Nanobubble technology to enhance energy recovery from anaerobic digestion of organic solid wastes: Potential mechanisms and recent advancements.

Nanobubble (NB) technology has gained popularity in the environmental field owing to its distinctive characteristics and ecological safety. More recently, the application of NB technology in anaerobic digestion (AD) systems has been proven to promote substrate degradation and boost the production of biogas (H2 and/or CH4). This review presents the recent advancements in the application of NB technology in AD systems. Meanwhile, it also sheds light on the underlying mechanisms of NB technology that contribute to the enhanced biogas production from AD of organic solid wastes. Specifically, the working principles of the NB generator are first summarized, and then the structure of the NB generator is optimized to accommodate the demand for NB characteristics in the AD system. Subsequently, it delves into a detailed discussion of how the addition of nanobubble water (NBW) affects AD performance and the different factors that NB can potentially contribute. As a simple and environmentally friendly additive, NBW was commonly used in the AD process to enhance the fluidity and mass transfer characteristics of digestate. Additionally, NB has the potential to enhance the functionality of different types of microbial enzymes that play crucial roles in the AD process. This includes boosting extracellular hydrolase activities, optimizing coenzyme F420, and improving cellulase function. Finally, it is proposed that NBW has development potential for the pretreatment of substrate and inoculum, with future development being directed towards this aim.

Preparation of a Novel Straw-Sludge Activated Biochar and Its Adsorption Mechanisms for Removal of VOCs.

To simultaneously achieve the objectives of waste resource utilization and clean production, a novel approach involving the utilization of corn straw-sludge hybrid biochar was proposed for the adsorption of VOCs emitted from biomass power plants. This study analyzed the effect of straw-sludge biochar on the adsorption characteristics of VOCs (toluene, isopentane, and ethylene) under different preparation conditions (raw material ratio, activation temperature, and activation time). The findings revealed that the adsorption efficiency of the mixed biochar was significantly superior to that of individual corn straw biochar and sludge biochar. The adsorption of methylbenzene, isopentane, and ethylene was 78.32, 40.81, and 41.18% higher, respectively, compared to the control groups consisting of pure sludge biochar and pure corn straw biochar. Moreover, the adsorption performance of the activated biochar followed the sequence of ethylene < isopentane < methylbenzene in terms of both saturation time and adsorption capacity. The adsorption capacity of VOCs on straw biochar-sludge biochar demonstrated a consistent correlation with the boiling point and molecular weight of the adsorbate, with higher adsorption capacities observed for adsorbates with larger boiling points and molecular weights, specifically methylbenzene > isopentane > ethylene.

Simultaneous addition of CO2-nanobubble water and iron nanoparticles to enhance methane production from anaerobic digestion of corn straw.

In this research, CO2-nanobubble water (CO2-NBW) and iron nanoparticles (Fe0NPs) were added simultaneously to exploit individual advantages to enhance the methanogenesis process from both the stability of anaerobic digestion (AD) system and the activity of anaerobic microorganism aspects. Results showed that the AD performance was enhanced by supplementing with CO2-NBW or Fe0NPs individually, and could be further improved by simultaneous addition of the two additives. The maximum methane yield was achieved in the CO2-NBW + Fe0NPs reactor (141.99 mL/g-VSadded), which increased by 26.16% compared to the control group. Similarly, the activities of the electron transfer system (ETS) and enzyme were improved. The results of microbial community structure revealed that the addition of CO2-NBW and Fe0NPs could improve the abundance of dominant bacteria (Anaerolineaceae, Bacteroidales, and Prolixibacteraceae) and archaea (Methanotrichaceae and Methanospirillaceae). Additionally, the functional metabolic prediction heatmap indicated that metabolic functional genes favorable for AD of corn straw were enhanced.

Supplementation of CO2-nanobubble water to enhance the methane production from anaerobic digestion of corn straw.

Nanobubble water (NBW) could improve methane production from anaerobic digestion (AD) of corn straw without secondary contamination. In this study, the effect of carbon dioxide nanobubble water (CO2-NBW) volumes (0%, 25%, 50%, 75%, 100%) on methane production from corn straw was investigated. The results showed that addition of CO2-NBW could improve methane production and promote substrate degradation in AD process. The highest cumulative methane production of 132.16 mL g-1VSadded was obtained in the 100% CO2-NBW added reactor, which was 17% higher than that in the control group. Additionally, the addition of CO2-NBW could mitigate the sharp decrease in pH by acting as a buffer. CO2-NBW could also enhance microorganism activity throughout the AD process. The electron transport system (ETS) activity was increased by 23%, while the ß-glucosidase, dehydrogenase (DHA), and coenzyme F420 activities were increased by 15%, 23%, and 11%, respectively, at optimum addition of CO2-NBW. Meanwhile, addition of CO2-NBW accelerated the production and consumption of reducing sugar and volatile fatty acids (VFAs), promoting the reduction rates of TS (Total solid) and VS (Volatile solid).

Enrichment culture combined with microbial electrochemical enhanced low-temperature anaerobic digestion of cow dung.

Enrichment culture combined with the microbial electrochemical system was used to co-enhance the low-temperature (20 °C) anaerobic digestion. The results showed that enrichment culture combined with microbial electrochemical system increased the cumulative methane production in low-temperature anaerobic digestion system by 39.64 % and 133.29 % compared to single and no enrichment culture, respectively. Enrichment culture combined with microbial electrochemical system increased the relative abundance of methanogenic archaea (Methanomassiliicoccus, Methanocorpusculum, unclassified Methanomicrobiaceae, Methanobacterium, Methanoculleus, Methanocalculus) and the relative abundance of cold-tolerant hydrolytic acidifying bacteria (unclassified Bacteroidetes, Treponema). The expressions of specific enzyme genes in the methanogenesis pathway were enhanced, including acetyl-CoA synthetase, formylmethanofuran dehydrogenase, methanol cobalamin methyltransferase, etc. These results indicated that enrichment culture combined with microbial electrochemical system enhanced low-temperature anaerobic digestion methanogenesis by altering microbial communities and stimulating enzyme gene expression to affect volatile fatty acids, pH, redox potential, and reducing sugar parameters.

Enhancement of anaerobic fermentation with corn straw by pig bone-derived biochar.

Different sources of biochar exhibit different effects on anaerobic fermentation. Here, the effects of activation temperature, activation time, impregnation ratio, and pickling times on the properties of pig bone-derived biochar additives were explored by orthogonal experiments. The pig bone-derived biochar with better performance was optimized to enhance the anaerobic fermentation. The results showed that when the preparation conditions of biochar were as follows: activation temperature of 700 °C, impregnation ratio of 2, activation time of 90 min, and pickling times of 2, the cumulative methane production of corn stalk by anaerobic fermentation exhibited the highest value of 164.54 mL/g VS, which was 68% higher than the control group. The correlation between the characteristics of biochar for promoting anaerobic fermentation and the performance of anaerobic fermentation was established. Interestingly, the pig bone-derived biochar can buffer pH value in straw anaerobic fermentation.

Enhancement of methane production by anaerobic digestion of corn straw with hydrogen-nanobubble water.

Hydrogen-nanobubble water was proposed to enhance methane production by anaerobic digestion (AD) with corn straw. The effects of H2-nanobubble water (H2-NBW) amounts (0%, 20%, 40%, 60%, 80%, and 100%) on methane production characteristics of corn straw were explored. The results showed that the methane yields were increased by 11.54%∼25.29% compared with the control group(CK), and the maximum cumulative methane production reached to 254.36 mL·g-VS-1 when the H2-NBW addition was of 60%. Interestingly, the maximum methane concentration increased by 4.37% compared with CK. H2-NBW addition can destroy the cellulose structure of corn straw, reduce the crystallinity of cellulose, and promote the hydrolysis. The degradation rate of cellulose and hemicellulose were increased by 20%∼33% and 13% ∼25.7% respectively, and the removal rate of TS and VS were increased by 6.82%-27.93% and 8.52%-21.47%, respectively. The modified Gompertz equation fitted the cumulative methane production curves very well, with high correlation coefficients (R2 > 0.992).

Investigation on the effect of different additives on anaerobic co-digestion of corn straw and sewage sludge: Comparison of biochar, Fe3O4, and magnetic biochar.

The co-digestion of corn straw and sewage sludge with different additives (biochar, magnetic biochar, Fe3O4) were investigated. The highest cumulative methane yield of 245.15 mL/g VSadded was obtained with the Fe3O4 addition ratio of 5 g/kg, which was 60.47% higher than that of the control run (without additives). The lag phase time was shortened from 5.46 to 3.82 days with a biochar dosage of 5 g/kg. The performance of Fe3O4 on methane production from the co-digestion process was better than that of the biochar and magnetic biochar. The direct interspecies electron transfer (DIET) was enhanced with regard to the increased concentration of acetic acid and decreased concentration of propionic acid. Microbial community analysis showed that the Geobacter and Methanosarcina were selectively enriched on the surface of Fe3O4, promoting the DIET and acetoclastic methanogenesis pathway. The cost-benefit analysis proved that the strategy of recycling Fe3O4 additive has the best economic benefit.

Exergy analysis and optimization of bio-methane production from corn stalk pretreated by compound bacteria based on genetic algorithm.

An exergy equilibrium model was established to obtain the exergy efficiency under different conditions of compound bacteria pretreatment and anaerobic digestion (AD) of corn stalk. The Genetic Algorithm (GA) was applied to optimize the exergy efficiency of the combination process of the pretreatment and AD. The maximum exergy efficiency with the GA was 19.04%, corresponding to the optimal pretreatment parameters: pretreatment temperature 33.34℃, stalk particle size 0.50 mm, ventilation rate 0.88 L/min, pretreatment time 169.03 h. The optimal AD parameters were: digestion temperature 38.08℃ and stirring rate 48.04 r/min. The validation experiment exergy efficiency reached to 19.25%, which was 24.37% higher as compared to that of the non-pretreatment process. Under these optimal conditions, the energy consumption of the compound bacteria pretreatment and the time of the bio-methane production process were effectively reduced.

One-pot conversion of wheat straw into biobased chemicals in methanol/water medium using cheap mixed acid catalyst.

BACKGROUND: Environmental concerns and the diminishing availability of unrenewable resources have spurred research into the use of agricultural waste as a feedstock for industrial applications. Efficient conversion of wheat straw into biobased chemicals is an important way to realize the potential value of renewable agricultural biomass. This study investigated one-pot conversion of wheat straw into two notable platform chemicals, levulinic acid (LA) and methyl levulinate (ML). RESULTS: A mixed acid catalyst system, including 1% H2 SO4 and 0.015 mol L-1 Al2 (SO4 )3 , was an efficient catalyst for the conversion of wheat straw due to the combination of Brønsted acid and Lewis acid. A ratio of wheat straw to methanol of 5 g/50 mL was identified as the preferred solid/liquid ratio, and a methanol/H2 O medium with 25% water content aided the simultaneous production of LA and ML from wheat straw. Under optimum conditions, the maximum total yield of LA and ML reached 23.01% at 220 °C and 3 h. The kinetics of biobased chemical formation and the reaction pathways in methanol/water were investigated. CONCLUSION: The presence of water in the methanol/H2 O medium affected the distribution of products and promoted hydrolysis reactions. The methanol/H2 O medium not only inhibited the side reactions but also promoted the degradation of wheat straw and increased the total yield of LA and ML. This study provides a feasible method for the conversion of wheat straw to prepare biobased chemicals. © 2021 Society of Chemical Industry.

Enhanced biohydrogen production from corn straw by basalt fiber addition.

The aim of this work was to study the impact of basalt fiber (BF) on hydrogen fermentation of corn straw. The maximum of hydrogen yield and corn straw conversion rate respectively achieved 323.94 mL and 5.23% by adding 1.5 g/L BF particle with the size of 300-400 mesh, which increased by 15.74% and 15.6% respectively than control group. The BF could promote the growth of photosynthetic bacteria, subsequently influencing the products distribution and hydrogen generation. Overall, this investigation demonstrated that BF addition is an effective way to enhance biohydrogen production from corn straw.

Prediction of methane production from co-digestion of lignocellulosic biomass with sludge based on the major compositions of lignocellulosic biomass.

In the present study, the simplex lattice mixture design method was adopted to design the artificial biomass with different ratios of three major components (cellulose, hemicellulose, lignin). The methane yield from the co-digestion of the artificial/ natural biomass (corn stover, wheat stover, rice straw, and peanut stalk) samples with the mixed sludge at the mixture ratio of 1:1 based on total solid (TS) content was recorded for 50 days. The original mathematical prediction models for estimating the cumulative methane production, maximum methane production rate, and lag phase time were established based on the experimental results from the co-digestion of artificial biomass with sludge. To investigate the influence of the structural features of biomass and interactions among the components of biomass which contributing to the inhibition of methane production, the macroscopic factor (MF) was proposed. The mathematical models which revealed the relationship between MF and the methane production parameters were developed by the combination of the prediction results from the original mathematical prediction model and experimental results from the co-digestion of natural biomass with sludge. Modification of the original mathematical prediction models was carried out by considering MF. After modification, the relative error (RE) and root mean square error (RMSE) of the prediction model for cumulative methane production were declined from 19.00 to 30.18% and 42.38 mL/g VSadded to that of - 1.93~7.14% and 4.36 mL/g VSadded, respectively.

Enhancement on enzymolysis of pigskin with ultrasonic assistance.

The fur is hard to decompose during the fermentation process of diseased swine carcasses. In order to enhance the enzymolysis of pigskin, the ultrasonic was proposed to use during the process of the enzymatic hydrolysis. The response surface optimization experiments were carried out with the DH (degree of hydrolysis) as the response value and the optimum conditions for enzymatic hydrolysis were determined. Based the optimum conditions, orthogonal experiments were carried out with ultrasonic frequency, power and time as variables, and optimal ultrasonic parameters were obtained. Without the assistance of ultrasonic, the descending order of influence factors on DH was, temperature＞SC(Substrate concentration)＞RES(The ratio of enzyme to substrate)＞pH. Moreover, the DH value is of 10.42% under the following optimal conditions: RES is of 16,006 U/g, the temperature is of 48.92°C, the SC is of 59.76 g/L and pH is of 10.43. Frequency has the greatest effect on DH, followed by power, and finally time. The optimum hydrolysis time is of 5 h, and the DH is of 22.94% were obtained under the following optimum ultrasonic pretreatment conditions: frequency combination is of (20,40,40), power is of 600 W and time is of 25 min. Comparing with the group without ultrasonic pretreatment, the DH for the ultrasonic assistance increased by 4%, the hydrolysis time was shorten by 3 h, and the total amino acids increased by 15.98%.

Biological pretreatment of corn straw for enhancing degradation efficiency and biogas production.

In order to explore the effect of pretreatment on corn straw degradation and biogas production, corn straw was pretreated with mixed microbes and composting at 30°C for 14 days. The characteristics of material were measured and analyzed in the pretreatment process. Then, the pretreated and untreated corn straw was digested by anaerobic fermentation. Gas production and methane content of corn straw were analyzed. The results showed that the biological pretreatment process with mixed microbes could accelerate the degradation rate of straw and increase the degradation efficiency of lignin. The pH value of material was more stable, and the content of organic matter in the material was higher in the pretreatment process of corn straw with mixed microbes. The Scanning Electron Microscope (SEM) images showed that the structure of the lignocellulose was changed by mixed microbes, increasing the exposed area of cellulose and hemicellulose, which was beneficial to improve the utilization efficiency of straw. The degradation rates of hemicellulose, cellulose and lignin were 44.4%, 34.9% and 39.2%, respectively, after the pretreatment process with mixed microbes. Pretreatment was more helpful to increase the methane content in the anaerobic fermentation process of corn straw pretreated with mixed microbes, and could also shorten the fermentation period.

Enhanced biodiesel production from diseased swine fat by ultrasound-assisted two-step catalyzed process.

In order to enhance the yield of high quality biodiesel form diseased swine fat, the ultrasound-assisted two-step catalyzed process was employed. First, three-dimensional ultrasound-assisted concentrated sulfuric acid pre-esterification experiment was carried out. Then, the transesterification reaction catalyzed by KOH was performed, and four parameters (catalyst concentration, reaction time, methanol/oil molar ratio and reaction temperature) were optimized using response surface methodology. The results showed that the optimal transesterification reaction conditions were catalyst concentration of 1.11 wt%, reaction temperature of 62.3 °C, methanol/oil molar ratio of 7.42:1, and reaction time of 116.14 min. The most significant factor affecting biodiesel purity was identified as catalyst concentration. Under the optimal conditions, the maximum biodiesel purity reached to 98% with the reaction time of 176.14 min, shortened by 63.3% compared with previous works. Furthermore, most of the biodiesel properties agreed the quality requirements established by Official Regulations of GB/25199-2017 of China.

Enhancement of converting corn stalk into reducing sugar by ultrasonic-assisted ammonium bicarbonate pretreatment.

In order to improve the yield of reducing sugar from corn stalk, ultrasonic-assisted ammonium bicarbonate pretreatment of corn stalk was proposed. Three ultrasonic factors (time (0-30 min), temperature (30-60 °C) and liquid/solid mass ratio (5-20)) were optimized by response surface experiment. The optimal conditions of ultrasonic pretreatment were obtained (liquid/solid mass ratio is 12:1, temperature is 42 °C and time is 11 min). The highest saccharification rate of corn stalk was of 82.61%, which was remarkably increased by 355% compared to the control group.

Ammonium bicarbonate pretreatment of corn stalk for improved methane production via anaerobic digestion: Kinetic modeling.

This work studied the effects of ammonium carbonate pretreatment on biogas production during sequential anaerobic digestion. The results showed that the addition of ammonium bicarbonate (8%, w/w) obtained the highest biogas production of 14,690 mL with a yield of 222.6 mL g-1, which was 31% higher than that of un-pretreated (control experiment). Interestingly, the COD removal was increased by 100%. Furthermore, it was also helpful in stabilizing the pH of anaerobically digested corn stalk. Based on the results, the modified Gompertz equation fitted the cumulative biogas production curves very well, with high correlation coefficients (>0.994).

Experimental study on the relationship between the mineral production capability and the physiochemical properties in the coproduction of Q phase-3CaO·3Al2O3·CaSO4 cement clinker.

A coproduction tests of quaternary (Q) phase(6CaO·4Al2O3·MgO·SiO2) -3CaO·3Al2O3·CaSO4 cement clinker and an experimental study on the relationship between the mineral production capability and the physiochemical properties are conducted in a two-stage multiphase reaction test bed with Changguang coal. X-ray diffractometer (XRD) analyses are performed on the coproduction clinker samples. The results demonstrate that, with the reduction in particle sizes of the coal powder and the additives and expanded screening level differences between them, both the proportion of Q phase and the mass of 3CaO·3Al2O3·CaSO4 in the clinker increase accordingly. When mixed coal powder particles are prepared through reducing particle sizes and expanding screening level differences between coal powder and additives, the additives CaO and MgO are more likely to be enclosed by coal powder to form globular polymerized particles. In addition, this preparation aids in polymerization and promotes even distribution of CaO, MgO and coal minerals, thus facilitating clinker mineral formation reactions of inorganic substances in the mixed coal powder. Target minerals, such as 2CaO·SiO2 and Q phase, are found in both industrial high-calcium limestone and low-calcium limestone coproduction clinker samples. A diffraction peak of free CaO is also evident in both samples. Compared with a coproduction clinker sample of high-calcium limestone, that of low-calcium limestone exhibits higher diffraction peaks for 2CaO·SiO2 and Q phase. With the current state of the art, it is not yet the optimum choice to substitute CaCO3 for CaO in Q-phase cement clinker coproduction. Before the technology matures and gains practical application, further study on the form and the mixing process of calcium-based additives for cement clinker coproduction will be required.

Effects of mass transfer and light intensity on substrate biological degradation by immobilized photosynthetic bacteria within an annular fiber-illuminating biofilm reactor.

In this work, effects of mass transfer and light intensity on performance of substrate biodegradation by cell-immobilized photosynthetic bacteria were investigated within an annular fiber-illuminating bioreactor (AFIBR). In AFIBR, stable biofilm of photosynthetic bacteria was generated on the surface of side-glowing optical fiber to provide sufficient light supply and uniform light distribution in cell-immobilized zone for continuous substrate biodegradation during hydrogen production process. To optimize operation parameters for substrate degradation, a two-dimensional mass transfer model based on experimental data to describe coupled processes of substrate transfer and biodegradation in biofilm with substrate diffusion and convection in bulk flow region was proposed. Investigations on influences of substrate concentration, flow rate and light intensity were carried out. It was showed that the optimum operational parameters for the substrate degradation in the AFIBR are: 10g/l substrate concentration, 100ml/h flow rate and 3.1W/m(2) light intensity.