A biogas plant crash rarely begins with one dramatic event. More often, organic acids accumulate, buffering capacity declines, methane production weakens, and operators discover that the digester can no longer process its normal feed rate.
This condition is commonly described as a sour digester, an overloaded digester or an anaerobic digestion process upset. If it is recognised early, operators may be able to restore stability by reducing the organic load, maintaining appropriate temperature and mixing, investigating the cause and restarting feed cautiously. If warning signs are missed, gas production may fall severely, and recovery can become lengthy and expensive.
Case-study context: This article originated from notes made from a YouTube video showing operators responding to a falling gas-holder level, deteriorating gas production and suspected overfeeding at an agricultural biogas plant. The original video can no longer be reliably identified. The incident is therefore used only as an anonymised operational scenario. Unverified names, instrument readings, feed quantities and recovery claims have been removed. The technical guidance below is based on published operating guidance and research rather than on the unidentified video.
What Is a Biogas Plant Crash?
“Biogas plant crash” is an informal expression rather than a precisely defined scientific term. Operators generally use it to describe a serious deterioration in biological performance that causes one or more of the following:
- a sustained fall in biogas production;
- a decline in methane concentration;
- accumulation of volatile fatty acids;
- loss of alkalinity or buffering capacity;
- falling pH;
- foaming or abnormal digestate behaviour;
- unstable gas pressure or gas-holder level;
- inability to supply a CHP engine, boiler or upgrading plant;
- the need to reduce or stop feeding while the biology recovers.
The failure is not necessarily permanent. Anaerobic microbial communities can recover, but the appropriate response depends on the cause, the severity of the imbalance and the digester design.
No operator should apply a generic internet restart recipe without first reviewing the plant’s process data, operating manual, feedstock history and advice from a competent process specialist.
What Does “Sour Digester” Mean?
A digester becomes “sour” when acid production exceeds the rate at which methanogenic microorganisms can convert the intermediate products into methane.
During anaerobic digestion, acid-forming organisms convert degradable organic matter into volatile fatty acids, or VFAs. Methanogens subsequently use products including acetate, hydrogen and carbon dioxide to produce methane.
Acid-forming organisms can respond rapidly when a readily degradable feed is added. Methanogens generally respond more slowly and are more sensitive to adverse conditions. A sudden rise in organic loading can therefore cause VFAs to accumulate faster than they are consumed.
As the digester’s buffering capacity is used, pH may eventually fall. Methane production then declines further because the methanogenic community is adversely affected, while acid production may continue. This creates the characteristic downward spiral associated with an overloaded or sour digester.[1]
Why pH Alone May Give a Late Warning
Operators sometimes assume that a stable pH proves that the digester is healthy. That is unsafe because bicarbonate and other buffering compounds can temporarily maintain pH while VFAs are already accumulating.
The US EPA AgSTAR operator guidebook describes pH as a lagging indicator of anaerobic digester instability. It recommends monitoring alkalinity because alkalinity indicates the liquid’s capacity to resist acid-driven pH change.[1]
A plant may therefore be moving towards instability while its pH remains within its familiar operating range. The trend in VFA concentration, alkalinity, gas flow, methane content and feed loading can provide an earlier warning than pH alone.
Principal Causes of a Sour Digester
1. Organic Overloading
Organic overloading occurs when the digester receives more biodegradable material than its microbial population can process under the prevailing conditions.
This may result from:
- increasing the daily feed quantity too quickly;
- receiving a feedstock with a higher volatile-solids content than expected;
- adding a highly degradable material without reducing other feeds;
- poor mixing that concentrates fresh feed in part of the tank;
- short-circuiting or loss of effective digester volume;
- reduced biological activity caused by falling temperature or inhibition.
The EPA operator guidebook warns that rapid changes in organic loading rate can adversely affect the biological balance and can contribute to foaming and process upset.[1]
Overfeeding should not be judged only by tonnes per day. Ten tonnes of dilute slurry may impose a very different organic load from ten tonnes of high-energy food-processing waste. Operators should consider volatile solids, chemical oxygen demand, biochemical methane potential, degradability and hydraulic loading.
2. Sudden Feedstock Changes
A plant may remain stable at a given tonnage and then become unstable after the feed composition changes.
Potentially troublesome changes include:
- a sudden increase in fats, oils or grease;
- highly degradable sugar or starch-rich wastes;
- protein-rich material that increases ammonia loading;
- acidic process wastes;
- salty material;
- cleaning chemicals, disinfectants or antibiotics;
- feed containing compounds toxic to anaerobic microorganisms.
Feedstock acceptance procedures should therefore address composition and contamination as well as mass and delivery documentation. The EPA recommends keeping detailed records for off-site co-digestion feedstocks and retaining representative samples where appropriate.[1]
3. Temperature Disturbance
Anaerobic digestion can operate over different temperature ranges, but each microbial community adapts to its established conditions. Mesophilic digesters commonly operate at approximately 35°C to 40°C, while thermophilic processes operate at higher temperatures.[2]
The most important operational requirement is often not an exact universal temperature but temperature stability. A rapid change can slow methanogenic activity, alter gas production and allow VFAs to accumulate.
Temperature problems may arise from:
- CHP or boiler failure;
- heat-exchanger fouling;
- loss of circulation;
- faulty sensors;
- cold feedstock additions;
- poor insulation;
- mixing failure that creates temperature gradients.
A fall below 30°C does not mean that all digester organisms immediately die. However, a substantial temperature decline in a plant adapted to warmer mesophilic operation can markedly reduce reaction rates. Continuing to feed at the former rate may then create an effective organic overload.
4. Ammonia and Other Inhibitors
Protein-rich feedstocks release ammoniacal nitrogen. Ammonia provides nutrients required by microorganisms, but excessive free ammonia can inhibit methanogenic activity.
The degree of inhibition depends on several interacting factors, including:
- total ammoniacal nitrogen;
- pH;
- temperature;
- microbial acclimatisation;
- feed composition;
- retention time.
Other inhibitors can include sulphide, salts, heavy metals, disinfectants, solvents and process chemicals. An unexplained crash following a new waste delivery should therefore trigger a feedstock investigation rather than an automatic assumption that tonnage alone caused the problem.
5. Mixing, Pumping or Hydraulic Failure
A biological failure may begin with a mechanical fault.
Inadequate mixing can cause:
- poor contact between microorganisms and substrate;
- temperature stratification;
- floating layers or sediment accumulation;
- localised acidification;
- loss of usable digester volume;
- blocked pipes and heat exchangers.
Excessive or inappropriate mixing can also create problems, particularly if it disrupts established biomass structures or increases foaming. Mixing performance should be assessed against the plant design rather than by assuming that more mixing is always better.
6. Loss of Active Biomass
Hydraulic overloading, unintended discharge, excessive solids removal or process short-circuiting may reduce the concentration of active microorganisms or the effective solids-retention time.
This is particularly serious for slow-growing methanogens. Restoring temperature and pH does not immediately replace biomass that has been washed out of the process.
Early Warning Signs of Process Instability
No single measurement is a universally reliable predictor of every digester failure. Research into food-waste digestion has found that the best warning system combines several complementary parameters rather than relying on one threshold.[3]
| Parameter | Possible Warning Trend | Important Limitation |
|---|---|---|
| Biogas flow | Sustained fall relative to feed input | May also be caused by leakage, meter failure or gas-use changes |
| Methane concentration | Declining methane with rising carbon dioxide | Must be interpreted with gas-flow data and calibrated equipment |
| Volatile fatty acids | Rising total VFA or accumulation of individual acids | Absolute limits vary between plants and analytical methods |
| Alkalinity | Declining buffering capacity | Results depend on the alkalinity fraction and test method used |
| VFA/alkalinity ratio | Rising ratio relative to the plant’s stable baseline | No single threshold is valid for every feedstock and digester |
| pH | Persistent downward movement | Often a late indicator because buffering can mask instability |
| Temperature | Unexpected change or increasing variation | Sensor location and calibration must be checked |
| Gas-holder level | Progressive deflation or failure to recover | Can reflect increased gas use, leakage or instrument error |
| Foaming | New or increasing foam | Can have biological, chemical and mechanical causes |
| Feedstock data | Change in dry matter, volatile solids or composition | Requires representative sampling and reliable records |
FOS/TAC, VFA/Alkalinity and the Danger of Universal Thresholds
Some European biogas plants use a two-stage titration commonly described as the FOS/TAC ratio. FOS represents an estimate associated with volatile organic acids, while TAC represents an estimate of buffering or carbonate alkalinity.
The term is sometimes incorrectly written as FOS/TOC. TOC normally means total organic carbon and is not the denominator in the conventional FOS/TAC digester-stability test.
Published guidance and equipment suppliers often provide indicative warning bands. However, results vary with:
- the titration method;
- sample preparation;
- feedstock type;
- ammonia concentration;
- digester configuration;
- laboratory calculation;
- the plant’s normal operating state.
For this reason, a ratio should be interpreted against a validated method and the individual digester’s historical baseline. The EPA guidebook, for example, advises increased testing when the VFA-to-total-alkalinity ratio reaches 0.25 or more, but this is guidance for the systems covered by that document and should not be misrepresented as a universal shutdown value.[1]
The trend and rate of change may be more useful than a single isolated result.
What Happens When a Digester Crashes?
Falling Gas Production and Revenue
The immediate commercial consequence is often a reduction in usable biogas. A CHP engine may derate or shut down if gas flow, methane concentration, pressure or contaminant levels fall outside its operating specification.
For a biomethane plant, unstable raw gas can also disrupt upgrading and grid-entry operations.
Loss of Process Heat
Where CHP heat maintains digester temperature, reduced gas production may create a feedback loop:
- gas output declines;
- the CHP engine runs for fewer hours or shuts down;
- less recovered heat reaches the digester;
- digester temperature falls;
- biological reaction rates decline further.
A correctly designed backup heating arrangement can be crucial. However, operators must follow the plant’s safe operating procedures and consider gas availability, boiler specification and the implications of any fuel switching.
Foaming and Loss of Working Volume
Organic overloading and rapid loading changes can contribute to foaming. Foam can enter gas pipework, contaminate instruments, activate pressure-control devices and reduce effective digester volume.
Foaming should be treated as a process and safety issue, not merely as a housekeeping inconvenience.
Digestate Quality and Storage Problems
An unstable digester may discharge material with a higher residual biodegradability and greater odour potential. If digestion is incomplete, further gas production can occur in storage.
Operators must continue to comply with permit conditions and digestate-management requirements during a process upset.
Safety Risks
Biogas is principally a mixture of methane and carbon dioxide and can contain hydrogen sulphide and other trace gases. Methane creates fire and explosion hazards; carbon dioxide can displace oxygen; and hydrogen sulphide is acutely toxic.
UK HSE enforcement notices have specifically identified inadequate assessments of methane, hydrogen sulphide, carbon dioxide, fire, explosion, asphyxiation and confined-space hazards at anaerobic digestion facilities.[4]
Falling biological performance does not remove these risks. Gas may still be present in tanks, pipework, gas holders and enclosed spaces.
Why Odour Is Not a Safe Diagnostic Method
A “sour” smell around a digester does not prove that the liquid pH has fallen. Odour may arise from VFAs, hydrogen sulphide, ammonia or other compounds.
Operators must never rely on deliberately smelling gas releases. Hydrogen sulphide can rapidly cause serious harm, and at high concentrations the sense of smell becomes unreliable. HSE warns that low-pressure releases containing hydrogen sulphide can accumulate under unfavourable dispersion conditions.[5]
Gas composition should be assessed with suitable, calibrated equipment under a safe system of work. Pressure-relief devices should not be used as informal sampling points unless the system was specifically designed and risk-assessed for that procedure.
Immediate Response to Suspected Digester Instability
The correct response must follow the plant’s operating manual, emergency procedures, permit and competent technical advice. A general response framework is as follows.
1. Confirm the Measurements
Before concluding that the biology has failed, check for:
- gas-flow meter faults;
- gas-analyser calibration problems;
- closed or incorrectly positioned valves;
- gas leakage;
- condensate blockage;
- gas-holder level-sensor faults;
- CHP or upgrading-plant instrumentation errors.
2. Review Recent Changes
Compare the onset of the problem with:
- feed quantities;
- new feedstocks or suppliers;
- dry-matter and volatile-solids results;
- temperature changes;
- mixing or pump outages;
- maintenance work;
- chemical additions;
- digestate withdrawal or recirculation changes.
3. Reduce the Organic Load Where Overfeeding Is Suspected
If the evidence points to organic overloading, reducing or temporarily suspending feed can prevent further acid accumulation.
There is no universal rule that every digester should be left unfed for exactly two days. The reduction and duration should be based on the plant’s VFA, alkalinity, pH, methane, temperature and gas-production trends, together with specialist advice.
4. Maintain Stable Temperature
Operators should protect the established operating temperature as far as safely practicable. Heating changes should be gradual, and the heating and mixing equipment should be checked for correct operation.
Attempting to recover lost temperature too aggressively can create local overheating or additional stress.
5. Continue Mixing as the Design Requires
Mixing may be needed to distribute heat, microorganisms and residual substrate. However, the appropriate mixing regime is plant-specific. Operators should not change mixer operation arbitrarily during a crisis.
6. Increase Sampling and Trend Review
Useful measurements may include:
- feed and digestate pH;
- total and bicarbonate alkalinity;
- total and individual VFAs;
- FOS/TAC where this is an established site method;
- temperature at representative locations;
- gas flow;
- methane and carbon-dioxide concentration;
- hydrogen sulphide;
- ammoniacal nitrogen;
- dry matter and volatile solids;
- foam level and gas-holder condition.
The EPA guidebook recommends recording and graphing operating data because trends are central to identifying deterioration and recovery.[1]
7. Protect People and Gas Equipment
During an upset, operators should maintain all DSEAR, COSHH, confined-space and process-safety controls. Uncontrolled biogas releases must be prevented, and gas storage and treatment equipment must be inspected and maintained.[6]
Personnel should not enter tanks, pits, chambers or other potentially confined spaces without the required risk assessment, atmospheric testing, isolation, ventilation, competent supervision and rescue arrangements.
Should Operators Add Lime or Another Alkali?
Buffer addition can sometimes be part of a professionally designed recovery plan, but adding lime, caustic soda, bicarbonate or ash without adequate analysis can create new problems.
Potential consequences include:
- raising pH too rapidly;
- increasing free-ammonia toxicity;
- localised extreme pH near the dosing point;
- precipitation and scaling;
- handling hazards;
- masking the continuing production of VFAs without correcting the cause.
Alkali addition does not remove the excess organic load. It only increases buffering or changes pH. The dose, chemical and addition rate should be determined from representative analysis and competent advice.
Restarting Feed After a Sour Digester Event
Feed should normally be restored in controlled stages rather than immediately returning to full design loading.
A defensible restart process may include:
- confirming that temperature and mixing are stable;
- checking that VFA accumulation has stopped or reversed;
- confirming that alkalinity is no longer falling;
- observing an improving gas-production and methane trend;
- reintroducing a familiar, consistent feedstock at reduced loading;
- holding each loading stage long enough to observe the biological response;
- increasing loading only when the agreed control parameters remain satisfactory;
- continuing enhanced sampling until normal stability is demonstrated.
The percentage of normal feed and duration of each stage must be set for the individual plant. Fixed tonnages quoted from another facility are unlikely to be transferable because digester volume, feed composition, biomass concentration and retention time differ.
Can Gas Blending Keep a CHP Engine Operating?
Some installations may have more than one gas holder, digester or gas source. Blending gas streams can sometimes help maintain a fuel specification, but it must not be improvised.
The feasibility depends on:
- the approved pipework and control design;
- gas pressure compatibility;
- methane concentration;
- hydrogen sulphide, moisture and siloxane levels;
- the CHP manufacturer’s fuel-gas specification;
- fire and explosion risk assessment;
- automatic isolation and pressure protection;
- the effect on flare and emergency systems.
A gas-holder reading described simply as “38 units” or “55 units” is not enough to design a safe blend. The units might refer to methane percentage, calorific value, pressure, level or an instrument-specific scale.
How to Prevent Future Biogas Plant Crashes
Establish a Normal Operating Baseline
Every digester develops its own normal range and seasonal pattern. Operators should establish baselines for:
- organic loading rate;
- hydraulic loading;
- gas yield per tonne or per kilogram of volatile solids;
- methane concentration;
- VFA and alkalinity;
- FOS/TAC where used;
- pH;
- temperature;
- ammoniacal nitrogen;
- mixing energy and pump performance.
Control limits should be based on the plant’s history, equipment requirements, process design and specialist interpretation—not copied uncritically from a different feedstock or digester.
Control Feedstock Change
New or variable feedstocks should be characterised before their full-scale introduction. A change-control procedure should consider:
- biodegradability;
- dry matter and volatile solids;
- nitrogen and sulphur content;
- fats, oils and grease;
- salinity;
- pH and alkalinity;
- potential inhibitors;
- physical contaminants;
- appropriate ramp-up rate.
Calibrate Instruments and Sample Correctly
Incorrect readings can cause operators either to miss a real upset or to make harmful interventions in a healthy digester.
Sampling locations and procedures should produce representative results. Gas analysers, temperature probes, pressure instruments and flow meters should be calibrated and maintained in accordance with their specifications.
Maintain Heating, Mixing and Backup Systems
Preventive maintenance should cover:
- CHP heat recovery;
- backup boilers;
- heat exchangers;
- circulation pumps;
- mixers;
- temperature sensors;
- gas-holder instrumentation;
- pressure and vacuum relief;
- flare availability;
- gas treatment equipment.
The Environment Agency (England) requires permitted AD facilities to prevent uncontrolled releases and to inspect, maintain, test and record the condition of gas-storage and treatment equipment.[6]
Use Multiple Early-Warning Indicators
Peer-reviewed research has repeatedly found that gas production, methane content, VFAs, alkalinity and combined ratios can all contribute to early warning, but no one indicator is universally sufficient.[3]
A robust dashboard should show trends and relationships rather than isolated numbers. For example, a falling gas yield combined with rising VFAs and declining alkalinity is more persuasive evidence of biological stress than a single unusual pH result.
Plan for Loss of CHP Heat
The plant’s contingency plan should explain how digester temperature will be protected if the CHP engine is unavailable. It should address:
- backup boiler capacity;
- alternative fuel availability;
- critical heat demand;
- cold-weather operation;
- safe reduction of feed;
- alarm and call-out arrangements.
Train Operators to Distinguish Biological and Mechanical Problems
A deflated gas holder does not by itself prove biological failure. It may be caused by increased gas consumption, a leak, a blocked line, a failed instrument or a pressure-control problem.
Operators need enough process and mechanical understanding to verify the evidence before intervening.
Biogas Plant Crash Prevention Checklist
- Record the mass and composition of every significant feedstock.
- Avoid abrupt increases in organic loading rate.
- Use a formal approval and ramp-up process for new feedstocks.
- Trend gas flow and methane content against volatile-solids loading.
- Monitor alkalinity and VFAs rather than relying on pH alone.
- Interpret FOS/TAC against a validated site method and baseline.
- Maintain stable digester temperature.
- Verify mixer, pump and heat-exchanger performance.
- Keep gas analysers and process instruments calibrated.
- Maintain backup heating and safe flare availability.
- Investigate foaming, odour and gas-holder changes promptly.
- Maintain DSEAR, COSHH and confined-space controls during process upsets.
- Document the cause, response and lessons from every significant instability event.
Frequently Asked Questions
What causes a sour digester?
A sour digester develops when acid production exceeds methanogenic conversion and the digester’s buffering capacity is progressively consumed. Common causes include organic overloading, sudden feedstock changes, temperature disturbance, inhibition, mixing failure and loss of active biomass.
Does a sour digester always have a low pH?
No. Buffering can maintain an apparently normal pH while VFAs accumulate. That is why pH is often a late indicator and should be assessed with alkalinity, VFA, gas-production and methane trends.
What is the correct FOS/TAC limit?
There is no single limit that can safely be applied to every digester. Indicative ranges exist, but the result depends on the method, feedstock and process. Operators should use a validated test and compare results with their plant’s stable historical baseline.
Should feeding always stop for 48 hours?
No. A reduction or pause may be appropriate when overloading is confirmed, but the duration should be based on process data and specialist advice. Forty-eight hours is not a universal recovery rule.
How long does recovery take?
Recovery may take days, weeks or longer. It depends on the severity and cause of the upset, whether methanogenic biomass has been lost, temperature, feedstock, alkalinity and the effectiveness of corrective action. A guaranteed four-to-eight-week period cannot be stated for all plants.
Can lime rescue a sour digester?
Alkaline chemicals may be used in some recovery plans, but indiscriminate dosing can worsen ammonia inhibition, cause scaling or create hazardous local pH conditions. Dosing should be based on representative analysis and competent advice.
Can operators judge hydrogen sulphide by smell?
No. Smell is not a safe or reliable exposure-control method. Suitable calibrated gas-detection equipment and safe working procedures are required.
Conclusion
A biogas plant crash is usually the end point of a developing imbalance rather than an unavoidable surprise. Organic overloading, changing feed composition, falling temperature, inhibition and equipment failure can all disrupt the relationship between acid-forming organisms and methanogens.
The most effective defence is disciplined process monitoring. Gas flow, methane concentration, temperature, volatile fatty acids, alkalinity, organic loading and equipment condition should be assessed together and trended over time.
When instability is detected, operators should verify the measurements, identify recent changes, reduce the organic load where appropriate, protect temperature and mixing, increase sampling and follow a site-specific recovery plan.
Prevention is normally faster and less expensive than rebuilding a seriously damaged microbial process. Stable feeding, representative sampling, properly maintained equipment and trained operators remain the foundations of dependable biogas production.
Reference Sources
- US Environmental Protection Agency, AgSTAR: Anaerobic Digester/Biogas System Operator Guidebook. Guidance covering organic loading, process monitoring, pH, alkalinity, VFAs, gas testing, data recording and operational troubleshooting.
- UK Health and Safety Executive: Disposal and Energy Recovery—Anaerobic Digestion. Overview of mesophilic and thermophilic operation and relevant UK health-and-safety legislation.
- Li, L. et al.: Early Warning Indicators for Monitoring the Process Failure of Anaerobic Digestion Systems of Food Waste, Bioresource Technology. Research evaluating gas production, methane, pH, VFAs, alkalinity and combined stability indicators.
- UK Health and Safety Executive: Improvement Notice Relating to Biogas, Fire, Explosion, Asphyxiation and Confined-Space Risks.
- UK Health and Safety Executive: High Concentrations of Hydrogen Sulphide—Safety Notice. Guidance on toxic exposure, monitoring and the danger from low-pressure gas releases.
- Environment Agency: Biological Waste Treatment Appropriate Measures—Waste Treatment, Biogas Treatment and Storage.
- UK Health Security Agency: Hydrogen Sulphide—Properties, Incident Management and Toxicology.
- Li, L., Peng, X., Wang, X. and Wu, D.: Anaerobic Digestion of Food Waste—A Review Focusing on Process Stability, Bioresource Technology.
- Wu, D. et al.: Anaerobic Digestion—A Review on Process Monitoring, Renewable and Sustainable Energy Reviews.
- Official UK Anaerobic Digestion Information Portal: About Anaerobic Digestion. General information on the process, feedstocks and the poor anaerobic biodegradability of lignified woody material.
Technical and safety notice: This article provides general information and does not replace a plant-specific operating manual, permit, DSEAR assessment, process-safety review or advice from the digester designer, CHP supplier, laboratory and competent biological-process specialist.
