Optimising the methane fermentation process in agricultural biogas plants

The stability of methane fermentation depends on several interrelated parameters: temperature, pH, the balance of acids and alkalinity (FOS/TAC) and the organic loading of the digester, and the most common operating mistake is increasing the substrate dose too quickly. Below we discuss the basic indicators, their typical ranges and rules of conduct, without promising universal “ideal” values, because every plant has its own characteristics.

Temperature

Methanogenic microorganisms work efficiently in two ranges: mesophilic (usually 35–42 °C) and thermophilic (50–60 °C). Most agricultural biogas plants operate in the mesophilic range, because it uses less heat to warm the digester and is less sensitive to disturbances; the thermophilic process is faster and sanitises better, but requires more energy and can be less stable (comparison: Mesophilic or thermophilic fermentation). More important than the value itself is constancy: abrupt fluctuations of more than a few degrees in a short time upset the balance between groups of microorganisms and lower biogas production.

pH and FOS/TAC

The optimum pH for methane fermentation is usually in the range of approx. 6.8–7.5. Below this range methanogens are inhibited. Stability is additionally assessed by indicators of alkalinity and acids, including FOS/TAC, i.e. the ratio of volatile fatty acids (expressed as acetic acid) to total alkalinity, measured by titration. It is often assumed that low values (in the order of 0.3–0.4) mean a stable process, and high ones (above approx. 0.6) signal a risk, but these ranges depend on the substrate, the measurement method and the specific plant. That is why the trend is more important than a single measurement. Details: FOS/TAC: how to measure and interpret.

Digester loading (OLR)

Organic loading is the mass of volatile solids delivered to the digester per day, per cubic metre of its working volume, in kg VS/(m³·d). In agricultural biogas plants typical values are in the range of approx. 2–4 kg VS/(m³·d). Excessive loading is a common cause of problems: readily degradable substrates acidify the digester faster than methanogens can consume the acids. The rules for selection and control are discussed in the entry OLR: organic loading of the digester.

Recipe and co-digestion

The fermentation recipe should take into account:

• dry matter (TS) and organic matter (VS),
• methane potential (BMP) and methane content of the biogas,
• the rate of degradation, which affects the required retention time,
• the carbon-to-nitrogen ratio and the content of ammonia, sulphur and trace elements.

Combining substrates with complementary properties (e.g. slurry with high buffer capacity with high-yielding waste) stabilises the process and makes it possible to even out seasonal fluctuations in supply; it does, however, require changes to be introduced gradually and mixtures to be verified in the laboratory (Co-digestion: planning the substrate mix).

Monitoring and documentation

The basic set of indicators worth recording regularly is:

• temperature in the digester and its fluctuations,
• quantity and composition of biogas (CH₄, CO₂, H₂S),
• pH, FOS/TAC, and possibly ammonia concentration,
• daily dose and composition of substrates,
• quantity and composition of digestate.

Interpreting gas flow and composition together makes it possible to catch changes quickly, before they develop into a serious disturbance. Documenting recipes and the corresponding performance builds a knowledge base about the plant.

Rules of conduct

1. Introduce changes to the recipe and loading gradually, observing the process response.
2. React to the trend of indicators, not a single reading.
3. Keep the temperature constant and the mixing efficient.
4. Check new substrates with a BMP test and, if possible, in a mixture test.
5. Plan regular laboratory tests and sensor inspections.

Typical disturbances and first steps

The most common disturbances include acidification (rising FOS/TAC, falling pH, decreasing methane content), ammonia inhibition (with a large share of protein substrates), foaming and trace element deficiency. When worrying symptoms appear, the dosing of the most readily degradable substrates is usually reduced or stopped, the temperature and mixing are checked, and control analyses are carried out. Only after the indicators have stabilised is the previous dose returned to, gradually. After any more serious failure it is worth keeping samples and data from the period preceding the disturbance, because they make it easier to find the cause.