over-processing (excessive preprocessing)

Over-processing in the CBG context refers to reducing feedstock particle size below the optimal range for anaerobic digestion — typically below 5-10 mm for fibrous material — through excessive shredding, milling or wet maceration. Counter-intuitively, finer is not always better; below the optimum the energy and equipment cost of further size reduction outweighs any incremental gain in biogas yield, and in some cases yield actually decreases.

The optimal particle size depends on feedstock type. Fibrous lignocellulosic material like paddy straw and sugarcane trash digests well at 5-20 mm particle size, where surface area is high enough for microbial colonisation but fibre structure remains for slow controlled release. Soft food waste at 10-30 mm digests well; below 10 mm, the slurry behaves more like a viscous paste with poor mass transfer. Cattle dung needs essentially no shredding because it is already well-broken-down by digestive enzymes. Press mud from sugar mills is fine enough at delivery; further milling adds no value.

Over-processing manifests in three failure modes. Energy waste: shredder power consumption rises non-linearly below 10 mm, doubling specific energy from 8-12 kWh/tonne at 20 mm to 25-40 kWh/tonne at 3 mm, eroding plant net energy balance. Equipment wear: hammer-mill rotor tips, screen mesh and bearings wear out faster at finer settings, raising spares cost by 1.5-2.5× and triggering more frequent unplanned downtime. Process-side problems: very fine particles increase slurry viscosity, settle in pipes causing clogs in pump suction lines, and form dense sediment layers in the digester that reduce active mixing volume. In extreme cases, excessive grinding releases too much soluble sugar at once, triggering VFA spikes and digester souring.

The disciplined approach is to set particle-size targets based on feedstock characterisation and biogas batch tests, then specify shredders with adjustable screens or two-stage cascades that allow trimming during commissioning rather than fixed-spec equipment. Periodic sieve analysis of digester feed during operation catches drift toward over-processing as screen wear opens up. A well-tuned shredder typically achieves 80-90% of biogas yield potential at 60-70% of the energy cost of an over-tuned one.