Trace elements in a biogas plant: cobalt, nickel, selenium, molybdenum and iron

Trace elements — cobalt, nickel, molybdenum, tungsten, selenium, iron and (in smaller amounts) manganese, zinc and copper — are components of enzymes in the pathways of methanogenesis and syntrophic acid oxidation. Their deficiency limits the rate of degradation even though the macronutrients (C, N, P, S) are normal. This applies especially to the digestion of mono-substrates, such as maize silage, with a small share of slurry.

The role of individual elements

The amounts needed are very small (on the order of fractions to single mg per kg of substrate), and the line between deficiency and toxicity can be narrow.

When to suspect a deficiency

• a constant or rising propionate concentration with correct pH and low ammonia,
• no response of gas yield to an increase in feed rate,
• a fall in CH₄ and a rise in H₂ in the gas,
• a plant running on maize silage without slurry, stillage and manures,
• a rise in FOS/TAC without any other clear cause.

Why total metal content is not enough

Sulphides, carbonates and phosphates bind metals into insoluble forms. What matters to microorganisms is the bioavailable fraction. A total analysis (after mineralisation) may therefore show "enough" metal even though the bacteria cannot see it. Laboratories use sequential extractions or determinations of the soluble fraction; interpretation requires experience.

Rules of supplementation

1. Start with analysis of the substrates and of the digester contents (total and soluble concentrations of Co, Ni, Se, Mo, Fe).
2. Introduce elements individually or as a package according to the result; do not copy someone else's recipe.
3. Dose regularly, in small portions, e.g. with the substrate, not in one go.
4. Observe the effect over several weeks: VFA, gas, pH.
5. Take fertiliser law and metal limits into account in the digestate that goes onto fields.
6. Remember the sulphur balance: high H₂S reduces the availability of metals, and iron in excess binds sulphides but can also bind phosphorus.

Ready-made preparations

Multi-component preparations are available on the market in liquid or powder form. When choosing, demand a declaration of composition, dosing instructions and information on purity. Differences between forms (e.g. chelates vs. salts) affect bioavailability, but independent comparisons are limited, so the assessment must be based on your own results.

An example approach in a plant

A plant runs on maize silage with a small addition of slurry. The observations: propionate approx. 1.5 g/l, FOS/TAC rising, CH₄ falling by 2 percentage points. Analysis shows low concentrations of soluble Co and Se. The operator introduces a supplement with cobalt and selenium in a small daily dose. After a few weeks propionate falls and the plant accepts a higher load. Such a scenario is typical, but not guaranteed; in another plant the cause could have been ammonia or temperature. Supplementation is therefore a hypothesis to be verified, not a first reflex.

Risks

• An excess of Ni, Co, Cu, Zn inhibits the process and may exceed metal limits in organic fertilisers.
• Additives containing sulphates increase H₂S.
• A lack of effectiveness control leads to costs without effect.

A three-step plan of action

1. Diagnosis. Collect the history of VFA, FOS/TAC and yield, and commission an analysis of soluble and total metal concentrations in the digester and in the main substrates. Compare with values given in the literature and with your own experience.
2. Trial. Introduce one preparation in a small dose for 3–4 weeks, without changing the mix or load at the same time. Record VFA and methane production weekly.
3. Assessment and correction. If the effect is clear, set a maintenance dose; if there is no improvement, look for the causes elsewhere (ammonia, temperature, mixing). Do not increase the dose indefinitely — metals can accumulate in the digestate and are subject to restrictions in fertiliser use.

It is worth keeping a register: date, preparation, dose, analysis results. After a year such documentation makes it possible to assess the profitability of supplementation and to negotiate supplier prices.

What a supplementation record should contain

For each trial it is worth keeping: the start date, the composition and batch number of the preparation, the daily dose, the dosing point, the results of VFA, FOS/TAC, methane production before and after the trial, and notes on changes in the mix. Without such a record it is hard to separate the effect of the supplement from seasonal changes in silage quality, let alone to justify further spending to the plant owner or supplier.