# 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
Moreover, two-stage HTL was better than three-stage HTL in terms of bio-oil yield, while the latter was preferred when high polysaccharide yields were required. [PERSON] et al. [83] were able to achieve the same biofuel yield as in a one-step process under milder conditions while obtaining polysaccharides/hydrolyzed sugars and proteins/amino acids as by-products from _Chlorella sorokiniana_. [PERSON] et al. [85] confirmed the higher overall yield and lower nitrogen content of biofuels from the processing of _Coelastrum_ sp. in a two-stage process in a semi-continuous reactor. [PERSON] et al. [80] investigated the processing of the high-protein microalgae _Chlorella vulgaris_, the two-stage processing of which reduced the nitrogen content in the fuel by 55%. [PERSON] et al. [86] investigated the possibility of using the extract obtained at the first stage for the cultivation of an acidophilic strain of _Galdieria sulphuraria_ microalgae. The algae productivity was 2.6 times higher in comparison with the productivity in a pure nutrient medium. [PERSON] et al. [78] carried out a two-stage conversion of the microalgae biomass under the following conditions:  
* first stage: 125-225 \({}^{\circ}\)C, 0.5-30 min
* second stage: 350 \({}^{\circ}\)C, 60 min  
At the same time, primary treatment at higher temperatures and a longer holding time (225 \({}^{\circ}\)C, 15 min) allowed for the extraction of up to 45% of all nitrogen, which ensured a decrease of its content in the fuel by 26-28% compared to the one-stage process [78]. Based on the above, we can conclude that the use of a two-stage process clearly has the following advantages over a single-stage process:  
* decrease in the amount of nitrogen and oxygen amount in oil phase;
* possibility of valuable component preliminary extraction;
* increase of biomass utilization rate. It is natural that the preliminary extraction of organic matter leads to the decrease in the yield of liquid fuel. However, the amount of solid residue is significantly reduced and, as a consequence, the effectiveness of algal biomass use is significantly higher. Extraction of valuable components at the first stage of the HTL process is possible and expedient in the case of using pure, possibly monocultural biomass. Processing washed ashore macroalgae, which are partially decomposed during long drifts in sea water and piled up on the shore, may be problematic due to difficulties with the extraction and purification of target components. The comparison of the fuel yield in a two-stage and one-stage process was carried out by [PERSON] and [PERSON] [87] using the microalga _Enteromorpha clathrata_ as an example. The process of HTL was carried out in three versions: a one-stage process at 200 and 300 \({}^{\circ}\)C (time 1 h) and a two-stage process, 0.5 h at 200 \({}^{\circ}\)C and then 0.5 h at 300 \({}^{\circ}\)C. In this work, the effect of the ratio biomass to solvent on the yield of liquid fuel was also evaluated, as well as the effectiveness of ethanol as a co-solvent. In the range of the solvent/biomass ratio 40/2.4-40/5.6, no significant change in the fuel yield was found. However, an increase in the share of ethanol resulted in a significant increase in oil yield from 8.12% (minimum oil yield in the absence of alcohol) to 38.4%. The maximum yield of liquid fuel was obtained in a one-stage process (40.3%), but it should be noted that in a one-stage process the proportion of the solid phase was higher than in a two-stage process. This is due to the formation at low temperatures of water-soluble products (alcohols, acids, ketones), which subsequently do not transform into biofuel and remain instead in the aqueous phase. The use of a two-stage process also led to a decrease in the number of nitrogen-containing compounds in the fuel from 37% to 21.6% when using water as a solvent and from 28.6% to 16.8% in the case of alcohol.