Hydrothermal liquefaction (HTL) is a promising technology to convert organic feedstocks into value-added fuel precursors known as biocrude, solids (hydrochar), and gaseous byproducts. The HTL process can efficiently break down mixed feedstocks, such as waste, into valuable chemicals. The current study has investigated the HTL conversion of three waste materials: municipal sewage sludge, waste plastics, forest residue and an algae strain. This research applied various gaseous environment beside traditional inert gas with red mud ( an iron-rich industrial waste) catalyst to enhance the biocrude production with improved quality. The municipal sewage sludge from the wastewater plant was the first feedstock of this study. This study applied ethylene gas and pretreated red mud as reaction environment and catalyst, respectively, to induce the stability in produced biocrude. With respect to the oxidation state, three modified red mud catalysts were prepared by calcination at 575°C (CRM), and reduction at 500°C (RRM500) and 700°C (RRM700). The HTL treatment of sludge was highly influenced by ethylene without any catalyst and produced 41.6 wt.% biocrude yields. The viscosity of the ethylene-derived biocrudes showed lower variances compared to biocrudes from an inert atmosphere. The RRM500 lowered the acidity by 14%, while the RRM700 minimized the viscosity by 47% compared to non-catalytic-inert biocrude samples. The reduced nitrogen content found the mutual effect of RRM500-ethylene in the biocrude. This study showed the potential of ethylene gas in improved biocrude production via catalytic HTL treatment. The second study explored the effects of ethylene, reducing, and oxidative gases to compare the influences of reaction ambiances over HTL conversion. This work utilized the "Tetraselmis sp." algae strain as feedstock and two forms of RM catalysts: RM reduced at 500℃ (RRM500) and nickel-supported RM (Ni/RM). The goal was to compare the catalytic activities of RRM500 and Ni/RM under four different reaction atmospheres for algae HTL conversion. The nickel metal on red mud (Ni/RM) catalyst maximized biocrude yield (37 wt.%) in an ethylene environment, generated the lowest total acid number (14 mgKOH/g) under inert atmosphere, and lowered sulfur (33-66%) and oxygen (18-30%) from biocrude products irrespective of environments. The RRM500 catalyst increased carbon content under the reducing environment and minimized the heavy metal and phosphorus transfer from the feedstock to biocrude in studied ambiances. Among the reaction environments, the reducing atmosphere optimized carbon content (54.3wt.%) and calorific value (28 MJ/kg) with minimum oxygen amount (27wt.%) in biocrudes without any catalyst. The household waste plastic mix was the third feedstock to evaluate the efficacy of HTL technology for waste plastic treatment. The chosen plastics were polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS). The reduced red mud at 500℃ (RRM500) was utilized as a catalyst for its proven efficiency in sludge and algae liquefaction. Before mixing, each plastic material was studied individually as control experiments. Without any catalyst, the HDPE generated the maximum crude yield of 76 wt.%, whereas the PET produced mainly solid (80wt.%) and gaseous products. The biocrude yield production from non-catalytic plastic conversion followed this trend: HDPE>PS>PP>LDPE. The mixed plastic feedstock produced approximately 22 wt.% of crudes and comparatively high solid residue of 35wt.%. The RRM500 catalyst generally suppressed the biocrude and solid formation from individual plastic feedstock but effectively reduced viscosity and acidity. After depolymerization, HDPE mainly decomposed into straight-chain alkanes, while PP and PS-derived crudes were composed of aromatic-cyclic compounds. The catalyst promoted straight-chain alkanes in LDPE biocrudes. Almost 36-92% of the plastic-derived oil had gasoline boiling range compounds. The HTL conversion of plastics could be a promising route for mixed plastic waste treatment with valuable fuel range chemical production. This study's fourth objective was to increase HTL biocrude production from lignocellulosic biomass (southern yellow pine). The Pine saw dust was liquified via HTL process using water and water-ethanol mixture at 250,300 and 350℃ reaction temperatures. Iron (Fe) (at zero-valent oxidation state) was used as a catalyst in the HTL system to enhance the catalytic activity. The biocrude yield was enhanced by increased ethanol concentration in a water-ethanol medium, and the pine HTL produced the maximum biocrude yield of 34wt.% at 300℃ temperature within 1:1(wt./wt.) water-ethanol mixture. The higher reaction temperature in pure water promoted biocrude yield without a catalyst. The highest biocrude yield from the water was 18wt.% at 350℃. The iron catalyst performed the best at 300℃ reaction temperature within the water and resulted in 27wt.% biocrude yield. Moreover, the catalyst improved the biocrude quality by lowering oxygen content and acidity. The pine-derived biocrudes were mainly composed of phenolic and acids. The ethanol neutralized the acids by an esterification reaction. The catalyst accelerated the esterification process. Overall this research has proved the potential of individual waste-based feedstock for liquid fuel production. The research findings will be beneficiary to mitigate waste materials by energy production.