An Overview: Troubleshooting is the systematic approach to solving a problem. It mainly deals with tracing and correcting faults in a mechanical or an electronic system. Troubleshooting is done when dealing with biogas output from anaerobic digestion systems. For this reason, troubleshooting is absolutely necessary, particularly in the event of a meltdown or a failure.
Problems that lead to Troubleshooting in Biogas Plants
Below are some of the problems that manifest themselves in biogas production, important points to look at, and finally, the application of remedies.
1. Scum Formation
Scum is a layer of dirt or froth on the surface of a liquid. With floating drum biodigesters, scum formation is easily identified by lifting the gasholder.
Remedies: Upon lifting the gasholder, scoop the scum out of the biodigester using a bucket. In the case of fixed dome biodigesters, an inspection would require opening the expansion chamber or the central manhole cover lid (Bensah, Antwi, Ahiekpor, 2021).
Depending on the size of the biodigester, a bucket or a water suction pump can be used to evacuate the biodigester for plants without a central manhole. Lastly, inoculate or treat the biodigester again after removing the scum.
2. Cracks in the Biodigester
Source of cracks: The source of cracks in the biodigester’s walls can be traced back to the construction phase. Owing to this, poor brick-layering standards lead to cracks in the biodigester. Examples are failing to confirm that each stacked brick is aligned with the radius and failing to correctly secure bricks with hooks when they begin to move. In addition, cracks can also form as a result of insufficient backfilling.
Backfilling in fact ensures that the stabilized soil around the biodigester balances the pressure inside. Roots of organic elements in the soil can also cause cracks by penetrating through open mortar joints and pulling water through them (Bensah et al., 2021).
Remedies in dealing with cracks: Because cracks in a biodigester might be difficult to repair, make sure you use a high-quality brick stacking standard during construction. In addition, use cement punning inside the biodigester to repair minor cracks. An example of this is Mr. Fix’s waterproof cement.
3. Leakage from gas pipes and fittings
Sources of leakages: Poor gas pipe networks can cause gas leakage. Once the gas begins to flow through the pipe, examine all joints for gas leaks. Subsequently, pour soapy water around the joints to accomplish this. Due to gas leakage, effervescence occurs around the joint.
Remedies in dealing with leakages: Firstly, close the main gas valve to disconnect the line. After that, remove and thoroughly examine the afflicted joint. Before screwing the treated end to its partner, apply a lot of Teflon tape. Lastly, remove and replace broken PVC pipes.
4. Inadequate biodigester feeding
Problem Source: Insufficient biodigester feeding is a cause of poor biodigester performance.
Remedies: Firstly, ensure that the biodigester receives the proper amount of feed on a daily basis. In addition, check the feeding logbook to make sure the biodigester is properly fed. As a result, the microorganism population will be depleted due to poor feeding, therefore resulting in limited gas output (Bensah et al., 2021).
5. Improper feed composition (Wrong C/N composition)
Carbon-Nitrogen ratio (C/N) is the measure of the amount of carbon and nitrogen present in a feedstock. Particularly, the concentrations of carbon and nitrogen influence the efficiency of the digestion process. In other words, carbon constitutes the energy source for the microorganisms in the biodigester and nitrogen serves to enhance microbial growth.
What to keep an eye on: C/N ratios ranging from 20 to 30 are considered perfect for anaerobic digestion with about 25 being the optimum. High C/N ratios, for instance, promote the growth of methanogens which results in the increased consumption of nitrogen to meet their protein requirements.
On the contrary, the methanogens will not react with the leftover carbon content of the substrate, consequently leading to low gas production. However, low C/N ratios imply high nitrogen. Therefore this leads to ammonia accumulation to levels that can poison the methanogens.
Remedy measures:
It is important to use feedstocks with the proper C/N ratio for optimal biogas generation. Avoid feed compositions with low Carbon: Nitrogen (C: N) ratio (Bensah et al., 2021).
Table 1: Feedstock and their Carbon- Nitrogen content
| Dung | C/N Ratio | Crop Residue | C/N Ratio |
| Cow manure | 16.5 -25 | Rice Straw | 51-67 |
| Pig manure | 6.2 – 13.5 | Bagasse (sugar/sorghum fibre) | 140-150 |
| Chicken manure | 5 – 9.6 | Soybean stalks | 33 |
| Human excreta | 6 – 10 | Water Lily | 11.4 |
6. Ammonia Content
Ammonia is a compound of nitrogen and hydrogen. Production of ammonia takes place during the digestion process.
What to watch out for: To avoid biogas plant difficulties, the operator should keep the ammonia content below 2000 ppm, or between 50 and 200 mg/L. Concentrations of 1500 and 3000 mg/L however, can obstruct the process and cause digestion failure in the biogas plant. However, if the mesophilic digester is properly adapted to ammonia, it can handle more than 3000 ppm (parts per million).
Ammonia levels that are higher than 3000 mg/L can be toxic for the process. When the ammonia concentration begins to rise, the operator should reduce the organic loading rate (Biogas world, 2022).
7. Volatile fatty acids (VFAs), or organic acids
These show the health of the digester. VFAs are also used as a source of nutrition for methane producers. Furthermore, they are soluble in water and are measured in milligrams of equivalent acetic acid. Therefore, the number of solids fed to the digester influence the amount of VFA produced.
What to keep an eye on: Volatile fatty acid concentrations should be less than 2000 parts per million (ppm). In contrast, a larger concentration can make the biodigester system hazardous. Indeed, when the concentration exceeds 300 ml/L, the digester can overload, leading to further biogas plant issues (Biogas world, 2022).
8. Methane gas production
Anaerobic digestion produces methane gas, which can be converted into fuel, heat, or electricity. Indeed, anaerobic digesters can produce between 0.8 and 1.1 m3/kg, or 13 and 18 ft3/lb (cubic foot/pound), of digested volatile solids. In addition, the normal composition of digester gas is approximately 65% methane and 35% carbon dioxide. Therefore, the heat value of the gas should stay between 19 and 23 Mega joules/cubic meter (MJ)/m3.
What to keep an eye on: Check the color of the flame at the waste gas burner. The digester produces quality methane if the flame is blue. If the flame turns yellow, on the contrary, this is an indication that the amount of carbon dioxide in the digester is rising. Hence, a buildup of carbon dioxide in the digester could indicate a problem with biogas digestion. Moreover, it could also cause equipment that runs on digester gas to malfunction.
9. Salt
Salt is produced during the anaerobic digestion process. However, its accumulation can cause production problems in biogas plants.
What to keep an eye on: The operator should make sure the sodium concentration stays between 3500 to 5500 parts per million (ppm).
10. Heavy metals
Heavy metals like copper can enter the digester because of the biogas industrial use. Traces of heavy metal are in fact beneficial for the anaerobic digestion process. However, a higher concentration can be toxic to the process. Hence, the operator should make sure that the soluble concentration of heavy metal stays below 0.5 milligrams per litre (mg/L).
11. Temperature
The temperature in the biogas plant should be maintained at a consistent level. The mesophilic temperature in a digesting system is usually between 30°C and 38°C (85°F and 100°F). Thermophilic digestion system temperature, on the other hand, is usually between 50°C and 60°C (122°F and 140°F) (Biogas world, 2022).
What to keep an eye on: During the anaerobic digestion process, the operator should take note of any temperature variations. For a mesophilic digesting system, the temperature should not vary more than 0.6 degrees Celsius. As a result, these systems should be kept at a temperature of 35°C to 37°C (95°F to 98°F). The retention period should be between 10 and 30 days.
Temperature variation greatly works up the bacteria in the biodigester. Thus, the operator should keep an eye out for any changes in a thermophilic digesting system. In fact, these systems should have a retention time of between 5 and 12 days. Furthermore, if the plant operator sees any temperature variations in the mesophilic and thermophilic digestion systems, he can inspect both the heating system and the Programmable Logic Controller (PLC) program.
12. Foam production in the digester
Foam production during the anaerobic digestion process can decrease its performance and cause safety issues. Moreover, foam causes damaged equipment and/or structures.
What to keep an eye on: The operator should pay attention to the mixing and temperature variations in the digester and inconsistent supply if there is foam. It should also be noted that foam is created by the transfer of filaments from the liquid process stream to the digesters.
Additionally, the operator should mix the contents of anaerobic digesters properly to make sure the temperature stays constant and that the supplied solids are well dispersed. If a mixer or pump doesn’t work, the operator should look in the user manual of the equipment for troubleshooting. He/she can also ask for technical assistance to repair any parts of the biogas plant (Biogas world, 2022).
13. Foreign Chemical Compositions(Struvite composition)
Struvite is a magnesium ammonium phosphate compound (NH4MgPO4·6H2O). 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 struvite deposits 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. Another way is the use of ferric chloride or ferrous chloride in digesters to prevent struvite deposits.
Struvite clogging of pipes is a prevalent problem in biogas plants, but there has been no environmentally appropriate solution to this problem so far. In general, it can be solved by adjusting the pH or the ratio of the three elements: magnesium, ammonia, and phosphate to 1:1:1.
Chemical composition study: The Woodman Point Wastewater Treatment Plant (WWTP) in Western Australia has had two separate problems that have resulted in avoidable maintenance costs. These are massive struvite deposits (MgNH4PO4 .6H2O) scaling downstream of the anaerobic digester and the formation of hydrogen sulfide (H2S) levels in the digester gas. In fact, these complications compromised gas engine operation and caused high operating costs on the gas scrubber. Lastly, the addition of iron dosing to digesters reduced hydrogen sulfide emissions effectively and cost-effectively, according to laboratory studies.
Remedy: Iron dosage addition did not appear to be a cost-effective method of preventing struvite formation through the production of iron phosphate. It is however feasible to lessen the risk of struvite condensation in and around the digester. This is done by increasing the pH to accelerate the struvite out before passing out through the digester, owing to the tendency of struvite to form in the liquid part in the biodigester. (Charles, Cord-Ruwisch, Ho, Costa, & Spencer, 2006).
A Summary of Problems, Causes, and Remedies in a Biogas Plant
The following table shows a summary of the problems that might be faced in a biogas environment and how to troubleshoot them
Table 2: A Summary of Problems, Causes, and Remedies in a Biogas Plant
| Problems | Causes | Remedy measures |
|---|---|---|
| Reduction in the gas yield | Reduced performance in methanogenic bacteria production. | Proper and quality substrate mixing. |
| Reduction in feedstock quality. | Check the heating system | |
| Temperature drop in the biodigester. | Check the level of potential inhibitor compounds | |
| Inhibition by foreign compounds. | Add bio-slurry from a foreign digester if the methanogenic bacteria has dropped | |
| Non-homogenous substrates | ||
| Methane concentration drop | Reduced feedstock quality. | Proper and quality substrate mixing. |
| Temperature drop. | Check the heating system in the biodigester. | |
| Inhibition by foreign compounds. | Check the composition of the inhibitor compounds in the feedstock | |
| Foaming problem | A new substrate with high protein content has been added | Reduce or stop feeding the biodigester |
| Air is introduced in the digestion chamber. | Analyze feedstock concentration | |
| Temperature changes in the biodigester. | Reduce the introduction of air in the biodigester. | |
| pH drop in the biodigester | Abnormal feeding rate in the biodigester | Reduce the input in the biogas plant until the system returns to normal |
| Change in operating temperatures | Use only manure until the system returns to normal | |
| Introduction of foreign compounds like soap | Check thoroughly what is entering the biodigester. | |
| Inhibition by foreign compounds. | Analyze the chemical composition of the feedstock and the input entering the bio-digester. | |
| Reduction in gas volume
(Insufficient gas pressure and maximum pressure not reached) |
Underfeeding of the plant. | Follow the feeding instructions. |
| Gas leaks. | Check for gas leaks | |
| Scum formation in the digester. | Check the biodigester climate/constituents. | |
| The improper mixture ratio of water and cow dung. | Follow normal mixing procedure preferably of the ratio 1:1. or 1:0.75. Do not make the mixture too thin or too compact. | |
| Siltation in the digester. | ||
| Bio-effluent smelling at the expansion chamber and the canal. | Overfeeding of the digester. | Follow feeding instructions. |
| Leakage in the bio-digester. | Check for gas leaks. | |
| Gas stove not burning well | Blocked primary ducts. | Clean the air ducts and burner holes. |
| Blocked flame holes. | Adjust the primary air knob to get the correct mixture. | |
| Incorrect gas mixture ratio. | Open the water drain valve to remove accumulated water in the pipe. | |
| The lamp is not bright enough. | Dirty glass screen. | Clean the lamp glass screen. |
| Cracked, blockage, and destroyed mantle. | Replace the cracked mantle. | |
| Presence of water in the system. | Adjust the primary air knob to get the right mixture. | |
| Clean the mantle holder hole. | ||
| Enforce traps along the piping lines. | ||
| Low gas quantity in the biodigester. | Underfeeding/ irregular feeding of the biodigester. | Follow feeding instructions as per the logbook. |
| Scum formation in the biodigester. | Proper mixing and removal of impurities. | |
| Remove the scum. | ||
| No gas reaching the appliances | Blocked water passage in the pipeline system. | Check for the presence of water. |
| No gas/insufficient gas is being produced in the digester. | Check for leaks. | |
| Disconnected gas pipeline. | Check for pipe disconnection and connect it back. | |
| Impurities in the digester. | Check the main gas valve. | |
| Closed main gas valve at the test unit chamber. | ||
| The feeding material does not enter into the digester | Blocked inlet pipe. | Poke through the inlet pipe. |
| The position of the inlet pipe is below the overflow point. | Ensure right vertical dimensions are used. | |
| Take out the inlet pipe for inspection. | ||
| Thick bio-slurry overflow | Incorrect water/dung mixing ratio. | Implement a good mixing ratio. |
| No hydraulic movement in the digester. | Check for water leakages. | |
| Water leakage in the digester. | Ensure daily use of the gas to allow hydraulic movement. | |
| Water leakage in the expansion chamber. | Check for cracks in the digester and the mixing and expansion chamber. | |
| Slurry entering the gas pipe | Gas outlet pipe placed below the overflow point. | Check slurry overflow point. |
| Reduce slurry overflow point to lower level. | ||
| Put a filter to allow only gas to pass through, blocking the slurry in the process. |
Conclusion
According to the article above, there are many reasons for a biogas plant to reduce its production rate. For instance, they may emanate from various points in the installation of the biogas project, an example being crack formation in the biogas plant construction. On the other hand, remedies are available for the various problems that may arise in the installation and the use of the biogas plant. They may be preventive or curative in nature. This article has gone ahead to cover many of them. Altogether, troubleshooting entails the application of these remedies for optimal gas production in a biogas plant.
References
Biogas world (2022). https://www.biogasworld.com/biogas-plant-troubleshooting
Charles, W., Cord-Ruwisch, R., Ho, G., Costa, M., & Spencer, P. (2006). Solutions to a combined problem of excessive hydrogen sulfide in biogas and struvite scaling. Water science and technology, 53(6), 203-211.
Edem Bensah, Edward Antwi, Julius Ahiekpor, (May 2021). Guide For The Design And Construction Of Fixed-Dome Biodigester. Retrieved from https://www.researchgate.net/publication/351818542
Suggested Citations in APA
Wanjohi, A.M. (2022) The Blue Flame: Biogas Training Blueprint. Nairobi: Kenya Projects Organization
For inquiries about Biogas installation for Domestic and Commercial use at household and institutional levels, kindly contact KENPRO Support Team or call us at +254725788400.
