The biogas construction project has been completed! On Tuesday I was able to connect the digesters to the gasholder, buffer the substrate, and add the seed cultures.

I found that buffering the substrate required much less alkaline material than I expected to use. It took 40grams of baking soda to buffer the clover/leaf batch inside digester David, while digester Ludwig used 35grams. This was enough to change the acidic solution (pH 5) to a more basic solution between pH 7-8. I checked the pH with litmus paper multiple times during the day and the pH of the clover/leaf substrate remained a constant 7-8. Farm manager, Mark Bomford, noted that the leafs inside the substrate have an excellent buffering capacity and will be able to absorb a lot of the baking soda. So, while the substrate seemed adequately buffered at 7-8 on the day that I added the seed cultures, it might become more acidic after the leaves absorb the baking soda water. I am nervous about his suggestion as we are now left to monitor how the buffering plays out.

Four different anaerobic seed cultures were used to inoculate the digesters. I used a culture of lama and sheep manure, a culture of lama and clover manure, a mixture of sheep and clover, and a mix of only clover and leaves. These cultures were maintained under protection inside a glass greenhouse for over five weeks. During that time I had observed gas production on the surface of all cultures using the sheep and lama dung and felt that those cultures offered a large population of methane forming bacteria. However, the mixture of only leafs and clover did not seem as active and I did not expect it to be a suitable seed. As I added the seed cultures to the digesters I checked the pH of each one. As I expected, the cultures using lama dung were defiantly basic at a pH of 7-8. The leaf and clover mix was very acidic at pH 5.

The acidic clover leaf culture lends us insight into the necessity of brewing appropriate bacteria cultures for seeding. Anaerobic bacteria can and will grow inside plant only mixtures, however, even in optimal growth conditions, it might take months. That is why a source of dung is useful when creating live cultures in a shorter period of time. I think it is reasonable to suggest that the plant only seed was stuck in the acid forming phase, where bacteria are breaking down the plant material into fatty acids. Without a suitable amount of methane forming bacteria to gobble up those acids the process gets stuck and the solution remains acidic. Literature suggests that this process will correct itself eventually because the bacteria that form the fatty acids will begin to drown in their own toxins, slowing their reproduction and allowing methane forming bacteria to make headway against the overabundance of the acids.

After the seed cultures were added to the digesters I put the lids on and sealed them closed. I stuck plumbers putty under the lip of the lids so that I could assure a gas tight seal and then bolted the lids down. Agitation of the material was not hampered by the lid or the temperature sensors and the entire system seemed to be functioning as expected.

Prototype Biogas Digesters (David and Ludwig)

Gasholder for Digesters