Application of additives for anaerobic digestion intensification: A comprehensive review on improving biogas production and methane yield

Anaerobic digestion (AD) is a key process in wastewater treatment since it breaks down the complex organic matter into methane-rich biogas through multiple steps, where different microorganisms are involved. The supplementation of additives is a promising technique to enhance biogas production and increase the organic loading rates due to the enhancement of microbial growth and diversity, direct interspecies electrons transfer pathways, and enzyme release. However, there is a lack of updated review for addition of enzymes, nano-materials, pH buffering materials, and nutrients as well as bioaugmentation using additives to improve the AD performance and potentially increase the organic loading rate. Therefore, in this review, the supplementation of these additives is reviewed from different perspectives including the mechanisms, efficiency gains, and governing factors on the performance of each group of additives in the AD process. In addition, the major advantages, disadvantages, and challenges associated with each additive are discussed. Based on the main outcomes of the current review, most of the additives were only used in lab-scale AD systems which highlights the importance of testing these additives in full-scale treatment applications to identify their potential in biogas upgrading on real-scale digesters. Also, additives require relatively high running costs which represents a major challenge in implementing this technology in large-scale AD applications. Considering the different types of additives, nano-materials showed higher performance in enhancing the methane production and yield compared to other additives, representing an efficient technology for AD intensification applications. The main insights and outcomes of this review can serve as a reference for the common additives used for intensifying the AD processes.