A critical review of experimental and CFD techniques to characterize the mixing performance of anaerobic digesters for biogas production

Biogas from anaerobic digestion (AD), as an important alternative to fossil fuels, has contributed to energy recovery and environmental sustainability. Incomplete or inefficient mixing within anaerobic reactors can result in poor biogas production or energy wastage. Thus, identifying mixing performance is meaningful for the digester design, operation and maximum biogas production. Over the years, various experimental and computational fluid dynamics (CFD) techniques have been developed to characterize the mixing performance of digesters. This review described the critical impact of mixing on biogas production in AD. Then, experimental techniques available on local and global scales were reviewed and compared in terms of their advantages, disadvantages, characterization capabilities and scope of application. Moreover, the implementation, reliability, indicators and application of CFD techniques in AD were thoroughly discussed. Based on the above discussion, mixing significantly affects AD methane production, and intermittent mixing is preferred for maximum biogas production. Local experimental techniques have been considered to be the simplest and cheapest for arbitrarily sized digesters; global experimental techniques relying on computer systems have received increasing attention for their applications in AD flow fields. More research efforts are needed to discover new experimental techniques that overcome the limitations of digestate opacity and industrial reactor geometries, in addition, compartmental models based on CFD to couple hydrodynamics with biokinetics are interesting and allow for greater implementation of CFD applications.

Graphical abstract