Maarten's Plastic Biodigester Manual

I’ve been remiss in not posting this much much sooner.

Maarten Graveland’s Polyethylene Tube Biodigester Manual [also available on his site here]

First, why do you want a plastic biodigester?

  • To implement an easy and cheap way to obtain the sustainable energy source BIOGAS, a mix of primarily CH4 (methane) and CO2 (carbon dioxide) from manure and organic matter.
  • To give an appropriate and affordable technique to poor rural areas.
  • To let rural people enjoy the pro’s of using combustible gas CH4 for cooking, heating and, on bigger scale, for running generators and pumps.
  • To cut down on use of wood for cooking and heating thereby slowing down deforestation.
  • To keep houses free of the toxic smoke of woodfires, and therefore improve welfare.
  • To give rural areas free, clean, quality fertilizer (effluent of the biodigester).
  • To set communities free from (pathological) diseases caused by fresh manure or feces.
  • To set communities free from flies, mosquitoes and maggots, attracted to fresh manure or feces.
  • To let communities be able to use all their organic waste (from kitchen, fields) as an energy source, by adding it to the biodigester
  • To prevent CH4 (which will exit from the manure eventualy anyway if runned of to a manure lagoon or septic system) from adding strongly to the global warming effect (20 times more than CO2) by burning it down to CO2.


An important goal is to make the biodigester of locally available materials, that are therefore affordable and easier to deal with for the local community. So materials that are suggested in this article might need a bit of editing when not available on the place where you want to build a digester. By the way, this manual is from a Guatemalan point of view.

What do you need
-a source of manure and/or organic matter and/or slaughter waste to feed the digester every day (for the first month(s) there needs to be fresh manure on the daily diet, to get a strong population of methane producing bacterias inside the biodigester);
-a need for combustible gas (CH4) to:
-keep warm (chicks for instance)
-run motorized pumps or generators;
-a use for first class clean natural liquid fertilizer (effluent of the biodigester);
-tubular PE (poly ethelene) plastic of 1,8 m (meters) whith on the roll, which makes 1.2 m in diameter, which gives it a circumfence of 3.66 m.
If only other sizes of tubular plastic are available, a lot of data in this manual has to be recalculated. But it is just as possible.
Length dependable on needs and availability of manure and organic matter. In general, the digester can hardly ever be to long;
-2 pipes of about 1 m, diameter between 3″ (inch) and 8″, material: type of plastic, concrete, not metal;
-some shoveling experience to dig a nice staight ditch, with accuired inclination, to put the biodigester in.
-a firm roof structure, made out of that what is available and much used. Just make sure that it will last for at least 10 years;
-some cement, sand, pebbles and blocks or stones, to make a decent entrance basin and exit basin of the digester. And maybe even a canal from the lifestock habitat to the biodigester (see furtheron).
-a floting gas deposit or a “plastic bag” gas deposit to capture and store the produced gas by the biodigester for later use;


-Start by finding a good place.
-Make it as close as possible to the source of manure. If possible, make a canal so the manure is able to run in by gravity. That will save you a lot of work!
-Take in to account that this basic (unheated) biodigester needs SPACE.

ruimte nodig / space needed / necesita espacio


VERY roughly you can say half a meter for each fattening pig. So the manure from twenty pigs will need a biodigester of 10 m (of the plastic size stated earlier). If you have cows, make it 1 m per animal, since the bacteria that are helping us creating the gas have a harder time digesting the manure from cows, and since a cow produces significantly more manure.

There are allthough a lot of things that have influence on the needed size;
1. Ambient temperature.
2. Dilution rate of manure with water.
3. Particle size of the material that enters.
4. Only manure or manure with urine.
5. Only manure (with or without urine), or also organic materials and/or slaughter waste.
6. Amount of food and water given to the amimals.
7. Type of food given to the animals.

1-> The bacteria that are helping us in the unheated plastic biodigester are of the mesophilic kind. That means that they are most productive at body temperature, 37°C. 5°C hotter and they slowly die, any degrees colder and they are less productive.
(around 20°C it is more the psychrophillic bacteria that gets into action)
2-> To less water, content inside biodigester gets to acid, which slows the bacterias down. Too much water, the bacterias can’t find their dinner.
3-> Particals to big, water and bacterias can’t reach the core of particels, proces is slowed down. Particals to small, that’s not possible, smaller is better.
4-> Urine contains a lot of N-based (Nitrogen-based) material, which are being transformed to acids in the biodigester. To acid content slows down the proces. Solution is to add a bit more water to feedstock to “dilute” later formed acids.
5-> Adding other things than manure changes retention time a bit, depending added materials. Most important is the change with slaughter waste. This could take the biodigester longer to “digest”. Especially when it is not put in in really smal particles.
6-> More food and more water is more manure.
7-> More concentrated food, less manure. More natural food, more manure.

So, to be more precize: What helps you best determening the needed size is to collect a days worth of feedstock (manure and/or feces and/or organic waste and/or slaugther waste). That would be an avarage of what you would like to be putting in the biodigester daily later on.
Mix that collection up with water. Not to much, not to little. You would like to have a mixture with a dry content matter of about 10%. Since manure has a dry content matter of about 20% (depending lots of things), you want to add the same amount of water to what you have collected, to get the dry content matter down to 10%.

PRACTICAL: If your mixture is runny like thin mud, like yoghurt, like canned spagetti sauce, you have a dry content matter of about 10%.

A plastic biodigester is a continuous system, meaning that the same amount you put in, will come out at the other end of the biodigester. Only, the fluid (liquid fertilizer) that comes out (called effluent) has been in in the biodigester a long time, producing gas and being sanitized. That time is called “retention time” or “turn over time”.
Depending the temperature inside the biodigester, retention time needs to be between 10 and 70 days.
10 days for a 55°C heated biodigester.
20 for a 37°C heated biodigester.
Between 30 and 70 days for a simpel and cheap non-heated plastic biodigester.
30 days when installed in a climate with an avarage temperature of 30°C.
40 days when installed in a climate with an avarage temperature of 25°C.
55 days when installed in a climate with an avarage temperature of 20°C.
70 days when installed in a climate with an avarage temperature of 15°C.

Any shorter and the bacterias don’t have enough time to transform all energy from feedstock (manure/feces/organic waste/slaugther waste) to methane AND (pathological) diseases and/or viruses and/or weed seeds are able to survive the time spend in the biodigester without O2 (oxigen).

So back to determining the needed size for the biodigester…
-You find, for example, that you can daily gather 3 buckets = 45 l (liter) of mixed up feedstock.
-The avarage ambient temperature is 20°C, meaning a retention time of minimal 55 days.
This means that there should be space in the biodigester for 55 times 45 l ≈ 2,500 l = 2.5 m3.
Depending on available size tubular PE plastic and shape of the ditch, you can calculate needed lenght of the biodigestor. We’ll get back on that later, in the “ditch” part.

PRACTICAL: ALWAYS be generous with the lenght. Maybe later on there will be more feedstock. And more lenght means more obligated time inside for the (pathological) diseases and viruses also. And those ones you rather have very dead than almost dead.
They get killed because they need O2 (oxigen) to live, and there is none inside a biodigester (see what biogas is made of). Also, the level of methane that is inside the biodigester is toxic for them.


-You would like a declination of about 2% from the beginning to the end of the biodigester. Since the content is not that runny as water, this 2% will help to evenly spread the content level from front to back in the biodigester. If there is a slope in the terrain, use it.

Making a mold
Now de shape of the ditch has to be determined.
The material used for the plastic biodigester is tubular PE (poly ethylene) which comes on rolls. Sizes (diameter and lenght) vary. In Guatemala, we use PE with a diameter of 1.16 m, that has a whith on the roll of 1.83 m, and has a circumfence of 3.66 m.
We are digging a ditch that can hold the total circumfence. That way, the biodigester is protected and insulated by dirt. Also, the roof structure can be (has to be) very low. The closer the roof is to the plastic, the easier it is to close all gaps around the roof. So dogs and cats cannot get on the plastic. Saving on material and increasing protection.

That for the scale of the size, now for the actual size. You can make the ditch very deep and narrow, saving space. But the walls will not be strong. You can make it very wide, and shallow. But that will cost more on the roof structure. Furthermore, any change of size from the ideal round shape will mean less content per lenght unit.
So you would want to be somewhere in the middle.
This is my “favorite shape”.

de vorm / the mold / el molde

Not to wide, not to deep. And with inclined walls for added strengh of the walls.
If you add the numbers up, you have the circumfence of the tubular plastic that I use for this manual; 130+90+2×73=266 cm

Set up construction lines
Set them up at both sides of the going to be ditch. As far apart as the maximum whith of the mold (1.37 m in the upper picture). And a bit longer than the desired (calculated) length of the ditch. Now for the inclination. These lines would have the declination of 2% in it.
-Set the first line level;
-take 2% of the distance between the post at the beginning and the post at the end;
-lower the line at the end post with that percentage of the distance.

So if you have the line of say 7.5 m, the end of the line should be 15 cm (centimeters) lower than the starting point. That’s where the 2% declination is.

You copy the setup of the first line to the second line at the other side of the going to be ditch, by leveling out the knots on the posts.

PRACTICAL: Eventhough the posts are temporaly, put them in deep, so you can trust the line and the line does not get false when somebody eventualy bumps in to it or leans against it.

NOW is a good time to put in the posts for the roof. If you do it later, chanches are that your nicely schaped walls will cave in, while hamering in the roof posts.

If you are building the biodigester in open field, so not side by side with a stucture (the stables or pigs facillity for example), you will have to make a self supporting roof. That means posts for the roofstructure at both sides of the going to be ditch.
If you can find a suitable place right next to a stucture, the roof wil be a lot easier to build. And you need only half the amount of posts.

If you can find FSC 2″x2″, 2′ long hard wood, that would be good to use for the posts. If not, I suggest galvanized steel 1/2″, 2′ long pipe.
Hard wood: Cut or saw a tip on the lower end, and drill a hole in the upper end.
Pipe: Smash the lower end flat with a hammer, and drill a hole in the upper end.
Now hamer them in, 5 cm away from the construction lines, untill the hole is at the same level as the construction line.

PRACTICAL: Protect the wood or pipe end, while hamering it in, with a piece of wood or something.

Start digging
Finally it is time for some hard work!
With the mold nearby, digging can begin.

PRACTICAL: Trow the excavated dirt far away, as you don’t want the walls of the ditch crumbling on the weight of excavated dirt lying right on the edge.

Round the ends of the biodigester off, so it is like a giant bath tub. This will be the form of the inflated plastic biodigester also, and therefore, will give maximum support to the plastic bag.


When that is all done, it is time to put together the roof. Depending on wether there is a structure near the biodigester or it is in open field, the structure will look something like this:

2 mogelijkheden / 2 options / 2 opciones
Important thing here is, that we designed the ditch to be able to hold the total circumfence of the used size of tubular PE.
BUT, there is a difference between a plastic biodigester filled up with water, and filled up with manure and gas:
het verschil / the difference / la diferencia

Same circumfence, same size of the ditch, but different shape of the plastic biodigester. So the roof should be at least 60 cm higher that ground level, at the center of the ditch.
The material used for the roofcover varies. I find that corrogated metal roofsheets work fastest. It helps also to give the roof a more rigid structure.