When you build up a Biogas digester, you have to apply some wax-oil mixture on the inside surface of the dome as a last step. This mixture fills the small holes in the plaster and makes the dome gastight. This sealing usually lasts for several years.
However, it is possible that the wax detaches from the surface and the digester starts to leak gas. In particular, this can happen if there are sharp things made of plastic or metal in the digester. Of course those things should not be in a digester but things might get washed into the digester by overland flow after heavy rain. It also happens that the instructions, which every customer gets, are not followed and consequently you can find nearly everything in a digester.
Now a customer had a problem with his agricultural biogas digester, which is fed with a lot of animal manure. There was no gas coming out anymore, so we could guess that there was a leakage, because the manure was still bubbling and producing gas, but the digester seemed to not holding the gas anymore. The solution was to get in the dome, find the cracks and renew the wax layer.
If you want to wax a biogas digester which is already in usage, you have to empty it until you can enter it. The emptying of a digester can only work if there is no too much sludge has accumulated, because then the vacuum sucker can not handle it. Often this is a problem in agricultural digesters. The site that we had to re-wax did not have this problem despite of the open inlet that allows to clean the stables of the pigs or other animals easily and to flush the sludge directly into the digester. After the honey-sucker had completed his work, we could start to melt the candle wax and mix it with the oil. Then four people prepared themselves to enter the dome. The manure was not sucked away completely so the stinky mass still stood up to the knees and the gumboots were only as high as the shinbone. Everyone can imagine what this means for the comfort of the workers: You have to stand in the animal manure like you are barefoot. Together with the dirt on your feet, there is of course also an unimaginable smell inside the digester.
The first look on the inside wall of the dome showed that there were indeed some cracks in the wax layer. Another problem was the blockage of the gas outlet pipe with dried manure. This problem can be handled by flushing the pipe with water.
The work we came for on that day was the re-waxing, so we put the necessary tools into the digester. At first you need a tool to scratch away the old layer of wax together with the dried dirt on the wall. Because you need a hot fan and a light after it, you have to make sure that you have electricity, but you also have to make sure that the electrical current gets no contact with the black water in which everybody is stands. That is the reason why one worker is needed to just make sure that the wire does not fall into the liquid shit. In addition, the wire connection is also isolated with plastic bags. Another worker is holding the lamp to make sure that there is enough light. Then there is one guy responsible for holding the can that is filled with the hot wax and oil mixture. The last person is heating the wall with the hot fan and coating it with wax. The worker who does the waxing has to be really tough, because the liquid wax is always dripping on the hands and even the face. During the time that you are in the digester, it gets really hot in there and the air becomes smoky and dirty because of the evaporated oil. So it is a really hard work to stay under these conditions for more than an hour.
After the work, we just made a group picture to remind this moment forever. We were all really happy to get in the sunlight and the fresh air again and enjoyed clean clothes and a shower like never before. All in all the re-waxing is a really dirty and hard job. That is of course one of the reasons why TED tries to avoid any problems with the digester and builds every system to high quality standards to avoid that the construction workers have to enter it when it is already in usage. But what makes a good company is that even such maintenance work is carried out very thoroughly and professionally.