Dear sir,
It is must to have dairy along and u should also have agriculture farm to make profit otherwise its waste.
The formation of biogas is through the microbial decomposition of organic matter under anaerobic conditions. In nature, this is a naturally occurring process; this is for instance what happens in the rumen of a cow. A biogas plant is therefore comparable to a rumen and it has to be fed just like a rumen. The process is complicated and involves many different bacteria. Simplistically it can be schematically described as follows:
RAW MATERIALS
liquid manure/slurry, etc
large molecules, e.g. cellulose
FIRST PHASE
Hydrolysis, enzymes from bacteria
Small molecules, e.g. glucose
SECOND PHASE
Acidogenesis, acid-forming bacteria
Alcohols, volatile acids
Hydrogen, carbon dioxide, etc
THIRD PHASE
Methanogenesis, methane-forming bacteria
BIOGAS
Methane (c. 65%) Residue:
Carbon dioxide (c. 35%) Undecomposed material
Others (0-1%) inorganic material, etc
The following conditions are important for the microbiological process:
1. Completely anaerobic conditions are the most important condition.
2. Temperature. The digestion process can take place at many different temperatures but each group of bacteria is active at only a certain range of temperature, which is why the temperature cannot be changed without requiring a long adaptation period. The process itself is fairly stable at stable temperatures. The temperature range below 25„aC is called the psychrophilic phase, between c. 25-40„aC it is called the mesophilic phase, and between c. 45-65„aC the thermophilic phase.
3. Acidity should be in the range pH 6.5-8, which is the range achieved in an efficiently run digester.
4. Substrate. All organic matter is decomposable in the digestion process, but the rate of decomposition differs between substrates. The figure on the following page shows the yield of gas from different substrates in relation to decomposition time, and the table gives specific yields for important groups of organic matter.
5. Carbon/nitrogen ratio. The C/N ratio should normally be less than 30/1, otherwise the supply of nitrogen becomes a limiting factor, but the relationship also depends on the chemical forms of the nitrogen. In substrates such as animal manure and slurry the C/N ratio is acceptable, but in pure straw it is not.
The management of a biogas plant has to take into consideration certain conditions affecting the operation.
1. Dry matter content
Biogas plants in Denmark operate exclusively on slurry, mainly because the raw material has to be pumped into the digester. The dry matter content must therefore not exceed 8-10%.
2. Stirring
Stirring in the biogas reactor is necessary to avoid the formation of a surface crust.
3. Inoculation
Not all organic material contains sufficient methane bacteria and in order to get a new plant started up quickly, it is often necessary to inoculate with bacteria that have been conditioned to the actual temperature.
4. Organic loading
As methane bacteria reproduce fairly slowly, the loading rate of new raw material must be balanced against the outflow, otherwise too many bacteria will be lost in the effluent and decomposition will stop. If there is a change of substrate, the change has to take place slowly to allow bacteria to adapt to the new conditions.
5. Hydraulic retention time
The more resistant to decomposition the substrate is, the longer the required retention time is in the digester.
1) ½ easily decomposable carbohydrate and ½ resistant carbohydrate
2) ODM (organic dry matter) = 80% of DM (dry matter)
(Source: Danish Energy Authority (1991): Biogas action plan. Background report no. 12)
Peter Jørgensen, PlanEnergi
The use of degassed slurry as an agricultural fertilizer has many advantages. There are, however, also some disadvantages, but these can be countered relatively easily.
When animal manure is treated in a biogas plant, the organic matter is broken down by microbiological activity, resulting in the production of methane. This decomposition process converts the organic nitrogen (N) into inorganic ammonium-N, thus increasing the ammonium-N content of the effluent. This is an advantage as ammonium-N is directly plant-available, as opposed to organic N. The resultant degassed slurry is therefore potentially more valuable as a fertilizer than untreated animal manure.
Unfortunately, the decomposition process also increases the pH of the mixture. The effect of a raised pH is that a higher proportion of ammonium-N is converted to ammonia, which can be lost through volatilization during storage or application.
A loss of up to 20% of the N content has been measured during storage of the slurry. This is due partly to the elevated concentration of ammonia, but also partly to the crust that naturally forms on the degassed slurry being very thin, thus allowing volatilization to take place. Fortunately, it is very easy to establish an artificial crust by using e.g. chopped straw, solid animal manure or Leca nuts. There is a high risk of ammonia volatilization during and particularly after application of the slurry. It is therefore important that it is incorporated into the soil as quickly as possible after application. The effluent is relatively watery and percolates fairly quickly into the soil. It is therefore best applied to growing crops where percolation is minimal, thus reducing the risk of leaching.
The content of organic matter and therefore also the content of organic N is low in the slurry. This reduces the level of N leaching as the level of leaching usually increases with increasing levels of organic N. This is because organic N can be converted to leachable inorganic N during the autumn when crop cover is often sparse. N loss through denitrification is also lower from degassed slurry. This is because denitrification requires the presence of a high level of readily decomposable organic matter in the soil and this has already been decomposed in the biogas digester.
Some farm-scale plants receive animal manure from both cattle and pig farms. It is often an advantage to both types of farm to share the resultant degassed mixed slurry. Pig manure can be rich in phosphorus, thus creating a farm surplus, but can be deficient in potassium. As the opposite is often the case for cattle farms, mixing the two types of manure can ensure a better distribution of phosphorus and potassium on the two types of farms. It may, however, increase the need for the cattle farm to import potassium in mineral fertilizer as its potassium requirement is fairly high.
A beneficial characteristic of the degassed slurry is that noxious smells at the time of application are much reduced, and the gasification process destroys many parasites and weed seeds. In April 2000, the Danish Agricultural Advisory Centre published a leaflet with a more comprehensive explanation of the content of the article. This can be obtained from the offices of the Advisory Centre or from the Biogas Secretariat.
Regards
Kirti s
