Biodigesteur domestique/en

Low-tech Lab

https://wiki.lowtechlab.org/wiki/Biodigesteur_domestique/en

Dernière modification le 29/04/2020

Difficulté

Medium Moyen

Durée

2 day(s) jour(s)

Coût

500 EUR (€)

Description

Produces natural gas and fertilizer from our organic waste.

Sommaire

biogaz, gaz naturel, digestat, organique, méthaniseur, biodigesteur, pouvoir méthanogène, autonomie, Low-tech Tour France en fr 1

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Introduction

A biodigestor is a solution to convert organic waste into fuel gas (biogas) and fertilizer (digestate). The biodigestor particularity is that digestion is done thanks to bacterias in an environment deprived from any oxygen. This situation is called anaerobic fermentation.

Biogas is a mix of different gases, containing mainly methane, which can be used for gas cookers, boilers or as fuel for engines.

Methanogen fermentation also exists in nature. For example, it happens in swamps when organic matter is decomposed underwater.

Biogas domestication happened in the beggining of the XIXth century, and the variety of biodigestors have considerably increased since then. They are particularly present in developing tropical countries, where farmers become autonomous in energy thanks to biogas production based on organic waste. Heat being an important catalyst of this reaction, small units are economically interesting in this area.

In France and other industrialized countries, the cost of energy being very low compared to workforce cost, only few small biodigestor units exist. However, many industrial units are present in wastewater treatment plants or around big breeding farms.

Different kind if biodigestors exist. They can be continuous or discontinuous, and also have different operation temperatures (psychrophilic : 15-25°C, mesophilic : 25-45°C or thermophilic : 45 – 65°C). In this tutorial, we are studying continuous mesophilic biodigestors à 38°C, which are the most commonly used in temperate regions.

The main feature of this system is its similarity to a digestive system. It also needs a certain temperature to be efficient, requires bacterias and receives food regularly.

In a compost, under aerobic conditions, decomposition of organic matter produces gas (H2S, H2, NH3) and an important amount of heat. Only decomposition deprived from air produces methane. It is one of the reasons why fermentation happens in a sealed tank.

In this tutorial, we will present the different components of a biodigestor (matter circuit and gas circuit) and how to use it.

This documentation realised with the association Picojoule describes fabrication of one of their micro-methanisation protypes. It does not provide full cooking gas autonomy but is a good introduction to methanisation. Hélie Marchand's half-burried digestor has a greater capacity : Biodigesteur.

These explanations are largely inspired from the work of Bertrand Lagrange in its books Biométhane 1 and 2, that we strongly recommand !

This work is free and open, do not hesitate to clarify and complete it based on your knowledge and experience.

Youtube

Matériaux

Matter circuit

• 1 60 L can
• 1 160 mm plug
• 1 160-100 mm reducer
• 1 100-50 mm reducer
• 1 meter PVC 50 mm tube
• 4 45° 50 mm PVC MF bends
• 2 50 mm separable connections
• 2 50 mm through wall connectors
• 2 MM 50 mm sleeves
• 1 50 mm PVC plug
• PVC glue
• Etchant
• Joint compound for plumbing

Gas circuit

• 2 planar nuts 1/2’ for through wall gas connector
• 1 batch of planar joints
• 1 threaded tube of 1/2’
• 1 brass MF 1/2’ bend
• 1 connector FF 1/2’ gland nut
• 1 gas valve 15x21 MM
• 1 batch of hose clamps
• 1 gas pin 1/2’ F
• 5 m gas pipe
• 1 water filter
• 1 sulfur filter made of clay balls
• 2 gas valves
• 3 T gas pipes
• 1 compressed air dismountable connector
• 1 manometer
• 1 flexible 150L water tank
• 1 gas compressor
• 1 gas cooker
• 1 heat mat

Outils

• saw
• drilling machine with cylinder saw
• screwer
• cutter
• compressor

Étape 1 - Matter circuit - Digestor

Sizing

For a good digestion, at 38°C, the organic matter must stay 30 days in the biodigestor. We will size the digestor volume based on this duration and on regular inputs.

Let's take an example: if the regular input is 2L per day, as the matter must stay at least 30 days, the digestor volume must be at least 60 liters.

Building

Degradation by the bacterias takes place in the digestor. The needed bacterias to produce methane are called methanogen bacterias. They grow in an environment deprived from oxygen, called an anaerobic environment. To remove organic matter from oxygen, it needs to be immerged into water.

• Drill two opposite holes in the digestor tank. They should be at one third of the tank height.

• Insert a through hole connector (greased beforehand) in each of the two holes.

• Grease the inside of the through wall connectors.

• Place a plate inside if the digestor, as separation between the input and output holes. Letting matter circulate above and below the plaque, it will increase the time spent by the organic matter inside of the digestor.

• Drill a hole in the tank cover and place there a through wall connector for gas.

Use teflon and planar joints on each side to seal the system.

• Grease the seal of the tank cover and close it. The grease should ensure the seal, and the cover maintains pressure.

• Intall a valve after the through wall connector for gas.

Étape 2 - Matter circuit - Input

The entry of the system will be the biodigestor mouth. The installation will first be done to verify dimensions of the system, and then will be dissasembled and definitively glued.

• Put a PVC pipe in one of the digestor openings, without putting it too much inside (this would reduce matter circulation).

• Create a 90° angle using two 45° bends. On small diameter tubes, it is better to have smooth angles, as a direct 90° angle would get stuck more easily.

• Build the "mouth" with large diameter pipes. The larger the mouth is, the easier it will be to feed the digestor. A first fermentation process happens in the mouth. An unscrewable lid closes the mouth.

• Link the mouth to the digestor, placing it higher than the digestor, so that matter can circulate in the system with gravity.

Étape 3 - Matter circuit - Overflow

The overflow represents the end of the digestive system. Each time the system is fed, the same volume of digestate leaves the digestor. To make maintenance easier, a low exit in created. It allows draining of the digestor.

• Place a PVC pipe in the second hole of the digestor. It should not be too much inside, as would again limit circulation of the matter.

• Place a Y connector.

• The horizontal part will be plugged to a pipe with a cap. This will be the draining pipe.

The second part will be continued vertically, until the top of the biodigestor, using 3 45° sleeves, again to prevent blockage of the system.

• One of the PVC pipe goes outside of the system, where the digestate flows.

• The overflow should be lower than the digestor cover. This will allow to maintain a "geaseous roof" and to avoid organic matter in the gas circuit.

Étape 4 - Matter circuit - Bonding and sealing

If the blank assembly is satisfaying, we will now glue PVC components together :

• Mark each of the connectors on their junction, so that you can reassemble the system respecting alignements.
• Clean areas that will be glued.

• Bond with PVC glue.

• Let it dry.

Then we will test the sealing :

• Block temporarily the overflow outpur (for example with an air chamber and a hose clamp), screw the cap of the matter input part ("the mouth") and the draining plug.

• Put the system under pressure with a compressor flowing in the gas valve.

• Spray soapy water on the junctions. If bubles form, the sealing is not correct and the system has to be rebuild.

Étape 5 - Heat and Isolation

Heat

This kind of biodigestor is mesophile, which means that bacterias develop between 25°C ans 45°C, ideally at 38°C. Unlike compost, biodigestion create only few heat. To reach these temperatures, heat will have to be provided to the system. It is possible to heat by different means :

• compost around the digestor,

• Solar heating,

• By burning part of the produced methane.

It our case, given the small volume of the system, we are using a bedwarmer placed below the digestor.

Isolation

To prevent the digestor from loosing energy, it is important to provide a proper isolation, so that only few heat has to be provided. In addition, a good isolation limits temperature variations, to which bacterias are very sensitive. Many differents ways exist for isolation. We chose to use corkboards, but is also possible to use straw for example, which is very cheap and provides good isolation.

Étape 6 - Gas circuit

We have presented the organic matter circuit, from the mouth to the digestate production. One of the main interests of th biodigestor is that it also produces methane. In this part, we will study the different components of the gas circuit, necessary to ensure a good production and purification of the fuel.

Étape 7 - Gas circuit - Digestor

In the digestor, bacterias will produce biomethane as they degradate the organic waste. In variable proportions, it is composed of :

• Methane (CH₄) 50 to 70%

• Carbon dioxyde (CO₂) 35 to 40%

• Hydrogen sulphide (H₂S) 1 à 3%

• Water vapor (H₂0) variable

There can be also traces of hydrogen, oxygen, carbon monoxyde, nitrogen, and other gases in very small quantities.

Étape 8 - Methane

Methane, CH₄, belongs to the hydrocarbons family C_nH_2n+2, as propane (C₃H₈) or (C₄H₁₀). It is very light (d=0,55), and does not accumulate on the ground, unlike butane and propane, which decreases risks of explosion. Natural gas is composed mainly by methane.

To become liquid, making it easier to transport, methane has to be cooled to -165°C or compressed at a pressure of 400bars. This is only possible with industrial means, so in our case we will keep it at a gaseous state.

In terms of mass, it is the best fuel for thermic energy (12 000 Kcal/kg), but itis also the most voluminous.

Our objective is to separate methane from other gas compounds.

Étape 9 - Carbon dioxyde

Its proportion depends on the bacterian reactions, the temperature and the organic waste. CO₂ disturbs combustion, but does not prevent it.

The easiest is to wash the gas with water. Carbon dioxyde is very soluble (878 cm³/l at 20°C), unlike methane (34 cm³/l). This water carrying CO2 can be used for agriculture irrigation, or for algeas farming such as spirulina.

• At the digestor exit, after the valve, create a gas circuit bringing the gas in a bubbler-reservoir.

• The reservoir must be filled with water.

• The gas arrival pipe dives into the water.

• The gas output pipe in on the top of the reservoir.

• A draining plug at the bottom of the bubbler-reservoir with allow collection of water carrying CO₂.

Étape 10 - Water vapour

It is advisable to have a minimum of water when combustion happens, as it already produces a lot. In addition, condensation in pipes risks to block the lower parts of the gas circuit. To remove water vapour :

• Place a waste collector at the lowest point of the system.

• If the gas circuit is long, install collectors every 5 meters, at the lowest points of the circuit.

• A draining plug at the bottom the collectors allows for regular removal of water.

The bubbler reservoir can play this role of water collector if it placed at the lowest of the gas circuit.

Étape 11 - Hydrogen sulphide

Hydrogen sulphide (H₂S) is burnable but produces sulfuric acid, which is highly corrosive. A good pH balance in the biodigestor avoids most of hydrogen sulphide production. To eliminate it, biomethane will go through iron oxide, or iron straw, that will be regenerated by exposing it to the air (sulfur will leave). Coal or clay balls can also serve as filters for hydrogen sulphide.

Étape 12 - Gas circuit - Storage

In the digestor, it is advised that fermentation takes place at a minimum pressure. Gas will therefore has to be evacuate along its production. Unless gas consumption is continuous and regular, it is necessary to have a storage place, supplying gas only when necessary.

Flexible reservoirs, "bladder" like, are interesting. Using a non-flexible container can be dangerous: because we will need to empty it from air befor introducing methane. Mixing air and methane can be explosive.

• Assemble the storage reservoir simultaneously to the gas circuit.

• Place a 100mbar safety valve as close as possible to the storage. It will remove gas in case of potentially dangerous overpressure.

Étape 13 - Gas circuit - Flashback

In any place where a flashback is feared, place a ball of iron straw or copper straw on the gas circuit. Through thermic conduction, it will decreases temperature and stifle the combustion. Be careful however not to compact too much the metallic straw in the pipes, otherwise it would impede a good circulation of gas.

In our case, to avoid flashbacks to the digestor and above all to the storage reservoir, we place iron straw in the closest pipe to the gas cooker.

Étape 14 - Gas circuit - Combustion

Burners setting

As it is mixed with non combustible carbon dioxyde, biomethane has a lower calorific power than propane, butane of pure methane for the same volume.

Therefore, devices functionning with these gases have a higher air admission than a gas cooker using biomethane.

To adpat standard burners to biomethane :

• Slightly close primary air arrival, with a metallic ring or with aluminium.

OR

• Disassemble the gas nozzle and use direct gas ejection.

CAUTION: flames from methane are less visible than from propane or butane, be careful not to get burned when using the gas cooker.

Other uses

Biomethane can also be used in gas lamps, heaters, or explosion engines: generators, agricultural machines, cars...

Pressure

The biodigestor and the storage are at atmospheric pressure, to ensure correct work of bacterias. The gas cooker will use 10mbar pressure. To provide this pressure:

• Install a compressor between the storage and the burner,

OR

• Press the flexible reservoir (10 cm of water). It will reduce methane production by 5% but is less expensive then buying a compressor.

Étape 15 - Gas circuit - Sealing

Each connector between a component of the circuit and a gas pipe has to be securized with a hose clamp.

Once the whole circuit is assembled, test the sealing, proceeding as for the matter circuit: put the circuit under pressure and spray soapy water on each junction. If fbubbles appear, there is a leak.

Étape 16 - Digestor usage - Feed

The biodigestor is a living system, composed by billions of bacterias, and needs to be taken care of.

Regular feeding

Ideally, the biodigestor has to be fed every day. It is possible to reduce it until once a week at the minimum. If the volume of organic matter is too important, it is advised to divide it in different "meals".

It is important to shred or grind the organic matter (knive, blender...) and to add their weight in water, in order to :

• Prevent the matter from blocking the system,

• Accelerate degradation and system productivity.

Alimentation équilibrée

Le biodigesteur est un complément très intéressant au composteur. En effet un compost a pour objectif de créer de l’humus, pour cela il a besoin d’un fort rapport carbone/azote, (entre 20 et 30), avec principalement de la cellulose et des composés ligneux. Un surplus de matière organique putrescible déstructure le compost.

A l’inverse, les matières humides et putrescibles sont les bienvenues dans un biodigesteur (fruits et légumes en décomposition, épluchures…). Il faut limiter les matières fibreuses, sèches et dures voire les éviter dans un petit digesteur. Ils risquent de boucher la circulation de matière, ils ont également tendance à flotter et à former une écume très difficile à faire disparaitre et, en formant des croutes ou en se déposant au fond, ils utilisent de la place inutilement.

Une alimentation très azotée est idéale, l’azote n’est que très peu présent dans le biométhane mais il participe fortement à sa synthèse via la stimulation de l’activité bactérienne. De plus il permet d’obtenir un fertilisant très riche avec le digestat.

Il est important d’apporter du « vert » au régime du digesteur, si les épluchures ou diverses fanes ne suffisent pas, de l’herbe tondue et broyée complète bien.

Les produits animaliers (viandes, lait, œufs…) doivent être évités dans un biodigesteur, ne montant pas en température comme un compost il ne détruit pas les germes pathogènes.

Les huiles alimentaires ont un très fort pouvoir méthanogène (780 litres de méthane par kilo d’huile !) mais acidifie le biodigesteur. S’il devient trop acide les bactéries vont mourir. A consommer avec modération.

L’eau de cuisson permet de réchauffer le système tout en fluidifiant le transit. Elle est également chargée en amidon (pommes de terre, céréales, pâtes, riz …) apprécié par les bactéries.

L’urine peut être utilisée régulièrement. Les excréments sont acceptés en petites doses mais ils ont un faible pourvoir méthanogène, une grande partie de leur valeur énergétique a été absorbée pendant la digestion.

pH

En milieu acide, l’activité enzymatique des bactéries est bloquée. Cette acidité est surtout due à l’accumulation d’acides organiques. En milieu basique, les fermentations produisent de l’hydrogène sulfuré (H2S) et de l’hydrogène (H2). La digestion peut s’effectuer entre des pH de 6,6 et 7,6 avec un optimum entre 7 et 7,2.

Étape 17 - Ensemencement

Nous avons vu précédemment que les excréments ont un faible pouvoir méthanogène car déjà digérés. Ils restent cependant importants pour lancer l’activité bactérienne dans le digesteur.

Une vache, à travers ses rots, génère à elle seule entre 60 et 200 litres de biogaz par jour. Nous allons donc récupérer une partie de la flore intestinale du ruminant dans … ses excréments.

Pour lancer la fermentation bactérienne dans le digesteur :

• Mélanger une bouse de vache fraiche à de l’eau et l’insérer à l’entrée du biodigesteur.

Si l’activité du biodigesteur est arrêtée à cause d’une longue période sans alimentation il faut à nouveau l’ensemencer de la même manière.

La stabilisation de la digestion jusqu’à une production régulière d’un gaz combustible peut durer plusieurs semaines, il est bon de ne pas trop perturber son alimentation.

Étape 18 - Digestat

Le digestat issu de biodigesteurs domestiques une fois stabilisé est un fertilisant liquide très riche en azote et minéraux.

Il peut être appliqué dilué à 10% sur toutes les plantes avec un intervalle d’un mois entre chaque utilisation.

Si des produits animaliers (viandes, lait, œufs…) font partis du régime du biodigesteur il ne faut pas appliquer de digestat sur les fruits et légumes mangés crus (fraises, salades, carottes…). Il trouvera son utilisation dans les vergers ou sur les plantes non-alimentaires.

Notes et références

• La première édition du tutoriel à été réalisée par Clément Chabot lors de l'escale Biodigesteur du Tour de France des Low-tech.

• La solution documentée a été réalisée avec Pierre et Thomas de l'association PicoJoule http://www.picojoule.org/ [1] https://www.facebook.com/Picojoule/?fref=ts

Sources

• Bernard LAGRANGE, Biométhane 1. Une alternative crédible ; 2. principes-techniques, utilisations
• https://fr.wikipedia.org/wiki/Pouvoir_m%C3%A9thanog%C3%A8ne
• https://fr.wikipedia.org/wiki/Biogaz

Notes

• tutoriel sur un digesteur semi-enterré d'Hélie Marchand à Madagascar : Biodigesteur

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