[Digestion] Design Example for biodigester

[Jaime Marti Herrero]

Hi everybody. I have translate, with my poor english, a example i have on the spanish handbook i have refer several times on this list.
This is my methodology and the way I design my biodigesters.... if you considerer other place the retention time must be change, and with other king of animals the calculation of fresh manuere and biogas produced change as well. I hope this example can help you.
Jaime Martí

Design Example for
biodigester 

Suppose a family who
have 5 cows, and they take the cow everyday to the fields. During the night the
cows sleep very near the house.

If you suppose that for
every 100kg of weight a cow produce 8 kg of manure every day, and you suppose
that the cows of the families have 325 kg of weight, this means that the 5 cows
produce every day 130 kg of manure.

But they are on the
field during all day, soy the family can only collect the manure that the cows
left during the night near the house. This is the 25% of the total manure.

So the family can
collect every day 32.5 kg of fresh manure.

 

If the family want to
use all the manure for the biodigestor, the will need to add 97.5 liter of
water to the 32.5kg of manure (1:3 manure:water), so the total daily load will
be 130 liters of mixed water and manure.

One kg of fresh cow
manure produces 35.3 liters of biogas, so the family will produce 1147 liters
of biogas every day. So this means that they could use it during 7 hours in a
stove and or gas lamp (similar consume on 130-140 l/h)

 

Now, this family live
in a high valley where the mean temperature is 15 ºC, and in winter they reach
the coolest night 0ºC. a good retention time (RT) for the biodigestor in these
conditions is 45 days.

So we can obtain the
liquid volume (Vl) multiplying the daily load with the RT, and we get 5.850
liters.

To know the total
volume (Vt) we have to add to the Vl the biogas bell. The biogas bell should be
about ¼ of the total Volume, so it is gonna be equal to 1/3 of the liquid
Volume.

So de Biogas bell is 1950
liters.

And the total volume is
7800 liters, or 7.8 m3.

 

Now, we have to decide what
kind of tubular polyethylene we use. We van find it with different Roll Width.
The roll width of the polyethylene corresponds with the half of the circumference
of the tubular section.

The best way is the make a
table with the different roll width, radios, diameters, and longitude needed to
reach in each case the total volume calculated. We will select the optimum biodigester
by the optimum longitude/diameter ratio, that must be between 5-10.

 

Roll width: Radio: Diameter: Cross
section(pixr2); longitude(Vt/CrossSecc); L/D

 

1          0.32    0.64    0.32
   24.5    38.5

1.25    0.40    0.80    0.50
   15.7    19.7

1.50    0.48    0.96    0.72
   10.9    11.4

1.75    0.56    1.12    0.97
   8          7.18

2          0.64    1.28    1.27
   6.1      4.8

 

The last column indicate the
L/D ratio, and you can see that there are only one with a ratio between 5 and
10, and is the biodigester for 1.75 roll width where 8 meters are needed to
reach the total volume calculated.

So now we know the dimensions
of the biodigestor, and we need to make a ditch adequate to these dimensions.
The ditch will be 8 meters long, 0.60 m withd at the bottom, 0.80 m width ant
the top, and 0.90 deep. This dimension depends on the roll width you are using.
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