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
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. |
