Effect of Agitator Operating Parameters on Biogas Yield and Process Stability in a Full-Scale Dry Anaerobic Digester

Mixing is one of the largest single consumers of parasitic electricity in a biogas plant. Yet, the quantitative effect of agitator operation on performance has been established almost exclusively at laboratory or pilot scale, and rarely for dry digestion. Unlike previous full-scale studies, which varied only the duration of mixing in mostly wet digesters, this study varied agitator rotational speed, run time, and pause time jointly (three levels each; a Taguchi L9 design run in two series, 18 runs in total) on an operating municipal dry (32–35% TS) horizontal-shaft digester treating source-segregated biowaste, with three-point (inlet/middle/outlet) pulp analyses. Specific biogas yield ranged from 71 to 106 m3/Mg across the nine settings. Analysis of variance identified rotational speed as the dominant factor (p < 0.001); yield increased monotonically with speed, most strongly between 0.2 and 0.4 rpm (yield = 68.0 + 50.2·speed[rpm], R2 = 0.64, p < 0.001). The yield-maximizing combination within the tested range (0.6 rpm; 400 s run; 150 s pause) increased biogas yield by 37% relative to the pre-experiment settings, and a confirmation run (104.4 m3/Mg) agreed with the additive-model prediction (105.1 m3/Mg). Because the mixing-energy index, which combines torque, speed, and duty cycle, was also highest at this setting, the yield-maximizing and energy-efficient operating points differ. The digester remained stable throughout (outlet FOS/TAC 0.13–0.17; over 98% net decrease in acetic acid from inlet to outlet), so the yield differences were not associated with acidification.