A review of the technologies used for preserving anaerobic digestion inoculum

Anaerobic digestion is considered biotechnology that is following greenhouse gas reduction policies and circular economy development goals. However, the activity and quality of inoculum are critical factors in guaranteeing the successful start-up and stable operation of an anaerobic digestion reactor. This review summarized the literature on relevant the categories of anaerobic digestion inoculum reflecting its merits and drawbacks, with a particular focus on livestock manure, digestate, and rumen fluid. The inoculum contains hydrolytic bacteria that may break down lignocellulose, such as Firmicutes, Bacteroidetes and Proteobacteria as well as methanogenic archaea like Euryarchaeota, Methanosaeta and Methanomicrobia. However, there is a risk that heavy metals and antibiotics in livestock manure and digestate may inhibit anaerobic digestion. The anaerobic digestion reactor in most situations lacks a cost-effective, ready-to-use, dedicated methanogenic culture when required. The inoculum is frequently stored for the start-up of a new reactor and bioaugmentation of an under-performing anaerobic digestion system. Numerous laboratory-scale evaluations have been carried out on preservation strategies for inoculums, including simple preservation in wet and dehydrated storage by centrifugation, heat drying, and freeze-drying. Temperature and time are the main factors affecting preservation effectiveness. However, preservation of anaerobic inoculum by freeze-drying with an effective cryoprotectant may provide the best results. Furthermore, the potential mechanisms by which dehydration techniques and storage conditions alter the chemical composition and microbial activity of inoculum, and the development of waste-based cryoprotectants and novel dehydration preservation techniques in the future should be made more investigations.