Flow field investigation of high solid anaerobic digestion by Particle Image Velocimetry (PIV)

High solid anaerobic digestion (HSAD) is a promising anaerobic digestion technology. Homogenization and mixing mechanism are essential for HSAD's performance, but relative knowledge still remains poor. In order to investigate HSAD's mixing behavior, a novel flow field measuring approach was proposed as following. Firstly, laponite suspension was selected as the model fluid of HSAD digestate, because the rheological properties and material structure they displayed were highly similar. Then, water and polyacrylamide (PAAm) solution were chosen as basic reference fluid and another non-Newtonian fluid respectively. Flow fields of the three fluids under different rotation speeds were measured via Particle Image Velocimetry (PIV). The evolution of working fluids did induce consecutively the significant flow and mixing behavior of HSAD, because their rheological properties and complexity were getting progressively closer to the real HSAD digestate. Results indicated that the flow field of simulated HSAD fluid was quite different from those of water and PAAm solution, i.e. only the fluid around the impeller could be mixed in HSAD. Besides, increasing rotation speed could not significantly enhance the mixing area of HSAD. Thus, multilayer impellers arranged abreast were recommended for HSAD's mixing. Considering that HSAD's flow field had never been measured before, this study proposed a novel flow field measuring method for such opaque non-Newtonian fluid for the first time. The visualization of HSAD's complex hydrodynamic conditions was also firstly achieved in this study, and thus could further help improve the homogenization of HSAD.