Dry Matter (DM) (Dry Matter)

Dry Matter (DM) is the portion of a material — expressed as a percentage of fresh weight — that remains after all water is removed by drying to constant mass at 103–105°C. It is the most basic and universal characterisation parameter for any biological feedstock or product because almost all subsequent chemical analyses (nutrient content, calorific value, ash content, organic carbon) are expressed on a dry-matter basis to allow comparison across materials with different moisture content.

In biogas and CBG operations, DM determines critical process design choices. Feedstock at less than 15% DM is called wet, and digestion proceeds in a Completely Stirred Tank Reactor (CSTR) at 6–10% DM in-reactor — typical of cattle dung, food waste, and dairy effluent feedstocks. Feedstock at 15–30% DM is called semi-dry, used in plug-flow digesters and some high-solids CSTRs — typical of solid manure, deep-litter poultry waste, and fibrous mixtures. Feedstock at 30%+ DM is called dry, processed in batch garage-type digesters or percolation systems — typical of agricultural residue and source-segregated organic municipal waste. Each system has different mixing, heating, retention-time, and dewatering requirements, so getting DM measurement wrong cascades into incorrect plant sizing.

DM is also the basis for digestate management. Raw digestate at 4–10% DM is a slurry that must be pumped, while separated solid fraction at 20–35% DM is stackable, and pelletised or composted fertiliser at 70–85% DM is bagged and retail-distributed. Each percentage point of DM increase in transported product reduces freight cost per kg of nutrient delivered — making mechanical separation and drying the central economic levers for any digestate-to-fertiliser business. In Indian biogas literature and in European reference documents, DM and Total Solids (TS) are used interchangeably — both refer to the same parameter, measured the same way. Volatile Solids (VS), which represents the biodegradable fraction, is a subset of DM (typically 70–85% of DM for typical feedstocks) and is the parameter that actually predicts biogas yield. So a project's biogas yield projection is built on a chain of measurements: fresh weight × DM% × VS% × biogas yield per kg VS — and an error anywhere in that chain propagates directly to the project bankability.