Over-mixing (excessive agitation)

Over-mixing is a common operational error in anaerobic digesters where excessive mechanical agitation — usually from running impeller mixers continuously at high speed — physically disrupts the loose microbial aggregates (granules and flocs) that sustain methane production, shearing apart the syntrophic associations between acid-forming bacteria and methanogens. The result is paradoxical: the operator intends to improve substrate contact and increase gas yield, but biogas output instead drops 10–30% and may take weeks to recover.

The mechanism is biological. Methanogenic archaea are slow-growing, strict anaerobes that depend on close spatial contact with hydrogen-producing acetogens to maintain low hydrogen partial pressures. When the mixer shears these granules apart, the symbiosis breaks down, volatile fatty acids accumulate, and the digester drifts toward souring. In Indian CSTR-type CBG plants of 50–200 m³ digester volume, recommended mixing regimes are typically:

• Intermittent mixing — 5–10 minutes every 1–2 hours rather than continuous
• Low tip speed — impeller tip speeds under 3 m/s to avoid shear damage
• Specific power input — 5–10 W per m³ of digester volume, well below the 20–40 W/m³ used in chemical reactors

Symptoms of over-mixing include falling daily gas yield despite stable feed, rising VFA-to-alkalinity ratio above 0.4, and gradual drop in methane percentage. The fix is straightforward — reduce mixer runtime and tip speed — but recovery of the microbial community can take 2–4 HRTs, during which gas yield remains depressed. The opposite failure, under-mixing, causes scum and crust formation, dead zones, and uneven temperature; both extremes hurt yield, which is why mixing optimisation is treated as a tuning exercise during commissioning, not a one-time design decision.