Volatile Solids (VS)

Volatile Solids (VS) are the fraction of total solids in a feedstock, slurry or digestate that is organic and degradable — the portion that can be broken down by anaerobic microorganisms into biogas, or alternatively combusted to release energy. It is the single most important compositional parameter for biogas yield prediction, because the inorganic balance (ash, sand, minerals) contributes nothing to methane production but still occupies digester volume.

The measurement follows a standardised lab procedure. A sample is dried at 105°C in a hot-air oven to constant weight, giving Total Solids (TS). The dried sample is then ashed at 550°C in a muffle furnace; the mass lost in this second step is the volatile fraction. VS is expressed as a percentage of either fresh weight or total solids. A typical feedstock report quotes both: paddy straw at 85-90% TS and 80-85% VS as fresh, or 88-92% VS on a dry-mass basis.

Typical Indian feedstock VS values are useful benchmarks. Cattle dung: 15-20% TS, 75-85% VS as percentage of TS. Press mud from sugar mills: 25-30% TS, 60-75% VS of TS. Food waste: 20-30% TS, 85-95% VS of TS. Paddy straw, dried: 85-90% TS, 88-92% VS of TS. Distillery spent wash: 8-12% TS, 70-80% VS of TS. The biogas yield potential is roughly 0.35-0.55 m³ biogas per kg VS for most agricultural feedstocks, with food waste at the higher end and woody material at the lower.

VS is the basis on which Organic Loading Rate (OLR) is calculated — kg VS per m³ of digester volume per day — and is therefore directly tied to digester sizing. A 5 TPD CBG plant requires roughly 6,000-9,000 kg VS per day, which at typical Indian feedstock compositions (mixed dung, press mud and food waste at 18-22% TS, 75-85% VS) translates to 30-50 tonnes of fresh feedstock per day. Mis-estimating VS at DPR stage is one of the most common reasons plants underperform: a project assuming 90% VS of TS on a feedstock that actually delivers 70% VS undersizes the gas plant by 22% and overstates revenue by an equivalent factor.