BIODIGESTER: WHAT IS IT FOR, TYPES, ADVANTAGES, DISADVANTAGES - ENVIRONMENT - 2024

A biodigester is a closed tank where methane gas and organic fertilizer are generated from the anaerobic fermentation of organic matter. The biological foundation is the decomposition of organic matter by the action of bacteria through hydrolysis, acidification, acetanogenesis and methanogenesis.

The biodigester provides the controlled conditions necessary for the biodigestion process. After this process, biogas (methane, carbon dioxide, nitrogen and hydrogen sulfide), biosol (solid fertilizer) and biol (liquid fertilizer) are obtained as final products.

Biogas system. Source: Renergon International AG

The basic operation starts from adding organic waste and water to an airtight container, in which the anaerobic fermentation process is generated. The biogas is then extracted for storage, direct use or as a fertilizer.

The three basic types of biodigesters according to their loading system are discontinuous, semi-continuous and continuous. The batch biodigesters are loaded with organic waste only once in each production process, then the fertilizer is extracted to start another cycle.

Those of semi-continuous load, are loaded in regular periods extracting the amount of fertilizer equivalent to the loaded volume. Continuous systems are industrial plants with a permanent load of organic matter, as well as the extraction of biogas and fertilizer.

Among the advantages of biodigesters are allowing proper management of organic waste, recycling it and reducing environmental risks. Additionally, energy (biogas) and organic fertilizers are produced, which generates an economic and environmental value.

However, there are also certain disadvantages such as water consumption, the difficulty of maintaining the ideal temperature levels and the presence of harmful substances (hydrogen sulfide, siloxenes). It also highlights the accumulation of raw material near the area and risks of explosions.

You can build a relatively inexpensive homemade biodigester and process organic kitchen waste. This only requires a barrel with a hermetic lid and some plumbing materials (PVC pipes, stopcocks, among others).

On a larger scale, in houses in rural areas the most economical and relatively easy to build system is the sausage. This system basically consists of a sealed polyethylene bag with the corresponding connections.

What is it for

- Treatment and recycling of organic waste

Biodigesters are very useful technological alternatives from the perspective of sustainable management of organic waste and the production of renewable energy. For example, they provide an alternative for the recycling of solid and liquid organic waste, which is transformed into raw material for the biodigester.

Recycling organic waste in this way reduces its polluting impact and generates savings in its management. Biodigesters are used for wastewater treatment, urban solid organic waste processing, and agricultural and livestock waste.

- Production of biogas and biofertilizers

The anaerobic digestion process generates biogas and organic fertilizer as products.

Biogas

Biogas has around 60% methane gas which is a high caloric fuel and can be used for energy production. It can be used for cooking, generating electrical energy (gas turbines), moving motors or heating.

Biofertilizers

Biofertilizers resulting from biodigesters are obtained in a state (biosol) and liquid (biol) with high levels of macro and micronutrients. Basic macronutrients (phosphorus, nitrogen and potassium) can be obtained in isolation from the biol through ultrafiltration and reverse osmosis processes.

Biol contains significant amounts of growth hormones useful for plant development such as indole-acetic acid, gibberellins and cytokinins, among others.

How does it work

The biodigester works by generating a biogasification process through anaerobic digestion, from decomposing the hydrated organic matter and in the absence of air. This occurs through a fermentation process whose main products are methane gas (CH4) and carbon dioxide (CO2).

- Loading the biodigester and shaking

It is carried out through the loading tank, which consists of a tank in which the organic matter is prepared to be added through the loading tube to the biodigester.

Processing of organic matter and load

The biodigester must be periodically fed with organic matter and enough water for its carrying capacity. In this sense, 25% of the volume of the biodigester must be left free for the accumulation of the gas produced.

In turn, the type and quality of organic matter will also influence productivity and the use or not of solid and liquid waste as fertilizer. Some organic wastes can cause problems in the fermentation process, such as citrus fruit residues that can acidify the medium too much.

The material must be crushed or reduced to the smallest possible size, and to facilitate fermentation, the mixture must contain 75% water and 25% organic matter. It must be stirred periodically in order to guarantee the homogeneity of the fermentation process in the mixture.

Temperature and retention time

The retention time of the organic matter in the biodigester to achieve its full fermentation will depend on the type of this and the temperature. The higher the ambient temperature, the faster the fermentation will be (for example, at 30 ºC it may take about 20 days to recharge the biodigester).

- Anaerobic digestion

Anaerobic digestion. Source: Tilley, E., Ulrich, L., Lüthi, C., Reymond, Ph., Zurbrügg, C.

Bacteria act in the process that require suitable environmental conditions such as the absence of air, temperatures above 20 ° C (ideally 30-35 ° C) and a medium that is not very acidic. Under these conditions, three phases develop:

Hydrolysis

Hydrolytic bacteria act in this process that secrete extracellular enzymes. Therefore, the complex chains of carbohydrates, proteins and lipids are broken down into smaller soluble pieces (sugars, amino acids and fats).

Acidification or fermentation stage

The soluble compounds of the previous phase are fermented to volatile fatty acids, alcohols, hydrogen and CO2.

Acetanogenesis

Acetogenic bacteria come into play that oxidize organic acids as a carbon source. They generate acetic acid (CH3COOH), hydrogen (H2) and carbon dioxide (CO2) and unpleasant odors are produced by the presence of hydrogen sulfide.

Methane formation or methanogenic phase

In the last phase, methanogenic bacteria act that decompose the products of acetanogenesis, generating methane. In nature these bacteria act in swamps, aquatic environments and in the stomach of ruminants.

At the end of this phase the mixture contains methane (45 to 55%), carbon dioxide (40 to 50%), nitrogen (2 to 3%) and hydrogen sulfide (1.5 to 2%).

- Discharge from the biodigester

The rate of biogas and fertilizer production depends on the type of biodigester, the organic matter that feeds it, and the temperature. The biogas accumulates in the upper part of the biodigester and is extracted through pipes to storage tanks.

Once the fermentation is finished, the sludge (a mixture of solids and liquid) is extracted through pipes. The discharge is produced by the principle of communicating vessels, that is to say, when loading new material, the pressure makes the surplus come out from the opposite side.

The ratio between the amount of material introduced (organic waste and water) and the output product (biosol and biol) is almost 1: 0.9. This is equivalent to a 90% yield, where the highest proportion corresponds to the biol (liquid).

- Biogas: purification

The gas produced must be purified to eliminate or reduce the content of hydrogen sulfide and water using traps to trap both compounds. This is necessary to reduce the risk of damage to the equipment due to the corrosive power of these components.

Water trap

The water carried away by the biogas precipitates when the pipe opens to a larger space and the gas continues through another constriction. This pipe ends in a large and hermetic container to contain the water that is later extracted by a drain cock in the lower part.

Hydrogen Sulfide Trap

The process to extract the hydrogen sulfide from the biogas is similar to that of the water trap, but the trap inserted in the path of the pipe must contain iron chips or sponges. When the biogas passes through the iron bed, the hydrogen sulfide reacts with it and precipitates.

- Fertilizer: separation and composting

The mixture of biosol and biol is subjected to a decantation process in order to separate both components. The biosol can be used alone or follow a process of mixing with composting for later use as a solid fertilizer.

Biol is used as a liquid foliar fertilizer or added to irrigation water, making it very useful in hydroponic systems.

Types

Biogas production in Germany. Source: Volker Thies (Asdrubal)

Biodigesters are classified according to their loading periodicity and structural shape. Due to its loading frequency we have:

- Discontinuous

The discontinuous or batch system consists of an airtight tank that is fully charged and not reloaded until it has stopped producing biogas. The gas accumulates in a floating collector attached to the top of the tank (gasometer).

This type of biodigester is used when the availability of organic waste is intermittent.

- Semi-continuous

Unlike the discontinuous system, loading and unloading are carried out at certain times during the biogas production process. According to its construction system there are three basic types:

Balloon or sausage biodigester

It is also called Taiwanese and consists of a flat concrete-lined pit where a polyethylene bag or cylinder is installed. Connections for the entry of organic waste and the exit of biogas must be installed in this bag.

The cylinder is filled with water and air and later the load of organic waste is added.

Fixed Dome Biodigesters

It is the so-called Chinese biodigester and consists of an underground tank built in brick or concrete. The tank is a vertical cylinder with convex or rounded ends and has a loading and unloading system.

The biogas accumulates in a space established for this purpose under the upper dome. The biodigester works with a variable pressure of biogas according to its production.

Floating dome biodigester

Called Hindu biodigester, it consists of an underground cylindrical tank with a loading and unloading system. It is built in brick or concrete and in its upper part there is a floating tank (gasometer) in which the biogas accumulates.

The stainless steel or plastic coated fiberglass gasometer floats above the mix thanks to the accumulated biogas. It has the advantage that it maintains a constant gas pressure.

Subsequently, the gasometer goes up and down depending on the level of the mixture and the amount of biogas. Therefore, it requires side rails or a central guide rod to avoid friction with the walls.

- Continuous

In this case, the loading and unloading of the biodigester is a continuous process, which requires permanent availability of organic waste. They are large industrial systems generally used for the processing of community wastewater.

For this, collection tank systems, pumps for transfer to biodigesters and fertilizer extraction are used. The biogas is subjected to a filtering system and distributed by compression to guarantee its distribution to the users.

Advantage

Recycling and pollution

The installation of a biodigester allows the recycling of organic waste, thus reducing environmental pollution and obtaining useful products. In the case of rural areas, it is particularly important for the management of animal excreta in livestock systems.

Obtaining biogas

Biogas represents an efficient and economical energy source, mainly in areas where the availability of other energy sources is not accessible. In rural areas of economically depressed countries, cooking is done with firewood, which impacts the environment.

The availability of biogas can help reduce the demand for firewood and therefore have a positive impact on the conservation of biodiversity.

Fertilizer production

By means of biodigesters, solid organic fertilizers (biosol) and liquid (biol) are obtained. These fertilizers have a lower environmental impact and reduce the costs of agricultural production.

Healthiness

By allowing proper management of organic waste, the risks they pose to health are reduced. It has been determined that 85% of pathogens do not survive the biodigestion process.

For example, fecal coliforms at 35 ° C are reduced by 50-70% and fungi by 95% in 24 hours. Therefore, being a closed process, bad odors are reduced.

Disadvantages

Water availability

The system is demanding in terms of water availability, since a mixture is required. On the other hand, the biodigester must be close to the source of raw material and the biogas consumption site.

Temperature

The biodigester must maintain a constant temperature close to 35 ° C and within a range between 20 and 60 ° C. Therefore, an external input of heat may be required.

Harmful by-products

It can produce hydrogen sulfide (H2S), which is toxic and corrosive, and silicone derived siloxenes contained in cosmetic products and in the mixture of organic waste. These siloxenes generate SiO2 (silicon dioxide), which is abrasive for machinery and components.

The presence and concentration of these by-products depends on the raw material used, the proportion of water and solid substrate, among other factors.

Accumulation of waste

It is necessary to accumulate waste near the biodigester, which brings with it logistical and sanitary problems that must be addressed.

Explosion risks

As it is a fuel gas generator system, it implies a certain risk of explosions if the proper precautions are not taken.

Costs

Although the maintenance and operation of the biodigester is relatively inexpensive, the initial installation and construction costs can be relatively high.

How to make a homemade biodigester

Home biodigester. Source: Kevinsooryan

A biodigester requires as basic elements and a tank for fermentation, loading and unloading pipes with their respective stopcocks. In addition, tanks for biogas and fertilizer are necessary.

It is important to note that the entire system must be airtight to prevent gas leaks. On the other hand, the system must be constructed of stainless materials such as PVC or stainless steel to avoid damage by water and hydrogen sulfide.

- Fermentation tank

A plastic barrel or tank can be used whose capacity will depend on the amount of organic waste to be processed. This tank must have a hermetic lid or, failing that, the lid must be sealed with high-temperature resistant plastic glue.

The tank must have four holes and all installations made in them must be sealed with high temperature silicone.

Loading cover

This hole is in the center of the tank cap, it must be at least 4 inches long and a threaded sanitary plug must be installed. This plug will be connected to a 4-inch PVC tube that will enter the tank vertically up to 10 cm before the bottom.

This entrance will serve to load the organic waste previously shredded or shredded.

Effluent drain hole 1

It is important to remember that 25% of the tank space must be left free for gas accumulation, so a hole must be opened in the side at that level. In this hole a tank adapter with a segment of 2-inch PVC pipe 15 cm long will be installed with a stopcock.

The function of this drain is to allow the supernatant biol to escape once the tank is recharged through the loading lid. The biol must be stored in suitable containers for later use.

Effluent drain hole 2

This second drain must go to the bottom of the tank in order to extract the densest part of the fermented product (biosol). Likewise, a segment of 2-inch PVC pipe of 15 cm in length with a stopcock will be used.

Biogas outlet

A 1/2-inch hole will be cut in the tank top to install an equal diameter PVC pipe using a tank adapter. This pipe will have a stopcock at the outlet.

- Biogas outlet and purification system

The biogas outlet tube must be at least 1.5 m long, in order to insert the water and hydrogen sulfide extraction systems in its path. This tube can then be extended if necessary to transfer the gas to its storage or use location.

Water extraction

To remove the water from the outlet, the pipe must be interrupted at 30 cm to insert a plastic or glass container with a hermetic lid. The gas transfer tube must have a bypass through a T connection, in order for the gas to penetrate the container.

In this way the gas fills the container, the water condenses and the gas continues its way through the pipe.

Hydrogen Sulfide Extraction

After the water trap, a 4-inch pipe segment is inserted in the next 30 cm by means of the corresponding reductions. This segment should be filled with iron shavings or commercial metal sponges.

The hydrogen sulfide will react with the metal and precipitate, while the biogas will continue its journey to the storage container or place of use.

References

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