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  Description of a Farm-Scale Biogas Plant  
  The principle layout of a farm-scale agricultural biogas plant is fairly simple. Manure is collected in a sump nearby the digester. From this sump the digester is feeded semi-continuously by a pump. The digester itself is a gas-tight completely sealed tank out of steel or concrete. It is insulated because there must be a fixed optimal temperature inside it for the microorganisms. This temperature can be either mesophilic with about 35° C or thermophilic with about 55° C.  
  Inside the digester there is an agitator. This agitator is responsible for complete mixing of digester's contents. The operator must ensure that there is no possibility for the development of swimming-layers and/or sedimentation. Additionally, the microorganisms must be supplied with all necessary nutrients. Fresh manure has to be fed to the digester several times a day, in small quantities. The average hydraulic retention time of the manure inside the digester is - depending on the substrate - between 20 and 40 days.  
  During this time the organic substances in the manure are metabolized by the microorganisms. There are two different outputs: biogas and digested substrate. The latter is stored in a standard manure storage tank. In Germany it is most common to use it as a fertilizer because of ist high ammonia concentration (NH4). The biogas is stored in a gas storage tank. Here all irregularities in quality and quantity will be equalized. From the gas storage tank there is continuous supply to a gas or diesel gas engine. Here heat and electricity is produced. top
  Farm-Scale Biogas Plant  
  Initially, the farmer uses the electricity produced on his own farm. He saves money because he needs no or less electricity from outside sources. Surplus electricity can be sold to the nearest public utility company - according to so-called Renewable Energy Law ("Erneuerbare Energien Gesetz"). This law garantees payment of 0,20 DM/kWh in Germany. Especially in such situations where there is no (or few) consumer on the farm that need electricity, the farmer earns money by selling the electricity from renewable energy ‚biogas'. The heat produced can be used for heating the stables, the farm-house, drying of grain and many other useful purposes. In general, about 10 to 30% of the electricity and heat produced have to be used for direct consumption by the biogas plant itself. Surplus electricity and heat are used for the financing of the biogas plant. Normal times for writing-off are 4 to 8 years.  
  Of course, there are other ways to utilise biogas. For example, all the biogas can be used for heat production: steam or hot water. This is attractive if there is need for large amounts of steam. Or the heat can be used to power refrigeration - this can be employed, for example, for cooling stored fresh milk in a dairy. top
  Please take a look at some Krieg & Fischer farm-scale biogas plant references  
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