Organic Loading Rate (OLR) (OLR)

Organic Loading Rate (OLR) is the mass of volatile solids fed into a biogas digester per unit of digester volume per day, expressed as kg VS / m³ / day. It is the throughput intensity metric for the digester — how hard the microbial community is being worked. Together with Hydraulic Retention Time, OLR is one of the two foundational design parameters, and the two are linked: at a given feedstock concentration of volatile solids, OLR rises as HRT falls.

The calculation is direct. A 1,500 m³ digester fed 75 m³/day of slurry containing 60 kg VS/m³ has an OLR of (75 × 60) ÷ 1,500 = 3.0 kg VS/m³/day. The same digester loaded at 90 m³/day of the same slurry runs OLR of 3.6 — closer to the upper safe limit for mesophilic wet AD.

Operational bands vary by mode and feedstock. Mesophilic wet AD on cattle dung: 1.5-3.5 kg VS/m³/day. Mesophilic AD on food waste with adequate buffering: 2.5-4.5 kg VS/m³/day. Mesophilic AD on press mud or pretreated lignocellulose: 2.0-4.0 kg VS/m³/day. Thermophilic AD: 4-7 kg VS/m³/day, tolerating higher load because kinetics accelerate. Dry AD batch reactors: 5-10 kg VS/m³/day on a working-volume basis.

Overloading is the most common operational mistake in CBG plants. Pushing OLR above the sustainable limit produces a short-term increase in gas yield for 3-7 days, after which VFA accumulates faster than methanogens can consume it, pH drops, methane fraction in biogas falls, and the digester sours. Recovery from severe overloading requires 50-70% load reduction for 2-4 weeks while buffering with sodium bicarbonate, during which the plant loses substantial revenue. The trade-off is therefore conservative: design for OLR 20-30% below the documented maximum for the feedstock, monitor VFA and pH weekly, and only push toward maximum once steady-state operation has been established for 3-6 months. Online VFA sensors and pH probes pay back capex within months by catching drift before it becomes failure.