Enrichment of bio-oil after hydrothermal liquefaction (HTL) of microalgae C. vulgaris grown in wastewater: Bio-char and post HTL wastewater utilization studies

Synthesis, delineation and technological advancements of algae biochar for sustainable remediation of the emerging pollutants from wastewater-a review

The use of algae for value-added product and biorefining applications is enchanting attention among researchers in recent years due to its remarkable photosynthetic ability, adaptability, and capacity to accumulate lipids and carbohydrates. Algae biomass, based on its low manufacturing costs, is relatively renewable, sustainable, environmentally friendly and economical in comparison with other species. High production rate of algae provides a unique opportunity for its conversion to biochar with excellent physicochemical properties, viz. high surface area and pore volume, high adsorption capacity, abundant functional groups over surface, etc. Despite several potential algal-biochar, a detailed study on its application for removal of emerging contaminants from wastewater is limited. Therefore, this technical review is being carried out to evaluate the specific elimination of inorganic and organic pollutants from wastewater, with a view to assessing adsorption performances of biochar obtained from various algae species. Species-specific adsorption of emerging pollutants from wastewater have been discussed in the present review. The promising methods like pyrolysis, gasification, dry and wet torrefaction for the production of algae biochar are highlighted. The strategies include chemical and structural modifications of algae biochar for the removal of toxic contaminants have also been considered in the current work. The overall aim of this review is to confer about the synthesis, technological advancements, delineation and application of algae biochar for the treatment of wastewater.

Application of microbial resources in biorefineries: Current trend and future prospects

The recent growing interest in sustainable and alternative sources of energy and bio-based products has driven the paradigm shift to an integrated model termed "biorefinery." Biorefinery framework implements the concepts of novel eco-technologies and eco-efficient processes for the sustainable production of energy and value-added biomolecules. The utilization of microbial resources for the production of various value-added products has been documented in the literatures. However, the appointment of these microbial resources in integrated resource management requires a better understanding of their status. The main of aim of this review is to provide an overview on the defined positioning and overall contribution of the microbial resources, i.e., algae, fungi and bacteria, for various bioprocesses and generation of multiple products from a single biorefinery. By utilizing waste material as a feedstock, biofuels can be generated by microalgae while sequestering environmental carbon and producing value added compounds as by-products. In parallel, fungal biorefineries are prolific producers of lignocellulose degrading enzymes along with pharmaceutically important novel products. Conversely, bacterial biorefineries emerge as a preferred platform for the transformation of standard cells into proficient bio-factories, developing chassis and turbo cells for enhanced target compound production. This comprehensive review is poised to offer an intricate exploration of the current trends, obstacles, and prospective pathways of microbial biorefineries, for the development of future biorefineries.

Modulating cultivation regimes of Messastrum gracile SVMIICT7 for biomass productivity integrated with resource recovery via hydrothermal liquefaction

The present study was designed to assess Messastrum gracile SVMIICT7 potential in treating dairy wastewater (autoclaved (ADWW) and raw (DWW)) with relation to nutrient removal, in-vivo Chl-a-based biomass, and bio-oil synthesis. Chlorophyll a fluorescence kinetics revealed improved photochemical efficiency (0.639, Fv/Fm) in M. gracile when grown with DWW. This may be owing to enhanced electron transport being mediated by an effective water-splitting complex at photosystem (PSII) of thylakoids. The increase in ABS/RC observed in DWW can be attributed to the elevated chlorophyll content and reduced light dissipation, as evident by higher values of ETo/RC and a decrease in non-photochemical quenching (NPQ). M. gracile inoculated in DWW had the highest Chl-a-biomass yield (1.8 g L-1) and biomolecules while maximum nutrient removal efficiency was observed in ADWW (83.7% TN and 60.07% TP). M. gracile exhibited substantial bio-oil yield of 29.6% and high calorific value of 37.19 MJ kg-1, predominantly composed of hydrocarbons along with nitrogen and oxygen cyclic compounds. This research offers a thorough investigation into wastewater treatment, illustrating the conversion of algal biomass into valuable energy sources and chemical intermediates within the framework of a biorefinery.

Hydrothermal co-liquefaction of microalgae, sugarcane bagasse, brewer's spent grain, and sludge from a paper recycling mill: Modeling and evaluation of biocrude and biochar yield

Biomass can be used as an energy source to thermochemical conversion processes to biocrude production. However, the supply and dependence on only one biomass for biocrude production can be an obstacle due to its seasonality, availability, and logistics costs. In this way, biomass waste and agroindustrial residues can be mixture and used as feedstock to the hydrothermal co-liquefaction (co-HTL) process as an alternative to obtaining biocrude. In this sense, the present paper analyzed the biocrude yield influence of the co-HTL from a quaternary unprecedented blend of different biomasses, such as sugarcane bagasse, brewer's spent grain (BSG), sludge from a paper recycling mill (PRM), and microalgae (Chlorella vulgaris). In this way, a simplex lattice design was employed and co-HTL experiments were carried out in a 2000 mL high-pressure stirred autoclave reactor under 275 °C for 60 min, considering 15% of feedstock/water ratio. Significant effects in each feedstock and their blends were analyzed aiming to increase biocrude and biochar yield. It was found that the addition of microalgae is only significant when considered more than 50% into the blend with BSG and PRM sludge to increase biocrude yield.

Advancement of Carotenogenesis of Astaxanthin from Haematococcus pluvialis: Recent Insight and Way Forward

The demand for astaxanthin has been increasing for many health applications ranging from pharmaceuticals, food, cosmetics, and aquaculture due to its bioactive properties. Haematococcus pluvialis is widely recognized as the microalgae species with the highest natural accumulation of astaxanthin, which has made it a valuable source for industrial production. Astaxanthin produced by other sources such as chemical synthesis or fermentation are often produced in the cis configuration, which has been shown to have lower bioactivity. Additionally, some sources of astaxanthin, such as shrimp, may denature or degrade when exposed to high temperatures, which can result in a loss of bioactivity. Producing natural astaxanthin through the cultivation of H. pluvialis is presently a demanding and time-consuming task, which incurs high expenses and restricts the cost-effective industrial production of this valuable substance. The production of astaxanthin occurs through two distinct pathways, namely the cytosolic mevalonate pathway and the chloroplast methylerythritol phosphate (MEP) pathway. The latest advancements in enhancing product quality and extracting techniques at a reasonable cost are emphasized in this review. The comparative of specific extraction processes of H. pluvialis biological astaxanthin production that may be applied to large-scale industries were assessed. The article covers a contemporary approach to optimizing microalgae culture for increased astaxanthin content, as well as obtaining preliminary data on the sustainability of astaxanthin production and astaxanthin marketing information.

A review on pretreatment methods for lipid extraction from microalgae biomass

Microalgal lipids are promising and sustainable sources for the production of third-generation biofuels, foods, and medicines. A high lipid yield during the extraction process in microalgae could be influenced by the suitable pretreatment and lipid extraction methods. The extraction method itself could be attributed to the economic and environmental impacts on the industry. This review summarizes the pretreatment methods including mechanical and non-mechanical techniques for cell lysis strategy before lipid extraction in microalgae biomass. The multiple strategies to achieve high lipid yields via cell disruption techniques are discussed. These strategies include mechanical (shear forces, pulse electric forces, waves, and temperature shock) and non-mechanical (chemicals, osmotic pressure, and biological) methods. At present, two techniques of the pretreatment method can be combined to increase lipid extraction from microalgae. Therefore, the extraction strategy for a large-scale application could be further strengthened to optimize lipid recovery by microalgae.

Hydrothermal liquefaction of municipal sludge and its products applications

Hydrothermal liquefaction (HTL) is an effective medium-temperature, high-pressure thermochemical process to dispose municipal sludge (MS), and biocrude (a crude bio-oil) is its main product. Many efforts are continued extensively to improve conversion efficiency and to promote industrial application of this technology. This work focuses on critical influencing factors (e.g., reaction temperature, residence time, atmosphere, solvent, catalyst, and pretreatment) and fundamental transformation mechanisms of main components (i.e., lipids, proteins, and carbohydrates) in MS HTL. It also analyzes migration behavior of heavy metals during MS HTL, which can provide a reference for subsequent recovery of nutrients from HTL products. Moreover, the applications of MS HTL products are systematically expounded, and potential challenges and opportunities are highlighted as well. It is necessary to develop advanced methods of catalyst recovery and innovative biocrude upgrading methods so as to reduce HTL investment and operating costs. Reusing aqueous phase and solid phase products as reaction medium and catalyst carrier separately after MS HTL is feasible to realize resource utilization of MS. This information can provide valuable guidance to promote MS HTL industrialization.

The magic of algae-based biochar: advantages, preparation, and applications

Compared with other biomass sources, the use of algae as a raw material to prepare biochar (BC) has important advantages including safety, high yield and economy. The protein content of algae cells is as high as 3.2 mg DCW/L, and the graphitic-N and N-O functional groups generated by the pyrolysis of proteins could effectively activate free radicals. Combined with the generated pore structure, the electron transfer/exchange capability was enhanced, which is conducive to improving its catalytic performance. Algae as a natural N source, the manuscript analyzed the surface properties and physicochemical properties of algae-based BC, and investigated its degradation effect on organic/inorganic pollutants in wastewater. Subsequently, the effect of nitrogen-doped BC on the adsorption/catalysis capacity was discussed. Finally, the directed preparation of algae-based BC applied in different scenarios was summarized. Algae-based BC has the property of N doping, which broadens its application efficiency in the environmental field. Overall, this manuscript reviews how to achieve efficient utilization of algae-based BC in wastewater.

Biochar production from microalgae: a new sustainable approach to wastewater treatment based on a circular economy

The generation of wastewater due to human activities are the main responsible for environmental problems. These problems are caused by the large amount of organic and inorganic pollutants related to the presence of pesticides, metals, pathogens, drugs and dyes. The photosynthetic treatment of effluents emerges as a sustainable and low-cost alternative for developing wastewater treatment systems based on a circular economy. Chemical compounds present in wastewater can be recovered and reused as a source of nutrients in microalgae cultivation to produce value-added bioproducts. The microalgal biomass produced in the cultivation with effluents has the potential to produce biochar. Biochar is carbon-rich charcoal that can be obtained by converting microalgae biomass through thermal decomposition of organic raw material under limited oxygen supply conditions. Pyrolysis, torrefaction, and hydrothermal carbonization are processes used for biochar synthesis. The application of microalgal biochar as an adsorbent material to remove several compounds present in effluents is an effective and fast treatment. This effectiveness is usually related to the unique physicochemical characteristics of the biochar, such as the presence of functional groups, ion exchange capacity, thermal stability, and high surface area, volume, and pore area. In addition, biochar can be reused in the adsorption process or applied in agriculture for soil correction. In this context, this review article describes the production, characterization, and use of microalgae biochar through a sustainable approach to wastewater treatment, emphasizing its potential in the circular economy. In addition, the article approaches the potential of microalgal biochar as an adsorbent material and its reuse after the adsorption of contaminants, as well as highlights the challenges and future perspectives on this topic.

Heavy metal tolerance in microalgae: Detoxification mechanisms and applications

The proficiency of microalgae to resist heavy metals has potential to be beneficial in resolving various environmental challenges. Global situations such as the need for cost-effective and ecological ways of remediation of contaminated water and for the development of bioenergy sources could employ microalgae. In a medium with the presence of heavy metals, microalgae utilize different mechanisms to uptake the metal and further detoxify it. Biosorption and the next process of bioaccumulation are two such major steps and they also include the assistance of different transporters at different stages of heavy metal tolerance. This capability has also proved to be efficient in eradicating many heavy metals like Chromium, Copper, Lead, Arsenic, Mercury, Nickel and Cadmium from the environment they are present in. This indicates the possibility of the application of microalgae as a biological way of remediating contaminated water. Heavy metal resistance quality also allows various microalgal species to contribute in the generation of biofuels like biodiesel and biohydrogen. Many research works have also explored the capacity of microalgae in nanotechnology for the formation of nanoparticles due to its relevant characteristics. Various studies have also revealed that biochar deduced from microalgae or a combination of biochar and microalgae can have wide applications specially in deprivation of heavy metals from an environment. This review focuses on the strategies adopted by microalgae, various transporters involved in the process of tolerating heavy metals and the applications where microalgae can participate owing to its ability to resist metals.

Microalgae-Based Biotechnology as Alternative Biofertilizers for Soil Enhancement and Carbon Footprint Reduction: Advantages and Implications

Due to the constant growth of the human population and anthropological activity, it has become necessary to use sustainable and affordable technologies that satisfy the current and future demand for agricultural products. Since the nutrients available to plants in the soil are limited and the need to increase the yields of the crops is desirable, the use of chemical (inorganic or NPK) fertilizers has been widespread over the last decades, causing a nutrient shortage due to their misuse and exploitation, and because of the uncontrolled use of these products, there has been a latent environmental and health problem globally. For this reason, green biotechnology based on the use of microalgae biomass is proposed as a sustainable alternative for development and use as soil improvers for crop cultivation and phytoremediation. This review explores the long-term risks of using chemical fertilizers for both human health (cancer and hypoxia) and the environment (eutrophication and erosion), as well as the potential of microalgae biomass to substitute current fertilizer using different treatments on the biomass and their application methods for the implementation on the soil; additionally, the biomass can be a source of carbon mitigation and wastewater treatment in agro-industrial processes.

Renewable adsorbents from the solid residue of sewage sludge hydrothermal liquefaction for wastewater treatment

Solid residue from hydrothermal liquefaction (HTL) of nutrient rich feedstock presents a promising source to recover valuable nutrients, such as phosphorus, in the solid form. The present work shows for the first time the potential of utilizing the waste residue remaining after nutrients extraction from HTL of sewage sludge, as renewable adsorbents. A parametric study was undertaken to investigate the influence of chemical activation conditions (temperature, residence time, activation agent loading, washing after activation) on raw and partially demineralized HTL solids. Kinetic and equilibrium adsorption investigation was undertaken for the removal of methylene blue (MB) from aqueous solution. For comparison purposes, a commercial activated charcoal (AC) was used. Demineralization was found to have a significant influence in the adsorption capacity of the resultant adsorbents. Three adsorbents were found to follow the Langmuir adsorption model, while the acid washed demineralized adsorbent had higher adsorption capacity than AC and was found to follow the Freundlich adsorption model. The superior performance of the acid washed demineralized adsorbent was verified from the kinetic study where all adsorbents were found to best fit the pseudo-second order model. Adsorption capacities for MB at equilibrium were 367.1, 332.3, 297.4 and 87.6 mg/g, for acid washed demineralized adsorbent, AC, demineralized adsorbent, and raw adsorbent, respectively. Finally, the most promising adsorbents were assessed for their adsorption capacity to remove pharmaceuticals present in a real wastewater treatment effluent. Results indicated ultimate concentration for all targeted compounds below the detection limits for acid washed demineralized adsorbent, AC and demineralized adsorbent. Future implementation of HTL technology in wastewater treatment facilities, will not only provide an efficient way to valorize sewage sludge into bio-crude and nutrients, but can also enhance technology integration by providing the precursors for renewable adsorbents needed in tertiary treatment of wastewater.

Extraction methodology of lignin from biomass waste influences the quality of bio-oil obtained by solvothermal depolymerization process

Lignin from sugarcane bagasse was extracted using three different methods such as Alkaline, Ethanosolv, and Hydrotropic extraction and the effect of each method on yield and quality of bio-oil obtained when the lignin was depolymerized through solvothermal liquefaction was studied using ethanol as solvent. The maximum lignin yield was obtained in the hydrotropic extraction method when Sodium Xylene Sulfonate was used as the hydrotropic solvent at a concentration of 1.43 M and a temperature of 90 °C. Hydrothermal experiments were performed at temperature of 250 °C with a residence time of 30 min and lignin to ethanol ratio of 1:200 g/mL respectively. Among the methods used, the Ethanosolv lignin showed the highest extent of depolymerization (86.7%) to yield bio-oil at 250 °C with reduced biochar formation at lignin to solvent ratio of 1:200. Biochar obtained was used in adsorption studies of Cadmium (Cd), Lead (Pb), Nickel (Ni), and Zinc (Zn) and results showed that more than 85% removal of all the metals under lower concentration levels.

Microalgal-based biochar in wastewater remediation: Its synthesis, characterization and applications

Microalgae are drawing attentions among researchers for their biorefinery use or value-added products. The high production rate of biomasses produced are attractive for conversion into volatile biochar. Torrefaction, pyrolysis and hydrothermal carbonization are the recommended thermochemical conversion techniques that could produce microalgal-based biochar with desirable physiochemical properties such as high surface area and pore volume, abundant surface functional groups, as well as functionality such as high adsorption capacity. The characterizations of the biochar significantly influence the mechanisms in adsorption of pollutants from wastewaters. Specific adsorption of the organic and inorganic pollutants from the effluent are reviewed to examine the adsorption capacity and efficiency of biochar derived from different microalgae species. Last but not least, future remarks over the challenges and improvements are discussed accordingly. Overall, this review would discuss the synthesis, characterization and application of the microalgal-based biochar in wastewater.

An Eco-friendly and Economical Approach for Removal of Remazol Blue, Malachite Green and Rhodamine B Dyes from Wastewater using Bio-char Derived from Chlorella Vulgaris Biomass

Background::

Rapid urbanization and industrialization has led to depletion of water resource and generation of enormous amount of wastewater. One among them is textile industry which discharge huge amount of dye wastewater into aquatic environment.

Methods::

This study deals with adsorption of Remazol blue, Malachite green and Rhodamine B dyes into bio-char derived from Chlorella vulgaris biomass cultivated from municipal wastewater. Column studies were performed to depict the industrial usage of bio-char for treatment of large quantity of wastewater. Effect of temperature, time, pH, dye concentration and adsorbent dosage on dye removal was studied in batch process.

Results::

The best batch adsorption conditions are temperature (25⁰C), time (60min), pH (7), dye concentration (100ppm) and adsorbent dosage (1g) with ± 5% for all three dyes. Dye removal percentage of bio-char increased with increased in adsorbent dosage to 94.5%, 88.2% and 90.1% for Remazol blue, Malachite green and Rhodamine B dyes at 1g/L adsorbent dosage. Freundlich isotherm exhibited correlation coefficient (R2) values of 0.99, 0.98 and 0.99 for Remazol blue, Malachite green and Rhodamine B dyes respectively. Kinetic studies revealed that all three dyes followed pseudo first order model. Increase in column bed height resulted in increased dye removal percentage since increase in bed height resulted in increase in bio-char quantity with more number of surface area.

Conclusion::

From the study, it can be concluded that Bio-char was the economical and eco-friendly alternative adsorbent for wastewater treatment process. Bio-char reusability study revealed that it could be used for 3-4 consecutive cycles.

Biomass as source for hydrochar and biochar production to recover phosphates from wastewater: A review on challenges, commercialization, and future perspectives

Excessive phosphate run-off with total phosphorus concentration greater than 20 μg P L^-1 triggers the growth of harmful algal species in waterbodies and potentially leads to eutrophication. This has severe negative implications on aquatic environment and impacts human health. The annual economic impact of harmful algal blooms is reported to be as high as $25 million for public health and commercial fishery sector, $29 million for recreation/tourism sector and $2 million for monitoring and management. Adsorption is widely considered as an effective and economic strategy to achieve extremely low concentration of phosphorus. The char produced by valorizing various waste biomasses have been gaining attention in phosphorus remediation owing to their availability, their ability to regenerate and reuse. This review paper exclusively focuses on utilizing hydrochar and biochar synthesized from waste biomass, respectively, through hydrothermal carbonization and slow pyrolysis to mitigate phosphorus concentration and potential strategies for handling the spent char. The key mechanisms involved in phosphate adsorption are electrostatic interaction, ion exchange and complexation. The maximum adsorption capacity of hydrochar and biochar ranges from 14-386 mg g^-1 and 3-887 mg g^-1, respectively. Hydrochar and biochar are cost-effective alternative to commercial activated carbon and spent char can be used for multiple adsorption cycles. Furthermore, extensive research studies on optimizing the feedstock, reaction and activation conditions coupled with technoeconomic analysis and life cycle assessment could pave way for commercialization of char-based adsorption technology.

Progress and challenges of contaminate removal from wastewater using microalgae biomass

The utilization of microalgae in treating wastewater has been an emerging topic focussed on finding an economically sustainable and environmentally friendly approach to treating wastewater. Over the last several years, different types of con microalgae and bacteria consortia have been experimented with to explore their potential in effectively treating wastewater from different sources. The basic features considered while determining efficiency is their capacity to remove nutrients including nitrogen (N) and phosphorus (P) and heavy metals like arsenic (As), lead (Pb), and copper (Cu). This paper reviews the efficiency of microalgae as an approach to treating wastewater from different sources and compares conventional and microalgae-based treatment systems. The paper also discusses the characteristics of wastewater, conventional methods of wastewater treatment that have been used so far, and the technological mechanisms for removing nutrients and heavy metals from contaminated water. Microalgae can successfully eliminate the suspended nutrients and have been reported to successfully remove N, P, and heavy metals by up to 99.6 %, 100 %, and 13%-100 % from different types of wastewater. However, although a microalgae-based wastewater treatment system offers some benefits, it also presents some challenges as outlined in the last section of this paper. Performance in eliminating nutrients from wastewater is affected by different parameters such as temperature, biomass productivity, osmotic ability, pH, O₂ concentration. Therefore, the conducting of pilot-scale studies and exploration of the complexities of contaminants under complex environmental conditions is recommended.

Enhanced production of hydrocarbons from lignin isolated from sugarcane bagasse using formic acid induced supercritical ethanol liquefaction followed by hydrodeoxygenation

This study involves the production of hydrocarbons from lignin extracted from sugarcane bagasse using Hydrothermal Liquefaction (HTL) followed by Hydrodeoxygenation (HDO). HTL of the lignin was studied under different solvents-methanol, ethanol and isopropanol in the presence of formic acid as an effective H-donor under varying lignin to solvent ratios (L:S = 1:15,1:30,1:40 g/mL), reaction temperatures (200 °C - 320 °C), reaction times (15, 30, 45,60 min) and ZnCl₂ catalyst concentrations (30, 40, 50, 60 wt%). A maximum of 86% lignin derived phenolics was obtained when ethanol was used as solvent at 250 °C under L:S = 1:30 at 30 min reaction time with 60 wt% ZnCl₂. The lignin-oil was upgraded by HDO process in the presence of Ni/Al₂O₃ catalyst and a maximum hydrocarbon yield of 73.5% was obtained with a HHV value of 48 MJ/kg. The hydrocarbons had excellent properties with a carbon range of C₆-C₁₂ with a purity of 51.2%.

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.

Insights into the potential impact of algae-mediated wastewater beneficiation for the circular bioeconomy: A global perspective

Algae-based technologies are one of the emerging solutions to societal issues such as accessibility to clean water and carbon-neutral energy and are a contender for the circular bioeconomy. In this review, recent developments in the use of different algal species for nutrient recovery and biomass production in wastewater, challenges, and future perspectives have been addressed. The ratio and bioavailability of nutrients in wastewater are vital parameters, which significantly impact nutrient recovery efficiency and algal biomass production. However, the optimum nutrient concentration and ratio may vary depending upon the microalgal species as well as cultivation conditions. The use of indigenous algae and algae-based consortia with other microorganisms has been proved promising in improving nutrient recovery efficiency and biomass production in pilot scale operations. However, environmental and cultivation conditions also play a significant role in determining the feasibility of the process. This review further focused on the assessment of the potential benefits of algal biomass production, renewable biofuel generation, and CO₂ sequestration using wastewater in different countries on the basis of available data on wastewater generation and estimated nutrient contents. It was estimated that 5-10% replacement of fossil crude requirement with algal biofuels would require ~952-1903 billion m³ of water, 10-21 billion tons of nitrogen, and 2-4 billion tons of phosphorus fertilizers. In this context, coupling wastewater treatment and algal biomass production seem to be the most sustainable option with potential global benefits of polishing wastewater through nutrients recycling and carbon dioxide sequestration.

Hydrothermal Liquefaction of Biomass as One of the Most Promising Alternatives for the Synthesis of Advanced Liquid Biofuels: A Review

The use of biofuels offers advantages over existing fuels because they come from renewable sources, they are biodegradable, their storage and transport are safer, and their emissions into the atmosphere are lower. Biomass is one of the most promising sustainable energy sources with a wide variety of organic materials as raw material. Chemical, biochemical, and thermochemical methods have been proposed to obtain biofuels from raw materials from biomass. In recent years, a thermochemical method that has generated great interest is hydrothermal liquefaction. In this paper, a brief review of the main sources for liquid biofuels and the synthesis processes is presented, with special emphasis on the production of biofuels using hydrothermal liquefaction by using waste generated by human activity as raw material.

Thermophilic biogas production from microalgae-bacteria aggregates: biogas yield, community variation and energy balance

Biogas production through anaerobic mesophilic digestion is the most straightforward biofuel production route integrated into microalgae-bacteria wastewater treatment plants. Improvement of this biofuel route without adding pretreatment units is possible through the temperature increase. This paper presents a comprehensive evaluation of the transitory effect of different temperatures (35 °C and 55 °C) and hydraulic retention times (HRT) of 15 and 30 d on the long-term methane production using non-pretreated microalgae-bacteria aggregates as a feedstock. The thermophilic transition from mesophilic inoculum adapted to microalgae-bacteria aggregate increased 1.7-fold the methane production (0.41 m³CH₄ kgVS^-1) at HRT of 30 d. A substantial decrease in the microbial community's diversity present in the anaerobic reactor was observed when thermophilic conditions were applied, explaining the long adaptation period needed. The increase of the operative temperature condition promotes changes in the dominance pathway of methanogenesis from hydrogenotrophic to acetolactic. The energy balance assessment showed a positive net energy ratio when the digester was operated at an HRT of 30 d. A maximum net energy ratio of 1.5 was achieved at mesophilic temperature. This study demonstrated, based on experimental data, that microalgal digestion with an HRT of 30 d favors energy self-sustainability in microalgal wastewater treatment plants.

Characterization and Evaluation of Hydrothermal Liquefaction Char from Alkali Lignin in Subcritical Temperatures

An evaluation of hydrothermal liquefaction (HTL) char is investigated in this work. Morphological studies, N₂ adsorption behavior, FTIR analysis, thermal behavior, and elemental composition are studied. The HTL char yield showed an increase with higher operating temperatures. It increased from 11.02% to 33% when the temperature increased from 573 K to 623 K. At lower temperatures, the residence time showed an impact on the yield, while close to the critical point, residence time became less impactful. Elemental analysis showed that both higher operating temperatures and longer residence times increased the nitrogen content of the chars from 0.32% to 0.51%. FTIR analysis suggested the char became more aromatic with the higher temperatures. The aliphatic groups present diminished drastically with the increasing temperature. Residence time did not show a significant impact as much as the temperature when considering the functional group elimination. An increase in operating temperatures and residence times produced thermally stable chars. HTL char produced at the lowest operating temperature and showed both the highest surface area and pore volume. When temperature and residence time increase, more polyaromatic char is produced due to carbonization.

Technical insights into the production of green fuel from CO2 sequestered algal biomass: A conceptual review on green energy

Algae a promising energy reserve due to its adaptability, cheap source, sustainability and it's growth ability in wastewater with efficient sequestration of industrial carbon dioxide. This review summarizes the pathways available for biofuel production from carbon sequestered algae biomass. In this regard, this review focuses on microalgae and its cultivation in wastewater with CO₂ sequestration. Conversion of carbon sequestered biomass into bio-fuels via thermo-chemical routes and its engine emission properties. Energy perspective of green gaseous biofuels in near future. This review revealed that algae was the pre-dominant CO₂ sequester than terrestrial plants in an eco-friendly and economical way with simultaneous wastewater remediation. Hydrothermal liquefaction of algae biomass was the most preferred mode for biofuel generation than pyrolysis due to high moisture content. The algae based fuels exhibit less greenhouse gases emission and higher energy value. This review helps the researchers, environmentalists and industrialists to evaluate the impact of algae based bio-energy towards green energy and environment.

Integrating micro-algae into wastewater treatment: A review

Improving the ecological status of water sources is a growing focus for many developed and developing nations, in particular with reducing nitrogen and phosphorus in wastewater effluent. In recent years, mixotrophic micro-algae have received increased interest in implementing them as part of wastewater treatment. This is based on their ability to utilise organic and inorganic carbon, as well as inorganic nitrogen (N) and phosphorous (P) in wastewater for their growth, with the desired results of a reduction in the concentration of these substances in the water. The aim of this review is to provide a critical account of micro-algae as an important step in wastewater treatment for enhancing the reduction of N, P and the chemical oxygen demand (COD) in wastewater, whilst utilising a fraction of the energy demand of conventional biological treatment systems. Here, we begin with an overview of the various steps in the treatment process, followed by a review of the cellular and metabolic mechanisms that micro-algae use to reduce N, P and COD of wastewater with identification of when the process may potentially be most effective. We also describe the various abiotic and biotic factors influencing micro-algae wastewater treatment, together with a review of bioreactor configuration and design. Furthermore, a detailed overview is provided of the current state-of-the-art in the use of micro-algae in wastewater treatment.

Hydrothermal liquefaction of Prosopis juliflora biomass for the production of ferulic acid and bio-oil

The objective of this work was to study the hydrothermal liquefaction (HTL) of Prosopis juliflora biomass for the production of ferulic acid and bio-oil. Biomass was processed with various solvents (NaOH, KOH, HCl and H₂SO₄) to produce ferulic acid (FA). FA oxidation was carried out using the Nano ZnO catalyst to produce an optimum vanillin yield of 0.3 g at 70 °C with 0.4% catalyst loading for a time of 60 min. The spent solid residue was then processed using HTL at 5 MPa pressure and a temperature range of 240-340 °C. Various biomass loading (2.5 g to 12.5 g) was taken for a fixed water content of 200 mL. Bio-oil optimum yield was 22.5 wt% for 10 g/200 mL of biomass loading ratio. The optimum temperature was 300 °C for a processing time of 1 h. The catalyst showed the reusable capability of two three consecutive cycles.

Review on sustainable production of biochar through hydrothermal liquefaction: Physico-chemical properties and applications

This review examines in detail the production and characteristics of biochar resulting from hydrothermal liquefaction. Specifically, the impact of feedstocks and different process parameters on the properties and yield of biochar by hydrothermal liquefaction has been thoroughly studied. Hydrothermal liquefaction derived biochars, relative to biochars from high-temperature thermochemical processes retain critical functional groups during carbonization and are therefore promising for a wide range of applications. Most of the review's efforts are to study possible hydrothermal liquefaction biochar applications in various fields, including fuel, metal and dye adsorption, pollutant reduction, animal feed, and biogas catalyst. The feasibility of biochar through the hydrothermal liquefaction process has been analysed via life-cycle assessment and energy evaluation. The article concludes with a brief discussion on possible issues and strategies for the sustainable development of hydrothermal liquefaction-based biochar.

Modification of NaY zeolite by lanthanum and hexadecyl trimethyl ammonium bromide and its removal performance for nitrate

Nitrate in the effluent of wastewater treatment plants (WWTPs) is the main nitrogen resource in natural water. The excessive nitrogen in natural water causes ecological issues such as aqueous eutrophication. A novel modified NaY zeolite (SMZ-La) with hexadecyl trimethyl ammonium bromide (HDTMA) and lanthanum (La) as modifying agents for NO 3 - -N adsorption was investigated in this study. Results showed that SMZ-La had a higher adsorption capacity (3.82 mg NO 3 - -N/g) than zeolite only modified with HDTMA or La (2.75 and 2.23 mg NO 3 - -N/g, respectively). Moreover, the adsorption process was endothermic with a maximum theoretic adsorption of 14.49 mg NO 3 - -N/g. X-ray photoelectron spectroscopy (XPS) analysis indicated that adsorption rate principally depended on chemisorption between SMZ and NO 3 - -N. Thermogravimetric analysis showed that HDTMA was loaded on the surface of NaY zeolite with double layer. Scanning electron microscope and X-ray spectroscopy analysis illustrated that La was primarily loaded in the pore of NaY zeolite, and the loading of HDTMA and La did not affect the original crystal structure of NaY zeolite. The novel adsorbent provided a promising perspective for nitrogen control in WWTPs and natural water. PRACTITIONER POINTS: A novel modified zeolite (SMZ-La) was prepared successfully with HDTMA and La. SMZ-La had an excellent adsorption capacity compared to SMZ and NaY-La. There were both physical and chemical adsorptions in the adsorption process of SMZ-La on NO 3 - -N.

Energy recovery and nutrients recycling from municipal sewage sludge

Hydrothermal Liquefaction (HTL) could be a promising and better alternative to other techniques for energy recovery from municipal sewage sludge (MSS). However, the nutrients (i.e., N, and P) recovery potential from the byproducts, generated in the HTL of MSS, needs to be studied so that a comprehensive sludge management practice could be adopted. In this study, HTL process temperature (275-400 °C), and reaction time (30-120 min) were first investigated for biocrude yield and release of the nutrients to the aqueous phase liquid (APL) and biochar. The maximum energy recovery (i.e., 59%) and maximum energy return on investment (i.e., 3.5) were obtained at 350 °C and 60 min of holding time. With the increase in HTL reaction time, the concentration of nitrogen in the APL increased (5.1 to 6.8 mg/L) while the concentration of phosphorus decreased (0.89 to 0.22 mg/L); the opposite was observed for the biochar. The nutrient recycling efficiency from the APL using microalgae was found to be strain-specific; nitrogen recycling efficiency by Picochlorum sp. and Chlorella sp. were 95.4 and 58.6%, respectively. The APL, derived from 1 kg MSS, could potentially produce 0.49 kg microalgal biomass. Since the concentrations of various metals in the biochar samples were substantially lower compared to their concentrations in raw MSS, the application of biochar as a soil conditioner could be very promising. Overall, net positive energy could be recovered from MSS using the HTL process, while the nutrients in the APL could be used to cultivate specific microalgae, and biochar could be applied to enhance the soil quality.

Catalytic pyrolysis and liquefaction behavior of microalgae for bio-oil production

Microalgae bio-oil production is related to the sustainable use of world energy in the future. In the present work, catalytic pyrolysis and liquefaction behavior of microalgae for bio-oil production were investigated. The results show that the rare earth compounds as catalysts contributed to significantly accelerating the pyrolysis of microalgae via reducing the activation energy of pyrolysis process. Ce(II)/HZSM-5 presented the optimal catalytic pyrolysis and liquefaction effects by helping cut the microalgae molecule chains. The maximum bio-oil yield amounted to 49.71 wt% at the catalyst concentration of 5 wt%. The chemical components of the Spirulina bio-oil were composed of carboxylic acids, ketones, olefins, amides, ethers, esters, and partially cyclic N-containing compounds. Although the combustion performances of the Spirulina bio-oil are worse than those of the diesel fuel, it is superior to the reported rice husk bio-oil, suggesting a promising potential application prospect.

Characterization of biofuel production from hydrothermal treatment of hyperaccumulator waste (Pteris vittata L.) in sub- and supercritical water

In this study, hyperaccumulator waste, i.e., Pteris vittata L. was converted into bio-oil, biogas and biochar via sub- and supercritical hydrothermal liquefaction processes. These products were characterized in terms of EI/MS, FTIR, TGA and GC to understand their chemical composition, thermal decomposition, structural properties and high biofuel reactivity. Characterization results revealed that the dominant chemical components in the heavy bio-oil were esters (40.22%), phenols (20.02%), alcohols (10.16%), organic acids (9.07%), nitrogenous compounds (8.83%) and ketones/aldehydes (6.42%), while the light oil was rich with a higher fraction of phenols (54.13%) and nitrogenous compounds (27.04%). Particularly, bio-oils obtained from supercritical conditions contained increased phenolic compounds and reduced oxygenated chemicals such as alcohols, aliphatic acid, ketones and aldehydes, suggesting the improved quality of bio-oil due to the reduction in oxygen contents. Meanwhile, H₂-rich syngas production with the H₂ yield of 38.87% was obtained at 535 °C for 20 min, and higher reaction temperature presented a positive influence on H₂ production during Pteris vittata L. liquefaction. Moreover, the remaining biochar product was analyzed to determine whether it could be used as a direct solid fuel or auxiliary fuel. This study provided full exploitation of this feedstock waste in energy and valuable chemical complexes.

This study evaluated the utilization of HTL to handle hyperaccumulator waste, i.e., Pteris vittata L. in both sub and supercritical water for the production of biofuels as a partial substitute for fossil fuels and valuable chemicals.

Microalgal bioenergy production under zero-waste biorefinery approach: Recent advances and future perspectives

In view of the globalization and energy consumption, an economic and sustainable biorefinery model is essential to address the energy security and climate change. From this perspective, renewable biofuel production from microalgae along with a wide range of value-added co-products define its potential as a biorefinery feedstock. However, economic viability of microalgal biorefinery at its current state is not considered sustainable. Reduce, recycle, and reuse of waste derived from algal bioenergy conversion process will lead to an energy efficient and sustainable zero-waste microalgal biorefinery. This review focuses on three major aspects of zero-waste microalgal biorefinery approach; (1) recent advances on microalgal bioenergy conversion processes (chemical, biochemical and thermochemical); (2) mitigation and transformation of liquid and solid waste and (3) techno-economic analysis (TEA) and lifecycle assessment (LCA). In addition, the study also focuses on the challenges and future perspectives for an advanced microalgal biorefinery model.

Low-temperature catalyst based Hydrothermal liquefaction of harmful Macroalgal blooms, and aqueous phase nutrient recycling by microalgae

The present study investigates the hydrothermal liquefaction (HTL) of harmful green macroalgal blooms at a temperature of 270 °C with, and without a catalyst with a holding time of 45 min. The effect of different catalysts on the HTL product yield was also studied. Two separation methods were used for recovering the biocrude oil yield from the solid phase. On comparision with other catalyst, Na₂CO₃ was found to produce higher yiled of bio-oil. The total bio-oil yield was 20.10% with Na₂CO₃, 18.74% with TiO₂, 17.37% with CaO, and 14.6% without a catalyst. The aqueous phase was analyzed for TOC, COD, TN, and TP to determine the nutrient enrichment of water phase for microalgae cultivation. Growth of four microalgae strains viz., Chlorella Minutissima, Chlorella sorokiniana UUIND6, Chlorella singularis UUIND5 and Scenedesmus abundans in the aqueous phase were studied, and compared with a standard growth medium. The results indicate that harmful macroalgal blooms are a suitable feedstock for HTL, and its aqueous phase offers a promising nutrient source for microalgae.

Hydrothermal liquefaction of Scenedesmus abundans biomass spent for sorption of petroleum residues from wastewater and studies on recycling of post hydrothermal liquefaction wastewater

In this study Scenedesmus abundans was used as a biosorbent material for removing hydrocarbons from simulated petroleum wastewater. Batch experiments resulted in the removal of 92.16% of hydrocarbons from simulated wastewater within 60 min. The spent biosorbent was converted to bio-oil through hydrothermal liquefaction process (HTL) at temperature range from 220 to 320 °C with 1 h holding time. Liquid hydrocarbons (bio-oil) yield was 43.4 wt% at 300 °C with 15 g of spent sorbent loading and possessed HHV of 39.10 MJ/Kg. Additionally the HTL wastewater (aqueous phase) was recycled as reaction medium and studied for its effects on bio-oil yield which increased till second cycle (47.91 wt%). HTL bio-char was employed as adsorbent to remove heavy metals from wastewater. It showed greater removal efficiency of 86.5% to Ni(II) ions. From the results it was concluded that the petroleum residues can be effectively recycled back into liquid hydrocarbons with simple waste management pathway.

Co-thermal liquefaction of Prosopis juliflora biomass with paint sludge for liquid hydrocarbons production

In this study, Prosopis juliflora biomass was co-liquefied with hydrocarbons rich paint waste for next generation fuel (bio-oil) production. Co-liquefaction (HTL) was performed at varying biomass to paint waste ratios (1:0, 0:1, 1:1, 2:1 and 1:2) at different temperatures from 340 to 440 °C for a holding time of 60 min. Bentonite catalyst was added from 1 to 5% wt. to the HTL reactor. Gas Chromatography-Mass Spectroscopy (GC-MS) and Fourier Transform Infrared Spectroscopy (FTIR) analysis were carried out for bio-oil and HTL aqueous phase. Maximum bio-oil yield was around 49.26% wt. at 420 °C, 2:1 blend and 4% wt. of bentonite catalyst. Energy and carbon recovery of bio-oil was around 70% and 96% respectively. Additionally, HTL aqueous phase was analysed and it showed presence of acids molecules in it. The gas from HTL process contained Carbon dioxide (46.25%), Carbon monoxide (6.38%), Methane (9.35%) and hydrogen (24.53%).

Co-liquefaction of Prosopis juliflora with polyolefin waste for production of high grade liquid hydrocarbons

In this study, co-liquefaction (HTL) of Prosopis juliflora (PJ) biomass with polyolefin waste (PO) was performed to produce bio-oil. HTL on bio-oil yield was studied at varying PJ to PO ratios (0:1, 1:0, 1:1, 2:1, 3:1, 4:1 and 5:1) and temperatures from 340 to 440 °C. Bio-oil and HTL by-products were characterized by Mass Spectroscopy (GC-MS) and Fourier Transform Infrared Spectroscopy (FTIR) analysis. Bio-oil yield was around 61.23%wt at 420 °C for 3:1 blends with 3 wt% of HCl activated bentonite catalyst at 60 min holding time. HHV value was 46 MJ/Kg with 88.23% purity (petro-diesel). Additionally gas possessed 26.28% of Hydrogen gas, 45.59% of Carbon dioxide gas, 7.1% of Carbon monoxide gas, 8.12% of Methane gas and other elements. The energy recovery (78%) and carbon recovery (94%) was higher for 3:1 blends bio-oil than PO and PJ processed bio-oils. HTL wastewater possessed higher degree of reusability nature as HTL medium.

Nutrient recovery from wastewaters by microalgae and its potential application as bio-char

The intensive agricultural practices are increasing the demand for chemical fertilizers, being currently produced from a non-environmental friendly way. Besides the environmental impacts, the nutrient uptake efficiency by the crops is very low, representing huge losses into the fields. Therefore, it is crucial to study alternatives for the current chemical fertilizers, which simultaneous improve nutrient efficiency and minimize environmental impacts. A sustainable solution is to recover nutrients from wastewater streams with microalgal cultures and the biomass conversion into bio-char for soil amendment. Wastewaters are loaded with nitrogen and phosphorus and can be used as culture medium for microalgae. Thus, nutrients can be recycled, reducing the requirement of chemical fertilizers. This paper aims to review nutrient recovery from wastewater using microalgae and the biomass conversion into bio-char. This process promotes nutrient recycling and the bio-char (when added to soil) improves the nutrient uptake efficiency by crops.