Review of explainable machine learning for anaerobic digestion.

Anaerobic digestion (AD) is a promising technology for recovering value-added resources from organic waste, thus achieving sustainable waste management. The performance of AD is dictated by a variety of factors including system design and operating conditions. This necessitates developing suitable modelling and optimization tools to quantify its off-design performance, where the application of machine learning (ML) and soft computing approaches have received increasing attention. Here, we succinctly reviewed the latest progress in black-box ML approaches for AD modelling with a thrust on global and local model interpretability metrics (e.g., Shapley values, partial dependence analysis, permutation feature importance). Categorical applications of the ML and soft computing approaches such as what-if scenario analysis, fault detection in AD systems, long-term operation prediction, and integration of ML with life cycle assessment are discussed. Finally, the research gaps and scopes for future work are summarized.

Nanocellulose aerogel for highly efficient adsorption of uranium (VI) from aqueous solution.

Cellulose nanofibers (CNFs) aerogel was prepared via simple covalent crosslinking and freeze-drying method. The porous cellulose aerogel possessed high specific surface area and high metal-chelating capacity, which showed fast adsorption kinetics and high adsorption capacity (440.60 mg g-1) in static uranium adsorption process. In the dynamic filtration system, the maximum adsorption capacity reached 194 mg g-1 with the initial concentration of 10 mg L-1. In addition, the CNFs aerogel possessed excellent selectivity and good regeneration ability for uranium adsorption. The integrated analyses of attenuated total reflection Fourier transform infrared (ATR-FTIR), X-Ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) suggested that the predominant UO22+ species formed inner-sphere surface complexes with two active carboxyl groups in the coordination model. This strategy may provide a sustainable route for development of efficient biomass-based adsorbents for selective uranium removal from aqueous solution.

Amidoximated cellulose fiber membrane for uranium extraction from simulated seawater.

Uranium extraction from seawater is considered as an efficient strategy to meet the increasing demands of uranium. Amidoxime has been reported as one of the most efficient groups for uranium affinity. Herein, amidoximated cellulose fibers were synthesized by grafting polyacrylonitrile (PAN) onto cellulose fibers followed by amidoxime modification. The amidoximated cellulose fibers showed maximum adsorption capacity of 52.88 mg g-1 (pH = 5.0), and its static adsorption process was well fitted with Langmuir model and Pseudo-second-order kinetics. The adsorption mechanism was attributed to the chelating reaction between uranyl complexes and amidoximated cellulose fibers. The prepared fibers were further fabricated into nonwoven membrane for dynamic adsorption, and the breakthrough curves were well fitted to Dose-Response model. The amidoximated cellulose fiber membrane showed a good adsorption capacity of 1.22 mg g-1 at pH 8.0 after filtrating 10.0 L simulated seawater, demonstrating promising efficient engineering materials for uranium extraction from seawater.

Preparation of three-dimensional fiber-network chitosan films for the efficient treatment of uranium-contaminated effluents.

A fiber-network chitosan film with three-dimensional interconnected structure was prepared in an alkali/urea solution and regenerated from an ethanol/water coagulation solution. The surface morphology and structure were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), N2 adsorption-desorption, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). Batch adsorption for uranium U(VI) was conducted to investigate the effects of pH, contact time and initial uranium concentration on adsorption capacity. The adsorption of CS-80% was in good agreement with the pseudo-second-order kinetic model and Langmuir isotherm model. The three-dimensional interconnected structure provided more active sites and favored the diffusion of uranium solute, and therefore enhanced the adsorption capacity. The maximum adsorption capacity at pH 5 was 196.735 mg/g. The adsorption mechanism was attributed to chelation and coordination of uranium with -NH2 and -OH groups on chitosan molecules.

Life cycle assessment and net present worth analysis of a community-based food waste treatment system.

Food waste management has been a global challenge with significant economic and environmental impacts. A community-based food waste treatment scheme for Glasgow, UK is proposed. The food waste was treated by small-scale wet, mesophilic anaerobic digestion. Biogas was combusted in a combined heat and power plant to generate heat and electricity for each community. 201.39 kWh of electricity and 246.09 kWh of thermal energy could be provided to local communities per tonne of food waste treated. A total of 52,762 tonnes of food waste were produced each year in the city. Net-present worth analysis was employed to evaluate the scheme's economic feasibility. The scheme's environmental impacts were evaluated using life cycle assessment. The entire system saved 92.27 kg CO2-eq. per tonne of food waste treated and had a net-present worth of £ 3.187 million with a carbon tax of 50 £ tonne-1 and a biogas yield of 190 m3 tonne-1.

Facile synthesis of TiO2/CNC nanocomposites for enhanced Cr(VI) photoreduction: Synergistic roles of cellulose nanocrystals.

In this study, TiO2 nanocrystals were synthesized in the scaffold of cellulose nanocrystal (CNC) using in situ hydrolysis, where the morphology and size of TiO2 was controlled by CNC's functional groups and surface charge. The resulting TiO2/CNC nanocomposites showed a superior photocatalytic activity for Cr(VI) reduction under visible light (λ > 420 nm) due to the combined effects of small TiO2 size and ligand-to-metal charge transfer (LMCT) complex between CNC and TiO2. It was found that the charge-enriched CNC not only acted as a template to direct the crystal growth of TiO2, but also played essential roles on light harvesting and charge transfer thereby promoting the photoreduction of Cr(VI). The demonstrated system represents a unique pathway to develop a lower cost and efficient purification material for remediation of Cr(VI).

Interaction and Kinetics Study of the Co-Gasification of High-solid Anaerobic Digestate and Lignite.

This study aims at investigating the interaction and kinetics behavior of the co-gasification of digestate and lignite. The co-pyrolysis performances of digestate and lignite blended by dry process were better than that blended by wet process, while the wet-blending process could improve the performance in co-gasification stage because of the larger pore diameter and pore volume. When anaerobic digestion (AD) time was 40 days, the synergistic interaction between digestate and lignite were the most remarkable based on the results of thermogravimetric analysis (TG) and the experiments in the lab-scale downdraft fixed bed gasifier. Kinetics study showed that the increase of AD time and the addition of digestate in lignite decreased the activation energy of the co-gasification reaction.

Anaerobic digestion of yard waste with hydrothermal pretreatment.

The digestibility of lignocellulosic biomass is limited by its high content of refractory components. The objective of this study is to investigate hydrothermal pretreatment and its effects on anaerobic digestion of sorted organic waste with submerged fermentation. Hydrothermal pretreatment (HT) was performed prior to anaerobic digestion, and three agents were examined for the HT: hot compressed water, alkaline solution, and acidic solution. The concentrations of glucose and xylose were the highest in the sample pretreated in acidic solution. Compared with that of the untreated sample, the biogas yields from digesting the samples pretreated in alkaline solution, acidic solution, and hot water increased by 364, 107, and 79%, respectively. The decrease of chemical oxygen demand (COD) in liquid phase followed the same order as for the biogas yield. The initial ammonia content of the treated samples followed the order sample treated in acidic solution > sample treated in alkaline solution > sample treated in hot water. The concentrations of volatile fatty acids (VFAs) were low, indicating that the anaerobic digestion process was running at continuously stable conditions.

Impact of pretreatment on solid state anaerobic digestion of yard waste for biogas production.

Solid state anaerobic digestion, as a safe and environment-friendly technology to dispose municipal solid wastes, can produce methane and reduce the volume of wastes. In order to raise the digestion efficiency, this study investigated the pretreatment of yard waste by thermal or chemical method to break down the complex lignocellulosic structure. The composition and structure of pretreated yard waste were analyzed and characterized. The results showed that the pretreatment decreased the content of cellulose and hemicelluloses in yard waste and in turn improved the hydrolysis and methanogenic processes. The thermal pretreatment sample (P1) had the highest methane yield, by increasing 88% in comparison with digesting the raw material. The maximum biogas production reached 253 mL/g volatile solids (VS). The largest substrate mass reduction was obtained by the alkaline pretreatment (P5). The VS of the alkaline-treated sample decreased about 60% in comparison with the raw material.

Enhancement of bunker oil biodesulfurization by adding surfactant.

Biodesulfurization (BDS) is a promising method to remove sulfur compounds from diesel and gasoline. However, the information on BDS of heavy oil is scanty, which might be due to their "undesirable" physical properties and more complicated sulfur diversities. In this study, the BDS of one kind of heavy oil, bunker oil MFO380 was investigated. The biocatalyst was obtained by the enrichment with oil sludge as the seed and using dibenzothiophene (DBT) as the sole sulfur source. The enriched biocatalyst (microbial mixed culture) could selectively remove sulfur from DBT and DBT was transformed into 2-hydroxybiphenyl, which indicates that the BDS process is beneficial to non-destructive carbon bonds and thus can maintain the calorific value. The bunker oil BDS results showed that after 7 days of incubation, the removal efficiency of sulfur in MFO380 was only 2.88 %, but this could be significantly improved by adding surfactants Triton X-100 or Tween 20. This effect could be attributed to greatly reduced viscosity of heavy oil and increased mass transfer of sulfur compounds in heavy oil into water. Adding Triton X-100 achieved the highest removal efficiency of sulfur, up to 51.7 % after 7 days of incubation. The optimal amount of Triton X-100 was 0.5 g/50 ml medium. When toluene was added as an organic solvent for MFO380, the BDS activity was improved, while lower than the effect of adding surfactants.

Reduction of hexavalent chromium by Pannonibacter phragmitetus LSSE-09 stimulated with external electron donors under alkaline conditions.

A novel Cr (VI) resistant bacterial strain LSSE-09, identified as Pannonibacter phragmitetus, was isolated from industrial sludge. It has strong aerobic and anaerobic Cr (VI)-reduction potential under alkaline conditions. At 37 °C and pH 9.0, growing cells of strain LSSE-09 could completely reduce 100 and 1000 mg L(-1) Cr (VI)-Cr (III) within 9 and 24h, respectively under aerobic condition. Resting cells showed higher anaerobic reduction potential with the rate of 1.46 mg g(-1)((dry weight))min(-1), comparing with their aerobic reduction rate, 0.21 mg g(-1)min(-1). External electron donors, such as lactate, acetate, formate, pyruvate, citrate and glucose could highly increase the reduction rate, especially for aerobic reduction. The presence of 3000 mg L(-1) acetate enhanced anaerobic and aerobic Cr (VI)-reduction rates up to 9.47 mg g(-1)min(-1) and 4.42 mg g(-1)min(-1), respectively, which were 5 and 20 times faster than those without it. Strain LSSE-09 retained high activities over six batch cycles and NO(3)(-) and SO(4)(2-) had slightly negative effects on Cr (VI)-reduction rates. The results suggest that strain LSSE-09 has potential application for Cr (VI) detoxification in alkaline wastewater.

Kinetic evaluation of products inhibition to succinic acid producers Escherichia coli NZN111, AFP111, BL21, and Actinobacillus succinogenes 130Z T.

Succinic acid is one of the platform compounds and its production via natural feedstocks has drawn worldwide concerns. To evaluate the inhibitory effects of fermentation products on the growth of Actinobacillus succinogenes 130Z(T) and Escherichia coli NZN111, AFP111, BL21, fermentations with addition of individual products in medium were carried out. The cell growth was inhibited when the concentrations of formate, acetate, lactate, and succinate were at range of 8.8-17.6 g/L, 10-40 g/L, 9-18 g/L, and 10-80 g/L, respectively. For these two species of bacteria, E. coli was more resistant to acid products than A. succinogenes, while both endured succinate rather than by-products. As a result of end product inhibition, succinate production yield by A. succinogenes decreased from 1.11 to 0.49 g/g glucose. Logistic and Monod mathematical models were presented to simulate the inhibition kinetics. The Logistic model was found more suitable for describing the overall synergistic inhibitory effects.

High-concentration sugars production from corn stover based on combined pretreatments and fed-batch process.

In this paper, high-concentration sugars were produced from pretreated corn stover. The raw corn stover was pretreated in a process combining steam explosion and alkaline hydrogen-peroxide. The hemicellulose and lignin were removed greatly. The cellulose content increased to 73.2%. Fed-batch enzymatic hydrolysis was initiated with 12% (w/v) solids loading and 20 FPU/g solids. Then, 6% solids were fed consecutively at 12, 36 and 60 h. After 144 h, the final concentrations of reducing sugar, glucose, cellobiose and xylose reached 220, 175, 22 and 20 g/L, respectively. The final total biomass conversion was 60% in fed-batch process.

Efficient conversion of crop stalk wastes into succinic acid production by Actinobacillus succinogenes.

Succinic acid is valued as a key platform chemical for use in a variety of synthetic applications. Efficient biosynthesis of succinic acid from renewable biomass resource is reported in this paper. Batch fermentations were carried out to analyze influence of several carbon sources on succinic acid production from feedstock wastes by Actinobacillus succinogenes BE-1. Crop stalk wastes, including corn stalk and cotton stalk, were enzymatically converted into a carbohydrate-rich feedstock, obtaining glucose concentrations approaching 65-80% of the total reducing sugar. For the anaerobic batch cultivation with cotton stalk hydrolysates, the production of succinic acid was 15.8 g l(-1) with a high yield of 1.23 g per g glucose. Glucose and xylose were utilized at same time, while cellubiose was not consumed until glucose and xylose were completely consumed.

pH neutralization while succinic acid adsorption onto anion-exchange resins.

Succinic acid is a useful chemical and its purification from fermentation broth by ion-exchange resins has widely drawn attention. In this study, pH neutralization in the process of adsorption of succinic acid from model solutions and fermentation broth by anion-exchange resin NERCB 04 has been tested. Adsorption capacity of NERCB 04 was about 0.41 g succinic acid/g resin at concentrations of succinic acid in the range of 10-50 g/L in packed column. In the process of succinic acid removal, pH of the system could also be neutralized. The neutralizing ability of the resin as a neutralizing agent has also been studied in the model cycle system and in the real fermentation cycle process. It was found that NERCB 04 showed stable adsorption capacity and pH neutralization ability after each regeneration. A certain amount of anion-exchange resin could neutralize the low pH values (pH 2-5) and maintain the system around pH 7.0. This means the anion-exchange resins have the function of neutralizing reagent in the process of adsorbing succinic acid.

[Expression of heterogenous pyruvate carboxylase in Escherichia coli with lactose as inducer and its effect on succinate production].

Escherichia coli strain DC1515, deficient in glucose phosphotransferase (ptsG), lactate dehydrogenase (ldhA) and pyruvate:formate lyase (pflA), is a promising candidate for the fermentative production of succinate. To further improve the succinate producing capability of DC1515, we constructed plasmid pTrchisA-pyc with heterogenous pyruvate carboxylase (pyc) from Bacillus subtilis 168 under the Trc promoter and introduced it into DC1515. We used lactose as a substitute of IPTG to induce pyc. We optimized the culture conditions such as the lactose addition time, the lactose concentration and the culture temperature after induction for succinate production. We also explored the effect of lactose supplement during the fermentation. The results showed that pyc can be expressed under lactose induction in the fermentative medium with 15 g/L glucose due to the deficient of ptsG in DC1515. Under optimized conditions, the final succinate concentration reached to 15.17 g/L, which was 1.78-fold higher than that of control strain. If complementing lactose twice to the concentration of 1 g/L during the fermentation, the final succinate concentration could further reach to 17.54 g/L. This work might provide valuable information for gene expression in E. coli strains using lactose as inducer for succinate production in a glucose-medium. Due to the reduced cost, E. coli is becoming a more promising strain for succinate production through fermentation.

In situ magnetic separation and immobilization of dibenzothiophene-desulfurizing bacteria.

In situ cell separation and immobilization of bacterial cells for biodesulfurization were developed by using superparamagnetic Fe(3)O(4) nanoparticles (NPs). The Fe(3)O(4) NPs were synthesized by coprecipitation followed by modification with ammonium oleate. The surface-modified NPs were monodispersed and the particle size was about 13 nm with 50.8 emu/g saturation magnetization. After adding the magnetic fluids to the culture broth, Rhodococcus erythropolis LSSE8-1 cells were immobilized by adsorption and then separated with an externally magnetic field. The maximum amount of cell mass adsorbed was about 530 g dry cell weight/g particles to LSSE8-1 cells. Analysis showed that the nanoparticles were strongly absorbed to the surface and coated the cells. Compared to free cells, the coated cells not only had the same desulfurizing activity but could also be easily separated from fermentation broth by magnetic force. Based on the adsorption isotherms and Zeta potential analysis, it was believed that oleate-modified Fe(3)O(4) NPs adsorbed bacterial cells mainly because of the nano-size effect and hydrophobic interaction.

Enhancement of biodesulfurization in two-liquid systems by heterogeneous expression of vitreoscilla hemoglobin.

The vgb gene, encoding Vitreoscilla hemoglobin (VHb), was introduced into a specific desulfurization bacterium, Rhodococcus erythropolis LSSE8-1. The VHb-specific spectrum was observed for the recombinant. Compared to the wild type, the strain bearing vgb showed a higher biomass yield and desulfurizing activity.

[Comparison of the desulfurization activity among several bacteria and analysis of the conservation of their desulfurization genes].

Several bacteria, Bacillus brevis R-6, Pseudomonas delafleldii R-8, Nocardia globerula R-9, Bacillus sphaericus R-16, Rhodococcus erythropolis LSSE8-1 and Gordonia nitida LSSEJ-1, which can convert dibenzothiophene into 2-hydroxybiphenyl and sulfate, were investigated. Desulfurization products were quantitively determined by HPLC. Result revealed that each of these bacteria desulfurize DBT at a different rate. In order to obtain more information, the fragments encoding desulfurizing enzymes were studied. Desulfurization genes of R-6 and R-8 were separately amplified via PCR with specific primers based on the related sequences of Rhodococcus sp. IGTS8. Both sequences areminimally 99% related to IGTS8 sequence. Afterwards, dsz operon of LSSEJ-1 and R-9 were amplified and cloned. Sequences are also highly conservative. Data shows that identity of dszA between R-9 and IGTS8 is 99.6%, and identity of dszA between LSSEJ-1 and IGTS8 is 99.9%; dszB sequence of R-9 and LSSEJ-1 is 99.6% similarity to their counterpart sequence from IGTS8;Identity of dszC between R-9 and IGTS8 is 99.9%, and identity of dszC between LSSEJ-1 and IGTS8 is 99.1% . It may be deduced that the origins of desulfurization genes from mesophilic bacteria are the same.