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La differenza fra la genialità e la stupidità è che la genialità ha dei limiti (A.Einstein).


Where does biogas come from?


Biogas is a result of the anaerobic treatment of organic waste. This gas, rich in methane, is used to produce electricity and heat.

Pre-treatments of organic matter such as grinding, sieving, homogenisation and elimination of impurities are necessary for the production of gas.

The fermentation of the biogas takes place in the digester, the main part of a biogas plant. During this process, the organic components are broken down by micro-organisms that produce biogas.

At the end of the process, the product receives further treatments, ranging from simple agricultural storage before use in fields, to dehydration and post-composting of the solid part. With comprehensive treatment in the plant, the water can be returned to a completely drinkable state.




Suitable materials for the anaerobic fermentation process have a high volume of organic substance. These products come not only from the livestock or agriculture industry but also from the food industry and the organic fraction of municipal waste. Some studies show that the transformation of all available waste into biogas/biomass could produce 8 billion m³ per year of bio-methane, equivalent to about 25 TWh of electricity per year.

The most common types of biomass used for energy production are the following:

Pig slurry: The solid substance in this manure is between 1-6%, depending on the type of breeding. The sewage of a pig of 100kg can produce approximately 25-35 m³ of gas per year.

Cattle slurry: The dry matter varies from 8-15%, depending on the type of farming, the amount of straw used, and the waste collecting system, with or without water use. The sewage of a 400kg  dairy cow can produce 200-300 m³ of gas per year.

Poultry manure: This type of manure is difficult for production because it has aggregates that settle at the bottom of the digesters, and a high concentration of ammonia that inhibits the process. However, since chicken manure is easy to collect, some farms use such waste for the production of gas with excellent results. This biomass has a high total solids content of 18-20% and for this reason a horizontal digester is the most suitable to effectively treat the matter.

Energy crops: In recent years have been many studies on cultivation specifically for the production of energy in the anaerobic process. Some crops belonging to the grass family not only offer high yields but also grow and spread faster. These crops easily fit different types of soils and have a high percentage of dry matter. The use of these crops can reduce agricultural overproduction, which in recent years has led to a lowering of prices. Uncultivated, recycled, or water irrigated areas can be used to grow energy crops.

Crop residues: This category includes agricultural products such as fruits, vegetables, low-quality forage, and straw from silos. Crop residues are excellent components for the anaerobic process when added to animal manure in cogeneration.

Foods industry waste: Processing of agricultural products creates large quantities of reusable waste in anaerobic digestion, including whey, slaughterhouse waste, fats, fruit juices and liquid products of the distillery. These materials can be added to the slurry, increasing the energy yields and producing fertilizer.

Organic fraction of municipal waste: Business such as restaurants, wholesale markets, and retail stores result in 25-35% of the total wet component of all the municipal waste. This material has a high percentage of humidity (about 65%) and decomposes quickly. Thus it is suitable for the anaerobic process, producing a high economic return from the gas and fertilizer.


The process


Studies and applications on anaerobic digestion with different types of biomass have developed various technologies for the production of biomass.


Anaerobic digestion:

Wet: the substrate in digestion has a dry matter content of less than 10%; this technique is ideal when used with slurry

Dry: the substrate in digestion has a dry matter content above 20%

The intermediate percentages of dry substance are referred to as semi-dry.


Depending on the type of product you can employ one of the following processes:

single-stage process: hydrolysis, fermentation, and methanogenic phases occur in a single tank

two-stage process: the first tank contains the hydrolysis and fermentation phases; the intermediate product is then transferred to a second tank for the methanogenic phase

Mesophilic conditions during anaerobic digestion maintains the temperature of the fermentation at about 35°C. Thermophilic conditions cause the temperature to rise to about 55°C. The material remains in the digester for 15-40 days for the mesophilic process, and less than 20 days for the thermophilic process.

A single stage process system can operate in psicrofilia conditions (10-25°C ), remaining in the digester for 30-90 days.


Depending on the type of product you can take one of the following types of processes :

single-stage process when hydrolysis , fermentation and methanogenic phases occur in a single tank;

two stages process when in first tank there are the hydrolysis and fermentation phases. The intermediate product is transferred to a second tank for the methanogenic phase .

In this phase anaerobic digestion in mesophilic conditions maintains the fermentation at a temperature of about 35 ° C, in thermophilic condition goes up to about 55 ° C. The choice between these two type determines the residence time of the material in the digester ranging from 15-40 days for mesophilic process to less than 20 days for thermophilic process.

When the system is single stage type you can also operate in psicrofilia condition (10-25 ° C ), with residence times longer than 30 days up to a maximum of 90 days .


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