Enzymatic treatment and digestion additives: what is documented and what is marketing

The market offers many preparations promising a higher biogas yield: enzymes, bacterial mixtures, "stimulants", trace elements, biochar, zeolite. What is soundly documented is mainly additives that correct a specific problem (a deficiency of trace elements, binding of hydrogen sulphide with iron, ammonia sorption); the effects of universal preparations are often small, variable and need to be verified at the scale of your own plant. The principles of assessment follow.

Hydrolytic enzymes

Preparations containing cellulases, hemicellulases, proteases, lipases or amylases are meant to accelerate hydrolysis. Laboratory studies have reported improved kinetics and sometimes improved yield for fibrous substrates; at technical scale the effect depends on:

• the dose and contact time (enzymes need time and optimum pH and temperature conditions),
• activity in the digester environment (pH, ammonia, proteases from other microorganisms that break the enzymes down),
• the type of substrate (greatest potential: straw, grasses, fibrous waste),
• the cost of the preparation in relation to the additional methane.

Trace elements

Cobalt, nickel, selenium, molybdenum and iron — an additive makes sense where a deficiency has been documented (see the separate article on trace elements). This is the best-supported group of additives, but only where there is a need.

Iron (salts, oxides)

It binds sulphides, lowering H₂S in the gas, and supplies Fe. Its effectiveness is easy to measure with an H₂S analyser.

Zeolite, biochar, activated carbon

Sorbent materials can bind ammonia, metals or volatile acids, provide a carrier for the biofilm and act as a buffer. Studies indicate possible benefits under inhibition, but the results are variable and the effect depends on the dose, the type of material and its quality. Biochar may contain contaminants (metals, PAHs), so certification matters.

Microbial preparations (bioaugmentation)

Adding bacterial cultures (e.g. hydrolytic bacteria, methanogens) can be effective at start-up or after a disturbance, especially if the culture is adapted to the given conditions (e.g. high ammonia). Commercial preparations of unclear composition are poorly documented. An inoculum from a working plant (digestate) is a cheaper and tried-and-tested alternative.

Antifoaming agents and pH correctors

These are process additives, not "stimulants" — they are covered in separate articles.

How to design a reliable test

1. Define the hypothesis (e.g. a 5% increase in yield from silage, a drop in propionate).
2. Run a BMP test of the substrate with and without the additive, in replicates (at least three), with a cellulose control, in line with the principles of such tests (separate articles). A BMP test has limited value for additives that act on the biomass in the digester, because the inoculum is already adapted.
3. Technical-scale test: a reference period before, a period with the additive and a period after withdrawal (each at least 2–3 HRT), with a constant mix.
4. Measure gas production, composition, VFA, FOS/TAC, TS/VS, energy consumption.
5. Evaluate the differences statistically or at least compare them with natural variability.

Example of an economic calculation

Assumptions: the additive costs PLN 3/kg and is dosed at 1 kg per 10 t of substrate (0.01%). For a plant with a feed of 60 t/d the cost is: 6 kg/d × PLN 3 = PLN 18/d, or about PLN 6,570 a year. If the energy yield rises by 1% with electricity production worth, say, PLN 4,000 a day (illustration), the gain will be PLN 40 a day, or about PLN 14,600 a year – worthwhile. If the effect is 0.3% (PLN 12 a day), the additive will not pay for itself. The difference between a significant and an apparent effect therefore requires measurement; the prices in this example are assumptions.

Red flags in an offer

• no composition and no safety data sheet,
• results only from the supplier's "own research" without a description of the method and control,
• promises independent of the substrate,
• no information on the effect on the digestate,
• pressure to sign long contracts without a trial period.

Legal compliance

Additives that end up in digestate used as fertiliser must not cause the permitted contaminant contents to be exceeded. Before use, check the requirements concerning the status of the digestate and the supplier's documentation.

Conclusion: additives are a supplementary tool, not a substitute for a good mix, even dosing and process control. Always test, measure, compare with a control.

Where the literature is strongest

The best-documented effects concern two situations: (1) correcting trace element deficiencies in plants running on mono-substrates and (2) binding hydrogen sulphide with iron salts. The other additives have mixed data: some studies show an improvement, others no effect or an effect dependent on conditions. Such variability is not an argument against testing, but for treating each preparation as a hypothesis to be checked.

A simple additive test sheet

Record: the name and composition of the preparation, the dose, the start date, the substrate mix, the process parameters before the test, the success indicators (e.g. a rise in CH₄ in the gas by x percentage points, a drop in propionate below y), the end criterion and the person responsible. This way the conclusion after a few weeks rests on data, not on impressions.