operating temperatures (digester temperature range)

Operating temperature in an anaerobic digester is the temperature range maintained inside the reactor — one of the three primary process variables along with pH and retention time. Methanogenic archaea, the microorganisms that produce methane from organic feedstock, are highly temperature-sensitive, and a stable operating temperature is essential for steady gas production.

Three regimes are defined. Psychrophilic digestion operates below 20°C and is used only in unheated rural household digesters — gas yields are low and retention times are long (60–90 days). Mesophilic digestion operates at 35–40°C and is the dominant choice for Indian commercial CBG plants. Mesophilic systems are tolerant to feedstock variability and temperature fluctuations of ±2°C, use modest heating energy, and deliver biogas yields of 0.4–0.6 Nm³ per kg of volatile solids destroyed. Retention time is 25–40 days for typical agro-feedstocks. Thermophilic digestion operates at 50–55°C with retention times of 15–20 days and higher organic loading rates (3–5 kg VS/m³/day versus 2–3 for mesophilic). Thermophilic systems achieve better pathogen kill — important when digestate is intended as agricultural fertiliser — but require more heating energy, are far more sensitive to feedstock and temperature shocks, and risk ammonia inhibition when handling high-protein feedstocks.

Maintaining temperature requires heat exchangers fed by hot water from the engine jacket of a biogas-fired CHP unit, or by a small fraction of the biogas itself. In the Indian climate, ambient temperature swings of 15–35°C make insulation and reliable temperature control critical — an unheated digester in north India can lose 30–50% of expected gas yield over winter months. Trade-offs centre on heat input (thermophilic uses 30–40% more), process stability (mesophilic is more forgiving), and product quality (thermophilic produces Class A digestate suitable for direct food-crop application under FCO 1985 norms).