### Recent Submissions

• #### Thermal Sensor Calibration for Unmanned Aerial Systems Using an External Heated Shutter

(Drones, MDPI AG, 2021-10-17) [Article]
Uncooled thermal infrared sensors are increasingly being deployed on unmanned aerial systems (UAS) for agriculture, forestry, wildlife surveys, and surveillance. The acquisition of thermal data requires accurate and uniform testing of equipment to ensure precise temperature measurements. We modified an uncooled thermal infrared sensor, specifically designed for UAS remote sensing, with a proprietary external heated shutter as a calibration source. The performance of the modified thermal sensor and a standard thermal sensor (i.e., without a heated shutter) was compared under both field and temperature modulated laboratory conditions. During laboratory trials with a blackbody source at 35 °C over a 150 min testing period, the modified and unmodified thermal sensor produced temperature ranges of 34.3–35.6 °C and 33.5–36.4 °C, respectively. A laboratory experiment also included the simulation of flight conditions by introducing airflow over the thermal sensor at a rate of 4 m/s. With the blackbody source held at a constant temperature of 25 °C, the introduction of 2 min air flow resulted in a ’shock cooling’ event in both the modified and unmodified sensors, oscillating between 19–30 °C and -15–65 °C, respectively. Following the initial ‘shock cooling’ event, the modified and unmodified thermal sensor oscillated between 22–27 °C and 5–45 °C, respectively. During field trials conducted over a pine plantation, the modified thermal sensor also outperformed the unmodified sensor in a side-by-side comparison. We found that the use of a mounted heated shutter improved thermal measurements, producing more consistent accurate temperature data for thermal mapping projects.
• #### On the Formation of Hydrogen Peroxide in Water Microdroplets

(arXiv, 2021-10-14) [Preprint]
Recent reports on the formation of hydrogen peroxide (H2O2) in water microdroplets produced via pneumatic spraying or capillary condensation have garnered significant attention. How covalent bonds in water could break under such conditions challenges our textbook understanding of physical chemistry and the water substance. While there is no definitive answer, it has been speculated that ultrahigh electric fields at the air-water interface are responsible for this chemical transformation. Here, we resolve this mystery via a comprehensive experimental investigation of H2O2 formation in (i) water microdroplets sprayed over a range of liquid flowrates, the (shearing) air flow rates, and the air composition (ii) water microdroplets condensed on hydrophobic substrates formed via hot water or humidifier under controlled air composition. Specifically, we assessed the contributions of the evaporative concentration and shock waves in sprays and the effects of trace O3(g) on the H2O2 formation. Glovebox experiments revealed that the H2O2 formation in water microdroplets was most sensitive to the air-borne ozone (O3) concentration. In the absence of O3(g), we could not detect H2O2(aq) in sprays or condensates (detection limit ≥250 nM). In contrast, microdroplets exposed to atmospherically relevant O3(g) concentration (10–100 ppb) formed 2–30 μM H2O2(aq); increasing the gas–liquid surface area, mixing, and contact duration increased H2O2(aq) concentration. Thus, the mystery is resolved –the water surface facilitates the O3(g) mass transfer, which is followed by the chemical transformation of O3(aq) into H2O2(aq). These findings should also help us understand the implications of this chemistry in natural and applied contexts.
• #### An ocean thermocline desalination system using the direct spray method

(Desalination, Elsevier BV, 2021-09-29) [Article]
Most of the existing desalination technologies are highly dependent on primary energy, limiting their applications to affluent areas. To address water scarcity in remote areas, it is crucial to exploit the potential of renewable energy sources for desalination. The natural temperature gradient between the surface and deep seabed, i.e., the ocean thermocline, can provide sufficient driving forces for low-temperature thermal desalination. This study presents a thermocline desalination system using the direct spray method. A single-stage spray desalination setup is designed and operated under the temperature ranges that are typical in ocean thermocline. The minimum temperature gradients to drive the system is
• #### A hybrid indirect evaporative cooling-mechanical vapor compression process for energy-efficient air conditioning

(Energy Conversion and Management, Elsevier BV, 2021-09-29) [Article]
The indirect evaporative cooler (IEC) is deemed an effective and sustainable alternative to existing mechanical vapor compression (MVC) chillers in cooling applications. However, IEC is a passive cooler that has no effective control over the supply air temperature and humidity. Also, the performance of IEC degrades severely when the humidity of the air is high. To overcome these limitations, we investigate a hybrid process that connects IEC and MVC in tandem. The outdoor air is firstly pre-cooled in the IEC by recovering energy from the room exhaust air, and then it is further processed to the desired condition using MVC. Such a hybrid IEC-MVC process benefits from IEC’s high energy efficiency and MVC’s capability of humidity and temperature control. A pilot IEC unit with the cross-flow configuration is firstly constructed and tested under assorted outdoor air conditions. Employing the room exhaust air as the working air in the wet channels, the IEC simultaneously cools and dehumidifies the outdoor air. Under the operating conditions considered, the outdoor air temperature can be reduced by 6–15 °C, and the humidity ratio drops by 0.5–4 g/kg. The coefficient of performance (COP) for IEC is 6–16, leading to an overall COP of 4.96–6.05 for the hybrid IEC-MVC process. Compared with a standalone MVC, the electricity consumption can be reduced by 19–135%.
• #### Structural properties and stability of the Betaine-Urea natural deep eutectic solvent

(Journal of Molecular Liquids, Elsevier BV, 2021-09-25) [Article]
This work focuses on the stability and supramolecular structure of the betaine-urea-water (B:U:W) natural deep eutectic solvent. Solutions spanning a range of molar ratios of betaine, urea, and water were prepared, varying the temperature and preparation times, and were analyzed by attenuated total reflection Fourier-transform infrared spectroscopy and Nuclear Magnetic Resonance. Density Functional Theory and the Natural Bond Orbital analysis were employed to obtain the most stable conformations for each mixture. The experimental results show that, in non-anhydrous conditions, betaine:urea (1:1), a minimum of two moles of water are needed to form a metastable transparent liquid, and a minimum of three moles of water is required to have a stable NADES. Comparison of the 13C-NMR spectra of B:U:W 1:1:2 and 1:1:3 shows for the latter that the carbonyl groups of betaine and urea form stronger hydrogen bonds with water, and that the CH3 group of betaine becomes more deprotected by the addition of the extra water molecule, making 1:1:3 a more stable solution. Our experimental and computational results show that water is of crucial importance to the NADES supramolecular structure and stability. A better understanding of the structural characteristics of NADES can lead to better envisage applications for these green solvents.
• #### A novel hybrid adsorption heat transformer – multi-effect distillation (AHT-MED) system for improved performance and waste heat upgrade

(Applied Energy, Elsevier BV, 2021-09-16) [Article]
Multi-effect distillation (MED) systems are considered to be the most energy-efficient thermal desalination methods. This paper introduces the development of a novel thermal desalination system for performance superior to MED systems for the same operating temperature limits. Such an unprecedented achievement was attained by upgrading the heat source using the chemical potential of adsorption phenomena. The proposed Adsorption Heat Transformer (AHT) cycle hybridized with Multi-effect distillation system (AHT-MED) exhibits higher performance ratio and water production rate than a conventional MED system for the same heating source and sink. The heat generated by the heat of adsorption with the temperature higher than the heat source is supplied to the first effect of the MED system, thus, extending the temperature difference between the Top Brine Temperature (TBT) and Bottom Brine Temperature (BBT). The higher temperature difference offers more number of effects, with the equivalent temperature difference between the effects (ΔTe) as the design parameter. Using the low-temperature heat source (as low as 58 °C), the system can employ an increased number of effects (as high as 11) due to the supply of heat at an increased temperature of around 80 °C. The proposed system achieves a higher performance ratio (approximately 5.4) and water production rate (2 kg/s) compared to the standalone MED system (PR: 4.2, WPR: 1 kg/s) with the number of effects of the hybrid system as 10 at constant interstage temperature difference between the standalone and hybrid systems. This novel AHT-MED system opens up new possibilities for low-temperature heat source-driven thermal desalination with significantly improved performance.
• #### Naturally Extracted Hydrophobic Solvent and Self-Assembly in Interfacial Polymerization

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2021-09-12) [Article]
Pharmaceutical, chemical, and food industries are actively implementing membrane nanofiltration modules in their processes to separate valuable products and recover solvents. Interfacial polymerization (IP) is the most widely used method to produce thin-film composite membranes for nanofiltration and reverse osmosis processes. Although membrane processes are considered green and environmentally friendly, membrane fabrication has still to be further developed in such direction. For instance, the emission of volatile solvents during membrane production in the industry has to be carefully controlled for health reasons. Greener solvents are being proposed for phase-separation membrane manufacture. For the IP organic phase, the proposition of greener alternatives is in an early stage. In this work, we demonstrate the preparation of a high-performing composite membrane employing zero vapor pressure and naturally extracted oleic acid as the IP organic phase. Its long hydrophobic chain ensures intrinsic low volatility and acid monomer dissolution, while the polar head induces a unique self-assembly structure during the film formation. Membranes prepared by this technique were selective for small molecules with a molecular weight cutoff of 650 g mol–1 and a high permeance of ∼57 L m–2 h–1 bar–1.
• #### Pulsating CO2 nucleation radically improves the efficiency of membrane backwash

(Desalination, Elsevier BV, 2021-09-11) [Article]
Although membrane filtration became a dominant water treatment technology globally, it suffers from membrane fouling which aggravates with time and imposes severe adverse effects on process performance, permeate quality and, eventually, its related costs. In this work, we introduce pulsating CO2 solution backwash with intermittent pressure drops to maximize CO2 bubbles yield and radically enhance membrane cleaning. The novel backwash technique was probed during ultrafiltration (UF) of feed waters containing sodium alginate, a model polysaccharide foulants, sea salts enriched in Ca2+, and SiO2. Transmembrane pressures (TMP) observed during the experiments with pulsating CO2 backwash acquired an up/down profile indicating that a considerable portion of TMP was recovered after each backwash cycle, in contrast to insufficient fouling removal and subsequent TMP build-up observed with continuous CO2 and Milli-Q backwashes. Notably, pulsating CO2 backwash alleviated irreversible membrane fouling in highly saline conditions with 30 g/L of sea salts and when it is combined with 1 mg/L of SiO2 (i.e., when conventional membrane backwash was not effective). Furthermore, intense cleaning of the membrane surface and its pores was resembled by a lower fouling resistance in the subsequent UF cycles implying potentially longer operation time with less cleaning frequency and substantial energy savings.
• #### Relative Importance of Stochastic Assembly Process of Membrane Biofilm Increased as Biofilm Aged

(Frontiers in Microbiology, Frontiers Media SA, 2021-09-10) [Article]
The relative importance of different ecological processes controlling biofilm community assembly over time on membranes with different surface characteristics has never been investigated in membrane bioreactors (MBRs). In this study, five ultrafiltration hollow-fiber membranes – having identical nominal pore size (0.1μm) but different hydrophobic or hydrophilic surface characteristics – were operated simultaneously in the same MBR tank with a constant flux of 10 liters per square meter per hour (LMH). In parallel, membrane modules operated without permeate flux (0 LMH) were submerged in the same MBR tank, to investigate the passive microbial adsorption onto different hydrophobic or hydrophilic membranes. Samples from the membrane biofilm were collected after 1, 10, 20, and 30days of continuous filtration. The membrane biofilm microbiome were investigated using 16S rRNA gene amplicon sequencing from DNA and cDNA samples. Similar beta diversity trends were observed for both DNA- and cDNA-based analyses. Beta diversity analyses revealed that the nature of the membrane surface (i.e., hydrophobic vs. hydrophilic) did not seem to have an effect in shaping the bacterial community, and a similar biofilm microbiome evolved for all types of membranes. Similarly, membrane modules operated with and without permeate flux did not significantly influence alpha and beta diversity of the membrane biofilm. Nevertheless, different-aged membrane biofilm samples exhibited significant differences. Proteobacteria was the most dominant phylum in early-stage membrane biofilm after 1 and 10days of filtration. Subsequently, the relative reads abundance of the phyla Bacteroidetes and Firmicutes increased within the membrane biofilm communities after 20 and 30days of filtration, possibly due to successional steps that lead to the formation of a relatively aged biofilm. Our findings indicate distinct membrane biofilm assembly patterns with different-aged biofilm. Ecological null model analyses revealed that the assembly of early-stage biofilm community developed after 1 and 10days of filtration was mainly governed by homogenous selection. As the biofilm aged (days 20 and 30), stochastic processes (e.g., ecological drift) started to become important in shaping the assembly of biofilm community.
• #### Thermo-Responsive Membranes from Blends of PVDF and PNIPAM-b-PVDF Block Copolymers with Linear and Star Architectures

(Macromolecules, American Chemical Society (ACS), 2021-09-10) [Article]
We report the synthesis of poly(n-isopropylacrylamide)-b-poly(vinylidene fluoride), (PNIPAM-b-PVDF), copolymers with linear and star structures, as well as the self-assembly and fabrication of thermo-responsive membranes from blends of these block copolymers and a linear PVDF homopolymer. The synthesis was achieved by reversible addition–fragmentation chain-transfer sequential copolymerization using mono- or multifunctional transfer agents. The self-assembly in bulk and selective solvents was investigated. The PVDF blocks are crystallizable and hydrophobic and the PNIPAM thermo-responsive in water. The morphology is dominated by the breakout crystallization of the PVDF block. Nanoporous membranes were fabricated by non-solvent-induced phase-separation method. The membranes revealed a macroscale zig–zag morphology, which is dependent on the block copolymer architecture. Due to the presence of PNIPAM, these membranes exhibited thermo-responsive behavior with water permeability and rejection alternately varying with the operating temperature, which is reversible in multiple heating–cooling cycles.
• #### Salt-solution-infused thin-film condenser for simultaneous anti-frost and solar-assisted atmospheric water harvesting

(Cell Reports Physical Science, Elsevier BV, 2021-09-10) [Article]
Frost is undesirable in many applications. Previous methods of defrosting are either energy intensive or ineffective in the long term. Meanwhile, frost is a valuable freshwater source whose production is delayed due to the solid state and lengthy accumulation of frost. Here, a salt-solution-infused thin-film condenser (SSTFC) is proposed for anti-frost at low energy consumption and simultaneous freshwater production. The SSTFC is composed of a highly permeable lower layer for salt solution infusion and an ultrathin hydrophobic top layer for constraining the infused solution and incorporating condensed vapor into the underlying solution at various low temperatures. The gradually diluted salt solution can be passively transported for regeneration and freshwater production by solar energy. The regenerated salt solution can be pumped back to the SSTFC for continuous operation of the system. Energy consumption can be saved by >50% compared to conventional methods.
• #### Real-time membrane fouling analysis for the assessment of reclamation potential of textile wastewater processed by membrane distillation

(Journal of Water Process Engineering, Elsevier BV, 2021-09-09) [Article]
Understanding the factors that specify the fouling development in membrane distillation (MD) plays a key role to develop effective control strategies with the aim of providing its widespread use in industrial applications, such as textile industry. The present study investigated the fouling mechanisms in textile wastewater treatment by direct contact MD (DCMD), employing an advanced in-situ optical coherence tomography (OCT) technology allowing to monitor MD fouling in real-time. The OCT analysis enabled evaluating the effect of feed temperature, flow rate, dye concentration on the membrane fouling and the long-term performance of MD operation that includes a periodical water flushing. The permeate flux decrease during the initial stages of experiments was attributed to the existence of attractive hydrophobic-hydrophobic interaction between the membrane and dye molecules as no cake fouling was observed at the early stages. Then, a flat and homogeneous cake layer was developed with time in all the fouled membranes regardless of the cake layer thickness. The long-term experiment resulted in both reversible and irreversible fouling and showed that water flushing had limited efficacy against reversible fouling. Additionally, electrostatic repulsive forces occurring between the membrane and textile dye molecules influenced the permeate flux depending on the dye concentration. Finally, among all the operating parameters, feed temperature had the highest impact on the membrane fouling and process performance, changed the heat transfer activity at the membrane-liquid frontier zone, in turn, leading to variations in the flux.
• #### UV and bacteriophages as a chemical-free approach for cleaning membranes from anaerobic bioreactors

(Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, 2021-09-07) [Article]
Anaerobic membrane bioreactor (AnMBR) for wastewater treatment has attracted much interest due to its efficacy in providing high-quality effluent with minimal energy costs. However, membrane biofouling represents the main bottleneck for AnMBR because it diminishes flux and necessitates frequent replacement of membranes. In this study, we assessed the feasibility of combining bacteriophages and UV-C irradiation to provide a chemical-free approach to remove biofoulants on the membrane. The combination of bacteriophage and UV-C resulted in better log cells removal and ca. 2× higher extracellular polymeric substance (EPS) concentration reduction in mature biofoulants compared to either UV-C or bacteriophage alone. The cleaning mechanism behind this combined approach is by 1) reducing the relative abundance of Acinetobacter spp. and selected bacteria (e.g., Paludibacter, Pseudomonas, Cloacibacterium, and gram-positive Firmicutes) associated with the membrane biofilm and 2) forming cavities in the biofilm to maintain water flux through the membrane. When the combined treatment was further compared with the common chemical cleaning procedure, a similar reduction on the cell numbers was observed (1.4 log). However, the combined treatment was less effective in removing EPS compared with chemical cleaning. These results suggest that the combination of UV-C and bacteriophage have an additive effect in biofouling reduction, representing a potential chemical-free method to remove reversible biofoulants on membrane fitted to an AnMBR.
• #### Tunable membranes incorporating artificial water channels for high-performance brackish/low-salinity water reverse osmosis desalination

(Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, 2021-09-07) [Article]
Membrane-based technologies have a tremendous role in water purification and desalination. Inspired by biological proteins, artificial water channels (AWCs) have been proposed to overcome the permeability/selectivity trade-off of desalination processes. Promising strategies exploiting the AWC with angstrom-scale selectivity have revealed their impressive performances when embedded in bilayer membranes. Herein, we demonstrate that self-assembled imidazole-quartet (I-quartet) AWCs are macroscopically incorporated within industrially relevant reverse osmosis membranes. In particular, we explore the best combination between I-quartet AWC and m-phenylenediamine (MPD) monomer to achieve a seamless incorporation of AWC in a defect-free polyamide membrane. The performance of the membranes is evaluated by crossflow filtration under real reverse osmosis conditions (15 to 20 bar of applied pressure) by filtration of brackish feed streams. The optimized bioinspired membranes achieve an unprecedented improvement, resulting in more than twice (up to 6.9 L·m−2·h−1·bar−1) water permeance of analogous commercial membranes, while maintaining excellent NaCl rejection (>99.5%). They show also excellent performance in the purification of low-salinity water under low-pressure conditions (6 bar of applied pressure) with fluxes up to 35 L·m−2·h−1and 97.5 to 99.3% observed rejection.
• #### Fluorinated thin-film composite membranes for nonpolar organic solvent nanofiltration

(Separation and Purification Technology, Elsevier BV, 2021-09) [Article]
Polyamide (PA) is highly effective as a selective layer in case of nanofiltration (NF) membranes, mainly for filtering water and other polar solvents. The incorporation of fluorinated monomers in a polyamide network is a novel strategy for obtaining membranes with enhanced permeability in case of nonpolar solvents. In this study, PA thin-film composite membranes were prepared by interfacially reacting trimesoyl chloride (TMC) and 4,4ʹ-(hexafluoroisopropylidene)bis(benzoyl chloride) (HFBC) in an organic phase with 5-trifluoromethyl-1,3-phenylenediamine (TFMPD) in an aqueous phase in a single step. The resulting membrane obtained using HFBC exhibited a considerably increased nonpolar solvent flux and selectivity in the nanofiltration range. Thus, the hydrophobicity of the PA layer and its permeance are effectively enhanced because of the incorporation of the fluorinated monomer. Therefore, high-performance membranes can be obtained for nonpolar solvent separation in petroleum refineries and purification in the pharmaceutical industry.
• #### Recovery of Critical Metals from Aqueous Sources

(ACS Sustainable Chemistry & Engineering, American Chemical Society (ACS), 2021-08-24) [Article]
Critical metals, identified from supply, demand, imports, and market factors, include rare earth elements (REEs), platinum group metals, precious metals, and other valuable metals such as lithium, cobalt, nickel, and uranium. Extraction of metals from U.S. saline aqueous, emphasizing saline, sources is explored as an alternative to hardrock ore mining. Potential aqueous sources include seawater, desalination brines, oil- and gas-produced waters, geothermal aquifers, and acid mine drainage, among others. A feasibility assessment reveals opportunities for recovery of lithium, strontium, magnesium, and several REEs from select sources, in quantities significant for U.S. manufacturing and for reduction of U.S. reliance on international supply chains. This is a conservative assessment given that water quality data are lacking for a significant number of critical metals in certain sources. The technology landscape for extraction and recovery of critical metals from aqueous sources is explored, identifying relevant processes along with knowledge gaps. Our analysis indicates that aqueous mining would result in much lower environmental impacts on water, air, and land than ore mining. Preliminary assessments of the economics and energy consumption of recovery show potential for recovery of critical metals.
• #### Ultrasound-assisted membrane technologies for fouling control and performance improvement: A review

(Journal of Water Process Engineering, Elsevier BV, 2021-08-20) [Article]
Membrane separation is widely used in wastewater treatment and desalination due to its high performance and ability to handle feed solutions of different qualities. Despite vast history of success, membrane fouling remains a major system deficiency that imposes substantial process limitations by reducing permeate production and increasing energy demand. Besides, chemical cleaning-in-place (CIP) adversely affects membrane integrity and generates an extra waste stream. Ultrasound (US) is a relatively new cleaning technique that improves process performance by mitigating fouling accumulation at a membrane surface and improving permeate flux by promoting mass and heat transfer. US-assisted membrane processes is an efficient method for fouling reduction and significant flux improvement. This study comprehensively reviews US applications in pressure-, thermally- and osmotic-driven membrane technologies and their impact on process performance. It also explores the impact of US operating conditions on membrane separation properties and how these parameters can be tuned to achieve the desirable outcome. To date, the application of US in membrane technologies is limited to laboratory tests. In the authors' opinion, there is a niche market for US-assisted membrane technology in heavily contaminated water such as wastewater and brine. After critical analysis of the literature, we found that there are still several aspects of the process need to be scrutinized carefully to make an adequate evaluation of its feasibility on an industrial scale. The most urgent one is the techno-economic evaluation of the technology based on large-scale and long-term tests. The study proposed a set of recommendations for future research directions of US applications in membrane technologies.
• #### Overcoming the Challenges of Thermal Infrared Orthomosaics Using a Swath-Based Approach to Correct for Dynamic Temperature and Wind Effects

(Remote Sensing, MDPI AG, 2021-08-18) [Article]
The miniaturization of thermal infrared sensors suitable for integration with unmanned aerial vehicles (UAVs) has provided new opportunities to observe surface temperature at ultra-high spatial and temporal resolutions. In parallel, there has been a rapid development of software capable of streamlining the generation of orthomosaics. However, these approaches were developed to process optical and multi-spectral image data and were not designed to account for the often rapidly changing surface characteristics inherent in the collection and processing of thermal data. Although radiometric calibration and shutter correction of uncooled sensors have improved, the processing of thermal image data remains difficult due to (1) vignetting effects on the uncooled microbolometer focal plane array; (2) inconsistencies between images relative to in-flight effects (wind-speed and direction); (3) unsuitable methods for thermal infrared orthomosaic generation. Here, we use thermal infrared UAV data collected with a FLIR-based TeAx camera over an agricultural field at different times of the day to assess inconsistencies in orthophotos and their impact on UAV-based thermal infrared orthomosaics. Depending on the wind direction and speed, we found a significant difference in UAV-based surface temperature (up to 2 °C) within overlapping areas of neighboring flight lines, with orthophotos collected with tail wind being systematically cooler than those with head wind. To address these issues, we introduce a new swath-based mosaicking approach, which was compared to three standard blending modes for orthomosaic generation. The swath-based mosaicking approach improves the ability to identify rapid changes of surface temperature during data acquisition, corrects for the influence of flight direction relative to the wind orientation, and provides uncertainty (pixel-based standard deviation) maps to accompany the orthomosaic of surface temperature. It also produced more accurate temperature retrievals than the other three standard orthomosaicking methods, with a root mean square error of 1.2 °C when assessed against in situ measurements. As importantly, our findings demonstrate that thermal infrared data require appropriate processing to reduce inconsistencies between observations, and thus, improve the accuracy and utility of orthomosaics.
• #### Long-Term Continuous Extraction of Medium-Chain Carboxylates by Pertraction With Submerged Hollow-Fiber Membranes

(Frontiers in Bioengineering and Biotechnology, Frontiers Media SA, 2021-08-13) [Article]
Medium-chain carboxylic acids (MCCAs), which can be generated from organic waste and agro-industrial side streams through microbial chain elongation, are valuable chemicals with numerous industrial applications. Membrane-based liquid-liquid extraction (pertraction) as a downstream separation process to extract MCCAs has been applied successfully. Here, a novel pertraction system with submerged hollow-fiber membranes in the fermentation bioreactor was applied to increase the MCCA extraction rate and reduce the footprint. The highest average surface-corrected MCCA extraction rate of 655.2 ± 86.4 mmol C m$^{−2}$ d$^{−1}$ was obtained, which was higher than any other previous reports, albeit the relatively small surface area removed only 11.6% of the introduced carbon via pertraction. This submerged extraction system was able to continuously extract MCCAs with a high extraction rate for more than 8 months. The average extraction rate of MCCA by internal membrane was 3.0- to 4.7-fold higher than the external pertraction (traditional pertraction) in the same bioreactor. A broth upflow velocity of 7.6 m h$^{−1}$ was more efficient to extract MCCAs when compared to periodic biogas recirculation operation as a means to prevent membrane fouling. An even higher broth upflow velocity of 40.5 m h$^{−1}$ resulted in a significant increase in methane production, losing more than 30% of carbon conversion to methane due to a loss of H2, and a subsequent drop in the H2 partial pressure. This resulted in the shift from a microbial community with chain elongators as the key functional group to methanogens, because the drop in H2 partial pressure led to thermodynamic conditions that oxidizes ethanol and carboxylic acids to acetate and H2 with methanogens as the syntrophic partner. Thus, operators of chain elongating systems should monitor the H2 partial pressure when changes in operating conditions are made.
• #### Ball milling as an important pretreatment technique in lignocellulose biorefineries: a review

(Biomass Conversion and Biorefinery, Springer Science and Business Media LLC, 2021-08-12) [Article]
The conversion of lignocellulosic biomass into bioethanol remains a challenging process due to the recalcitrant structure of lignocellulose. The presence of the sturdy lignin protective sheath, complex structure, and partial crystallinity of cellulose often reduces the enzymatic susceptibility of lignocellulosic biomass. Therefore, pretreatment is aimed to increase accessibility by improving the physicochemical properties and composition of lignocellulosic biomass. It is the first and the most critical step that needs to be carefully selected and designed to overcome the constraints and improve the overall efficiency of bioethanol production. In recent years, ball milling has been applied as an emerging technique to produce bioethanol from lignocellulosic biomass efficiently and in an environment-friendly manner. Furthermore, ball milling technique coupled with chemical and physicochemical pretreatments has been shown to facilitate lignin removal, reduce cellulose crystallinity, and increase the specific surface area which ultimately improves the digestibility of lignocellulosic biomass. Over the last decade, several reports have been published on the application of ball milling to intensify the pretreatment process. However, a compiled report showing the progress of the technology in bioethanol processing is absent. In this review, a critical analysis and evaluation of published works on ball milling and ball milling–assisted chemical/physicochemical pretreatments are presented. It also addresses the synergistic effects of combining ball milling and chemical/physicochemical treatments to bring desirable characteristics of lignocellulosic biomass that will eventually improve hydrolysis yield and reduce chemical and energy consumption in bioethanol production.