Thermophilic anaerobic digestion of food waste: Impact of mixing speed

Inadequate mixing of anaerobic digesters is often associated with accumulation of inhibiting byproducts, short circuiting of substrate and reduction of the effective digester volume. In contrast, higher mixing speeds are thought to increase the distance between syntrophic microorganisms and reduce the efficiency of the system. This study examines the stability and efficiency of thermophilic digesters treating food waste under various mixing velocities applied for a period of one hydraulic retention time. Two digesters were operated for 98 days under steady conditions, during which the mixing intensity was varied between 0 and 160 rpm. The results show that high velocities (120 and 160 rpm) and no mixing were harmful to the digestion process with a methane generation as low as 0.13 and 0.07 l.gVS-1, respectively, compared to an average of 0.24 l.gVS-1 under milder mixing schemes (50 and 80 rpm); and high VFA concentrations of 4.8±1.7 and 15.6±2.2 g.l-1, respectively (3 to 10 times higher than the mild stirring option). SCOD and TCOD concentrations were highest under the no mixing option with concentrations as high as 35 and 57 g.l-1, respectively. Similarly, organic matter removal was lower under high velocities, achieving higher SCOD (average of 13.1 g.l-1 ) and TCOD (average of 26.8 g.l-1) concentrations when compared to medium speeds (average of 5 g.l-1 SCOD and 17 g.l-1 TCOD). The mild mixing options resulted in a more stable process, with an IA/PA ratio that below 0.4, whereas other velocities surpassed this threshold. Accordingly, the best efficiency and stability of thermophilic anaerobic digestion of food waste is achieved with mild mixing (50 and 80 rpm). In particular, the 50 rpm stirring speed resulted in the highest methane generation (0.29 l.gVS-1, compared to 0.21 l.gVS-1 for 80 rpm).