Loading calculations

Once you have acquired your biodigester, this must be loaded with organic matter and water to allow the proliferation of methanogenic bacteria. These bacteria are supplied by the manure and are necessary to produce biogas. 

Summarising the previous blog post (click here to read it) I acquired an 8.5 m3 biodigester and determined that it should be fed daily with 19 kg of pig manure and 41 kg of water for a 100 days Hydraulic Retention Time, 8.0 % dry matter and an Organic Loading Rate of 0.63 kg organic matter/m3/day. Now we need to determine how to load it.

The first step is defining the useful volume. We already did that in the Feeding Calculations blog post by defining a 70 % useful volume (6.0 m3). Then, we determine how much manure and water is needed to fill the six cubic metres of useful volume. But first of all, let's define that the mix of water and manure has a 10 % dry matter. 

Why cow manure? Can't we use any other substrate to load the biodigester?

Cow manure is ideal because ruminants faeces contain huge amounts of methanogenic bacteria, which means that we will not need to wait too much to obtain a flammable biogas. We can also use other manures, although I haven't, but I presume they may take a bit longer to produce a flammable biogas. So for a household biodigester, we can use dog faeces. This may not be rich in methanogenic bacteria, but it has some, for sure. or you can use the household wastewater, but you may not want to mess up with that. It is also suggested to add ruminal content (10 % of the total volume, approx), this is extremely rich in methanogenic bacteria. The problem is that ruminal content can only be acquired in slaughterhouses which are usually located outside the cities.

Do I need to do laboratory tests? Are they expensive?

If you want to be sure what you are doing is right, do it. Dry matter and organic matter determination are the cheapest analysis. You can also follow the classic water:manure proportion of 2:1 (in kilograms), but this only works for fresh manure. For slurries (manure mixed with water), the proportion might be 1:1 or even 1:2, and for dried manure (including rabbit, guinea pig and horse fresh manure) it can be 3:1 - 5:1. The density is easy to determine by adding a determined amount of manure into a volumetric bucket of water and recording the water displacement.

Dry matter determination

What happens after the biodigester is loaded?

I will tell you the whole biodigester story in the next blog post, but once the biodigester is loaded, this must be left resting as a if it were a batch system (no feeding) until it reaches a pH of 6.8 or starts producing a flammable biogas. Once you get to this point, you start feeding the biodigester progressively. During this resting period, the organic matter of the manure will be fermented and methanogenic bacteria will start reproducing without the feeding pressure. So every time, more and more methane will be produced (click here to know about the conversion of organic matter into methane). A flammable biogas contains from 50 to 70 % of methane, many factors affect this proportion.