Umunnawuike Chika creator author. text my 2026 2026 monographic eng xv, 175 pages : illustration ; 30 cm.+ 1 CD-ROM Majority of the oilwells in around the world have reached late stage of their production with decreasing oil rate and rising water cut. However, a sizeable amount of the initial oil in place are still left unrecovered at abandonment. Oil producing companies will continue to abandon these oil wells in the coming years due to low production rate and high cost of plugin and decommissioning. Nevertheless, residual hydrocarbons present in oil wells can be used as carbon source for biological production of hydrogen. Therefore, the objective of this research is to investigate the role of external thermophilic bacteria in fermenting hydrocarbon to hydrogen. In this study different methods such as acid, heat and ozonolysis were used to pretreat mixed culture consortia from palm oil mill effluent (POME) to enhance microbial activity and hydrocarbon biodegradation potential. Then, the physicochemical properties like total solids (TS), suspended solids (SS), volatile suspended solids (VSS), chemical oxygen demand (COD), and biochemical oxygen demand (BOD) analyses were evaluated. Likewise, morphological changes in microbial communities were assessed using Field emission scanning electron microscopy (FESEM). Metagenomic profiling was conducted to identify the dominant microbial taxa capable of producing biohydrogen under reservoir conditions. Thereafter, the cultivated thermophilic bacteria were used to ferment light crude oil in a bioreactor at 70 °C to depict reservoir condition. Afterward, the biogas produced was analysed via Micro Gas Chromatography with Thermal Conductivity Detector (Agilent 990 MicroGC-TCD). FESEM images revealed structural adaptations among microbial cells after pretreatment. Microbial analysis of the enriched POME showed abundance of Anaerolineae, (25%) Firmicutes (15%) and Proteobactria (23%) as the dominant phylum for acid, heat, and ozonolysis pretreatment, respectively. The results shows that the highest hydrogen production rate was (97.40 ± 2.92 mL/L) for heat pretreatment compared to (25.68 ± 1.80 mL/L) for untreated. Furthermore, hydrogen production increased with concentration of crude oil and hydraulic retention times (HRT) until substrate utilization was achieved at 72 hours. Hence, this research will contribute significantly to produce and store hydrogen on large scale for industrial application, reduce carbon emission, provide clean and affordable energy. specialized Umunnawuike Chika Faculty of Chemical and Process Engineering Technology Thesis (Master of Science) -- Universiti Malaysia Pahang Al Sultan Abdullah - 2026 Include bibliographical reference Faculty of Chemical and Process Engineering Technology Dissertations Universities and colleges Dissertations Theses Dissertations THE0010813 (Local) UMPSA 261006 20261006092116.0 eng