Anaerobic digestion, nothing to do with stomach pains or heavy lunches and dinners, it is an alternative process to that of composting, completely anaerobic, indeed, on the contrary, strictly aerobic.
Anaerobic digestion: what it is
For anaerobic digestion we mean the degradation of the organic substance when it occurs by the action of microorganisms and in conditions of anaerobiosis. There are two types of microorganisms that can be the protagonists and the temperature of anaerobic digestion varies according to them.
It remains between 20-45 ° C, with an optimal range of 37-41 ° C, if i act mesophilic bacteria, the most widespread bacteria in general and which are found in very different environments: soil, water, including marine, sewage, animals and the human body. The temperatures are instead between 50-52 ° C, with temperatures that can also be relatively high and exceed 70 ° C, for thermophilic bacteria: set of organisms, belonging to the largest class of extremophiles, which live and multiply a relatively high temperatures.
Let's see the type of devices that host the anaerobic digestion: the most common are the continuous ones, in which the material is mixed with mechanical or hydraulic tricks and the excesses are continuously extracted to maintain a constant volume despite the addition of organic material.
Alternatively you can opt for the discontinuous digester, simpler but smells. Furthermore, the emptying cycles of this device are problematic and, once the material is initially inserted, it is closed and cannot be reopened for the entire duration of the process.
Anaerobic digestion of waste
When the anaerobic digestion of waste the residence time in a device depends very much on the quantity of material to be treated but also on the type of waste and the operating temperature. Another important parameter when it comes to anaerobic digestion of waste is the pH value.
As for the duration of the process, in general if you are using mesophilic bacteria it takes between 15 and 30 days. In the case of waste water treatment the anaerobic digestion with mesophilic bacteria it has times that depend on the composition, because one day is enough for the liquid part while the solid one takes many days, maximum 90.
If we move on to thermophilic bacteria, however, we work at higher temperatures so everything is faster: in two weeks everything is done. But it costs more, it takes more energy and the process is more critical. Today mesophiles are the recommended choice for anaerobic digestion.
Biomass anaerobic digestion
This kind of anaerobic digestion, in which there is also biodegradable organic material, implies the use of many different species of bacteria occurring in nature. As in one digestive relay, each has its leading role in a phase of the anaerobic digestion process and it is important to be able to monitor and know the actual and optimal operating conditions from time to time.
The phases that follow one another in anaerobic digestion are hydrolysis, from which simple compounds such as monosaccharides, amino acids and fatty acids are obtained, acidogenesis, where further cleavage takes place into even simpler molecules and ammonia is produced, carbon dioxide and hydrogen sulphide. Then there is acetogenesis where there is a further one anaerobic digestion, again, which produces carbon dioxide, hydrogen and mainly acetic acid. Finally, methanogenesis gives rise to a methane, carbon dioxide and water.
Dry anaerobic digestion
There anaerobic digestion it can be carried out both wet and dry, the dry one is that which concerns mixtures of materials with a minimum solids content of 30%. Conversely, it is wet when the treated material has a minimum of 15% solids content. In addition to the solid content of the material, to define the operating conditions in which a process can take place anaerbic digestion for example, the thermal reaction conditions and the biological phases.
Biogas anaerobic digestion
The main by-products of anaerobic digestion there are three including biogas. Then there is the acidogenic and methanogenic digestate. During the anaerobic digestion the biogas it is not produced constantly: the maximum level is that reached in the central phase. At the beginning, however, the biogas produced is scarce, because the bacteria have not yet reproduced enough, but towards the end it is little because the remaining material is more difficult to digest.
The biogas obtained can be used for the production of thermal energy, through combustion in the boiler, or also for the production of electricity, through cogeneration groups. Once the fate of this biogas is known, it is also known what kind of purification process it must undergo.
The biogas is produced in the section of the plant anaerobic digestion dedicated to the preparation of the substrate, to the anaerobic digestion to the production of energy. It is the penultimate, then there is the dehydration one from which, leaving the digester, a liquid mud not fully stabilized. Once dehydrated, the liquid fraction can be purified while the rest, biostabilized, can be further refined so that it can still be used, for example in agriculture.
Anaerobic digestion of sludge
In anaerobic digestion, the substances present in the mud, in the absence of oxygen, are reduced by fermentation processes which gradually stabilize more and more lead us to the production of methane and carbon dioxide. There digestion some mud is currently the most used, it is based on fermentation and for fermentation we mean the set of chemical and biological phenomena that produce the transformation of organic substances into simpler, organic and inorganic ones.
Fermentation can be acidic or alkaline. If left to itself, the mud tends to alkaline, also known as putrefaction, resulting in the production of foul-smelling, viscous mass, not much less voluminous than the one we started from. When the anaerobic digestion it is instead through alkaline fermentation that it relies on the presence of methanogenic microorganisms, the volume is reduced and the substance becomes viscous, giving off a bad smell, especially due to hydrogen sulphide.
The sludge digested in this way is more fluid and can be extracted from the tanks with great ease dehydratable and non-putrescible: can be used without danger as fertilizer for any kind of cultivation.
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