Hydrothermal Liquefaction Aqueous Phase Research Articles

Electrolysis of HTL-AP for nutrient recovery by converting cyclic nitrogen to nitrate-N fertilizer

Valorization of hydrothermal liquefaction aqueous phase (HTL-AP) can be achieved through its use as a nutrient source for lettuce production in hydroponic systems after being treated to reduce the nutrient imbalance. Removing nitrogen cyclic compounds in HTL-AP may impact the availability of some nutrients, such as nitrate-N, that are necessary for plant growth. Previous studies indicate that electrolysis enables nitrate-N accumulation in algal-HTL-AP. In this study, HTL-AP derived from food waste was electrolyzed to convert available nitrogenous compounds into nitrogen forms that are preferred by plants such as nitrate-N. Biochemical properties were assessed for the HTL-AP samples before and after two years of storage. Results from this study show that it is viable to convert heterocyclic amines in HTL-AP into inorganic nitrogen forms such as nitrite-N, nitrate-N, ammonia-N, and fatty acids. Specifically, this study showed that accumulation of 609 mg/L of nitrate-N in the HTL-AP with an initial concentration of 25 mg/L was achieved at the lowest current density. Additionally, electrolysis treatment removed 48%–61% of COD from the HTL-AP at different current densities. Furthermore, water quality characterization before and after storage for two years showed decreased organic matter in the HTL-AP, leading to reduced inorganic nitrogen recovery. Overall, this study indicates that electrolysis can increase the concentration of inorganic nitrogen in the HTL-AP both before and after long-term storage.

Adsorption enhanced biological treatment of hydrothermal liquefaction aqueous phase derived from municipal sludge

Hydrothermal liquefaction (HTL) is a promising method for municipal sludge valorization through waste minimization and biofuel production. The process wastewater, HTL aqueous, presents a significant challenge for scale-up due to recalcitrant compounds. In this study, granular activated carbon (GAC) was used to remove potential inhibitors from HTL aqueous through adsorption to enhance aerobic and anaerobic biological treatment. GAC removed up to 61 % chemical oxygen demand (COD), 50 % biochemical oxygen demand (BOD) and potential inhibitors, such as total phenolic compounds (87 %) and N-heterocycles (90 % of pyridines) at 100 g/L. Conversely, most volatile fatty acids remained in HTL aqueous. Subsequently, mesophilic and thermophilic specific methane potential increased by up to 97 % and 83 %, respectively. BOD increased by up to 50 %, which enhanced BOD/COD ratio from 81 % to 93 % before and after adsorption. This study established the groundwork for HTL aqueous adsorption, described mechanism for pollutant removal, and provided insights for biological treatment.

Hydrothermal liquefaction aqueous phase mycoremediation to increase inorganic nitrogen availability

Hydrothermal liquefaction aqueous phase (HTL-AP) is a waste product from a thermochemical process where wet biomass is converted into biocrude oil. This nutrient-rich wastewater may be repurposed to benefit society by assisting crop growth after adequate treatment to increase inorganic nitrogen, especially NO3−. This study aims to increase HTL-AP inorganic nitrogen, specifically NH3/NH4+ and NO3−, through fungal remediation for further use in hydroponic systems. Trametes versicolor, a white-rot fungus known for degrading a range of organic pollutants, was used to treat a diluted (5 %) HTL-AP for 9 days. No fungal growth was observed, but T. versicolor activity was suspected by laccase activity throughout cultivation time. NO3−-N and NH3/NH4+-N increased by 17 and 8 times after three days of fungal treatment, which was chosen as the appropriate time for HTL-AP fungal treatment as it resulted in the highest concentration of NO3−-N. The addition of nitrifying bacteria to the fungal treatment resulted in a twofold increase in NO3−-N concentration compared to the fungal treatment alone, indicating an enhancement in treatment efficacy. COD decreased by 51.33 % after 24 h, which may be related to the fungus’ capacity to reduce the concentration of organics in the wastewater; nonetheless, COD increased in the following days, which may be related to the release of fungal byproducts. T. versicolor shows promise as a potential candidate for increasing inorganic nitrogen in HTL-AP. However, future studies should primarily address HTL-AP toxicity, reducing NH3/NH4+-N while increasing NO3−-N, and hydroponics crop production after fungal treatment.

Ultrafiltration fractionation of potentially inhibitory substances of hydrothermal liquefaction aqueous phase derived from municipal sludge

Hydrothermal liquefaction (HTL) is a promising thermo-chemical technology for municipal sludge treatment due to its potential for biocrude oil recovery and minimizing biosolids management costs. However, the process generates a high volume of an aqueous byproduct that needs to be treated due to its high chemical oxygen demand (COD) and various organic and inorganic compounds. Although the aqueous phase is known to contain recalcitrant and potentially inhibitory substances that may affect its biological treatment, their molecular weight distribution (MwD) and its impact on anaerobic biodegradability are poorly understood. Ultrafiltration (UF) was conducted to fractionate HTL aqueous into different molecular weight (Mw) fractions using 300, 100, 10, and 1 kDa membranes. Mesophilic biochemical methane potential (BMP) assays were conducted to assess the anaerobic biodegradability of each fraction, and the first-order model was used to calculate the degradation kinetics of potential inhibitory compounds. The highest percentage of organics (65 %) was found in the Mw<1 kDa range, whereas the 10>Mw>1 kDa had the lowest percentage (8 %). There was no significant difference in the cumulative specific methane produced from various Mw fractions (p>0.05). The Mw<1 kDa fraction had the highest first-order specific methane production rate (0.53 day−1), whereas the unfiltered HTL had the lowest (0.38 day−1). Although UF fractionation increased the rate of anaerobic degradation of HTL aqueous for the Mw<1 kDa fraction, the observed methane potential was only 55 % of the theoretical value. This implies that 45 % of COD remains undegraded even after permeation through the lowest Mw cut-off membrane. Therefore, further characterization of HTL aqueous is needed for compounds with molecular weights below 1 kDa to fully understand the nature of inhibitory organics and their impact on anaerobic digestion. Furthermore, pretreatments utilizing techniques such as adsorption and advanced oxidation may be necessary to enhance the specific methane yields from various HTL aqueous fractions, thereby bringing them closer to the theoretical yield.

Investigating the Impacts of Wastewaters on Lettuce (Lactuca sativa) Seed Germination and Growth

There is an opportunity for agriculture to utilize the many different waste streams in our world and capitalize on what would otherwise be viewed as waste products. Hydrothermal liquefaction (HTL) is an emerging technology for converting wet biomass to bio-crude oil, while aquaponics is a practice tracing back to indigenous communities around the world; both technologies have the potential to sustainably provide the necessary nutrients for crop growth. Food systems worldwide are actively transitioning to address the many challenges of climate change in a sustainable and efficient manner. Urban agriculture (UA) has the potential to generate localized crops in densely populated areas year-round, but has its challenges, involving high capital requirements, especially for vertical farming and controlled-environment agriculture, and being energy intensive due to artificial lighting and fossil fuel-based synthetic fertilizers. This study investigated the potential for aquaponic and HTL effluents to be used in hydroponic systems through a seed germination screening experiment. Buttercrunch lettuce (Lactuca sativa L.) seeds were placed in Ziploc plastic bags on paper towels saturated with the wastewater treatments for 10 days while their total length of growth was routinely measured from the tip of the root to the tip of the cotyledons. The Chicago High School for Agricultural Sciences (CHSAS) aquaponic effluent with a 5.8× times higher nitrate concentration and 4.25× higher ammonia concentration outperformed the Bevier aquaponic effluent and improved any other source water it was combined with. Results also showed that seed germination was not inhibited in the presence of 2–8% solutions of hydrothermal liquefaction aqueous phase (HTL-AP), which performed on par with standard hydroponic fertilizer; solutions of a higher percentage, though, may lead to inhibitory effects in plants, and those of a lower percentage may not provide enough nutrients in the proper forms to sustain plant growth. However, the nutrient analyses revealed that there is still much to investigate regarding the combination of wastewaters to provide a complete, well-rounded, and sustainable source for hydroponic crop production.

Aqueous phase recycling: impact on microalgal lipid accumulation and biomass quality.

The potential success of microalgal biofuels greatly depends on the sustainability of the chosen pathway to produce them. Hydrothermal liquefaction (HTL) is a promising route to convert wet algal biomass into biocrude. Recycling the resulting HTL aqueous phase (AP) aims not only to recover nutrients from this effluent but also to use it as a substrate to close the photosynthetic loop and produce algal biomass again and process this biomass again into new biocrude. With that purpose, the response to AP recycling of five Chlorellaceae strains was monitored over five cultivation cycles. After four successive cycles of dynamic growth under nutrient-replete conditions, the microalgae were cultivated for a prolonged fifth cycle of 18 days in order to assess the impact of the AP on lipid and biomass accumulation under nutrient-limited conditions. Using AP as a substrate reduced the demand for external sources of N, S, and P while producing a significant amount of biomass (2.95-4.27 g/L) among the strains, with a lipid content ranging from 16 to 36%. However, the presence of the AP resulted in biomass with suboptimal properties, as it slowed down the accumulation of lipids and thus reduced the overall energy content of the biomass in all strains. Although Chlorella vulgaris NIES 227 did not have the best growth on AP, it did maintain the best lipid productivity of all the tested strains. Understanding the impact of AP on microalgal cultivation is essential for further optimizing biofuel production via the HTL process.

Evaluation of on-site biological treatment options for hydrothermal liquefaction aqueous phase derived from sludge in municipal wastewater treatment plants

Aerobic treatment, mesophilic anaerobic digestion, thermophilic anaerobic digestion, and dark fermentation were evaluated for on-site biological treatment of municipal sludge derived HTL aqueous. For all four described batch test scenarios, municipal sludge-derived HTL aqueous samples obtained under 290–360 °C and 0–30 min retention time were used. In the aerobic respirometric tests, HTL aqueous samples resulted in a five-day biochemical oxygen demand range of 40.75 g/L (350 °C-25.6 min) to 54 g/L (325 °C-0 min). The calculated aerobic biodegradability index showed that approximately 50 % of the organics in HTL aqueous were easily biodegradable. Mesophilic and thermophilic biochemical methane potential tests resulted in specific yields of 151–179 mL CH4/g chemical oxygen demand (COD) and 103–122 mL CH4/g COD, respectively. HTL aqueous obtained under 360 °C-15 min condition caused total inhibition in both mesophilic and thermophilic anaerobic digestion. Possible causes for this inhibition were pyridine, pyrrolidinone, piperidinone, pyridinol, and phenolic compounds, which were higher in abundance in the 360 °C-15 min sample. HTL aqueous was found unfit for hydrogen production in dark fermentation due to inhibitory composition. In summary, on-site biological treatment of HTL aqueous was found to be most suitable under aerobic and mesophilic anaerobic conditions.

Massive components in renewable hydrothermal liquid trigger ultra-high differentially expressed genes and diverse pathways for pathogen control

Hydrothermal liquefaction aqueous phase (HTL-AP) greatly hindered the sustainable development of HTL technology due to its high output and diverse compound distribution. Herein, the antimicrobial behavior, application scenario and acton mechanism of HTL-AP were clarified since an emerging pathogen reduction approach by HTL-AP attracts increasing attention. We studied the molecular cognition and underlying mechanism for phytopathogen control provoked by HTL-AP via multiscale analysis including mycelial morphology, intracellular metabolites and transcriptome. HTL-AP in a very low concentration (only 1.5%) completely inhibited the growth of Botrytis cinerea (B. cinerea) and showed promising potential for seed-borne fungi control. Biochemical analysis revealed that the morphology was significantly changed, the contents of four intracellular compounds were all largely disordered, and activities of six enzymes simultaneously decreased in mycelium after uptake of HTL-AP. Further, the transcriptome analysis revealed the disturbance of the gene expression of B. cinerea in response to HTL-AP stress. Ultra-high differentially expressed genes were enriched, which was significantly distinguished from the reported fungicide agent. HTL-AP mainly acted on metabolic processes of B. cinerea while disruption of genetic information processes and cellular processes were also performed. All four main antimicrobial modes were observed in HTL-AP action, and multiple action pathways of HTL-AP exhibited a synergistic interaction in growth inhibition. The multiscale analysis in this study refreshed the knowledge and cognition of HTL-AP functioned for pathogen control, which was speculated due to the multiple active compounds. HTL-AP showed a high potential for seed-borne fungi control, contributing to the novel renewable and suatainable fungicide agent development and new antimicroial target discovery.

Effects of biochar, granular activated carbon, and magnetite on the electron transfer of microbials during the anaerobic digestion process: Insights into nitrogen heterocyclic compounds degradation

Anaerobic digestion (AD) of the hydrothermal liquefaction aqueous phase is often inhibited by high levels of nitrogen heterocyclics, which hinder its valorization. Despite the potential benefits of conductive materials in promoting the degradation of toxic compounds through the establishment of direct interspecies electron transfer (DIET), there is still a gap in the understanding of their role and related mechanisms in nitrogen heterocyclic degradation. In this study, the AD process was improved by the addition of conductive materials. Subsequently, the cumulative methane production increased by 7.2% (with biochar), 15.4% (with granular activated carbon), and 24.3% (with magnetite). Besides, The addition of conductive materials significantly improved the complexity and stability of the microbial communities, especially the DIET potential microorganisms—Methanosaetaceae and Syntrophobacteraceae, Anaerolineaceae, Clostridiaceae, Geobacteraceae, Desulfovibrionaceae. Thermodynamic analysis and pyridine degradation pathways further supported enhanced pyridine degradation via the addition of conductive materials.

Evolving tolerance of Yarrowia lipolytica to hydrothermal liquefaction aqueous phase waste.

Hydrothermal liquefaction (HTL) is an emerging method for thermochemical conversion of wet organic waste and biomass into renewable biocrude. HTL also produces an aqueous phase (HTL-AP) side stream containing 2-4% light organic compounds that require treatment. Although anaerobic digestion (AD) of HTL-AP has shown promise, lengthy time periods were required for AD microbial communities to adapt to metabolic inhibitors in HTL-AP. An alternative for HTL-AP valorization was recently demonstrated using two engineered strains of Yarrowia lipolytica, E26 and Diploid TAL, for the overproduction of lipids and the polyketide triacetic acid lactone (TAL) respectively. These strains tolerated up to 10% HTL-AP (v/v) in defined media and up to 25% (v/v) HTL-AP in rich media. In this work, adaptive laboratory evolution (ALE) of these strains increased the bulk population tolerance for HTL-AP to up to 30% (v/v) in defined media and up to 35% (v/v) for individual isolates in rich media. The predominate organic acids within HTL-AP (acetic, butyric, and propionic) were rapidly consumed by the evolved Y. lipolytica strains. A TAL-producing isolate (strain 144-3) achieved a nearly 3-fold increase in TAL titer over the parent strain while simultaneously reducing the chemical oxygen demand (COD) of HTL-AP containing media. Fermentation with HTL-AP as the sole nutrient source demonstrated direct conversion of waste into TAL at 10% theoretical yield. Potential genetic mutations of evolved TAL production strains that could be imparting tolerance were explored. This work advances the potential of Y. lipolytica to biologically treat and simultaneously extract value from HTL wastewater. KEY POINTS: • Adaptive evolution of two Y. lipolytica strains enhanced their tolerance to waste. • Y. lipolytica reduces chemical oxygen demand in media containing waste. • Y. lipolytica can produce triacetic acid lactone directly from wastewater.

Analyses of the efficiency of a recombinant vanillyl alcohol oxidase in producing vanillin in the hydrothermal liquefaction aqueous phase

In hydrothermal liquefaction (HTL) of biomass, only the oil and the char phases are considered imperative while the aqueous phase is discarded. However, the aqueous phase is rich in phenolic compounds like guaiacol and vanillyl alcohol that can be upgraded to vanillin. Commercially vanillin is synthesized from phenolic compounds derived from fossil fuels using chemical synthesis methods. In this investigation, the method was developed to use raw materials and enzymatic conversion of substrates which is sustainable and renewable. The purpose of this investigation is to evaluate the applicability of enzymatic vanillin synthesis within the HTL aqueous phase major components. Vanillyl alcohol oxidase was expressed in E. coli BL21 (DE3) bacteria cells and purified to characterize the production of vanillin from vanillyl alcohol. The effect on the enzyme reaction of each of the major components of the aqueous phase as detected by HPLC (High Performance Liquid Chromatography) analysis was evaluated. The components evaluated in this study included different phenolic compounds namely guaiacol, vanillic acid, vanillyl alcohol, phenol, catechol and acetic acid. These components were evaluated individually as well as in different combinations. Vanillic acid increased the reaction rate while guaiacol inhibited the reaction rate. Acetic acid exhibited the largest inhibition of the enzyme reaction after 15 min. It is known that acids are often used to stop enzyme reactions as it results in precipitation of the enzyme. Therefore, to successfully convert vanillin from vanillyl alcohol using a recombinantly expressed vanillyl alcohol oxidase, the acetic acid would first need to be removed from the HTL aqueous phase.

Multi-cycle anaerobic digestion of hydrothermal liquefaction aqueous phase: Role of carbon and iron based conductive materials in inhibitory compounds degradation, microbial structure shaping, and interspecies electron transfer regulation

Anaerobic digestion is commonly performed to treat hydrothermal liquefaction aqueous phase (HTL-AP), but the conversion is limited by the inhibitory compounds. This study was conducted to investigate the effects of adding granular activated carbon, biochar, and magnetite on the digestion of HTL-AP. Moreover, the enriched aggregates and conductive materials were recovered after experiment, and then reused to study the effects of multi-cycle digestion on organics degradation, microbial activity, and methane production. Results showed that the conductive materials significantly contributed to the digestion performance and maintained high activity after recycling. The degradation of organic matters, especially the removal of toxic compounds, the enrichment of functional microbes, as well as the production of methane can be further enhanced with the reuse of conductive materials. Specifically, a 77% higher methane production rate was observed in magnetite assisted digester after recycling. More than 80% of the nitrogen organics were degraded with biochar, and all potential inhibitors were completely removed with GAC addition. The diversity and abundance of microbes were improved, and the dominant structure was established in the first batch, then the enrichment was further stabilized after the recycling. Meanwhile, the sludge conductivity was increased by 28.1 and 8.5 times in GAC and magnetite digesters in the first batch and was further enhanced by 8.0% and 16.4% after the recycling, indicating the improved expression of conductive pili and cytochromes of the microbes. These results point to the establishment of direct interspecies electron transfer. This study showed promising prospects for the utilization and recycling of conductive materials in the anaerobic digestion of HTL-AP and its long-term applications.

Adsorption or direct interspecies electron transfer? A comprehensive investigation of the role of biochar in anaerobic digestion of hydrothermal liquefaction aqueous phase

Hydrothermal liquefaction (HTL) is promising for the conversion of biowaste into biofuels, but the energy recovery from the HTL aqueous phase (HTL-AP) by anaerobic digestion is limited due to its degradability resistance. Adding biochar was reported to facilitate digestion, but its role has not been explicitly determined. Direct interspecies electron transfer (DIET) was reported to participate and dominate the digestion process; however, the adsorption and detoxification effects of biochar cannot be ignored. This study is conducted to confirm the exact role of biochar and its primary mechanism on the digestion process. Results showed that the total pore volume and adsorption capacity of biochar played the most influential role. In comparison, DIET was very likely not dominant due to the limited electrical conductivity and electron-donating/accepting capacities of biochar. The microbial analysis further indicated that mediated interspecies electron transfer remained the primary mechanism rather than DIET. The addition of facilitative biochar promoted the enrichment of Thermovirga and Methanosaeta, whereas a suppressive biochar addition shifted the dominant microbes to Asaccharospora, Clostridium, and Methanobacterium. Furthermore, a Random Forest prediction model was developed, with an accuracy of 87%, to forecast whether DIET dominantly influenced methane generation with biochar addition. This study proved that the effect of biochar on anaerobic digestion of HTL-AP relied mainly on adsorption, mediated interspecies electron transfer was more effectively enhanced rather than DIET, and a modeling approach was developed to verify the presence of DIET.

Modeling and optimization of an algal-based sewage treatment and resource recovery (STaRR) system

This paper presents modeling and optimization of an algal-based sewage treatment and resource recovery (STaRR) system composed of the following subprocesses: S,1) mixotrophic algal sewage treatment; S,2) hydrothermal liquefaction (HTL) of the resulting algal biomass; S,3) phosphorus recovery from the solid HTL char via leaching and precipitation; and S,4) nitrogen recovery from the HTL aqueous phase via a gas-permeable membrane (GPM). Specific models for the above subprocesses have been developed and validated in previous studies. Here, those models are integrated to optimize the STaRR system based on operating cost and profit to reduce wastewater treatment costs. The following operational conditions were identified as optimal for the STaRR system: 1) fed-batch cycle time of 3 d for S,1; 2) temperature of 350 °C, process time of 1 h, and biomass content of 20 wt% for S,2; 3) temperature of 60 °C, process time of 72 h, alkali concentration of 0.5 M NaOH, alkali eluent-to-char (L:S) ratio of 20 with mixing at 20 rpm to recover calcium phosphate in S,3; and 4) process time of 70 h and a membrane area:feed volume ratio of 2.32 m2/m3 for S,4. Under these optimal conditions, the STaRR system incurred 20% lower operating costs than the pre-optimization version with a different P recovery process and without N-recovery via GPM. The optimized STaRR system recovered 69.1 ± 2.4% of P in the primary effluent as calcium phosphate and 42.9 ± 1.9% of N as ammonium sulfate at an operating cost of $0.20/m3 of primary effluent.

Construct a novel anti-bacteria pool from hydrothermal liquefaction aqueous family

Hydrothermal liquefaction aqueous phase (HTL-AP) is complex and toxic, which severely hinders the scale-up of HTL technology. Distinguished from degrading organics and extracting chemical energy or nutrients from HTL-AP via biological fermentation or algae cultivation, here, we propose an innovative strategy to valorize the HTL-AP as a powerful anti-bacterial pool. Six model ingredients, i.e. lipids, cellulose, xylan, lignin, protein and the mixture were employed, to obtain a thirty-HTL-AP pool for characteristics database construction. We found that the xylan group at 230 °C on Escherichia coli (E. coli) and at 200 °C on Staphylococcus aureus (S. aureus) exhibited the highest anti-bacterial activities via plate experiments, nearly equal to 100 μg/ml streptomycin which far exceeded the working concentration of streptomycin (10–50 μg/ml). The liquid cultivation studies further revealed HTL-APs from the mixture feedstock, protein, real biomass microalgae and cornstalk had more stable anti-bacterial activities as chemically stable substances. Interestingly, the Gram-positive strain S. aureus was more susceptible than the Gram-negative E. coli on the HTL-APs, probably owing to the outer selectively permeable membrane difference and the strong reducibility and acidity of HTL-APs. This study provides a new vision to seek the anti-bacterial potential of HTL aqueous, supporting further investigations on its molecular mechanism and new bactericide development.

Hydrothermal liquefaction aqueous phase treatment and hydrogen production using electro-oxidation

Post-hydrothermal liquefaction (HTL) aqueous phase treatment is an emerging issue and must be addressed for the commercial application of the HTL process. This study investigates the simultaneous production of hydrogen and reduction of chemical oxygen demand by applying electro-oxidation using boron doped diamond electrodes. Two types of hydrothermal liquefaction aqueous phases produced from wheat straw and sewage sludge are investigated and the largest chemical oxygen demand removal (82 and 99%) and hydrogen production rates (2.0 and 1.8 NL/h) were attained at the highest current density. However, a positive energy return on investment of an integrated HTL-electro oxidation process can only be achieved at the lowest current density where the removal of organics and hydrogen production is lower. Analysis via GC–MS and GC-FID shows that complex aromatic compounds are degraded first through the formation of organic acids that are oxidized only with longer residence times and more severe current densities.

Review on hydrothermal liquefaction aqueous phase as a valuable resource for biofuels, bio-hydrogen and valuable bio-chemicals recovery

Hydrothermal liquefaction (HTL) of biomass results in the formation of bio-oil, aqueous phase (HTL-AP), bio-char, and gaseous products. Safer disposal of HTL-AP is difficult on an industrial scale since it comprises low molecular acid compounds. This review provides a comprehensive note on the recent articles published on the effective usage of HTL-AP for the recovery of valuable compounds. Thermo-chemical and biological processes are the preferred techniques for the recovery of biofuel, platform chemicals from HTL-AP. From this review, it was evident that the composition of HTL-AP and product recovery are the integrated pathways, which depend on each other. Substitute as reaction medium in HTL process, growth medium for algae and microbes are the most common mode of reuse and recycle of HTL-AP. Future research is needed to depict the mechanism of HTL process when HTL-AP is used as a reaction medium on an industrial scale. Need to find a solution for the hindrance in commercializing HTL process and recovery of value-added compounds from HTL-AP from lab scale to industry level. Integrated pathways on reuse and HTL-AP recycle helps in reduced environmental concerns and sustainable production of bio-products.

Green hydrogen from microalgal liquefaction byproducts with ammonia recovery and effluent recycle for developing circular processes

Hydrothermal liquefaction is a promising technology for microalgae-based biofuel production. However, hydrothermal liquefaction’s aqueous wastes have little established reuse, and contain significant fractions of toxic ammoniacal nitrogen. Careful reuse of this waste can assure microalgae-based biofuels are produced with less environmental impact and larger energy efficiency. Microbial electrolysis cells were investigated to valorize this waste product by converting the leftover organics into hydrogen and remove ammonia. Waste hydrothermal liquefaction aqueous phase from two microalgal strains, Tetraselmis sp. and Chlorella sp. were used as feedstocks for hydrogen production in microbial electrolysis cells. Chlorella and Tetraselmis aqueous phase-fed microbial electrolysis cells reach an average current density of 5.1 ± 0.19 A/m2 and 3.8 ± 0.08 A/m2. Compound removal rates and mass removal percentages were also investigated for each feedstock. Acetic acid, propionic acid, ethanol, and glycerol were effectively removed from the aqueous byproduct. Further, microbial electrolysis cells separated up to 34.3% of ammoniacal nitrogen present in the aqueous phase. Charge transfer analysis indicated that proton transfer, not ammonium transfer, contributed to the majority of the hydrogen production in the cathode. Finally, the microbial electrolysis cell effluent was reused to grow the same microalgal strains, leading to the development of a circular biofuel production system. Microalgae regrowth studies using microbial electrolysis cell effluent showed nearly complete removal of total organic carbon, but significantly less removal of total nitrogen. Tetraselmis sp. growth occurred with the Tetraselmis-derived MEC effluent, however, the control medium without effluent produced the most growth. These findings support the possibility of a circular biofuel framework using MECs, but additional constraints, including the removal of inorganic contaminants, are necessary to realize the circular processes.

Enhancing energy recovery via two stage co-fermentation of hydrothermal liquefaction aqueous phase and crude glycerol

Hydrothermal liquefaction (HTL) is a promising method to convert wet biomass into biocrude oil which can further be upgraded into transportation fuel. Approximately 20–40% of the total energy still remains in the aqueous phase after the HTL process. While conventional anaerobic digestion has demonstrated a limited conversion efficiency, two stage co-fermentation with crude glycerol was developed in this study to process HTL aqueous phase (HTL-AP) into hydrogen and methane, aiming to enhance biogas generation and energy recovery. Compared with single stage operation, two stage HTL-AP fermentation improved the biogas production by 25.5%. Subsequently, the addition of co-substrate crude glycerol helped relieve the acidic stress, adjusted the nutrient supply, and diluted the toxic concentration of chemicals in HTL-AP within the reactors. The biogas production was further enhanced by 1.85 times from single stage when the HTL-AP to crude glycerol ratio was 1:1. The initial pH value of the two stage operation was also controlled to optimize the metabolic pathways during the first stage of hydrogen production and to provide desirable intermediates for methanogenesis. Results showed that an initial pH of 5.5 resulted in the highest hydrogen production in this study. Accompanied with the enhanced biogas yield, the organic conversion, energy generation, and energy recovery from two stage co-fermentation were improved by 48.6%, 84.9%, and 40.1% compared to single stage fermentation, respectively. The enhanced biogas production, especially the hydrogen generation, provided a promising direction for wet biomass conversion. Specifically, downstream two stage treatment of HTL-AP could be integrated with upstream HTL by utilizing the produced hydrogen for upgrading biocrude oil via hydrocracking, and the methane could be used as a heating source for the HTL process.

Current status of biomethane production using aqueous liquid from pyrolysis and hydrothermal liquefaction of sewage sludge and similar biomass

Pyrolysis and hydrothermal liquefaction (HTL) are potential technologies for renewable energy production and waste valorization using municipal wastewater sewage sludge and other lignocellulosic biomass. However, the organic-rich aqueous pyrolysis liquid (APL) and HTL aqueous phase (HTL-AP) produced currently have no apparent use and are challenging to manage. Furthermore, the toxic organic compounds in them can be harmful to the environment. Anaerobic digestion (AD) may be a viable method to manage the liquids and recover energy in APL and HTL-AP in form of methane-rich biogas. Integrating thermochemical processes with AD could promote a circular economy by recovering resources and reducing environmental pollution. The challenge, however, is the presence of toxic compounds recalcitrant to anaerobic biodegradation such as phenols and nitrogen-containing organics that can inhibit methane-producing microbes. This review presents information on APL and HTL-AP characterization and biodegradability. Feedstock composition and process operational parameters are major factors affecting APL and HTL-AP composition, subsequent toxicity, and degradability. Feedstocks with high nitrogen content as well as increased thermochemical processing temperature and retention time result in a more toxic aqueous liquid and lower methane yield. Dilution and low AD organic loading are required to produce methane. More comprehensive APL and HTL-AP chemical characterization is needed to adopt suitable treatment strategies. Pretreatments such as overliming, air stripping, partial chemical oxidation, adsorption, and solvent extraction of toxic constituents as well as co-digestion and microbial acclimatization successfully reduce toxicity and increase methane yield.