High-rate volatile fatty acid (VFA) production by a granular sludge process at low pH

Why do lactic acid bacteria thrive in chain elongation microbiomes?

Efficient waste management is necessary to transition towards a more sustainable society. An emerging trend is to use mixed culture biotechnology to produce chemicals from organic waste. Insights into the metabolic interactions between community members and their growth characterization are needed to mediate knowledge-driven bioprocess development and optimization. Here, a granular sludge bioprocess for the production of caproic acid through sugar-based chain elongation metabolism was established. Lactic acid and chain-elongating bacteria were identified as the two main functional guilds in the granular community. The growth features of the main community representatives (isolate Limosilactobacillus musocae G03 for lactic acid bacteria and type strain Caproiciproducens lactatifermentans for chain-elongating bacteria) were characterized. The measured growth rates of lactic acid bacteria (0.051 ± 0.005 h-1) were two times higher than those of chain-elongating bacteria (0.026 ± 0.004 h-1), while the biomass yields of lactic acid bacteria (0.120 ± 0.005 g biomass/g glucose) were two times lower than that of chain-elongating bacteria (0.239 ± 0.007 g biomass/g glucose). This points towards differential growth strategies, with lactic acid bacteria resembling that of a r-strategist and chain-elongating bacteria resembling that of a K-strategist. Furthermore, the half-saturation constant of glucose for L. mucosae was determined to be 0.35 ± 0.05 g/L of glucose. A linear trend of caproic acid inhibition on the growth of L. mucosae was observed, and the growth inhibitory caproic acid concentration was predicted to be 13.6 ± 0.5 g/L, which is the highest reported so far. The pre-adjustment of L. mucosae to 4 g/L of caproic acid did not improve the overall resistance to it, but did restore the growth rates at low caproic acid concentrations (1-4 g/L) to the baseline values (i.e., growth rate at 0 g/L of caproic acid). High resistance to caproic acid enables lactic acid bacteria to persist and thrive in the systems intended for caproic acid production. Here, insights into the growth of two main functional guilds of sugar-based chain elongation systems are provided which allows for a better understanding of their interactions and promotes future bioprocess design and optimization.

Steering the product spectrum in high-pressure anaerobic processes: CO2 partial pressure as a novel tool in biorefinery concepts

BACKGROUND

Elevated CO₂ partial pressure (pCO₂) has been proposed as a potential steering parameter for selective carboxylate production in mixed culture fermentation. It is anticipated that intermediate product spectrum and production rates, as well as changes in the microbial community, are (in)directly influenced by elevated pCO₂. However, it remains unclear how pCO₂ interacts with other operational conditions, namely substrate specificity, substrate-to-biomass (S/X) ratio and the presence of an additional electron donor, and what effect pCO₂ has on the exact composition of fermentation products. Here, we investigated possible steering effects of elevated pCO₂ combined with (1) mixed substrate (glycerol/glucose) provision; (2) subsequent increments in substrate concentration to increase the S/X ratio; and (3) formate as an additional electron donor.

RESULTS

Metabolite predominance, e.g., propionate vs. butyrate/acetate, and cell density, depended on interaction effects between pCO₂-S/X ratio and pCO₂-formate. Individual substrate consumption rates were negatively impacted by the interaction effect between pCO₂-S/X ratio and were not re-established after lowering the S/X ratio and adding formate. The product spectrum was influenced by the microbial community composition, which in turn, was modified by substrate type and the interaction effect between pCO₂-formate. High propionate and butyrate levels strongly correlated with Negativicutes and Clostridia predominance, respectively. After subsequent pressurized fermentation phases, the interaction effect between pCO₂-formate enabled a shift from propionate towards succinate production when mixed substrate was provided.

CONCLUSIONS

Overall, interaction effects between elevated pCO₂, substrate specificity, high S/X ratio and availability of reducing equivalents from formate, rather than an isolated pCO₂ effect, modified the proportionality of propionate, butyrate and acetate in pressurized mixed substrate fermentations at the expense of reduced consumption rates and increased lag-phases. The interaction effect between elevated pCO₂ and formate was beneficial for succinate production and biomass growth with a glycerol/glucose mixture as the substrate. The positive effect may be attributed to the availability of extra reducing equivalents, likely enhanced carbon fixating activity and hindered propionate conversion due to increased concentration of undissociated carboxylic acids.

A plant wide simulation of polyhydroxyalkanoate production from wastewater and its conversion to methyl crotonate

This work simulates the production of methyl crotonate from various industrial wastewaters. In the upstream process, wastewater is fermented into volatile fatty acids which are then converted into polyhydroxyalkanoates (PHA) by means of mixed microbial cultures. In the downstream, PHA undergoes a series of thermolysis and esterification reactions to produce methyl crotonate. The origin of the wastewater was found to have a great influence on the composition of the PHA with the effluent of a candy bar factory producing a high polyhydroxybutyrate/polyhydroxyvalerate ratio of 86/14 in favour of methyl crotonate production. It was observed that the use of intracellular polyhydroxybutyrate, instead of purified, significantly lowers the number of separation steps and yet reduces the methyl crotonate recovery by only 20 %. An operating pressure higher than 18 bar led to more transesterification of polyhydroxybutyrate, producing byproducts instead of methyl crotonate. Finally, a 3 h reaction was found sufficient for completion of polyhydroxybutyrate conversion.

High-rate conversion of lactic acid-rich streams to caproic acid in a fermentative granular system

Lactic acid-driven chain elongation enables upgrading low-value organic streams into caproic acid. Recently, volumetric production rates over 0.5 g L^-1 h^-1have been reported for carbohydrate-rich streams in expanded granular sludge bed (EGSB) reactors. However, many target streams contain mixtures of carbohydrates and lactic acid, and little is known about their impact on product profile and microbial ecology, or the importance of carbohydrates as substrate to achieve high rates. This manuscript investigated varying glucose-to-lactate ratios and observed that decreasing glucose-content eliminated odd-chain by-products, while glucose omission required acetic acid addition to support lactic acid conversion. Decreasing the glucose-content fed resulted in decreasing amounts of granular biomass, with the disappearance of granules when no glucose was fed. Lowering the HRT to 0.3 days while feeding only lactic and acetic acid likely triggered re-granulation, enabling the highest lactic acid-driven caproic acid production rates reported thus far at 16.4 ± 1.7 g L^-1 d^-1.

Effect of carbohydrates on protein hydrolysis in anaerobic digestion

This study aimed to assess the effect of carbohydrates on protein hydrolysis and potential implications for the design of anaerobic reactors for treatment of protein-rich wastewaters. Batch experiments were carried out with dissolved starch (Sta) and gelatine (Gel) at different chemical oxygen demand (COD) ratios ranging from 0 to 5.5 under methanogenic conditions for methane production and up to 3.8 under non-methanogenic conditions for volatile fatty acids (VFA), both at 35 °C. The Sta/Gel did not have a direct effect on the gelatine hydrolysis rate constants under methanogenic (0.51 ± 0.05 L g VSS^-1 day^-1) and non-methanogenic conditions (0.48 ± 0.05 L g VSS^-1 day^-1). However, under non-methanogenic conditions, gelatine hydrolysis was inhibited by 64% when a spectrum of VFA was added at a VFA/Gel (COD) ratio of 5.9. This was not caused by the ionic strength exerted by VFA but by the VFA itself. These results imply that methanogenesis dictates the reactor design for methane production but hydrolysis does for VFA production from wastewater proteins.

A waste-to-wealth initiative exploiting the potential of Anabaena variabilis for designing an integrated biorefinery

The current research work was an innovative approach providing dual advantages of waste bioremediation and an effective biorefinery. The study attempted to exploit wastewater like aqua discharge and solid wastes like poultry litter/cow dung for cyanobacterial cultivation. Aqua discharge appended with 7.5 g L⁻¹ poultry litter turned out as the best combination generating 46% higher carbohydrate yield than BG-11 control. A. variabilis cultivation in this waste-utilized medium also revealed its excellent bioremediation ability. While 100% removal was observed for nitrite, nitrate, and orthophosphate, a respective 74% and 81% reduction was noted for ammonium and total organic carbon. Chemical and biological oxygen demands were also reduced by 90%. This work was also novel in developing a sequential design for the production of bioethanol and co-products like exopolysaccharides, sodium copper chlorophyllin, C-phycocyanin, and poly-β-hydroxybutyrate from the same cyanobacterial biomass. The developed biorefinery implementing the waste-utilized medium was one of its kind, enabling biomass valorization of 61%. Therefore, the present study would provide a leading-edge for tackling the high production costs that limit the practical viability of biorefinery projects. The recyclability of the bioremediated wastewater would not only curtail freshwater usage, the waste disposal concerns would also be mitigated to a great extent.

Recovery Techniques Enabling Circular Chemistry from Wastewater

In an era where it becomes less and less accepted to just send waste to landfills and release wastewater into the environment without treatment, numerous initiatives are pursued to facilitate chemical production from waste. This includes microbial conversions of waste in digesters, and with this type of approach, a variety of chemicals can be produced. Typical for digestion systems is that the products are present only in (very) dilute amounts. For such productions to be technically and economically interesting to pursue, it is of key importance that effective product recovery strategies are being developed. In this review, we focus on the recovery of biologically produced carboxylic acids, including volatile fatty acids (VFAs), medium-chain carboxylic acids (MCCAs), long-chain dicarboxylic acids (LCDAs) being directly produced by microorganisms, and indirectly produced unsaturated short-chain acids (USCA), as well as polymers. Key recovery techniques for carboxylic acids in solution include liquid-liquid extraction, adsorption, and membrane separations. The route toward USCA is discussed, including their production by thermal treatment of intracellular polyhydroxyalkanoates (PHA) polymers and the downstream separations. Polymers included in this review are extracellular polymeric substances (EPS). Strategies for fractionation of the different fractions of EPS are discussed, aiming at the valorization of both polysaccharides and proteins. It is concluded that several separation strategies have the potential to further develop the wastewater valorization chains.

Influence of feedstock mix ratio on microbial dynamics during acidogenic fermentation for polyhydroxyalkanoates production

The nature of microbial populations plays an essential role in the production of volatile fatty acids (VFA) during acidogenesis, the first stage in polyhydroxyalkanoates (PHA) production using mixed cultures. However, the composition of microbial communities is generally affected by substrate alterations. This work aimed to unravel the microbial dynamics in response to a gradual change in the feedstock composition in an acidogenic reactor, with subsequent PHA production. To achieve this, co-digestion of cheese whey and brewery wastewater (BW) was carried out for the production of VFA, in which the ratio of these feedstocks was varied by gradually increasing the proportion of BW from 0 up to 50% of the organic content. Bacteria such as Megasphaera, Bifidobacterium or Caproiciproducens were the most abundant in the first stages of the co-digestion. However, when BW reached 25% of the organic load, new taxa emerged and displaced the former ones; like Selenomonas, Ethanoligenens or an undefined member of the Bacteroidales order. Accordingly, the production of butyric acid dropped from 52 down to 27%, while the production of acetic acid increased from 36 up to 52%. Furthermore, the gradual increase of the BW ratio led to a progressive drop in the degree of acidification, from 72 down to 57%. In a subsequent approach, the VFA-rich streams, obtained from the co-digestion, were used as substrates in PHA accumulation tests. All the tests yielded similar PHA contents, but with slightly different monomeric composition. The overall results confirmed that the microbiome was altered by a gradual change in the feedstock composition and, consequently, the VFA profile and the monomeric composition of the biopolymer also did.

Valorization of agro-industrial wastes to produce volatile fatty acids: combined effect of substrate/inoculum ratio and initial alkalinity

ABSTRACTAgroindustry generates huge amounts of wastes leading to environmental problems in the zones where they are disposed. One of the strategies for the valorization of these wastes is the acidogenic fermentation used to produce volatile fatty acids. In this study, four agroindustrial wastes generated in different Spanish industries were selected for evaluating their acidogenic potential in batch assays. The selected wastes were potato solid waste, grape marc distilled, grape marc and brewery spent grain. Potato solid waste and grape marc presented the highest degree of acidification (69% and 54%, respectively) with the predominance of acetic, butyric and propionic acids in their VFA profiles. In the potato solid waste, the influence of two parameters, substrate/inoculum ratio and initial alkalinity added, on the degree of acidification and on the VFA profile was evaluated. The maximum VFA production (higher than 70% of the total COD added) was obtained at substrate/inoculum ratios of 1.5 and 2.8 g VS substrate g^-1 VS inoculum and at the highest concentration of initial alkalinity (3 g L^-1 as CaCO₃). Additionally, it was demonstrated that an increase of initial alkalinity, at all S/I ratios, can shift the VFA profile obtained, decreasing the relative amount of butyric and propionic acids and increasing the amount of acetic acid.

High-rate ethanol production at low pH using the anaerobic granular sludge process

In this study, we investigated the operational performance and product spectrum of glucose-fermenting anaerobic granular sludge reactor at pH 4. A selective environment for the growth of granules was implemented by the introduction of a 2 min settling phase, a hydraulic retention time of 6 h and a solid retention time of 12 ± 3 days. The fermentation products were ethanol, lactate, and volatile fatty acids (VFA) with yields of 0.55 ± 0.03, 0.15 ± 0.02, and 0.20 ± 0.04 gram chemical oxygen demand (gCOD)/gCOD glucose, respectively. The obtained product spectrum was remarkably different from the VFA-dominated product spectrum reported in a previous study when the same system was operated at higher pH (4.5-5.5). The shift in product spectrum coincided with a shift in the microbial community structure with the dominance of eukaryotic Candida tropicalis, Pichia jaroonii, and prokaryotic Lactobacillus species instead of the Clostridia species obtained at higher pH-values. The control of the microbiomes and the associated product spectra provides bioprocess engineers with the option to tailor a suitable precursor compound mixture for subsequent chain elongation fermentation or PHA biopolymer production.

Salinity enhances high optically active L-lactate production from co-fermentation of food waste and waste activated sludge: Unveiling the response of microbial community shift and functional profiling

Lactic acid (LA), a versatile platform molecule, can be fermented from organic wastes, such as food waste and waste activated sludge. In this study, an efficient approach using salt, a component of food waste as an additive, was proposed to increase LA production. The LA productivity was increased at 10 g NaCl/L and optical pure L-lactate was obtained at 30 g NaCl/L. The enhancement of LA was in accordance with the increased solubilization and the critical hydrolase activities under saline conditions. Moreover, high salinity (30-50 g NaCl/L) changed the common conversion of LA to volatile fatty acids. In addition, the key LA bacteria genera (Bacillus, Enterococcus, Lactobacillus) were selectively enriched under saline conditions. Strong correlations between salinity and functional genes for L-LA production were also observed. This study provides a practical way for the enrichment of L-LA with high optical activity from organic wastes.

Enhanced lactic acid production from the anaerobic co-digestion of swine manure with apple or potato waste via ratio adjustment

The valorization of organic waste into lactic acid (LA) via co-digestion has attracted tremendous research interests in recent years. This study investigated the feasibility of intensifying the LA accumulation from anaerobic digestion (AD) of swine manure (SM) by adding apple waste (AW) or potato waste (PW). Results indicated that AW or PW obviously enhanced the accumulation of LA, and when the optimal mixing ratio of AW or PW to SM of 75:25, the maximum concentrations of LA were 27.61 and 8.91 g COD/L, which were around 3.53- and 1.14-folds of that of the mono-digestion of SM, respectively. Meanwhile, the co-digestion of SM and AW showed significantly higher LA production than that of SM and PW (p < 0.05). High reducing sugar content of AW contributed to LA accumulation in AD process. In addition, AW increased the relative abundance of Lactobacillus and Clostridium, thus benefited the production of LA.

Factors influencing volatile fatty acids production from food wastes via anaerobic digestion

Volatile fatty acids (VFAs) are intermediate products in anaerobic digestion. The effect of substrate loading or inoculum to substrate ratio (ISR), the addition of methanogen inhibitor, O₂ presence, control the reactor’s pH, and inoculum adaptation on the VFAs production from food waste through acidogenesis process was investigated in this study. Addition of 2-bromoethane sulfonic (BES) as methanogen inhibitor suppressed VFA consumption by methanogens at ISR 1:1. At higher substrate loading (ISR 1:3), methane production can be suppressed even without the addition of BES. However, at high substrate loading, controlling the pH during acidogenesis is important to achieve high VFAs yield. Acclimatization of inoculum is also one of the strategies to achieve high VFA yield. The highest VFAs yield obtained in this work was 0.8 g VFA/g VS _added at ISR 1:3, controlled pH at 6, with the presence of initial O₂ (headspace unflushed).

Transformation of pulp and paper mill sludge (PPMS) into a glucose-rich hydrolysate using green chemistry: Assessing pretreatment methods for enhanced hydrolysis

Pulp and paper mill sludge is a waste stream derived from the pulp and paper making industry, comprised of organic and inorganic material in the form of cellulose, hemicellulose, lignin and ash. In South Africa, approximately fivefour hundred thousand wet tonnes are produced per annum and is currently disposed via landfilling or incineration. However, these disposal methods raise environmental and financial concerns. This waste stream is an attractive feedstock for fermentable sugars, mainly glucose, recovery and can be redirected for valorisation as a feedstock for microbial fermentation to produce value-added products. Sugar recovery by enzymatic hydrolysis, as opposed to acidic hydrolysis, is a promising approach but is hampered by the lignin and inorganic material found in pulp and paper mill sludge. Several treatment steps to reduce or remove these components prior to enzymatic hydrolysis are assessed in this review. Pretreatment improves hydrolysis of cellulosic fibres and ensures a substantial yield of sugars.

A comprehensive study of volatile fatty acids production from batch reactor to anaerobic sequencing batch reactor by using cheese processing wastewater

Volatile fatty acids (VFAs) has great potential for closed-loop production in dairy industries via resource recovery from waste-streams. In the current study, the transition of VFA production from batch reactor to anaerobic sequencing batch reactor (ASBR) by using cheese industry wastewater under alkali pH was evaluated with respect to seed sludge structure, microbial diversity and reactor type. The transition from the batch reactor to the ASBR demonstrated that the maximum VFA production yield (g COD/g SCOD) was comparable in two reactors (batch: 0.97; ASBR: 0.94), whereas, the dominant acid type was different (batch: 49% lactic acid; ASBR: 80% propionic acid). There was a significant correlation between the productions of butyric acid with Gracilibacteraceae and Desulfovibrionaceae; propionic acid with Desulfovibrionaceae and Synergistaceae; lactic acid with Pseudomonadaceae and Rhodocyclaceae. The high VFA production efficiency can be achieved by long term reactor operation, which enables the shift from industrial waste-streams to biorefineries.

Cheese whey fermentation into volatile fatty acids in an anaerobic sequencing batch reactor

The present research explored the optimization of volatile fatty acids (VFA) production from cheese whey in an anaerobic sequencing batch reactor (AnSBR). For that purpose, changes of solid and hydraulic retention times (SRT and HRT) were applied. Moreover, the experiments were coupled to metagenomic analyses by 16S rRNA sequencing. The results showed an enhancement of the process effectiveness at longer SRT and shorter HRT. The degree of acidification (DA) improved from 0.73 to 0.83 when increasing the SRT from 5 to 15 days. It also increased from 0.79 to 0.83 when lowering the HRT from 3 to 1 day. The acidification yield (Y_VFA/S) improved from 0.78 to 0.87 and from 0.86 to 0.90 g COD-VFA g COD-Lactose^-1 when increasing the SRT from 5 to 15 days and decreasing the HRT from 3 to 1 day, respectively. Hydrolytic bacteria dominated the microbial community at the shortest SRT, although they were replaced by acidogenic bacteria at longer SRT.

Value Proposition of Untapped Wet Wastes: Carboxylic Acid Production through Anaerobic Digestion

Although traditional anaerobic digestion (AD) process to produce methane-rich biogas from wet waste is deep-rooted, high carbon footprint and its low value as compared with other renewable sources demand advanced strategies to avoid its production. An emerging conversion pathway to arrest methanogenesis for producing value-added fuels and chemicals instead of biogas is sought as a sustainable alternative. This research provides a comprehensive analysis on current technology development, process challenges, applications, and economics for producing high-value short-chain carboxylic acids from AD of wet wastes. We show that (1) the theoretical energy yields of acids equal or exceed biogas, and (2) the cost of these acids is competitive with those produced from chemical markets, making this economically viable for mass production. With global abundance of wet waste feedstocks, this process of short-chain acid production provides a promising alternative to conventional biogas production technology, while achieving waste management and carbon mitigation goals.

Stabilizing lactate production through repeated batch fermentation of food waste and waste activated sludge

Bio-valorization of organic waste streams, such as food waste and waste activated sludge, to lactic acid (LA) has recently drawn much attention. It offers an opportunity for resource recovery, alleviates environmental issues and potentially turns a profit. In this study, both stable and high LA yield (0.72 ± 0.15 g/g total chemical oxygen demand) and productivity rate (0.53 g/L•h) were obtained through repeated batch fermentation. Moreover, stable solubilization and increase in the critical hydrolase activities were achieved. Depletions of ammonia and phosphorus were correlated with the LA production. The relative abundance of the key LA bacteria genera (i.e., Alkaliphilus, Dysgonomonas, Enterococcus and Bifidobacterium) stabilized in the repeated batch reactor at a higher level (44.5 ± 2.53%) in comparison with the batch reactor (26.2 ± 4.74%). This work show a practical way for the sustainable valorization of organic wastes to LA by applying the repeated batch mode during biological treatment.

Selecting for lactic acid producing and utilising bacteria in anaerobic enrichment cultures

Lactic acid‐producing bacteria are important in many fermentations, such as the production of biobased plastics. Insight in the competitive advantage of lactic acid bacteria over other fermentative bacteria in a mixed culture enables ecology‐based process design and can aid the development of sustainable and energy‐efficient bioprocesses. Here we demonstrate the enrichment of lactic acid bacteria in a controlled sequencing batch bioreactor environment using a glucose‐based medium supplemented with peptides and B vitamins. A mineral medium enrichment operated in parallel was dominated by Ethanoligenens species and fermented glucose to acetate, butyrate and hydrogen. The complex medium enrichment was populated by Lactococcus, Lactobacillus and Megasphaera species and showed a product spectrum of acetate, ethanol, propionate, butyrate and valerate. An intermediate peak of lactate was observed, showing the simultaneous production and consumption of lactate, which is of concern for lactic acid production purposes. This study underlines that the competitive advantage for lactic acid‐producing bacteria primarily lies in their ability to attain a high biomass specific uptake rate of glucose, which was two times higher for the complex medium enrichment when compared to the mineral medium enrichment. The competitive advantage of lactic acid production in rich media can be explained using a resource allocation theory for microbial growth processes.

Volatile fatty acid production from semi-synthetic milk processing wastewater under alkali pH: The pearls and pitfalls of microbial culture

Volatile fatty acids (VFA) are one of the most promising sustainable and environmentally friendly bioproduct owing to their wide usage area and high market demand. For this reason, in this study, the evaluation of VFA production from pure and mixed bacterial cultures was aimed. Three different mixed cultures with C. aceticum, C. butyricum and P. acidipropionici as pure cultures were used for inoculation of milk processing wastewater fermentation under pH 10 for 15 days. The mixed culture fermentation had the highest VFA production efficiency whereas the highest amount of acetic, butyric and propionic acid productions were obtained by C. aceticum, C. butyricum and P. acidipropionici, respectively. Also, the mixed cultures demonstrated faster pH regulation and acclimation than the pure cultures tested. Therefore, development of synthetic cultures may offer a useful approach to produce VFA mixtures with one-dominant acid type and with high production efficiency.

Impact of Organic Loading Rate in Volatile Fatty Acids Production and Population Dynamics Using Microalgae Biomass as Substrate

Volatile fatty acids (VFAs) are regarded as building blocks with a wide range of applications, including biofuel production. The traditional anaerobic digestion used for biogas production can be alternatively employed for VFAs production. The present study aimed at maximizing VFAs productions from Chlorella vulgaris through anaerobic digestion by assessing the effect of stepwise organic loading rates (OLR) increases (3, 6, 9, 12 and 15 g COD L^-1 d^-1). The biological system was proven to be robust as organic matter conversion efficiency into VFAs increased from 0.30 ± 0.02 COD-VFAs/COD_in at 3 g COD L^-1 d^-1 to 0.37 ± 0.02 COD-VFAs/COD_in at 12 g COD L^-1d^-1. Even though, the hydrolytic step was similar for all studied scenario sCOD/tCOD = 0.52-0.58), the highest OLR (15 g COD L^-1 d^-1) did not show any further increase in VFAs conversion (0.29 ± 0.01 COD-VFAs/COD_in). This fact suggested acidogenesis inhibition at 15 g COD L^-1d^-1. Butyric (23-32%), acetic (19-26%) and propionic acids (11-17%) were the most abundant bioproducts. Population dynamics analysis revealed microbial specialization, with a high presence of Firmicutes followed by Bacteroidetes. In addition, this investigation showed the microbial adaptation of Euryarchaeota species at the highest OLR (15 g COD L^-1d^-1), evidencing one of the main challenges in VFAs production (out-competition of archaea community to avoid product consumption). Stepwise OLR increase can be regarded as a tool to promote VFAs productions. However, acidogenic inhibition was reported at the highest OLR instead of the traditional hydrolytic barriers. The operational conditions imposed together with the high VFAs and ammonium concentrations might have affected the system yields. The relative abundance of Firmicutes (74%) and Bacteroidetes (20%), as main phyla, together with the reduction of Euryarchaeota phylum (0.5%) were found the best combination to promote organic matter conversion into VFAs.

Microalgae Biomass as a Potential Feedstock for the Carboxylate Platform

Volatile fatty acids (VFAs) are chemical building blocks for industries, and are mainly produced via the petrochemical pathway. However, the anaerobic fermentation (AF) process gives a potential alternative to produce these organic acids using renewable resources. For this purpose, waste streams, such as microalgae biomass, might constitute a cost-effective feedstock to obtain VFAs. The present review is intended to summarize the inherent potential of microalgae biomass for VFA production. Different strategies, such as the use of pretreatments to the inoculum and the manipulation of operational conditions (pH, temperature, organic loading rate or hydraulic retention time) to promote VFA production from different microalgae strains, are discussed. Microbial structure analysis using microalgae biomass as a substrate is pointed out in order to further comprehend the roles of bacteria and archaea in the AF process. Finally, VFA applications in different industry fields are reviewed.

Volatile fatty acids production via mixed culture fermentation: Revealing the link between pH, inoculum type and bacterial composition

The aim of the study was to investigate the effects of operational parameters, inoculum type and bacterial community on mixed culture fermentation to produce one dominant acid type in the mixture of volatile fatty acids (VFA). The study was performed using three different inocula (large&small granular and slurry) with glucose under various initial pH. The VFA production efficiency reached to 0,97 (gCOD/gSCOD) by granular sludge. VFA composition was changed by initial pH: in neutral conditions, acetic acid; in acidic conditions, acetic and butyric acids, in alkali conditions butyric acid were dominated, respectively. The VFA production was positively affected by the high relative abundance of Firmicutes. On the contrary, a negative correlation was seen between VFA production and the relative abundance of Chloroflexi. The results revealed the physical sludge structure of inoculum was the key factor for production efficiency, whereas, pH was the most important parameter to affect VFA composition.

Protein hydrolysis and fermentation under methanogenic and acidifying conditions

BACKGROUND

Many kinds of wastewaters contain appreciable quantities of protein. Anaerobic processes are suitable for the treatment of wastewater high in organics to achieve pollution control and recovery of energy as methane and hydrogen, or intermediates for production of biofuels and valuable biochemicals. A distinction between protein hydrolysis and amino acid fermentation, especially for dissolved proteins, is needed to target which one is truly rate-limiting and to effectively harvest bioproducts during anaerobic conversion of these wastewaters. This study explored mesophilic anaerobic hydrolysis and amino acid fermentation of gelatine, as a model for dissolved proteins, at pH 7 and at pH 5.

RESULTS

The results showed that at pH 7, protein hydrolysis (first-order rate of 0.15 h^-1) was approximately 5 times faster than acidification of the hydrolysis products (first-order rate of 0.03 h^-1), implying that not hydrolysis but acidification was the rate-limiting step in anaerobic dissolved protein degradation. This was confirmed by (temporary) accumulation of amino acids. Nineteen different amino acids were detected during the first 8 incubation hours of gelatine at neutral pH and the total chemical oxygen demand (COD) of these 19 amino acids was up to approximately 40% of the COD of the gelatine that was added. Protein hydrolysis at pH 5 was 2-25 times slower than at pH 7. Shifting the initial pH from neutral to acidic conditions (pH 5) inhibited protein degradation and changed the volatile fatty acids (VFA) product profile. Furthermore, the presence or absence of methanogenic activity did not affect the rates of protein hydrolysis and acidification.

CONCLUSIONS

The findings in this study can help to set a suitable solid retention time to accomplish anaerobic degradation of protein-rich wastewaters in continuous reactor systems. For example, if the target is harvesting VFAs, methanogens can be washed-out for a shorter retention time while amino acid fermentation, instead of hydrolysis as assumed previously, will govern the design and solutions to improve the system dealing with dissolved proteins.

An improved hydrodynamic model for percolation and drainage dynamics for household and agricultural waste beds

This study focuses on the hydrodynamic modelling of percolation and drainage cycles in the context of solid-state anaerobic digestion and fermentation (VFA platform) of household solid wastes (HSW) in leach bed reactors. Attention was given to the characterization of the water distribution and hydrodynamic properties of the beds. The experimental procedure enabled the measurement of water content in waste beds at different states of compaction during injection and drainage, and this for two types of HSW and for two other type of wastes. A numerical model, set up with experimental data from water content measurements, highlighted that a capillary-free dual-porosity model was not able to correctly reproduce all the hydrodynamic features and particularly the drainage dynamics. The model was improved by adding a reservoir water fraction to macroporosity which allowed to correctly simulate dynamics. This model, validated with data obtained from agricultural wastes, enabled to explain more precisely the water behaviour during percolation processes and these results should be useful for driving either solid-state anaerobic digestion or fermentation reactors. Indeed, this implies that the recirculation regime will impact the renewal of the immobile water fraction in macroporosity, inducing different concentration levels of fermentation products in the leachate.

Key sub-community dynamics of medium-chain carboxylate production

BACKGROUND

The carboxylate platform is a promising technology for substituting petrochemicals in the provision of specific platform chemicals and liquid fuels. It includes the chain elongation process that exploits reverse β-oxidation to elongate short-chain fatty acids and forms the more valuable medium-chain variants. The pH value influences this process through multiple mechanisms and is central to effective product formation. Its influence on the microbiome dynamics was investigated during anaerobic fermentation of maize silage by combining flow cytometric short interval monitoring, cell sorting and 16S rRNA gene amplicon sequencing.

RESULTS

Caproate and caprylate titres of up to 6.12 g L-1 and 1.83 g L-1, respectively, were achieved in a continuous stirred-tank reactor operated for 241 days. Caproate production was optimal at pH 5.5 and connected to lactate-based chain elongation, while caprylate production was optimal at pH 6.25 and linked to ethanol utilisation. Flow cytometry recorded 31 sub-communities with cell abundances varying over 89 time points. It revealed a highly dynamic community, whereas the sequencing analysis displayed a mostly unchanged core community. Eight key sub-communities were linked to caproate or caprylate production (rS > | ± 0.7|). Amongst other insights, sorting and subsequently sequencing these sub-communities revealed the central role of Bifidobacterium and Olsenella, two genera of lactic acid bacteria that drove chain elongation by providing additional lactate, serving as electron donor.

CONCLUSIONS

High-titre medium-chain fatty acid production in a well-established reactor design is possible using complex substrate without the addition of external electron donors. This will greatly ease scaling and profitable implementation of the process. The pH value influenced the substrate utilisation and product spectrum by shaping the microbial community. Flow cytometric single cell analysis enabled fast, short interval analysis of this community and was coupled with 16S rRNA gene amplicon sequencing to reveal the major role of lactate-producing bacteria.

Copper (II) addition to accelerate lactic acid production from co-fermentation of food waste and waste activated sludge: Understanding of the corresponding metabolisms, microbial community and predictive functional profiling

Bio-refinery of food waste and waste activated sludge to high value-added chemicals, such as lactic acid, has attracted particular interest in recent years. In this paper, the effect of copper (II) dosing to the organic waste fermentation system on lactic acid production was evaluated, which proved to be a promising method to stimulate high yield of lactic acid (77.0% higher than blank) at dosage of 15 μM-Cu²⁺/g VSS. As mechanism study suggested, copper addition enhanced the activity of α-glycosidase and glycolysis, which increased the substrate for subsequent acidification; whereas, the high dosage (70 μM-Cu²⁺/g VSS) inhibited the conversion of lactic acid to VFA, thus stabilized lactic acid concentration. Microbial community study revealed that small amount of copper (II) at 15 μM/g VSS resulted in the proliferation of Lactobacillus to 82.6%, which mainly produced lactic acid. Finally, the variation of functional capabilities implied that the proposed homeostatic system II was activated at relatively low concentration of copper. Meanwhile, membrane transport function and carbohydrate metabolism were also strengthened. This study provides insights into the effect of copper (II) on the enhancement of lactic acid production from co-fermentation of food waste and waste activated sludge.

Efficient bioconversion of organic wastes to high optical activity of l-lactic acid stimulated by cathode in mixed microbial consortium

Lactic acid is one of the emerging top biomass derived platform chemicals that can be fermented from organic wastes. This study evaluated the potential of Cathodic Electro-Fermentation (CEF) as a novel approach to enhance the yield of high optical activity (OA) of l-lactic acid from organic wastes using mixed microbial consortium. The fermentation process was stimulated through the cathode applied with -100 mV versus standard hydrogen electrode (SHE), which contributed to 4.73 times higher lactic acid productivity (0.6578 g L^-1 h^-1) compared to that in the open circuit control (0.1392 g L^-1 h^-1), and an improved OA of l-lactic acid was also observed (42.3% vs. 3.6% of the open circuit control). The study elucidated that the optimal voltage at -100 mV promoted the conversion of pyruvate to l-lactate by 77.9% compared to the Blank, which triggered the generation of l-lactic acid to occur rapidly even at low concentration of pyruvate. The significant variation of microbial community in family- and genus-level distributions were observed in CEF system. Furthermore, the open-circuit operation test demonstrated that the cathode providing in-situ electron supply was essential to achieve high efficient bioconversion of organic wastes to lactic acid. Our work highlights the feasibility of CEF to steer high value-added fermentation products deriving from organic wastes by the mixed microbial consortium.

Continuous liquid fermentation of pretreated waste activated sludge for high rate volatile fatty acids production and online nutrients recovery

Raw sludge was pretreated, and the separated sludge liquid was used as substrate in a continuous operated up-flow anaerobic sludge blanket reactor to produce volatile fatty acids (VFAs). The highest VFA productivity of continuous fermentation with sludge liquid at an organic loading rate (OLR) of 10.0 kg COD/m³/d was about 5.0-fold and 4.0-fold higher than batch and semi-continuous fermentation with pretreated sludge slurry, respectively. Moreover, the liquid fermentation with an OLR of 10.0 kg COD/m³/d consumed the least energy, which was about 10.57% and 12.12% of batch and semi-continuous sludge fermentation, respectively. When combined with online nitrogen and phosphorus recovery, VFA production further increased by 20.67% and struvite recovery efficiency reached 1.98  ±  0.28 g/g PO₄^3-. The process showed high VFA production, low energy consumption and good nutrients recovery by continuous liquid anaerobic fermentation, significantly increasing the economic potential of VFA production from waste activated sludge.

Organic acid production from potato starch waste fermentation by rumen microbial communities from Dutch and Thai dairy cows

BACKGROUND

Exploring different microbial sources for biotechnological production of organic acids is important. Dutch and Thai cow rumen samples were used as inocula to produce organic acid from starch waste in anaerobic reactors. Organic acid production profiles were determined and microbial communities were compared using 16S ribosomal ribonucleic acid gene amplicon pyrosequencing.

RESULTS

In both reactors, lactate was the main initial product and was associated with growth of Streptococcus spp. (86% average relative abundance). Subsequently, lactate served as a substrate for secondary fermentations. In the reactor inoculated with rumen fluid from the Dutch cow, the relative abundance of Bacillus and Streptococcus increased from the start, and lactate, acetate, formate and ethanol were produced. From day 1.33 to 2, lactate and acetate were degraded, resulting in butyrate production. Butyrate production coincided with a decrease in relative abundance of Streptococcus spp. and increased relative abundances of bacteria of other groups, including Parabacteroides, Sporanaerobacter, Helicobacteraceae, Peptostreptococcaceae and Porphyromonadaceae. In the reactor with the Thai cow inoculum, Streptococcus spp. also increased from the start. When lactate was consumed, acetate, propionate and butyrate were produced (day 3-4). After day 3, bacteria belonging to five dominant groups, Bacteroides, Pseudoramibacter_Eubacterium, Dysgonomonas, Enterobacteriaceae and Porphyromonadaceae, were detected and these showed significant positive correlations with acetate, propionate and butyrate levels.

CONCLUSIONS

The complexity of rumen microorganisms with high adaptation capacity makes rumen fluid a suitable source to convert organic waste into valuable products without the addition of hydrolytic enzymes. Starch waste is a source for organic acid production, especially lactate.

Acidogenic fermentation of the organic fraction of municipal solid waste and cheese whey for bio-plastic precursors recovery - Effects of process conditions during batch tests

The problem of fossil fuels dependency is being addressed through sustainable bio-fuels and bio-products production worldwide. At the base of this bio-based economy there is the efficient use of biomass as non-virgin feedstock. Through acidogenic fermentation, organic waste can be valorised in order to obtain several precursors to be used for bio-plastic production. Some investigations have been done but there is still a lack of knowledge that must be filled before moving to effective full scale plants. Acidogenic fermentation batch tests were performed using food waste (FW) and cheese whey (CW) as substrates. Effects of nine different combinations of substrate to inoculum (S/I) ratio (2, 4, and 6) and initial pH (5, 7, and 9) were investigated for metabolites (acetate, butyrate, propionate, valerate, lactate, and ethanol) productions. Results showed that the most abundant metabolites deriving from FW fermentation were butyrate and acetate, mainly influenced by the S/I ratio (acetate and butyrate maximum productions of 21.4 and 34.5g/L, respectively, at S/I=6). Instead, when dealing with CW, lactate was the dominant metabolite significantly correlated with pH (lactate maximum production of 15.7g/L at pH = 9).

Enhancing sludge biodegradability and volatile fatty acid production by tetrakis hydroxymethyl phosphonium sulfate pretreatment

A new pretreatment method based on tetrakis hydroxymethyl phosphonium sulfate (THPS) biocide was tried to enhance sludge disintegration, and improved sludge biodegradability and subsequent volatile fatty acid (VFA) production. Sludge activity decreased to less than 10% after 2 days pretreatment using 20mg/g-TSS THPS, which also obviously destroyed EPS and cell membrane, and dissolved more biodegradable substances (48.8%) than raw sludge (19.7%). Moreover, 20mg/g-TSS THPS pretreatment shortened fermentation time to 4days and improved VFA production to 2778mg COD/L (4.35 times than that in control). Therein, the sum of n-butyric, n-valeric and iso-valeric acids unexpectedly accounted for 60.5% of total VFA (only 20.1% of that in control). The more high molecular weight VFAs (C4-C5) than low molecular VFAs (C2-C3) resulted from THPS pretreatment benefited to subsequent medium-chain volatile acids (C6-C12) generation to realize the separation and recovery of organic carbon more efficiently.

Electrochemically Driven Fermentation of Organic Substrates with Undefined Mixed Microbial Cultures

Growing scientific interest in mixed microbial culture-based anaerobic biotechnologies for the production of value-added chemicals and fuels from organic waste residues requires a parallel focus on the development and implementation of strategies to control the distribution of products. This study examined the feasibility of an electrofermentation approach, based on the introduction of a polarized (-700 mV vs. the standard hydrogen electrode) graphite electrode in the fermentation medium, to steer the product distribution during the conversion of organic substrates (glucose, ethanol, and acetate supplied as single compounds or in mixtures) by undefined mixed microbial cultures. In batch experiments, the polarized electrode triggered a nearly 20-fold increase (relative to open circuit controls) in the yield of isobutyrate production (0.43±0.01 vs. 0.02±0.02 mol mol^-1 glucose) during the anaerobic fermentation of the ternary mixture of substrates, without adversely affecting the rate of substrate bioconversion. The observed change in the fermentative metabolism was most likely triggered by the (potentiostatic) regulation of the oxidation-reduction potential of the reaction medium rather than by the electrode serving as an electron donor.

Esters production via carboxylates from anaerobic paper mill wastewater treatment

This paper describes a new option for integrated recovery and esterification of carboxylates produced by anaerobic digestion at a pH above the pK_a. The carboxylates (acetate, propionate, butyrate, valerate and lactate) are recovered using a strong anion exchange resin in the bicarbonate form, and the resin is regenerated using a CO₂-expanded alcohol technique, which allows for low chemicals consumption and direct esterification. Paper mill wastewater was used to study the effect of pH and the presence of other inorganic anions and cations on the adsorption and desorption with CO₂-expanded methanol. Calcium, which is present in paper mill wastewater, can cause precipitation problems, especially at high pH. Esters yields ranged from 1.08±0.04mol methyl acetate/mol of acetate_in to 0.57±0.02mol methyl valerate/mol of valerate_in.

Performance, kinetics behaviors and microbial community of internal circulation anaerobic reactor treating wastewater with high organic loading rate: Role of external hydraulic circulation

Performance of internal circulation anaerobic reactor (IC) treating wastewater at high organic loading rate (OLR) and role of external hydraulic circulation were evaluated. When the OLR was increased from 2.50 to 18.94kgCOD/m³/d, COD removal decreased to 85% slightly and methane production increased to 4.49L/L/d with the upflow velocity of 1.0m/h resulted from the additional hydraulic circulation. Withdrawal of external hydraulic circulation led to decrease of COD removal to lower than 60% drastically and methane production by 81%. Accumulation of volatile fatty acids caused decline of pH to below 6.0 and the shift of substrate metabolic pathway to the hybrid fermentation. In addition, both maximum methane production rate and maximum substrate degradation rate obtained from mathematical models decreased significantly. Hydrogenotrophic methanogens including Methanobacterium and Methanocorpusculum predominated in the anaerobic sludge and the shift of microbial community was also observed.

Different substrates and starter inocula govern microbial community structures in biogas reactors

The influence of different starter inocula on the microbial communities in biogas batch reactors fed with fresh maize and maize silage as substrates was investigated. Molecular biological analysis by Denaturing Gradient Gel Electrophoresis (DGGE) of 16S rRNA gene fragments showed that each inoculum bore specific microbial communities with varying predominant phylotypes. Both, bacterial and archaeal DGGE profiles displayed three distinct communities that developed depending on the type of inoculum. Although maize and silage are similar substrates, different communities dominated the lactate-rich silage compared to lactate-free fresh maize. Cluster analysis of DGGE gels showed the communities of the same substrates to be stable with their respective inoculum. Bacteria-specific DGGE analysis revealed a rich diversity with Firmicutes being predominant. The other abundant phylotypes were Bacteroidetes and Synergistetes. Archaea-specific DGGE analysis displayed less diverse community structures, identifying members of the Methanosarcinales as the dominant methanogens present in all the three biogas digesters. In general, the source of inoculum played a significant role in shaping microbial communities. Adaptability of the inoculum to the substrates fed also influenced community compositions which further impacted the rates of biogas production.