Effect of pH on lactic acid production from acidogenic fermentation of food waste with different types of inocula

Lactate production from lactose-rich wastewater: A comparative study on reactor configurations to maximize conversion rates and efficiencies

About 90 % of global lactate production is derived from bacterial fermentation of sugars via pure homofermentative cultures in batch mode. Acid whey, which is a lactose-rich wastewater from the yogurt industry, can be used as an alternative substrate for commercial lactate production. Operating reactor microbiomes reduces the lactate production costs by circumventing sterilization, while continuous operation with biomass retention achieves higher productivity at shorter production times. To find the best reactor configuration with biomass retention for lactate production from acid whey, we operated three different reactor configurations: (1) an upflow anaerobic sludge blanket (UASB) reactor; (2) an anaerobic filter reactor (AFR); and (3) an anaerobic continuously stirred tank reactor (CSTR) with a hollow-fiber membrane module. We operated at different hydraulic retention times (HRTs) to find the optimum production parameters at a temperature of 50 °C and a pH of 5.0. We did not use an inoculum but enriched the endogenous D-lactate-producing Lactobacillus spp. that later dominated the reactor microbiomes (> 90 % relative abundance). Undissociated lactic acid concentrations of more than 60 mmol C L-1 inhibited the microbiomes. We alleviated the inhibition effect by shortening the HRT to 0.6 days and using diluted acid-whey substrate (1.67-fold dilution) to achieve almost complete conversion of the acid-whey sugars to lactate. At the 0.6-day HRT, the AFR and CSTR performed better than the UASB reactor due to their better cell retention abilities. During the period between Day 365-384, we experienced an error in the pH control of the CSTR system during which the pH value dropped to 4.3. After this pH-error period, the lactose and galactose-into-lactate (LG-into-LA) conversion efficiency for the CSTR considerably improved and surpassed the AFR. We achieved the highest lactate conversion rate of 1256 ± 46.3 mmol C L-1 d-1 (1.57 ± 0.06 g L-1 h-1) at a LG-into-LA conversion efficiency of 82.2 ± 3.4 % (in mmol C), with a yield of 0.85 ± 0.02 mmol C mmol C-1 (product per consumed substrate) for the CSTR.

Converting methanogenesis into chain elongation for caproic acid production from food waste: Roles of pH and hydraulic retention time

Caproic acid production from food waste is a promising pathway for waste valorization, but how to restrict methanogenesis and enrich chain elongators during long-term continuous operation by adjusting operating parameters was not fully revealed. In this study, the impacts of pH and hydraulic retention time (HRT) on methane production and chain elongation were investigated. Results showed that pH 6 and 5.5 at long HRT would result in methanogenesis without caproic acid production. After a long-term acclimation at pH 5, methanogens could be effectively restricted, and chain elongation could be activated. When pH further increased to 5.5 at an HRT of 50 days, caproic acid content could be promoted to 52.94 mmol/L by enriching Caproiciproducens as the dominant chain elongators. However, short HRT (25 days) will cause the accumulation of lactic acid bacteria and finally reduce the caproic acid yield. This study proposed an effective pathway for establishing chain elongation from pit mud through operation adjustment, which provided useful information for caproic acid production from organic wastes.

Fermentation of the organic fraction of municipal solid waste under different pH values and composition of microbial communities

The organic fraction of municipal solid waste (OFMSW) must be stored for hours or days before being fed to the anaerobic digestion reactors. This storage leads to spontaneous lactic acid fermentation, and volatile fatty acids (VFAs) and ethanol are produced by naturally occurring microorganisms. This research deals with fermentation and hydrolysis by controlling the OFMSW storage (silage) conditions. Using only naturally occurring microorganisms as inoculum, OFMSW fermentation in a semi-continuous reactor at pH values of 4, 5, and 6 was performed. During 6 days, samples were collected and analyzed daily for VFAs, ethanol, and lactic acid. At pH 4, the main products were ethanol, lactic acid, and acetic acid; at pH 5, lactic acid predominated, decreasing after day 4; at pH 6, acetic acid formed rapidly and after day 1, the concentration remained constant. At pH 6, butyric acid reached the highest concentration of all VFAs. The microbial diversity increased with pH. Metataxonomic analysis supports the possibility that the fungus of the Pichia genus is responsible for ethanol production and that various bacteria are responsible for VFAs, lactic acid production, and acetogenesis. Acetogenesis was the main pathway for the decrease in lactic acid and ethanol over time.

Pre-fermentation and filtration pretreatments enhance hydrogen production from food waste through microbial electrolysis

Bioavailable organic-rich food waste (FW) is a promising feedstock for renewable hydrogen production. However, its highly suspended and complex nature presents substantial challenges for producing high-purity hydrogen in dual-chamber microbial electrolysis cells (MECs). This study examined the effects of pretreating FW through pre-fermentation and/or filtration on its microbial electrolysis. Both methods enhanced the exoelectrogenic utilization of FW, with pre-fermentation being especially effective by conditioning substrate composition, while filtration alone was less advantageous due to associated energy loss. The MECs fed with pre-fermented FW exhibited significantly higher performances, achieving the highest hydrogen yield of 1,029 mL/g chemical oxygen demand fed (39.1 % increase over raw FW) when pre-fermentation was followed by filtration. Bioanodes across all MECs were dominated by exoelectrogenic bacteria, mainly Geobacter and Desulfovibrio, with significantly greater abundance observed with pre-fermentation. These findings highlight the value of pretreatment, particularly pre-fermentation, and warrant further optimization research to maximize FW conversion into hydrogen.

Performance improvements and increased ruminal microbial interactions in Angus heifers via supplementation with native rumen bacteria during high-grain challenge

Feedlot cattle may be subjected to digestive disorders, including ruminal acidosis, due to high concentration of grain in their diet. Therefore, novel feeding strategies are required to maximize animal performance and mitigate economic losses in the operation. This study employed a two-period crossover design to assess the effect of direct ruminal administration of native rumen microorganisms (NRM) inoculation on cattle that underwent a high-grain challenge. The NRM inoculation consisted of six microorganisms (1.70 M CFU /day/animal) isolated from the rumen of healthy feedlot cattle: Succinivibrio dextrinosolvens ASCUSBF53, Prevotella albensis ASCUSBF41, Chordicoccus furentiruminis ASCUSBF65, Bacteroides xylanisolvens ASCUSBF52, Clostridium beijerinckii ASCUSBF26, and Syntrophococcus sp. ASCUSBF60. The trial consisted of 16 Angus heifers receiving NRM (n = 8) or a CON (CON = Carrier Buffer; n = 8) inoculation daily for 14-days as pre-challenge while on a high-grain diet and continued daily for a 21-day treatment period. The combined 35 days of microbial supplementation resulted in an improved average daily gain (ADG) of 29% (P = 0.037) and a tendency toward a 19% decrease in the feed efficiency metric, gain to feed ratio (G: F) (P = 0.055). Additionally, administration of NRM to animals on a high-grain diet, improved ruminal microbiome stability (P < 0.001), potentially encouraging the conversion of rumen lactate to propionate over time via the succinate pathway and alleviating metabolic stress.

Lactic acid production from food waste: Advances in microbial fermentation and separation technologies

China generates over 100 million tons of food waste annually, leading to significant environmental pollution and health risks if not managed properly. Converting FW into a high-value-added platform molecule, lactic acid (LA), through fermentation offers a promising approach for both waste treatment and resource recovery. This paper presents a comprehensive review of recent advancements in LA production from FW, focusing on pure strains fermentation and open fermentation technologies, metabolic mechanisms, and problems in fermentation. It also assesses purification methods, including molecular distillation, adsorption, membrane separation, precipitation, esterification and hydrolysis, solvent extraction, and in-situ separation, analyzing their efficiency, advantages, and disadvantages. However, current research encounters several challenges, including low LA yield, low optical purity of L-(+)-LA, and difficulties in the separation and purification of LA. The integration of in-situ separation technology coupled with multiple separation methods is highlighted as a promising direction for future advancements.

Promotion of polyhydroxyalkanoates-producing granular sludge formation by lactic acid using anaerobic dynamic feeding process

To promote the formation of granular sludge with high polyhydroxyalkanoates (PHAs) synthesis ability, an anaerobic dynamic feeding process (AnDF) was proposed. This process combines the feast-famine mode with an anaerobic plug flow feeding process and involving variations in cycle length and settling time. The effects of lactic acid (LA) content (0 %, 20 %, and 40 % COD) on sludge granulation and PHAs production were investigated using three AnDF reactors (R1, R2, and R3). The results showed that the AnDF process feeding with LA not only effectively promoted sludge granulation but also improved its PHAs synthesis ability. The granules were quickly observed in R3 after 50 days of cultivation, with an average diameter of 0.69 mm. The maximum PHAs content reached 47.0 wt% in R3, representing a 30.09 % increase compared to R1. Additionally, extracellular polymeric substances (EPS)-producing bacteria observed in granular sludge may be the prime drivers of the formation of PHAs-producing granular sludge (PHAGS), which was defined as granular sludge with an average particle size larger than 0.30 mm and PHAs content above 40 % cell dry weight (CDW) of sludge samples.

Bioprocesses for lactic acid production from organic wastes toward industrialization-a critical review

Lactic acid (LA) is a crucial chemical which has been widely used for industrial application. Microbial fermentation is the dominant pathway for LA production and has been regarded as the promising technology. In recent years, many studies on LA production from various organic wastes have been published, which provided alternative ways to reduce the LA production cost, and further recycle organic wastes. However, few researchers focused on industrial application of this technology due to the knowledge gap and some uncertainties. In this review, the recent advances, basic knowledge and limitations of LA fermentation from organic wastes are discussed, the challenges and suitable envisaged solutions for enhancing LA yield and productivity are provided to realize industrial application of this technology, and also some perspectives are given to further valorize the LA fermentation processes from organic wastes. This review can be a useful guidance for industrial LA production from organic wastes on a sustainable view.

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

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

Lactic acid production with two types of feedstocks from food waste: Effect of inoculum, temperature, micro-oxygen, and initial pH

Lactic acid (LA) is an important chemical with broad market applications. To optimize LA production, food waste has been explored as feedstock. Due to the wide variety of food waste types, most current research studies have obtained different conclusions. This study focuses on carbohydrate-rich fruit and vegetable waste (FVW) and lipid-rich kitchen waste (KW), and the effect of inoculum, temperature, micro-oxygen, and initial pH were compared. FVW has a greater potential for LA production than KW. As an inoculum, lactic acid bacteria (LAB) significantly increased the maximum LA concentration (27.6 g/L) by 50.8 % compared with anaerobic sludge (AS). FVW exhibited optimal LA production at 37 °C with micro-oxygen. Adjustment of initial pH from 4 to 8 alleviated the inhibitory effect of accumulated LA, resulting in a 46.2 % increase in maximum LA production in FVW. The expression of functional genes associated with metabolism, genetic information processing, and environmental information processing was higher at 37 °C compared to 50 °C.

Synergistic effects of peracetic acid and free ammonia pretreatment on anaerobic fermentation of waste activated sludge to promote short-chain fatty acid production for polyhydroxyalkanoate biosynthesis: Mechanisms and optimization

Peracetic acid (PAA) combined with free ammonia (FA) pretreatment can be utilized to promote anaerobic fermentation (AF) of waste activated sludge (WAS) to produce short-chain fatty acids (SCFAs), and the resulting SCFAs are desirable carbon sources (C-sources) for polyhydroxyalkanoate (PHA) biosynthesis. This work aimed to determine the optimum conditions for PAA + FA pretreatment of sludge AF and the feasibility of using anaerobic fermentation liquor (AFL) for PHA production. To reveal the mechanisms of integrated pretreatment, the impacts of PAA + FA pretreatment on different stages of sludge AF and changes in the microbial community structure were explored. The experimental results showed that the maximum SCFA yield reached 491.35 ± 6.02 mg COD/g VSS on day 5 after pretreatment with 0.1 g PAA/g VSS +70 mg FA/L, which was significantly greater than that resulting from PAA or FA pretreatment alone. The mechanism analysis showed that PAA + FA pretreatment promoted sludge solubilization but strongly inhibited methanogenesis. According to the analysis of the microbial community, PAA + FA pretreatment changed the microbial community structure and promoted the enrichment of bacteria related to hydrolysis and acidification, and Proteiniclasticum, Macellibacteroides and Petrimonas became the dominant hydrolytic and acidifying bacteria. Finally, after alkali treatment, the AFL was utilized for batch-mode PHA production, and a maximum PHA yield of 55.05 wt% was achieved after five operation periods.

Enhanced lactic acid production from household food waste under hyperthermophilic conditions: Mechanisms and regulation

An annual production of about 500 million tons of household food waste (HFW) has been documented, resulting in significant implications for human health and the environment in the absence of appropriate treatment. The anaerobic fermentation of HFW in an open system offers the potential to recover high value-added products, lactic acid (LA), thereby simultaneously addressing waste treatment and enhancing resource recovery efficiency. Most of LA fermentation studies have been conducted under mesophilic and thermophilic conditions, with limited research on the production of LA through anaerobic fermentation under hyperthermophilic conditions. This study aimed to produce LA through anaerobic fermentation from HFW under hyperthermophilic conditions (70 ± 1 °C), while varying pH values (5.0 ± 0.1, 7.0 ± 0.1, and 9.0 ± 0.1), and compare the results with LA production under mesophilic (35 ± 1 °C) and thermophilic (52 ± 1 °C) conditions. The findings of this study indicated that the combination of hyperthermophilic conditions and a neutral pH (pH7_70) yielded the highest concentration of LA, measuring at 17.75 ± 1.51 g/L. The mechanism underlying the high yield of LA at 70 °C was elucidated through the combined analysis of organics dissolution, enzymes activities, and 16S rRNA microbiome sequencing.

Hydrogen production promotion and energy saving in anaerobic co-fermentation of heat-treated sludge and food waste

The objective of this paper is to gain insights into the synergistic advantage of anaerobic co-fermentation of heat-treated sludge (HS) with food waste (FW) and heat-treated food waste (HFW) for hydrogen production. The results showed that, compared with raw sludge (RS) mixed with FW (RS-FW), the co-substrate of HS mixed with either FW (HS-FW) or HFW (HS-HFW) effectively promoted hydrogen production, with HS-HFW promoted more than HS-FW. The maximum specific hydrogen production (MSHP) and the maximum hydrogen concentration (MHC) of HS-HFW were 40.53 mL H2/g dry weight and 57.22%, respectively, and 1.21- and 1.45-fold as high as those from HS-FW. The corresponding fermentation was ethanol type for HS-HFW and butyric acid type for HS-FW. The net energy production from RS-FW and HS-FW was both negative, but it was positive (2.57 MJ) from 40% HFW addition to HS-HFW. Anaerobic fermentation was more viable for HS-HFW.

Top-down and bottom-up microbiome engineering approaches to enable biomanufacturing from waste biomass

Growing environmental concerns and the need to adopt a circular economy have highlighted the importance of waste valorization for resource recovery. Microbial consortia-enabled biotechnologies have made significant developments in the biomanufacturing of valuable resources from waste biomass that serve as suitable alternatives to petrochemical-derived products. These microbial consortia-based processes are designed following a top-down or bottom-up engineering approach. The top-down approach is a classical method that uses environmental variables to selectively steer an existing microbial consortium to achieve a target function. While high-throughput sequencing has enabled microbial community characterization, the major challenge is to disentangle complex microbial interactions and manipulate the structure and function accordingly. The bottom-up approach uses prior knowledge of the metabolic pathway and possible interactions among consortium partners to design and engineer synthetic microbial consortia. This strategy offers some control over the composition and function of the consortium for targeted bioprocesses, but challenges remain in optimal assembly methods and long-term stability. In this review, we present the recent advancements, challenges, and opportunities for further improvement using top-down and bottom-up approaches for microbiome engineering. As the bottom-up approach is relatively a new concept for waste valorization, this review explores the assembly and design of synthetic microbial consortia, ecological engineering principles to optimize microbial consortia, and metabolic engineering approaches for efficient conversion. Integration of top-down and bottom-up approaches along with developments in metabolic modeling to predict and optimize consortia function are also highlighted.

ONE-SENTENCE SUMMARY

This review highlights the microbial consortia-driven waste valorization for biomanufacturing through top-down and bottom-up design approaches and describes strategies, tools, and unexplored opportunities to optimize the design and stability of such consortia.

Acidogenic fermentation of food waste to generate electron acceptors and donors towards medium-chain carboxylic acids production

This study investigated the optimum pH, temperature, and food-to-microorganisms (F/M) ratio for regulating the formation of electron acceptors and donors during acidogenic fermentation to facilitate medium-chain carboxylic acids (MCCAs) production from food waste. Mesophilic fermentation at pH 6 was optimal for producing mixed volatile fatty acids (719 ± 94 mg COD/g VS) as electron acceptors. Under mesophilic conditions, the F/M ratio (g VS/g VS) could be increased to 6 to generate 22 ± 2 g COD/L of electron acceptors alongside 2 ± 0 g COD/L of caproic acid. Thermophilic fermentation at pH 6 was the best condition for producing lactic acid as an electron donor. However, operating at F/M ratios above 3 g VS/g VS under thermophilic settings significantly reduced lactic acid yield. A preliminary techno-economic evaluation revealed that converting lactic acid and butyric acid generated during acidogenic fermentation to caproic acid was the most profitable food waste valorization scenario and could generate 442-468 €/t VS/y. The results presented in this study provide insights into how to tailor acidogenic fermentation reactions to desired intermediates and will help maximize MCCAs synthesis.

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.

Caproic acid production from food waste using indigenous microbiota: Performance and mechanisms

Caproic acid (CA) production from food waste (FW) is a promising way for waste recycling, while the fermentation processes need further exploration. In this study, FW acidogenic fermentation under different pH (uncontrolled, 4, 5, 6) using indigenous microbiota was investigated. Result showed that substrate hydrolysis, carbohydrate degradation and acidogenesis increased with the increase of pH. Although various microbial communities were observed in FW, lactic acid bacteria (Lactobacillus and Limosilactobacillus) were enriched at pH lower than 6, resulting in lactic acid accumulation. CA (88.24 mM) was produced at pH 6 accounting for 31.23% of the total product carbon. The enriched lactic acid bacteria were directionally replaced by chain elongators (Caproicibacter, Clostridium_sensu_stricto, unclassified_Ruminococcaceae) at pH 6, and carbohydrates in FW were firstly transformed into lactic acid, then to butyrate and CA through lactate-based chain elongation processes. This work provided a novel CA fermentation pathway and further enriched the FW valorization.

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

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

Multi-omics analysis unravels effects of salt and oil on substance transformation, microbial community, and transcriptional activity in food waste anaerobic digestion

In this study, through quantitative detection of key substances and enzyme activities, an integrated analysis of 16S rRNA sequencing and metatranscriptomics revealed the mechanisms by which salt and oil influence the biotransformation process during anaerobic digestion (AD). The results demonstrated that a salt concentration of 6 g/L promoted lipid metabolism and hydrogenotrophic methanogenesis, while inhibiting the acetoclastic pathway. An oil concentration of 5 g/L facilitated the expression of key enzyme-encoding genes involved in β-oxidation of long-chain fatty acids, transcription, and acetoclastic methanogenesis. It also promoted the enrichment of syntrophic propionate/butyrate oxidation bacteria (Syntrophomonas and DMER64). Salt/oil co-addition enhanced the expression of genes related to glucose metabolism, amino acid metabolism, organic acid synthesis, and quorum sensing. Furthermore, salt/oil co-addition inhibited the secretion of key enzymes related to methanogens by impeding the transcription process. Collectively, these findings provide systematic insights into how salt and oil affect the biochemical metabolic mechanisms of AD.

Analysing the mechanism of food waste anaerobic digestion enhanced by iron oxide in a continuous two-stage process

The food waste (FW) digestion performance can be enhanced by introducing iron oxide (IO) into digesters. However, the role of IO in continuous two-stage digesters in enhancing the FW anaerobic digestion remains unclear. In this study, the effect of IO on the bioenergy recovery from a two-stage digestion process was investigated. The bioenergy recovery was significantly increased by up to 208.43 % with IO addition. The activities of dehydrogenase, α-amylase, and protease increase by 0.82-1.44, 7.24-14.56 and 7.97-20.45 times, respectively, as compared with that of the blank. With IO addition, the metabolic pathway in hydrolytic-acidogenic (HA) reactor shifted from lactic acid fermentation to butyric fermentation, which promoted stable methane production in methanogenic (MG) reactor. The activity of coenzyme F420 increased by 19.19-39.01 times, indicating that IO facilitated FW digestion by promoting hydrogenotrophic methanogenesis. The enhancement in the enzyme activity was attributable to the Fe2+ generated by dissimilatory iron reduction. According to the microbial analysis, IO enhanced interspecies hydrogen transfer between Methanobacterium and Syntrophomonas. Furthermore, IO improved direct interspecies electron transfer between Geobacter sulfurreducens and Methanosarcina. The effluent recirculation strategy greatly facilitated the hydrolysis and acidification of FW, which was critical for improving the two-stage process performance.

The role of hydrochloric acid pretreated activated carbon in chain elongation of D-lactate to caproate: Adsorption and facilitation

Medium chain fatty acids (MCFA) generation is attracting growing interest due to fossil fuel depletion. To promote the production of MCFA, especially caproate, hydrochloric acid pretreated activated carbon (AC) was introduced into chain elongation fermentation. In this study, the role of pretreated AC on caproate production was investigated using lactate and butyrate as electron donor and electron acceptor, respectively. The results showed that AC did not improve the chain elongation reaction at beginning but promoted the caproate production at later stage. The addition of 15 g/L AC facilitated reactor reaching the peak of caproate concentration (78.92 mM), caproate electron efficiency (63.13%), and butyrate utilization rate (51.88%). The adsorption experiment revealed a positive correlation between the adsorption capacity of pretreated AC and the concentration as well as the carbon chain length of carboxylic acids. Moreover, the adsorption of undissociated caproate by pretreated AC contributed to a mitigated toxicity towards microorganisms, thereby facilitating the production of MCFA. Microbial community analysis revealed an increasing enrichment of key functional chain elongation bacteria, including Eubacterium, Megasphaera, Caproiciproducens, and Pseudoramibacter, but a suppression on acrylate pathway microorganism Veillonella, as the dosage of pretreated AC increasing. The findings of this study demonstrated the substantial impact of the adsorption effect of acid-pretreated AC on promoting caproate production, which would aid to the development of more efficient caproate production process.

Fermentation broth of food waste: A sustainable electron donor for perchlorate biodegradation

Microbial reduction has been considered an effective way to remove perchlorate (ClO₄^-), during which, additional electron donors and carbon sources are required. This work aims to study the potential of fermentation broth of food waste (FBFW) serving as an electron donor for ClO₄^- biodegradation, and further investigates the variance of the microbial community. The results showed that FBFW without anaerobic inoculum at 96 h (F-96) exhibited the highest ClO₄^- removal rate of 127.09 mg/L/d, attributed to higher acetate and lower ammonium contents in the F-96 system. In a 5 L continuous stirred-tank reactor (CSTR), with a 217.39 g/m³·d ClO₄^- loading rate, 100% removal efficiency of ClO₄^- was achieved, indicating that the application of FBFW in the CSTR showed satisfactory performance for ClO₄^- degradation. Moreover, the microbial community analysis revealed that Proteobacteria and Dechloromonas contributed positively to ClO₄^- degradation. Therefore, this study provided a novel approach for the recovery and utilization of food waste, by employing it as a cost-effective electron donor for ClO₄^- biodegradation.

Valorisation of fresh waste grape through fermentation with different exogenous probiotic inoculants

The disposal of fresh waste grape berries restraining the sustainable development of vineyards. The aims of this study were to evaluate the effects of different exogenous probiotic inoculants on the fermentation of fresh waste grape berries. In the fermentation process, the variations of pH and EC value, chemical characteristics of the fermentation products, as well as the microbial communities' composition were simultaneously observed. In addition, the feasibility of using the fermentation products as chemical fertilizer substitute in agricultural production also has been verified in this study. The results indicated that the different probiotic inoculants has shown clear impacts on the variation trends of pH and EC value in the grape waste fermentation. Lactobacillus casei and Zygosaccharomyces rouxii are ideal probiotics for the fermentation of waste grape, which enhanced the contents of free Aa and other nutrients in fermentation products. Compared with Fn treatment (without exogenous inoculants), the total free Aa contents in Fs (inoculation with Z. rouxii) and Fm (inoculation with L. casei and Z. rouxii mixture) treatments have improved by 199.1% and 325.5%, respectively. The microbial communities' composition during the fermentation process also been greatly influenced by the different inoculants. At the genus level, Lactobacillus and Pseudomonas were the dominant bacteria, while Saccharomyces and Candida were the dominant fungi in the fermentation. Using the fermentation products as chemical fertilizer substitute has enhanced the quality of Kyoho grape. Compared with traditional chemical fertilization treatment (T1), application with fermented grape waste (T2) has significantly improved VC and soluble solid contents in grape berries by 16.89% and 20.12%, respectively. In conclusion, fermentation with suitable probiotics was an efficient approach for the disposal and recycling of fresh waste grape in vineyards.

Lactic acid production from food waste at an anaerobic digestion biorefinery: effect of digestate recirculation and sucrose supplementation

Low lactic acid (LA) yields from direct food waste (FW) fermentation restrict this production pathway. However, nitrogen and other nutrients within FW digestate, in combination with sucrose supplementation, may enhance LA production and improve feasibility of fermentation. Therefore, this work aimed to improve LA fermentation from FWs by supplementing nitrogen (0-400 mgN·L-1) as NH4Cl or digestate and dosing sucrose (0-150 g·L-1) as a low-cost carbohydrate. Overall, NH4Cl and digestate led to similar improvements in the rate of LA formation (0.03 ± 0.02 and 0.04 ± 0.02 h-1 for NH4Cl and digestate, respectively), but NH4Cl also improved the final concentration, though effects varied between treatments (5.2 ± 4.6 g·L-1). While digestate altered the community composition and increased diversity, sucrose minimised community diversion from LA, promoted Lactobacillus growth at all dosages, and enhanced the final LA concentration from 25 to 30 g·L-1 to 59-68 g·L-1, depending on nitrogen dosage and source. Overall, the results highlighted the value of digestate as a nutrient source and sucrose as both community controller and means to enhance the LA concentration in future LA biorefinery concepts.

The dissolution of polysaccharides and amino acids enhanced lactic acid production from household food waste during pretreatment process

A large amount of household food waste (HFW) is produced yearly, resulting in environmental problems and financial burdens. Bio-production of lactic acid (LA), a high value-added platform chemical, from HFW by anaerobic fermentation is a promising way of resource recovery. However, the LA production yield from HFW is low. This paper compared several pretreatment methods (hydrothermal pretreatment, chemical pretreatment, and combined hydrothermal and chemical pretreatment) to improve LA production from HFW. The result showed that the combined pretreatment (alkali-thermal pretreatment at pH 10 and 120 °C) significantly increased the LA production than single hydrothermal and chemical pretreatment. The pretreatment process promoted the dissolution of organics, especially the polysaccharides and amino acids, and further influenced the LA production by Lactobacillus rhamnosus ATCC 7469. Among the amino acids, aspartic acid (Asp), threonine (Thr), glutamic acid (Glu), glycine (Gly), alanine (Ala), cystine (Cys), valine (Val), isoleucine (Ile), arginine (Arg), and proline (Pro) significantly correlated with LA concentration.

Highly efficient oriented bioconversion of food waste to lactic acid in an open system: Microbial community analysis and biological carbon fixation evaluation

The valorization of organic solid waste to lactic acid (LA) in open fermentation systems has attracted tremendous interest in recent years. In this study, a highly efficient oriented LA bioconversion system from food waste (FW) in open mode was established. The maximum LA production was 115 g/L, with a high yield of 0.97 g-LA/g-total sugar. FW is a low-cost feedstock for LA production, containing indigenous hydrolysis and LA-producing bacteria (LAB). Saccharification and real-time pH control were found to be essential for maintaining LAB dominantly in open systems. Furthermore, microbial community analysis revealed that Enterococcus mundtii adapted to complex FW substrates and dominated the subsequent bioconversion process. The oriented LA bioconversion exhibited the capacity for biological carbon fixation by reducing CO₂ emissions by at least 21 kg per ton of FW under anaerobic conditions.

The liquid-liquid extractive fermentation of L-lactic acid in a novel semi-partition bioreactor (SPB)

Fermentation technology is commonly used as a mature process to produce numerous products with the help of micro-organisms. However, these organisms are sometimes inhibited by the accumulation of these products or their by-products. One route to circumvent this is via extractive fermentation, which combines the fermentation process with extraction. To facilitate this, novel bioreactor designs are required, such as the semi-partition bioreactor (SPB) which has been recently proposed for in-situ extractive fermentation. The latter combines a fermentation and an extraction unit into a single vessel using a mixer-settler principle. Where the bioproduct is produced in the mixer and removed continuous in the settler. As the SPB functionality is a subject of interest, this study builds on demonstrating different process conditions in the production of a sample bioprocess (lactic acid (LA)) which is susceptible to product inhibition. The results showed a 34.5 g/L LA concentration was obtained in the pH-controlled condition. While LA production can suffer from product inhibition, neutralizing agents can be easily used to curb inhibitory problems, however, the LA fermentation is a simple (and well-studied) example, which can demonstrate an alternative route to avoiding product inhibition (for systems which cannot be rescued using pH control). Hence, to replicate a scenario of product inhibition, two different process conditions were investigated, no pH control with no extraction (non-integrated), and no pH control with integrated extractive fermentation. Key findings showed higher LA concentration in integrated (25.10 g/L) as compared to the non-integrated (14.94 g/L) case with improved yield (0.75 gg^-1 (integrated) versus 0.60 gg^-1 (non-integrated)) and overall productivity (0.35 gL^-1h^-1(integrated) versus 0.20 gL^-1h^-1(non-integrated)) likewise. This is the first demonstration of an SP bioreactor, and shows how the reactor can be applied to improve productivity. Based on these results, the SPB design can be applied to produce any product liable to product inhibition.

New efficient meta-fermentation process for lactic acid production from municipal solid waste

BACKGROUND

The global market for lactic acid is witnessing growth on the back of increasing applications of lactic acid for manufacturing polylactic acid. Indeed, the lactic acid market is expected to reach 9.8 billion US dollars by 2025. The new concept of meta-fermentation has been proposed in recent years as an alternative to fermentation with pure cultures, due to multiple advantages such as lower susceptibility to contamination, no need for sterilization of culture media or lower raw material costs. However, there are still challenges to overcome to increase the conversion efficiency, decrease formation of by-products and facilitate fermentation control. In this context, the purpose of the study was to develop a robust meta-fermentation process to efficiently produce lactic acid from the OFMSW, stable at pre-industrial scale (1500 L). To maximize lactic acid production, operating conditions (pH, HRT) were modified, and a novel bioaugmentation strategy was tested.

RESULTS

A LAB-rich inoculum was generated with LAB isolated from the digestate and grown in the laboratory with MRS medium. After feeding this inoculum to the digester (bioaugmentation), lactic acid accumulation up to 41.5 gO2/L was achieved under optimal operating conditions. This corresponds to more than 70% of the filtered COD measured in the digestate. The amount of lactic acid produced was higher than the volatile fatty acids under all feeding strategies applied.

CONCLUSIONS

The operating conditions that enhanced the production of lactic acid from mixed cultures were 55ºC, 2 days HRT and pH 4.8-5.7, with pH-control once a day. The bioaugmentation strategy improved the results obtained in the prototype without applying reinoculation. Lactic acid was the main product along with other carboxylic acids. Further improvements are needed to increase purity as well as lactic acid concentration to reach economic feasibility of the whole process (digestion of OFMSW and downstream).

Direct lactic acid production from household food waste by lactic acid bacteria

China is vigorously promoting garbage classification, but the treatment of classified waste, especially household food waste (HFW) has yet to be studied. Lactic acid (LA), a high value-added platform molecule has broad market prospects. Although there have been many studies on the production of LA from food waste, open fermentation often produces lots of by-products, while the traditional fermentation under a pure bacteria system often requires the saccharification process, which increases the production cost. We sought to analyze the comprehensive properties of classified HFW in Shanghai, then to produce LA by inoculating lactic acid bacteria (LAB) directly. The effects of strains, temperature, sterilized or not, initial pH, inoculum size, and substrate concentration on LA production were investigated. HFW was rich in nutrients and growth factors which provided the possibility for direct LA production from HFW by inoculating LAB. The results showed that Lactobacillus rhamnosus ATCC 7469, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus all could be used as the inoculum, however, no significant synergistic effect of the three strains on LA production was found. LA concentration of 30.25 g/L at 37 °C, pH 6.8 could be obtained by inoculating Lactobacillus rhamnosus ATCC 7469 from sterilized HFW. High inoculum size and substrate concentration resulted in high LA concentration, but not high LA yield. The result of ANOVA indicated that there was a significantly positive relationship between substrate concentration and LA concentration (r = 0.942, p < 0.01), while no statistically significant difference between these groups at different inoculum size was evident (p = 0.318). In addition, an average LA concentration of 26.8 g/L, LA yield of 0.20 g/g TCOD was obtained by repeated batch fermentation for 32 d.

Fungal mash enzymatic pretreatment combined with pH adjusting approach facilitates volatile fatty acids yield via a short-term anaerobic fermentation of food waste

As an alternative for commercial enzyme, crude enzyme of fungal mash could promote food waste (FW) hydrolysis, but its specific effects coupled pH adjusting on the production of volatile fatty acids (VFAs) remains unknown. The crude enzyme produced from an Aspergillus awamori, named complex-amylase (CA), was added to short-term anaerobic system of FW fermentation. Results showed that adding CA significantly improved the solubility and degradability of biodegradable and non-biodegradable organics in FW, where the SCOD concentration with adding CA increased by 116.9% relative to the control but a marginal enhancement on VFAs yield. In contrast, adding CA combined with adjusting pH 8 markedly increased the VFAs production to 32.0 g COD/L, almost 10 times as much as the control. Besides, pH adjusting altered the metabolic pathway from lactate-type to butyrate-type. Adding CA coupled pH adjusting significant increase the component of butyrate compared with pH adjusting alone. Moreover, microbial community analysis indicated that adding CA reinforced proportion of the butyrate-producing bacteria (e.g., Dialister) under basic conditions, thus enhancing the butyrate metabolic pathways. This study demonstrated that fungal mash pretreatment coupled pH conditioning could be an economical way to enhance VFAs yield for FW valorization during anaerobic fermentation.

Lactic acid fermentation of food waste at acidic conditions in a semicontinuous system: effect of HRT and OLR changes

Lactic acid production through fermentation is an established technology, however, improvements are necessary to reduce the process costs and to decrease its market price. Lactic acid is used in many industrial sectors and its market has increased in the last decade for its use as the raw material for polylactic acid product. Using food waste as a cheap and renewable substrate, as well as fermentation at uncontrolled pH, helps to make the production cheaper and to simplify the downstream purification process. Lactic acid production at acidic conditions and the role of varying organic loading rate (OLR) and hydraulic retention time (HRT) were tested in two different semicontinuous batch fermentation systems. Reactor performances indicated that lactic acid fermentation was still possible at pH < 3.5 and even up to a pH of 2.95. The highest lactic acid production was recorded at 14-day HRT, 2.14 g VS/L·day OLR, and pH 3.11 with a maximum lactic acid concentration of 8.72 g/L and a relative yield of 0.82 g lactate/g carbohydrates. The fermentation microbial community was dominated by Lactobacillus strains, the organism mainly responsible for lactic acid conversion from carbohydrates. This study shows that low pH fermentation is a key parameter to improve lactic acid production from food waste in a semicontinuous system. Acidic pH favored both the selection of Lactobacillus strains and inhibited VFA producers from utilizing lactic acid as primary substrate, thus promoting the accumulation of lactic acid. Finally, production yields tend to decrease with high OLR and low HRT, while lactic acid production rates showed the opposite trend.

Two-stage anaerobic digestion of food waste: Enhanced bioenergy production rate by steering lactate-type fermentation during hydrolysis-acidogenesis

This study proposed a lactate-based two-stage anaerobic digestion (AD) process to enhance bioenergy production rate from food waste (FW) and investigated the effect of inoculum addition and culture pH on hydrolysis-acidogenesis and further methanization. A series of batch fermentations were performed with an enriched lactate-producing consortium and without inoculum addition under controlled (5.7) and uncontrolled pH (initial 6.7) conditions. The interplay between the studied factors dictated the fate of lactate, particularly if it is produced and accumulated in the fermentation broth or is consumed by butyrogenic bacteria. Only the self-fermentation of FW with uncontrolled pH resulted in lactate accumulation (0.2 g/g volatile solid (VS) fed) with limited off-gas production (0.32 NL/L) and VS losses (≈16%). Such lactate-rich broth was successfully digested through biochemical methane potential tests, resulting in a maximum bioenergy production rate of 2891 MJ/ton-VS fed per day, which was two-fold higher compared to that achieved by one-stage AD.

Hydrodynamic cavitation pre-treatment of urban waste: Integration with acidogenic fermentation, PHAs synthesis and anaerobic digestion processes

Urban waste can be valorized within a biorefinery approach, producing platform chemicals, biopolymers and energy. In this framework, hydrodynamic cavitation (HC) is a promising pre-treatment for improving biodegradability due to its high effectiveness and low cost. This paper deals with the effect of HC pre-treatment on the acidogenic co-fermentation process of thickened sewage sludge from a WWTP and seasonal vegetable waste from a wholesale market. Specifically, HC was assessed by testing two sets of parameters (i.e., treatment time of 30 and 50 min; vacuum pressure 1.4 and 2.0 bar; applied power 8 and 17 kW) to determine its effectiveness as a pre-treatment of the mixture. The highest increase in sCOD (+83%) and VFAs (from 1.93 to 17.29 gCOD_VFA L^-1) was gained after 50 min of cavitation. Fermentations were conducted with not cavitated and cavitated mixtures at 37 °C on 4 L reactors in batch mode, then switched to semi-continuous with OLR of 8 kg_TVS m-³ d^-1 and HRT of 5-6.6 d. Good VFAs concentrations (12.94-18.27 gCOD_VFA L^-1) and yields (0.44-0.53 gCOD_VFA g_VS(0)^-1) were obtained, which could be enhanced by pre-treatment optimization and pH control. The organic acid rich broth obtained was then assessed as a substrate for PHAs storage by C. necator. It yielded 0.37 g g^-1 of polyhydroxybutyrate, such biopolymer resulted to have analogous physicochemical characteristics of commercial equivalent. The only generated side-stream would be the solid-rich fraction of the fermented effluent, which valorization was assessed through BMP tests, showing a higher SGP of 0.42 Nm³ kgTVS^-1 for the cavitated.

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

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

A review on anaerobic membrane bioreactors for enhanced valorization of urban organic wastes: Achievements, limitations, energy balance and future perspectives

Sustainable urban development is threatened by an impending energy crisis and large amounts of organic wastes generated from the municipal sector among others. Conventional waste management methods involve greenhouse gas (GHG) emission and limited resource recovery, thus necessitating advanced techniques to convert such wastes into bioenergy, bio-fertilizers and valuable-added products. Research and application experiences from different scale applications indicate that the anaerobic membrane bioreactor (AnMBR) process is a kind of high-rate anaerobic digester for urban organic wastes valorization including food waste and waste sludge, while the research status is still insufficiently summarized. Through compiling recent achievements and literature, this review will focus on the following aspects, including AnMBR treatment performance and membrane fouling, technical limitations, energy balance and techno-economic assessment as well as future perspectives. AnMBR can enhance organic wastes treatment via complete retention of functional microbes and suspended solids, and timely separation of products and potential inhibitory substances, thus improving digestion efficiency in terms of increased organics degradation rates, biogas production and process robustness at a low footprint. When handling high-solid organic wastes, membrane fouling and mass transfer issues can be the challenges limiting AnMBR applications to a wet-type digestion, thus countermeasures are required to pursue extended implementations. A conceptual framework is proposed by taking various organic wastes disposal and final productions (permeate, biogas and biosolids) utilization into consideration, which will contribute to the development of AnMBR-based waste-to-resource facilities towards sustainable waste management and more economic-environmental benefits output.

Changes in volatile fatty acid production and microbiome during fermentation of food waste from hospitality sector

Due to the increasing demand for low carbon-footprint bioproducts in the markets, innovative processes technologies and products are needed. The objective of this study was to assess the quality and potential of food waste (FW) from the hospitality sector to produce volatile fatty acids (VFAs). A batch type acid fermentation system was used to study VFA production in different process conditions (a decreased pH and increased organic loading rate). The evolution of VFAs and long-chain fatty acids was followed. Amplicon sequencing of the 16S rRNA gene was used to investigate the bacterial and archaeal community, and elucidate microbial communities in different FW and process conditions. The results show that high VFA concentrations (of up to 18 g/L) were achieved in overloaded conditions, which were also affected by the activity and composition of the inoculum. FW played an important role in modulating microbial composition, especially the bacterial communities belonging to the lactic acid bacteria group.

R, Maitra S, Tarafdar A, Alphy MP, Kumar AN, Madhavan A, Sirohi R, Awasthi MK, Sindhu R, Varjani S, Binod P. Waste-Derived Fuels and Renewable Chemicals for Bioeconomy Promotion: A Sustainable Approach

Bio-based fuels and chemicals through the biorefinery approach has gained significant interest as an alternative platform for the petroleum-derived processes as these biobased processes are noticed to have positive environmental and societal impacts. Decades of research was involved in understanding the diversity of microorganisms in different habitats that could synthesize various secondary metabolites that have functional potential as fuels, chemicals, nutraceuticals, food ingredients, and many more. Later, due to the substrate-related process economics, the diverse low-value, high-carbon feedstocks like lignocellulosic biomass, industrial byproducts, and waste streams were investigated to have greater potential. Among them, municipal solid wastes can be used as the source of substrates for the production of commercially viable gaseous and liquid fuels, as well as short-chain fattyacids and carboxylic acids. In this work, technologies and processes demanding the production of value-added products were explained in detail to understand and inculcate the value of municipal solid wastes and the economy, and it can provide to the biorefinery aspect.

Roles of micro-aeration on enhancing volatile fatty acids and lactic acid production from agricultural wastes

Micro-aeration was proven to be an environmentally friendly strategy for efficiently enhancing volatile fatty acids (VFAs) and lactic acid (LA) production. The roles of micro-aeration on mono-digestion of swine manure (SM) for VFAs production and co-digestion of SM with corn silage (CS) for LA production were investigated, respectively. In this study, micro-aeration increased the maximum VFAs concentration by 20.3% to 35.71 g COD/L, and shortened the time to reach the maximum from 18 days to 10 days. Micro-aeration limited the conversion of LA into VFAs, leading to LA accumulation effectively to be 26.08 g COD/L. Microbial community analysis suggested that Clostridium and Terrisporobacter were always the dominant bacteria with or without micro-aeration for VFAs production, but the relative abundance increased notably during the same period. However, Bifidobacterium, which could use the higher productivity metabolism pathway, i.e., Bifidum pathway to produce LA, increased from lower than 1% to 22.9% by micro-aeration.

Lactic acid production from mesophilic and thermophilic fermentation of food waste at different pH

It is promising to recover lactic acid (LA) from fermentation of food waste (FW). In this study, pH and temperatures were investigated comprehensively to find their effects on LA fermentation, and microbial analyses were used to take insight to the variation of LA production. The results showed that mesophilic fermentation benefited hydrolysis and acidification, leading to a high yield of LA, while thermophilic conditions restricted other producers at low pH, leading to a high purity of LA. Lactobacillus amylolyticus was the main LA producer under thermophilic conditions, but Thermoanaerobacterium thermosaccharolyticum boomed at pH 5.0-6.0 and it converted LA partly to butyric acid. Simultaneously, Bacillus coagulans also increased and improved the optical purity (OP) of L-LA. From a series of this study, an operational condition of pH 5.5 and temperature of 52 °C would be potentially suitable for lactate fermentation of FW with high purity of 89%, while a stable LA production with an OP of 68% was achieved at 55 °C and pH 6.0.

Microbial production of lactic acid from food waste: Latest advances, limits, and perspectives

A significant amount of food waste (FW) is produced every year. If it is not disposed of timeously, human health and the ecological environment can be negatively affected. Lactic acid (LA), a high value-added product, can be produced by fermentation from FW as a substrate, realizing the concurrent treatment and recycling of FW, which has attracted increasing research interest. In this paper, the latest advances and deficiencies were presented from the following aspects: microorganisms involved in LA fermentation and the metabolic pathways of Lactobacillus, fermentation conditions, and methods of enhanced biotransformation and LA separation. The limitations of the LA fermentation of FW are mainly associated with low LA concentration and yield, the low purity of L(+)-LA, and the high separation costs. The establishment of biorefineries of FW with lactic acid as the target product is the future development direction, but there are still many research studies to be done.

The biochemical mechanism of enhancing the conversion of chicken manure to biogenic methane using coal slime as additive

To improve the efficiency of methane production from chicken manure (CM) anaerobic digestion, the mechanism of coal slime (CS) as an additive on methane production characteristics were investigated. The results showed that adding an appropriate amount of CS quickened the start of the fermentation and effectively increased the methane yield. In addition, the pH changed in a stable manner in the liquid phase, and the concentrations of total ammonia nitrogen (TAN) and free ammonia nitrogen (FAN) were reduced. Moreover, organic matter was decomposed and volatile fatty acids (VFAs) were consumed effectively. The abundance of Bacteroides in the bacterial community and Methanosarcina in the archaea was increased. In addition, the reduction of CO₂ was the main methanogenic pathway, and adding CS raised the abundance of genes for key enzymes in metabolic pathways during methane metabolism. The results provide a novel method for the efficient methane production from CM.

Metabolic cascade of complex organic wastes to medium-chain carboxylic acids: A review on the state-of-the-art multi-omics analysis for anaerobic chain elongation pathways

Medium-chain carboxylic acid (MCCA) production from organic wastes has attracted much attention because of their higher energy contents and diverse applications. Anaerobic reactor microbiomes are stable and resilient and have resulted in efficient performance during many years of operation for thousands of full-scale anaerobic digesters worldwide. The method underlying how the relevant microbial pathways contribute to elongate carbon chains in reactor microbiomes is important. In particular, the reverse β-oxidation pathway genes are critical to upgrading short-chain fermentation products to MCCAs via a chain elongation (CE) process. Diverse genomics and metagenomics studies have been conducted in various fields, ranging from intracellular metabolic pathways to metabolic cascades between different strains. This review covers taxonomic approach to culture processes depending on types of organic wastes and the deeper understanding of genome and metagenome-scale CE pathway construction, and the co-culture and multi-omics technology that should be addressed in future research.

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.

Resource recovery from sugarcane vinasse by anaerobic digestion - A review

The increase in biofuel production by 2030, driven by the targets set at the 21st United Nations Framework Convention on Climate Change (COP21), will promote an increase in ethanol production, and consequently more vinasse generation. Sugarcane vinasse, despite having a high polluting potential due to its high concentration of organic matter and nutrients, has the potential to produce value-added resources such as volatile fatty acids (VFA), biohydrogen (bioH₂) and biomethane (bioCH₄) from anaerobic digestion. The objective of this paper is to present a critical review on the vinasse treatment by anaerobic digestion focusing on the final products. Effects of operational parameters on production and recovery of these resources, such as pH, temperature, retention time and type of inoculum were addressed. Given the importance of treating sugarcane vinasse due to its complex composition and high volume generated in the ethanol production process, this is the first review that evaluates the production of VFAs, bioH₂ and bioCH₄ in the treatment of this organic residue. Also, the challenges of the simultaneous production of VFA, bioH₂ and bioCH₄ and resources recovery in the wastewater streams generated in flex-fuel plants, using sugarcane and corn as raw material in ethanol production, are presented. The installation of flex-fuel plants was briefly discussed, with the main impacts on the treatment process of these effluents either jointly or simultaneously, depending on the harvest season.

Effect of pH on volatile fatty acid production and the microbial community during anaerobic digestion of Chinese cabbage waste

The effects of pH on the production of volatile fatty acids (VFAs) and the evolution of microbial community structure were studied via anaerobic fermentation of Chinese cabbage waste. The results showed that the concentration of total VFAs was highest at 20,241.4 mg COD/L at pH 6.0, followed by pH 7.0. Ethanol, acetate and butyrate were dominant under the acidic condition. The main products at pH 7.0 were acetate, propionate, and butyrate. Ethanol, acetate and butyrate were rapidly produced during the initial stage. The hexanoate concentration increased quickly from day 6 due to the chain extension between ethanol and butyrate, and was 4,885.1 mg COD/L on day 8, accounting for 30.4% of the total VFAs. As fermentation was extended, Bacteroidia and Clostridia were dominant at pH 6.0 and the uncontrolled pH, respectively. Clostridium IV, Ruminococcus, and Candida, were suspected to be related to hexanoate production.

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

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

Effects of different temperatures and pH values on volatile fatty acids production during codigestion of food waste and thermal-hydrolysed sewage sludge and subsequent volatile fatty acids for polyhydroxyalkanoates production

The effects of temperature (35 °C and 55 °C) and pH (uncontrolled, 7 and 10) on volatile fatty acid (VFA) yields from anaerobic codigestion of food waste, and thermal-hydrolysed sewage sludge were investigated in this study. The results revealed that optimal conditions for VFA production occurred at 35 °C at pH 7 and at 10 and 55 °C at pH 7. The dominant bacterial genera associated with VFA production significantly differed when the temperature and pH were altered, including Prevotella, Lactobacillus, Bifidobacterium Megasphaera, Clostridium XlVa, and Coprothermobacter. A temperature of 35 °C at pH 7 favoured mixed acid-type fermentation, while a temperature of 35 °C at pH 10 and 55 °C at pH 7 favoured butyric acid-type fermentation. The maximal polyhydroxyalkanoate content accounted for 54.8% of the dry cell at 35 °C with pH 7 fermentative liquids and comprised 58.9% 3-hydroxybutyrate (3HB) and 41.1% 3-hydroxyvalerate (3HV).