A review on two-stage anaerobic digestion options for optimizing municipal wastewater sludge treatment process

Although two-staged anaerobic digestion (AD) is not a new process, information about process stability and optimum operating conditions for improving its overall performance and resource recovery is limited and often inconsistent. The review examined the critical operating conditions and the various AD treatment options for municipal wastewater treatment plants to identify the best possible two-staged AD systems. Contour plots, multi-criteria evaluation, principal component analysis (PCA), energy balances, and economic analysis were the basis of identifying the best two-staged AD systems in this study from published literature. Contour plots generated using 29 data points from the literature showed higher volatile fatty acids (VFAs) yield prediction at lower temperatures (30-34 degrees C), relatively higher sludge retention time (SRT) (4-9 days), and at either acidic (5 6) or alkaline pH (7.7-8) for the acid-stage AD. Also, the PCA showed that SRT and pH have a more significant influence on VFA yield than the temperature in the acid phase of two-staged ADs. A multi-criteria analysis performed on 51 data points obtained from the literature showed the thermophilic-mesophilic (T-M) two-staged AD as the best option for overall % improvement in volatile solids removal and methane yield over the control among the six two-staged configurations. Additionally, T-M also achieves higher pathogen reduction and better dewaterability. Lastly, energy and economical analysis of the different two-staged AD configurations revealed a higher positive energy balance for single-stage thermophilic, thermophilic-thermophilic, and T-M than the others. This article provides a guide for the implementation and practical applicability of the two-staged AD process for wastewater treatment plants.