Operational Breakdown: Activities & Resources
A five-step operational guide for wet anaerobic digestion in a CBG plant — from pre-processing feedstock to de-sludging the reactor — with the key activities and essential equipment at each stage.
| Step | Key Activities | Essential Resources |
| 1. Pre-Processing | Sorting, shredding, and mixing waste with water/slurry to reach <15% Total Solids. | Shredders, Slurry Mixers, Feedstock Bunkers. |
| 2. Feeding | Continuous or batch-wise pumping of the liquid slurry into the primary digester. | Heavy-duty Slurry Pumps, Flow Meters. |
| 3. Digestion | Maintaining constant temperature (Mesophilic: 35–40°C) and mechanical stirring. | CSTR (Continuous Stirred Tank Reactor), Agitators, Heating Coils. |
| 4. Gas Extraction | Capturing raw biogas from the top of the reactor dome for further upgrading. | Gas Blowers, Pressure Sensors, Moisture Traps. |
| 5. De-Sludging | Removing digested slurry (digestate) to make room for new feedstock. | Screw Press/Decanters (for solid-liquid separation). |
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How to read this table
- Steps are sequential — each step must be operational for the next to function correctly.
- Key Activities describe what happens at that step; Essential Resources list the equipment required.
- This table covers wet digestion only — the process differs for dry digestion (above 20% Total Solids) and co-digestion (mixed feedstock types).
About this table
Wet anaerobic digestion — the most common process for Compressed Biogas (CBG) plants in India — runs as a continuous five-step sequence. This table maps the key activities and essential equipment at each step, giving plant operators and new developers a clear view of what happens inside the plant between feedstock arrival and gas capture.
Step 1 (Pre-Processing) is where raw feedstock is prepared for digestion. Sorting removes contaminants, shredding breaks up fibrous materials, and mixing with water brings Total Solids content below 15% — the pumpable threshold for most Continuously Stirred Tank Reactor (CSTR) systems. This step has the highest labour and equipment demand, and shortcuts here frequently cause downstream problems. Step 2 (Feeding) uses heavy-duty slurry pumps and flow meters to push the prepared slurry continuously or in batches into the primary digester. Flow metering supports both process control and record-keeping against authorised daily input volumes.
Step 3 (Digestion) is the heart of the process — maintaining 35–40°C (mesophilic temperature) with mechanical stirring in the CSTR. Temperature and agitation are the two variables operators adjust daily to maintain stable gas production. Step 4 (Gas Extraction) captures raw biogas from the reactor dome, passes it through moisture traps, and delivers it to Zone 3 for cleaning and upgrading. Step 5 (De-Sludging) removes spent digestate to make room for new feedstock — using screw presses or decanters to separate solid and liquid fractions for Fermented Organic Manure (FOM) production.
Key insights
- Pre-processing is the most resource-intensive step — inadequate shredding or mixing at Step 1 is the most common cause of downstream digester problems in Indian CBG plants.
- The CSTR at Step 3 must be held at 35–40°C continuously — a temperature drop of more than 2°C can significantly slow methanogenesis and reduce gas yield.
- Flow metering at the feeding step is both a process control tool and a regulatory compliance requirement, establishing the daily Organic Loading Rate on record.
- De-sludging equipment capacity must match the feedstock type and daily digestate volume — not just the plant TPD rating — to avoid bottlenecks at Step 5.
Methodology & sources
Operational steps described represent standard practice for wet Continuously Stirred Tank Reactor (CSTR) systems operating at mesophilic temperature (35–40°C). Equipment choices vary with plant scale, feedstock type, and technology vendor. Thermophilic systems (55–60°C) follow the same five steps but require different heating systems.
Related data tables
CO-digestion process: Operational Breakdown: Activities & Resources
A four-step operational guide for co-digestion in a CBG plant — combining two or more feedstock types — covering Carbon-to-Nitrogen ratio optimisation, multi-feed mixing, digestion management, and nutrient recovery from the enriched digestate.
Dry digestion process - Operational Breakdown: Activities & Resources
A five-step operational guide for dry anaerobic digestion — the process used for high-solids feedstocks like straw and crop stalks — covering preparation, tunnel loading, leachate-based fermentation, gas capture, and dry digestate discharge.
Process Differences by Feedstock type
A comparison of five feedstock types for CBG plants showing the primary digestion challenge for each and the unique pre-treatment step that addresses it — from lignin-heavy agro-waste to pH-sensitive industrial effluents.