2.2 General Classifications

Input

Digesters can be classified based on their input. For example, agricultural biogas digesters use animal manure and crop residuals, converting them into biogas and nutrient-rich digestate that can be used as organic fertiliser. Faecal sludge digesters treat human waste, producing biogas and reducing environmental pollution. Co-digestion systems combine different feedstocks, enhancing biogas production and waste management sustainability.

Total Solids (TS) content

Digesters can be classified based on the Total Solids (TS) content of the input material they handle. Wet fermentation (TS <15%) digesters operate with higher moisture content and are suitable for processing materials like food waste or faecal sludge that are pumpable and mixed with water. Dry fermentation digesters (TS > 20%) handle feedstocks with lower moisture content, such as agricultural residues or solid organic waste that will be stacked inside the digester.

Feeding mode

Digesters can be categorised according to their feeding method.

• Continuous-feeding digesters are supplied with a constant stream of feedstock.
• Semicontinuous, the feedstock supply occurs several times per day.
• Batch-feeding digesters operate in batches (no substrate added during dwell time).

Continuous feeding is ideal for consistent waste streams, while batch feeding is utilised for smaller-scale or intermittent feedstock availability.

Operating temperature

Digesters can be classified based on their operating temperature.

• The psychrophilic temperature range is <20°C and the digestion rate slows down at lower temperatures. The type of digester does not require additional energy but a larger reactor volume.
• Mesophilic digesters operate at moderate temperatures (20-40°C), offering a balanced trade-off between biogas production, digester volume and system stability, but would usually require additional energy for operation.
• Thermophilic digesters operate at higher temperatures (50-70°C), resulting in higher biogas production rates and reduced digester volume but requiring more careful operational control due to a sensitive bacteria environment and increased energy input.

Number of stages

Digesters can be categorised based on the number of stages involved. Single-stage digestion means the anaerobic digestion takes place in a single digester, while in multi-stage digestion, multiple digesters or tanks are installed to have separated functional zones for the different steps during anaerobic digestion. Multi-stage digestion is especially advisable for codigestion and enables a higher treatment efficiency.

Digester types

Digesters can be classified based on their design, which refers to their specific configuration and structural characteristics—for example, a fixed dome digester or floating drum.