Co-digestion: how to plan a substrate mix without overloading the digester

Co-digestion is the simultaneous fermentation of several substrates, usually slurry or manure with the addition of materials of higher methane yield, such as silages, production waste or bio-waste. The mix is planned so as to combine a base substrate providing buffer and water with more energetic ones, while maintaining a C/N of about 20–30, a safe OLR and a low risk of inhibition. Reliable substrate analyses, a mass balance for the whole year and gradual introduction of changes are key.

Step 1: get to know the substrates

For each component collect data from the laboratory, not from a catalogue:

Step 2: define the role of the substrates

• Base: cattle or pig slurry — buffer, water, trace elements, low energy.
• Energy carriers: maize and grass silages, whole-crop silage (GPS), beet pulp, waste from the food industry.
• "Difficult" substrates: poultry manure (high nitrogen), fats (high yield, foaming risk), whey (rapid acidification), straw (slow hydrolysis).

Rule: easily degradable substrates are introduced in a limited proportion and evenly, because the risk of acidification rises with the rate of hydrolysis.

Step 3: balance and example

Assumptions: a 3,000 m³ digester, target OLR about 3.3 kg VS/(m³·d), i.e. about 9,900 kg VS/d. The mix:

OLR ≈ 3.2 kg VS/(m³·d); HRT at 65 m³/d is about 46 days. The slurry content provides 61% of the fresh mass, which helps with dilution and alkalinity. Next check the C/N (balance spreadsheet), TAN and free NH₃, fat content and sulphur.

Step 4: safe limit values

Establish for your own installation:

• the maximum share of easily degradable substrates in VS,
• the maximum OLR for the mix,
• the maximum TAN and free NH₃,
• the minimum share of slurry (buffer),
• the maximum rate of change of the mix (e.g. no more than a dozen or so per cent of VS per week).

The values follow from the operating history: the best source is notes from previous changes and analysis results.

Step 5: introducing changes

1. Change one factor at a time.
2. Start with a low share of the new substrate (a few per cent of VS).
3. Observe VFA, FOS/TAC, pH, gas composition and foam for 2–3 weeks.
4. Increase the share in stages if the parameters are stable.
5. Record the process's reactions — this will be the knowledge base.

Logistics

A mix on paper has to be covered by deliveries: contracts with substrate suppliers, storage space (silos, tanks), mass measurement, quality. The annual plan should take into account:

• fluctuations in crop yields (drought, excessive rainfall) — a silage reserve of one to two months,
• seasonality of processing waste,
• travel time and transport cost,
• the possibility of replacing a substrate with an equivalent one.

Test before the decision

Before you sign a contract for a new substrate, carry out a BMP test or a continuous laboratory-scale test with the planned mix. The result will not replace a technical-scale trial, but it will indicate the inhibition risk and an approximate yield. Remember that BMP does not take into account the limits of mass transfer and HRT in a real digester.

The most common mistakes

• planning the mix from tables, without analyses,
• a sudden increase in the share of fat or poultry manure,
• ignoring sulphur and ammonia,
• no contingency plan for the absence of one substrate,
• failing to consider the digestate: co-digestion with waste may change the status and legal requirements for the digestate.

A well-planned mix is not only energetically optimal but also resilient: if one component fails, the installation can replace it without disturbing the process.

Contingency scenarios in the mix plan

A good plan contains variants: what to do when silage runs out (replacement with grass, GPS, beet pulp), when the share of manure rises (dilution, C/N correction), when a waste supplier delivers a batch of poorer quality (quarantine, a test on a small dose). Write down the decision thresholds and the persons responsible. It is worth keeping a reserve of ready, stable substrate that makes it possible to quickly lower the share of risky components without a fall in energy loading.

Assessing the profitability of components

Compare substrates not by price per tonne, but by cost per m³ of methane: the price of a delivered tonne divided by the CH₄ yield per tonne. Free waste with a low yield and high risk may be more expensive than silage. Add to the calculation the costs of transport, storage, any treatment and the effect on the digestate.