Bioengineering of anaerobic digestion for volatile fatty acids, hydrogen or methane production: A critical review

Integrated biorefinery routes of biohydrogen: Possible utilization of acidogenic fermentative effluent

Biohydrogen production and integration possibilities are vital towards hydrogen economy and sustainability of the environment. Acidogenic fermentation is acquiring great interest and it is one of the prime pathways to produce biohydrogen and short chain carboxylic acids. In addition to hydrogen recovery, simultaneously nearly 60 percent of the organics may get converted to ethanol, 1,3propanediol and organic acids. Besides, these organics (fermentative effluents) can be used indirectly as a raw material for the generation of value- added products such as biolipid, polyhydroxyalkanoates, excess hydrogen, methane and electrical energy recovery. In this regard, this review has been assessed as a valuable biorefinery for biofuel and value- added products recovery. The biorefinery can be used to minimize entire cost of the approach by obtaining extra profits.

Effect of different aerobic hydrolysis time on the anaerobic digestion characteristics and energy consumption analysis

Aerobic hydrolysis of stover before anaerobic digestion is beneficial to improve the biodegradability of corn stover. Aerobic hydrolysis of corn stover at 43 °C was conducted to investigate the effects of hydrolysis time (0 h, 8 h, 16 h, and 24 h) on the degradation of lignocellulose from corn stover and material conversion. Further anaerobic digestion and energy consumption analysis with the digestion temperature of 36 °C were carried out. The accumulation rate of volatile fatty acids began to slow down after 16 h of hydrolysis, and the concentration of acetic acid reached 221.85 mmol/L at 24 h of hydrolysis. The degradation rate of lignocellulose was obviously increased after hydrolysis. When the hydrolysis time was 16 h, it reached the maximum cumulative methane production with 268.75 ml/g VS. In terms of biogas production and energy conversion efficiency, it is more appropriate to choose 16 h as hydrolysis time in biogas engineering.

Methane Saline Ameliorates Traumatic Brain Injury through Anti-Inflammatory, Antiapoptotic, and Antioxidative Effects by Activating the Wnt Signalling Pathway

OBJECTIVE

Methane saline (MS) can be used to treat many diseases via its anti-inflammatory, antiapoptotic, and antioxidative activities. However, to date, there is no published evidence as to whether MS has any effect on traumatic brain injury (TBI). The Wnt signalling pathway regulates cell proliferation, differentiation, migration, and apoptosis; however, whether the Wnt signalling pathway regulates any effect of MS on TBI is unknown. This study was designed to explore the role of MS in the treatment of TBI and whether the Wnt pathway is involved.

METHODS

Sprague-Dawley rats were randomly divided into five groups: sham, TBI, TBI+10 ml/kg MS, TBI+20 ml/kg MS, and TBI+30 ml/kg MS. After induction of TBI, MS was injected intraperitoneally once daily for seven consecutive days. Neurological function was evaluated by the Neurological Severity Score (NSS) at 1, 7, and 14 days after TBI. Haematoxylin-eosin (HE) staining, inflammatory factors, neuron-specific enolase (NSE) staining, oxidative stress, and cell apoptosis were measured and compared 14 d after TBI to identify the optimal dose of MS and to investigate the effect of MS on TBI. In the second experiment, Sprague-Dawley rats were randomly divided into four groups: sham, TBI, TBI+20 ml/kg MS, and TBI+20 ml/kg MS+Dickkopf-1 (DKK-1, a specific inhibitor of the Wnt pathway). NSE, caspase-3, superoxide dismutase (SOD), Wnt3a, and β-catenin were detected by real-time PCR and Western blotting. The results from each group were compared 14 d after TBI to determine the regulatory role of the Wnt pathway.

RESULTS

Methane saline significantly inhibited inflammation, oxidative stress, and cell apoptosis, thus protecting neurons within 14 days of TBI. The best treatment effect against TBI was obtained with 20 ml/kg MS. When the Wnt pathway was inhibited, the treatment effect of MS was impaired.

CONCLUSION

Methane saline ameliorates TBI through its anti-inflammatory, antiapoptotic, and antioxidative effects via activation of the Wnt signalling pathway, which plays a part but is not the only mechanism underlying the effects of MS. Thus, MS may be a novel strategy for treating TBI.

Glucose fermentation with biochar-amended consortium: microbial consortium shift

The effects of adding biochar rice husk (R), white popinee (WP), bamboo (BB), or coconut (CT) on microbial community in fermentation broths from glucose were investigated. The added biochars acted as biofilm carriers on which Sporolactobacillus spathodeae, Clostridium sensu stricto 11 sp., Clostridium sensu stricto 12 sp., Clostridium sensu stricto 1 sp., and Clostridium sensu stricto 5 sp. were enriched. Fermentation reactions substantially increased the amounts of acid-producers in biofilm. The homoacetogens, Clostridium carboxidivorans and Clostridium drakei, were identified in the biofilm in the first two batches of fermentation with biochars as electron conductors between acid-producers and homoacetogens to assist homoacetogenesis. The heterotrophic bacteria overcompeted the acid-producers in the biofilm in long-term fermentation.

Comparison of the microbial communities in anaerobic digesters treating high alkalinity synthetic wastewater at atmospheric and high-pressure (11 bar)

High-pressure anaerobic digestion is an appealing concept since it can upgrade biogas directly within the reactor. However, the decline of pH caused by the dissolution of CO₂ is the main barrier that prevents a good operating high-pressure anaerobic digestion process. Therefore, in this study, a high-pressure anaerobic digestion was studied to treat high alkalinity synthetic wastewater, which could not be treated in a normal-pressure anaerobic digester. In the high-pressure reactor, the pH value was 7.5 ~ 7.8, and the CH₄ content reached 88% at 11 bar. Unlike its normal-pressure counterpart (2285 mg/L acetic acid), the high-pressure reactor ran steadily (without volatile fatty acids inhibition). Furthermore, the microbial community changed in the high-pressure reactor. Specifically, key microbial guilds (Syntrophus (11.2%), Methanosaeta concilii (50.9%), and Methanobrevibacter (26.8%)) were dominant in the high-pressure reactor at 11 bar, indicating their fundamental roles under high-pressure treating high alkalinity synthetic wastewater.

Metagenomics for taxonomy profiling: tools and approaches

The study of metagenomics is an emerging field that identifies the total genetic materials in an organism along with the set of all genetic materials like deoxyribonucleic acid and ribose nucleic acid, which play a key role with the maintenance of cellular functions. The best part of this technology is that it gives more flexibility to environmental microbiologists to instantly pioneer the immense genetic variability of microbial communities. However, it is intensively complex to identify the suitable sequencing measures of any specific gene that can exclusively indicate the involvement of microbial metagenomes and be able to advance valuable results about these communities. This review provides an overview of the metagenomic advancement that has been advantageous for aggregation of more knowledge about specific genes, microbial communities and its metabolic pathways. More specific drawbacks of metagenomes technology mainly depend on sequence-based analysis. Therefore, this 'targeted based metagenomics' approach will give comprehensive knowledge about the ecological, evolutionary and functional sequence of significantly important genes that naturally exist in living beings either human, animal and microorganisms from distinctive ecosystems.

Influence of fine coal gasification slag on greenhouse gases emission and volatile fatty acids during pig manure composting

This study was evaluated industrial waste fine coal gasification slag (FCGS) as an additive on pig manure composting by parameters of greenhouse gases, NH₃, volatile fatty acids (VFAs) and maturity. Six treatments of FCGS (0%, 2%, 4%, 6%, 8% and 10%) were added into the mixture raw material and composted 42 days. Results illustrated that the FCGS amendment could prolong sanitation stage and promote the degree of maturity, germination index and C/N ratio during composting. With the increasing amendment of FCGS, GI was increased from 9.97 to 28.45%. Compared with control, increasing of FCGS proportion could reduce the mitigation of global warming potential (N₂O and CH₄), NH₃ and cumulative of VFAs from 8.89-77.04%, 3.81-71.65%, 5.18-28.02% and 8.79-83.33%. Finally, present study results revealed that 10%FCGS could improve composting and reduced the maturity period as well as compost quality, thus recommended as effective dosage for efficient pig manure composting.

Swine manure valorization in fabrication of nutrition and energy

Renewable energy can boost the growing population's need and rapid budgetary development. To reduce fossil fuel consumption is the initial purpose of renewable and sustainable energy, producing valuable bio-based products. The fermenters, using for pretreatment of swine manure, and involvement of swine carcasses are reported to enhance organic loading rate followed by improved biogas yield on household digesters. The compositions such as animal residues, pathogenic microbes, pharmaceutical residues and nutrient compositions including undigested feed are still confused. Therefore, it is mandatory to optimize and stabilize anaerobic practice and digestate filtration purification for consequential fertilizer consumption. The effective bio-methane recovery from energy-rich compounds is challenging due to slow degradation procedures. The pretreatment procedure could enhance lipid depolymerization and improve anaerobic fermentation. This article deeply focuses on biodegradation of swine manure. The components of this manure were evaluated and established several approaches to improve biogas production. Furthermore, recycling of co-digestates was discussed in detail as fertilizer consumption including hygienic aspects of manure and pretreatment strategies of biomass residues. KEY POINTS: • Co-digestion of manure and carcasses enhance bio-methane production. • Removel of ammonia from biogas digester may improve bio-methane gas. • A strong antimicrobial influence has been reported on biogas production.

Batch-Mode Analysis of Thermophilic Methanogenic Microbial Community Changes in the Overacidification Stage in Beverage Waste Treatment

Biogasification by methane fermentation is an important and effective way to utilize beverage wastes. Beverage wastes are good feedstocks for methane fermentation because of their richness in sugars and proteins, although overacidification and inhibition of methane production caused by high substrate loading often become problematic. This study investigated changes in microbial communities in the overacidification state of the thermophilic methane fermentation process with beverage waste by establishing a simulated batch culture. We assessed 20 mL-scale batch cultures using a simulant beverage waste mixture (SBWM) with different amounts of addition; high cumulative methane production was achieved by adding 5 mL of SBWM (11358 mg-chemical oxygen demand-COD/L of organic loading), and overacidification was observed by adding 10 mL of SBWM (22715 mg-COD/L of organic loading). The results of 16S rRNA amplicon sequence analysis using nanopore sequencer suggested that Coprothermobacter proteolyticus, Defluviitoga tunisiensis, Acetomicrobium mobile, and Thermosediminibacter oceani were predominantly involved in hydrolysis/acidogenesis/acetogenesis processes, whereas Methanothrix soehngenii was the major acetotrophic methane producer. A comparison of microbial population between the methane-producing cultures and overacidification cultures revealed characteristic population changes especially in some minor species under 0.2% of population. We concluded that careful monitoring of population changes of the minor species is a potential indicator for prediction of overacidification.

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

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

Effects of Sulfamethoxazole on the Microbial Community Dynamics During the Anaerobic Digestion Process

Anaerobic digestion (AD) treatment of cattle manure and slurry makes it possible to produce biogas, a renewable and storable biofuel, as well as digestate, a residual organic matter that can be used to replace chemical fertilizers. On the other hand, the intense use of antibiotics (e.g., sulfamethoxazole) in animal husbandry practices is showing increasing negative impacts resulting from the release of still metabolically active molecules into agroecosystems. In the present study, cattle manure collected from an AD plant-feeding tank was used as feedstock for AD experiments in which some batches were spiked with 5 mg L^-1 of sulfamethoxazole (SMX). Adding the antibiotic affected the microbial community dynamic; in particular, the efficiency of the acidogenic and acetogenic phases of the process corresponded to higher CH₄ and H₂ production than in the control. SMX was also degraded, and at the end of the experiment (69 days), just 20% of its initial concentration was found. The relative abundance (ARG/16S) of resistance genes sul1, sul2, and the proxy intI1 initially found in the ingestate decreased during the AD in both the spiked and control batches, suggesting that this process lowers the likelihood of antibiotic resistance genes spreading.

Enhancing the production of biogas through anaerobic co-digestion of agricultural waste and chemical pre-treatments

Large amounts of agricultural solid wastes (ASWs) and animal dung are produced annually causing serious environmental problem that requires proper treatment. The present study proposes a strategy for optimizing the anaerobic co-digestion of ASWs and cow dung (CD), identifies the key factors governing the co-digestion performance and evaluates the effect of NaHCO₃ alkalinity treatment on improving the economy and performance of anaerobic digestion (AD). The results revealed that the highest cumulative methane production (CMP) of 297.99 NL/kgVS can be generated by co-digestion of ASWs and CD at a ratio of 60:40. Further improvement was achieved via alkalinity treatment with 1.0 g of NaHCO₃/gVS leading to decrease in lignin, cellulose, and hemicellulose contents of feedstock by 3.5%, 10.5% and 15.9%, respectively, converting them to soluble fractions and improving the CMP by 11.2-29.7% based on substrate quality. The improved CMP in the chemically treated substrates reflects a 19% increase in the generated revenue. The kinetics of the AD process was successfully fitted to modified Gompertz model with very low standard deviation residuals (SDR) ≤ 5.21 and R² ≥ 0.979. Results confirm that the proposed strategy is an effective method for producing biogas from co-digestion of ASWs and CD.

Volatile fatty acid production from saline cooked mussel processing wastewater at low pH

The production of VFA using as substrate the wastewater produced in a cooked mussel processing factory, containing large COD (13.7 ± 3.2 g COD/L), salt concentrations (21.8 ± 2.8 g NaCl/L) and characterized by low pH (4.6 ± 0.6) was evaluated. This wastewater was fed to a 5-L completely stirred tank reactor operated in continuous mode. The conversion efficiency of its COD content into volatile fatty acids (VFA) was evaluated. The maximum acidification of 43% (total VFA on soluble COD basis) was obtained when an organic loading rate of 2.5 ± 0.4 g COD/(L·d) was applied to the reactor and corresponded to a VFA volumetric productivity of 0.72 ± 0.07 g COD_VFA/(L·d). Under steady-state conditions, the obtained mixture of VFA was composed by 80:18:2 as acetic:propionic:butyric acids (percentage of VFA on soluble COD basis). Carbohydrates were degraded up to 96% while protein fermentation did not take place, probably due to the low pH value, limiting the maximum acidification of the wastewater. Batch experiments showed that the increase of the pH from 4.2 to 4.9 by the addition of NaHCO₃ resulted in the improvement of the acidification and changed the VFA mixture composition. Thus, this study demonstrates the opportunity of using complex substrates, as cooked mussel processing wastewater, to produce rich-VFA streams under unfavourable operational conditions, such as high salinity and low pH.

Utilization of food waste-derived volatile fatty acids for production of edible Rhizopus oligosporus fungal biomass

Rhizopus oligosporus is an edible filamentous fungus that can contribute to meet the growing demand for single-cell protein. Volatile fatty acids (VFAs) are favorable potential substrates for producing R. oligosporus biomass due to their capacity to be synthesized from a wide range of low-value organic solid wastes via anaerobic digestion. The goal of this work was to cultivate R. oligosporus using food waste-derived VFAs as the sole carbon source. To maintain the requisite low substrate concentrations, the fed-batch cultivation technique was applied. This resulted in a four-fold improvement in biomass production relative to standard batch cultivation. Maximum biomass yield of 0.21 ± 0.01 g dry biomass/g VFAs _{COD eq. consumed}, containing 39.28 ± 1.54% crude protein, was obtained. In the bubble-column bioreactors, the complete uptake of acetic acid was observed, while the consumptions of caproic and butyric acids reached up to 97.64% and 26.13%, respectively.

Manure anaerobic digestion effects and the role of pre- and post-treatments on veterinary antibiotics and antibiotic resistance genes removal efficiency

This review was aimed to summarize and critically evaluate studies on removal of veterinary antibiotics (VAs), antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) with anaerobic digestion (AD) of manure and demonstrate areas of focus for improved removal efficiency. The environmental risks associated to the release of the same were also critically evaluated. The potential of AD and advanced AD of manure on removal rate of VAs, ARGs and MGEs was thoroughly assessed. In addition, the role of post and pre-AD treatments and their potential to support VAs and ARGs removal efficiency were evaluated. The overall review results show disparity among the different groups of VAs in terms of removal rate with relatively higher efficiency for β-lactams and tetracyclines compared to the other groups. Some of sulfonamides, fluoroquinolones and macrolides were reported to be highly persistent with removal rates as low as zero. Within group differences were also reported in many literatures. Moreover, removal of ARGs and MGEs by AD was widely reported although complete removal was hardly possible. Even in rare scenarios, some AD conditions were reported to increase copies of specific groups of the genes. Temperature pretreatments and temperature phased advanced AD were also reported to improve removal efficiency of VAs while contributing to increased biogas production. Moreover, a few studies also showed the possibility of further removal by post-AD treatments such as liquid-solid separation, drying and composting. In conclusion, the various studies revealed that AD in its current technological level is not a guarantee for complete removal of VAs, ARGs and MGEs from manure. Consequently, their possible release to the soils with digestate could threaten the healthcare and disturb soil microbial ecology. Thus, intensive management strategies need to be designed to increase removal efficiency at the different manure management points along the anaerobic digestion process.

Chemically Stressed Bacterial Communities in Anaerobic Digesters Exhibit Resilience and Ecological Flexibility

Anaerobic digestion is a technology known for its potential in terms of methane production. During the digestion process, multiple metabolites of high value are synthesized. However, recent works have demonstrated the high robustness and resilience of the involved microbiomes; these attributes make it difficult to manipulate them in such a way that a specific metabolite is predominantly produced. Therefore, an exact understanding of the manipulability of anaerobic microbiomes may open up a treasure box for bio-based industries. In the present work, the effect of nalidixic acid, γ-aminobutyric acid (GABA), and sodium phosphate on the microbiome of digested sewage sludge from a water treatment plant fed with glucose was investigated. Despite of the induced process perturbations, high stability was observed at the phylum level. However, strong variations were observed at the genus level, especially for the genera Trichococcus, Candidatus Caldatribacterium, and Phascolarctobacterium. Ecological interactions were analyzed based on the Lotka–Volterra model for Trichococcus, Rikenellaceae DMER64, Sedimentibacter, Candidatus Cloacimonas, Smithella, Cloacimonadaceae W5 and Longilinea. These genera dynamically shifted among positive, negative or no correlation, depending on the applied stressor, which indicates a surprisingly dynamic behavior. Globally, the presented work suggests a massive resilience and stability of the methanogenic communities coupled with a surprising flexibility of the particular microbial key players involved in the process.

Removal of hydrogen sulfide from biogas using activated carbon synthesized from different locally available biomass wastes - a case study from Palestine

The main aim of this study was to remove hydrogen sulfide (H₂S) from biogas by adsorption using synthesized activated carbon prepared using locally available biomass. The effect of the type of precursors, impregnation reagent and bed height was studied in continuous reactors. Three types of biomass wastes (almond shells, eucalyptus and coffee grains) were collected, grinded, sieved, pyrolyzed at 500°C and impregnated with chemical reagents such as potassium hydroxide or zinc chloride. Adsorption tests were performed using a fixed bed filter filled with the produced activated carbon. The highest biochar yield of 36% was obtained eucalyptus followed by almond shells (28.5%) and coffee grains (24%), respectively. The highest adsorption capacity and removal efficiency were obtained with eucalyptus followed by almond shells and coffee grains, respectively. For instance, eucalyptus showed an adsorption capacity of ~690 (mg hydrogen sulfide/g adsorbent) followed by almond (230 mg hydrogen sulfide/g adsorbent) and coffee grains (22 mg hydrogen sulfide/g adsorbent). As an impregnation reagent, potassium hydroxide gave the highest adsorption efficiency and capacity than zinc chloride. Furthermore, the breakthrough time with KOH (180 min) was higher than ZnCl₂ (70 min). Increasing the bed height during continuous breakthrough tests increased the adsorption capacity and hydrogen sulfide removal efficiency. The results of this study showed that the adsorption efficiency of the synthesized activated carbon and consequently the hydrogen sulfide removal efficiency could be fine-tuned by selecting an appropriate biomass precursor and proper impregnation reagent.

Pretreatment technologies for anaerobic digestion of lignocelluloses and toxic feedstocks

Several feedstocks for anaerobic digestion (AD) have challenges that hamper the success of AD with their low accessible surface area, biomass recalcitrance, and the presence of natural inhibitors. This paper presents different types of pretreatment to address those individual challenges and how they contribute to facilitate AD. Organosolv and ionic liquid pretreatments are effective to remove lignin without a significant defect on lignin structures. To deal with accessible surface area and crystallinity, comminution, steam explosion, pretreatment using N-methyl-morpholine-N-oxide methods are suggested. Moreover, solid extraction, simple aeration, and biological treatments are capable in removing natural inhibitors. Up to date, methods like comminution, thermal process, and grinding are more preferable to be scaled-up.

Nutritional, Energy and Sanitary Aspects of Swine Manure and Carcass Co-digestion

Renewable energy can assist the management of the effects of population growth and rapid economic development on the sustainability of animal husbandry. The primary aim of renewable energy is to minimize the use of fossil fuels via the creation of environmentally friendly energy products from depleted fossil fuels. Digesters that treat swine manure are extensively used in treatment systems; and inclusion of swine carcasses can increase the organic loading rate (OLR) thereby improving biogas yield and productivity on farms. However, the characteristics of the components including animal residues, proteins, lipids, remains of undigested feed items, antimicrobial drug residues, pathogenic microorganisms and nutrient contents, are complex and diverse. It is therefore necessary to manage the anaerobic process stability and digestate purification for subsequent use as fertilizer. Efficient methane recovery from residues rich in lipids is difficult because such residues are only slowly biodegradable. Pretreatment can promote solubilization of lipids and accelerate anaerobic digestion, and pretreatments can process the swine carcass before its introduction onto biodigesters. This review presents an overview of the anaerobic digestion of swine manure and carcasses. We analyze the characteristics of these residues, and we identify strategies to enhance biogas yield and process stability. We consider energy potential, co-digestion of swine manure and carcasses, physical, chemical, and biological pretreatment of biomass, sanitary aspects of swine manure and co-digestates and their recycling as fertilizers.

MBR-Assisted VFAs Production from Excess Sewage Sludge and Food Waste Slurry for Sustainable Wastewater Treatment

The significant amount of excess sewage sludge (ESS) generated on a daily basis by wastewater treatment plants (WWTPs) is mainly subjected to biogas production, as for other organic waste streams such as food waste slurry (FWS). However, these organic wastes can be further valorized by production of volatile fatty acids (VFAs) that have various applications such as the application as an external carbon source for the denitrification stage at a WWTP. In this study, an immersed membrane bioreactor set-up was proposed for the stable production and in situ recovery of clarified VFAs from ESS and FWS. The VFAs yields from ESS and FWS reached 0.38 and 0.34 gVFA/gVSadded, respectively, during a three-month operation period without pH control. The average flux during the stable VFAs production phase with the ESS was 5.53 L/m2/h while 16.18 L/m2/h was attained with FWS. Moreover, minimal flux deterioration was observed even during operation at maximum suspended solids concentration of 32 g/L, implying that the membrane bioreactors could potentially guarantee the required volumetric productivities. In addition, the techno-economic assessment of retrofitting the membrane-assisted VFAs production process in an actual WWTP estimated savings of up to 140 €/h for replacing 300 kg/h of methanol with VFAs.

Narrowing the Gap for Bioplastic Use in Food Packaging: An Update

Plastic production has outgrown most other man-made materials, with more than 90% being petroleum-based and nonbiodegradable. Packaging, primarily food packaging, consumes the most plastic and is the largest contributor to municipal solid waste. In addition, its dependence on crude oil feedstock makes the plastic industry unsustainable and renders plastic markets vulnerable to oil price volatility. Therefore, the development of bioalternatives to conventional plastics is now a priority of the food packaging industry. Bioplastics are polymers that are either biobased (fully or partially), or biodegradable, or both. This review aims to provide an insightful overview of the most recent research and development successes in bioplastic materials, focusing on food packaging applications. Bioplastics are compared to their conventional counterparts with respect to their mechanical, thermal, barrier, and processability properties. The gaps between bio- and conventional plastics in food packaging are elucidated. Potential avenues for improving bioplastic properties to broaden their food packaging applications are critically examined. Furthermore, two of the most controversial topics on bioplastic alternatives, sustainability assessment and their impact on the plastic waste management system, are discussed.

Effect of Inoculum Pretreatment on the Composition of Microbial Communities in Anaerobic Digesters Producing Volatile Fatty Acids

Volatile fatty acids (VFAs) are intermediates in the methane formation pathway of anaerobic digestion and can be produced through the fermentation of organic wastes. VFAs have become an anticipated resource- and cost-effective way to replace fossil resources with higher added value and more versatile fuels and chemicals. However, there are still challenges in the production of targeted compounds from diverse and complex biomasses, such as urban biowastes. In this study, the aim was to modulate the microbial communities through inoculum treatment to enhance the production of green chemicals. Thermal and freeze-thaw treatments were applied to the anaerobic digester inoculum to inhibit the growth of methanogens and to enhance the performance of acidogenic and acetogenic bacteria. VFA fermentation after different inoculum treatments was studied in batch scale using urban biowaste as the substrate and the process performance was assessed with chemical and microbial analyses. Inoculum treatments, especially thermal treatment, were shown to increase VFA yields, which were also correlating with the dynamics of the microbial communities and retention times of the test. There was a strong correlation between VFA production and the relative abundances of the microbial orders Clostridiales (families Ruminococcaceae, Lachnospiraceae and Clostridiaceae), and Lactobacillales. A syntrophic relationship of these taxa with members of the Methanobacteriales order was also presumed.

Temperature regulations impose positive influence on the biomethane potential versus digesting modes treating agricultural residues

Temperature regulations (mesophilic/thermophilic) and digesting modes (mono-/co-digestion) play key roles in the biomethane potential of anaerobic digestion, but limited research focus on the synergetic effects on microbial interconnections of the biomethane process. In this study, the pineapple and maize residues under different operations were monitored by batch biogas assays and 16S high-throughput sequencing to explore: 1) biomethane potential regarding different operations, 2) microbial communities in different treated reactors, and 3) significant factors determine microbial distribution. Results showed that the co-digestion had higher methanogenic abundance and biomethane production (~3300 mL_n) versus mono-digestion under mesophilic condition. To the thermophilic condition, the co-digestion had less methanogenic abundance but more biomethane production (~5000 mL_n). Statistical evidence uncovered that the Clostridiaceae and Thermoanaerobacteraceae dominated pathways linked closely with methanogenesis which may contribute the more biomethane production in the thermophilic condition. This study demonstrated the temperature regulations drove rare taxa as major contributors for biomethane production.

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

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

Resource recovery and circular economy from organic solid waste using aerobic and anaerobic digestion technologies

With the inevitable rise in human population, resource recovery from waste stream is becoming important for a sustainable economy, conservation of the ecosystem as well as for reducing the dependence on the finite natural resources. In this regard, a bio-based circular economy considers organic wastes and residues as potential resources that can be utilized to supply chemicals, nutrients, and fuels needed by mankind. This review explored the role of aerobic and anaerobic digestion technologies for the advancement of a bio-based circular society. The developed routes within the anaerobic digestion domain, such as the production of biogas and other high-value chemicals (volatile fatty acids) were discussed. The potential to recover important nutrients, such as nitrogen through composting, was also addressed. An emphasis was made on the innovative models for improved economics and process performance, which include co-digestion of various organic solid wastes, recovery of multiple bio-products, and integrated bioprocesses.

Combined freezing-thawing pretreatment and microbial electrolysis cell for enhancement of highly concentrated organics degradation from dewatered sludge

The influence of freezing-thawing (F/T) pretreatment on the degradation of highly concentrated organic matters from dewatered sludge (DS) in microbial electrolysis cell (MEC) was investigated in this study. Extended freezing disintegrated the DS matrix and resulted in accelerated hydrolysis rate. The biogas production and stabilization were increased due to the pretreatment by 25–70% of H₂ production rate and 17.8–33.8% of COD reduction rate, respectively. Fourier transform infrared spectroscopy analysis indicated that the pretreatment was unable to alter the bioelectrochemical reactions except for accelerating degradation rate. Excitation and emission matrix (EEM) spectra showed that aromatic protein and soluble microbial products (SMPs)-like materials in DS were increasingly solubilized by the pretreatment and significantly removed during electrogenesis. The F/T-pretreated DS favored the enrichment of exoelectrogens in MEC.

Acetate Production from Cafeteria Wastes and Corn Stover Using a Thermophilic Anaerobic Consortium: A Prelude Study for the Use of Acetate for the Production of Value-Added Products

Efficient and sustainable biochemical production using low-cost waste assumes considerable industrial and ecological importance. Solid organic wastes (SOWs) are inexpensive, abundantly available resources and their bioconversion to volatile fatty acids, especially acetate, aids in relieving the requirements of pure sugars for microbial biochemical productions in industries. Acetate production from SOW that utilizes the organic carbon of these wastes is used as an efficient solid waste reduction strategy if the environmental factors are optimized. This study screens and optimizes influential factors (physical and chemical) for acetate production by a thermophilic acetogenic consortium using two SOWs-cafeteria wastes and corn stover. The screening experiment revealed significant effects of temperature, bromoethane sulfonate, and shaking on acetate production. Temperature, medium pH, and C:N ratio were further optimized using statistical optimization with response surface methodology. The maximum acetate concentration of 8061 mg L-1 (>200% improvement) was achieved at temperature, pH, and C:N ratio of 60 °C, 6, 25, respectively, and acetate accounted for more than 85% of metabolites. This study also demonstrated the feasibility of using acetate-rich fermentate (obtained from SOWs) as a substrate for the growth of industrially relevant yeast Yarrowia lipolytica, which can convert acetate into higher-value biochemicals.

Recycling strategies for polyhydroxyalkanoate-based waste materials: An overview

The plastics market is dominated by fossil-based polymers, but their gradual replacement by bioplastics (e.g., polyhydroxyalkanoates) is occurring. However, recycling strategies need to be developed to truly unveil the impact of bioplastics on waste accumulation. This review provides a state of the art of recycling strategies investigated for polyhydroxyalkanoate-based polymers and proposes future research avenues. Research on mechanical and chemical recycling is dominated by the use of extrusion and pyrolysis, respectively, while that on biodegradation of polyhydroxyalkanoates is related to soil and aquatic samples, and to anaerobic digestion towards biogas production. Research gaps exist in the relationships between polymer composition and ease of use of all recycling strategies investigated. This is of utmost importance since it will influence the need for separation at the source. Therefore, research emphasis needs to be given to the area to follow the continuous improvement of the process economics towards widespread commercial production of polyhydroxyalkanoates.

Comparative Study on the Energetic and Ecologic Parameters of Dual Fuels (Diesel–NG and HVO–Biogas) and Conventional Diesel Fuel in a CI Engine

The Article presents the results of the experimental research and numerical analysis of a compression ignition (CI) engine adapted for running on dual fuels of different composition (diesel and natural gas, diesel and biogas, biodiesel and natural gas, and biodiesel and biogas). The main goal was to find out the impact of different dual fuels on energy performance and emissions depending on the start of injection (SOI) of diesel and the crank angle degree (CAD). Pure conventional diesel fuel and second generation hydrotreated vegetable oil (HVO) (Neste) was used in the research. Natural gas contained 97 vol. % of methane. Biogas (biomethane) was simulated using a methane and carbon dioxide blend consisting of 60 vol. % of methane and 40 vol. % of carbon dioxide. Dual (liquid and gaseous) fuels were used in the tests, with the energy share of liquid fuels accounting for 40% and gas for 60%. The research results have shown that having replaced conventional diesel fuel with dual fuel, engine’s BTE declined by 11.9–16.5%. The use of methane in the dual fuel blend reduced CO2 volumetric fraction in the exhaust gases by 17–20%, while biomethane increased CO2 volumetric fraction by 10–14%. Dual fuel significantly increased CO and HC emissions, but NOx volumetric fraction decreased by 67–82% and smoke by 23–39%. The numerical analysis of the combustion process revealed changes in the ROHR (Rate of Heat Release) that affected engine efficiency and exhaust emissions was done by AVL (Anstalt für Verbrennungskraftmaschinen List) BOOST program.