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

Continuous medium chain carboxylic acids production from excess sludge by granular chain-elongation process

Short chain carboxylic acids (SCCAs) production is one of the primary ways to recycle excess sludge (ES). However, the high cost for the SCCAs separation/extraction due to its complete miscibility in water hinders the practical application of SCCAs and the popularization of this recycling way. To overcome this barrier, this study performed an emerging chain elongation (CE) technology to upgrade the SCCAs-rich sludge fermentation broth into the highly hydrophobic medium chain carboxylic acids (MCCAs). In a continuous expanded granule sludge bed (EGSB) reactor, a maximal MCCAs yield of 67.39 % and the corresponding concentration of 9.80 g COD/L (224.97 mM C/L) were achieved. By supplying CO₂ at a loading rate of 2 [Formula: see text] to lower the hydrogen partial pressure, the ethanol utilization rate and the resulting MCCAs yield were further improved. In addition, three branched-MCCAs including iso-caproate, iso-heptylate, and iso-caprylate were obtained the first time from waste biomass with the average proportions of 6.17 %, 3.65 %, and 0.8 %, respectively. The branched-MCCAs came from the CE of branched-SCCAs. The granule sludges performing CE were mainly consisted of rod-shaped cells, and dominated by Clostridium sensu stricto and Clostridium IV. This study is expected to lay a foundation for recycling ES to MCCAs.

Volatile fatty acids production from waste activated sludge during anaerobic fermentation: The effect of superfine sand

Superfine sand in waste activated sludge (WAS) increased the uncertainty of anaerobic fermentation. Experiments showed that VFAs production from WAS was positively affected by superfine sand, with an increase from 2513 mg COD/L in the control (without superfine sand) to 3002 mg COD/L with superfine sand. A mechanism study demonstrated that the main factor responsible for the improved VFAs accumulation in response to superfine sand was acetic acid, which increased by nearly 30%. Further investigation exhibited that the process of solubilization and acidification were facilitated by superfine sand and the abundance of anaerobic functional microorganisms was greatly increased. Moreover, the activities of acetate kinase (AK) as well as the quantity of AK encoding gene were greatly promoted by superfine sand. The heat release during WAS anaerobic fermentation with superfine sand was higher than that without superfine sand (25.8 × 10^-3 versus 24.7 × 10^-3 W·min at about 70 min).

Co-Digestion of Grape Marc and Cheese Whey at High Total Solids Holds Potential for Sustained Bioenergy Generation

At the end of fermentation, wine contains approximately 20% (w/v) of solid material, known as grape marc (GM), produced at a yield of 2 t/ha. Cheese manufacture produces cheese whey (CW), which is over 80% of the processed milk, per unit volume. Both waste types represent an important fraction of the organic waste being disposed of by the wine and dairy industries. The objective of this study was to investigate the bioenergy potential through anaerobic codigestion of these waste streams. The best bioenergy profile was obtained from the digestion setups of mixing ratio 3/1 GM/CW (wet weight/wet weight). At this ratio, the inhibitory salinity of CW was sufficiently diluted, resulting in 23.73% conversion of the organic material to methane. On average, 64 days of steady bioenergy productivity was achieved, reaching a maximum of 85 ± 0.4% CH₄ purity with a maximum cumulative methane yield of 24.4 ± 0.11 L CH₄ kg⁻¹ VS. During the fermentation there was 18.63% CODt removal, 21.18% reduction of conductivity whilst salinity rose by 36.19%. It can be concluded that wine and dairy industries could utilise these waste streams for enhanced treatment and energy recovery, thereby developing a circular economy.

Production of volatile fatty acids and H2 for different ratio of inoculum to kitchen waste

The aim of this study was to evaluate the effect of different inoculum ratio on the dark fermentation of kitchen waste in terms of volatile fatty acids (VFAs) and H₂ production. The experiments were performed in batch bioreactors of effective volume 1 L without pH regulation. The ratio between the DS and KW was being increased from 0.11 to 0.51 on a volatile solids (VS) basis, while the initial content of KW was equal to 34.1 g VS/L. Increase of the DS/KW ratio from 0.11 to 0.28 resulted in the rise of VFAs and H₂ production. Further increase in the amount of added DS did not cause a significant change in the production of VFAs and H₂. In the bioreactor with the DS/KW ratio of 0.28, the production of VFAs and H₂ was equal to 16.0 g/L and 68.1 mL/g VS, respectively. Acetic and butyric acids were produced in the largest amount and their content, for DS/KW ratio of 0.28, were equal 37% and 43%, respectively. At the ratio of DS/KW above 0.4, the caproic acid content attained the level of 25%. Based on the DS and KW microbiological analysis, it was observed that dominant bacteria were Bacteroidetes, Firmicutes, Proteobacteria, Spirochaetes and WWE1 at the phylum level.

Enhanced anaerobic fermentation of waste activated sludge by reverse osmosis brine and composition distribution in fermentative liquid

This study reported a "treating waste by waste" strategy to dispose waste activated sludge (WAS), i.e. reverse osmosis (RO) brine-enhanced anaerobic fermentation. RO brine was hazardous by-product from seawater desalination process, which contains numerous Na⁺. After 4-day RO brine-enhanced anaerobic fermentation at Na⁺ concentration of 0.33 mol/L, 5.0 g/L VSS reduction (37.9% of VSS) was achievable, leading to considerable soluble chemical oxygen demand (SCOD) release of 349.6 mg/g VSS. Acetic acid was predominant component in SCOD (31.1%), followed by propionic, butyric, valeric acids and proteins (14.0-17.6%). Sludge solubilization and SCOD composition in the enhanced anaerobic fermentation with RO brine and NaCl agent were similar, whereas less nutrient release and extracellular polymeric substance (EPS) disruption were achieved by RO brine, attributing to the Ca²⁺&Mg²⁺-caused skeleton strengthening on EPS matrix. Such RO brine-based strategy provided environmental and economic benefits, e.g. none chemical consumption, synchronous disposal of WAS and RO brine.

Oriented Fermentation of Food Waste towards High-Value Products: A Review

Food waste has a great potential for resource recovery due to its huge yield and high organic content. Oriented fermentation is a promising method with strong application prospects due to high efficiency, strong robustness, and high-value products. Different fermentation types lead to different products, which can be shifted by adjusting fermentation conditions such as inoculum, pH, oxidation-reduction potential (ORP), organic loading rate (OLR), and nutrients. Compared with other types, lactic acid fermentation has the lowest reliance on artificial intervention. Lactic acid and volatile fatty acids are the common products, and high yield and high purity are the main targets of food waste fermentation. In addition to operational parameters, reactors and processes should be paid more attention to for industrial application. Currently, continuously stirred tank reactors and one-stage processes are used principally for scale-up continuous fermentation of food waste. Electro-fermentation and iron-based or carbon-based additives can improve food waste fermentation, but their mechanisms and application need further investigation. After fermentation, the recovery of target products is a key problem due to the lack of green and economic methods. Precipitation, distillation, extraction, adsorption, and membrane separation can be considered, but the recovery step is still the most expensive in the entire treatment chain. It is expected to develop more efficient fermentation processes and recovery strategies based on food waste composition and market demand.

Cation-exchange resin regeneration waste liquid as alternative NaCl source for enhancing anaerobic fermentation of waste activated sludge: Compositions of dissolved organic matters and chemical conditioning performance

This study reported the NaCl-enhanced anaerobic fermentation with cation-exchange resin regeneration waste liquid (CRWL) as alternative NaCl source for waste activated sludge (WAS) disposal and carbon recovery. Through 4-day CRWL-enhanced anaerobic fermentation at Na⁺ concentration of 0.34 mol/L, the Na⁺-caused sludge disintegration triggered numerous release of dissolved organic matters (DOMs), i.e. 371.6 mg/g VSS, with composition distribution: acetic acid (28.2%) > butyric acid (16.1%) ≈ valeric acid (17.8%) ≈ proteins (16.4%) > propionic acid (14.4%) > unknown (3.2%) > carbohydrates (3.9%). Satisfying chemical conditioning performance for the fermented sludge was observed at the FeCl₃ dosage of 0.3 g/g DS, attributing to the roles of double-layer compression and electric neutralization. The capillary suction time (CST) and sludge cake moisture content were decreased to 60.3 s and 75.1%, against those of 607 s and 93.5% before conditioning, respectively. Such "treating waste by waste" strategy could provide numerous environmental and economic benefits.

Enhanced anaerobic fermentation of waste activated sludge by NaCl assistant hydrolysis strategy: Improved bio-production of short-chain fatty acids and feasibility of NaCl reuse

This study developed an economical approach for enhancing short-chain fatty acids (SCFAs) production from waste activated sludge (WAS) by NaCl assistant anaerobic fermentation. With NaCl addition at 20 g/L, sludge disintegration with extracellular polymeric substance (EPS) disruption and cell lysis were induced owing to the attack of osmotic pressure, which facilitated WAS solubilization with release of biodegradable organic matters. The SCOD sharply increased to 4092 mg/L (SCOD/TCOD = 23.9%) after 2-day hydrolysis, against 1462 mg/L in the control. After 4-day anaerobic fermentation, considerable SCFAs production of 288.2 mg COD/g VSS was achievable. More than 60% of the SCFAs was composed of acetic and propionic acids. The feasibility of bio-electrogenesis in microbial fuel cell (MFC) utilizing fermentative liquid was assessed. As such, the produced SCFAs could be consumed with energy recovery, thereby the used NaCl was reusable, which created environmental and economic benefits, e.g. reduced NaCl consumption and cost, negligible residual NaCl.

An innovative alkaline protease-based pretreatment approach for enhanced short-chain fatty acids production via a short-term anaerobic fermentation of waste activated sludge

This study reported a novel pretreatment approach with combination of alkaline protease (AP) and pH 10 for enhancing short-chain fatty acids (SCFAs) production from waste activated sludge (WAS). Through the AP-based pretreatment, WAS flocs were disintegrated with cell lysis, leading to release of biodegradable organic matters. At the external AP dosage of 5%, SCOD of 5363.7 mg/L (SCOD/TCOD = 32.5%) was achievable after 2-h pretreatment. More than 66% of SCOD was composed of proteins and carbohydrates. Considerable SCFAs of 607 mg COD/g VSS was produced over a short-term anaerobic fermentation of 3 days, which was 5.4 times higher than that in the control. Acetic and propionic acids accounted for 74.1% of the SCFAs. The AP-based approach increased endogenous protease and α-glucosidase activities, facilitating biodegradation of dissolved organic matters and SCFAs production. Such approach is promising for WAS disposal and carbon recovery, the produced SCFAs might supply 60% of carbon gap in wastewater.

Production of volatile fatty acids through co-digestion of sewage sludge and external organic waste: Effect of substrate proportions and long-term operation

Volatile fatty acids (VFAs) are intermediates of anaerobic digestion with high value and wide range of usage. Co-digestion of sewage sludge and external organic waste (OW) for VFA production can help achieve both resource recovery and ensure sustainable and innovative waste management. In view of this, the effect of substrate proportions on VFA production from co-digestion of primary sewage sludge and OW is studied. Long-term operation in a semi-continuous reactor was performed to assess the resilience of such a system and the VFA-rich effluent was tested for its ability to be used as carbon source for denitrification. Co-digestion was initially carried out in batch reactors with OW proportion of 0%, 25%, 50%, 75%, 100% in terms of COD and scaled up in a semi-continuous reactor operation with 50% OW. In the short-term operation in the batch mode, acetic acid dominated, however, increasing OW fraction resulted in increased valeric and caproic acid production. Moreover, in the long-term semi-continuous operation, caproic acid dominated, accounting for ≈55% of VFAs. The VFA-rich effluent from the semi-continuous reactor achieved the highest denitrification rate as a carbon source when compared with acetic acid and methanol. The results demonstrate that co-fermentation can increase VFA yield and shift products from acetic acid to caproic acid in long-term operation and the VFAs can be used within wastewater treatment plants to close the loop.

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.

A novel approach of synchronously recovering phosphorus as vivianite and volatile fatty acids during waste activated sludge and food waste co-fermentation: Performance and mechanisms

This research proposed an innovative approach to synchronously enhance the recovery of phosphorus (P) as vivianite and volatile fatty acids (VFAs) during waste activated sludge (WAS) and food waste (FW) co-fermentation. A high performance was achieved under 30% FW addition and pH uncontrolled, which gained 83.09% of TP recovery as high-purity vivianite (93.90%), together with efficient VFAs production (7671 mg COD/L). The FW supplement could enhance VFAs production and subsequently lower pH to contribute to the release of Fe2+ and PO43-. Also, it could dampen disrupting effects of strong acidic pH on microbial cells (lowering LDH release). Moreover, the flexible pH variation caused by biological acidification could maintain relatively higher microbial activities (increasing enzymes' activities), which was advantageous to the biological effects involved in Fe2+ and PO43 release and VFAs generation. Therefore, this research provide a promising and economic alternative to dispose of WAS and FW simultaneously for valuable resource recovery.

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

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

Influence of temperature on enhancement of volatile fatty acids fermentation from organic fraction of municipal solid waste: Synergism between food and paper components

This study explores individual contributions and synergistic effects of food and paper, main components of organic fraction of municipal solid waste (OFMSW) towards volatile fatty acids (VFA) fermentation under different temperatures (25, 37, 42 and 52 °C). Thanks to the synergism of food and paper component (FC & PC), the results revealed that OFMSW is suitable for VFA production. Maximum VFA production was noticed to be 21.5 mg/L at 42 °C, ~2.1, and 1.42 times higher than fermentation of PC and FC. Enhanced hydrolysis of PC occurred at >37 °C, increasing alkalinity in leachate to 6.7 g/L at 42 °C, thus maintaining a stable pH (5.4-5.6) during acidogenic fermentation. Additionally, 74% of COD is hydrolyzed, of which 79% is converted to VFA based on biodegradable carbon at 42 °C. It is suggested that co-existence of FC and PC can enhance VFA production of OFMSW, and targeted VFA production can be maximized through process optimization.

Effect of Aso limonite on anaerobic digestion of waste sewage sludge

The effect of Aso volcanic limonite was explored in anaerobic digestion using waste sewage sludge (WSS). In this study, methane and hydrogen sulfide were remarkably inhibited when Aso limonite was mixed with WSS as well as a significant reduction of ammonia. Although pH was lowered after adding Aso limonite, methane was still inhibited in neutralized pH condition at 7.0. Hydrolysis stage was not influenced by Aso limonite as supported by the result that a high protease activity was still detected in the presence of the material. However, acidogenesis stage was affected by Aso limonite as indicated by the different productions of organic acids. Acetic acid, was accumulated in the presence of Aso limonite due to the inhibition of methane production, except in the highest concentration of Aso limonite which the production of acetate may be inhibited. Besides, the production of propionate and butyrate reduced in accordance to the increased concentration of Aso limonite. In addition, Archaeal activity (methanogens) in WSS with Aso limonite was low in agreement with the low methane production. Thus, these results indicate that Aso limonite influences the acidogenesis and methanogenesis processes, by which the productions of methane and ammonia were inhibited. On the other hand, in the contactless of Aso limonite during the anaerobic digestion of WSS (Aso limonite was placed in the area of headspace in the vial), Aso limonite had the adsorptive ability for hydrogen sulfide from WSS, but not for methane. This contactless system of Aso limonite may be a practical means to remove hydrogen sulfide without inhibiting methane production as an important bioenergy source.

Cation exchange resin-induced hydrolysis for improving biodegradability of waste activated sludge: Characterization of dissolved organic matters and microbial community

This study reported an efficient and green approach towards facilitating hydrolysis of waste activated sludge (WAS) using cation exchange resin (CER) as a recyclable additive. Through CER-mediated removal of multivalent cations, WAS flocs were disintegrated into small particles with extracellular polymeric substance (EPS) solubilization. At CER dosage of 1.75 g/g SS, SCOD increased to 2579 mg/L (SCOD/TCOD = 15.9%) after 8-h hydrolysis. Afterwards, CER displayed further sludge hydrolysis performance lasting 2 days, i.e. SCOD/TCOD = 34.2%. Meanwhile, proteins, carbohydrates and other organics in dissolved organic matters (DOMs) were major contributors for volatile fatty acids (VFAs) accumulation, with composition percentage: VFAs (58.9%) > proteins (21.8%) > other organics (8.8%) > humic acids (5.9%) > carbohydrates (4.4%). The biodegradable tryptophan-like and tyrosine-like proteins were major proteins, while other organics included amino acids, aliphatic and metabolic intermediates. More than 85.2% of DOMs were easily biodegradable. Moreover, CER-induced hydrolysis modified microbial community structure through inhibiting VFAs-utilizing microbes, while hydrolytic-acidogenic bacteria were enriched, responsible for DOMs biodegradation.

Effects of dissimilatory iron reduction on acetate production from the anaerobic fermentation of waste activated sludge under alkaline conditions

Anaerobic digestion of waste activated sludge (WAS) to produce acetate has recently attracted growing interest, while the slow hydrolytic acidification of sludge and the consumption of acetate by methanogens both decrease the yield of acetate. In this study, Fe₃O₄ was added to a WAS anaerobic digester under alkaline conditions (pH = 10). The concentration of short-chain fatty acids (SCFA) during WAS anaerobic fermentation was found to be affected positively by Fe₃O₄. The maximal SCFA production of the Fe₃O₄-added digester was 3619.4 mg/L, while the maximal SCFA production in the control was 2899.7 mg/L. The increase of SCFA with Fe₃O₄ was mainly resulted from the increase in acetate accumulation (accounting for 90%), because Fe₃O₄ stimulated dissimilatory iron reduction (DIR) that participated in the decomposition of complex organics and the transformation of pronionate and butyrate into acetate. Further investigation showed that each step of hydrolytic-acidification process was promoted except the homoacetogenesis. The activity of enzymes and abundance of microbes relevant to hydrolysis and acidification were in agreement with the above results.

Optimizing the production of short and medium chain fatty acids (SCFAs and MCFAs) from waste activated sludge using different alkyl polyglucose surfactants, through bacterial metabolic analysis

Alkyl polyglucose is an environmentally-friendly biosurfactant, which is able to enhance short-chain fatty acids production with different carbon chain lengths and concentrations, during sludge anaerobic fermentation. This presents a promising strategy for sludge re-utilization by effectively converting hazardous sludge into value-added compounds. The maximum yield of short-chain fatty acids produced from sludge was 479.3 and 462.2 mg COD/g VSS, following pretreatment with APG06 and APG1214, respectively. To the best of our knowledge, the short-chain fatty acid production performance by sludge fermentation reported here, achieved a higher level than reported in previous studies. Additionally, these findings indicate that the production of medium-chain fatty acids from sludge can be induced by alkyl polyglucoses. Finally, the microbial community and enzyme activity were also assessed to reveal the mechanism of short-/medium-chain fatty acids biosynthesis under alkyl polyglucose pretreatment. This study demonstrates that alkyl polyglucose provides an environmentally-friendly and effective strategy for enhancing the production of short-/medium-chain fatty acids from waste activated sludge. These findings are useful for the assessment of alkyl polyglucose-assisted production of short-/medium-chain fatty acids, as well as for understanding the interactions between short-/medium-chain fatty acids and microbial communities with key enzymes, to establish short-/medium-chain fatty acids metabolic pathways during sludge fermentation.

Archaea inhibition: Strategies for the enhancement of volatile fatty acids production from microalgae

In the present study, anaerobic sludge was subjected to thermal and chemical pretreatments to favour VFAs production from a protein-rich waste (i.e. microalgae biomass). Sludge pretreatments have been previously used in hydrogen production; however, information about how they can affect VFAs production from microalgae is still lacking. Thermal pretreatment was studied at: (i) 80 °C for 10 and 30 min; (ii) 120 °C for 10 and 30 min; and (iii) 100 °C for 20 min. 2-bromoethanesulfonate (BES) at 10 mM and 30 mM was used as chemical pretreatment. Besides, a combination of both pretreatment methods (80 °C and 120 °C at 10 mM and 30 mM BES) was also tested. Thermal pretreatment increased organic matter conversions into VFAs (up to 71% COD-VFAs/COD_in) when compared to control values (40% in the untreated anaerobic sludge). Acetic acid was the most abundant VFAs at high temperatures (120 °C) and when BES was employed (up to 60% and 40%, respectively, in terms of COD). On the other hand, propionic acid was the most abundant product at low temperatures and in the untreated anaerobic sludge (up to 60% in terms of COD). This research work might set guidelines in order to choose a suitable sludge pretreatment for VFAs production from microalgae.

Overview of key operation factors and strategies for improving fermentative volatile fatty acid production and product regulation from sewage sludge

In recent years, volatile fatty acid (VFA) production through anaerobic fermentation of sewage sludge, instead of methane production, has been regarded as a high-value and promising roadmap for sludge stabilization and resource recovery. This review first presents the effects of some essential factors that influence VFA production and composition. In the second part, we present an extensive analysis of conventional pretreatment and co-fermentation strategies ultimately addressed to improving VFA production and composition. Also, the effectiveness of these approaches is summarized in terms of sludge degradation, hydrolysis rate, and VFA production and composition. According to published studies, it is concluded that some pretreatments such as alkaline and thermal pretreatment are the most effective ways to enhance VFA production from sewage sludge. The possible reasons for the improvement of VFA production by different methods are also discussed. Finally, this review also highlights several current technical challenges and opportunities in VFA production with spectrum control, and further related research is proposed.

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

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

Effects of nanoscale zero-valent iron on the performance and the fate of antibiotic resistance genes during thermophilic and mesophilic anaerobic digestion of food waste

The effects of nanoscale zero-valent iron (nZVI) on the performance of food waste anaerobic digestion and the fate of antibiotic resistance genes (ARGs) were investigated in thermophilic (TR) and mesophilic (MR) reactors. Results showed that nZVI enhanced biogas production and facilitated ARGs reduction. The maximum CH₄ production was 212.00 ± 4.77 ml/gVS with 5 g/L of nZVI in MR. The highest ARGs removal ratio was 86.64 ± 0.72% obtained in TR at nZVI of 2 g/L. nZVI corrosion products and their contribution on AD performance were analyzed. The abundance of tetracycline genes reduced significantly in nZVI amended digesters. Firmicutes, Chloroflexi, Proteobacteria and Spirochaetes showed significant positive correlations with various ARGs (p < 0.05) in MR and TR. Redundancy analysis indicated that microbial community was the main factor that influenced the fate of ARGs. nZVI changed microbial communities, with decreasing the abundance bacteria belonging to Firmicutes and resulting in the reduction of ARGs.

Enhanced hydrolysis-acidification of high-solids and low-organic-content sludge by biological thermal-alkaline synergism

In this study, a biological thermal-alkaline synergistic system was successfully established to enhance the hydrolysis-acidification efficiency of high-solids and low-organic-content sludge (HS-LOC-S). The results indicated that the highest hydrolysis rate was obtained at pH of 12 (52.62%) leading to the highest production of soluble chemical oxygen demand (SCOD) and soluble protein (SP). The highest acidification rate was observed at pH of 10 (32.15%), leading to the highest production of volatile fatty acids (VFAs). At pH of 10, average sludge size reduced by 24.60%, and the proportion of biodegradable dissolved organic matter (DOM) produced by synergistic system increased by 15.82%, when compared with those of raw sludge. Moreover, results of 16S rRNA clearly validated that the relative abundance of hydrolytic and acidogenic microbes (e.g. Tepidimicrobium, Coprothermobacter) abundantly enriched at pH of 10 (49.88%) was greatly higher than others, which was the main reason for its maximum VFAs accumulation.

Hydrolysis and decomposition of waste activated sludge with combined lysozyme and rhamnolipid treatment: Effect of pH

Effect of pH on waste activated sludge (WAS) hydrolysis and decomposition treating with lysozyme and rhamnolipid combined (Ly + RL) was investigated in this study. Results showed that Ly + RL system could significantly improve the release of soluble organic matters at the optimal RL dosage of 0.3 g/gSS and lysozyme dosage of 0.15 g/gSS. Alkali conditions showed better effect than that of acid on the release of soluble organics, improvement of WAS biodegradability and reduction of big floc size within Ly + RL treatment system and the optimal pH was 10. And 9591.6 mg/L soluble chemical oxygen demand (SCOD), 1612.0 mg/L protein and 1211.6 mg/L polysaccharide were released at pH10 after 12 h co-digestion. 83.7% bacteria and 92.2% archaea were decomposed at pH10. Class Gammaproteobacteria (82.4%) was the predominant bacteria after treated by Ly + RL system, and the treated WAS was beneficial for the subsequent organics bio-degradation and volatile fatty acids accumulation.

Volatile fatty acids production from saccharification residue from food waste ethanol fermentation: Effect of pH and microbial community

In this study, residue from saccharification and centrifugation of food waste ethanol fermentation was used as substrate to produce volatile fatty acids. The effects of different pH (5.5, 6.5, and uncontrolled) on the VFAs concentration, composition, acidogenic efficiency and microbial community distribution were investigated. The results showed that the highest concentration of VFAs was 267.8 ± 8.9 mg COD/g VS at pH of 6.5, and the highest percentage of butyric acid (79.8%) was followed by propionic acid and acetic acid at the end of the reaction. Microbial analysis showed that the contents of Vagococcus and Actinomyces increased, while the contents of Bacteroides and Fermentimonas decreased during anaerobic fermentation. The comparative high pH induced the accumulation of butyric acid. This study provides a new idea for the step anaerobic fermentation of food waste to produce alcohol and acid simultaneously.

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.

Concentrating lactate-carbon flow on medium chain carboxylic acids production by hydrogen supply

Upgrading lactate/carbohydrate-rich waste biomass into medium-chain carboxylic acids (MCCAs) by chain elongation (CE) technology exhibits economic and environmental benefits. However, the largely dispersive lactate-carbon-flow decreases MCCAs yield. This work discovered appropriate H₂ supply could significantly reduce lactate-carbon-flow loss and improve MCCAs production (∼1.65 times) when the system is not operated according to well-defined operating conditions, and revealed corresponding mechanism. Hydrogen (H₂) supply largely enhanced electron efficiency and electron transfer capacity, and H₂ could reduce propionate (from competing acrylate pathway, which should be prevented, but when not possible, the carbon recovery from propionate is possible) to propanol, which was used as electron donor to elongate acetate and propionate. Moreover, H₂ could react with CO₂ (from CE process) to sequentially generate acetate and ethanol, which further contributed to caproate/caprylate generation. Comparing with non-H₂-supplemented test, the lactate-carbon-flow used for MCCAs production was enhanced by ∼28.4% after H₂ supply, and Clostridium spp. were the key discriminative microorganisms.

Enhancement of excess sludge hydrolysis and decomposition by combined lysozyme and rhamnolipid pretreatment

Feasibility of combined lysozyme and rhamolipid (RL) pretreatment on the enhancement of excess sludge (ES) hydrolysis and decomposition was assessed in this study. Results showed lysozyme and RL combined treatment could significantly promote ES hydrolysis and decomposition, an additional 1196.9 mg/L soluble chemical oxygen demand (SCOD), 792.5 mg/L protein and 133.5 mg/L polysaccharide were released compared with the sum of sole RL and sole lysozyme treatment at the optimal RL dosage of 0.3 g/gSS and lysozyme dosage of 0.15 g/gSS after 8 h co-digestion. 45.3% bacteria and 84.5% archaea decomposition degree were gained under the combined treatment at the optimal RL dosage. Class Gammaproteobacteria and genus Methanothrix were the predominant bacteria and archaea with the relative abundance of 72.4% and 60.8%, respectively. After the combined pretreatment, ES was beneficial for volatile fatty acids accumulation and acetic acid dependent methane generating inferred from the results of microbial community composition.

Aged refuse enhances anaerobic fermentation of food waste to produce short-chain fatty acids

In this work, a new technology of intensifying anaerobic fermentation of food waste to accumulate SCFA by using landfill degradation product aged refuse (AR) was reported. The experimental results showed that AR enhanced the accumulation of SCFA, and when the optimal dosage of AR was 300 mg/g, the maximum accumulation of SCFA was 32.5 g/L, which was around 1.9 times that of the blank group. Mechanism investigations had shown that AR accelerated the disintegration of food waste to release soluble carbohydrates and proteins. The model experiment of synthetic wastewater revealed that AR improved both hydrolysis and acidification efficiencies but inhibited the methanogenesis process, thereby accumulating SCFA. 454 high-throughput sequencing showed that AR increased the relative abundance of Clostridium and Sporanaerobacter, which benefited anaerobic hydrolysis of food waste.

Three-stage anaerobic co-digestion of food waste and waste activated sludge: Identifying bacterial and methanogenic archaeal communities and their correlations with performance parameters

A three-stage anaerobic digester setup was configured and evaluated for enhanced methane production during co-digestion of food waste and waste activated sludge and the corresponding bacterial and methanogen communities were characterized. Results showed that the average methane yield (0.496 L/gVS) in the three-stage digester was 13-52% higher than that of one- and two-stage digesters. Compared to controls, an increase of 12-47% in volatile solids reduction was achieved in the three-stage digester (69.3 ± 6.7%). Bacterial phyla Proteobacteria, Firmicutes and Bacteroidetes dominated in one-, two- and three-stage digester while genera Pseudomonas, Tissierella, and Petrimonas were selectively enriched in the three-stage digester due to functional segregation. Taxonomic analysis identified 8 dominant methanogen genera, of which Methanosarcina, Methanosaeta, Methanobacterium and Methanolinea collectively accounted for 80%. With increasing OLR and digester stage number, the dominant methanogenic pathway shifted from hydrogenotrophic pattern to acetoclastic pattern and reached a final synergy of these two.

Upgrading volatile fatty acids production through anaerobic co-fermentation of mushroom residue and sewage sludge: Performance evaluation and kinetic analysis

Due to complex inherent structure of lignocellulosic biomass, inefficient hydrolysis and acidification limits fermentative volatile fatty acids (VFA) production of mushroom residues. Meanwhile, the mushroom residues present insufficient nutrient with a high C/N ratio. To solve this issue, anaerobic co-fermentation of cellulose-rich Oyster champost and sewage sludge was tested to enhance the VFA production, and the effect of proportion of mixed substrate was investigated in this study. The results indicated that the sewage sludge yielded higher VFAs than the Oyster champost in single-substrate fermentative system. The maximal VFA yield of 595 mgCOD/gVS_added was achieved when the proportion of sewage sludge increased to 50% in the mixed substrate. In the co-fermentation system, the optimal C/N ratio and features of mixed substrate contributed to the enhancement in hydrolysis and acidification in terms of organic solubilization and VFA production, respectively. But the co-fermentation could not increase VFA/SCOD ratio, probably due to the existence of refractory products such as humic-like and protein-like materials. Besides, this co-fermentation system had strong buffer capacity and it was not necessary to dose chemicals to control the system pH for stable VFA production. Acetate was the dominant VFA product in co-fermentation systems. A modified-Logistic model fitted co-fermentation of sludge and Oyster champost well, and presented a faster rate and higher efficiency of VFA production.

Continuous acidogenic fermentation: Narrowing the gap between laboratory testing and industrial application

This study explored the potential of acidogenic fermentation of sewage sludge (SS) in an 80 L automatized pilot scale platform. A high-rate VFA production was obtained at HRT 5 d and pH 9, with a volatile fatty acid (VFA) yield of 336 mg VFA g^-1 VS and a VFA productivity of 2.15 kg VFA m^-3 d^-1. During co-fermentation of SS with OFMSW, a reversible pH shift from pH 9 to pH 6, evidenced a higher acidogenic activity which promoted the butyrate metabolic pathway, with 13.97 g COD L^-1 of butyric acid and a VFA peak 23.2 g COD L^-1. The results show the degree of flexibility of mixed culture fermentation systems, where other pH control methods other than steady control could be used to enhance the fermentation process. Ultrafiltration was a feasible technology to obtain a VFA rich permeate where 12.3-26.6 g COD L^-1 could be recovered.

Optimization of urban waste fermentation for volatile fatty acids production

The problem of waste disposal has recently focused on practices for waste recycling and bio-resources valorization. Organic waste produced in urban context together with biological sludge produced in wastewater treatment plants (WWTPs) can be used as renewable feedstock for the production of building blocks of different products, from biopolymers to methyl esters. This paper deals with the optimization of the fermentation process in order to transform urban organic waste (a mixture of pre-treated food waste and biological sludge) into added-value volatile fatty acid (VFA) rich stream, useful for biological processes within a biorefinery technology chain. Different temperatures, pH, hydraulic retention times (HRTs) and organic loading rates (OLRs) were tested both in batch and continuous trials. Batch tests showed the best working conditions at 37 °C and pH 9, using the bio-waste feedstock thermally pre-treated (76 h at 72 °C). These conditions were applied in continuous process, where higher HRT (6.0 d) and lower OLR [7.7 kg VS/(m³ d)] gave the best performances in terms of process yield and maximum VFA level achieved: 0.77 COD_VFA/VS₍₀₎ and 39 g COD_VFA/L. An optimized fermentation process is crucial in a biorefinery perspective since it has to give a final stream of constant composition or tailored products suitable for further applications.

Magnetic biochar catalysts from anaerobic digested sludge: Production, application and environment impact

Regulated disposal or re-utilization of dewatered sludge is of economic benefits and can avoid secondary contamination to the environment; however, feasible and effective management strategies are still lacking. In this study, a peroxydisulfate/zero-valent iron (PDS-ZVI) system is proposed to destroy proteins in soluble extracellular polymeric substances (S-EPS) and loosely bound EPS (LB-EPS) in anaerobic digested sludge (ADS) to improve the dewaterability. Moreover, ADS derived biochars supported via iron oxides (Fe-ADSBC) were generated by dewatering and thermal annealing. Intriguingly, the iron species was discovered to gradually transform from Fe₃O₄ to FeO with increased pyrolysis temperatures from 600 to 1000 °C. The manipulated iron species on the biochar can remarkably impact the catalytic activity in PDS activation and degradation of sulfamethazine (SMT). The in situ radical scavenging and capturing tests revealed that the principal reactive oxygen species (ROS) in Fe-ADSBC/PDS system experienced a variation from OH into SO₄^- at higher annealing temperature (1000 °C). In addition, the carbonaceous ADSBC can promote the catalytic activity of iron oxides by synergistically facilitating the adsorption of reactants and charge transfer through COFe bonds at the interfaces. This study enables the first insights into the properties and catalytic performance of Fe-ADSBC, meanwhile unveils the mechanism, reaction pathways, and environmental impacts of the ultimate transformation products (TPs) from SMT degradation in the Fe-ADSBC/PDS system. The study also contributes to developing value-added green biochar catalysts from bio-wastes towards environmental purification.

Polyamidoamine dendrimer grafted forward osmosis membrane with superior ammonia selectivity and robust antifouling capacity for domestic wastewater concentration

Developing a forward osmosis (FO) membrane with superior ammonia selectivity and robust antifouling performance is important for treating domestic wastewater (DW) but challenging due to the similar polarities and hydraulic radii of NH₄⁺ and water molecules. Herein, we investigated the feasibility of using polyamidoamine (PAMAM) dendrimer to simultaneously enhance the ammonia rejection rate and antifouling capacity of the thin-film composite (TFC) FO membrane. PAMAM dendrimer with abundant, easily-protonated, terminal amine groups was grafted on TFC-FO membrane surface via covalent bonds, which inspired the TFC-FO membrane surface with appreciable Zeta potential (isoelectric point: pH = 5.5) and outstanding hydrophilicity (water contact angle: 39.83 ± 0.57°). Benefiting from the electrostatic repulsion between the protonated amine layer and NH₄⁺-N as well as the concentration-induced diffusion resistance, the introduction of PAMAM dendrimer endowed the grafted membrane with a superior NH₄⁺-N rejection rate of 98.23% and a significantly reduced the reverse solute flux when using NH₄Cl solutions as feed solution. Meanwhile, the perfect balance between the electrostatic repulsion to positively-charged micromoleculer ions (metal ions and NH₄⁺-N) and the electrostatic attraction to negatively-charged macromolecular organic foulants together with the hydrophilic nature of amine groups facilitated the enhancement of the grafted membranes in antifouling capacity and hence the NH₄⁺-N selectivity (rejection rate of 91.81%) during the concentration of raw DW. The overall approach of this work opens up a frontier for preparation of ammonia-selective and antifouling TFC-FO membrane.

Medium chain carboxylic acids production from waste biomass: Current advances and perspectives

Alternative chemicals to diverse fossil-fuel-based products is urgently needed to mitigate the adverse impacts of fossil fuel depletion on human development. To this end, researchers have focused on the production of biochemical from readily available and affordable waste biomass. This is consistent with current guidelines for sustainable development and provides great advantages related to economy and environment. The search for suitable biochemical products is in progress worldwide. Therefore, this review recommends a biochemical (i.e., medium chain carboxylic acids (MCCAs)) utilizing an emerging biotechnological production platform called the chain elongation (CE) process. This work covers comprehensive introduction of the CE mechanism, functional microbes, available feedstock types and corresponding utilization strategies, major methods to enhance the performance of MCCAs production, and the challenges that need to be addressed for practical application. This work is expected to provide a thorough understanding of the CE technology, to guide and inspire researchers to solve existing problems in depth, and motivate large-scale MCCAs production.

A viable approach for commercial VFAs production from sludge: Liquid fermentation in anaerobic dynamic membrane reactor

A novel strategy of liquid fermentation using anaerobic dynamic membrane reactor (AnDMBR) was proposed to enhance volatile fatty acids (VFAs) production from sewage sludge. Results indicated that liquid sludge fermentation in AnDMBR had the potential in commercial VFAs production. VFAs productivity and concentration as well as substrate conversion rate could reach as high as 7.8 kg VFA-COD/m³ d, 60 g/L and 0.38 kg VFA-COD/kg VS, respectively. Moreover, dynamic membrane was stably operated for approximately 70 days. During the operational period, membrane flux was increased from 6.25 to 25 L/m² d and only once online membrane cleaning was implemented. Results of microbial analyses showed bacterial richness and evenness in AnDMBR were increased by membrane separation and organic loading rate (OLR) increase, but reduced by excessive OLR, which should led the variations in the performances of AnDMBR. Furthermore, the necessity of liquid sludge fermentation for VFAs production was further confirmed by economic assessment and the bioavailability analysis of the residual solids in pretreated sludge. The residual solid was proved to be not conducive to enhance VFAs yield. Conversely, the energy consumption for VFAs production could be reduced from over 100 to below 20 kwh/kg VFAs by avoiding the "useless" residual solids entering into fermenters.

Engineering of phosphoenolpyruvate: carbohydrate phosphotransferase system increased acetate assimilation in Escherichia coli

Acetate is a potential low-cost carbon source and can be used for microbial production of valuable chemicals in Escherichia coli. In this study, separate and simultaneous inactivation of the ptsG, ptsI, and ptsP genes involved in the phosphoenolpyruvate:carbohydrate phosphotransferase system was performed in E. coli and the effects on cell growth and acetate assimilation were evaluated. The mutant strain with double deletion of ptsG and ptsP exhibited faster acetate use than the other mutants. Inactivation of ptsI seriously reduced acetate consumption. This work provides a novel engineering target for improving acetate use rates.

The effect of redox capacity of humic acids on hexachlorobenzene dechlorination during the anaerobic digestion process

Hexachlorobenzene (HCB) dechlorination affected by humic acids (HA) was evaluated in terms of HA redox capacity, HA concentrations, and microbial community, as well as the correlation between HA redox capacity values and HCB concentrations. With addition of HA in the initial stage, redox capacity values increased by 2.19 meq/L (80 mg/L of HA addition, HA₈₀), 2.51 meq/L (120 mg/L of HA addition, HA₁₂₀), and 3.64 meq/L (200 mg/L of HA addition, HA₂₀₀), respectively. The addition of HA could prominently enhance the HCB degradation rate. However, the concentration and the redox capacity of HA decreased during the anaerobic digestion process. Illumina MiSeq sequencing showed that microbial community affected by HA. Bacillus, Comamonas, and Pseudomonas were the predominant genera during the HCB dechlorination treatment. Moreover, Bacillus and Pseudomonas can improve HA electron transfer capability and promote the dechlorination of HCB.

The effect of ISR on OFMSW during acidogenic fermentation for the production of AD precursor: kinetics and synergies

Acidogenic fermentation of organic fraction of municipal solid waste (OFMSW) and it's components (food waste and paper wastes) was studied in batch percolator reactor without artificial pH adjustment.

Bio-based volatile fatty acid production and recovery from waste streams: Current status and future challenges

Bio-based volatile fatty acid (VFA) production from waste-stream is getting attention due to increasing market demand and wide range usage area as well as its cost-effective and environmentally friendly approach. The aim of this paper is to give a comprehensive review of bio-based VFA production and recovery methods and to give an opinion on future research outlook. Effects of operation conditions including pH, temperature, retention time, type of substrate and mixed microbial cultures on VFA production and composition were reviewed. The recovery methods in terms of gas stripping with absorption, adsorption, solvent extraction, electrodialysis, reverse osmosis, nanofiltration, and membrane contractor of VFA were evaluated. Furthermore, strategies to enhance bio-based VFA production and recovery from waste streams, specifically, in-line VFA recovery and bioaugmentation, which are currently not used in common practice, are seen as some of the approaches to enhance bio-based VFA production.

Effect of pH and temperature on microbial community structure and carboxylic acid yield during the acidogenic digestion of duckweed

BACKGROUND

Duckweeds (Lemnaceae) are efficient aquatic plants for wastewater treatment due to their high nutrient-uptake capabilities and resilience to severe environmental conditions. Combined with their rapid growth rates, high starch, and low lignin contents, duckweeds have also gained popularity as a biofuel feedstock for thermochemical conversion and alcohol fermentation. However, studies on the acidogenic anaerobic digestion of duckweed into carboxylic acids, another group of chemicals which are precursors of higher-value chemicals and biofuels, are lacking. In this study, a series of laboratory batch experiments were performed to determine the favorable operating conditions (i.e., temperature and pH) to maximize carboxylic acid production from wastewater-derived duckweed during acidogenic digestion. Batch reactors with 25 g/l solid loading were operated anaerobically for 21 days under mesophilic (35 °C) or thermophilic (55 °C) conditions at an acidic (5.3) or basic (9.2) pH. At the conclusion of the experiment, the dominant microbial communities under various operating conditions were assessed using high-throughput sequencing.

RESULTS

The highest duckweed-carboxylic acid conversion of 388 ± 28 mg acetic acid equivalent per gram volatile solids was observed under mesophilic and basic conditions, with an average production rate of 0.59 g/l/day. This result is comparable to those reported for acidogenic digestion of other organics such as food waste. The superior performance observed under these conditions was attributed to both chemical treatment and microbial bioconversion. Hydrogen recovery was only observed under acidic thermophilic conditions, as 23.5 ± 0.5 ml/g of duckweed volatile solids added. More than temperature, pH controlled the overall structure of the microbial communities. For instance, differentially abundant enrichments of Veillonellaceae acidaminococcus were observed in acidic samples, whereas enrichments of Clostridiaceae alkaliphilus were found in the basic samples. Acidic mesophilic conditions were found to enrich acetoclastic methanogenic populations over processing times longer than 10 days.

CONCLUSIONS

Operating conditions have a significant effect on the yield and composition of the end products resulting from acidogenic digestion of duckweed. Wastewater-derived duckweed is a technically feasible alternative feedstock for the production of advanced biofuel precursors; however, techno-economic analysis is needed to determine integrated full-scale system feasibility and economic viability.

Production of succinate by recombinant Escherichia coli using acetate as the sole carbon source

Acetate is a potential low-cost carbon source that can be generated by biological and chemical processes. In this study, deletion of sdhAB encoding succinate dehydrogenase, iclR encoding the isocitrate lyase regulator, and maeB encoding the malic enzyme, and overexpression of acs encoding acetyl-CoA synthetase, gltA encoding citrate synthase, and acnB encoding aconitate hydratase in the wild-type Escherichia coli MG1655 strain yielded the recombinant E. coli strain WCY-7, which could synthesize succinate from acetate. After 48 h batch fermentation, this strain accumulated 11.23 mM succinate from 50 mM sodium acetate. This work indicates that microbial fermentation using acetate as the sole carbon source may be a suitable route to produce high yields of the valuable chemicals.

Assessing the fermentation quality and microbial community of the mixed silage of forage soybean with crop corn or sorghum

The silage quality of forage soybean (FS) rich in protein with crop corn (CN) or sorghum (SG) rich in water soluble carbohydrate was investigated, and microbial community after ensiling was analyzed. Results showed that pH in mixed silages dropped to 3.5-3.8 lower than 100%FS silage (4.5). Microbial analysis indicated that mixed ensiling could influence the microbial community. Although Lactobacillus and Weissella were the dominant genera in all silage samples, Lactobacillus abundance in mixed silages (33-76%) was higher compared with 100%FS silage (27%). In conclusion, FS ensiled with CN or SG could be an alternative approach to improve FS silage quality.

A novel process for volatile fatty acids production from syngas by integrating with mesophilic alkaline fermentation of waste activated sludge

The present study proposed and demonstrated a novel process for the bioconversion of syngas (mainly CO and H₂) to valuable volatile fatty acids (VFA) by integrating with mesophilic alkaline fermentation of waste activated sludge (WAS). The results showed that although pH 9 was suitable for VFA production from WAS, 62.5% of the consumed CO was converted to methane due to the presence of hydrogenogenic pathway for CO conversion. The increase of pH from 9 to 9.5 inhibited the methane production from CO because of the possible presence of only acetogenic pathway for CO conversion. However, methane was still produced from H₂ contained in syngas through hydrogenotrophic methanogenesis, and around 32-34% of the consumed syngas was converted to methane. At both pH 9 and 9.5, methane was produced by hydrogenotrophic methanogens Methanobacteriales. Further increase of pH to 10 effectively inhibited methane production from syngas, and efficient VFA (mainly acetate with the concentration of around 135 mM) production by simultaneous conversion of syngas and WAS was achieved. High acetate concentrations (>150 mM) were shown to have serious negative effects on the conversion of syngas. The addition of syngas to the mesophilic alkaline fermentation of WAS at pH 10 not only resulted in the enrichment of some known bacteria related with syngas conversion, but also changed the microbial community compositions for the fermentation of WAS.

From sewage sludge and agri-food waste to VFA: Individual acid production potential and up-scaling

Volatile fatty acid (VFA) production through anaerobic fermentation may constitute an innovative solution for organic waste management within the context of circular economy. In the present study, the evolution of individual VFA during laboratory-scale fermentation of sewage sludge (SS), winery wastewater (W_ww) and meat and bone meal (MBM) was assessed, focusing on the effect of pH (5.5 and 10) and temperature (35 and 55 °C). Up-scaling of the fermentation process was evaluated in batch operation. The latter showed that specific VFA could be produced, giving similar individual evolution to lab-scale testing. To be precise, acetic acid percentage ranged within 30-65% and increased up to 5900 mg O₂ L^-1 during SS fermentation at 55 °C and pH 9. In addition, 60% butyric acid was reached during W_ww acid fermentation at 55 °C, which corresponded to 6670 mg O₂ L^-1 concentration in the fermentation broth. Regarding valeric acid, over 20% proportion and 2700 mg O₂ L^-1 were reached in MBM acid fermentation at 35 °C. Finally, iso-valeric maximum level ranged within 15-17% in SS alkaline fermentation at 55 °C, which represented a concentration close to 2000 mg O₂ L^-1. Interestingly, co-fermentation of agri-food waste and SS at thermophilic temperature and alkaline pH, boosted the VFA concentration 1.7-2 fold, which suggests that anaerobic co-fermentation of substrates from different nature could give promising outcomes in full-scale operation.

Direct Growth of Bacteria in Headspace Vials Allows for Screening of Volatiles by Gas Chromatography Mass Spectrometry

Bacterially produced volatile organic compounds (VOCs) can modify growth patterns of eukaryotic hosts and competing/cohabiting microbes. These compounds have been implicated in skin disorders and attraction of biting pests. Current methods to detect and characterize VOCs from microbial cultures can be laborious and low-throughput, making it difficult to understand the behavior of microbial populations. In this work we present an efficient method employing gas chromatography/mass spectrometry with autosampling to characterize VOC profiles from solid-phase bacterial cultures. We compare this method to complementary plate-based assays and measure the effects of growth media and incubation temperature on the VOC profiles from a well-studied Pseudomonas aeruginosa PAO1 system. We observe that P. aeruginosa produces longer chain VOCs, such as 2-undecanone and 2-undecanol in higher amounts at 37°C than 30°C. We demonstrate the throughput of this method by studying VOC profiles from a representative collection of skin bacterial isolates under three parallel growth conditions. We observe differential production of various aldehydes and ketones depending on bacterial strain. This generalizable method will support screening of bacterial populations in a variety of research areas.