kg VS/m³/day (Organic Loading Rate)

The unit kg VS / m³ / day — kilograms of volatile solids fed into a digester per cubic metre of working volume per day — is the standard measure of Organic Loading Rate (OLR) used in CBG and biogas engineering worldwide. It is the throughput-intensity metric that determines how hard the methanogenic community is being worked: the higher the number, the more degradable matter the microbes must consume per day per litre of reactor.

The three components must be quantified precisely. Volatile solids (VS) is the organic, combustible fraction of total solids in the feed — typically 70-90% of total solids in agricultural and food residues, 60-75% in animal dung, lower in sludge with mineral content. VS is measured by drying a sample at 105°C to constant weight (giving total solids) then ashing at 550°C (the loss is VS). Working volume is the active liquid volume in the digester at normal operating level, typically 75-85% of the geometric tank volume after accounting for gas headspace and dead zones. Daily feed is the volume of slurry pumped in per 24 hours.

The metric translates plant economics directly. A 5 TPD CBG plant produces roughly 5,000 kg of CBG per day at 50 MJ/kg, which requires approximately 15,000-20,000 m³ of biogas at 60-65% methane, which in turn requires processing approximately 6,000-9,000 kg VS per day depending on feedstock yield. If the plant runs OLR at 3.0 kg VS/m³/day, the digester working volume must be 2,000-3,000 m³. Designing the digester to a lower OLR (1.8 kg VS/m³/day) gives more margin but raises civil capex by 65%; a higher OLR (4.5 kg VS/m³/day) cuts capex but raises washout risk.

The trade-off is therefore between capital efficiency and operational resilience. Indian plants on cattle dung typically design at 2.0-2.5 kg VS/m³/day for safety margin against feedstock variability; plants on standardised food waste with active buffering and online monitoring can design at 3.0-3.5; thermophilic plants on press mud reach 4.0-5.0. The most common error is treating OLR as nameplate and operating at it from day one — methanogenic communities take 3-6 months to reach full strength, and ramp-up should start at 40-50% of design OLR and rise in 10% steps weekly with VFA monitoring.