In current conditions of extreme and constant necessity of energy and high environmental risk, the anaerobic treatment
with biogas recycling results a system of great appeal, and it can offer several advantages:
- ENERGETIC RECYCLING from renewable sources: the anaerobic treatment in controlled conditions leads to the
degradation of the organic matter and to the production of biogas, a mix formed of 50-70% and methane and, for the remaining share, almost only of carbon dioxide. The cogeneration of electric energy and heat through biogas combustion is very convenient;
- REDUCTION OF GAS CARBON
- LOWER TOXICITY OF SLURRY FOR PLANTS
- SLURRY STABILIZATION The reduction of the carbon organic charge reached with the anaerobic digestion
gives to slurry a sufficient stability even in the periods next to the storage; there is a slowing down of the
degradation and fermentation processes, with a consequent reduction of the production of nasty compounds.
- REDUCTION OF THE PATHOGENIC CHARGE : the anaerobic digestion in mesophile can partially reduce the
pathogenic charge that can be found in the slurry. On the contrary, in thermophile it is possible to obtain the
complete hygienization of the slurry with the total destructionl of the pathogens;
- LESS ODOUR IMPACT reduction of odours and polluting emissions (NH3 e CH4). The nasty materials formed
during the process (hydrogen sulphide, mercaptans, ammonia) are conveyed to the combustion with the biogas.
A good reduction of the odours can be reached with plants in which the process of anaerobic digestion is led in
mesophilic conditions (38-42°C) and thermophilic (50-55°C). Good results can also be reached with the
digestion at lower temperatures, between 10-25°C, as long as the permanence times are appropriate;
- BETTER HYGIENIC LEVEL;
- CONVENIENT USE OF THE SINGLE PRODUCTS
- RISE OF THE GENERAL ECONOMICAL LEVEL
WHY MAKE A BIOGAS PLANT?
A further reduction of organic matter transformable in biogas comes from the slurry pre-treating operations (sieve) necessary for the removal of the rougher solids that can cause problems of surface scabs in the non-mixed reactors.
Then, for the calculation of the biogas yield, it is necessary to make a stechiometric analysis, from which results that for each g of COD destroyed, there is the production of 0.35l of methane in standard conditions (volume calculated at 0°C and at pressure 1 of absolute atmosphere) and report the reactor in standard conditions.
Actually this value has to be corrected, as a fraction estimated on average at 5% of the COD distroyed is used for the cell growth of the anaerobic biomass responsible of the process. Therefore the conversion factor lowers to 0.33.
As biogas is usually measured at a temperature and a pressure different from the standard conditions, this value has to be multiplied for a factor equal to (273 + T) /273 where T is the temperature espresse in °C, and divided for a factor (10.33 + P) /10.33 where P is the pressure expressed in mm of water column (the opposite procedure has to be made if it is necessary to take a measure in reactor's conditions to a measure in standard conditions.
To simplify, we list the values and the yields of some of the main matrix and organic biomasses used as food substrate for the biogas plants.
INFLUENCE OF THE QUALITY OF SLURRY ON THE BIOGAS YIELDS
The total biodegradability of slurry, when analized at level of collection tank or sewer can change a lot, between 60 and
80%, depending on the age of slurry and the type of food. A further classification of the biodegradable fractions allows
the distinction inside the soluble fraction between a dissolved fraction immediately biodegradable (about 20% of SSV)
and one not immediately biodegradable, and inside the suspended fractions between a suspended fraction easily
hydrolizable and one slowly hydrolizable.
The datas resulting from long-term lab tests, with normal conditions of the anaerobic reactor, with limited times of
hydraulic permanence, show levels of transformation of the organic matter in biological gas that vary from 70 and 90% of the maximum biodegradabilità of the slurry. Low levels of transformation in biogas can be caused by low temperatures,by too short times of hydraulic retention (or by too high organic charges) according to the temperature of the process, by bad hydrodynamic behaviours of the reactor with formation of dead areas and by-pass flows between entry and outlet.
During this stage, starting from the products of hydrolisis, takes place the production of volatile organic acids. In fact, the "acid-forming" facoltative anaerobic bacteria lead to the formation of acetic, propanoic and butirri acid. From these
precursor then come aldeide, alcols, carbon dioxide and hydrogen.
The gassification stage can be considered as composed by two main sub-stages:
- the first includes the transformation of the products from acidification in amine, ammonia, acid carbonates, carbon
dioxide, methane, hydrogen, nitrogen, mercaptans, indole, skatole and hydrogen sulphide.
- the second stage is characterized from the transformation of the chemical products resulting from the first phase in
methane and carbon dioxide.
N.B: WITH AN OPERATING PROCESS THE STAGES 1 – 2 – 3 ARE SIMULTANEOUS AND CONNECTED BY A
Stages of biological process
The main stages of the process can be synthetized as follows:
The hydrolisis is the stage which has the highest influence on the general speed of the system. This first metabolic stage is managed both by anaerobic and by facoltative bacteria. Hydrolisis of the organic macro-molecules is made by extracell enzymes produced from the same bacteria.
the demolition process of the organic matter that leads to the formation of biogas can be approximately synthetized in the relation
The gas resulting from the biological demolition of the organic matter, by the bacteria strain working in anaerobic environment, is a gaseous mixing essentially composed by methane and carbon dioxide, with traces of other gas.
Medium percentage of methane in the biogas produced from animals' dejections and other biomasses
|CH4 (% in vol.)|
The following groups of bacteria act in the process:
• hydrolytic bacteria, that break the biodegradable macromolecules in more simple substances
• acidogenic bacteria that use as substrate the simple organic compounds released from the hydrolytic bacteria
and they produce organic acids in short chain, which in turn represent the substrate for the next bacterial groups
• acetogen bacteria obbligate producers of hydrogen (OPHA: Obbligate Hydrogen Producing Acetogens) that use
as substrate the products of the acidogenic bacteria causing acetate, hydrogen and carbon dioxide
• omoacetogen bacteria that synthetize acetate starting from carbon dioxide and hydrogen
• methanigen bacteria, divided in two groups:
a. aceto clastics: they produce methane and carbon dioxide from acetic acid, called acetoclastics
b. hydrogenotrophic: they produce methane starting from carbon dioxide and hydrogen, called hydrogenotrophic
While methane is almost completely released in gas phase, due to its little solubilità in water, carbon dioxide concurs to the balance of the carbonates included in the biomass in reaction. The interactions between the different bacterical types are very close and the products of the metabolism of some species can be used in other species as substrate or as factors of growth.
Bacteria strains responsible for the process of anaerobic digestion and contaminable substrates
COMPOSIZIONE MEDIA DEL BIOGAS
The anaerobic digestioni s a complex biological process through which, in absence of oxygen, the organic matter is transformed into biogas or biological gas, mainly composed of methane and carbon dioxide. The percentage of methan in biogas varies according to the type of the organic matter digested and to the conditions of the process, from a minimum of 50% up to about 70%.
For the activation of the process, it is necessary to have the action of different groups of micro-organisms that can transform the organic matter into intermediate compounds, especially acetic acid, carbon dioxide and hydrogen, usable from the methanigen micro-organisms that finish the process with the production of methane.
The anaerobic micro-organisms present low growth speeds and low reaction speeds, so it is necessary, if possible, to keep best conditions of the reaction environment. Even with these expedients, the time of the process are relatively long if compared to those of other biological processes; therefore, the advantage of the processi s that the complex organic matter is converted in methane and carbon dioxide and so it take sto the final production of a renewable source of energy as combustibile gas with high calorific power. The reaction environment, usually called digestor (or anaerobic reactor), to allow the simultaneous growth of all the micro-organisms involved, has to strike a balance among the needs of the single microbic groups. For instance, the ideal pH is about 7/7.5, the ideal temperature of the process is about 40°C, if dealing with mesophile bacteria, or about 55°C if dealing with termophile bacteria.
BIOGAS : what is?
Biogas springs from the decomposition of the organic matter through the process of anaerobic digestion that takes place in absence of oxygen.
The ideal temperature for the process is about 35 - 42 ° C (mesophile field).
FUNCTIONAL PARAMETERS OF THE PROCESS
The most important parameters that influence the process and therefore they must be constantly and carefully controlled, can be synthetized as follows:
• LOADING UNITY (m3 o T);
• METABOLIC SUBSTRATES;
• TEMPERATURE (°C);
• ACIDITY' (Ph);
• RETENTION TIME (HRT);
• CONVERSION FACTORS;
• BIOGAS QUALITY.
Set of the single materials measured in tons and/or cubic meter according to the specific characters that ar loaded in the digestor on a daily basis. Defined also as the specific charge and it represents the total of the organic matter loaded daily.
It is important not to exceed with high specific charges to avoid that, in the process, there is a predominante of acid phase compared to the methanigen one. It is very important the possibility to distribute the loading uniformally during the whole day.
With too low percentages of dry matter there's the risk to have not sufficient metabolic substrates for the proper development of the microbic populations. On the contrary, with a too high solids content, the problems will concern the movement of the biomass, most of all during the loading phase.
3 < S.S. < 18%
The working field of the different cathegories of bacteria is very wide, but obviously there are ideal metabolic
conditions for each of them and they're espressed at
defined thermal ranges within which their potentiality is the highest.
The process can be kept active within the three pre-settled thermal ranges:
- PSICROFILIA : 1 0 ÷ 25 °C
- MESOFILIA : 30 ÷ 42 ° C
- TERMOFILIA : 48 ÷ 58 °C
pH is a very important element that influences the process, and it should be kept within neutral values, pH at limits of 6,5 ÷ 7,5 allow to consider the process as stable.
Others parameters to control are the potential oxidreduction (rH) that controls the proper anaerobic conditions and the ammonia concentrations that, at higher levels, can block the process.
RETENTION TIME (HRT)
The time for the hydraulic retention (HRT) (Hydraulic Retention Time) is the medium permanence time in the mixing ofthe loading unit inside the digestors. This time must be enouth to obtain the total demolition and degradation of the organic matter in biogas. The variabilità of the retention time is obviously connected to several factors, among them the most important are the characteristics of the loaded mixing and the range in which the process takes place. In mesophile condition we can consider :
• Pig slurry (2-4 % s.s) : 20/25 days;
• Cow slurry (6-8 % s.s.) : 25/35 days;
• Slurry and Biomass (12-14 % s.s.) : 50/60 days;
• Slurry and Biomasses (15 - 18 % s.s.) : 60/70 days.
FATTORI DI CONVERSIONE
Values that indicate the productions espresse in Nm3 of biogas for each kg of S.V.
Approximately, the values referred in the following table
can be considered as for the animals' dejections as well as for some biomasses involved.
The biogas mixing is composed of methane, carbon dioxide and other gas.
The biogas quality is higher as much as the percentage of methane is higher. A variation in the methane content can be foreseen between 50 ÷ 70 % and a good biogas quality is considered when the methane values are about 60 %.
A computerized system for the control of the process in terms of quantity, quality and times of immission is provided for each plant; the monitoring is constant also with remote control for the single performances. The production levels and the quality of gas are continuously controlled, as well as the efficiency of the single components, the ideal parameters and the cogenerators' yields.