Loading calculations

Loading calculations

By Jose Alfredo López   3:03   1 comentario:

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. 

Read More

The four stages of anaerobic digestion

The four stages of anaerobic digestion

By Jose Alfredo López   23:43   No hay comentarios:

Knowing the four stages of anaerobic digestion will allow you to explain why biodigesters fail when they are overloaded, how to fix this issue, why biogas do not burn in the beginning and many other things. So let's start:

In the early years of anaerobic digestion research, Omelianskii (1906) described the Methanobacillus omelianskii as the responsible bacteria for converting organic matter into methane. By this date, methane was a well-known gas because people related it with the awareness of not lighting a fire in swamps. Anyway, later in 1967, Bryant found out that the Methanobacillus omelianskii was not a single bacteria, but an association of two microorganisms. Indeed, two types of bacteria participate in the conversion of organic matter into methane and carbon dioxide: acidic and methanogenic bacteria.

The following stages occur all at the same time; however, they are called stages because the products released by bacteria in one phase is substrate for the bacteria participating in the next one.

Source: Van Lier (2014)

Hydrolysis: This phase is slow and consists in the breakdown of all the big and complex molecules (polymers) into smaller units (monomers and oligomers). This breakdown is done by enzymes released by hydrolytic bacteria. Proteins are broken down into amino acids, fats into long chain fatty acids, and sugars into monosaccharides.

Acidogenesis: This is the quickest stage. Here, acidogenic bacteria convert all the monomers and oligomers into volatile fatty acids (VFA). During the process carbon dioxide is released. Because carbon dioxide does not burn, a non-flammable biogas means this stage is prevailing in the biodigester.

Acetogenesis: Some authors combine this stage with Acidogenesis. In this phase, volatile fatty acids are converted into acetate. Carbon dioxide and hydrogen are also released during the process.

Methanogenesis: Methanogenic bacteria are very slow to duplicate which is a disadvantage compared with acidogenic bacteria which need few hours to replicate, methanogenic require days.There are many ways in which methane is formed, but two routes prevail. The biggest part of the methane comes from the acetate (70%), and the rest of the carbon dioxide and hydrogen conversion (30 %).

The main aspects from the fours stages of anaerobic digestion are:

- All these phases occur at the same time in the biodigester

- Acidogenic bacteria duplicate quicker than methanogenic bacteria

So, we can now easily explain what happens in a Batch System. Once the substrate is mixed with the water inside the biodigester, the different types of bacteria that the substrate contains start degrading the organic matter. In the beginning, hydrolysis will prevail because complex molecules mostly compose substrates like manure. However, there are also simpler molecules which are substrate for acidogenic bacteria. The release of carbon dioxide from this stage is the main constituent of the biogas after the first day of digestion. As days pass by, most of the complex molecules of the manure are already broken into simpler molecules, and acidogenic bacteria start processing them. Methanogenic bacteria are also working since the very beginning because there is also acetate in the manure, but as days passes by, more acetate, carbon dioxide, and hydrogen are formed, and more methanogenic bacteria are produced to deal with them to generate methane. 

How do you think the biodigester will work if you feed it daily? Don't you think a non-flammable biogas is more likely to be produced if acidogenic bacteria have an advantage over methanogenic bacteria because of the duplication rate) Well, we can avoid this by regulating the amount of feed we put inside the biodigester. If we feed the biodigester with moderate amounts of substrate, then methanogenic bacteria can reach an equilibrium with acidogenic bacteria and a flammable biogas (rich in methane) will continuously be produced. Now, you may be thinking how much is low and how much is high? The anaerobic digestion parameters (HRT and OLR) will help us with that, but we will see this in another post.

References

Van Lier, J. 2014. Anaerobic Fundamentals and COD Balance. TU Delft. [Retrieved from https://www.youtube.com/watch?v=Yr6LIu379Yw&list=PLrwuNGSwGLHcrfxlTiJ2zTGWi6-fEZV8m&index=5]

Read More