Also sometimes called wet torrefaction. Hydrothermal carbonization (HTC) (also referred to as "aqueous carbonization at elevated temperature and pressure") is a chemical process for the conversion of organic compounds to structured carbons. It can be used to make a wide variety of nanostructured carbons, simple production of brown coal substitute, synthesis gas, liquid petroleum precursors and humus from biomass with release of energy. Technically the process imitates, within a few hours, the brown coal formation process. [\[Wiki\]](https://en.wikipedia.org/wiki/Hydrothermal_carbonization) **Highlights:** * Hydrothermal carbonization (HTC) has received much attention in recent years as a process to **convert wet organic waste into carbon-rich hydrochar**. The process also generates an aqueous phase that is still largely considered a burden. The success of HTC is dependent on finding solutions for the aqueous phase. In the present study, we provide the first investigation of recirculation of the aqueous phase from HTC of poultry litter as a means to concentrate nutrients and its subsequent application to agriculture as a fertilizer. [ \[Art. #ARTNUM\]](#article-96378-2965652869) * Hydrothermal carbonization is an attractive thermochemical method for upgrading organic waste and biomass. Hydrothermal carbonization's improvement of the upgrading and dewatering of fuel mixed with sewage sludge and low rank coal as peat was evaluated at **temperatures ranging from 200 to 350 °C and at 60 min reaction time**. **The moisture content of mixed fuel (50:50 wt %) of sludge: peat was approximately 80.7%.** **Hydrothermal carbonization can improve sludge with a high moisture content as well as the mixed fuels increasing the latter's calorific value by reducing the hydrogen and oxygen contents of the solid products.** [\[Art. #ARTNUM\]](#article-96378-2760148100) * In this document a new biomass conversion process called hydrothermal carbonization (HTC) is presented, which has the potential to convert organic waste into a hydrophobic solid of reduced mass and increased fuel value (hydrochar) to provide at least renewable energy and/or increase soil organic matter. As with every emerging technology, hydrothermal carbonization is currently hardly a competitive stand alone process on the open market. **But if the process can be implemented in an existing infrastructure e.g. compost plant, sewage plant or other businesses which are confronted with large amounts of wet organic waste, HTC is already today a financially feasible process.** Altogether, HTC offers not only a new green and sustainable technology for the treatment of biowaste. It is a promising research and development field leading to new functional materials based on renewable resources. [\[Art. #ARTNUM\]](#article-96378-2806731263)

Sometimes called hydrothermal pyrolysis or wet pyrolysis. Hydrothermal liquefaction (HTL) is a thermal depolymerization process used to convert wet biomass into crude-like oil -sometimes referred to as bio-oil or biocrude- under moderate temperature and high pressure. The crude-like oil (or bio-oil) has high energy density with a lower heating value of 33.8-36.9 MJ/kg and 5-20 wt% oxygen and renewable chemicals.[\[Wiki\]](https://en.wikipedia.org/wiki/Hydrothermal_liquefaction) **Highlights:** * Conversion of wet biomass and waste products via hydrothermal liquefaction (HTL) has been evolving as an alternative thermochemical technology for the production of liquid biofuels. Processing of biomass slurries with approximately 20 % solids content under high temperature and pressure mimics the natural formation of fossil crude on earth. With reaction times of around 10 to 30 minutes, **temperatures of 350 °C and pressures of around 200 bar**, HTL converts any biomass feedstock to a liquid bio-crude. This raw product roughly resembles petroleum, but exhibits higher oxygen contents (\~10 %) and has a higher viscosity. Therefore, development of the hydrothermal liquefaction technology has concentrated on the upgrading of bio-crude via hydrotreatment to reduce its heteroatom content, viscosity, boiling point and density. Upgraded bio-crude can then be further refined via distillation or other established processes into renewable gasoline, diesel and jet fuel. [\[Art. #ARTNUM\]](#article-96337-2744095134) * These results suggest that AD is a feasible approach to treat post-hydrothermal-liquefaction wastewater (PHWW), and to improve the energy efficiency of the HTL processes.[ \[Art. #ARTNUM\]](#article-96337-2210221651)

Using catalysts in hydrothermal liquefaction can improve the conversion and also lower temperature and pressure required. Catalysts that have been used are Nickel-based, Aluminum-based, bentonite, transition metal chlorides, carbonates, hydroxides, iron etc. Also enzymatic processes have been developed. **Highlights:** * Javier Remón et al. reported Microwave-Assisted, Catalytic, Hydrothermal Liquefaction (MAC-HTL) of a mixture of pine and spruce using a Ni–Co/Al–Mg catalysts at moderate temperatures (150–250 °C) using pressurized reactors (50–120 bar). **In their study, the catalyst influenced the product distribution (gas, oil yield) and bio-oil properties**, whereas overall biomass conversion remained the same as non-catalytic HTL. By optimizing reaction conditions, 27% of biomass was converted into phenolic bio-oil with high HHV (20 MJ/kg) at a temperature of 250 °C, a pressure of 80 bar using a ratio of 0.25 g catalyst/g biomass for 1.9 h. * Arun et al. reported **co-liquefaction of plant wood biomass (*Prosopis juliflora*) with waste polyolefin plastics. In their HTL process activated bentonite clay was used as the catalyst** and experiments were conducted under different catalyst loading (1–5 wt%) and various temperatures 340–440 °C at 50 bar. With a 3:1 feedstock ratio (wood: polyolefin) and 3 wt% catalyst loading, a maximum of 61.2 wt % bio-oil was produced at a temperature of 420 °C. Up to 88.2 wt% of C~8~–C~21~ range hydrocarbons were found in the organic phase whereas low molecular organic acids and polyols were detected in the aqueous phase of the HTL products. The authors mentioned the benefits of having low molecular organic acids in the product as these could be recycled back to the HTL process to further promote the degradation of the lignocellulosic portion of the wood-polyolefin blend. [\[Art. #ARTNUM\]](#article-96405-2976637787) * One of the novel technologies for conversion of wet biomass and waste streams is hydrothermal liquefaction. The research described in this thesis deals with the CatLiq process; a hydrothermal liquefaction technology developed by the Danish company SCF Technologies A/S, which operates a continuous 20L/h capacity pilot plant in Copenhagen. **This company uses organic matter as for instance DDGS (Dried Distillers Grains with Solubles) as a feedstock to produce bio-oil in the presence of a homogeneous (K2CO3) and a heterogeneous (Zirconia) catalyst at subcritical conditions** (T = 280-350 C, P = 22.5-25.0 MPa). [\[Art. #ARTNUM\]](#article-96405-2115943201) * This plant features new continuous and energy-efficient technology developed for pretreatment and liquefaction of lignocellulosicbiomass and has now been operated and optimized for four years with promising results.In the IBUS process, biomass is converted using steam and enzymes only. The process is energy efficient due to very high dry matter content in all process steps and by integration with a power plant. [\[Art. #ARTNUM\]](#article-96405-2132472358)

Liquefaction under supercritical conditions can be beneficial to the yield. There are good opportunities for the use of biomass feedstocks and supercritical technology in the production of liquid fuels for the transportation sector, as well as renewable chemicals for the chemical industry. [\[Art. #ARTNUM\]](#article-96406-2061939403) **Highlights:** * Licella is an Australian company developing the Cat-HTR^TM^ process which involves forming a slurry of water and organic matter, heating the slurry to a temperature of between about 350 and 420 °C and pressurising it to greater than 220 bar so that the water phase becomes supercritical. In some embodiments, the hemicellulose of the biomass is first solvated from the cellulose and lignin in sub-critical water at temperatures between about 120 and 190 °C and pressures of about 40 bar, prior to the hydrothermal reaction of the cellulose and lignin at temperatures above 350 °C. However, this pre-treatment step does not seem to be the preferred process line up at large scale, wherein complete processing of the biomass in a single step is more attractive. [\[Art. #ARTNUM\]](#article-96406-2976637787)

Different hydrothermal biomass gasification processes are under development. In contrast to biomass gasification processes without water, biomass with the natural water content (“green biomass”) can be converted completely and energetically efficiently to gases. Depending on the reaction conditions, methane or hydrogen is the burnable gas produced. Some processes use catalysts. In recent years, significant progress was achieved in the development of various hydrothermal biomass gasification processes.[\[Paper\]](https://www.sciencedirect.com/science/article/pii/S0896844608003501) **Highlights:** * **Supercritical water gasification is a promising technology to convert coal/biomass/organic wastes to hydrogen cleanly and efficiently.** Extensive investigations on supercritical water gasification were conducted in China. State Key Laboratory of Multiphase Flow in Power Engineering (SKLMFPE) together with other universities/institutes established experimental device with the reactor type of quartz tube reaction system, tubular reactor and fluidized bed reactor. The fluidized bed reactor system solved the blocking problems to guarantee continuous and stable gasification. [ \[Art. #ARTNUM\]](#article-96336-2053968199) * Supercritical water gasification (SCWG) is a process that has been examined in the past years for its potential implementation in waste treatment technologies. SCWG can convert organic waste streams from industrial activities into energy. **A one of a kind vertical continuous apparatus was constructed at UCT Prague for testing this process.** Several experiments with tall soap and black liquor on the hydrothermal gasification apparatus were performed. In this paper, results that permit to compare the influence of different process conditions (residence time, catalyst addition). In addition, practical gasification experiences with real industry waste streams treated in the specific vertical reactor made from stainless steel are presented. All acquired data were obtained from experiments carried at temperature 500 °C and pressure 25 MPa. The reached carbon efficiency when using tall soap and black liquor were 11% and 24%, respectively. [\[Art. #ARTNUM\]](#article-96336-2783804797)

**In recent years, biomass steam gasification has become an area of growing interest because it produces a synthesis gas with relatively higher hydrogen content.** [\[Paper\]](https://pubs.acs.org/doi/10.1021/cr200024w) **Highlights:** * The thermochemical conversion processes, such as pyrolysis, gasification, and steam gasification are available for converting the biomass to a more useful energy. The yield from steam gasification increases with increasing water-to-sample ratio. The yields of hydrogen from the pyrolysis and the steam gasification increase with increase of temperature. Hydrogen-powered fuel cells are an important enabling technology for the hydrogen future and more efficient alternatives to the combustion of gasoline and other fossil fuels. Hydrogen has the potential to solve two major energy problems: reducing dependence on petroleum and reducing pollution and greenhouse gas emissions. [\[Art. #ARTNUM\]](#article-96466-2061451393)

**Highlights:** * PNNL developed catalytic hydrothermal gasification (CHG) for use with aqueous streams within the pyrolysis biorefinery. These aqueous streams included the aqueous phase separated from the fast pyrolysis bio-oil and the aqueous byproduct streams formed in the hydroprocessing of the bio-oil to finished products. The purpose of this project was to demonstrate a technically and economically viable technology for converting renewable biomass feedstocks to sustainable and fungible transportation fuels.[ \[Art. #ARTNUM\]](#article-96517-2339007579) * Gasification of the biomass in a hydrothermal environment is an emerging technology that offers many advantages over gas-phase conversion processes or anaerobic digestion. **Heterogeneous catalysis is the key to a successful hydrothermal gasification process for the synthesis of methane.** Only a few metals, including Ru, Ni, Rh and Pt, are useful under these conditions. Pd and Co catalysts might also be suitable but conclusive data are lacking. Alloying is another approach that holds promise to yield active and stable catalysts. The mechanism for the gasification of the organic compounds to CO and H2 is likely to follow a Mars–van-Krevelen redox cycle with two oxides of the catalytic metal involved. Based on this knowledge, a continuous catalytic hydrothermal gasification process is under development at PSI featuring continuous on-line salt precipitation and removal before the catalytic reactor.[ \[Art. #ARTNUM\]](#article-96517-1936405539)

Wet oxidation is a form of hydrothermal treatment. It is the oxidation of dissolved or suspended components in water using oxygen as the oxidizer. It is referred to as "Wet Air Oxidation" (WAO) when air is used. The oxidation reactions occur in superheated water at a temperature above the normal boiling point of water (100 °C), but below the critical point (374 °C). [\[Wiki\]](https://en.wikipedia.org/wiki/Wet_oxidation) **Highlights:** * A commercial case study on hydrothermal technology (Terax™) revealed that the challenge of sewage sludge management can be addressed successfully using a combination of biological and hydrothermal processes.[ \[Art. #ARTNUM\]](#article-96387-2886693062) * The oxidation of harmful organic compounds contained in aqueous waste effluents known as **super critical water oxidation, SCWO**, has been worked out since the 1980s. This highly efficient end of the pipe process operates at pressures and temperatures above 221 bar and 374 °C, the critical point of water. R&D experience and the technological state including economical and regulatory aspects are reviewed and further R&D needs are discussed in this article. Future applications are also seen in coupling supercritical CO2 extraction with oxidation to treat contaminated materials and in supercritical water gasification, SCWG, to convert biomass and organic wastes to hydrogen. [\[Art. #ARTNUM\]](#article-96387-1984630463)

Torrefaction is a thermochemical pretreatment process at 200–300 °C in an inert condition which transforms biomass into a relatively superior handling, milling, co-firing and clean renewable energy into solid biofuel. **Highlights:** * This increases the energy density, water resistance and grindability of biomass and makes it safe from biological degradation which ultimately makes easy and economical on transportation and storing of the torrefied products. Torrefied biomass is considered as improved version than the current wood pellet products and an environmentally friendly future alternative for coal. Torrefaction carries devolatilisation, depolymerization and carbonization of lignocellulose components and generates a brown to black solid biomass as a productive output with water, organics, lipids, alkalis, SiO2, CO2, CO and CH4. During this process, 70 % of the mass is retained as a solid product, and retains 90 % of the initial energy content. The torrefied product is then shaped into pellets or briquettes that pack much more energy density than regular wood pellets.[ \[Art. #ARTNUM\]](#article-96419-2017865626)

Shu's improvements to torrefaction involve using higher temperatures and molten salts as the heat transfer fluid. Supertorrefaction (STR) submerges wood chips or other forms of shredded biomass into a bath of molten salts -- NaOAc/KOAc for example -- that is held at an average temperature of 450 ℃.[\[Source\]](https://www.forbes.com/sites/rodadams/2016/09/16/sequestering-carbon-using-mass-quantities-of-small-scale-supertorrefaction-systems/#60044caf4e04) **Highlights:** * There are various techniques that can be applied to convert biomass into biofuels but recently the thermochemical conversion of Biomass in a molten salt medium has received a lot of attention. In this work, it will be attempted to carry out the liquefaction of kraft lignin in ZnCl2-KCl-NaCl salt mixture. At ZnCl2:KCl:NaCl= 60:20:20 in mole fraction composition, the salt should exhibit a eutectic temperature of 203°C, which will be advantageous for the liquefaction process.[ \[Art. #ARTNUM\]](#article-96468-2995696136)

Pyrolysis is the thermal decomposition of materials at elevated temperatures in an inert atmosphere. It involves a change of chemical composition.[\[Wiki\]](https://en.wikipedia.org/wiki/Pyrolysis) **Highlights:** * Pyrolysis is a promising technology for the production of marketable energy products from waste mixtures, as it decomposes heterogeneous material into homogenous fuel products. **This research assessed the ability of slow pyrolysis to convert three waste streams, composed of fibre residues contaminated with different plastic mixtures, into char and tarry phase products** at three different temperatures (300, 425 and 550 °C). Significant amounts of hydrocarbon plastics in the feed materials increased the calorific values of the char (up to 32.9 MJ/kg) and tarry phase (up to 42.8 MJ/kg) products, comparable to high volatile bituminous A coal and diesel respectively. [\[Art. #ARTNUM\]](#article-96339-2563570629) * This paper presents a mobile autothermal pyrolysis system for locally converting biomass feedstock into bio-oil that can be transported easily. The system includes a compact internally interconnected fluidized bed (IIFB) reactor, a biomass pretreatment facility, and a product recovery unit. On the basis of modified chemical kinetic models, the pyrolysis process of common forestry and agricultural residues in this mobile system has been simulated. The pyrolytic product distribution from simulation has good agreements with experimental results. Then, the techno-economic performance of the mobile pyrolysis system in China is evaluated and compared with other liquid biofuel production facilities, i.e., fixed biomass pyrolysis plants and Fischer–Tropsch liquids production via biomass gasification (BG-FT). The results indicate that the biomass feedstock cost of mobile pyrolysis systems can be effectively reduced. Compared with the fixed biofuel production plant, the labor cost is higher for the mobile plant. [\[Art. #ARTNUM\]](#article-96339-2788065379) * The purpose of this article is to demonstrate how the generated sludge from fermentation can be further converted into useful products such as hydrogen, methane and carbon monoxide, all produced in their maximum possible concentrations. [\[Art. #ARTNUM\]](#article-96339-2026844222)

Fast pyrolysis is a process in which organic materials are rapidly heated to 450 - 600 °C in the absence of air. Under these conditions, organic vapors, pyrolysis gases and charcoal are produced. The vapors are condensed to bio-oil. Typically, 60-75 wt. % of the feedstock is converted into oil.[\[Source\]](https://www.btgworld.com/en/rtd/technologies/fast-pyrolysis#:\~:text=Fast%20pyrolysis%20is%20a%20process,feedstock%20is%20converted%20into%20oil.) In flash pyrolysis the heatin rates are even more rapid. **Highlights:** * **Fast pyrolysis of biomass is praised as an efficient and feasible process to selectively convert lignocellulosic biomass into bio-fuels and bio-chemicals.** Pith of sugarcane bagasse could be an attractive lignocellulosic waste from depithing process from pulp and paper mill, which can utilize for production of biofuel and added value products. [ \[Art. #ARTNUM\]](#article-96416-2771828318) * UOP LLC proposed to demonstrate a fast pyrolysis based integrated biorefinery. Pacific Northwest National Laboratory (PNNL) has expertise in an important technology area of interest to UOP for use in their pyrolysis-based biorefinery. This CRADA project provides the supporting technology development and demonstration to allow incorporation of this technology into the biorefinery. PNNL developed catalytic hydrothermal gasification (CHG) for use with aqueous streams within the pyrolysis biorefinery. These aqueous streams included the aqueous phase separated from the fast pyrolysis bio-oil and the aqueous byproduct streams formed in the hydroprocessing of the bio-oil to finished products. The purpose of this project was to demonstrate a technically and economically viable technology for converting renewable biomass feedstocks to sustainable and fungible transportation fuels. [\[Art. #ARTNUM\]](#article-96416-2339007579)

In hydropyrolysis, the reducing H~2~ gas generates hydrogen radicals which react with volatiles released by the biomass, usually in the presence of a catalyst, removing oxygen which can be released in the form of water, CO, and CO~2~, and producing hydrocarbons.[\[Paper\]](https://www.sciencedirect.com/science/article/abs/pii/S0920586116300025) **Highlights:** * To increase the stability of pyrolysis bio-oil, pyrolysis can be carried out under a hydrogen environment called hydropyrolysis. In the early days, hydropyrolysis was performed without a catalyst, but the products of hydropyrolysis had similar disadvantages as those derived from conventional pyrolysis. After these studies, several direct routes for the production of gasoline and diesel range hydrocarbons or blending components were developed using catalytic hydropyrolysis, integrated hydropyrolysis and hydroconversion (integrated hydropylrolysis and hydroconversion (IH^2^®)) and a two-step biofuel process (H~2~Bioil).[\[Art. #ARTNUM\]](#article-96404-2976637787)

Lignocellulosic waste material serves as a considerable renewable feedstock that may be used to replace oil refineries with biorefineries. Indeed, all biomass components can be converted into platform chemicals, bioenergy and materials. However, thermo-chemical and conventional catalytic conversions suffer from a number of drawbacks. Enabling technologies, such as microwaves (MW), can reduce process times and energy consumption, leading to improvements in product quality and yields. The remarkable advantages of MW over conventional heating, which originate from its direct dielectric interaction with biomass, are documented in this comprehensive survey. Moreover, the use of alternative solvents that interact strongly with MW in biphasic systems can circumvent additional upgrading and separation steps. Finally, this review discusses some of the challenges that MW irradiation faces, including the poor dielectric properties of some substrates and issues related to its large-scale application in pyrolysis, hydrothermal conversion and catalytic routes to biofuels, materials and platform chemicals. Waste biomass may well be the benchmark feedstock for the development of a circular bioeconomic approach.[ \[Art. #ARTNUM\]](#article-96414-2915203621)

A form of catalytic pyrolysis. **Highlights:** * The TCR ® process combines intermediate pyrolysis, using an auger reactor to heat the material under moderate temperatures (350–450 °C) and moderate solid residence times (minutes) in the complete absence of Oxygen, with post catalytic reforming in a fixed bed reactor at 700 °C. Pelletised co-form ® rejects were successfully converted into 12 wt% bio-oil, 9 wt% aqueous phase liquid, 8 wt% char and 71 wt% syngas products.[ \[Art. #ARTNUM\]](#article-96390-2797202079) * In the operating conditions, pyrolysis temperature and HZSM-5 catalyst had significant effect on products and distributions. An increase in the pyrolysis temperature from 400°C to 550°C led to an increase in the yield of phenolic compounds (6.3%, w/w%), followed decrease at higher temperature. The maximum carboxylic acids (10.6%) and furfural (3.5%) were obtained at lower temperature. [\[Art. #ARTNUM\]](#article-96390-2771828318)

Gasification is a process that converts organic- or fossil fuel-based carbonaceous materials into carbon monoxide, hydrogen and carbon dioxide. This is achieved by reacting the material at high temperatures (>700 °C), without combustion, with a controlled amount of oxygen and/or steam.[\[wiki\]](https://en.wikipedia.org/wiki/Gasification#:\~:text=Gasification%20is%20a%20process%20that,of%20oxygen%20and%2For%20steam.) **Highlights:** * The gasification technique comprises chemical reaction in an environment which is oxygen-deficient. This process involves biomass heating at extreme temperatures (500–1400 °C), from atmospheric pressures up to 33 bar and with low/absent oxygen content to yield combustible gas mixtures. Gasification process transforms carbonaceous constituents into syngas comprising hydrogen, carbon monoxide, carbon dioxide, methane, higher hydrocarbons, and nitrogen with the presence of a gasification agent and catalyst. By utilizing this syngas, various types of energy/energy carriers are supplied for examples biofuel, hydrogen gas, biomethane gas, heat, power and chemicals. * **It is reported that gasification process is the most efficient technique in the production of hydrogen gas from biomass.** * **Gas composition produced from gasification process varies according to type of gasifier, gasification agent, catalyst type and size of particle.** [\[Art. #ARTNUM\]](#article-96410-2947495040) * **Co-pyrogasification of plastics and biomass mixtures, as opposed to separately converting these waste streams, offers several advantages including an improvement in syngas quality and composition (H2/CO ratio) in relation to the desired application, and an easier reactor feeding of plastics.** Furthermore, many studies have shown that co-pyrogasification promotes the conversion of waste to gas rather than char and tar. However, in order to achieve the desired product distribution or syngas composition, operating parameters such as the reactor temperature, equivalence ratio (air or oxygen), steam/fuel ratio and catalyst, have to be optimized. [\[Art. #ARTNUM\]](#article-96410-2793895765)

Staged gasification is an efficient method of converting biomass, producing a gas with a low concentration of tar and with high process efficiency. This is achieved with reasonable simplicity and is cost-effective, making staged gasification ideal for power production at small-to-medium scale. Several staged-gasification systems have been developed based on fixed/moving beds. In the present work, a new staged-gasification system based on a fluidized-bed design is presented and modeled. The process includes three main stages: devolatilization of the fuel, homogeneous gas reforming/oxidation of volatiles, and heterogeneous reforming of gas over in situ generated char. Each thermochemical stage is modeled using kinetics data obtained in dedicated tests in a laboratory-scale fluidized bed or taken from the literature. The fluid-dynamics is characterized using the results from a cold rig constructed to model the gas and solids flows in the new system. The numerical model developed is employed to evaluate the performance of the new system during gasification of dried sewage sludge. The significant improvement achieved in the new staged gasifier compared to a single-stage unit is shown, as well as the potential optimization of the system under different operating conditions.[ \[Art. #ARTNUM\]](#article-96622-1966625754)

***Hydrogasification*** is gasification in a hydrogen-rich environment, often used for the production of synthetic natural gas (SNG) from coal or other gasifier feedstocks.[\[Source\]](https://www.netl.doe.gov/research/Coal/energy-systems/gasification/gasifipedia/hydro) **Highlights:** * In this paper the integration of the energy production from programmable (biomass, waste) and not programmable (solar, wind) renewable sources is examined as an opportunity for increasing the share of electricity from renewable power plants, in order to overcome the major obstacles to their extensive penetration into the grid. **The integration is performed by using hydrogen from intermittent renewable energy powered-electrolysis as gasification medium in conventional or advanced gasification systems for the waste treatment.** The proposed integrated energy system consists of three main sections: i) the hydrogen production island; ii) the gasification island; iii) the power island. The assessment of the system performance has been conducted by considering two gasification technologies: hydro-gasification and hydro-plasma gasification. The performances comparison has been carried out in terms of syngas composition, energy consumptions and electric efficiency. Results have pointed out that the electric efficiencies of the integrated energy systems are in the range of 40% and 43%.[ \[Art. #ARTNUM\]](#article-96511-2173720029)

Biodrying has emerged recently as an energy-effective technology for converting organic wastes into solid recovered fuel (SRF). In biodrying, water removal is achieved using metabolic heat produced from microbiological degradation of organics and forced airflow to promote heat and mass transfer. Factors affecting biodrying include moisture content (MC) and free air space (FAS) of the wastes, use of bulking agents, aeration rate, mechanical turning as well as microbial activities. These factors independently or collectively determine water removal capacities of various biodrying processes. In this review article, studies on these factors are surveyed extensively and reviewed. Additionally, details of reactor configurations, operation modes, and modeling studies on biodrying are summarized. Finally, health and environmental risks and their management associated with off-gases and final products of biodrying processes are discussed.[ \[Art. #ARTNUM\]](#article-96392-2616124719)

Plasma treatments use a plasma conversion zone in either gasification or pyrolysis. **Highlights:** * An emerging technology involving thermal plasma gasification for MSW treatment has received increasing attention recently. The advantages are mainly related to improved energy recovery efficiency associated with the fast reaction times, low amount of oxidant, and high heat flux densities. Studies have shown that the high temperature of the plasma arc can reduce the formation of tar and other undesirable products in the syngas \[129\]. The solid residues are produced in the form of a vitrified slag, which can be utilized in construction. One such demonstration plant has been operating in Korea since 2010 \[129\]. An integrated furnace equipped with nontransferred thermal plasma torches was successfully applied for the direct treatment of MSW with a capacity for gasification of MSW at 10 ton/d.[\[Paper\]](https://www.sciencedirect.com/science/article/pii/S0717345817300040#s0015) * This paper outlines the principles of thermal plasma pyrolysis processes and discusses recent research activities about organic waste treatment using thermal plasma pyrolysis technology. Different kinds of organic wastes, varying from plastic and used tires to agricultural residue and medical waste, have been subjected to thermal plasma pyrolysis tests in laboratory and pilot scale projects. Plasma pyrolysis of organic waste usually gives two product streams: a combustible gas having a calorific value in the range of 4–9 MJ/Nm^3^ and a carbonaceous residue. Pyrolysis conditions as well as some technical measures such as the quenching process and steam reforming have significant influences on the properties of these pyrolysis products. Research results indicated that thermal plasma pyrolysis may be a useful way of waste management for energy and material recovery.[\[Paper\]](https://www.sciencedirect.com/science/article/pii/S0196890406002512)

**Hydrocracking** is a two-stage process that combines catalytic cracking and hydrogenation. The heavier feedstocks are cracked in the presence of hydrogen to produce more desirable products. The process employs high pressure, high temperature, a catalyst, and hydrogen.[\[Source\]](https://www.sciencedirect.com/topics/engineering/hydrocracking#:\~:text=Hydrocracking%20is%20a%20two%2Dstage,%2C%20a%20catalyst%2C%20and%20hydrogen.) **Highlights:** * The presence of tar content in the product gas is a major problem with the biomass gasification process as it prevents its further utilization. Heterogeneous cracking of tar using catalyst is the most effective way to overcome this problem. **The present study provides specially a method for converting biomass to hydrogen-rich syngas in a two-stage process.** The first stage refers to pyrolysis process and the second stage refers to gasification process. The heterogeneous experiments of rice husk tar cracking were performed in a two-stage gasifier using different catalysts such as char formed from pyrolysis of rice husk in the same reactor, commercial activated carbon, sand silica, and nickel containing stainless steel turnings obtained from mechanical workshop. The products of heterogeneous tar cracking were evaluated for optimizing hydrogen-rich syngas formation considering the effects such as temperature, carrier gas and quantity of catalyst. The mild steel turnings were found to be the best catalyst giving 0.31 vol% tar, 53.60 vol% of hydrogen, 22.73 vol% of carbon monoxide, 0.0 vol% of methane and 23.35 vol% of carbon dioxide in product gas at 900 °C. [\[Art. #ARTNUM\]](#article-96382-2778827371)

Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. [\[Wiki\]](https://en.wikipedia.org/wiki/Anaerobic_digestion) **Highlights:** * Oily-biological sludge (OBS) generated from petroleum refineries has high toxicity. Therefore, it needs an appropriate disposal method to reduce the negative impacts on the environment. The anaerobic co-digestion process is an effective method that manages and converts organic waste to energy. For effective anaerobic digestion, a co-substrate would be required to provide a suitable environment for anaerobic bacteria. In oily-biological sludge, the carbon/nitrogen (C/N) ratio and volatile solids (VS) content are very low. Therefore, it needs to be digested with organic waste that has a high C/N ratio and high VS content. This study investigates the use of sugarcane bagasse (SB) as an effective co-substrate due to its high C/N ratio and high VS content to improve the anaerobic co-digestion process with oily-biological sludge. The sugarcane bagasse also helps to delay the toxicity effect of the methane bacteria. [\[Art. #ARTNUM\]](#article-96338-3010195771) * Anaerobic Digestion (AD) is a well-developed sustainable technology to convert organic waste streams and energy crops to produce renewable gaseous biofuels, while recycling nutrients and mitigating greenhouse gas emissions. In this study, the environmental and economic impacts of an integrated-state AD technology (i-AD) producing Compressed Natural Gas (noted as BioCNG) were investigated from dairy-manure, food-wastes, and miscanthus biomass feedstocks, and compared with that of stand-alone liquid-state (LS-AD) and solid-state (SS-AD) AD technologies. A coupled life-cycle assessment and techno-economic analysis (LCA‐TEA) approach was used to estimate the Global Warming Potential (GWP) and the Minimum Selling Price (MSP) of BioCNG ‒ a renewable alternative to fossil-CNG. The results illustrated that the Fossil Energy Ratios (FERs) for BioCNG were between 2.3 and 3.3 in the increasing order as LS‐AD [\[Art. #ARTNUM\]](#article-96338-2972178027) The dual digestion process consists of an autothermal thermophilic aerobic digestion (ATAD) process ahead of an anaerobic digestion process. [[Art. #ARTNUM]](#article-96338-150671618)

In this two-stage process, high-solid codigestion of food waste and chicken manure was carried out in the first stage and then, transferred and codigested with grass in the second stage. **Highlights:** * It was found that high-solid digestion of food waste failed after 3 days because of the accumulation of volatile fatty acids (VFAs). The two-stage process could be optimized by adjusting the mass ratio (based on volatile solids (VS)) among food waste, chicken manure, and grass, and a ratio of 4:5:5 of food waste to chicken manure to grass could lead to the highest biogas yield and efficiency. The biogas yield of the two-stage process 83.25% higher and the duration of digestion was 18 days shorter than those of the codigestion. [\[Paper\]](https://www.sciencedirect.com/science/article/abs/pii/S0306261917305469) * Two-stage anaerobic digestion is a substantial breakthrough in the field of renewable and sustainable energy technology that potentially transforms organic wastes into biohythane and simultaneously resolves the threat of energy crisis and waste disposal. Two-stage anaerobic system provides optimal process stability, increased energy efficacy and better control over crucial parameters governing assured performance and energy recovery. This review mainly focuses on the parameters to be monitored and controlled for maximum performance, influence on efficiency and process stability. The current findings and advanced research prospects in the field of two-stage anaerobic digestion of food wastes are analyzed. Increasing interest in utilizing this method for food waste management lies in understanding the influence of process parameters for maximal benefits.[ \[Art. #ARTNUM\]](#article-96377-2988722300) * **Zero-valent iron supplementation and two-stage anaerobic digestion are enhanced ways for CH4 production in anaerobic treatment to convert organic wastes into renewable energy.** In this study, different zero-valent iron supplementation modes were applied during acidogenic stage or methanogenic stage in swine manure anaerobic digestion. Adding zero-valent iron in acidogenic stage can increase the pH and improve volatile fatty acids production by 16.5%; zero-valent iron addition on methanogenic stage is more beneficial for CH4 production than acidogenic stage. The results show that adding zero-valent iron at both acidogenic stage and methanogenic stage could achieve CH4 yield an increase of 27.3% than control. The microbial sequencing analysis showed that zero-valent iron supplementation could enrich hydrogenotrophic Methanomassiliicoccus by 39.2-92.0%.[ \[Art. #ARTNUM\]](#article-96377-2985895162)

Anaerobic digestion (AD) is an important energy strategy for converting organic waste to CH 4 . A major factor limiting the practical applicability of AD is the relatively long hydraulic retention time (HRT) which declines the treatment efficiency of digesters. A coupling process of anaerobic digestion and ‘electromethanogenesis’ was proposed to enhance anaerobic digestion at a short HRT in this study. Microorganisms analysis indicated that the electric-biological reactor enriched hydrogenotrophic methanogens in both cathodic biofilm and suspended sludge, helping achieve the high organic removal (71.0% vs 42.3% \[control reactor\]) and CH 4 production (248.5 mL/h vs 51.3 mL/h), while the additional electric input was only accounted for 25.6% of the energy income from the increased CH 4 production. This study demonstrated that a bioelectrochemical enhanced anaerobic reactor could improve the CH 4 production and organic removal at a short HRT, providing an economically feasible scheme to treat wastewater.[ \[Art. #ARTNUM\]](#article-96628-2471479672)

**Highlights:** * At shorter HRTs and upper manure-to-dog food ratio, the thermophilic CSAD marginally outperformed the biomethane production rates and substrate stabilization of the mesophilic CSAD. The increased fiber content relative to lipids at upper manure-to-dog food ratios improved the stability and performance of the thermophilic process by decreasing the concentration of LCFAs in solution, likely adsorbed onto the manure fibers. Overall, results of this study show that stability of the thermophilic co-digestion process is highly dependent on the influent substrate composition, and particularly for this study, on the proportion of manure to lipids in the influent stream.[\[Paper\]](https://www.sciencedirect.com/science/article/abs/pii/S0043135414000736) * In this study the anaerobic co-digestion of organic fraction of municipal solid waste, with thickened waste activated sludge (50:50, w/w based on total volatile solids) was investigated using anaerobic digestion thermophilic and hyper-thermophilic biochemical methane potential (BMP) assays. The hyper-thermophilic BMP assays outperformed the thermophilic BMP assays by providing faster biogas production rates, higher cumulative biogas productions and methane yields. Additionally, 10, 20 and 30% FOG (based on total volatile solids) were added to the co-digestion mixtures in order to boost the biogas production and methane yield in three hyperthermophilic assays. 30% FOG in the co-digestion mixture enhanced the biogas methane content for sample TWAS:OFMSW:30%FOG(H) to 66.4% compared to 60.1% for the control sample TWAS(T), and accordingly improved the methane yield to be 84.4% higher than the methane yield of the control.[ \[Art. #ARTNUM\]](#article-96395-2755803738)

**Highlights:** * Conventional anaerobic digestion is a widely applied technology to produce biogas from organic wastes and residues. The biogas calorific value depends on the CH4 content which generally ranges between 55 and 65%. Biogas upgrading to so-called ‘green gas’, with natural gas quality, generally proceeds with add-on technologies, applicable only for biogas flows >100 m3/h. **In the concept of autogenerative high pressure digestion (AHPD), methanogenic biomass builds up pressure inside the reactor. Since CO2 has a higher solubility than CH4, it will proportion more to the liquid phase at higher pressures. Therefore, AHPD biogas is characterised by a high CH4 content, reaching equilibrium values between 90 and 95% at a pressure of 3–90 bar.** In addition, also H2S and NH3 are theoretically more soluble in the bulk liquid than CO2. Moreover, the water content of the already compressed biogas is calculated to have a dew point <−10 °C. Ideally, high-quality biogas can be directly used for electricity and heat generation, or injected in a local natural gas distribution net. In the present study, using sodium acetate as substrate and anaerobic granular sludge as inoculum, batch-fed reactors showed a pressure increase up to 90 bars, the maximum allowable value for our used reactors. However, the specific methanogenic activity (SMA) of the sludge decreased on average by 30% compared to digestion at ambient pressure (1 bar). Other results show no effect of pressure exposure on the SMA assessed under atmospheric conditions. These first results show that the proposed AHPD process is a highly promising technology for anaerobic digestion and biogas upgrading in a single step reactor system.[ \[Art. #ARTNUM\]](#article-96631-2002182342) * The two-stage autogenerative high-pressure digestion technique is a novel and promising approach for the production of gaseous fuels or upgraded biogas. This new technique is described in the patent DE 10 2011 015415 A1 and integrates biogas production, its upgrading and pressure boosting in one process. Anaerobic digestion under elevated pressure conditions leads to decreasing pH-values in the digestate due to the augmented formation of carboxylic acid. Model calculations carried out to evaluate the two-stage design showed that the pH-value in the pressurized anaerobic filter has a major influence on the methane content of the biogas produced. Within this study, the influence of the nitrogen content as one of the most important buffering substances on the performance of the system has been tested. The results show that higher NH 4 contents lead to higher pH-values in the digester and as a consequence to higher methane contents.[ \[Art. #ARTNUM\]](#article-96631-2022204724)

Anaerobic digestion is a succession of processes in which biodegradable material is broken down in absence of oxygen, leading to the production of biogas containing methane and of a residue called digestate. Anaerobic digestion represents an interesting treatment process for bio-waste, allowing the production of energy trough the recovery of biogas and the generation of fertilizers from the digestate. While centralised anaerobic digestion plants are common in Europe to treat municipal bio-waste, the DECISIVE project aims at developing **an eco-designed, micro-scale anaerobic digestion treatment process** with the objective to propose a local, **inexpensive, flexibile and easy-to manage treatment** option for urban bio-waste. Currently, micro-scale anaerobic digestion plants are widely used in rural areas in China and India, as well as in farms across Europe. To be implemented in European urban reas where winter cold can be challenging for the AD process and to comply with the European environmental and sanitary regulation, several improvements need to be brought to the process. The micro-AD units designed by the DECISIVE projects will be coupled with **Stirling motors** to maximise their energy recovery and produce a local source of renewable energy.[\[Source\]](https://www.decisive2020.eu/the-project/micro-scale-anaerobic-digestion/) **Highlights:** * This paper describes the analysis of an AD plant that is novel in that it is located in an urban environment, built on a micro-scale, fed on food and catering waste, and operates as a purposeful system. The results showed that the plant was capable of stable operation despite large fluctuations in the rate and type of feed. Another innovative aspect of the plant was that it was equipped with a pre-digester tank and automated feeding, which reduced the effect of feedstock variations on the digestion process. Towards the end of the testing period, a rise in the concentration of volatile fatty acids and ammonia was detected in the digestate, indicating biological instability, and this was successfully remedied by adding trace elements. [ \[Art. #ARTNUM\]](#article-97034-2587750466)

Aerobic digestion is a process in sewage treatment designed to reduce the volume of sewage sludge and make it suitable for subsequent use. More recently, technology has been developed that allows the treatment and reduction of other organic waste, such as food, cardboard and horticultural waste.[\[Wiki\]](https://en.wikipedia.org/wiki/Aerobic_digestion) Can be used as pretreatment for anaerobic digestion or as a process for the breakdown of biomass, followed by fractionation of components. **Highlights:** * Due to the auto thermal thermophilic aerobic digestion the VFA production was enhanced which ultimately increased the biogas production by 39.72%, as compared to the control system.[ \[Art. #ARTNUM\]](#article-97348-2324572544) * Autothermal thermophilic aerobic digestion (ATAD) is an exothermic process where sludge is subjected to temperatures greater than 55 °C for at least 4 hours, over 6–10 days. Organic solids are degraded and the heat released during the microbial degradation is used to bring the process temperature within the thermophilic range.[ \[Art. #ARTNUM\]](#article-97348-2046787021) * A process for the conversion of biomass into a biomass product which is suitable for use as a fuel, wherein the biomass is of plant origin and comprises microorganisms naturally occurring in the biomass, which process comprises - preparing a slurry by dispersing the biomass comprising the naturally occurring microorganisms in an aqueous liquid, - maintaining the slurry at conditions suitable for aerobic digestion by the microorganisms to obtain a slurry comprising the biomass product as a dispersed solid phase, and - recovering the biomass product, which recovering comprises washing using water as a washing liquid and drying the biomass product.[ \[Art. #ARTNUM\]](#article-97348-EP2606140B1)

Transesterification is the reaction where fats and oils are converted to form esters and glycerol in the presence of catalysts. The physical characteristics of the fatty acid methyl ester (FAME) produced would then be comparable with the commercial petroleum fuel and the by-product glycerol also has commercial value.  **Highlights:** * Generally, there are three groups of catalysts exploited for biodiesel production, namely acid, base and enzymes. Base-catalyzed transesterification is frequently applied for commercial production as it gives higher FAME yield rapidly with mild reaction conditions contrasting to acid-catalyzed transesterification. On the other hand, enzyme catalysts are more environmental friendly and can produce high quality products, but its slow reaction rate and high cost needs further development to make it more feasible. The fabrication of nanocomposites, containing both acid and base sites, higher surface to volume ratio and larger pore distribution, can also contribute to the commercialization of biodiesel production. [\[Art. #ARTNUM\]](#article-97048-2947495040) * Preparation of biodiesel using in situ transesterification has been extensively conducted for agricultural, microbial and algal biomass, while few works have been performed using aquatic animal tissue. In this work, fish processing wastes were collected to perform in situ transesterification using grass carp (Ctenopharyngodon idellus) biomass as a representative with which to optimize the reaction conditions.[ \[Art. #ARTNUM\]](#article-97048-3014328127) * The high lipid-containing municipal sewage sludge (MSS) can be considered as a potential feedstock owing to its low cost and abundant availability. The lipids are transformed to biodiesel via transesterification[ \[Art. #ARTNUM\]](#article-97048-2978992766)

 Chain elongation is an anaerobic open-culture biotechnological process that converts volatile fatty acids (VFAs) and an electron donor into more valuable medium chain fatty acids (MCFAs). The conversion of VFAs into MCFAs with ethanol as electron donor is done by chain elongating micro-organisms (e.g., *Clostridium kluyveri*) that use the reverse β-oxidation pathway. In this pathway, 1 additional mole of ethanol is oxidized into acetate for every 5 chain elongation reactions (Equation 1). Reverse β−oxidation pathway: 5CxH(2x−1)O2− +6C2H6O                  → 5C(x+2)H(2x+3)O2− + C2H3O2− + 4H2O+ H+ +2H2 [\[Paper\]](https://www.frontiersin.org/articles/10.3389/fbioe.2018.00050/full) \ **Highlights:** * In chain elongation, short-chain fatty acids (SCFAs; saturated fatty acids containing less than six carbons) and ethanol are converted by microorganisms into medium-chain fatty acids (MCFAs; saturated fatty acids containing six to 12 carbons). It was found that chain elongation can be performed under a nonsterile condition and in a continuous production mode. Moreover, the use of SCFAs, like acetate and butyrate, in chain elongation yields caproate as the most dominant end-product with a high production rate and specificity. Both acetate and butyrate are the main intermediates from anaerobic degradation of mixed organic waste like OFMSW. Ethanol addition during the anaerobic degradation of OFMSW has been shown to stimulate chain elongation of these SCFAs and the added ethanol to caproate as the main end-product. The highest caproate production rate via this process was 26 g/L/day with a concentration up to 12.6 g/L, which approximates the solubility of caproic acid in water and is advantageous to the downstream processes.[\[Art. #ARTNUM\]](#article-96394-2615639733) * Chain elongation from AFW and ethanol is more effective at long HRT than at short HRT not only because it results in a higher concentration of MCFAs but also because it leads to a more efficient use of ethanol and base. The HRT did not influence the n-caproate production rate. The obtained n-caproate concentration is more than twice as high as the maximum solubility of n-caproic acid in water which is beneficial for its separation from the fermentation broth. This study does not only set the record on the highest n-caproate concentration observed in a chain elongation process to date, it notably demonstrates that such high concentrations can be obtained from AFW under practical circumstances in a continuous process.[\[Paper\]](https://www.frontiersin.org/articles/10.3389/fbioe.2018.00050/full)

Dark fermentation is the fermentative conversion of organic substrate to biohydrogen. It is a complex process manifested by diverse groups of bacteria, involving a series of biochemical reactions using three steps similar to anaerobic conversion. Dark fermentation differs from photofermentation in that it proceeds without the presence of light. Fermentative/hydrolytic microorganisms hydrolyze complex organic polymers to monomers which are further converted to a mixture of lower-molecular-weight organic acids and alcohols by obligatory producing acidogenic bacteria.[\[Wiki\]](https://en.wikipedia.org/wiki/Dark_fermentation) **Highlights:** * In recent years, coupling bacterial dark fermentation (DF) and heterotrophic cultivation of microalgae (HCM) has been pointed out as a promising sustainable approach for producing both gaseous and liquid biofuels. **Complex organic waste and effluents that are not susceptible to be directly degraded by microalgae are first converted into volatile fatty acids (VFAs) and hydrogen by DF**.[ \[Art. #ARTNUM\]](#article-96400-2333824632) * Low biohydrogen (H2) yields and use of process by-products from dark fermentation (DF) of waste biomass is limiting its scaled-up application. This study aims to investigate the effects of culture pH, combination of substrate concentration and culture pH, pre-treatment of substrate and inoculum adaptation in H2 yields during the DF of three different wastes biomass. The study showed that the biodegradability of the substrates is important for the selection and application of optimum operational parameters aimed at enhancing H2 production. **Moreover, long-term operational feasibility and stability of dark fermentative H2 production was demostrated using food waste and cheese whey in two semi-continuous thermophilic DF reactors.** [\[Art. #ARTNUM\]](#article-96400-2559195065)

Photofermentation is the fermentative conversion of organic substrate to biohydrogen manifested by a diverse group of photosynthetic bacteria by a series of biochemical reactions involving three steps similar to anaerobic conversion. Photofermentation differs from dark fermentation because it only proceeds in the presence of light. [\[Wiki\]](https://en.wikipedia.org/wiki/Photofermentation) **Highlights:** * We used NIR light sources and optically resonant gold–silica core–shell nanoparticles to increase the light utilization of the bacteria to convert waste organic acids such as acetic and maleic acids to hydrogen. The batch growth studies for the small cultures (40 mL) of Rhodopseudomonas palustris demonstrated >2.5-fold increase in hydrogen production when grown under an NIR source (167 ± 18 μmol H2) compared to that for a broad-band light source (60 ± 6 μmol H2) at equal light intensity (130 W m−2). The addition of the mPEG-coated optically resonant gold–silica core–shell nanoparticles in the solution further improved the hydrogen production from 167 ± 18 to 398 ± 108 μmol H2 at 130 W m−2. The average hydrogen production rate with the nanoparticles was 127 ± 35 μmol L−1 h−1 at 130 W m−2. [\[Art. #ARTNUM\]](#article-96372-2996198659) * Some biomass such as microalgae have the natural ability to produce H~2~ gas in the presence of light. In photosynthesis, microalgae transform water molecules into O~2~ and H^+^. Hydrogenase enzymes then reduce the H^+^ into H~2~ gas under anaerobic conditions. The O~2~ released during photosynthesis rapidly inhibits the hydrogenase enzymes and interrupts the release of H~2~ gas. This implies that anaerobic condition is necessary for the culturing microalgae for the H~2~ gas production. There are two key methods to extract photosynthetic H~2~ using microalgae. First approach is the simultaneous production of O~2~ and H~2~ gas in the presence of light. The electrons generated from oxidation of water molecules are used by hydrogenase enzymes to yield H~2~ gas. Theoretically, this method has higher yields than the second approach, but the H~2~ production is rapidly inhibited by the O~2~ production. The second approach is to utilize a two-phase system, where the first phase is culturing microalgae under normal conditions and the second phase is promoting continuous H~2~ generation under anaerobic and sulfur-deprived conditions. Sulfur deprivation engages the microalgae in a survival state where the energy required by the cells are obtained through the release of H~2~. In the two-phase system, the H~2~ production would begin to decline after 60 h of operation, and the theoretical maximum H~2~ yield could reach 198 kg H~2~ ha^− 1^ day^− 1^. [\[Art. #ARTNUM\]](#article-96372-2947495040)

Gas fermentation is a technology that uses biocatalysts to convert gaseous feedstocks such as carbon monoxide (CO), carbon dioxide (CO2), syngas, methane (CH4), or biogas, into platform chemicals, fuels, polymers, etc.[\[Source\]](https://vito.be/en/product/gas-fermentation#:\~:text=Gas%20fermentation%20is%20a%20technology,%2C%20fuels%2C%20polymers%2C%20etc.) **Highlights:** * Increasing demand for renewable feedstock-based biofuels is driving the interest and rapid development of processes to produce fuels and chemicals from biomass-generated syngas. Biomass is gasified to produce syngas that can be converted via thermochemical routes to fuels and chemicals such as alcohols, olefins, and fuel grade hydrocarbons. An alternate route to produce liquid products from syngas is through gas fermentation, a hybrid thermochemical/ biochemical process. Biomass gasification, thermochemical syngas conversion routes, and gas fermentation are described including a comparison for converting biomass to ethanol via thecurrent thermochemical route versus gas fermentation.[ \[Art. #ARTNUM\]](#article-96463-2059176864) * Fermentation of syngas is a means through which unutilized organic waste streams can be converted biologically into biofuels and commodity chemicals. Despite recent advances, several issues remain which limit implementation of industrial-scale syngas fermentation processes. At the cellular level, the energy conservation mechanism of syngas fermenting microorganisms has not yet been entirely elucidated. Furthermore, there was a lack of genetic tools to study and ultimately enhance their metabolic capabilities. Recently, substantial progress has been made in understanding the intricate energy conservation mechanisms of these microorganisms. Given the complex relationship between energy conservation and metabolism, strain design greatly benefits from systems-level approaches. Numerous genetic manipulation tools have also been developed, paving the way for the use of metabolic engineering and systems biology approaches. Rational strain designs can now be deployed resulting in desirable phenotypic traits for large-scale production.[ \[Art. #ARTNUM\]](#article-96463-2126579076)

Membrane bioreactor is the combination of a membrane process like microfiltration or ultrafiltration with a biological wastewater treatment process, the activated sludge process. It is now widely used for municipal and industrial wastewater treatment. [\[Wiki\]](https://en.wikipedia.org/wiki/Membrane_bioreactor) **Highlights:** * The concept proposed here is the coupling of two main compartments. In the first one, lignocellulosic biomass is transformed by dark fermentation into hydrogen and VFAs. The medium used in the first compartment is transferred via an immersed membrane bioreactor to the second compartment where the production of single-cell oil with oleaginous yeast occurs using VFAs as carbon source. This study aims at presenting the feasibility of this bioprocess, through hydrogen production from glucose followed by lipid production by the oleaginous yeast *Cryptococcus curvatus* grown on the VFA-enriched supernatant resulting from the dark fermentation step.[\[Art. #ARTNUM\]](#article-96389-2566232385) * Light energy is one of the major costs for phototrophic systems. This study evaluated the photoreactor efficiency of purple phototropic bacteria anaerobic membrane bioreactor (PAnMBR) at low irradiance for the treatment of municipal wastewater. Infrared irradiance levels of 3.0 and 1.4 W/m2 produced by an infrared (IR) lamp emitting in the 800–900 nm wavelength range were investigated, with the ultimate goal of optimizing the irradiance energy demand. Experimental and modeling results demonstrated the ability of PPB to grow and treat raw municipal wastewater at the applied low irradiances, with effluent quality below target limits of TCOD˂50 mg/L, TN˂10 mg/L, and TP˂1 mg/L.[ \[Art. #ARTNUM\]](#article-96389-3003638816) * **MBR enhancement:** Chemical enhancers (e.g., electroassisted membranes, ozonation) induce high nitrogen and organic removal, biogas production, and energy recovery. Biocarrier enhancers (e.g., powdered activated carbon, zeolite) improve removal of various types of pollutants and energy recovery. Microbial enhancers (e.g., entrapped biomass technology, functional bacterium) accomplish effective removal of pollutants, realize high methane production, and encourage enrichment of functional microbial communities and presence of new emerging bacteria to resist extreme conditions. Membrane fouling control enhancers (e.g., electric field, quorum quenching technology) change sludge properties and inhibit the deposition of sludge flocs on membrane surface. Development of new flocculants, new carriers, and novel electrically conductive membranes, and application of quorum quenching technology in hybrid AnMBR systems are suggested to be future research direction.[ \[Art. #ARTNUM\]](#article-96389-3005465132)

Besides food preservation, γ-irradiation is taking place for novel applications, especially involving the enhancement of food fermentation processes, by directly irradiating the medium, or generating performant genetically modified strains.[\[Paper\]](https://link.springer.com/chapter/10.1007/978-3-319-42457-6_7) **Highlights:** * The present invention relates to a method comprising irradiating a biomass feedstock to change a molecular and/or a supramolecular structure of the biomass feedstock; cooling the irradiated biomass feedstock; and then re- irradiating the biomass feedstock with a second dose of ionizing radiation. A method comprising: irradiating a biomass feedstock to change a molecular and/or a supramolecular structure of the biomass feedstock; cooling the irradiated biomass feedstock; and then re-irradiating the biomass feedstock with a second dose of ionizing radiation. The method of claim 1, further comprising processing the re-irradiated biomass feedstock to produce a fuel. The method of claim 2, wherein processing comprises converting the re-irradiated biomass feedstock utilizing a microorganism having the ability to convert at least about 1 percent by weight of the biomass to the fuel.[ \[Art. #ARTNUM\]](#article-96465-EP3546587A1)

**Example:** **Mycelium composites comprise of networks of filamentous hyphae, utilising biological growth rather than expensive energy intensive manufacturing processes to convert low-cost organic wastes into economically viable and environmentally friendly materials.** Although generally characterised as polymer grade foams and used primarily for limited packaging and construction applications, the mechanical performance of these materials varies significantly and is governed by hyphal architecture, cell wall composition, composite constituents and growth kinetics which are in turn influenced by inherent and exogenous factors. A range of potential applications have been proposed including acoustic dampers, super absorbents, paper, textiles, structural and electronic parts.[ \[Art. #ARTNUM\]](#article-96393-2745793211)

Microwave hydrodiffusion and gravity (MHG) is a combination of microwaves for hydrodiffusion of essential oils from the inside to the exterior of biological material and earth gravity to collect and separate.[\[Paper\]](https://www.sciencedirect.com/science/article/abs/pii/S0021967308004032) **Highlights:** * Components like essential oils, phenolic compounds and pectin can be extracted from plant biomass using this technique.[\[Art. #ARTNUM\]](#article-96383-2946440646)

A chemical reaction involving the decomposition of a molecule, with the addition of the elements of an acid to the molecule; the reaction is comparable to hydrolysis or alcoholysis, in which water or alcohol, respectively, is used in place of the acid.[\[Source\]](https://encyclopedia2.thefreedictionary.com/acidolysis#:\~:text=acidolysis,in%20place%20of%20the%20acid.) **Highlights:** * Here, we present the successful scale-up demonstration of the **acid-assisted IL deconstruction on feedstock blends of municipal solid wastes and agricultural residues** (corn stover) by 30-fold, relative to the bench scale (6 vs 0.2 L), at 10% solid loading. By integrating IL pretreatment and acid hydrolysis with subsequent centrifugation and extraction, the sugar and lignin products can be further recovered efficiently. This scale-up development at Advanced Biofuels/Bioproducts Process Demonstration Unit (ABPDU) will leverage the opportunity and synergistic efforts toward developing a cost-effective IL-based deconstruction technology by drastically eliminating enzyme, reducing water usage, and simplifying the downstream sugar/lignin recovery and IL recycling.[\[Paper\]](https://link.springer.com/article/10.1186/s13068-016-0694-8) * In this paper, three types of lignocellulosic biomass (softwood, hardwood and herbaceous biomass) were processed by **microwave-assisted acidolysis to produce high quality lignin.** The lignin from the softwood was isolated largely intact in the solid residue after acidolysis. For example, a 10 min microwave-assisted acidolysis treatment produced lignin with a purity of 93% and in a yield of 82%, which is superior to other conventional separation methods reported. To assess the suitability of this methodology as part of a biorefinery system, the aqueous phase, produced after acidolysis of the softwood, was characterised and assessed for its suitability for fermentation. The broth contained some mono- and di-saccharides but mainly contained organic acids, oligosaccharides and furans. **This preliminary work demonstrates new protocols of microwave-assisted acidolysis and therefore offers an effective approach to produce high purity lignin and fermentable chemicals, which is a key step towards developing a zero-waste lignocellulosic biorefinery.**[\[Art. #ARTNUM\]](#article-97022-2601982806)

Supercritical fluid extraction is the process of separating one component from another using supercritical fluids as the extracting solvent. Extraction is usually from a solid matrix, but can also be from liquids.[\[Wiki\]](https://en.wikipedia.org/wiki/Supercritical_fluid_extraction) **Highlights:** * SFE has been used to **extract lipids** from various biomass which includes spent coffee grounds using SC-CO~2~, soybean using SC-CO~2~, linseed using supercritical-ethanol, residual corn material using SC-CO~2~, organosolv lignin using supercritical-ethanol, shrimp waste using SC-CO~2~, and white pinewood. Apart from that, SFE has been extensively studied for the extraction of lipid from third generation feedstock as well. Lipid from milk was also extracted using a supercritical anti-solvent fractionation technology (SAFT). The SAFT is a versatile process which consists of a water-miscible organic solvent and water. The resulting solution from the process undergoes extraction using a supercritical solvent. High lipid yields was obtained by SAFT using dimethyl ether and the lipid contents were found to be around 70% neutral and 30% phospholipids. * **Supercritical transesterification is a simplified biofuel production process that requires no catalyst.** This technique was developed to overcome certain drawbacks associated with catalysed transesterification, such as long reaction time, catalyst poisoning, catalyst regeneration, high operation cost, saponification and biodiesel washing. Supercritical fluid has been used in commercial production of biodiesel, which indicates its scalability potential and its readiness to be deployed as a feasible technology. [\[Art. #ARTNUM\]](#article-97049-2947495040)

Solutions can separate into distinc layer, which can be used for fractionation and reaction purposes. **Highlights:** * Lignocellulosic biomass, such as corn stover, pulp and paper mill waste, and switchgrass, is a readily available feedstock for the production of monomeric sugars and platform chemicals that can then be transformed into valuable organic molecules. However, efficiently fractionating lignocellulosic biomass is difficult due to the recalcitrance of lignin at mild reaction conditions and the reactive sugars/platform chemicals at more severe conditions. **Biphasic systems present a possible solution to creating an economically viable biomass upgrading process since sugars prefer the aqueous phase while the lignin and furans partition to the organic phase.** [ \[Art. #ARTNUM\]](#article-97054-2890619490) * **The fractionation of lignocellulose in its three main components, hemicellulose, lignin and cellulose pulp can be achieved in a biphasic system comprising water and bio-based 2-methyltetrahydrofuran (2-MeTHF) as solvents and oxalic acid as catalyst at mild temperatures (up to 140 °C). This so-called OrganoCat concept relies on selective hemicellulose depolymerization to form an aqueous stream of the corresponding carbohydrates, whereas solid cellulose pulp remains suspended and the disentangled lignin is to a large extent extracted in situ with the organic phase.** Economic analysis of the process reveals that the improved biomass loading significantly reduces capital and energy costs in the solvent recycle, indicating the importance of process integration for potential implementation. The procedure was successfully scaled-up from the screening on bench scale to 3 L reactor. The feedstock flexibility was assessed for biomasses containing moderate-to-high hemicellulose content.[ \[Art. #ARTNUM\]](#article-97054-1991412117)

Black soldier fly larvae (Hermetia illucens; BSFL) can convert organic wastes into a nutrient-rich biomass suitable in animal feed, which could be a way to achieve more sustainable production of food. **Highlights:** * In this study, BSFL were fed 11 diets based on four different organic waste sources (mussels, bread, fish and food waste). Fatty acid and proximate composition (dry matter, crude fat, crude protein and ash) were analysed in the diets, in two-week-old larvae and substrate residues. Larval weight, survival and feed conversion were also recorded. The diet was found to affect all parameters investigated. Irrespective of diet, the larval fat consisted mainly of lauric acid and other saturated fatty acids and these were found to be synthesised by the larvae. However, both the fatty acid composition of the substrate, and the larval weight were found to affect the fatty acid profile of the larvae. In general, larvae with a higher weight contained a higher percentage of saturated fatty acids and a lower percentage of unsaturated fatty acids, such as eicosapentaenoic (EPA) and docosahexaenoic acid (DHA). It was concluded that the possibilities to tailor the fatty acid composition of the BSFL through the diet are limited; thus, the BSFL fat may not be suitable to replace fish oil, but has potential of inclusion in other food, feed and fuel products.[ \[Art. #ARTNUM\]](#article-96374-2981556263) * An innovative solution is to use insects for the management of organic waste. Here, we used black soldier fly to convert organic waste into animal feed protein, as fly larvae, and plant fertilizer, as compost residue. A continuous fly reactor was monitored for 9 weeks. We analyzed physicochemical and microbial parameters, and we evaluated the sanitary risk. Results show 55.1 % of material degradation and 11.8 % of biomass conversion based upon total solids. We observed higher levels of N and P in the treatment residue than in the inflow material. Results also show a lower concentration of Salmonella spp. and viruses. Compost treatment with black soldier fly is therefore an efficient system for nutrient recycling.[ \[Art. #ARTNUM\]](#article-96374-1990613273)

Biological decomposition for the production of compost can be done by different composting techniques, such as vermicomposting and hyperthermophilic composting. **Highlights:** * The present study aimed to evaluate the behavior of materials such as sewage sludge and green waste during composting, their compatibility in mixtures and the quality of the end product. Also, the study highlighted the technical and operational characteristics of a composting system on a laboratory scale. The main advantage of this laboratory scale experiment is the reduced processing time and the fact that it can be fast adapted to different experimental conditions. The pilot scale experiment allowed to investigate the dynamics of main parameters during the composting process and provide the basis for an efficient design process. The elaborated composting process is a convenient, cost-effective and environmentally friendly process for biodegradable organic waste management, being also a feasible and controllable process. The obtained results show that all composting variants lead to the production of a quality compost that can be used in agriculture.[ \[Art. #ARTNUM\]](#article-96379-2976627460) * Here, we demonstrated for the first time that hyperthermophilic composting ( h TC) was able to mitigate nitrogen loss by 40.9% compared to c TC after 44 days of composting in a full-scale plant. Results demonstrate a decrease in NH 3 volatilization (52.4%), together with an inhibitory effect on protease (19.4–87.5%) and urease (9.1–75.2%) enzyme activities and the ammonification rate (5.2–80.1%) for h TC. Additionally, this study found that h TC could accelerate the humification process, thereby enhancing the formation of the recalcitrant nitrogen reservoir (mainly in the form of nitrogenous humic substances) and reducing the substrate for ammonification reactions. These findings suggest that h TC can significantly reduce nitrogen loss and provide insights into the role of humic substances in nitrogen retention in composting systems. [\[Art. #ARTNUM\]](#article-96379-2957144356)

**Photocatalysis** is a type of catalysis that results in the modification of the rate of a photoreaction - a chemical reaction that involves the absorption of light by one or more reacting species - by adding substances (catalysts) that participate in the chemical reaction without being consumed.[\[Source\]](https://www.nature.com/subjects/photocatalysis#:\~:text=Photocatalysis%20is%20a%20type%20of,chemical%20reaction%20without%20being%20consumed.) **Highlights:** * **This study explores a novel photocatalytic approach for the conversion of complex biomass into smaller molecular units for their utility in generation of biofuel precursors.** Following hydrothermal method, TiO 2 /Bi 2 WO 6 nanocomposite photocatalyst for different ratio of TiO 2 viz. 15% and 25% by weight have been synthesized. The efficiency of photodegradation of organic molecule is determined. Composite loaded with 25% TiO 2 found to represent maximum photocatalytic efficiency of 99.9%. Using this composite, solar energy is harvested to perform photocatalytic fragmentation of a biopolymer, namely starch, to derive smaller molecule precursors. The results demonstrate photocatalytic pretreatment of starch by nanocomposite of TiO 2 /Bi 2 WO 6 resulted in the formation of organic precursors that are common feedstock for the microbiota of anaerobic methanation and ethanol fermentation. We therefore conclude that photocatalytic pretreatment of biomass can be utilized for hydrolysis and partial acedogenesis of biomass in biofuel conversion processes. Such an approach represent environmentally benign way to convert biomass waste to biofuel.[ \[Art. #ARTNUM\]](#article-96401-2802720600) * As previously mentioned, an interesting valorization protocol to develop would be a photocatalytic approach. To accomplish such photocatalytic strategies, TiO~2~, Pt/CdS/TiO~2~ composite materials, TiO~2~/Ni(OH)~2~ clusters may be used depending on the target, samples, and reaction conditions. Photodegradation has been shown to be possible toward many environmental pollutants such as chlorofluorocarbons, CO~2~, and NO, but whether these photoactive composites could degrade the stable polymeric structure of lignin/protein/carbohydrates is yet to be seen and perhaps understood. \[...\] **This study provides proof of concept that band gap engineering of semiconductors can lead to the development of photoactive materials that may be used selectively for waste valorization. A photocatalytic approach will most importantly address one of the major drawbacks of industrial valorization which is on the relatively large amounts of energy needed for processing and purification of products.**[\[Paper\]](https://onlinelibrary.wiley.com/doi/10.1002/ese3.9)

Flow chemistry, continuous processing, or continuous flow chemistry, begins with two or more streams of different reactants pumped at specific flow rates into a single chamber, tube, or microreactor. A reaction takes place, and the stream containing the resultant compound is collected at the outlet.[\[Source\]](https://www.mt.com/my/en/home/applications/L1_AutoChem_Applications/continuous.html#:\~:text=Flow%20chemistry%2C%20continuous%20processing%2C%20or,is%20collected%20at%20the%20outlet.) **Highlights:** * One among such promising waste valorization strategies is the application of flow chemical technology to process waste to valuable products. A recent review of Ruiz et al. highlighted various **advantages of continuous flow processes particularly for biomass and/or food waste valorization which included reaction control, ease of scale‐up, efficient reaction cycles producing more yield, and no required catalyst separation.** Although flow chemistry has been known to be used in industries for other processing methodologies, it still remains to be used in biomass/waste valorization – a limitation caused by the large energy needed to degrade highly stable biopolymers and recalcitrant compounds (e.g., lignin). These requirements are not simple to satisfy and various techniques (e.g., microwave irradiation) need to be combined to satisfy the prerequisites for a successful transformation of waste. However, the main challenge for this combination is on the scale‐up itself. As conceptualized by Glasnov et al. microwave and flow chemistries may be coupled by attaching back‐pressure regulators to flow devices. **This approach can revolutionize industrial valorization since it will synthesize products fast (due to microwave heating) on one continuous run (flow process).** Although the approach presented is possible, the main challenge of temperature transfer from microwave to flow remains to be solved. A buildup of temperature gradient inside the instrument could lead to various instrument inefficiencies. [\[Paper\]](https://onlinelibrary.wiley.com/doi/10.1002/ese3.9) * Continuous flow chemical processes offer several advantages as compared to batch chemistries. These are particularly relevant in the case of heterogeneously catalyzed transformations of biomass-derived platform molecules into valuable chemicals and fuels. This work is aimed to provide an overview of key continuous flow processes developed to date dealing with a series of transformations of platform chemicals including alcohols, furanics, organic acids and polyols using a wide range of heterogeneous catalysts based on supported metals, solid acids and bifunctional (metal + acidic) materials.[\[Paper\]](https://www.mdpi.com/2078-1547/3/2/114)

Biomass liquefaction in organic solvents is an alternative to hydrothermal liquefaction. **Highlights:** * In specific cases, (supercritical) hydrocarbon solvents have been demonstrated to possess similar physical properties (high density, low dielectric point and radical scavenger) to water and could be considered for the biomass depolymerization at milder critical temperature and pressure conditions. A large variety of supercritical solvents have been used, e.g. alcohols, ketones, hydrocarbons and CO~2~, with the objective of obtaining higher quality and yields of biocrude. Like water, alcohols and hydrocarbons such as ethanol, methanol or isopropanol and tetraline or decaline are considered as hydrogen donor solvents and have been used for research studies on biomass liquefaction. Further information on the effects of hydrogen donors for biomass upgrading is available elsewhere. * **In the last decade, a small number of start-up companies have begun developing technologies using high boiling point solvents for the liquefaction of lignocellulose or waste to produce diesel**. Examples of these technologies are the BioCRACK process (BDI-Bioenergy international Gmbh), the KDV process (Alphakat Gmbh), the MECC process (Global Ecofuels Solutions), CTC process (RECENSO GmBH) and the READi process (Nexxoil Gmbh). [\[Art. #ARTNUM\]](#article-96408-2976637787) * Here, potato peel waste was subjected to direct acid liquefaction at 160 °C in 2-ethylhexanol and diethylene glycol using p-toluene sulfonic acid as catalyst.[ \[Art. #ARTNUM\]](#article-96408-2599683166)