Municipal wastewater treatment plants generate large amounts of sludge after a set of unit processes. The sewage sludge is an important resource for energy production because of its high level of biodegradability. Sewage sludges are generally made of non-toxic and biodegradable organic compounds mixed with a small fraction of non-toxic and toxic inorganic compounds having a very low biodegradability. The large fraction of biodegradable matter constitutes a pool for green/clean energy to be used

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... for industrial and domestic applications. The generated energy can also be used in the wastewater treatment plant. Currently, fossil fuels are leading the energy world, however, they are being depleted and are considered to be among the main causes contributing to climate change and global warming. Domestic sewage sludge can be converted sustainably into bio-hydrogen and bio- methane. This conversion is achievable through anaerobic digestion, combustion, pyrolysis and gasification. With regard to the last three conversion processes, the organic and inorganic toxic compounds are eliminated. Production of biogas from sewage sludge is being undertaken worldwide on small, medium, and large scales. However, hydrogen production from sludge is still developing. There is an existence of substantial knowledge in this field, the production of hydrogen and biogas from sewage sludge is gaining interest. This study analyses various possibilities of sewage sludge conversion into clean energy. The analysis focuses on the technology strengths, weaknesses and gaps to be improved in future studies.