Domestic methanization: focus on the operating principles

 Present in France for many years, the methanization process is now increasingly reaching the domestic sphere: factories and farmers are no longer the only ones who can produce their biogas, any average family can also do so. TO DO ! It is in fact now possible to acquire small domestic methanization units, sometimes called individual digesters or micro-methanizers.

How do these domestic methanization systems work? And besides, what is methanization? What are the different types of digesters available? Greenwatt takes stock of all this!

What is methanization?

Before analyzing in more detail the operation of a domestic methanizer, we must already understand what methanization is, this natural biological process which allows us to produce biogas. To learn more about this compound – also called green gas or renewable gas – we advise you to read our file on how biogas works .

Anaerobic digestion: operating principle

Anaerobic digestion is a process which creates biogas in an oxygen-free environment, thanks to the decomposition of putrescible materials (which can rot) carried out by bacteria. This is called anaerobic digestion or fermentation .

Etymology of anaerobic:

• privative prefix “an-”, from ancient Greek ἀν-;
• prefix “aero-”, from ancient Greek ἀήρ, air;
• suffix “-bie”, from the ancient Greek βίος, life.

Anaerobic therefore means “which can develop or take place in the total absence of air or oxygen”, as opposed to “aerobic”.

Sources: Littré and CNRTL

Waste

Several types of waste, both liquid and solid, can be used during anaerobic fermentation:

• effluent (wastewater)
  • of the food industry
  • livestock (we then speak of slurry, when the water is mixed with animal excrement)
  • urban eras
  • sewage sludge
• solid waste
  • agricultural (plants, animal excrement, etc.)
  • industrial
  • municipal (green waste, packaging, etc.)

Process

All this waste is stored in a tank, the famous methanizer or digester, which is empty of all oxygen, but does however contain a certain number of anaerobic bacteria. Several biological reactions then occur:

1- Hydrolysis

During this stage, the covalent bonds of complex molecules are broken, in order to make them simpler molecules: proteins and polysaccharides become, for example, fatty acids and amino acids.

2- Acidogenesis

The compounds resulting from hydrolysis are transformed into alcohols, organic acids, hydrogen and carbon dioxide.

3- Acetogenesis

Acetate is made from compounds from the acidogenesis stage.

4- Methanogenesis

Thanks to the acetate, hydrogen and carbon dioxide produced, the bacteria present in the tank will allow the creation of methane.

Products

Once finished, methanization produces biogas , composed mainly of methane (between 50 and 70%) and carbon dioxide (between 30 and 50%), and a digestate , a kind of residue which serves as compost.

Biogas, once purified, can be used to produce heat, electricity, vehicle fuel or be injected into the natural gas network (find out more about this ). The digestate is used as a fertilizer.

Advantages and disadvantages of methanization

Anaerobic digestion allows for efficient treatment of organic waste , while helping to reduce greenhouse gas emissions , since the energy produced is 100% renewable.

However, this process requires bulky installations, which must be well maintained. The waste must be sorted, while the biogas and digestate must be purified, transported and stored. This therefore requires significant material, financial and human resources

What is domestic methanization?

Many people already carry out an aerobic transformation process at home, sometimes without being aware of it... in fact, composting , very common among families with a garden, is none other than the fermentation of organic waste in an oxygenated environment.

Ultimately, anaerobic digestion takes up more or less this principle, but in an environment without oxygen. So why not equip yourself with a micro-methanizer at home? This is precisely the whole principle of domestic methanization.

Functioning

For several years, many companies have gradually developed systems aimed at offering households waste recovery solutions, using the methanization process.

Yield

You should know that a French person throws away on average 391 kg of waste per year, including 152 kg of organic waste (food scraps, peelings, etc.), a quantity of waste which could produce around 200 m 3 of methane .

However, 1 m 3 of methane can create electrical energy of 6 kWh, which would be 1200 kWh of electricity created thanks to one person's waste, in one year.

With a domestic methanizer, a family of 4 people can hope to produce 4800 kWh of electrical energy per year , to be used for example for the water heater or a gas stove. A micro-methanizer works on the same principle as a classic digester, but in a reduced format.

Purchasing a domestic methanization system can therefore prove to be a good way for individuals to reduce their ecological impact while making some savings!

Cost

However, domestic methanization has a certain price  : a methanization unit with solar panels can cost up to €20,000, making it a profitable investment in the long term. However, you can obtain a tax credit of €5,000 with such a purchase.

However, a few start-ups also offer for sale smaller formats of methanizers, which are used only to produce gas for cooking food, but which “only” cost €600 or €700: proof that the market is growing. little by little and has not finished evolving!

Constraints

However, the installation of such a system is not necessarily possible everywhere: a large domestic methanizer requires at least 4 or 5 m2 of land for its installation. You should also know that this type of system must be supplied daily , and in sufficient quantity, otherwise the bacteria will die. Finally, the outside temperature and the nature of the waste fermented strongly influence the performance of a methanization system . It is therefore difficult to predict what the exact yield will be.

This is why large domestic methanization units are undoubtedly more useful in an eco-neighborhood or used by catering professionals, while more flexible and smaller solutions will be more suitable in a family setting

Domestic anaerobic digestion: what the law says

The conquest of the domestic methanization sector by manufacturers is still recent, so that regulations have not yet had time to follow.

However, the law of August 17, 2015 modified the regulations relating to the recovery of green waste, which now indicates:

Source: Légifrance

Sorting waste at source will therefore become widespread within a few years: there is no doubt that the methanization process is likely to be even more successful.

If the installation of methanization units in the agricultural and industrial sector is indeed subject to precise regulations and requires long project preparation stages (see this ADEME pdf on on-farm methanization units) , the regulations seem more vague with regard to domestic methanization.

According to the newspaper La Tribune , a domestic methanization installation requires ICPE authorization (Installation Classified for Environmental Protection). While according to the online magazine We Demain  : “If only plant waste is recycled, an authorization requested from the prefecture is sufficient. But if other materials are added (meat waste, droppings, etc.), the installation is considered a waste treatment plant”, which would involve requesting a registration procedure which “then costs 10,000 euros! ".

It is therefore preferable, when you wish to install a micro-methanizer at home, to obtain information from an official body regarding the regulations in force .

Examples of domestic methanization units

There are many types of domestic digesters on the world market, but we quickly notice that, if we look at the subject, two of them come up quite often in the French press: the Méthatec system from the company French SCTD Industries and HomeBiogas, offered by the Israeli start-up of the same name