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.
| Step | Key Activities | Essential Resources |
| 1. Recipe Optimization | Calculating the Carbon-to-Nitrogen (C:N) ratio. Ideally, we aim for 25:1 to 30:1 to keep bacteria healthy. | Lab testing kits, Nutrition software. |
| 2. Multi-Feed Mixing | Feeding liquid-rich waste (manure) and carbon-rich waste (straw) into a centralized mixing pit to adjust moisture. | Multi-chamber intake pits, High-torque shredders. |
| 3. Synergistic Digestion | Managing the digester to handle increased gas production rates and varying pH levels caused by co-substrates. | CSTR or Plug-flow reactors, pH sensors, Acid-dosing systems. |
| 4. Nutrient Recovery | Processing the digestate, which is now richer in N, P, and K (Nitrogen, Phosphorus, Potassium) due to the variety of inputs. | Screw press, Enrichment tanks. |
Beyond definitions
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How to read this table
- Steps are sequential within the co-digestion workflow — recipe optimisation must precede mixing, which must precede digestion.
- The C:N ratio (Step 1) is the single most important number to get right — values below 20:1 cause ammonia inhibition; values above 35:1 cause low gas yield.
- This table covers the additional operational complexity that co-digestion adds relative to single-feedstock wet digestion.
About this table
Co-digestion means feeding two or more feedstock types into the same digester simultaneously. It is not just an operational convenience — it is a deliberate strategy to achieve a stable Carbon-to-Nitrogen (C:N) ratio of 25:1 to 30:1, which is the range at which anaerobic bacteria are most productive. This table outlines the four specific steps that differentiate co-digestion operations from single-feedstock digestion.
Step 1 (Recipe Optimisation) is the analytical foundation. Before a batch or feed change, operators use laboratory testing to measure the C:N ratio of each feedstock and calculate the blend that hits the 25:1–30:1 target. Carbon-rich feedstocks (crop straw, corn cobs) are paired with nitrogen-rich feedstocks (animal manure, food waste) to balance each other. Step 2 (Multi-Feed Mixing) receives different feedstocks into separate intake chambers and uses high-torque shredders and multi-chamber mixing pits to combine them at the target ratio before feeding the digester. Getting this blend right at Step 2 determines whether the digester runs smoothly or whether operators spend the next two weeks correcting pH imbalances.
Step 3 (Synergistic Digestion) manages the digester under the changed conditions. Co-digestion typically produces higher gas volumes and faster digestion kinetics than single-feedstock operation, which also means pH swings and Volatile Fatty Acid accumulation are more likely. pH sensors and acid-dosing systems are essential tools for keeping the process stable. Step 4 (Nutrient Recovery) processes the resulting digestate — which is richer in nitrogen, phosphorus, and potassium (NPK) than single-feedstock digestate — using screw presses and enrichment tanks to produce higher-quality Fermented Organic Manure (FOM).
Key insights
- The Carbon-to-Nitrogen ratio of 25:1 to 30:1 is the operating target for co-digestion — achieving it consistently requires regular lab testing and recipe adjustment as feedstock quality changes seasonally.
- Co-digestion generally increases gas production rate compared to either feedstock alone — the synergistic effect is measurable and well-documented.
- Higher gas production in Step 3 also means higher risk of pH swings — pH sensors and acid-dosing systems are essential additions for co-digestion plants.
- Digestate from co-digestion is typically richer in NPK than single-feedstock digestate, increasing its value as an organic fertiliser product.
Methodology & sources
Operational steps described are based on standard co-digestion practice for CSTR and plug-flow reactors at mesophilic temperature. The optimal C:N ratio range of 25:1–30:1 is well-established in anaerobic digestion literature and applies regardless of feedstock combination. Site-specific ratios should be confirmed through laboratory analysis of the actual feedstocks available to the plant.
Related data tables
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.
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.
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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.