Novel Sludge Processing Methods Focusing on Advantages and Disadvantages, Energy Production, and Bioplastics: A Systematic Review

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Context: Today, the growth of excessive energy consumption and concerns about the conservation of natural resources has led to the identification and exploitation of new energy sources. One of the new energy sources is the production of energy from biomass, where sludge forms a significant source of biomass. Objectives: This study aims to review novel methods of sludge processing, such as dark fermentation, photofermentation, anaerobic digestion, and bioelectrochemical processes, in addition to their advantages and disadvantages. Data Sources: A literature search was conducted on PubMed, ScienceDirect, and Web of Science databases between 2000 - 2022 to find related articles. A total of 315 records were identified, of which 80 studies were retained that met the review’s inclusion criteria. Search engines were searched using some keywords, including biohydrogen, sludge processing, dark fermentation, photofermentation, bioelectrochemical system, microbial fuel cell, anaerobic digestion, microbial electrolysis cell, bioplastic, and polyhydroxyalkanoates. Results: The reviewed studies have shown that dark fermentation alone cannot produce high levels of biohydrogen. However, the level of biohydrogen production increases significantly when dark fermentation is coupled with anaerobic digestion, optical fermentation, or bioelectrochemical processes. The advantages of integrating dark fermentation with other processes can be mentioned as an increase in biohydrogen production, the use of dark fermentation end products in electricity generation by bioelectrochemical systems, and bioplastics production. Conclusions: With the development of biohydrogen production methods from renewable energy sources, dependence on fossil fuel systems will be reduced in the future. A review of various studies showed that combining the dark fermentation process with other processes also has disadvantages. Therefore, biohydrogen production strategies at an appropriate economic level need to be developed, and further research is required.