CH₄ falling, CO₂ rising: what it means and how to find the cause

A fall in methane content with a rise in CO₂ in biogas is a signal to check, but it is not a diagnosis. The most common biological cause is an imbalance between the stages of fermentation: acid-forming bacteria produce volatile fatty acids (VFA) faster than methanogenic archaea consume them, which lowers the pH and the methane share. But it may equally well be a measurement error, a change of substrate or air ingress. That is why the order is always followed: measurement, trend, process parameters, cause.

Step one: is the reading reliable

Before dosing starts to be changed, artefacts must be ruled out:

• Analyser calibration. Sensor drift, a worn electrochemical sensor or a lack of calibration can give lowered CH₄.
• Condensate and moisture in the sampling line. The gas may be diluted or disturbed by condensate.
• Presence of O₂ and N₂. If the rise in CO₂ does not correspond to the fall in CH₄ (the sum CH₄+CO₂ falls), and oxygen and nitrogen appear, air is entering (e.g. from biological desulphurisation, a leak or blower operation). See Biological desulphurisation.
• Sampling point. The gas composition differs between the digester, the store and the feed to the CHP.

Step two: trend and context

A reading from a single moment is not enough. The CH₄ and CO₂ content should be set against the gas quantity, dosing, substrate changes, temperature and pressure. It is good practice to compare with the typical variability of composition described in the text Variability of biogas composition and quantity: the composition naturally depends on the substrate, so a new batch of silage or of waste with a high carbohydrate content can temporarily shift the biogas composition (including the CO₂ share).

Step three: process parameters

VFA and alkalinity indicators are best interpreted as a trend, not as a single threshold; thresholds depend on the plant and the substrate (see VFA in a biogas plant). The measurement of pH and its buffering role is described in the text pH in the digester.

Possible biological causes

1. Overloading with organic load. Increasing the dose of easily fermentable substrates too quickly.
2. A change in substrate composition. A rise in the share of carbohydrates or fats.
3. Inhibitors. Ammonia, hydrogen sulphide, disinfectants, antibiotics in slurry.
4. Trace element deficiency. See Trace elements.
5. Temperature or mixing disturbances. In winter especially after a heating failure; see Heat balance of a biogas plant in winter.
6. Shortened retention time. Too intensive dosing or too rapid an outflow.

What to do

The response depends on the diagnosis. A typical set of actions is to reduce or temporarily stop dosing the most reactive substrates, restore stable parameters (temperature, mixing), sample and test more often, and, in the case of acidification, the actions described in the text on digester acidification. Agents added to the digester (buffers, additives) are chosen on the basis of analyses and laboratory recommendations, not “by eye”. Sudden changes in dosing in either direction make the situation worse.

How to reduce the risk

• Introduce changes in doses and substrates gradually.
• Keep a regular schedule of analyses of VFA, alkalinity, pH and ammonium nitrogen.
• Calibrate the analyser according to plan and keep a log of changes.
• Set alarm thresholds for gas composition and gas quantity and an escalation procedure.
• Use BMP data for new substrates before increasing their share; see What BMP is.

How to build a simple response procedure

A plant can adopt a short internal procedure: an alarm threshold for a fall in CH₄ (e.g. a deviation from the average of recent days), the order of checks (measurement, gas quantity, dosing, laboratory parameters), the decision-makers and the conditions for calling in a laboratory or process adviser. In this way the response does not depend on who happens to be on shift. The procedure should also specify how long the trend is observed before dosing is changed, to avoid overreacting to a single reading.

What not to do

• Do not increase dosing to make up for a fall in gas production. In the case of overloading this deepens the problem.
• Do not add chemical agents to the digester without analysis and recommendations.
• Do not ignore the trend on the assumption that the analyser has made a mistake, without checking the calibration.
• Do not introduce several changes at once, because then it is not known which one worked.