# Review of Studies on Joint Recovery of Macroalgae and Marine Debris by Hydrothermal Liquefaction
## 2 Experience of Hydrothermal Liquefaction of Macroalgae
### Application of a Two-Stage Hydrothermal Liquefaction Process for Algae Conversion  
One of the disadvantages of HTL is that the proteins and carbohydrates contained in algae greatly affect the quality of the final product, leading to high oxygen and nitrogen content, which in turn increases the acidity of the fuel, the tendency to polymerize, and the viscosity, and reduces the proportion of light fractions [77]. These undesirable reactions ultimately represent the loss of nutrients from biomass processing systems and further limit the potential applications of biofuels [17]. Proteins from algae have a balanced composition, not inferior in properties to proteins from traditional sources [78]. Therefore, preliminary removal and recovery of protein compounds and carbohydrates from biomass is useful and necessary in the implementation of HTL processes. There are a number of different methods for reducing the content of nitrogenous compounds in algal biomass, including enzymatic, acidic, or alkaline hydrolysis, but these methods are associated with the expensive subsequent separation of proteins and their derivatives [78]. Several authors [78; 79; 80] proposed the use of low-temperature pretreatment in an aqueous medium, leading to the transition of a part of nitrogen-containing compounds into the aqueous phase without significant lipid destruction. One of the options for implementing this approach is a two-stage HTL process. The two-stage HTL process was developed for the simultaneous extraction of proteins and carbohydrates from algal biomass in the first stage at low temperatures (140-180 \({}^{\circ}\)C) and the production of biofuels through the conversion of lipids in the second stage (220-280 \({}^{\circ}\)C) [51]. The general technological scheme of the two-stage process is shown in Figure 1.  
Fine tuning of the first stage reaction conditions can enhance favorable reaction pathways for the selective recovery of compounds such as polysaccharides, proteins, amino acids, pigments, and inorganic nutrients. Moreover, after preliminary removal, the mass transfer between water and lipid molecules is improved, which ultimately improves the efficiency of fuel recovery. Figure 1: Scheme of carrying out a two-stage HTL process. The cascade extraction of valuable components fully implements the Zero Waste approach, which is a fundamental requirement to implement the bioeconomy amongst EU countries [81]. The key requirement for biomass processing is the primary extraction of the most valuable bioactive substances and nutrients, followed by biofuel production from the residues [82]. The process of cascade hydrothermal extraction of valuable components is subsequently carried out in two reactors. Water is used as a solvent. At the first stage, the process is carried out at 150-160 \({}^{\circ}\)C with a processing time of 20-30 min. After the first stage reaction, the reaction mixture is filtered and the treated algae in solid form is loaded into the second reactor with an additional dose of water at the desired ratio. In the second stage, the treated biomass is processed at temperatures ranging from 240 to 250 \({}^{\circ}\)C and 3.5 MPa, which is much lower than in the one-stage HTL process. Most studies on the implementation of the two-stage process are devoted to the processing of microalgae. [PERSON] et al. [83] concluded that the extraction of higher polysaccharides is achieved at the first stage at 160 \({}^{\circ}\)C in 20 min with a biomass ratio of 1:9. In the second stage, 240 \({}^{\circ}\)C, 20 mins, and a loading ratio of 1:9 are sufficient. [PERSON] et al. [84] studied three different scenarios of hydrothermal liquefaction using the example of _Chlorella sorokiniana_: one stage, two stage, and three stage. Although the one-stage process produced the highest bio-oil yield (28%), the two-stage process produced a high yield of polysaccharides (26%) as well as a relatively high amount of bio-oil (23%), while the bio-char yield was 64% lower than in the one-stage process.