Carboxylic Acid Production from Organic Waste: Integrating Substrate Composition, Reactor Configuration, Inoculum, and Future Perspectives

Acidogenic fermentation is a promising biotechnology for converting organic wastes into carboxylic acid (CA), which has significant commercial value and diverse applications in the food, chemical, pharmaceutical, and cosmetic industries. However, major challenges such as limited substrate hydrolysis and lower CA production hinder further development of this biotechnology towards full-scale implementation. This review provides a comprehensive overview of the current status of acidogenic fermentation, focusing on substrate composition, inoculum, and reactor design, along with potential strategies to overcome reactor-specific limitations and enhance CA production. It was found that the substrate composition, particularly its carbohydrate, protein, and lipid contents, strongly influences both CA production and yield. Specifically, carbohydrate-rich substrates yield higher CA production compared to protein- and lipid-rich substrates. These substrates have been investigated in different reactor configurations for CA production. Among them, the leachate bed reactor and anaerobic membrane bioreactor have demonstrated superior performance, achieving higher CA production with acetic and butyric acids as the dominant CA composition. These reactors are generally operated using three types of inocula: aerobic and anaerobic inoculum, enriched inoculum, and rumen microorganisms. Interestingly, rumen microorganisms are effective in degrading complex substrates, whereas enriched inoculum accelerates hydrolysis and acidogenesis processes within a shorter fermentation time. The findings presented herein will provide valuable information for addressing the challenges associated with acidogenic fermentation and lay the foundation for future research aimed at upscaling this biotechnology to a commercial scale.