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. 

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Feeding calculations

Feeding calculations

By Jose Alfredo López   22:23   No hay comentarios:

In this post, we are going to size a biodigester for a small farm. Where I come from, Latin America, the most popular biodigester is the plug-flow tubular biodigester because it lasts up to 10 years, it is relatively cheap, and it is easy to install. So, let's start:

This example is a real case; it was the second tubular biodigester I installed. The smallest tubular biodigester size available at the store was 8.5 m3.

8.5 m3 Plug-flow tubular biodigester

Just as you see the biodigester in the image above, it is easy to determine some measures like the length and its width. If we multiply the width by two, this is the measure of the circumference perimeter. In the image below, the measures in black colour are the ones we already know. In red colour, we apply some basic geometric formulas to determine the radius (r) of the circumference and the area

Knowing the measures of the biodigester we can now determine how much it can be fed daily using pig manure. For this, we will use the parameters stated in this blog post: Parameters of design (click here).

As said in the post, we will leave a 30 % space for the gas and the rest will be the space occupied by the digestate.

The digestate space is also the useful volume. Why does it have to be 30 % and not less? That depends on you, take into consideration that the gas space in the biodigester also works as a gas reservoir. Sometimes I also use 20 % but no less than that because the bubbles formed in the upper layer of the biodigester can block the gas pipes.

Now we need to determine the Hydraulic Retention Time (HRT), in the Parameters of Design Post, I said it is comprehended between 30 and 100 days.  Here is a consideration to take into account when working with manures: the lower HRT, the more contaminated with pathogens (coliform bacteria) the digestate will result (Al Seadi, 2008). This researcher also claims that the Hydraulic Retention Time should be chosen taken into account the atmospheric temperature. The colder the temperature, the higher the Hydraulic Retention Time should be. Therefore, because I want to obtain a clean digestate and the average temperature of my city is 20 degrees Celsius, let's choose 100 days of retention time.

Dividing the useful volume by the Hydraulic Retention Time (6000 litres / 100 days) it results in a daily feeding of 60 litres.

60 litres of feed is not only pig manure, it is also water. As stated in the Parameters of Design post,the dry matter of the digestate should be lower than 10 %. Therefore, the pig manure + water mix should have a dry matter content lower than 10 %. If higher, the biodigester can get obstructed. Let's use 8 % dry matter.

In the laboratory I determined the following:

Pig Manure dry matter: 25.3 % of total weight

This goes beyond the calculation, but the 19 kg of manure should be entered into the biodigester progressively. I mean, during the first week of feeding you will use 5 kg of manure and 10 kg of water, during the second one, 10 kg of manure and 20 kg of water. Keep doing that until you reach the 19 kg. I usually suggest doing this progressive feeding during five weeks.

As you can see from the calculation, the proportion water: manure is 2:1, now you know where this classic feeding balance comes from. However, this is not a rule because it can change based on our assumptions for dry matter and HRT. So far, we have determined that our 8.5 m3 biodigester will be fed with 19 kg of manure and 41 kg or litres of water. Now let's calculate the Organic Loading Rate (my favourite parameter). For this we need the organic matter of pig manure:

Pig Manure organic matter: 20.25 % of total weight

A 0.64 kg Organic Matter per cubic metre of useful biodigester per day is a good parameter to start, but It could be better. We could try with a higher value like 0.8 kg OM/m3/day to obtain a greater amount of biogas. Now we can start our calculation all again but state the amount of OLR we want (maximum 1.0, this is my personal suggestion) and we will see that the HRT and the dry matter will change. Here we should not forget that the maximum amount of dry matter of the manure + water mix is 10 %. You will notice something interesting like it is not possible to get a lower HRT and high OLR without exceeding the 10 % dry matter rule. As stated before this will have an effect on the digestate microbiological quality and the biogas production. So what did I chose?

I selected a high OLR (1.0) with a little HRT of around 30 days. Of course, I obtained a pretty contaminated digestate, but this one was only applied to the soil and not plants (except for grass). The decisions you make will depend on your purpose. Of course, if you are planning to sell your digestate, make sure it is clean, or you will get into trouble. You can also use the biogas to boil the digestate if you do not mind about the biogas, this is up to you. As you can see, there are many things involved, and you should wisely choose your digestion parameters based on your goals (produce biogas or produce digestate).

Final Result. The bag on top is a 2 m3 gas storage

The final result.

Before starting to feed the biodigester with the 60 litres of manure + water mix, it should be loaded with digestate. The calculations are much simpler, but we will see this in the next blog post.

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