Optimization of methane production from solid tuna waste: Thermal pretreatment and co-digestion

Fish canning industries generate large amounts of solid waste during their processing operations, creating a significant environmental challenge. Nonetheless, this waste can be efficiently and sustainably treated through anaerobic digestion. In this study, the potential of biogas production from anaerobic digestion of thermally pretreated and co-digested solid tuna waste was investigated. The thermal pretreatment of raw fish viscera resulted in a 50 % increase in methane yield, with a production of 0.27 g COD-CH4/g COD added. However, this pretreatment did not lead to a significant increase in biogas production for cooked tuna viscera. When non-thermally pretreated raw viscera was tested, a large accumulation of volatile fatty acids and long chain fatty acids was observed, with levels reaching 21 and 6 g COD/L, respectively. On the other hand, anaerobic co-digestion of cooked tuna viscera with fat waste significantly enhanced methane production, achieving 0.87 g COD-CH4/g COD added. In contrast, co-digestion of cooked tuna viscera with dairy waste and sewage sludge resulted in notably lower yields of 0.36 and 0.46 g COD-CH4/g COD added, respectively. These results may be related to the C/N ratio, which was found to be within the optimal range for anaerobic digestion only in the tuna and fat waste co-digestion assay.

Effect of electron acceptors on product selectivity and carbon flux in carbon chain elongation with Megasphaera hexanoica

Megasphaera hexanoica is a bacterial strain following the reverse β-oxidation pathway to synthesize caproate (CA) using lactate (LA) as an electron donor (ED) and acetate (AA) or butyrate (BA) as electron acceptors (EA). Differences in the type and concentration of EA lead to distinctions in product distribution and energy bifurcation of carbon fluxes in ED pathways, thereby affecting CA production. In this study, the effect of various ratios of AA, BA, and AA+BA as EA on carbon flux and CA specific titer during the carbon chain elongation in M. hexanoica was explored. The results indicated that the maximum levels of CA were 18.81 mM and 31.48 mM when the molar ratios of LA/AA and LA/BA were 10:1 and 3:1, respectively. Meanwhile, when AA and BA were used as combined EA (LA, AA, and BA molar amounts of 100, 23, and 77 mM), a maximum CA production of 39.45 mM was obtained. Further analysis revealed that the combined EA exhibited a CA production carbon flux of 49 % (4.3 % and 19.5 % higher compared to AA or BA, respectively) and a CA production specific titer of 45.24 mol (80.89 % and 58.51 % higher compared to AA or BA, respectively), indicating that the effective carbon utilization rate and CA production efficiency were greatly improved. Finally, a scaled-up experiment was conducted in a 1.2 L (working volume) automated bioreactor, implying high biomass (optical density at 600 nm or OD600 = 1.809) and a slight decrease in CA production (28.45 mM). A decrease in H2 production (4.11 g/m3) and an increase in CO2 production (0.632 g/m3) demonstrated the appropriate metabolic adaptation of M. hexanoica to environmental changes such as stirring shear.

Unlocking the potential of one-carbon gases (CO2, CO) for concomitant bioproduction of β-carotene and lipids

This study investigates the use of a Yarrowia lipolytica strain for the bioconversion of syngas-derived acetic acid into β-carotene and lipids. A two-stage process was employed, starting with the acetogenic fermentation of syngas by Clostridium aceticum, metabolising CO, CO2, H2, to produce acetic acid, which is then utilized by Y. lipolytica for simultaneous lipid and β-carotene synthesis. The research demonstrates that acetic acid concentration plays a pivotal role in modulating lipid profiles and enhancing β-carotene production, with increased acetic acid consumption leading to higher yields of these compounds. This approach showcases the potential of using one-carbon gases as substrates in bioprocesses for generating valuable bioproducts, providing a sustainable and cost-effective alternative to more conventional feedstocks and substrates, such as sugars.

Carbon dioxide as key player in chain elongation and growth of Clostridium kluyveri: Insights from batch and bioreactor studies

Chain elongation technology allows medium-chain fatty acids (MCFAs) production as an alternative to fossil resources. Clostridium kluyveri generates n-caproate primarily from ethanol and acetate, presumably requiring CO2 for growth. Here, the impact of CO2 on C. kluyveri was explored. Bottle studies revealed the bacterium's adaptability to low CO2 levels, even in conditions with minimal dissolved NaHCO3 (0.0003 M) and unfavorable pH (below 6) under 1 bar CO2. Bioreactor investigations demonstrated a direct correlation between CO2 availability and bacterial growth. The highest n-caproate production (11.0 g/L) with 90.1 % selectivity was achieved in a bioreactor with continuous CO2 supply at 3 mL/min. Additional bottle experiments pressurized with 1 bar CO2 and varying ethanol:acetate ratios (1:1, 2:1, 4:1) also confirmed CO2 consumption by C. kluyveri. However, increasing the ethanol:acetate ratio did not enhance n-caproate selectivity, likely due to overly acidic pH conditions. These findings provide insights into chain-elongators responses under diverse conditions.

Unveiling novel pathways and key contributors in the nitrogen cycle: Validation of enrichment and taxonomic characterization of oxygenic denitrifying microorganisms in environmental samples

Microorganisms play a crucial role in both the nitrogen cycle and greenhouse gas emissions. A recent discovery has unveiled a new denitrification pathway called oxygenic denitrification, entailing the enzymatic reduction of nitrite to nitric oxide (NO) by a putative nitric oxide dismutase (nod) enzyme. In this study, the presence of the nod gene was detected and subsequently enriched in anaerobic-activated sludge, farmland soil, and paddy soil samples. After 150 days, the enriched samples exhibited significant denitrification, and concomitant oxygen production. The removal efficiency of nitrite ranged from 64.6 % to 79.0 %, while the oxygen production rate was between 15.4 μL/min and 18.6 μL/min when exposed to a sole nitrogen source of 80 mg/L sodium nitrite. Additionally, batch experiments and kinetic analyses revealed the intricate pathways and underlying mechanisms governing the oxygenic denitrification reaction by using CARBOXY-PTIO, 18O-labelled water, and acetylene to unravel the intricacies of the reaction. The quantitative polymerase chain reaction (qPCR) results indicated a significant surge in the abundance of nod genes, escalating from 7.59 to 10.12-fold. Moreover, analysis of 16S ribosomal DNA (rDNA) amplicons revealed Proteobacteria as the dominant phylum and Thauera as the main genus, with the presumed affiliation. In this study, a new nitrogen conversion pathway, oxygenic denitrification, was discovered in environmental samples. This process provides the possibility for the control of nitrous oxide in the treatment of nitrogenous wastewater.

Sequential bioconversion of C1-gases (CO, CO2, syngas) into lipids, through the carboxylic acid platform, with Clostridium aceticum and Rhodosporidium toruloides

Syngas (CO, CO2, H2) was effectively bioconverted into lipids in a two-stage process. In the first stage, C1-gases were bioconverted into acetic acid by the acetogenic species Clostridium aceticum through the Wood-Ljungdahl metabolic pathway in a stirred tank bioreactor, reaching a maximum acetic acid concentration of 11.5 g/L, with a production rate of 0.05 g/L·h. Throughout this experiment, samples were extracted at different periods, i.e., different concentrations, to be used in the second stage, aiming at the production of lipids from acetic acid. The yeast Rhodosporidium toruloides, inoculated in the acetogenic medium, was able to efficiently accumulate lipids from acetic acid generated in the first stage. The best results, in terms of lipid content, dry biomass, biomass yield (Y(X/S)) and lipid yield (Y(L/S)) were 39.5% g/g dry cell weight, 3 g/L, 0.35 and 0.107, respectively. In terms of abundance, the lipid profile followed the order: C18:1 > C16:0 > C18:2 > C18:0 > Others. Experiments were also performed to determine the toxicity exerted by high concentrations of acetic acid on R. toruloides, resulting in inhibition at initial acid concentrations around 18 g/L leading to a higher lag phase and being lethal to the yeast at initial acetic acid concentrations around 22 g/L and above. This research paves the way for a novel method of growing oleaginous yeasts to produce sustainable biofuels from syngas or C1-pollutant gases.

Integrated fermentative process for lipid and β-carotene production from acetogenic syngas fermentation using an engineered oleaginous Yarrowia lipolytica yeast

An engineered Yarrowia lipolytica strain was successfully employed to produce β-carotene and lipids from acetic acid, a product of syngas fermentation by Clostridium aceticum. The strain showed acetic acid tolerance up to concentrations of 20 g/L. Flask experiments yielded a peak lipid content of 33.7 % and β-carotene concentration of 13.6 mg/g under specific nutrient conditions. The study also investigated pH effects on production in bioreactors, revealing optimal lipid and β-carotene contents at pH 6.0, reaching 22.9 % and 44 mg/g, respectively. Lipid profiles were consistent across experiments, with C18:1 being the dominant compound at approximately 50 %. This research underscores a green revolution in bioprocessing, showing how biocatalysts can convert syngas, a potentially polluting byproduct, into valuable β-carotene and lipids with a Y. lipolytica strain.

Neural network-based performance assessment of one- and two-liquid phase biotrickling filters for the removal of a waste-gas mixture containing methanol, α-pinene, and hydrogen sulfide

The performance of one- and two-liquid phase biotrickling filters (OLP/TLP-BTFs) treating a mixture of gas-phase methanol (M), α-pinene (P), and hydrogen sulfide (H) was assessed using artificial neural network (ANN) modeling. The best ANN models with the topologies 3-9-3 and 3-10-3 demonstrated an exceptional capacity for predicting the performance of O/TLP-BTFs, with R2 > 99%. The analysis of causal index (CI) values for the model of OLP-BTF revealed a negative impact of M on P removal (CI = -2.367), a positive influence of P and H on M removal (CI = +7.536 and CI = +3.931) and a negative effect of H on P removal (CI = -1.640). The addition of silicone oil in TLP-BTF reduced the negative impact of M and H on P degradation (CI = -1.261 and CI = -1.310, respectively) compared to the OLP-BTF. These findings suggested that silicone oil had the potential to improve P availability to the biofilm by increasing the concentration gradient of P between the air/gas and aqueous phases. Multi-objective particle swarm optimization (MOPSO) suggested an optimum operational condition, i.e. inlet M, P, and H concentrations of 1.0, 1.1, and 0.3 g m-3, respectively, with elimination capacities (ECs) of 172.1, 26.5, and 0.025 g m-3 h-1 for OLP-BTF. Likewise, one of the optimum operational conditions for TLP-BTF is achievable at inlet concentrations of 4.9, 1.7, and 0.8 g m-3, leading to the optimum ECs of 299.7, 52.9, and 0.072 g m-3 h-1 for M, P, and H, respectively. These results provide important insights into the treatment of complex waste gas mixtures, addressing the interactions between the pollutant removal characteristics in OLP/TLP-BTFs and providing novel approaches in the field of biological waste gas treatment.

Effect of pH and medium composition on chain elongation with Megasphaera hexanoica producing C4-C8 fatty acids

Introduction

Chain elongation technology, which involves fermentation with anaerobic bacteria, has gained attention for converting short and medium chain substrates into valuable and longer-chain products like medium chain fatty acids (MCFAs). In the recent past, the focus of studies with pure chain elongating cultures was on species of other genera, mainly Clostridium kluyveri. Recently, other chain elongators have been isolated that deserve further research, such as Megasphaera hexanoica.

Methods

In this study, batch studies were performed in bottles with two different media to establish the optimal conditions for growth of M. hexanoica: (a) a medium rich in different sources of nitrogen and (b) a medium whose only source of nitrogen is yeast extract. Also, batch bioreactor studies at pH values of 5.8, 6.5 and 7.2 were set up to study the fermentation of lactate (i.e., electron donor) and acetate (i.e., electron acceptor) by M. hexanoica.

Results and discussion

Batch bottle studies revealed the yeast extract (YE) containing medium as the most promising in terms of production/cost ratio, producing n-caproate rapidly up to 2.62 ± 0.24 g/L. Subsequent bioreactor experiments at pH 5.8, 6.5, and 7.2 confirmed consistent production profiles, yielding C4-C8 fatty acids. A fourth bioreactor experiment at pH 6.5 and doubling both lactate and acetate concentrations enhanced MCFA production, resulting in 3.7 g/L n-caproate and 1.5 g/L n-caprylate. H2 and CO2 production was observed in all fermentations, being especially high under the increased substrate conditions. Overall, this study provides insights into M. hexanoica's behavior in lactate-based chain elongation and highlights optimization potential for improved productivity.

Comparative assessment of the performance of one- and two-liquid phase biotrickling filters for the simultaneous abatement of gaseous mixture of methanol, α-pinene, and hydrogen sulfide

A gaseous mixture of methanol (M), α-pinene (P), and hydrogen sulfide (H) was treated in one/two-liquid phase biotrickling filters (OLP/TLP-BTFs) at varying inlet concentrations and at an empty bed residence time (EBRT) of 57 s. The performance of TLP-BTF [BTF (A)] improved significantly in terms of M and P removal due to the presence of silicone oil at 5% v/v. The maximum elimination capacities (ECs) of M, P, and H in BTF (A) were obtained as 309, 73, and 56 g m-3 h-1, respectively. While, the maximum ECs achieved in the BTF operated without silicone oil [BTF (B)] were 172, 28, and 21 g m-3 h-1 for M, P, and H removal, respectively. Increasing the inlet concentration of H from 32 to 337 ppm inhibited P removal in both the BTFs. The presence of silicone oil enhanced gas-liquid mass transfer, prevented the BTF from experiencing substrate inhibition effects and allowed reaching high ECs for M and P. The experiments showed promising results for the long-term operation of removal of M, P, and H mixture in a one-stage TLP-BTF with the decreasing negative effects of M and H on P degradation.

Production of short-chain fatty acids (SCFAs) as chemicals or substrates for microbes to obtain biochemicals

Carboxylic acids have become interesting platform molecules in the last years due to their versatility to act as carbon sources for different microorganisms or as precursors for the chemical industry. Among carboxylic acids, short-chain fatty acids (SCFAs) such as acetic, propionic, butyric, valeric, and caproic acids can be biotechnologically produced in an anaerobic fermentation process from lignocellulose or other organic wastes of agricultural, industrial, or municipal origin. The biosynthesis of SCFAs is advantageous compared to chemical synthesis, since the latter relies on fossil-derived raw materials, expensive and toxic catalysts and harsh process conditions. This review article gives an overview on biosynthesis of SCFAs from complex waste products. Different applications of SCFAs are explored and how these acids can be considered as a source of bioproducts, aiming at the development of a circular economy. The use of SCFAs as platform molecules requires adequate concentration and separation processes that are also addressed in this review. Various microorganisms such as bacteria or oleaginous yeasts can efficiently use SCFA mixtures derived from anaerobic fermentation, an attribute that can be exploited in microbial electrolytic cells or to produce biopolymers such as microbial oils or polyhydroxyalkanoates. Promising technologies for the microbial conversion of SCFAs into bioproducts are outlined with recent examples, highlighting SCFAs as interesting platform molecules for the development of future bioeconomy.

Production of biofuels from C1 -gases with Clostridium and related bacteria-Recent advances

Clostridium spp. are suitable for the bioconversion of C₁ -gases (e.g., CO₂ , CO and syngas) into different bioproducts. These products can be used as biofuels and are reviewed here, focusing on ethanol, butanol and hexanol, mainly. The production of higher alcohols (e.g., butanol and hexanol) has hardly been reviewed. Parameters affecting the optimization of the bioconversion process and bioreactor performance are addressed as well as the pathways involved in these bioconversions. New aspects, such as mixotrophy and sugar versus gas fermentation, are also reviewed. In addition, Clostridia can also produce higher alcohols from the integration of the Wood-Ljungdahl pathway and the reverse ß-oxidation pathway, which has also not yet been comprehensively reviewed. In the latter process, the acetogen uses the reducing power of CO/syngas to reduce C₄ or C₆ fatty acids, previously produced by a chain elongating microorganism (commonly Clostridium kluyveri), into the corresponding bioalcohol.

Accumulation of lipids by the oleaginous yeast Yarrowia lipolytica grown on carboxylic acids simulating syngas and carbon dioxide fermentation

Volatile fatty acids (VFAs) can be considered as low-cost carbon substrates for lipid accumulation by oleaginous yeasts. This study demonstrates that a common mixture of VFAs, typically obtained from the anaerobic fermentation of C1-gases by some acetogenic bacteria, can be used in a second aerobic fermentation with the yeast Yarrowia lipolytica to obtain lipids as precursors of biodiesel. In the batch experiments, the preference of Yarrowia lipolytica W29 for acetic acid over butyric and caproic acids was demonstrated, with the highest consumption rate reaching 0.664 g/L·h. In the bioreactor experiments, the amount initial biomass inoculated, as well as the initial acid concentration, were found to have a significant influence on the process. Though the lipid content was relatively low, it can be optimized and further improved. Oleic, linoleic and palmitic acids accounted for about 80 % of the fatty acids in the lipids, which makes them suitable for biodiesel.

Effect of Endogenous and Exogenous Butyric Acid on Butanol Production From CO by Enriched Clostridia

Butanol is a potential renewable fuel. To increase the selectivity for butanol during CO fermentation, exogenous acetic acid and ethanol, exogenous butyric acid or endogenous butyric acid from glucose fermentation have been investigated using CO as reducing power, with a highly enriched Clostridium sludge. Addition of 3.2 g/L exogenous butyric acid led to the highest 1.9 g/L butanol concentration with a conversion efficiency of 67%. With exogenous acetate and ethanol supply, the butanol concentration reached 1.6 g/L at the end of the incubation. However, the presence of acetic acid and ethanol favoured butanol production to 2.6 g/L from exogenous butyric acid by the enriched sludge. Finally, exogenous 14 g/L butyric acid yielded the highest butanol production of 3.4 g/L, which was also among the highest butanol concentration from CO/syngas fermentation reported so far. CO addition triggered butanol production from endogenous butyric acid (produced from glucose, Glucose + N₂) with as high as 58.6% conversion efficiency and 62.1% butanol yield. However, no efficient butanol production was found from glucose and CO co-fermentation (Glucose + CO), although a similar amount of endogenous butyric acid was produced compared to Glucose + N₂. The Clostridium genus occupied a relative abundance as high as 82% from the initial inoculum, while the Clostridia and Bacilli classes were both enriched and dominated in Glucose + N₂ and Glucose + CO incubations. This study shows that the supply of butyric acid is a possible strategy for enhancing butanol production by CO fed anaerobic sludge, either via exogenous butyric acid, or via endogenous production by sugar fermentation.

Selective butanol production from carbon monoxide by an enriched anaerobic culture

An anaerobic mixed culture able to grow on pure carbon monoxide (CO) as well as syngas (CO, CO₂ and H₂), that produced unusual high concentrations of butanol, was enriched in a bioreactor with intermittent CO gas feeding. At pH 6.2, it mainly produced acids, generally acetic and butyric acid. After adaptation, under stress conditions of CO exposure at a partial pressure of 1.8 bar and low pH (e.g., 5.7), the enrichment accumulated ethanol, but also high amounts of butanol, up to 6.8 g/L, never reported before, with a high butanol/butyric acid molar ratio of 12.6, highlighting the high level of acid to alcohol conversion. At the end of the assay, both the acetic acid and ethanol concentrations decreased, with concomitant butyric acid production, suggesting C₂ to C₄ acid bioconversion, though this was not a dominant bioconversion process. The reverse reaction of ethanol oxidation to acetic acid was observed in the presence of CO₂ produced during CO fermentation. Interestingly, butanol oxidation with simultaneous butyric acid production occurred upon production of CO₂ from CO, which has to the best of our knowledge never been reported. Although the sludge inoculum contained a few known solventogenic Clostridia, the relative taxonomic abundance of the enriched sludge was diverse in Clostridia and Bacilli classes, containing known solventogens, e.g., Clostridium ljungdhalii, Clostridium ragsdalei and Clostridium coskatii, confirming their efficient enrichment. The relative abundance of unassigned Clostridium species amounted to 27% with presumably novel ethanol/butanol producers.

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.

Efficient production of n-caproate from syngas by a co-culture of Clostridium aceticum and Clostridium kluyveri

In recent years, the possibility of merging technologies for waste recovery such as those based on syngas fermentation and chain elongation has been studied for the production of medium chain fatty acids (MCFAs) and bioalcohols, in an attempt to integrate the concept of circular economy in the industry. Nevertheless, one of the main issues of this approach is the pH mismatch between acetogens and chain elongating microorganisms. This work reports, for the first time, the suitability of a co-culture of C. aceticum and C. kluyveri metabolizing syngas at near neutral pH in stirred tank bioreactors. For this purpose, bioreactor studies were carried out with continuous syngas supply. In the first experiment, maximum concentrations of n-butyrate and n-caproate of 7.0 and 8.2 g/L, respectively, were obtained. In the second experiment, considerable amounts of n-butanol were produced as a result of the reduction, by C. aceticum, of the carboxylates already formed in the broth. In both experiments, ethanol was used as an exogenous electron agent at some point. Finally, batch bottle assays were performed with a pure culture of C. aceticum grown on CO in presence of n-butyrate to assess and confirm its ability to produce n-butanol, reaching concentrations up to 951 mg/L, with a n-butyrate conversion efficiency of 96%, which had never been reported before in this species. Therefore, this work contributes to the state of the art, presenting a novel system for the bioproduction of MCFAs by combining syngas fermentation and chain elongation at near neutral pH, as opposed to the acidic pH range used in all previously reported literature.

Enhanced Ethanol Production From Carbon Monoxide by Enriched Clostridium Bacteria

Carbon monoxide (CO)-metabolizing Clostridium spp. were enriched from the biomass of a butanol-producing reactor. After six successive biomass transfers, ethanol production reached as much as 11.8 g/L with minor accumulation of acetic acid, under intermittent gas feeding conditions and over a wide pH range of 6.45-4.95. The molar ratio of ethanol to acetic acid exceeded 1.7 after the lag phase of 11 days and reached its highest value of 8.6 during the fermentation process after 25 days. Although butanol production was not significantly enhanced in the enrichment, the biomass was able to convert exogenous butyric acid (3.2 g/L) into butanol with nearly 100% conversion efficiency using CO as reducing power. This suggested that inhibition of butanol production from CO was caused by the lack of natural butyric acid production, expectedly induced by unsuitable pH values due to initial acidification resulting from the acetic acid production. The enriched Clostridium population also converted glucose to formic, acetic, propionic, and butyric acids in batch tests with daily pH adjustment to pH 6.0. The Clostridium genus was enriched with its relative abundance significantly increasing from 7% in the inoculum to 94% after five successive enrichment steps. Unidentified Clostridium species showed a very high relative abundance, reaching 73% of the Clostridium genus in the enriched sludge (6th transfer).

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.

Autotrophic (C1-gas) versus heterotrophic (fructose) accumulation of acetic acid and ethanol in Clostridium aceticum

The influence of the carbon source on the metabolism and growth of Clostridium aceticum was investigated, supplying either CO or fructose as sole carbon source. The acid and solvent production patterns were determined under either autotrophic or heterotrophic conditions, elucidating the effect of pH on the substrate's bioconversion pattern. The highest maximum specific growth rate was observed with CO, under the organism's optimal growth conditions, reaching 0.052 h^-1 and an acetic acid concentration of 18 g·L^-1. The production of 4.4 g·L^-1 ethanol was also possible, after medium acidification, during CO bioconversion. Conversely, formic acid inhibition was observed during fructose fermentation under optimal growth conditions. In the latter experiments, it was not possible to stimulate solvent production when growing C. aceticum on fructose, despite applying the same medium acidification strategy as with CO, showing the selective effect of the carbon source (autotrophic vs heterotrophic) on the metabolic pattern and solventogenesis.

Treatment of waste gas contaminated with dichloromethane using photocatalytic oxidation, biodegradation and their combinations

The treatment of waste gas (WG) containing dichloromethane (DCM) using advanced oxidation processes (AOPs) [UV and UV-TiO₂], biological treatment (BT), and their combination (AOPs-BT) was tested. AOP tests were performed in an annular photo-reactor (APHR), while BT was conducted in a continuous stirred tank bioreactor (CSTBR). The effects of gas flow rate (Q_gas), inlet DCM concentration ([DCM]_i), residence time (τ), photocatalyst loading (PH-C_L) and % relative humidity (% RH) on the AOPs performance and the removal of DCM (%DCM_r) were studied and optimized. The UV process exhibited %DCM_r ≤ 12.5 % for tests conducted at a [DCM]_i ≤ 0.45 g/m³, Q_gas of 0.12 m³/h and τ of 27.6 s, respectively, and < 4 % when the [DCM]_i ≥ 4.2 g/m³. The UV-TiO₂ achieved a %DCM_r ≥ 71 ± 1.5 % at Q_gas of 0.06 m³/h, [DCM]_i of 0.45 g/m³, τ of 55.2 s, PH-C_L of 10 g/m², and %RH of 50, respectively. The BT process removed ∼97.6 % of DCM with an elimination capacity (EC) of 234.0 g/m³·h. Besides, the high %DCM_r of ∼98.5 % in the UV-BT and 99.7 % in the UV-TiO₂-BT processes confirms AOPs-BT as a promising technology for the treatment of recalcitrant compounds present in WG.

Valorization of sewage sludge in co-digestion with cheese whey to produce volatile fatty acids

The present work explored the production of volatile fatty acids through the anaerobic co-digestion of sewage sludge (SS) and cheese whey (CW). Two batch experiments were conducted to evaluate the effect of the substrate mixing ratio (SS%:CW% of total COD of feedstock) and the initial pH on the acidogenic fermentation of SS with CW at different temperatures. The first batch experiment showed that a decrease of the SS proportion in the co-digestion with CW led to a higher degree of acidification observing a synergistic effect at a SS:CW mixing ratio of 25:75 (SS25:CW75). In the second batch experiment, three temperatures (30 °C, 38 °C and 50 °C) and two initial pH (5.5 and 9) were studied at SS60:CW40 and SS25:CW75 substrate mixing ratios. Maximum degrees of acidification of 56% and 73% were achieved, at 50 °C and initial pH of 5.5, for the SS60:CW40 and SS25:CW75 substrate mixing ratios, respectively. Finally, the performance of a semi-continuous reactor was demonstrated at laboratory scale reactor. Different hydraulic retention times (HRT) (10 and 20 days), pH (uncontrolled, 5.5 and 9) and the effect of a thermal pre-treatment of the SS was studied. The maximum degree of acidification in the lab-scale reactor was 45% at 37 °C, HRT of 20 days and pH of 5.5. Under these conditions, the volatile fatty acids (VFA) profile was dominated by butyric and acetic acids.

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.

Effect of tungsten and selenium on C1 gas bioconversion by an enriched anaerobic sludge and microbial community analysis

The effect of trace metals, namely tungsten and selenium, on the production of acids and alcohols through gas fermentation by a CO-enriched anaerobic sludge in a continuous gas-fed bioreactor was investigated. The CO-enriched sludge was first supplied with a tungsten-deficient medium (containing selenium) and in a next assay, a selenium-deficient medium (containing tungsten) was fed to the bioreactor, at a CO gas flow rate of 10 mL/min. In the absence of tungsten (tungstate), an initial pH of 6.2 followed by a pH decrease to 4.9 yielded 7.34 g/L acetic acid as the major acid during the high pH period. Subsequently, bioconversion of the acids at a lower pH of 4.9 yielded only 1.85 g/L ethanol and 1.2 g/L butanol in the absence of tungsten (tungstate). A similar follow up assay in the same bioreactor with two consecutive periods at different pH values (i.e., 6.2 and 4.9) with a selenium deficient medium yielded 6.6 g/L acetic acid at pH 6.2 and 4 g/L ethanol as well as 1.88 g/L butanol at pH 4.9. The results from the microbial community analysis showed that the only known CO fixing microorganism able to produce alcohols detected in the bioreactor was Clostridium autoethanogenum, both in the tungsten and the selenium deprived media, although that species has so far not been reported to be able to produce butanol. No other solventogenic acetogen was detected.

Effect of pH, yeast extract and inorganic carbon on chain elongation for hexanoic acid production

Several anaerobic bioconversion technologies produce short chain volatile fatty acids and sometimes ethanol, which can together be elongated to hexanoic acid (C6 acid) by Clostridium kluyveri in a secondary fermentation process. Initiatives are needed to further optimize the process. Therefore, five strategies were tested aiming at elucidating their influence on hexanoic acid production from mixtures of acetic acid, butyric acid and ethanol. pH-regulated bioreactors, maintained at pH 7.5, 6.8 or 6.4 led to maximum C6 acid concentrations of, respectively, 19.4, 18.3 and 13.3 g L^-1. At pH 6.8, yeast extract omission resulted in a decrease of the hexanoic acid concentration to 12.0 g L^-1 while the addition of an inorganic carbon source, such as bicarbonate, for pH control, increased the C6 acid concentration up to 21.4 g L^-1. This research provides guidelines for efficient improved production of hexanoic acid by pure cultures of C. kluyveri, contributing to the state of art.

Solventogenesis in Clostridium aceticum producing high concentrations of ethanol from syngas

The ethanol production capability of Clostridium aceticum was investigated and optimized, in order to evaluate the ability of that organism to produce high concentrations of fuel-ethanol. The results showed that C. aceticum can produce significant amounts of ethanol when a natural pH drop occurs in the fermentation broth as a consequence of acetic acid production in a first stage. Applying different pH-regulating strategies allowed to optimize ethanol production, which proved to be more efficient in case of natural acidification due to acetic acid, reaching up to 5.6 g/L ethanol, compared to artificial pH adjustment through the addition of hydrogen chloride. Playing with the pH value and the bioreactor operating conditions showed that, under specific conditions, C. aceticum is able to perform the reverse reaction as well and convert ethanol, produced at low pH, back to acetic acid, impeding, under those specific conditions, further accumulation of ethanol in the fermentation broth.

Effect of salinity on C1-gas fermentation by Clostridium carboxidivorans producing acids and alcohols

Clostridium carboxidivorans can produce acids and/or alcohols through syngas fermentation. In that C1-gas fermentation process, the production of acids takes place at higher pH (acetogenesis) (e.g., around 6.00), while the conversion of accumulated acids into alcohols (solventogenesis) is more favourable at a lower pH (e.g., 4.75–5.00). The pH drop, when switching from acetogenesis to solventogenesis, can either be natural—and result from the production of acids—or artificial. In the latter case, for the acidification process, a strong acid (HCl) was added to a syngas fermenting bioreactor in this study, while NaOH was added to increase the pH whenever needed. Cycles of high and low pH were applied in order to switch from acetogenic to solventogenic stages. This pH adjustment procedure leads to the accumulation of salts. The possible inhibitory effect exerted by changes in salinity in the bioreactor was estimated in batch bottles assays, carried out with different salinities (media with different concentrations of sodium chloride) using C. carboxidivorans and CO as a carbon source. At NaCl concentrations below 9 g/L, maximum growth rates around 0.055 h⁻¹ were obtained, whereas increasing the concentration of sodium chloride had a negative effect on bacterial growth and CO consumption. In the case of the most concentrated bottles, above 15 g/L NaCl no relevant growth was observed. Also, the IC₅₀, i.e. concentration yielding 50% growth inhibition, was estimated, and reached a value of 11 g/L sodium chloride.

Influence of electron acceptors on hexanoic acid production by Clostridium kluyveri

Clostridium kluyveri was used for chain elongation of C2C4 fatty acids in stirred tank bioreactors. The influence of different electron acceptors (acetic acid, butyric acid and the mixture of both) on C6 fatty acid production was evaluated in presence of ethanol using similar molar alcohol/acid ratios around 3.5. Bottle batch assays without pH regulation and with only acetic acid as electron acceptor yielded a final C6 fatty acid concentration of 6.8 ± 0.6 g L^-1. Then, pH-regulated bioreactors were operated at constant pH of 6.8. Under such conditions, the maximum growth rate was 0.039 h^-1 obtained using acetic acid and butyric acid as electron acceptors, whereas the lowest growth rate was 0.010 h^-1 with only butyric acid as electron acceptor. The maximum growth rate with acetic acid only, was similar, though slightly lower, as with the mixture of C2C4 fatty acids. Besides, the maximum productions of hexanoic acid were 11.8 g L^-1, 13.1 g L^-1 and 21.2 g L^-1 using, respectively, acetic acid, butyric acid and the mixture of both acids as electron acceptors. Thus, the use of a mixture of acetic acid and butyric acid in presence of ethanol for chain elongation, at constant pH, proved to be efficient for hexanoic acid production.

Selective anaerobic fermentation of syngas into either C2-C6 organic acids or ethanol and higher alcohols

Clostridium carboxidivorans produces alcohols from C1 gases (CO, CO₂), converting them first into fatty acids, and subsequently into alcohols. This research identified conditions that allow to selectively produce either fatty acids or alcohols. The conversion of gases into acids and then into alcohols is catalysed by metalloenzymes, stimulated by specific trace metals. Therefore, different bioreactors were set-up, either with or without addition of tungsten (W) or selenium (Se) and at different pHs. Combining the presence of those trace metals with a low pH (5.0) allowed to accumulate high amounts of alcohols as major end products (8038 mg/L total alcohols; 3027 mg/L total acids). Instead, maintaining a higher pH (6.2), in the absence of those trace metals, allowed to selectively produce organic acids (9577 mg/L) and almost no alcohols (676 mg/L). Omitting W, but not Se, at high pH (6.2), led to a still higher concentration of acids (11303 mg/L).

Enrichment of a solventogenic anaerobic sludge converting carbon monoxide and syngas into acids and alcohols

An anaerobic granular sludge was acclimatized to utilise CO in a continuously gas-fed stirred tank bioreactor by applying operating conditions expected to stimulate solventogenesis, i.e. the production of alcohols, and allowing to enrich for solventogenic populations. A cycle of high (6.2) and low (4.9) pH was applied in order to produce volatile fatty acids first at high pH, followed by their bioconversion into alcohols at low pH. The addition of yeast extract stimulated biomass growth, but not necessarily solventogenesis. The highest concentrations of metabolites achieved were 6.18 g/L acetic acid (30th day), 1.18 g/L butyric acid (28th day), and 0.423 g/L hexanoic acid (32nd day). Subsequently, acids were metabolized at lower pH, producing alcohols at concentrations of 11.1 g/L ethanol (43rd day), 1.8 g/L butanol (41st day) and 1.46 g/L hexanol (42nd day), confirming the successful enrichment strategy. Similarly, the enriched sludge could also convert syngas into acids and alcohols.

Organic loading rate effect on the acidogenesis of cheese whey: a comparison between UASB and SBR reactors

Volatile fatty acids (VFA) production and degree of acidification (DA) were investigated in the anaerobic treatment of cheese whey by comparison of two processes: a continuous process using a laboratory upflow anaerobic sludge blanket (UASB) reactor and a discontinuous process using a sequencing batch reactor (SBR). The main purpose of this work was to study the organic loading rate (OLR) effect on the yield of VFA in two kinds of reactors. The predominant products in the acidogenic process in both reactors were: acetate, propionate, butyrate and valerate. The maximum DA obtained was 98% in an SBR at OLR of 2.7 g COD L^-1 d^-1, and 97% in the UASB at OLR at 15.1 g COD L^-1 d^-1. The results revealed that the UASB reactor was more efficient at a medium OLR with a higher VFA yield, while with the SBR reactor, the maximum acidification was obtained at a lower OLR with changes in the VFA profile at different OLRs applied.

Production of acids and alcohols from syngas in a two-stage continuous fermentation process

A two-stage continuous system with two stirred tank reactors in series was utilized to perform syngas fermentation using Clostridium carboxidivorans. The first bioreactor (bioreactor 1) was maintained at pH 6 to promote acidogenesis and the second one (bioreactor 2) at pH 5 to stimulate solventogenesis. Both reactors were operated in continuous mode by feeding syngas (CO:CO₂:H₂:N₂; 30:10:20:40; vol%) at a constant flow rate while supplying a nutrient medium at different flow rates of 8.1, 15, 22 and 30 ml/h. A cell recycling unit was added to bioreactor 2 in order to recycle the cells back to the reactor, maintaining the OD₆₀₀ around 1 in bioreactor 2 throughout the experimental run. When comparing the flow rates, the best results in terms of solvent production were obtained with a flow rate of 22 ml/h, reaching the highest average outlet concentration for alcohols (1.51 g/L) and the most favorable alcohol/acid ratio of 0.32.

Glucose bioconversion profile in the syngas-metabolizing species Clostridium carboxidivorans

Some clostridia produce alcohols (ethanol, butanol, hexanol) from gases (CO, CO₂, H₂) and others from carbohydrates (e.g., glucose). C. carboxidivorans can metabolize both gases as well as glucose. However, its bioconversion profile on glucose had not been reported. It was observed that C. carboxidivorans does not follow a typical solventogenic stage when grown on glucose. Indeed, at pH 6.2, it produced first a broad range of acids (acetic, butyric, hexanoic, formic, and lactic acids), several of which are generally not found, under similar conditions, during gas fermentation. Medium acidification did not allow the conversion of fatty acids into solvents. Production of some alcohols from glucose was observed in C. carboxidivorans but at high pH rather than under acidic conditions, and the total concentration of those solvents was low. At high pH, formic acid was produced first and later converted to acetic acid, but organic acids were not metabolized at low pH.

Integrated bioconversion of syngas into bioethanol and biopolymers

Syngas bioconversion is a promising method for bioethanol production, but some VFA remains at the end of fermentation. A two-stage process was set-up, including syngas fermentation as first stage under strict anaerobic conditions using C. autoethanogenum as inoculum, with syngas (CO/CO₂/H₂/N₂, 30/10/20/40) as gaseous substrate. The second stage consisted in various fed-batch assays using a highly enriched PHA accumulating biomass as inoculum, where the potential for biopolymer production from the remaining acetic acid at the end of the syngas fermentation was evaluated. All of the acetic acid was consumed and accumulated as biopolymer, while ethanol and 2,3-butanediol remained basically unused. It can be concluded that a high C/N ratio in the effluent from the syngas fermentation stage was responsible for non-consumption of alcohols. A maximum PHA content of 24% was reached at the end of the assay.

Modelling the removal of volatile pollutants under transient conditions in a two-stage bioreactor using artificial neural networks

A two-stage biological waste gas treatment system consisting of a first stage biotrickling filter (BTF) and second stage biofilter (BF) was tested for the removal of a gas-phase methanol (M), hydrogen sulphide (HS) and α-pinene (P) mixture. The bioreactors were tested with two types of shock loads, i.e., long-term (66h) low to medium concentration loads, and short-term (12h) low to high concentration loads. M and HS were removed in the BTF, reaching maximum elimination capacities (EC_max) of 684 and 33 gm^-3h^-1, respectively. P was removed better in the second stage BF with an EC_max of 130 gm^-3h^-1. The performance was modelled using two multi-layer perceptrons (MLPs) that employed the error backpropagation with momentum algorithm, in order to predict the removal efficiencies (RE, %) of methanol (RE_M), hydrogen sulphide (RE_HS) and α-pinene (RE_P), respectively. It was observed that, a MLP with the topology 3-4-2 was able to predict RE_M and RE_HS in the BTF, while a topology of 3-3-1 was able to approximate RE_P in the BF. The results show that artificial neural network (ANN) based models can effectively be used to model the transient-state performance of bioprocesses treating gas-phase pollutants.

Carbon monoxide fermentation to ethanol by Clostridium autoethanogenum in a bioreactor with no accumulation of acetic acid

Fermentation of CO or syngas offers an attractive route to produce bioethanol. However, during the bioconversion, one of the challenges to overcome is to reduce the production of acetic acid in order to minimize recovery costs. Different experiments were done with Clostridium autoethanogenum. With the addition of 0.75 μM tungsten, ethanol production from carbon monoxide increased by about 128% compared to the control, without such addition, in batch mode. In bioreactors with continuous carbon monoxide supply, the maximum biomass concentration reached at pH 6.0 was 109% higher than the maximum achieved at pH 4.75 but, interestingly, at pH 4.75, no acetic acid was produced and the ethanol titer reached a maximum of 867 mg/L with minor amounts of 2,3-butanediol (46 mg/L). At the higher pH studied (pH 6.0) in the continuous gas-fed bioreactor, almost equal amounts of ethanol and acetic acid were formed, reaching 907.72 mg/L and 910.69 mg/L respectively.

Ethanol and acetic acid production from carbon monoxide in a Clostridium strain in batch and continuous gas-fed bioreactors

The effect of different sources of nitrogen as well as their concentrations on the bioconversion of carbon monoxide to metabolic products such as acetic acid and ethanol by Clostridium autoethanogenum was studied. In a first set of assays, under batch conditions, either NH4Cl, trypticase soy broth or yeast extract (YE) were used as sources of nitrogen. The use of YE was found statistically significant (p < 0.05) on the product spectrum in such batch assays. In another set of experiments, three bioreactors were operated with continuous CO supply, in order to estimate the effect of running conditions on products and biomass formation. The bioreactors were operated under different conditions, i.e., EXP1 (pH = 5.75, YE 1g/L), EXP2 (pH = 4.75, YE 1 g/L) and EXP3 (pH = 5.75, YE 0.2 g/L). When compared to EXP2 and EXP3, it was found that EXP1 yielded the maximum biomass accumulation (302.4 mg/L) and products concentrations, i.e., acetic acid (2147.1 mg/L) and ethanol (352.6 mg/L). This can be attributed to the fact that the higher pH and higher YE concentration used in EXP1 stimulated cell growth and did, consequently, also enhance metabolite production. However, when ethanol is the desired end-product, as a biofuel, the lower pH used in EXP2 was more favourable for solventogenesis and yielded the highest ethanol/acetic acid ratio, reaching a value of 0.54.

Optimization of the performance of a thermophilic biotrickling filter for alpha-pinene removal from polluted air

Biodegradation of alpha-pinene was investigated in a biological thermophilic trickling filter, using a lava rock and polymer beads mixture as packing material. Partition coefficient (PC) between alpha-pinene and the polymeric material (Hytrel G3548 L) was measured at 50 degrees C. PCs of 57 and 846 were obtained between the polymer and either the water or the gas phase, respectively. BTF experiments were conducted under continuous load feeding. The effect of yeast extract (YE) addition in the recirculating nutrient medium was evaluated. There was a positive relationship between alpha-pinene biodegradation, CO2 production and YE addition. A maximum elimination capacity (ECmax) of 98.9 g m(-3) h(-1) was obtained for an alpha-pinene loading rate of about 121 g m(-3) h(-1) in the presence of 1 g L(-1) YE. The ECmax was reduced by half in the absence of YE. It was also found that a decrease in the liquid flow rate enhances alpha-pinene biodegradation by increasing the ECmax up to 103 gm(-3) h(-1) with a removal efficiency close to 90%. The impact of short-term shock-loads (6 h) was tested under different process conditions. Increasing the pollutant load either 10- or 20-fold resulted in a sudden drop in the BTF's removal capacity, although this effect was attenuated in the presence of YE.

Transient-state studies and neural modeling of the removal of a gas-phase pollutant mixture in a biotrickling filter

The removal efficiency (RE) of gas-phase hydrogen sulfide (H), methanol (M) and α-pinene (P) in a biotrickling filter (BTF) was modeled using artificial neural networks (ANNs). The inlet concentrations of H, M, P, unit flow and operation time were used as the model inputs, while the outputs were the RE of H, M and P, respectively. After testing and validating the results, an optimal network topology of 5-8-3 was obtained. The model predictions were analyzed using Casual index (CI) values. M removal in the BTF was influenced positively by the inlet concentration of M in mixture (CI=3.79), while the removal of P and H were influenced more by the time of BTF operation (CI=25.36, 15.62). The BTF was subjected to different types of short-term shock-loads: 5-h shock-load of HMP mixture simultaneously, and 2.5-h shock-load of either H, M, or P, individually. It was observed that, short-term shock-loads of individual pollutants (M or H) did not significantly affect their own removal, but the removal of P was affected by 50%. The results from this study also show the sensitiveness of the well-acclimated BTF to handle sudden load variations and also revival capability of the BTF when pre-shock conditions were restored.

Steady- and transient-state performance of a thermophilic suspended-growth bioreactor for α-pinene removal from polluted air

The removal of α-pinene from polluted air was examined under thermophilic conditions (50°C) in one- and two-liquid phase continuous stirred tank bioreactors (CSTBs). Steady-state experiments were performed at different gas residence times, and α-pinene concentrations, corresponding to inlet loading rates between 0.5 and 38gm(-3)h(-1) in the one-liquid phase CSTB, and between 2.9 and 176gm(-3)h(-1) in the two-liquid phase CSTB. The presence of a second liquid-phase (5% v/v silicone oil) increased the maximum elimination capacity (85.7gm(-3)h(-1)) by ∼6-fold compared to the one-liquid phase CSTB. During transient-state experiments, the CSTB with silicone oil could withstand a higher α-pinene shock load (110gm(-3)h(-1)) than the CSTB operated without silicone oil (70gm(-3)h(-1)). Besides, the thermophilic specific substrate utilization rates were estimated from batch assays, reaching 89 and 340μgα-pinenemgbiomass(-1)d(-1), in the absence of or presence of silicone oil, respectively.

One-stage biotrickling filter for the removal of a mixture of volatile pollutants from air: performance and microbial community analysis

The biodegradation of gas-phase mixtures of methanol, α-pinene and H2S was examined in a biotrickling filter (BTF), inoculated with a microbial consortium composed of an autotrophic H2S-degrading culture, and pure strains of Candida boidinii, Rhodococcus erythropolis, and Ophiostoma stenoceras. The inlet concentrations of methanol, α-pinene and H2S varied from 0.05 to 3.3 gm(-3), 0.05 to 2.7 gm(-3), and 0.01 to 1.4 gm(-3), respectively, at empty bed residence times (EBRT) of either 38 or 26s. The maximum elimination capacities (ECmax) of the BTF were 302, 175, and 191 gm(-3)h(-1), with 100%, 67%, and >99% removal of methanol, α-pinene and H2S, respectively. The presence of methanol showed an antagonistic removal pattern for α-pinene, but the opposite did not occur. For α-pinene, inlet loading rates (ILRs) >150 gα-pinenem(-3)h(-1) affected its own removal in the BTF. The presence of H2S did not show any declining effect on the removal of both methanol and α-pinene.

Evaluation of the biomethane potential of solid fish waste

Manufacturing processes in fish canning industries generate a considerable amount of solid waste that can be digested anaerobically. The aim of this research was to study the biochemical methane potential of different solid fish waste. For tuna, sardine and needle fish waste, around 0.47g COD-CH(4)/g COD(added) was obtained in batch experiments with 1%TS; whereas for mackerel waste, the methane production attained 0.59g COD-CH(4)/g COD(added). The increase in the waste/inoculum ratio, from 1.1-1.3 to 2.8-3.3g VS(waste)/g VS(inoculum), led to overload due to VFA and LCFA accumulation. Afterward, co-digestion assays of fish waste with gorse were undertaken but the biochemical methane potential did not improve.

Biodegradation of BTEX in a fungal biofilter: influence of operational parameters, effect of shock-loads and substrate stratification

The effect of relative humidity (RH: 30% to >95%) of a gas-phase mixture composed of benzene, toluene, ethylbenzene and para-, meta- and ortho-xylenes (BTEX), inlet concentrations (0.2-12.6 g m(-3)), and empty bed residence times (EBRTs) (48-144 s) was tested in a fungi-dominant biofilter. A maximum elimination capacity (EC(max)) of 244.2 gBTEX m(-3) h(-1) was achieved at a total inlet loading rate (ILR(T)) of 371.2 gBTEXm(-3) h(-1) (RH: 65%). The transient-state response was tested by increasing the ILR(T), in two steps, from ~50 to 850 gm(-3) h(-1) and from ~50 to 320 g m(-3) h(-1), at a constant EBRT of 41.7s. Increasing the ILR(T) reduced the total BTEX removal efficiency (RE(T)) from >97% to 35%, and from >90% to 60% during medium and high shock-load, respectively. When subjected to short (4d) and long-term (7d) shut-down periods, the biofilter was able to recover high EC(max) of, respectively, 200 and 72 gBTEX m(-)3 h(-1) after resuming operation.

Biological conversion of carbon monoxide to ethanol: effect of pH, gas pressure, reducing agent and yeast extract

A two-level full factorial design was carried out in order to investigate the effect of four factors on the bioconversion of carbon monoxide to ethanol and acetic acid by Clostridium autoethanogenum: initial pH (4.75-5.75), initial total pressure (0.8-1.6 bar), cysteine-HCl·H(2)O concentration (0.5-1.2 g/L) and yeast extract concentration (0.6-1.6 g/L). The maximum ethanol production was enhanced up to 200% when lowering the pH and amount yeast extract from 5.75 to 4.75 g/L and 1.6 to 0.6 g/L, respectively. The regression coefficient, regression model and analysis of variance (ANOVA) were obtained using MINITAB 16 software for ethanol, acetic acid and biomass. For ethanol, it was observed that all the main effects and the interaction effects were found statistically significant (p<0.05). The comparison between the experimental and the predicted values was found to be very satisfactory, indicating the suitability of the predicted model.

Combined biological and physicochemical waste-gas cleaning techniques

This review presents a general overview of physical, chemical and biological waste-gas treatment techniques such as adsorption, absorption, oxidation and biodegradation, focusing more extensively on combined processes. It is widely recognized that biological waste-gas treatment devices such as biofilters and biotrickling filters can show high performance, often reaching removal efficiencies above 90 % for pollutant concentrations below 5 g/m(3). However, for concentrations exceeding this limit and under transient shock-load conditions that are frequently encountered in industrial situations, a physicochemical gas cleaning process can sometimes be advantageously combined with a biological one. Besides improving the overall treatment efficiency, the non-biological, first-stage process could also serve as a load equalization system by reducing the pollutant load during periodic shock-loads, to levels that can easily be handled in the second-stage bioreactor. This article reviews the operational advantages of integrating different non-biological and biological processes, i.e., adsorption pre-treatment+bioreactor, bioreactor+adsorption post-treatment, absorption pre-treatment+bioreactor, UV pre-treatment+bioreactor, and bioreactor/bioreactor combinations, for waste-gas treatment, where different gas-phase pollutants have been tested.

Biotreatment of a gas-phase volatile mixture from fibreglass and composite manufacturing industries

Acetone, toluene and styrene (ATS) are representative air pollutants emanating during the production process in fibreglass and composite manufacturing industries. In this study, the performance of a steady-state biofilter inoculated with the fungus Sporothrix variecibatus was tested at different empty bed residence times (EBRTs), and at different inlet concentrations of ATS, corresponding to total pollutant loading rates ranging from 30 to 490 gm(-3)hour(-1). Styrene was somewhat better removed (47-100%) in the biofilter than acetone (34-100%) and toluene (42-100%), with maximum elimination capacities (EC(max)) of 108, 72 and 144 gm(-3)hour(-1), for ATS, respectively. Besides, it was observed that, although increasing the concentration of ATS decreased their removal, the presence of toluene also decreased the EC(max) of both acetone and toluene in the ternary mixture. During transient operations, the biofilter was subjected to intermittent shutdown and re-start operations where the gas-phase pollutant flow was stopped for either 5 or 16d. It was observed that, for longer shutdown periods (16d), the biofilter required nearly 8-10d to reach similar removal patterns to those observed before the shutdown phase. Batch biodegradation tests were conducted, using Sporothrix-like microorganisms present in the leachate of the biofilter, with a mixture of ATS as the sole carbon source. Complete removal of ATS was observed within the test period of 168 hours. Styrene was degraded faster, with a specific substrate utilization rate of 0.9 mg styrenemg biomass(-1)hour(-1), followed by toluene (0.6) and acetone (0.44). The effectiveness of the biofilter to reach high total EC (321.3 gm(-3)hour(-1)), and withstand transient operations shows the robustness of this fungal-bioreactor and its suitability to handle emissions from a fibreglass and composite manufacturing industry.