DM (DM)

Dry Matter (DM) is the fraction of a material's mass remaining after all moisture has been driven off by oven-drying at 105°C until constant weight. Expressed as a percentage of fresh weight, it is the European biogas industry's standard composition metric and is numerically identical to Total Solids (TS) used in North American and Indian practice. A feedstock listed as 25% DM contains 25 kg of solids per 100 kg of as-received material.

DM determines reactor design more than almost any other feedstock parameter. Wet anaerobic digestion (the dominant Indian configuration) operates at 8–15% DM, requiring liquid recirculation and continuous stirring. Dry digestion (used for stackable agricultural waste and FOG-free MSW) operates at 25–35% DM and uses garage-type batch reactors with leachate recirculation. Pushing wet systems above 15% DM causes pump cavitation, foaming, and stratification; running dry systems below 25% leaks leachate uncontrollably.

DM also drives nutrient calculations. A digestate quoted at 4% N on dry basis means 40 g of nitrogen per kg of solids — but if the digestate is 6% DM, the fresh-weight nitrogen content is only 2.4 g per kg, which is the figure that matters for tanker-load fertilizer planning. Indian FCO 1985 specifications for Fermented Organic Manure are written on fresh-weight basis, so converting between lab reports (dry basis) and product labels (fresh basis) is a routine source of compliance error. Typical Indian feedstock DM values: cattle dung 12–18%, press mud 25–30%, food waste 18–25%, paddy straw 85–90%, poultry litter 60–70%.

• DM is the solid fraction after drying at 105°C; identical to TS in Indian and US practice.
• Wet digestion: 8–15% DM. Dry digestion: 25–35% DM.
• Convert dry-basis lab values to fresh-basis before sizing tankers or quoting fertilizer NPK on FCO labels.
• Mixing high-DM (paddy straw) with low-DM (cattle dung) feedstocks is how Indian co-digestion plants balance the reactor.