Can I Put This in My Biogas Digester? – Part 1, Waste Cooking Oil

Hi, Arvin from the Do-Engineering team here! Recently we installed a biogas digester in a certain farmer house at Munggu, 10-15 minutes from the office. The farmer, Rudi is potentially one of the model farmers for our biogas project. He separates his household waste perfectly, entrepreneur-minded, actively asking and updating his biogas-related activity. His biogas digester is the first digester that mainly uses organic waste as the feedstock. Unlike feedstock from animal dung, it cannot produce biogas alone because of the lack of biogas-producing bacteria. Thus, we added methanogen bacteria. With the right amount, it will digest the food waste and producing biogas!

A few days ago, Rudi asking about the right feedstock to use for biogas. From goat dung to various food waste. There is one particular food waste that caught my attention: coconut oil. Logically speaking, since cooking oil relatively releases a large amount of energy from combustion because of the high carbon content in lipid, it must be a great energy source. However, in biogas digester, there is another important thing that we must consider: the bacteria's well-being.

The main purpose of the digester is to release methane, and then this methane is the actual combustion source for cooking.

Cooking oil – including coconut oil – is not a good main feedstock for biogas, because it contains long-chain fatty acid (LCFA). Most methanogen bacteria could digest short-chain fatty acid (SCFA) and medium-chain fatty acid (MCFA), but requires enormous energy to digest LCFA. SCFA consists of <6 carbon atoms, MCFA consist of 6-10 carbon atoms, and LCFA >10 carbon atoms.

According to Sousa et al (access here: https://aem.asm.org/content/79/14/4239), trace amount of LCFA could provide a larger energy source for the last period of HRT (Hydraulic Retention Time, ideal time needed by bacteria to digest the overall organic waste). However, large amounts of LCFA inhibit the metabolism pathway of methanobacteria. Fats (triglycerides) in food waste must be oxidized first to gain glycerol and fatty acid. After that, glycerol and fatty acids can continue to the next step, acidosis and acetogenesis. This is where large amounts of LCFA inhibit biogas production.

According to Labatut, et al (access here: https://www.sciencedirect.com/science/article/abs/pii/S0043135414000736?via%3Dihub) SCFA, MCFA, and LCFA are processed through bacteria’s β-oxidation pathway, which releases acetyl-CoA molecule per 2 carbon atoms in the fatty acid. The longer the chain, the more acetyl-CoA released. Further accumulation of acetyl-CoA will thermodynamically limit the other type of metabolism as well, thus the bacteria technically will “die of starvation”.

So, if you want to use food waste as biogas digester feedstock, please avoid using waste cooking oil. If your food waste mostly consists of oily food, you can drain it with a filter or simply put it on a tissue to absorb the oil.

I will continue this blog next time. If you want to have a discussion of this topic, or even questions about “can I put this in my biogas digester?”, you can comment on this blog or email me via arvin.nanda@su-re.co.