Diagnosing low biogas production

Start with a reliable baseline

Establish what fell: raw-gas generation, methane content, upgrading throughput or saleable product. Check meter operation and units, the reporting period and downtime. Compare like-for-like data before attributing the loss to the digester.

Review feed and process together

Examine changes in feed quantity, moisture, organic fraction, contamination and preparation. Relate them to temperature, gas composition and the laboratory trends used by the plant. pH is useful, but it should not be the only indicator used to judge biological condition.

Check the equipment and gas path

Review actual feeding, mixing availability, transfer restrictions, storage, upgrading stoppages and compression. Gas that was produced but could not be processed is a different problem from gas that was never generated. Inspection and fault-finding must follow site safety procedures.

• Reconcile measured feed and production over matching periods.
• Record instrument faults and missing readings.
• Separate biological constraints from downstream downtime.
• Define an owner, due date and verification method for each action.

Make controlled corrections

An improvement plan should explain the suspected cause, the proposed intervention and how the outcome will be assessed. Avoid large unassessed loading or chemical changes. Plant-specific decisions require the responsible operating team and technical adviser, with the approved procedures governing execution.

Distinguish the symptom from the root cause

A low gas reading can result from lower production, an instrument fault or a changed reporting basis. A high electricity-per-kg figure can follow an output decline even without additional electricity consumption. Treat these as observations to investigate, rather than diagnoses.

Build a timeline of feed changes, equipment events, laboratory results and gas output. A short period of apparently normal pH should not override other evidence of a developing process problem. Site-specific trends and the established operating baseline are more informative than isolated values.

Close the investigation with a verified result

The corrective-action register should state the suspected cause, the action, the responsible person and the expected measurement response. Distinguish a temporary restoration of service from a permanent change that addresses repeated failure.

After the intervention, compare equivalent operating periods and record any other changes that could influence the result. If evidence remains inconclusive, say so and identify the next measurement or controlled trial needed. Unsupported claims of improvement make the next failure harder to investigate.