During the operation of biogas digesters, various issues may occur that affect gas production and fertilizer use, which can significantly reduce the efficiency and lifespan of the system. One common problem is leakage, which can be identified through pressure testing—either using a pressure gauge or a water test. When the pressure reaches a certain level and is then held, if the pressure drops quickly, it indicates that the digester is full of water. If the pressure decreases at a steady rate, it suggests there's a leak.
Leakage often results from poor foundation preparation, improper sealing at pipe joints, or weak structural integrity. When the pool is filled with water, uneven settlement of the foundation can cause cracks in the inlet and outlet pipes, leading to serious leaks. Additionally, if bricks are not properly sealed or the cement mortar isn't compacted, air pockets may form, causing further damage. In cases of severe leakage, the water level in the pool will stabilize at a certain point, and the leak location can usually be found near that level. Cracks should be repaired promptly with appropriate sealing methods.
Air leakage, on the other hand, is often due to inadequate sealing of the gas chamber. Reapplying a sealing coating or cement can help resolve this issue. During operation, if the water column moves toward the inlet, it indicates a water leak; if it stops moving or remains stable, it suggests a gas leak. In such cases, the digester should be drained for maintenance, and potential leak points—including the main chamber, the active cover, or pipelines—should be inspected and fixed.
Another common issue is when a new digester fails to produce gas. This could be due to insufficient inoculum, low feed temperature (below 12°C), or an overly concentrated fermentation mixture, which can lead to acidification and inhibit gas production. In winter, low temperatures can also cause reduced gas output. To address these issues, new digesters and gas systems should undergo pressure tests to ensure they are leak-free. Gas-containing tanks should be treated by adding microorganisms like activated sludge or old digester residue. Adjusting the pH of the fermentation mixture to between 6.8 and 7.5 is also essential. A yellow or sour flame during combustion can indicate acidification.
To adjust the pH, you can add grass ash or alkaline substances like ammonia or lime water through the feed inlet. However, direct addition of lime is not recommended—only lime water should be used. Too much lime water can cause calcium carbonate precipitation, reducing carbon dioxide levels and affecting gas production.
If the pressure gauge rises slowly, it may indicate either low gas production or a leak. To diagnose this, perform a positive and negative pressure test. If the water column rises to 15 cm within 24 hours, disconnect the gas pipe and release the gas. Install a U-tube pressure gauge, create a vacuum by removing manure, and observe the pressure change. If the water column height matches the initial reading, the issue is likely slow gas production. If it’s lower, it suggests a leak. Check the gas pipeline for additional leaks. If no leaks are found, the problem may be due to insufficient raw materials, poor quality, or an imbalanced mix. In such cases, fresh materials should be added to restore proper function.
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