Proposal for a Conceptual Biorefinery for the Conversion of Waste into Biocrude, H₂ and Electricity Based on Hydrothermal Co-Liquefaction and Bioelectrochemical Systems

Abstract

Microalgal biomass contributes to the valorization of urban and agro-industrial solid waste via hydrothermal co-liquefaction (co-HTL) for the production of biocrude, a sustainable substitute for petroleum. Tropical and populous countries like Brazil generate a lot of agro-industrial waste, such as sugarcane bagasse and malt bagasse, as well as sludge from sewage treatment plants. Such residues are potential sources of biocrude production via thermochemical conversion. To increase biocrude productivity, microalgal biomass has been successfully used in mixing the co-HTL process feed with different residues. In addition to biocrude, co-HTL generates an aqueous phase that can be used to produce H₂ and/or electricity via microbial energy cells. In this sense, this paper aims to present the potential for generating energy from solid waste commonly generated in emerging countries such as Brazil based on a simplified scheme of a conceptual biorefinery employing algal biomass co-HTL together with sugarcane bagasse, malt bagasse, and sludge. The biorefinery model could be integrated into an ethanol production plant, a brewery, or a sewage treatment plant, aiming at the production of biocrude and H₂ and/or electricity by bioelectrochemical systems, such as microbial electrolysis cells and microbial fuel cells.

1. Introduction

In recent years, emerging economies have often been hailed as the “engine of world economic growth”, attracting the interest of many researchers. Nevertheless, the discharge of carbon dioxide (CO₂) in these economies has become an urgent issue to be solved, since air pollution is more associated with the growing trend instead of the level of development of a country [1,2]. This is the case in Brazil, where the emissions correspond to a fast-growing economy, while European countries and the United States of America (USA) have been able to slow down their carbon emissions while growing their economies. Therefore, with more mature technologies and renewable energy, emerging economies can keep growing without compromising the environment and public health [2].

Biomass valorization is an important renewable that could significantly reduce the global dependence on fossil fuels and contribute to sustainability. There is a range of different types of biomasses that could be used in biorefineries to produce biofuels, such as agricultural residues, forestry wastes, grasses, and energy crops [3]. Also, multiple routes can process biomass such as pyrolysis, gasification, combustion, and hydrothermal liquefaction (HTL) [4,5].This flexibility has attracted the attention of many researchers because it shows the possibility of implementing these technologies in different scenarios, regarding the country and culture or biomass availability. Therefore, microalgal biomass, sewage sludge, and different kinds of agro-industrial wastes, such as malt bagasse and sugarcane bagasse, are available in Brazil and have the potential to produce biofuel. To mix these different biomasses, the hydrothermal co-liquefaction (co-HTL) process is a route that can be taken to produce biocrude or bio-oil, which demands a high pressure and a high temperature for a certain amount of time [6]. In addition, the direct utilization of wet biomass is considered the main advantage of this process, which significantly reduces the energy input needed for biomass drying [5,6,7]. Additionally, biocrude is a complex mixture of oxygenated compounds, with higher heating values (HHVs) that can be upgraded or used directly as fuel [8,9].

Although the co-HTL process is still underexplored when compared to the HTL process, research has shown that mixing different types of biomasses can increase the biocrude yield, with an emphasis on combinations of lignocellulosic waste with microalgal biomass. Therefore, this paper aims to propose a simplified scheme of a conceptual biorefinery employing algal biomass co-HTL together with other abundant biomass residues in a developing country like Brazil. The biorefinery model could be integrated into an ethanol production plant, a brewery, or a sewage treatment plant, aiming at the production of biocrude.

2. Consumption and Energy Demand

For the past decades, the increase in global population and economic growth has been triggering several environmental problems, causing irreversible impacts and health issues in the world. In 2024, the global population estimate is 8 billion people with the prospect of reaching the mark of 9.6 billion in 2050 [10]. Over the last 100 years, the population has more than quadrupled, and nowadays, its growth is still fast; around 82 million people are added to the world every year [11]. In addition, economic growth describes an increase in the quantity of the economic goods and services that a society produces and consumes. As a consequence, consumption has stabilized in the world and is encouraged by capitalism.

Energy, food, household goods, clothes, transport, services, and commodities are examples of marketable sectors and industries that are needed in human life. However, as a country becomes richer, overconsumption is noticed among the citizens and more disposable products are encouraged by the economic system. In the meanwhile, the environment is impacted by natural resource capture, water and soil pollution, residue disposal, and gaseous pollutant emissions. That is why the United Nations (UN) declared the twelfth global goal as “Responsible Consumption and Production” to ensure sustainable consumption and production patterns until 2030. It was estimated that the global population will reach 9.6 billion by 2050, and the equivalent of almost three planets could be required to provide the natural resources needed to sustain current lifestyles [12]. In the same vein, the seventh goal regarding “Affordable and Clean Energy” was also adopted because economic development and population growth rely on energy consumption, which is the largest contributor to global greenhouse gases (GHGs) and environmental pollution [13,14,15]. In this context, the UN also addressed some global issues that transcend national boundaries and cannot be resolved by any country acting alone. Among these global issues, there is climate change, which is the reason for many environmental impacts and anomalous events and they are encouraged by GHG emissions into the atmosphere [16].

Natural GHGs are essential for living on Earth and keeping part of the sun radiation retained in the atmosphere for human survival; however, GHGs are accumulating more and more because of their intense release from human activities and this is causing global warming [16]. There are many GHGs in the atmosphere; nevertheless, the most abundant GHG is CO₂. Over the millenniums, the atmospheric concentrations of CO₂ did not exceed 300 parts per million (ppm) and were kept practically constant. This changed with the Industrial Revolution and the increase in human emissions of CO₂ from burning fossil fuels. A rapid increase in global CO₂ concentrations was recorded over the past few centuries, and in recent decades in particular, with over 400 ppm. In terms of global emissions, in 1750, 9.35 million tons (t) of CO₂ were emitted and in 2019, 34.81 billion tons were released into the atmosphere. Regarding countries, China had the highest contribution of CO₂ emission in 2019 with 10.67 billion tons, the USA with 4.71 billion tons, and Brazil, in twelfth position with 467.38 million tons, corresponding to 1.34% of the global release [11]. Analyzing the CO₂ emissions by energy sectors, power generation and transport together are responsible for over two-thirds of the total emissions registered in 2019 [17].

In the past years, fossil fuels have been the cheapest source of energy, justifying why they are the most used around the world. In 2019, the total primary energy supply globally was divided into 31% oil, 27% coal, 23% gas, 14% renewables, and 5% nuclear. Additionally, North, Central, and South America are more dependent on oil and oil products, while Africa has the highest renewable energy share due to hydropower and traditional biomass use for heating and cooking [18]. Differences between the richest and poorest countries are found when analyzing the energy consumers since the poorest rely on traditional biomass—crop residues, wood, and other organic matter—and have a lower energy consumption per capita. However, energy consumption will continue to grow year after year following the global economic development and population growth [19]. Despite massive use of fossil resources, which include oil, coal, and gas, the fuel price has reached records triggered by an increase in demand, finite petroleum with easily accessible deposits, and environmental concern [20,21,22,23,24]. In contrast to the present situation, where fuel demand depends mainly on a single source, a flexible system composed of multiple possibilities should be attractive for a long-term solution. Therefore, the necessity of studying alternative and sustainable sources of energy arises aiming to replace fossil-based fuels with green fuels. In this context, the utilization of biomass has been highlighted by several researchers for being a potential solution for the problem [20].

3. Biomass and Residues

Biomass resources refer to all organic materials, such as wood and wood waste, energy crops, aquatic plants, crops, animal wastes, sawdust, manure, sewage sludge, and some industrial and household organic wastes, that can be designated for biofuel production [21,24,25,26,27], excluding material embedded in geological formations and transformed to fossils [28]. In addition, another purpose for biomass also lies in the production of chemical products that can have a cheaper price than petroleum-based chemicals [20,21]. The dependence on petroleum reduction is mainly stimulated by the low cost, which can vary by feedstock, conversion process, scale of production, and region. Thereby, the objective is to produce inexpensive biomass that can be used to make a range of fuels, chemicals, and other materials that are cost-competitive with conventional commodities [29]. In addition to that, the environmental appeal is also taken into account, since the photosynthetic biomass consumes CO₂, enabling it to reduce or neutralize greenhouse gas emissions, and the biofuel consumption restricts the SO_x and NO_x released into the atmosphere [20,21,24,27,30,31,32].

Biofuels, which are produced from biomass, are usually grouped into different categories known as the first-, second-, and third-generations depending on the feedstock used for their production. First-generation biofuels are produced primarily from food crops such as grains, sugar beet, oilseed, and sugarcane. They are mainly made from sugar, starch, and vegetable oil, and their sustainability is questionable under the possibility of creating undue competition for land and water used for food. Second-generation biofuels are bio-based products that come from non-food feedstock that is known as lignocellulosic biomass such as agricultural and forestry feedstock, including cereal straw, bagasse, forest residues, and purpose-grown energy crops such as vegetative grasses and short-rotation forests. Third-generation biofuels are produced from the aquatic photosynthetic cultivated feedstock, such as microalgae [33,34]. Therefore, the agriculture sector is strongly related to biofuel production because the industry is responsible for biomass production. In the world, around 37% of the total land area is shared by agriculture, with cereals as the main group of crops produced in 2019, with slightly less than 33% of the total, followed by sugar crops (24%) and vegetables and oil crops with 12% each [35].

Countries with the highest share of forest area are located mostly in tropical zones, and they also tend to be developing countries, such as Brazil. However, over the years, with civilization development, deforestation has taken place for urbanization and agriculture, especially in Brazil, where deforestation was the strongest as 53 million ha of forest were lost in the 2000–2019 period. In 2019, 27.8% (236,878.8 thousand ha) of Brazil’s national territory was covered with agriculture, with approximately 7.46% designated as cropland and 20.37% as permanent meadows and pastures [35]. Additionally, the country ranks in top 3–4 of countries in world in terms of biomass production from plantation crops, i.e., sugarcane, corn, soybean, rice, and cotton [3], and is the biggest sugarcane producer [35]. Due to this, agricultural residues are massively produced in the country with a high reuse potential, designated especially to the energy sector in order to substitute fossil fuels. Some of these agricultural residues are discussed in the following sections, such as sugarcane and malt bagasse. Furthermore, other biomasses that are also being discussed at length in the literature about energy and fuel production are sewage sludge and microalgae. Sewage sludge is the final residue of wastewater treatment and is also targeted for the sustainable recovery of materials and energy [36]. Lastly, microalgae are ideal for producing liquid fuels due to their advantages among other terrestrial crops. As a consequence, there has been a recent interest in third-generation algae biofuel projects in the literature [7,37].

4. Sugarcane and Malt Bagasse

Sugarcane is considered an important option in the biofuel sector due to its high potential for ethanol production and subproducts. In Brazil, ethanol production in 2021 was about 24.8 billion liters [38]. The USA is the biggest ethanol producer being responsible for 54.9% of the global production, and in second position is Brazil, responsible for approximately 27.5% in 2021. The vast majority of the USA’s ethanol is produced from corn, while Brazil primarily uses sugarcane [39]. In addition, sugarcane is also destined for sugar production, where Brazil has the highest contribution in the world with 36 million metric tons produced in the 2021/2022 season, corresponding to 19.88% of global production [40]. It is worth noting that the sugarcane harvest is seasonal and occurs in Brazil in the period of March and December for the South–Central region and from September to March for the North–Northeast region; that is why a season is named with two years, i.e., the 2021/2022 season [41].

Sugarcane accounted for 21% of global crop production over the 2000–2019 period. The Americas is the leading region in the production of sugarcane with 52% of the world’s total. At the top of the world ranking is Brazil with the highest sugarcane production in the world, reaching almost 40% of the global production [35]. In data, the estimated sugarcane production in Brazil in the 2021/2022 harvest was approximately 568 million tons, with productivity of 68.8 Mg/ha, and a planted area of 8264.43 thousand ha [42], representing 3.5% of the national territory covered with agriculture in 2019. In general, 120–140 kg of sugarcane straw and 270–300 kg of sugarcane bagasse are generated per ton of processed sugarcane.

Sugarcane straw includes tops, green leaves, and secondary stalks; however, bagasse is composed of mill residues after the sugarcane is processed [3,41]. In 2020, bagasse production in Brazil was 181 million tons and 7.9% of the final energy consumption was derived from sugarcane bagasse [43] since it is usually burned to supply all the energy needed in the mills [41]. Nevertheless, an important part of the produced bagasse is underutilized, and it could be destined for other sustainable applications [44].

Barley is one of the most important crop plants worldwide, representing the fourth most broadly cultivated cereal after wheat, maize, and rice [45]. The barley grain production in Brazil is concentrated in the south of the country and, in 2021, approximately 415 thousand tons of barley were harvested, corresponding to approximately 0.26% of the total production, since the world produced 160,703.72 thousand tons in the same year [42,46] . In addition, the productivity of the barley culture and the planted area were 3.74 Mg/ha and 111.5 thousand ha, respectively, around 0.05% of the national territory covered with agriculture [42].

In Brazil, the grain is mainly used for malting and brewing applications; however, it can also be destined for animal feed [47]. Malting is a process that favors the germination of the seeds in controlled conditions of different temperatures and humidities and involves three stages: steeping, germination, and drying. The major purpose of germination is synthesizing hydrolytic enzymes to decompose the cell wall and proteins and starch compounds leading to a desirable enhancement of the fragility of the barley grains [48]. Thereby, malted barley is the main source of sugars for fermentation in the production of beer and generates solid residues such as spent grains or malt bagasse in the process [49].

Sugarcane and malt bagasse, like any other lignocellulosic biomass, are composed mainly of lignin, cellulose, and hemicellulose [44]. Cellulose is a homopolymer consisting of b-D-glucopyranose units linked via b-glycosidic bonds and is the main constituent of the plant cell wall, comprising 40–50% of the lignocellulosic biomass. Hemicellulose compromises 15–30% of the lignocellulosic biomass, is an amorphous polymer consisting of short-branched chains of sugars and may consist of pentoses, hexoses, and uronic acids. Lignin makes up 15–30% of the biomass weight, contains approximately 40% of the possible energy of the biomass due to its high carbon content, and is an amorphous large molecular structure containing cross-linked phenolic polymers. These components are associated with each other and vary depending on the type of lignocellulosic biomass and its particularity [50,51]. For example, for sugarcane bagasse, Ahmed Baloch et al. [52] and Sangjan et al. [53] found 39.0% and 37.36% cellulose, 24.9% and 22.17% hemicellulose, and 23.1% and 19.5% lignin, respectively. Regarding the elemental composition, values in the order of 40% C, 0.5% N, 45% O, and 5% H were found in the literature [52,53,54]. However, the malt bagasse composition may vary according to barley variety, harvest time, brewery technology, and quality and type of adjunct added during the fermentation process, which is unique to each brewery [55]. For example, Mello and Mali [56] found a cellulose content of 12.29 ± 1.02%, hemicellulose content of 23.41 ± 1.23%, and lignin content of 26.13 ± 1.89%, and Mendes et al. [57] reported 28.4 ± 0.9% cellulose, 32.6 ± 2.1% hemicellulose, and 26.0 ± 0.9% lignin. In terms of elemental composition, 44.66% C, 6.34% H, 0.46% N, and 47.97% O were found by Zhu et al. [58]. As verified, malt bagasse presents a low cellulose content compared to other typical lignocellulosic biomasses since values lower than 40% were found in the literature.

5. Sewage Sludge

Global water capture has increased almost two times faster than the world’s population over the last century, and a recent study has also suggested that the global production of municipal wastewater is expected to increase by 24% by 2030 and 51% by 2050 over the current levels [59,60]. Currently, Brazil has only 43% of wastewater that is collected and treated, while 12% has an alternative and individual solution, 18% is collected but not treated, and 27% is neither collected nor treated [61]. In terms of quantities, 9.1 tons are generated currently in Brazil and in 2015, 40,684.813 million m³ of wastewater was generated [61,62]. Despite the lack in the country, the importance of safely treating wastewater is clear: beyond public health, there are social, environmental, and economic benefits that lead to a sustainable environment for future generations [62].

Many technologies may be used and routes taken to treat wastewater. Multiple parameters are manageable to best suit a case in terms of the nature of the treatment, following anaerobic and/or aerobic routes and technologies, and geographic aspects, such as climate and land area available to install the infrastructure. Therefore, understanding the nature of the wastewater, local regulations, site geography, and other crucial aspects are fundamental to designing an appropriate treatment technology to ensure the safety, efficacy, and quality of the treated wastewater [63].

Sewage sludge contains a broad variety of organic and inorganic matter, including proteins, carbohydrates, fats, oil, microorganism, and metal, and its composition varies in each studied case [64,65]. Some of the physicochemical characteristics of the sludge in terms of elemental characterization are shown in

Table 1. Characterization of the municipal sewage sludge on dry basis, including the analysis of the moisture content.

Proximate Analysis	(wt.%)
Moisture Content	82.4 ± 1.2
Volatile Matter	52.9 ± 0.7
Fixed Carbon by Difference	17.3 ± 2.92
Elemental Composition (wt.%):
C	33.1 ± 0.3
H	5.5 ± 0.1
N	5.0 ± 0.1
O	25.9 ± 0.1

Source: Adapted from Rahman et al [67].

. In terms of proteins, carbohydrates, and lipids, Huang et al. [66] found values of 33.6%, 20.3%, and 6.9%, respectively.

This information is relevant to this area of study because these factors can significantly affect HTL products. To illustrate, the feedstock characterization shown in Table 1 reveals a high nitrogen content, which leads to the necessity of understanding nitrogen compound distribution in HTL products. In this context, the presence of nitrogen heavily impacts the upgrading of biocrude products and the safe disposal of the aqueous phase [67].

The availability, high volatile content, and high calorific value make the municipal sewage sludge a promising feedstock for renewable energy production. Nevertheless, in Brazil, the main form of the final disposal of sewage sludge is still through landfills [65,67]. Sludge reuse and subproduct recovery is an interesting topic from an economic point of view since the residue represents 1–2% of the final volume of the treated wastewater and 60% of the wastewater treatment plant’s (WWTP) total operational cost, which is destined for sludge management, processing, and final disposal [68].

6. Microalgae

Microalgae are photosynthetic microorganisms from either marine or freshwater environments and can include bacteria, diatoms, other protists, and unicellular plants. They exist as individual cells or as chains of cells but do not form differentiated multicellular organisms, as do macroalgae [69]. Algal biomass is also commercialized in multiple sectors, such as a source of human nutrition, animal feed, cosmetic products, bio-fertilizers, nutraceuticals, pigments, and biofuels. Currently, microalgae are consumed as a dietary supplement in pills or tablets and they have also been incorporated into animal feed and human food products such as noodles, cookies, biscuits, sweets, and bread. This supplementation occurs because they are considered a rich source of proteins, lipids, carbohydrates, vitamins, and minerals. In cosmetics, microalgal extracts are assumed to be potential ingredients for boosting the skin’s rejuvenation process and removing toxins from the skin. In agriculture, microalgae can improve the soil’s electrical conductivity and pH, together with the residual carbon and nitrogen of the soil. In addition, microalgae have antiviral, antibacterial, anti-HIV, anticancer, and numerous neurological properties that can be used in medicine, and they are considered to be the most promising sources of natural pigments such as carotenoids, chlorophylls, and phycobiliproteins [70,71]. Finally, microalgal biomass is also used in the energy sector and has been extensively researched and has received attention over the years for producing third-generation biofuels, such as biodiesel and biocrude [25,72,73,74]. The advantages of microalgae use are related to the non-competition with human food production, the possibility of using wastewater and gas flue as a source of nutrients and carbon, and the higher photosynthetic efficiency, which leads to higher growth rates, biomass production, and CO₂ mitigation [7,25,26,74,75,76]. In the literature, for example, some studies focus on the production of biofuels using microalgae [77,78,79], and some focus on reconciling the production with the treatment of wastewater [80,81,82], where the microalgae utilize the nutrients of these waste streams to produce their biomass.

Algae production combined with wastewater treatment is a promising way to fulfill technological and economic gaps because it has several advantages such as stable culture and production because of the mixed culture developed with diverse tolerance levels; the efficient use of nutrients with complementarity between species; cheap and easy operation and maintenance; and the elimination of the sludge treatment [83,84]. Algae cultivation in industrial WWTPs, such as breweries and sugar/alcohol industries, is an interesting alternative to treat liquid wastewater rich in nutrients. In addition to that, a biorefinery would be more integrated considering that the cultivated microalgae and the solid wastes are destined for biocrude production [85,86].

The composition of microalgae grown under normal conditions, that is, without nutrient limitation, primarily encompasses an average of 30–50% proteins, 20–40% carbohydrates, and 8–15% lipids. However, the concentration of these compounds varies a lot according to the species and reflects the influence of the chosen culture conditions (like nutrient availability, temperature, and pH) and the stage of growth of the culture [4].

Table 2. Major composition of microalgae according to some species.

Species	Proteins (%)	Carbohydrates (%)	Lipids (%)
Anabaena cylindrica	43–56	25–30	4–7
Aphanizomenon flosaquae	62	23	3
Chlamydomonas rheinhardii	48	17	21
Chlorella pyrenoidosa	57	26	2
Chlorella vulgaris	51–58	12–17	14–22
Dunaliella salina	57	32	6
Euglena gracilis	39–61	14–18	14–20
Porphyridium cruentum	28–39	40–57	9–14
Scenedesmus obliquus	50–56	10–17	12–14
Spirogyra sp.	6–20	33–64	11–21
Arthrospira maxima	60–71	13–16	6–7
Spirulina platensis	46–63	8–14	4–9

Source: Adapted from Salam, Velasquez-Orta and Harvey [89].

shows the major chemical composition of different algae species and, among them, Chlorella vulgaris is the most cultivated microalgae due to its suitability to produce many products, including fuels [71]. In addition, regarding the elemental composition of Chlorella vulgaris, Guo et al. [87] and Xu et al. [88] realized the characterization and found contents of 47.7% and 40.31% for C, 8.4% and 9.14% for N, 7.5% and 5.99% for H, and 35.9% and 27.74% for O.

7. Hydrothermal Co-Liquefaction Process and Biocrude as Product

Different routes can be taken to convert biomass into biofuels, such as combustion, gasification, pyrolysis, and hydrothermal liquefaction (HTL) [27,30,72,90]. The direct combustion of biomass releases hazardous products, such as ammonia and NO_x, which puts this process at a disadvantage in terms of environmental protection. Gasification is a process of partial oxidation in high temperatures to produce syngas. Pyrolysis involves the production of biocrude, syngas, and biochar in the absence of air at medium to high temperatures [7,24,91]. However, the HTL process has been widely studied as it is not necessary to dry the biomass as a pretreatment, saving investments, energy, and time in the operation, and also because it has biocrude as the main product [72,91,92,93]. HTL is a technique that works in the presence of water as the solvent at temperatures between 250 and 550 °C and pressures between 5 and 28 MPa with or without an added catalyst (e.g., Fe, Zn, Pt/Al, Na₂CO₃, and K₂CO₃) during a residence time between 10 and 60 min. The high operating conditions cause dehydration of the biomass components breaking them into small molecules, which are reactive and can re-polymerize into oily compounds, eliminating the need to pre-dry the matter [24,72,91,94].

The reactions involved in the HTL are complex and vary according to the operating parameters. However, the process involves three main steps: depolymerization, decomposition, and recombination [72,95]. The first step of depolymerization is a sequential dissolving of the macromolecules, which is responsible for changing the structure of the long chain of polymers into shorter-chain hydrocarbons. The decomposition is characterized by the hydrolyzation of the biomass macromolecules into oligomers and monomers and the loss of water molecules (dehydration), CO₂ (decarboxylation), and amino acids (deamination), which leads to the formation of liquids, gases, and solid products. During the last reaction step, the recombination of available free radicals is essential to stabilize the molecules and form products with a high molecular weight [95,96].

Lignocellulosic biomass is the most widely used feedstock for biocrude production via HTL. There are two main aspects that interfere in the biocrude yield: biomass lignin content and temperature. The decomposition of lignin produces radicals that tend to re-combine into solid residues. So, the HTL of biomasses with higher lignin contents has a higher biochar yield than biocrude [97]. Also, the temperature is an important parameter since higher values benefit the formation of biochar and syngas, where such products are encouraged in pyrolysis and not in HTL [21]. The temperature range for the HTL process is already established in the literature, and several studies have analyzed its influence on the biocrude yield using different feedstock [98,99,100]. Recently, scientists discovered that the combination of two or more biomasses being co-processed might result in a better result, due to the adjustment of the biochemical composition of the feedstock mixture [101]. The presence of microalgae in the co-liquefaction (co-HTL), for example, enhanced the conversion efficiency, alleviated the reaction conditions, and improved the quality of the products [8,102,103]. That happens because microalgae have lower thermal resistance and higher conversion into the biocrude of microalgae compounds. Proteins, lipids, and carbohydrates are the major components of microalgae, which are less thermal resistant compared to those of the main components of lignocellulosic biomass including cellulose, hemicellulose, and lignin [102,103,104]. Biller and Ross [4] found that the biocrude yield from a range of model biochemical compounds followed the trend lipids > proteins > carbohydrates. Lipids form an oil yield of 80–55%, protein of 18–15%, and carbohydrates of 15–16%. Additionally, the co-HTL process has several advantages because its flexibility of using two or more biomasses compensates for seasonal variations in terms of the quantity and quality of the biomass. Thus, co-HTL can significantly reduce the logistic costs associated with collection and transportation [8,105].

As the main product of the HTL process, biocrude is a mixture of water, char fines, fatty acids, alcohols, aldehydes, ketones, furans, esters, phenols, and multifunctional compounds, such as hydroxyacetic acids, hydroxyaldehydes, and hydroxyketones, derived from biomass. The potential biocrude applications include heat and power generation from utilization in boilers, engines, and turbines; biofuel transportation such as biodiesel and bio kerosene by upgrading processes; and various chemical compounds such as preservatives, resin precursors, additives in pharmaceutical industries, flavoring agents in food industries, and acetic acid. Biocrude has been considered an alternative fuel comparable to petroleum, mainly for the generation of heat and energy, on average 35 MJ/kg [4].

However, the biocrude obtained via HTL has high viscosity and the presence of unwanted heteroatoms such as oxygen, nitrogen, and sulfur compromise its quality and prevent its direct use. It usually has an oxygen content of 10–20 wt.% and a heating value of about 28–38 MJ/kg, which can be further improved to liquids similar to diesel and jet fuel [27,29,30]. The high oxygen content can justify some undesirable qualities to the oil product, such as a lower energy content, poor thermal stability, lower volatility, higher corrosivity, and tendency to polymerize. Consequently, this fact dictates that biocrude be used in direct combustion or upgraded [30,75,106]. For the upgrading, it is necessary to use techniques and biocrude refining operations such as hydrothermal treatment. Hydrothermal treatment, for example, basically involves the reaction of biocrude with water at high temperatures with or without using specific catalysts. Additionally, it is considered effective and economically viable for the removal of heteroatoms, reduction in viscosity, and increase in calorific value [4,107,108].

The quality and yield of biocrude are the most relevant parameters that must be taken into account for the process to achieve desirable results.

Table 3. Operational parameters and biocrude yields of HTL and co-HTL experiments using different biomasses.

Process	Biomass	Mixture Ratio	Dry Biomass/Solvent (w/v)	Temp. (°C)	Time (min)	HTL Biocrude Yield	Ref.
HTL	Chlorella vulgaris	-	20%	300	5	40%	Yang et al. [109]
				325	5	42%
				350	5	43%
				300	10	49%
				325	10	47%
				350	10	46%
				300	30	48%
				325	30	44%
				350	30	41%
HTL	Chlorella vulgaris	-	15%	350	60	29%	Faeth e Savage [110]
	Neochloris oleoabundans					34%
	Botryococcus braunii					40%
	Nannochloropsis sp.					38%
HTL	Sugarcane bagasse	-	-	240	60	46.0%	Ahmed Baloch et al. [52]
				260		49.0%
				280		51.8%
HTL	Swine manure	-	20%	260	15	15%	Xiu et al. [111]
				280		18%
				300		21%
				340		24%
				360		20%
				340	5	17%
					15	24%
					30	23%
					60	21%
					90	12.5%
HTL and co-HTL	Rice straw/sewage sludge	8/0	8%	300	20	22.74%	Leng, Li and Yuan et al. [112]
		6/2				27.39%
		5/3				31.33%
		4/4				32.45%
		3/5				29.92%
		2/6				22.78%
		0/8				23.67%
	Wood sawdust/sewage sludge	8/0				26.73%
		6/2				32.02%
		5/3				28.80%
		4/4				27.63%
		3/5				26.48%
		2/6				39.46%
		0/8				23.67%
HTL and co-HTL	Spent mushroom compost/aspen wood sawdust	1/0	20%	400	15	35.05%	Jasiūnas et al. [113]
		1/0				47.85%
		2/1				23.00%
		1/1				21.23%
		½				17.90%
		1/3				15.52%
		0/1				20.65%
HTL and co-HTL	Cyanidioschyzon merolae/Galdieria sulphuraria	1/0	20%	150	30	2.6%	Dandamudi et al. [114]
				200		4.4%
				250		16.4%
				300		18.8%
		0/1		150		0.5%
				200		2.4%
				250		8.2%
				300		14.0%
		4/1		300		25.5%
		1/1				16.5%
		¼				13.0%
		4/1			15	20.0%
					30	26.0%
					45	24.0%
					60	23.0%
Co-HTL	Spirulina platensis/Entermorpha prolifera	1/1	-	300	40	17%	Jin et al. [115]
				330		19%
				360		21%
				340	20	30%
					40	22%
					60	20%
					120	20%
HTL and co-HTL	Chlorella vulgaris/sewage sludge	0/1	10%	350	15	12%	Ellersdorfer [116]
		1/1				16.4%
	Chlorella vulgaris/green waste	0/1				4.4%
		1/1				10.9%
	Chlorella vulgaris/food waste	0/1				18.2%
		1/1				17.1%
	Chlorella vulgaris/grease residue	0/1				76.3%
		1/1				48.1%
	Chlorella vulgaris	-				18.3%
HTL and co-HTL	Mixed-culture algal biomass from wastewater pond/swine manure	1/0	25%	300	60	26.5%	Chen et al. [6]
		3/1				25.8%
		1/1				22%
		1/3				35.7%
		0/1				39%
HTL and co-HTL	Chlorella sp./sewage sludge	1/0	10%	340	30	21.5%	Xu et al. [117]
		1/3				24.5%
		1/1				26.8%
		3/1				23.5%
		0/1				23%
HTL and co-HTL	Chlorella sp./sawdust	3/1	10%	270	10	27%	Yang et al. [118]
				320		35%
		1/1		270		22%
				320		28%
		1/3		270		19%
				320		25%
		0/1		270		25.3%
				320		21.1%
	Chlorella sp./spent coffee grounds	3/1		270		29%
				320		34%
		1/1		270		29%
				320		37.2%
		1/3		270		25.0%
				320		30%
		0/1		270		23.2%
				320		25.9%
	Chlorella sp.	-		270		24.1%
				320		34.9%

summarizes some operational parameters and biocrude production yields from studies that performed HTL and/or co-HTL experiments using different biomasses. It is noticeable that the biocrude yield is influenced by the operational parameters, such as mixture ratio, temperature, and time. Moreover, the biocrude composition may vary according to the biomass used as feedstock.

In this way, co-HTL can be used in a biorefinery context to explore the bioenergetic potential of algal biomass together with biomass residues in order to meet part of the energy demand while minimizing the risks of soil pollution, offering a destination for solid residues. Residues such as malt and sugarcane bagasse, WWTP sludge, and algal biomass have been co-processed in pairs via co-HTL or separately via HTL, demonstrating the technical feasibility of the process [81,116,119,120]. Figure 1 proposes a simplified scheme of a conceptual biorefinery and its context employing algal biomass co-HTL together with other biomass residues aiming at the production of biocrude. Such a biorefinery model could be integrated into an ethanol/sugar production plant, a brewery, or a WWTP. Since in Brazil, the most common technologies for domestic wastewater treatment involve sludge generation, it is suitable to consider its supply to the biorefinery. However, an industrial WWTP encompasses various technologies, being able to supply either sludge or microalgae.

8. Aqueous Phase from HTL and Co-HTL

In HTL processes, water serves as the reaction medium, with the feed content (dry basis) typically ranging from 5 to 25%. This process generates large amounts of wastewater, commonly referred to as the aqueous phase (AP). The utilization and treatment of the AP pose significant challenges in the application of hydrothermal technologies. The AP is rich in carbon, primarily in the form of soluble organic compounds, and contains other elements such as N, K, and P. If the AP is discarded, valuable resources are wasted, leading to environmental contamination. In another scenario, treating the AP as wastewater requires a substantial additional investment, potentially undermining the viability of biofuel production from the hydrothermal process. However, valorizing the AP presents an opportunity to reduce the extra investment needed for waste treatment and generate income. Therefore, it is crucial to identify suitable methods for harnessing the energy present in the AP to maximize the potential of HTL applications [121,122].

Some methods for valorizing the AP are already known, while many others are still under development. One way to valorize the AP is through the separation of value-added chemicals. Organic compounds in the AP have a wide variety of applications. However, since the AP is a complex aqueous mixture consisting of dozens of chemicals, its separation and chemical concentration pose a critical barrier to extracting value-added chemicals. Therefore, the separation of high-value organic compounds from the AP has been the focus of various recent studies [123]. Inorganic compounds present in the AP, such as N, P, and K, can also be separated for commercial applications, such as fertilizer production. Indeed, separating nutrients from the AP is an interesting and appealing approach. Although this method is feasible, further optimization of the separation methods is still necessary [121].

Another way to valorize the AP is through its hydrothermal conversion. One of these thermochemical methods for valorizing the AP is hydrothermal gasification, which allows the transformation of organic matter into a usable product. In another possibility, studies employ the AP as a valuable resource to enhance the outputs of thermochemical conversion. In this approach, the AP can be recycled within the HTL process itself as a dilution agent, which can influence the process yield and reduce the operational costs of hydrothermal liquefaction [121,124].

An alternative possibility for valorizing the AP is through its biological conversion. With this approach, it is possible to produce hydrogen, methane, electricity, and chemicals, potentially enhancing the productivity and cost-effectiveness of HTL technology. The primary challenge in the biological conversion of the AP lies in the presence of potential inhibitors, which can significantly reduce conversion efficiency or even lead to conversion failure. Therefore, pretreatment is necessary to remove or convert inhibitors into easily biodegradable compounds, facilitating AP biological conversion. The main pretreatment methods include partial oxidation, adsorption, and extraction [125,126,127].

Among the biological methods for valorizing the AP, another possibility is obtaining nutrients for biomass production. Since the AP contains high concentrations of N, P, K, and micronutrients, it can be used in growth media for algae production, for example. However, there are challenges to algae cultivation using HTL-AP, especially the high content of inhibitors, including organic compounds and heavy metals in the AP. Therefore, significant dilutions of the AP are required for this application, resulting in a large consumption of clean water and potentially rendering algae cultivation using the AP economically unfeasible. Upon further exploration of the biological methods for valorizing the AP, it is recognized that the AP can be utilized in microbial growth media, with the resulting biomass subsequently used as raw material for biofuels and value-added chemicals. However, once again, due to the complex and multivariate composition of the AP, additional studies are needed to solidify this application [128,129,130].

Another method of valorizing the AP through biological conversion involves applying anaerobic fermentation. This methodology relies on the diverse characteristics of microbial communities involved in anaerobic fermentation. Two main approaches are recognized: anaerobic digestion for methane production and two-stage fermentation for simultaneous production of hydrogen and methane. Notwithstanding, as previously recognized, the intricate composition of the AP can pose a limitation. Abundant nitrogen organic compounds, furfurals, and inorganic compounds are present in the AP, unlike wastewater typically treated by anaerobic fermentation. Therefore, more thorough investigations into the utilization of anaerobic digestion to treat the AP are necessary [121].

Table 4. References that have employed the methods discussed above and their main results.

Method for AP Valorization	Main Results	Reference
Separation of organics	Through the extraction of phenolic compounds from rice straw HTL-AP, the total phenolic compound content in the aqueous solution increased from 18% to 78%.	Chen et al. [123]
	Through the separation of rice straw HTL-AP, the residue was fractionated into glucose concentrate, monophenol and cyclopentenone concentrate, and acetic acid permeate.	Lyu et al. [131]
Separation of inorganics	Through the collection of N and P from HTL-AP as struvite, 99% of the P and 40–100% of the ammonium nitrogen could be separated.	Shanmugam, Adhikari and Shakya [132]
	Phosphate recovery up to 75% was achieved from microalgae HTL-AP using struvite.	Mcginn et al. [133]
Algae cultivation	Promising results for the co-production of food and energy through the utilization of HTL-AP as a medium for the growth of algae and rice co-culture	Mazur [134]
Microbe cultivation	Escherichia coli and Pseudomonas putida grown using 10–40 vol.% AP from liquefaction of algae	Nelson et al. [135]
Anaerobic fermentation	A methane yield of 314 mL CH4/g COD was obtained from rice straw HTL-AP.	Chen et al. [136]
	A hydrogen yield of 29.3 mL/g COD and a methane yield of 254.3 mL/g COD were achieved from cornstalk HTL-AP using two-stage fermentation.	Si et al. [137]
Hydrothermal gasification	Through catalytic hydrothermal gasification of wastewater–algal biomass HTL-AP, 98.2 ± 0.4% of the COD and 97.2 ± 0.4% of the TOC were removed.	Li et al. [138]
Recycling	The production of biocrude oil rose from 14% to 42% following six consecutive cycles of microalgae HTL-AP recycling.	Ramos-Tercero, Bertucco and Brilman [139]

presents the references that have employed the methods discussed above and their main results.

A growing interest in the field of AP valorization through biological methods involves the utilization of bioelectrochemical systems, including microbial electrolysis cells (MECs) and microbial fuel cells (MFCs). MEC is a microbial electrochemical technology that employs anaerobic bacteria to produce electrons from biodegradable waste. The generated electrons are then used to reduce protons and produce H₂ with the assistance of a low external voltage. MEC technology can be an effective approach to treat the AP. For example, Shen and collaborators studied the utilization and degradation of the AP derived from cornstalk HTL with simultaneous hydrogen production through an MEC. In this study, chemical oxygen demand removal rates were over 60% under different applied voltages, reaching a maximum of 80.2%, and a hydrogen production rate of 3.92 mL/L/d was achieved. According to the authors, the work suggested that it is feasible to degrade effluents from cornstalk HTL and simultaneously produce hydrogen through MEC [140]. In a subsequent study, Sheng and collaborators [141] treated the AP generated by swine manure HTL in MEC. In this study, the researchers achieved the removal of organic compounds ranging from 90 to 98% and nitrogen removal ranging from 57 to 93%, with the highest H₂ production rate being 168.01 ± 7.01 mL/L/d.

MFCs are also electrochemical devices. While MECs partially reverse the bioelectrochemical process to generate hydrogen from organic material by applying an electric current, MFCs aim to produce electrical energy in the form of an electric current from the microbial decomposition of organic compounds. Thus, it can be considered that in MFCs, chemical energy from organic compounds is converted into electrical energy through the metabolism of microorganisms growing on electrodes under anaerobic conditions [142]. The general structure of an MFC comprises two compartments: the first one is anaerobic and houses an anode; the second one is aerobic and contains a cathode. These electrodes are internally separated by a barrier that prevents the diffusion of oxygen gas (O₂) from the cathodic chamber to the anodic one, but they are externally connected by an electrical circuit [143]. Figure 2 depicts the general structure of a microbial fuel cell.

Previous studies have demonstrated that toxic and persistent compounds, such as N-heterocyclic compounds, commonly found in the AP, can be efficiently converted using bioelectrochemical systems. This discovery holds significance as alternative methods, including anaerobic digestion and algae cultivation, have shown limited success in breaking down these compounds. Further research has also affirmed the feasibility of electricity production through the AP application in MFC reactors, achieving simultaneous substrate degradation and COD removal efficiencies of up to 88% [144].

Table 5. Studies utilizing MFCs for AP treatment, along with their primary outcomes.

Dispositive	Main Results	Reference
MFC	Through the operation of a fixed-bed MFC constructed with carbon nanotubes using HTL-AP derived from cornstalk, a power density of 680 mW/m3 and a COD removal rate exceeding 80% were achieved.	Liu et al. [145]
	The power generated in an MFC fed with raw industrial wastewater from wood hydrothermal treatment was 70 mW/m2, and it increased to 360 mW/m2 when municipal wastewater was introduced into the reactor.	Toczyłowska-Mamińska, Szymona and Kloch [146]
MEC	MEC converted furfural, HMF, dimethyl phthalate, and diethyl phthalate from cornstalk HTL-AP, achieving a hydrogen production rate of 3.92 mL/L/d.	Shen et al. [140]
	The conversion of swine manure HTL-AP in a two-chamber fixed-bed MEC resulted in over 90% removal of organics and a hydrogen production rate of 168.01 ± 7.01 mL/L/d.	Ruixia et al. [147]
	When treating hydrothermal liquefied wastewater in an MEC, a COD removal of up to 83.84% was achieved, with a maximum hydrogen production rate of 3.92 mL/Ld.	Sheng, Wang and Yang [148]

provides an overview of studies utilizing MFCs for the AP treatment, along with their primary outcomes.

Although bioelectrochemical systems are highly relevant for converting the AP into electricity or hydrogen, there are still some limitations that must be overcome to enable their large-scale commercial application with economic and sustainable viability. Among the necessary advancements are reducing startup times, optimizing electrode materials, progressing in process scaling, and developing lower-cost proton exchange membranes [121].

9. Challenges and Prospects in HTL-AP Valorization Technologies

After analyzing various HTL-AP valorization methods, a comprehensive and macroscopic overview can be constructed regarding the challenges and prospects associated with technologies for valorizing HTL-AP. In the realm of obtaining value-added chemicals from the HTL-AP, a promising approach emerges. Methodologies yielding favorable results include the separation and concentration of chemicals from HTL-AP via nanofiltration and resin utilization. These methods exhibit potential for commercializing solvents, acids, and other chemicals. However, current research remains confined to simplified chemical compositions of HTL-AP, necessitating further exploration into applications involving HTL-AP with more complex compositions. Additionally, there is potential in integrating separation and biological conversion techniques, as organic residues produced post-separation could serve as feedstock for subsequent biological processes [121,131].

Biomass cultivation using HTL-AP as a growth medium demonstrates effective nutrient recovery and economic viability for fertilizer production. Nonetheless, challenges persist with high-organic-content HTL-AP, including inhibition and inefficient organic compound utilization. In addition, commercializing biomass produced via this method faces obstacles such as environmental risks from heavy metal accumulation and substantial land area requirements for cultivation [149]. Bioelectrochemical systems offer various advantages, such as conversion of recalcitrant compounds (phenols, furan derivatives, and nitrogen heterocycles) and production of energy, gas, and electricity. However, there are still some limitations that must be overcome to enable their large-scale commercial application with economic and sustainable viability. Among the necessary advancements are reducing startup times, optimizing electrode materials, progressing in process scaling, and developing lower-cost proton exchange membranes [121,150].

Recycling HTL-AP represents a promising strategy to increase the biocrude oil yield while recovering energy. It is straightforward to implement, cost-effective, and requires minimal scaling effort. However, this approach may exacerbate AP toxicity and complicate the disposal of the resultant waste. To mitigate these challenges, integrating this technique with complementary valorization methods presents a viable alternative. Anaerobic fermentation of HTL-AP is already considered a mature and well-developed technology with potential for commercial application. However, the pretreatment of HTL-AP, enrichment of functional microbes, and bio-augmentation need to be emphasized to enhance HTL-AP anaerobic fermentation. Additionally, this method requires the residual effluent to be treated using other valorization approaches. At last, the gasification approach presents the advantage of being readily integrated into an HTL biorefinery to enhance the commercial potential of biocrude oil production technology. However, it comes with high energy and financial demands and needs to address challenges such as low efficiency and yields before it can be considered a conventional treatment technique for HTL-AP [121,149].

10. Conclusions and Future Prospects

Agriculture residues are largely generated as a by-product in Brazil such as sugarcane and malt bagasse. However, they are still underused or are being used in unsustainable routes such as burning. Since the population in the country is growing, it has become interesting to investigate the use of malt and sugarcane bagasse for energy production. Green energy is widely being studied by many researchers because of the necessity of substituting fossil fuels and its benefits to the environment. A reduction in or neutralization of GHG emissions and a restriction of SOx and NOx released into the atmosphere are some of the benefits of using energy derived from renewable feedstock. In addition, the use of residues from other economic activities for biofuel production also contributes to the environment since no landfill used for food competition is occupied. Additionally, sewage sludge is another underused residue in Brazil that has great potential for energy production. Sewage sludge is commonly generated in WWTPs as the final residue, especially to treat domestic wastewater. However, some technologies employ the use of microalgae, which are suitable to treat ethanol/sugar and beer industries. Considering a biorefinery for energy production, a technology that allows the insertion of multiple feedstocks available to supply the demand is an interesting option to cover season variations and logistics. A technology that is considered to produce biocrude, a petroleum substitute, is hydrothermal liquefaction (HTL). This technology works with biomass under high pressure and temperature for a certain amount of time and has been the focus of multiple studies in terms of optimizing the parameters and facilitating market availability. A simplified scheme of a conceptual biorefinery is shown in the present study in order to integrate the utilization of four different types of feedstocks: sugarcane bagasse, malt bagasse, sewage sludge, and microalgae. Nevertheless, some obstacles are still hindering the advancement of the biorefinery proposal. Future research should be focused on improving the efficiency of co-HTL for the four different types of feedstocks. Life cycle and economic studies should be carried out in order to analyze its economics and environmental effects. Considerations of producing different co-products in the biorefinery also must be taken into account in order to better integrate the possibilities. Green energy and sustainable routes are important for the present and future generations and must be further considered in academic studies to reach a real application in the market. This study also highlighted that while some methods for valorizing the AP are already known, many others are still under development. Among these methods, approaches such as separation of organics, separation of inorganics, algae cultivation, microbe cultivation, anaerobic fermentation, hydrothermal gasification, and recycling can be mentioned. The survey conducted has shown that bioelectrochemical systems could become effective alternatives for treating HTL and co-HTL effluents, generating H₂ as a valuable product or electricity, while still achieving excellent performance in organic matter removal. However, few research studies in this area have been found. Therefore, it is important to emphasize in this discussion that future studies should include further investigations into reducing startup times, optimizing electrode materials, advancing process scaling, and developing lower-cost proton exchange membranes.