic acid

Municipal organic waste (biowaste) consists of food derived starch, protein and sugars, and lignocellulose derived cellulose, hemicellulose, lignin and pectin. Proper management enables nutrient recycling and sustainable production of platform chemicals such as lactic acid (LA). This review gathers the most important information regarding use of biowaste for LA fermentation covering pre-treatment, enzymatic hydrolysis, fermentation and downstream processing to achieve high purity LA. The optimal approach was found to treat the two biowaste fractions separately due to different pre-treatment and enzyme needs for achieving enzymatic hydrolysis and to do continues fermentation to achieve high cell density and high LA productivity up to 12 g/L/h for production of both L and D isomers. The specific productivity was 0.4 to 0.5 h^-1 but with recalcitrant biomass, the enzymatic hydrolysis was rate limiting. Novel purification approaches included reactive distillation and emulsion liquid membrane separation yielding purities sufficient for polylactic acid production.

Biochar amendment to advance contaminant removal in anaerobic digestion of organic solid wastes: A review

Anaerobic digestion (AD) has been widely applied to convert organic solid wastes into biogas, a renewable energy, and digestate, a bio-fertilizer, to sustain waste management. Nevertheless, several vexing contaminants in OSWs restrict digestate application in agriculture. Biochar has been evidenced to effectively improve AD by promoting organic biodegradation and alleviating the accumulation of inhibitory substances (e.g. ammonia and volatile fatty acids). Furthermore, biochar could advance contaminant removal in AD given its highly porous, conductive and alkaline features. Thus, this review aims to highlight the role of biochar amendment to advance contaminant removal in AD of OSWs. Key contaminants, such as antibiotics, heavy metals, microplastics, polycyclic aromatic hydrocarbons, furfural and 5-hydroxy methyl furfural (5-HMF) that ubiquitously present in OSWs were demonstrated. The underlying mechanisms of biochar to amend the removal of these contaminants by AD were discussed. Furthermore, future perspectives to the development of biochar-assisted AD for OSWs treatment were provided.

Promote lactic acid production from food waste fermentation using biogas slurry recirculation

It is a promising method to recover lactic acid from food waste (FW) fermentation, but the bottleneck problem is the low yield when using mixed inoculation. In this study, laboratorial biogas slurry (LBS) and industrial biogas slurry (IBS) were used as the additive in semi-continuous FW fermentation, aiming to promote the production of lactic acid. According to the research results, the addition of LBS or IBS promoted the production of lactic acid significantly from FW, especially carbohydrate, because it increased the pH values, maintained low OPR levels, and increased microbial number and diversity in the fermentation systems. IBS performed better than LBS because of higher pH, more diverse microbial community and more functional microorganisms. The best ratio of IBS to feedstock was 0.2, and the lactic acid yield reached 0.42 g/gVS_added. An excessively high dose would alter the fermentation pathways, reduce the ratio of lactic acid.

Cultivation of edible filamentous fungus Aspergillus oryzae on volatile fatty acids derived from anaerobic digestion of food waste and cow manure

In a circular economy approach, edible filamentous fungi (single cell protein) can be cultivated on volatile fatty acids (VFAs) derived from anaerobic digestion (AD) of organic-rich waste streams. In this study, the effect of pH, concentration/distribution of VFAs, nutrient supplementation, and type of waste on Aspergillus oryzae cultivation on synthetic VFAs, and actual VFAs derived from AD of food waste and cow manure were investigated. The optimal pH for A. oryzae growth on VFAs were 6 and 7 with maximum acetic acid consumption rates of 0.09 g/L.h. The fungus could thrive on high concentrations of acetic (up to 9 g/L) yielding 0.29 g dry biomass/gVFAs_fed. In mixed VFAs cultures, A. oryzae primarily consumed caproic and acetic acids reaching a biomass yield of 0.26 g dry biomass/gVFAs_fed (containing up to 41% protein). For waste-derived VFAs at pH 6, the fungus successfully consumed 81-100% of caproic, acetic, and butyric acids.

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.

Inoculum microbiome composition impacts fatty acid product profile from cellulosic feedstock

Conversion of organic wastes to fatty acids rather than methane through anaerobic digestion-based technologies has considerable promise. However, the relationships between microbiome structure and fatty acids produced from cellulosic feedstocks are not well understood. This study investigated the nature of those relationships for anaerobic digester sludge, bison rumen, and cattle rumen inocula grown on cellulose. Acetic acid production was highest in anaerobic sludge reactors, while propionic acid production was highest in cattle rumen reactors. Butyric and pentanoic acid were produced at the highest rates in bison rumen before Day 5. Reactor microbiomes remained distinct, despite identical operating conditions. Novel associations linked Alistipes with butyric acid production and Eubacterium nodatum and Clostridiales bacterium with pentanoic acid production. This study provides new insights into the ability of microbiomes to convert cellulose to different fatty acid mixtures and adds impetus for the rewiring of anaerobic digestion to generate high-value products.

Synergistic association between cytochrome bd-encoded Proteiniphilum and reactive oxygen species (ROS)-scavenging methanogens in microaerobic-anaerobic digestion of lignocellulosic biomass

Intermittent (every other day) microaerobic [picomolar oxygen by oxidation-reduction potential (ORP) set at +25 mV above anaerobic baseline] digestion of lignocellulosic biomass showed higher digestibility and better stability at a high organic loading rate (OLR) of 5 g volatile solids (VS)/L/d than that under strict anaerobic conditions. However, the microbial mechanisms supporting the delicate balance under microaeration remain underexplored. On the basis of our previous findings that microbial communities in replicate experiments were dominated by strains of the genus Proteiniphilum but contained diverse taxa of methanogenic archaea, here we recovered related genomes and reconstructed the putative metabolic pathways using a genome-centric metagenomic approach. The highly enriched Proteiniphilum strains were identified as efficient cellulolytic facultative bacterium, which directly degraded lignocellulose to carbon dioxide, formate, and acetate via aerobic respiration and anaerobic fermentation, alternatively. Moreover, high oxygen affinity cytochromes, bd-type terminal oxidases, in Proteiniphilum strains were found to be closely associated with such picomolar oxygen conditions, which has long been overlooked in anaerobic digestion. Furthermore, hydrogenotrophic methanogenesis was the prevalent pathway for methane production while Methanosarcina, Methanobrevibacter, and Methanocorpusculum were the dominant methanogens in the replicate experiments. Importantly, the two functional groups, namely cellulolytic facultative Proteiniphilum strains and methanogens, encoded various antioxidant enzymes. Energy-dependent reactive oxygen species (ROS) scavengers (superoxide reductase (SOR) and rubrerythrin (rbr) were ubiquitously present in different methanogenic taxa in response to replicate-specific ORP levels (-470, -450 and -475 mV). Collectively, cytochrome bd oxidase and ROS defenders may play roles in improving the digestibility and stability observed in intermittent microaerobic digestion.

Rapid Two Stage Anaerobic Digestion of Nejayote through Microaeration and Direct Interspecies Electron Transfer

Corn is one of the main food products in Mexico. The elaboration of corn-derived products generates wastewater with a high organic load (nejayote). Anaerobic digestion is an indicated treatment for wastewater with high organic loads. The results of this study show that the application of microaeration in the hydrolysis-fermentative reactor increased the percentage of volatile fatty acids (VFA) available in the medium by 62%. The addition of a conductive material, such as granulated activated carbon (GAC), promotes DIET (Direct interspecies electrons transfer) in the methanogenic UASB reactor increasing the methane yield by 55%. Likewise, a great diversity of exoelectrogenic bacteria, with the ability to donate electrons DIET mechanisms, were developed in the GAC biofilm, though interestingly, Peptoclostridium and Clostridium (17.3% and 12.75%, respectively) were detected with a great abundance in the GAC biofilm. Peptoclostridium has not been previously reported as a participant in DIET process.

Using air instead of biogas for mixing and its effect on anaerobic digestion of animal wastewater with high suspended solids

This study proposed a new mixing method for anaerobic digestion treating animal wastewater using air as gas source of agitation (named "air mixing") and demonstrated its feasibility by comparing with other mixing modes. The results indicated that the methane production for air mixing was increased by 6.4%, 11.9% and 19.6% compared with biogas mixing, mechanical mixing and no mixing. Air mixing improved the mass transfer and the homogeneous mixing time was shortened from 10 min of mechanical mixing to 1.5 min at the same power input. A transient microaerobic environment was created by air mixing, which increased the hydrolysis efficiency by 1.7-11.4% compared with biogas mixing and facilitated VFAs generation and consumption, as well as promoted the syntrophic relationship between facultative bacteria and hydrogenotrophic methanogens. The relative contribution of the improvement of mass transfer and the reaction of microaerobic environment to methane production was 62.9% and 37.1%, respectively.

Product Concentration, Yield and Productivity in Anaerobic Digestion to Produce Short Chain Organic Acids: A Critical Analysis of Literature Data

In order to make anaerobic digestion-based processes for short chain organic acid (SCOA) production attractive, the key performance variables, i.e., concentration, yield, and productivity of the produced SCOAs need to be maximised. This study analysed recent literature, looking for the effect of process operating parameters (feed concentration, pH, temperature, and residence time) on the performance variables. Data from 551 experiments were analysed. Mean values of the SCOA concentration, yield, and productivity were 10 g l−1, 32% (chemical oxygen demand (COD) COD−1), and 1.9 g l−1 day−1, respectively. Feed concentration and residence time had the most important effect. Higher feed concentration corresponded to higher product concentration and productivity, but to lower yield. The mean feed concentration was 109 gCOD l−1 and 19 gCOD l−1 in the experiments with the highest product concentrations and in the experiments with the highest yields, respectively. Shorter residence times corresponded to higher productivity. The mean HRT (hydraulic residence time) in the experiments with the highest productivities was 2.5 days. Sequencing batch reactors gave higher values of the performance variables (mean values 29 g l−1, 41% COD COD−1, and 12 g l−1 day−1 for product concentration, yield, and productivity, respectively) than processes without phase separation.

Shifts of microbial community and metabolic function during food wastes and waste activated sludge co-fermentation in semi-continuous-flow reactors: Effects of fermentation substrate and zero-valent iron

The effects of food wastes (FW) composition and zero-valent iron (ZVI) on the volatile fatty acids (VFAs) generation, bacterial community succession and related metabolic functions during long-term FW and waste activated sludge (WAS) co-fermentation were investigated. The VFAs production in the carbohydrate-enriched reactor was approximately 3.0-folds of that in FW reactor. The ZVI contributed to the VFAs promotion by 3.6- and 6.7-folds in carbohydrate-enriched and FW reactors, respectively. Firmicutes (20.1-74.7%), Actinobacteria (0.9-26.3%), Bacteroidetes (3.4-65.7%), and Proteobacteria (9.1-28.5%) were the main bacteria in different fermentation reactors, and they were closely associated with the fermentation substrates and ZVI. Further analysis demonstrated that the key metabolic capacity (i.e. amino acid, carbohydrate and energy metabolism) and the genetic expressions of enzymes (i.e. fabA, fabZ, accA and accB) involved in VFAs generation were also related to FW composition, and were improved by the ZVI, which accounted for the significant VFAs promotion.

Microbial community in an anaerobic membrane bioreactor and its performance in treating organic solid waste under controlled and deteriorated conditions

The adoption of anaerobic membrane bioreactors (AnMBRs) for organic solid waste management is important for the recovery of energy and high-quality treated water. However, few studies have focused on AnMBR treatment of high-strength organic solid waste and the microorganisms involved under deteriorated operating conditions. In the present study, a 15-L bench-scale AnMBR was operated using a model slurry of high-strength organic solid waste with the organic loading rate (OLR) increasing from 2.3 g chemical oxygen demand (COD) L^-1 day^-1 (represented as a controlled condition) to 11.6 g COD L^-1 day^-1 (represented as a deteriorated condition), and microbial community dynamics over 120 days of operation were analyzed. The abundances of methanogens and bacteria that were dominant under the controlled condition decreased as a result of both high organic loading and sludge withdrawal under the deteriorated condition and did not recover thereafter. Instead, numbers of putative volatile fatty acid (VFA)-producing bacterial operational taxonomic units (OTUs) related to the genus Prevotella increased rapidly, reaching a relative abundance of 43.2%, leading to the deterioration of methanogenic AnMBR operation. Considering that the sequences of these OTUs exhibited relatively low sequence identity (91-95%) to those of identified Prevotella species, the results strongly suggest that the accumulation of VFAs by novel VFA-producing bacteria in the digestion sludge promotes the disruption of the methanogen community under deteriorated conditions.

Simultaneous H2S mitigation and methanization enhancement of chicken manure through the introduction of the micro-aeration approach

The impact on H₂S alleviation and methane yield enhancement after submitting the anaerobic digestion of chicken manure to a finite amount of air was investigated. The largest reduction in the H₂S biogas content (58% lower) occurred when air intensity of 30 ml/g VS_in was injected into the reactors. Consequently, a maximum methane yield (335 mL-g VS_in^-1), which was 77% higher than the control, was concurrently achieved. Slight sulfate accumulation (<330 mg L^-1) was observed inside the micro-aerated digesters with higher air intensities, suggesting a suppression of sulfide inhibition. Bacterial diversity/richness was enhanced in these digesters while the relative abundance of Methanocelleus increased by 36%. The most important contributing factor to enhancement was the synergistic effect resulting from increments in the hydrolysis rate and the suppression of sulfide inhibition. The results highlighted the potential of in situ H₂S mitigation with the added benefit of methane yield enhancement.

Different micro-aeration rates facilitate production of different end-products from source-diverted blackwater

The effects of micro-aeration on the performance of anaerobic sequencing batch reactors (ASBR) for blackwater treatment were investigated in this study. Different micro-aeration rates, 0, 5, 10, 50, and 150 mg O₂/L-reactor/cycle, and their effect on the hydrolysis, acidogenesis, and methanogenesis of blackwater were evaluated and compared at ambient temperature. Source-diverted blackwater (toilet water) contains high organic contents which can be recovered as biogas. Previous studies have found that anaerobic digestion of blackwater without micro-aeration can only recover upwards of less than 40% of chemical oxygen demand (COD) to methane at room temperature due to the low hydrolysis rate of biomass content in blackwater. This study achieved increases in blackwater hydrolysis (from 34.7% to 48.7%) and methane production (from 39.6% to 50.7%) with controlled micro-aeration (5 mg O₂/L-reactor/cycle). The microbial analysis results showed that hydrolytic/fermentative bacteria and acetoclastic methanogens (e.g. Methanosaeta) were in higher abundances in low-dose micro-aeration reactors (5 and 10 mg O₂/L-reactor/cycle), which facilitated syntrophic interactions between microorganisms. The relative abundance of oxygen-tolerant methanogen such as Methanosarcina greatly increased (from 1.5% to 11.4%) after oxygen injection. High oxygen dosages (50 and 150 mg O₂/L-reactor/cycle) led to reduced methane production and higher accumulation of volatile fatty acids, largely due to the oxygen inhibition on methanogens and degradation of organic matters by aerobic growth and respiration, as indicated by the predicted metagenome functions. By combining reactor performance results and microbial community analyses, this study demonstrated that low-dose micro-aeration improves blackwater biomethane recovery by enhancing hydrolysis efficiency and promoting the development of a functional microbial population, while medium to high-dose micro-aeration reduced the activities of certain anaerobes. It was also observed that medium-dose micro-aeration maximizes VFA accumulation, which may be used in two-stage anaerobic digesters.

Enhanced hydrogen production from food waste dark fermentation by potassium ferrate pretreatment

Hydrogen generation from food waste anaerobic dark fermentation is identified as a promising strategy for resource recovery. In this work, an innovative strategy of using potassium ferrate (PF), a strong oxidant, to promote anaerobic dark fermentation of food waste to produce hydrogen has been reported. The experimental results revealed that PF enhanced the hydrogen production from food waste, the maximal hydrogen yield was 173.5 mL/g, and the optimal PF dosage was 0.4 g/g total suspended solids. PF shortened the lag phase for hydrogen generation from 120 to 96 h. Mechanisms investigation revealed that PF accelerated the disintegration of organic compounds and increased the soluble organic matter in the liquid phase. The strong oxidation of PF inhibited the processes of hydrolysis, acidification, acetogenesis, homoacetogenesis, and methanogenesis by using synthetic wastewater in the fermentation process. The inhibition of PF on these processes was further verified by the enzyme activity analysis. Economic analysis indicated that 0.1 g/g PF was the optimal dosage. PF treatment is a promising strategy to enhance the production of hydrogen from food waste dark fermentation.

Effect of pH, substrate loading, oxygen, and methanogens inhibitors on volatile fatty acid (VFA) production from citrus waste by anaerobic digestion

Citrus waste from e.g., juice production is a potential substrate for anaerobic digestion (AD). However, due to the toxic citrus peel oil content, citrus waste has several challenges in biogas production. Hence, volatile fatty acids (VFAs) are very interesting intermediate products of AD. This paper was aimed to investigate VFA production from citrus wastes by boosting its production and inhibiting methane formation. Therefore, the effects of inoculum to substrate ratio (ISR), O₂ presence, pH, and inhibitor for methanogens, in VFA production from citrus waste through acidification process were studied. The addition of 2 g/L methanogens inhibitor and the presence of O₂ in the reactors were able to reduce methane production. The highest yield of VFA (0.793 g VFA/g VS_added) was achieved at controlled pH at 6 and low substrate loading (ISR 1:1). Acetic acid (32%), caproic acid (21%), and butyric acid (15%) dominate the VFA composition in this condition.

Two Phase Anaerobic Digestion System of Municipal Solid Waste by Utilizing Microaeration and Granular Activated Carbon

In an anaerobic digestion (AD) process, the hydrolysis phase is often limited when substrates with high concentrations of solids are used. We hypothesized that applying micro-aeration in the hydrolysis phase and the application of granular activated carbon (GAC) in the methanogenesis phase could make the AD process more efficient. A packed bed reactor (PBR) coupled with an up-flow anaerobic sludge blanket (UASB) was conducted, and its effects on methane generation were evaluated. The micro-aeration rate applied in PBR was 254 L-air/kg-Total solids (TS)-d was compared with a control reactor. Micro-aeration showed that it reduced the hydrolysis time and increased the organic matter solubilization as chemical oxygen demand (COD) increasing 200%, with a volatile fatty acids (VFAs) increment higher than 300%, compared to the control reactor (without aeration). Our findings revealed that the implementations of microaeration and GAC in the two-phase AD system could enhance methane production by reducing hydrolysis time, increasing solid waste solubilization.

Co-production of DHA and squalene by thraustochytrid from forest biomass

Omega-3 fatty acids, and specifically docosahexaenoic acid (DHA), are important and essential nutrients for human health. Thraustochytrids are recognised as commercial strains for nutraceuticals production, they are group of marine oleaginous microorganisms capable of co-synthesis of DHA and other valuable carotenoids in their cellular compartment. The present study sought to optimize DHA and squalene production by the thraustochytrid Schizochytrium limacinum SR21. The highest biomass yield (0.46 g/g_substrate) and lipid productivity (0.239 g/g_substrate) were observed with 60 g/L of glucose, following cultivation in a bioreactor, with the DHA content to be 67.76% w/w_{total lipids}. To reduce costs, cheaper feedstocks and simultaneous production of various value-added products for pharmaceutical or energy use should be attempted. To this end, we replaced pure glucose with organosolv-pretreated spruce hydrolysate and assessed the simultaneous production of DHA and squalene from S. limacinum SR21. After the 72 h of cultivation period in bioreactor, the maximum DHA content was observed to 66.72% w/w_{total lipids} that was corresponded to 10.15 g/L of DHA concentration. While the highest DHA productivity was 3.38 ± 0.27 g/L/d and squalene reached a total of 933.72 ± 6.53 mg/L (16.34 ± 1.81 mg/g_CDW). In summary, we show that the co-production of DHA and squalene makes S. limacinum SR21 appropriate strain for commercial-scale production of nutraceuticals.

Inoculum Source Determines Acetate and Lactate Production during Anaerobic Digestion of Sewage Sludge and Food Waste

Acetate production from food waste or sewage sludge was evaluated in four semi-continuous anaerobic digestion processes. To examine the importance of inoculum and substrate for acid production, two different inoculum sources (a wastewater treatment plant (WWTP) and a co-digestion plant treating food and industry waste) and two common substrates (sewage sludge and food waste) were used in process operations. The processes were evaluated with regard to the efficiency of hydrolysis, acidogenesis, acetogenesis, and methanogenesis and the microbial community structure was determined. Feeding sewage sludge led to mixed acid fermentation and low total acid yield, whereas feeding food waste resulted in the production of high acetate and lactate yields. Inoculum from WWTP with sewage sludge substrate resulted in maintained methane production, despite a low hydraulic retention time. For food waste, the process using inoculum from WWTP produced high levels of lactate (30 g/L) and acetate (10 g/L), while the process initiated with inoculum from the co-digestion plant had higher acetate (25 g/L) and lower lactate (15 g/L) levels. The microbial communities developed during acid production consisted of the major genera Lactobacillus (92–100%) with food waste substrate, and Roseburia (44–45%) and Fastidiosipila (16–36%) with sewage sludge substrate. Use of the outgoing material (hydrolysates) in a biogas production system resulted in a non-significant increase in bio-methane production (+5–20%) compared with direct biogas production from food waste and sewage sludge.