Quick answer
Fresh feedstock (kg/day) = Methane needed ÷ (BMP × plant efficiency) ÷ (TS% × VS%), where methane needed = CBG (kg/day) × 1.25 Nm³ ÷ upgrading recovery. For a 5 TPD plant on Napier grass with 25% TS, 92% VS and a BMP of 0.28 Nm³ CH₄/kg VS, that is about 6,450 Nm³ of methane a day and roughly 115–120 tonnes of fresh grass a day.
What TS and VS mean
- TS (total solids): the dry matter left after drying a sample at 105 °C, as % of the fresh weight. Moisture = 100 − TS. Fresh Napier grass is about 20–25% TS; air-dry straw is about 88–92%.
- VS (volatile solids): the organic part of the dry matter, which burns off at 550 °C. It is usually reported as % of TS. Only VS can turn into biogas; the rest is ash, sand and minerals.
- BMP (biochemical methane potential): how much methane 1 kg of VS gives in a 30–45 day lab test, in Nm³ CH₄ per kg VS.
Watch the units: some labs report VS as % of fresh weight. If so, VS % of TS = VS (fresh) ÷ TS × 100. Example: TS 25% and VS 23% of fresh weight means VS = 92% of TS.
TS & VS feedstock calculator
| Material | TS % of fresh | VS % of TS | BMP Nm³ CH₄/kg VS | Share of gas % | Quantity t/day |
|---|---|---|---|---|---|
Design assumptions (change if you know better)
Planning estimate only. Use TS, VS and BMP from a lab test of representative, seasonal samples of your own feedstock. Ask us to test your feedstock.
How the calculation works, step by step
- Methane inside 1 kg of CBG. CBG at 96% methane weighs about 0.77 kg per Nm³, so 1 kg of CBG holds about 1.25 Nm³ of methane (at 0 °C).
- Methane the digesters must produce. The upgrading unit loses 2–4% of the methane, so: methane needed = CBG (kg/day) × 1.25 ÷ 0.97. For 5 TPD: 5,000 × 1.25 ÷ 0.97 ≈ 6,450 Nm³ CH₄ a day. At 55% methane, that is about 11,700 Nm³ of raw biogas.
- Methane per kg of VS in a real plant. Lab BMP × plant efficiency. Napier grass: 0.28 × 0.85 ≈ 0.24 Nm³ CH₄ per kg VS. A real digester rarely captures all of the lab potential within its retention time.
- VS needed a day. 6,450 ÷ 0.24 ≈ 27,100 kg VS a day.
- From VS back to dry matter, then to fresh feed. TS = VS ÷ VS%: 27,100 ÷ 0.92 ≈ 29,500 kg TS. Fresh feed = TS ÷ TS%: 29,500 ÷ 0.25 ≈ 118 tonnes of fresh Napier grass a day, or about 24 kg of grass per kg of CBG.
- Check the digester. Slurry at 10% solids = 29.5 t TS ÷ 0.10 ≈ 295 m³ a day. At a 35-day retention time, the digesters need about 10,300 m³ of working volume. Organic loading rate = 27,100 ÷ 10,300 ≈ 2.6 kg VS/m³/day, which is a safe range for fibrous feed.
Use this to check any quotation: work backwards from the supplier's feedstock quantity and digester volume. If their CBG figure needs more methane per kg of VS than your lab BMP can give, or their loading rate is above about 3.5, the output guarantee is at risk. See our CBG quotation checklist.
Typical TS, VS and BMP values
| Feedstock | TS % of fresh | VS % of TS | BMP, Nm³ CH₄/kg VS |
|---|---|---|---|
| Napier grass (fresh, 60–75 days) | 20–25 | 88–93 | 0.25–0.32 |
| Paddy straw (air-dry) | 88–92 | 78–85 | 0.22–0.28 (pretreated) |
| Wheat / mustard straw | 88–92 | 82–88 | 0.20–0.27 |
| Cattle dung (fresh) | 15–20 | 75–85 | 0.18–0.25 |
| Press mud | 25–35 | 70–80 | 0.25–0.30 |
| Food / mandi waste | 12–20 | 85–92 | 0.38–0.48 |
| Poultry litter | 50–70 | 60–70 | 0.25–0.30 |
Indicative ranges from published literature and Biovardhan project work. Values change with season, harvest age, storage and soil contamination, so always test your own samples.
How to get TS, VS and BMP tested
- TS: weigh a fresh sample, dry it at 105 °C until the weight stops changing, and weigh again.
- VS: burn the dried sample at 550 °C; the weight lost is the VS and the remainder is ash.
- BMP: a 30–45 day batch test with inoculum at 37 °C. Test several samples across seasons, because harvest age and storage change the result.
- Also ask for lignin, C/N ratio and sand or inert content for fibrous materials; they decide pretreatment and how much of the BMP a real plant can reach.
Next steps: compare with indicative yields on our feedstock-to-CBG yield calculator, see sizing for a 5 TPD or 10 TPD plant, or read the CBG plant machinery list.
Frequently asked questions
TS (total solids) is the dry matter left after drying a sample at 105 °C, as a percentage of the fresh weight. VS (volatile solids) is the organic part of that dry matter that burns off at 550 °C, usually given as a percentage of TS. Only the VS can turn into biogas.
Methane needed = CBG (kg/day) × about 1.25 Nm³ ÷ upgrading recovery. VS needed = methane needed ÷ (BMP × plant efficiency). Fresh feedstock = VS needed ÷ (TS% × VS% of TS). For example, 5 TPD of CBG from Napier grass with 25% TS, 92% VS and a BMP of 0.28 needs about 115–120 tonnes of fresh grass a day.
BMP (biochemical methane potential) is the volume of methane a kilogram of volatile solids produces in a laboratory test, in Nm³ CH₄ per kg VS. A full-scale plant usually captures about 80–90% of the lab BMP.
CBG with 96% methane has a density of about 0.77 kg/Nm³, so 1 kg of CBG holds about 1.25 Nm³ of methane at 0 °C. Allowing for 2–4% upgrading losses, a plant needs about 1.29 Nm³ of methane, or 2.3–2.4 Nm³ of raw biogas, per kg of CBG.
For fibrous feedstocks like Napier grass and paddy straw in a CSTR digester, a design OLR of about 2–3.2 kg VS per m³ per day is common. Above about 3.5, the risk of acid build-up and digester upset rises sharply.