Maintaining a biogas plant covers the combined heat and power (CHP) unit, process biology, mechanical parts such as agitators and pumps, and mandatory safety and environmental inspections. The CHP unit takes the most effort: it is serviced by operating hours, with oil changes and spark plug replacement every few hundred to several thousand hours and a major overhaul after roughly 40,000 to 80,000 operating hours. Whether you do the work yourself or sign a full service contract depends on your time, your expertise and how much risk of major damage you are prepared to carry.
Daily routine on the plant
A biogas plant needs attention every day, even though modern controls handle much of the work automatically. The daily routine usually includes a walk round with visual checks of the digester, gas store, pipework and pumps, a look at the CHP operating data, a gas quality check and recording the feedstock quantities used. For small manure plants, complete records of quantities are also needed to prove the manure share to the grid operator.
Many operators use remote monitoring that sends an alert to a mobile phone when a fault occurs. This does not replace the walk round, but it greatly shortens response time when the CHP unit trips.
An operating log recording faults, services, readings and anything unusual has proved its worth. It helps detect gradual changes, makes working with service technicians easier and is important evidence during official inspections or insurance claims. Also decide who looks after the plant during holidays or illness, and train that person in good time.
CHP maintenance
The CHP unit is the most heavily stressed component of the plant. At around 8,000 operating hours a year, the engine runs almost continuously. Service intervals are set by the manufacturer. The values below are rough guidelines that can differ considerably depending on engine, size and gas quality:
| Task | Typical interval (indicative) |
|---|---|
| Oil change | roughly 500 to 2,500 operating hours, depending on oil volume and oil analysis |
| Replace spark plugs | roughly 1,500 to 4,000 operating hours |
| Check and adjust valve clearance | usually together with larger inspections |
| Overhaul cylinder heads | roughly 15,000 to 30,000 operating hours |
| Major overhaul | roughly 40,000 to 80,000 operating hours, earlier for dual fuel engines |
| Replace activated carbon | when loaded, depending on the sulphur content of the gas |
Regular oil analysis shows whether the oil is still fit for use and whether wear or coolant ingress is occurring. Many failures announce themselves there early. Good gas conditioning is also essential: too much hydrogen sulphide and moisture in the gas shorten oil life and damage the engine. The guide to CHP units for biogas explains the engine technology.
Monitoring the biology
The microbiology in the digester is the real heart of the plant. Disturbances often show up late in gas output, so regular monitoring pays off. Common parameters are:
- FOS/TAC: the ratio of volatile organic acids to buffering capacity. Many operators measure it weekly themselves or have it measured. The trend matters, not a single reading.
- pH and temperature: sudden changes are a warning sign.
- Gas quality: methane, hydrogen sulphide and oxygen are usually measured continuously.
- Dry matter and organic dry matter: these show whether the digester contents are becoming too thick or too thin.
- Laboratory analyses: at longer intervals, analyses of acid spectrum, ammonium and trace elements give a more detailed picture.
Slurry-based plants are usually biologically more robust than plants with a high share of energy crops, because slurry brings trace elements and buffering capacity. Still, keep an eye on ration changes in the barn, new feedstocks or changes in quantity, and introduce them gradually.
Agitators, pumps and tanks
Agitators keep the digester contents moving, prevent floating and settled layers and let the gas escape. Typical wear parts are bearings, seals and propellers. On many plants, submersible motor agitators can be pulled out through service openings without emptying the tank. Make sure at the planning stage that agitators and pumps are easy to reach.
Over the years, sand and stones accumulate on the digester floor. Eventually cleaning is needed, which means downtime and cost. Feedstocks with few contaminants and good agitation extend the intervals. Gas storage membranes, overpressure and underpressure relief devices and condensate pits also need regular checks.
Pumps and solids feeders are wear parts too. Progressive cavity pumps lose performance through sand and foreign objects, while augers and feeders suffer from stones and metal in manure. A small stock of critical spares such as seals, stators or V-belts shortens downtime considerably, especially at weekends and on public holidays.
Safety and environmental inspections
Besides technical maintenance, there are inspection duties arising from various regulations. Which apply to your plant depends on size, permit and location. Common ones are:
- Explosion protection: risk assessment, explosion protection document and recurring inspections under the Industrial Safety Regulation (Betriebssicherheitsverordnung), including gas detectors and ventilation.
- Plant safety: the Technical Rule for Plant Safety TRAS 120 sets out safety requirements for biogas plants, including expertise, operating instructions and inspections.
- Water law: under the Ordinance on Facilities for Handling Substances Hazardous to Water (AwSV), biogas plants are inspected by approved experts before commissioning and at recurring intervals. The intervals follow from Section 46 AwSV in conjunction with Annex 5, with stricter rules in protected areas.
- Electrical installations: regular testing of fixed electrical installations.
- Emissions: emission measurements on the CHP unit where the permit or a regulation requires them.
- Emergency flare and safety devices: regular function tests.
A maintenance and inspection calendar listing all deadlines helps ensure nothing is missed. The guide to biogas plant safety covers hazards and protective measures.
Full service contract or doing it yourself?
| Criterion | Full service contract | Own work |
|---|---|---|
| Cost | higher but predictable (often per operating hour or per kilowatt hour) | lower but variable |
| Major damage and overhaul | usually included | borne by the operator |
| Availability | often guaranteed in the contract | depends on own response and spare parts |
| Time required | low | high |
| Expertise | with the contractor | must be available on the farm |
Between the two extremes there are mixed models. Under an inspection contract, the contractor carries out the larger services while the operator handles simple tasks such as visual checks or oil level checks. Separate advisory and laboratory contracts are available for process biology.
For a fair comparison, look at costs over the whole term, not just one year. With own work, a reserve for the major overhaul, your own working time and the risk of longer downtime belong in the calculation. A full service contract looks more expensive at first glance but can pay off if it secures high availability and covers costly one-off repairs.
Check the contract carefully before signing: which services and wear parts are included, is the major overhaul covered, what response time is promised, what happens if the guaranteed availability is not reached, and how are prices adjusted? The guide to choosing a biogas contractor helps with comparing quotes. Service companies are listed under service and maintenance.
Frequently asked questions
How often does a biogas CHP unit need servicing?
Intervals depend on operating hours and the manufacturer's specifications. Oil changes and spark plug replacement are due roughly every 500 to 4,000 operating hours, depending on the engine and oil volume. At around 8,000 operating hours a year, that means several services annually. A major overhaul is due after roughly 40,000 to 80,000 operating hours.
What does the FOS/TAC value mean?
FOS/TAC is the ratio of volatile organic acids to buffering capacity in the digester. A value that rises over a longer period points to overloading or a disturbance of the biology. The trend over several weeks matters more than any single reading.
Is a full service contract worthwhile for a small plant?
That depends on your time, technical experience and appetite for risk. A full service contract costs more than doing the work yourself, but usually also covers expensive repairs and the major overhaul. For small CHP units, an inspection contract combined with doing simple tasks yourself can be a sensible middle way.
Which inspections are mandatory for a biogas plant in Germany?
Depending on the plant and its permit, they include explosion protection inspections under the Industrial Safety Regulation, inspections by experts under water law (AwSV), electrical installation checks and emission measurements on the CHP unit. The specific duties follow from your permit and the applicable regulations.
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