# Review of Studies on Joint Recovery of Macroalgae and Marine Debris by Hydrothermal Liquefaction
## 2 Experience of Hydrothermal Liquefaction of Macroalgae
### Using Macroalgae for Biorfied Production in a One-Step Process
The use of alkalis only provides a decrease in the likelihood of repolymerization of products, which ensures a decrease in the yield of biochar and an increase of liquid fuel yield. Acid catalysis, as a rule, leads to negative consequences, increasing the rate of repolymerization of molecules in solution, reducing the yield of biofuel, and increasing the yield of the solid fraction [31; 69; 72]. The significant advantage of using mineral and metal-containing heterogeneous catalysts is the possibility of their regeneration and multiple use. The mechanism of metal-containing catalysts is in adsorption on the surface of organic molecules, which leads to a decrease in the activation energy, and, as a consequence, causes increase in the yield of products and a decrease in the required temperature and pressure. The data in Table 2 indicate the unequivocal effectiveness of this type of catalysis in relation to any type of algae. Hydrothermal processing of the macroalgae _Gracilaria corticata_ was carried out using a series of iron-nickel catalysts at 260-300 \({}^{\circ}\)C using water, methyl, and ethyl alcohol as solvents which allowed for establishing that the maximum fuel yield (56.2 wt%) is achieved when using a Ga/NiFe -LDO/AC catalyst at 280 \({}^{\circ}\)C [67]. [PERSON] et al. [68] studied the _Sargassum tenterrimum_ macroalgae as an object of hydrothermal processing. The process was carried out at different temperatures (260, 280, and 300 \({}^{\circ}\)C), the pressure varied from 4.5 to 12 MPa in the presence of heterogeneous basic catalysts (CaO, applied to CeO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and ZrO\({}_{2}\)) and without a catalyst. The catalyst dose varied from 5 to 25 wt%. The contact time was 15 min. Water, ethanol, and a mixture of water and ethanol were used as solvents. The fuel yield in the non-catalytic process was 3.3 wt%, 23.3 wt%, and 32.0 wt% when using water, ethanol, and their mixture, respectively. In the catalytic process, the best results were obtained using CaO/ZrO\({}_{2}\) as a catalyst (10.0 wt%), which provided a fuel yield of 25.2 wt% and 33.0 wt% when using ethanol and a mixture of ethanol and water, respectively. The fuel obtained as a result of the catalytic process had a HHV of 27.9 MJ/kg (in a non-catalytic process with water as a solvent, the HHV was 20% lower and amounted to 22.4 MJ/kg) [68]. The use of zeolites as a catalyst (dose from 10-20% by weight) allowed the group of authors [73] to increase the fuel yield during the processing of _Ulva prolifera_ from 16.6 wt% to 29.3 wt% and to increase the HHV of the fuel from 21.2 MJ/kg to 32.2-34.8 MJ/kg under other identical conditions. The process was carried out at 260-300 \({}^{\circ}\)C for 15-45 min and a 4.3-7.0 MPa pressure. The innovations in the HTL process described above include the use of heterogeneous and homogeneous catalysis, as well as the use of co-solvents (alcohols, ketones). Both of these areas face a range of technological challenges. Heterogeneous catalysis is associated with the problem of catalyst deactivation. The use of homogeneous catalysts and/or co-solvents leads to their irreversible loss in the technological process and, as a consequence, high capital costs. Recently, one of the most important trends in biomass processing has been the concept of cascade conversion, which allows for the consistent production of the greatest number of valuable products. Macroalgae can potentially be used in various sectors of biotechnology, such as the production of biofuels, food, and cosmetic and medical products [17; 76]. This strategy for algae consists of the initial production of bioactive compounds (lipids, vitamins) and the subsequent conversion of biomass (hydrolysis, pyrolysis, fermentation, and liquefaction) to produce fuel with the subsequent disposal of fermented or recycled waste in the form of compost, sorbents, etc. [17]. One of the options for organizing a cascade technology for processing macroalgae is considered in the next section.