Troubleshooting: Biogas Plants problems

Operators can encounter biogas plants problems. These problems can inhibit the biological process of the plant and have consequences on its production capacity and profitability.

Here is your biogas plant troubleshooting guide, including a list of factors the operator should pay attention to, examples of problems that can occur and how to solve them.

Please note: This page is writen for general information only using publicly available sources mentionned below. If you are planning a biogas project, please make sure to connect with a professional company to discuss applicable standards in your country.

What factors can cause biogas plant problems?

When the biogas plant is not working or to prevent problems with the process, the operator should investigate or verify these factors.

Ammonia (NH3)

Ammonia is a compound of nitrogen and hydrogen that is produced during the digestion process. Factors such as temperature, inputs composition, pH and bacteria acclimation influence the effect of ammonia on this process.

What to keep an eye on:

The operator should make sure the ammonia concentration stays under 2000 ppm or between 50 to 200 mg/L to avoid biogas plant problems. Concentration levels that are between 1500 and 3000 mg/L can inhibit the process and cause digestion failure. However, mesophilic digesters can support more than 3000 ppm if it is well acclimated to ammonia. Ammonia levels that are higher than 3000 mg/L can be toxic for the process.

When the operator notices the concentration of ammonia is climbing, he should reduce the organic loading rate.

Alkalinity (ALK)

The capacity of water to neutralize acids in the biogas plant is called alkalinity (ALK). We measure ALK in milligrams of equivalent calcium carbonate per liter. It is used to measure carbonates, bicarbonates, hydroxides and, sometimes, borates, silicates and phosphates.

What to keep an eye on:

The operator should make sure the ALK stays between 1500 and 5000 mg/L. The buffering capacity of the digester content influences the stability of the digestion process. It should also be noted that small pH variations can affect methanogens. Acid producers can function with a wide range of pH.

Ratio between volatile acids and alkalinity

The operator can calculate the ratio between volatile acids and alkalinity to control the digestion process better.

What to keep an eye on:

The operator can use the following formula to calculate this ratio:

If the ratio is:

  • Below 0.35: Digester operations are proper
  • Between 0.1 and 0.35: The digester is well-operated
  • + 0.35: The digester experiences issues like increased organic loading, hydraulic overloading or others.


The anaerobic digestion process produces some oxygen even though it occurs with the absence of it.

What to keep an eye on:

The anaerobic digestion process requires below 0.1 ppm of oxygen in the environment, which is very little.

Volatile fatty acids (VFA)

Volatile fatty acids, or organic acids, shows the health of the digester. VFAs are also used as a food for the methane formers. They are measured in milligrams of equivalent acetic acid and are soluble in water. The production of VFA varies according to the quantity of solids supplied to the digester.

What to keep an eye on:

The concentration of volatile fatty acids should be below 2000 ppm. A higher concentration can off-balance the biology of the system and be toxic. The digester can overload, and the operator can experience other biogas plant problems when the concentration is over 300 ml/L.

Organic loading

The operator can measure organic loading based on mass or weight of volatile solids per unit of digester volume per day, or kg/m3 x d and lb/100 ft3 x d.

What to keep an eye on:

The organic loading of anaerobic digestion systems can range between 1.6 and 6.4 kg/m3 x d, or 100 and 400 lb/100 ft3 x d. In any case, however, the operator should supply the digesters at a consistent and constant rate.

Gas production

Anaerobic digestion produces methane gas, which can be converted into fuel, heat or electricity. Anaerobic digesters can produce between 0.8 and 1.1 m3/kg, or 13 and 18 ft3/lb, of volatile solids destroyed. The normal composition of digester gas is approximatively 65% methane and 35% carbon dioxide.  The heat value of the gas stays between 19 and 23MJ/m3.

What to keep an eye on:

The operator should verify the color of the flame at the waste gas burner. If the flame is blue, the digester produces quality methane. If the flame is yellow, the level of carbon dioxide the digester contains is increasing.

Too much carbon dioxide in the digester could hint at a digestion process problem. It could also cause malfunction for the equipment that is fueled with digester gas.

Salt Level

The anaerobic digestion process also produces salt, but its accumulation can affect the production and cause other kinds of biogas plant problems.

What to keep an eye on:

The operator should make sure the sodium concentration stays between 3500 to 5500 ppm.

Heavy metal

Heavy metal like copper can enter the digester because of industrial users. Traces of heavy metal is beneficial for the anaerobic digestion process. However, a higher concentration can be toxic to the process.

What to keep an eye on:

The soluble concentration of heavy metal should stay below 0.5 mg/L.

Temperature variation

The temperature in the biogas plant should be constant.

For example:

  • The temperature in a mesophilic digestion system is normally between 30°C and 38°C (85 °F and 100 °F)
  • The temperature in a thermophilic digestion system is normally between 50°C and 60 °C. (122 °F and 140°F).

What to keep an eye on:

The operator should also pay attention to any temperature variation that occurs during the anaerobic digestion process. It should not vary more than 0.6 °C for a mesophilic digestion system.

The temperature is these systems should ideally stay between 35°C and 37°C (95°F to 98°F). The retention time should range between 10 and 30 days.

The operator should look for any changes in a thermophilic digestion system, as temperature variation is especially hard on these microorganisms. The retention time for these systems should range between 5 and 12 days.

The operator of the plant can check the heating system and the PLC program if he notices any temperature variation in the mesophilic and thermophilic digestion systems.


The production of foam during the anaerobic digestion process can decrease its performance and cause safety issues, damaged equipment and/or structures.

What to keep an eye on:

The operator should pay attention to the mixing, temperatures variations in the digester and/or incorrect or inconsistent supplying if there is foam. It should also be noted that the transfer of filaments from the liquid process stream to the digesters can create foam.


The operator should mix the contents of anaerobic digesters to make sure the temperature stays constant and that the supplied solids are well dispersed. The mixing should be good and thorough for optimal operation.

What to keep an eye on:

If a mixer or pump doesn’t work, the operator should look in the user manual of the equipment for troubleshooting. He can also ask for technical assistance to repair any parts.

Struvite creation

Struvite is a magnesium ammonium phosphate compound (MgNH4PO4). It forms scale deposits in anaerobic digesters and in the downstream dewatering system. When this happens, struvite can cause maintenance problems, such as clogging pipes, valves, heat exchangers and more.

What to keep an eye on:

The operator should pay attention to any deposits of struvite in the digesters because they are difficult to remove.  There are ways to remove these deposits, such as acid washing, but it is time-consuming and can be a safety issue.  Other facilities use ferric chloride or ferrous chloride in digesters to prevent struvite deposits.

What are the problems that can occur in biogas plants?

Here are examples of problems that can occur in a biogas plant, their causes, and solutions to solve them.


Most likely causes

Recommended action

Gas yield has dropped
  • Drop in quality of substrates
  • Drop of temperature
  • Compounds inhibition
  • Non-homogenous substrates
  • Drop of methanogenic bacteria
  • Assure substrates mixing and quality
  • Check heating system
  • Check level of potential inhibitor compounds
  • Add digestate from another digester if the methanogenic bacteria as drop
Methane concentration dropped
  • Drop in quality of substrates
  • Drop of temperature
  • Compounds inhibition
  • Assure substrates mixing and quality
  • Check heating system
  • Check level of potential inhibitor compounds
Foaming problem
  • A new substrates with high protein content has been added
  • Air is introduced in the digestion
  • Temperature is changing
  • Reduce or stop feeding
  • Analyze substrates
  • Reduce air introduction
pH dropped
  • Feeding rate is too high or variable
  • Operating temperature have changed
  • Agitation is not working
  • Reduce substrates until system returns to normal
  • Use only manure until system returns to normal
FOS/TAC ratio has increased
  • VFA rate is too high
  • Change in feedstock
  • Lack of buffer
  • Compounds inhibition
  • Reduce OLR
  • Use more manure in the mixte


Water Environment Federation

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