# Review of Studies on Joint Recovery of Macroalgae and Marine Debris by Hydrothermal Liquefaction
## 4 Discussion  
This paper analyzes macroalgae composition. It was found that macroalgae are carbohydrate-rich (concentration varies from 43-50% d.m.) due to their cellulose and hemicellulose contents in the cell wall. This leads to macroalgae having a lower bio-crude yield compared to the microalgae, which are richer in lipids (average content is four times higher). Due to a high content of carbohydrates, excessive organic acids are formed during the HTL process and decrease the pH. Acidic conditions during HTL cause a shift towards solid products rather than oil [31].  
From an economic and technological point of view, hydrothermal liquefaction technology has been recognized as promising for the transformation of the wet biomass of macroalgae since it does not require preliminary drying of the biomass.  
Five key points were established based on a comprehensive analysis of the conditions and efficiency of macroalgae hydrothermal liquefaction:  
* The recommended process temperature for pure algae without the use of catalytic systems is 260-280 \({}^{\circ}\)C, which provides a maximum biofuel yield up to 16% [27, 61, 63].
* Maximum oil yield during the HTL process could be achieved by using alcohols as co-solvents, since the presence of alcohols leads to an esterification reaction with carboxylic acids [31];
* As catalytic systems, it is worth paying attention to heterogeneous catalysis with the use of mineral and metal-containing catalysts, which provide an increase in the bio-oil yield and \"soften\" the conditions of the process.
* The presence of polymers, typical representatives of marine waste (TeraPak, high- and low-pressure polyethylene, polypropylene, and nylon) do not provide a significant synergistic effect, and with a significant proportion of polymers in the macroalgae biomass (more than 10% by weight in terms of dry matter), an increase in the yield of non-target solid fraction and a decrease in the yield of biofuel should be expected;
* Optimum process temperatures, providing the maximum yield of liquid fraction, for most polymers are in the range of 350-400 \({}^{\circ}\)C, while for macroalgae the optimum temperature is in the range of 260-290 \({}^{\circ}\)C.  
The organization of a cascade process, including the production of a number of valuable components (proteins, vitamins, pigments, biologically active substances) with the subsequent processing of residual biomass into liquid fuel, should be considered the most promising direction for optimizing the technology for processing macroalgae biomass, which meets modern principles of circular economy and resource management. However, additional practical research is needed to assess the effect of impurities on the quality and quantity of recovered components from drifting algae. Without a doubt, solid residue can be included in the resource cycle as a sorbent or bio-char.  
The advantages of using a two-stage process for the processing of contaminated biomass are also not obvious since the advantages are clear only when valuable components are extracted in the first stage. If there is no such task, then the two-stage treatment will only lead to a decrease in the fuel yield. Assessment of potential consumers of synthetic oil will make it possible to understand the need for preliminary extraction of proteins and carbohydrates to control the quality of oil (nitrogen, oxygen, and HHV content).  
Conceived and designed the research--Y.K., O.B. and N.S.; analyzed and interpreted the data--Y.K., S.I. and N.S.; contributed reagents, materials, analysis tools or data--Y.K., S.S. and N.S.; wrote the paper--Y.K., O.B., S.I. and N.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Science and Higher Education of the Russian Federation, project number FZWM-2021-0016.  
Not applicable.  
The authors declare no conflict of interest.