# Review of Studies on Joint Recovery of Macroalgae and Marine Debris by Hydrothermal Liquefaction
## 1 Introduction
Despite said general dependency of the reaction type on the water conditions, there are studies reporting that ionic reactions can still happen in supercritical water, as was shown by [PERSON] et al. [22]. The process is typically carried out in an aqueous medium at temperatures ranging from 250 \({}^{\circ}\)C to 380 \({}^{\circ}\)C and pressures ranging from 4 to 25 MPa [23]. The usual dry matter content in the introduced mixture is from 5% to 20% [24], so the biomass can be processed without preliminary drying. Hydrothermal liquefaction, like conventional pyrolysis, yields three main products: bio-oil, solid residue, and gas phase, as well as an aqueous phase containing soluble products. Liquid organic fraction (bio-oil) is obviously the most valuable product among them. In this regard, the majority of current research is devoted to the search for methods to increase the yield of bio-oil, including the use of catalysts [25; 26], the selection of solvents for the extraction of bio-oil from an aqueous solution [27], and the joint processing of various types of raw materials [28]. Using the HTL process typically results in higher quality fuel after drying. As a result, the fuel has a lower oxygen content and a higher calorific value (HHV 33-36 MJ/kg) and thus requires much less effort to condition than pyrolysis oil [29]. According to the literature, hydrothermal liquefaction [30; 31; 32; 33] can also provide fuel with a lower cost price of $0.57-1.2 per L (for gasification processes this figure is 1.1-1.73 [12] and for pyrolysis is 1.54-1.78 [12]), while the fuel will be of reasonably good quality. The ability to process wet raw materials with a moisture content of up to 80% (for example, sewage sludge, manure, algae, etc.) without the need for drying or other pretreatment is one of the benefits of HTL [34]. Thus, the HTL process should be considered the most promising for the direct transformation of wet algal biomass. According to the literature data, a mixture of drifting macroalgae is inevitably contaminated with impurities of foreign fractions to one degree or another. The world's total plastic reserves in the oceans amount to five trillion elements weighing more than 0.25 million tons, while in the surface layer of water, the concentration of plastic reaches half a million elements per square kilometer [35]. Plastic marine debris significantly affects the quality and safety of the aquatic environment, posing a threat to aquatic organisms and human health, and becoming a problem on the path to ensuring food safety [32; 33; 34; 35; 36; 37; 38; 39]. Polymers account for approximately 60-90% of the total mass of marine waste, according to an analysis of the component composition of marine debris [38; 39]. Typical polymers and polymer-containing composite in marine debris are high- and low-pressure polyethylene, polyethylene temephthalate, Tetra Pak, polypropylene, and nylon [39; 40]. There are few studies on the hydrothermal liquefaction of polymers or their mixtures [40; 41; 42; 43; 44; 45; 46; 47; 48; 49], and only a few studies are devoted to the joint recovery of biomass of macroalgae and polymers to produce liquid fuel [50; 51]. This article provides an overview of studies on producing fuel from macroalgae and an assessment of the possibility of their joint recovery with marine debris. The article begins with a brief description of the experience of hydrothermal liquefaction of macroalgae, followed by a critical review of recent research in the processing of polymers (as the main representatives of marine debris) and their mixtures with various types of biomasses, including macroalgae. This article also touches upon algae composition, bio-oil yield, product quality, and methods of intensifying the process of hydrothermal liquefaction of biomass.