Hydrothermal liquefaction (HTL) of sewage sludge is considered in the article as an analogue of the natural processes of oil formation (catagenesis). A comparison of the physicochemical composition of primary and secondary sludge with type II kerogen (natural precursor of oil) showed their similarity. Both types of sludge have a slightly higher level of oxygen and nitrogen. The study tested the hypothesis that the elements included in the inorganic part of the oil source rocks can have a catalytic effect on the oil formation processes. For the conducted studies of sludge HTL, the catalysts containing cations and substances found in oil source rocks were chosen: as homogeneous catalysts (KOH, NaOH, NH4Fe(SO4)2, CoCl6, NiSO4, CuSO4, ZnSO4, MoO3) and as heterogeneous catalysts (MgO, Zeolite, Al2O3). The effectiveness of catalysts containing metal ions, zeolite and aluminum oxide has been proven. The highest biocrude yield was achieved in a process with NiSO4 as the catalyst in a dose 2 g per 10 g of sludge: oil yield increased by 34.9% and 63.4% in the processing of primary and secondary sludge, respectively. The use of catalysts provided an increase in fuel HHV by 10.8–12.5%, which is associated with a decrease in oxygen content (by 10.8–43.2%) with a simultaneous increase in carbon (by 7.9–10.9%) and hydrogen (by 6.5–18.7%) content.

The excess biomass of drifting algae and their casting to the Baltic Sea coast imposes a significant environmental burden. The analysis of beach-cast algae showed that the dominant species are macroalgae Ulva sp., Furcellaria lumbricalis, Cladophora sp., and Polysiphonia fucoides. The biomass of Furcellaria and Polysiphonia algae, containing 25.6% and 19.98% sugars, respectively, has the greatest resource potential in terms of obtaining carbohydrates. Fucose, glucose, and galactose were found to be the most common carbohydrates. The lipid content did not exceed 4.3% (2.3–4.3%), while the fatty acid composition was represented by saturated fatty acids (palmitic, stearic, methyloleic, behenic, etc.). The highest content of crude protein was found in samples of macroalgae of the genus Polysiphonia and amounted to 28.2%. A study of the elemental composition of drifting algae revealed that they have a high carbon content (31.3–37.5%) and a low hydrogen (4.96–5.82%), and sulfur (1.75–3.00%) content. Red algal biomass has the most resource potential in terms of biofuel generation, as it has a high number of lipids and proteins that can produce melanoidins during hydrothermal liquefaction, enhancing the fuel yield. The study noted the feasibility of using the biomass of the studied algae taxa to produce polysaccharides and biofuels. The analyses of antioxidant properties, fat content, and fat composition do not provide convincing evidence of the viability of using the aforementioned macroalgae for their production.

At the moment, macroalgae blooms in sea waters, the rotting of which causes greenhouse gas emissions and contributes to the formation of a negative ecological and economic situation in coastal zones, which has become a serious problem. Fuel production through hydrothermal liquefaction (HTL) of macroalgae and marine debris is a promising solution to this ecological problem. The 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 optimal process conditions and their technological efficiency were evaluated. The article shows the feasibility of using heterogeneous catalysis and co-solvent to increase the yield of bio-oil and improve its quality. An assessment of the possibility of joint processing of waste macroalgae and marine debris showed the inexpediency of this direction. The high degree of drift macroalgae contamination also raises the question of the appropriateness of the preliminary extraction of other valuable components for nutrition use, such as fats, proteins, carbohydrates, and their derivatives.

The pulp and paper industry leads to the formation of significant amounts of bark and wood waste (BWW), which is mostly dumped, causing negative climate and environmental impacts. This article presents an overview of methods for recycling BWW, as well as the results of assessing the resource potential of old bark waste based on physicochemical and thermal analysis. It was found that using BWW as a plant-growing substrate is challenging because it was observed that bark waste is phytotoxic. The C:N waste ratio is far from optimum; moreover, it has a low biodegradation rate (less than 0.15% per year). The calorific value content of BWW ranged from 7.7 to 18.9 MJ/kg on d.m., the ash content was from 4% to 22%, and the initial moisture content was from 60.8% to 74.9%, which allowed us to draw conclusions about the feasibility of using hydrothermal methods for their processing to obtain biofuel and for the unreasonableness of using traditional thermal methods (combustion, pyrolysis, gasification).