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  Description of a Digestion Plant  
    The situation in Germany
Description of a Digestion Plant
cofermentation biogas plant references
 
       
  The situation in Germany - a little bit of history and explanation  
  Many years ago people realised that native-organic waste dumped on landfills is responsible for the majority of all the methane emissions from landfills. Together with all the well-known emissions to the groundwater and into the air, landfills also make a contribution to the greenhouse effect, too. Incineration of native-organics is not supposed to be an intelligent solution because of their high water content.
       
  Therefore, as early as the seventies, people tried to remove all native-organics from the waste and compost it. From a technical point of view there has been no big problem. But detailed investigations demonstrated that there were large quantities of heavy metals within compost produced this way. As a result of this problem the compost produced got a bad image. There was no chance to sell it either in landscape-gardening or in gardening or privately.  
       
  Somewhere in the mid-eighties first municipalities in Germany introduced a special bin for collection of so-called biowaste in each individual household. Biowaste is a mixture of garden and kitchen waste and was first collected for composting. Although this path proved to be quite stony, it could be shown that this was the most senseful and best way. Meanwhile in most municipalities so called "bio-bins" are standard operating procedure in Germany. top
       
  Parallel to the invention of bio-bins the first composting plants had to be engineered and constructed. It began with simple windrow composting. Later on these old plants gradually became more automated. Still later on, these plants were enclosed to avoid, or more exactly, to minimize odour emissions. Nowadays enclosed composting plants, more and more working automated and on a high technical level are state of the art.  
       
  But only at the beginning of the nineties was digestion of biowaste initiated. Similar to developmental history of composting plants, the path to reliable and efficient digestion plants has been long and stony. Around the mid-nineties comparison of composting and digestion plants showed an equal development level.  
       
  At this time it could be demonstrated that housed and automated composting plants and digestion plants with an identical throughput have similar investment costs. The difference is that operation of digestion plants is cheaper - due to the production of electricity and heat. Again the Electricity Input Law ("Stromeinspeisegesetz") was very valuable for the breakthrough. Comparison: Digestion and Composting.  
       
  Today composting and digestion plants are presumed to be equal in their technical level and costs in Germany. Rural municipalities which tend to have more garden waste inside their bio-bins prefer composting plants; cities that tend to have more kitchen waste inside the bio-bin prefer digestion plants.  
       
  Even today biogas technology is being improved day by day. But the basic problems have been solved. If there are still many more composting plants than digestion plants in Germany, the only reason for this is that composting started around 1985 and digestion about 10 years later. And there are some cities now that complain about their composting plant ..... top
       
  Description of a Digestion Plant  
  Large-scale industrial digestion plants are used for biowaste and kitchen waste digestion. Biowaste is collected in special bins in every single household in Germany. Depending on the season it consists more or less out of kitchen and/or garden waste. Kitchen waste is collected in restaurants, hotels, universities and so on. Years ago most of the kitchen waste was fed to pigs. But nowadays, as there are more and more restrictions because of hygiene it is becoming increasingly more common to put kitchen waste into biogas plants.  
       
  Before digestion, biowaste and kitchen waste have to be treated. This pretreatment includes foreign objects removal, grinding, sieving and homogenisation. Plastics, stones, forks, knives - any of which may hamper the digestion process itself or damage any pump, mixer or whatever have to be taken out of the process. Grinding means to reduce particle size so that all parts can be pumped and mixed readily. As kitchen waste has to be hygienised - normally at a temperature of 70° C with a retention time of one hour - particle size must be no greater then 1 cm. A sieve is used to remove all large particles just before entry into digestion. Of course, everything has to be slurried with liquid to make it pumpable. All the pretreatment is performed within a hall.  
       
  To digest biowaste is much more complicated than manure - and kitchen waste digestion is even more difficult. Depending on the amount of kitchen waste included in the biowaste, you may have to use a two-stage digestion process technology. In such a case a so called hydrolisis tank is necessary. In it the first biological processes take place. The hydrolisis gas consists mainly of CO2 and can either be added to the biogas from the digester or to a biofilter for odour treatment before being emitted to the environment. top
       
  After hydrolisation is finished the substrate is pumped to the digester. In it all the biogas production takes place. The retention time depends on the substrate, the temperature and the intention of the operator. As there are tendencies for the substrate to develop swimming and sedimentation layers, one of the most important tasks of the process technique is to avoid exactly this. Complete mixing is abolutely necessary. Tanks are mainly coated steel tanks; the standard is an industrial one.  
       
  The produced biogas is treated in a manner very similar to farm-scale and cofermentation biogas plants. Gas engines and diesel gas engines are in operation. But in most cases gas engines are used. Power can be several MW. Large-scale cofermentation biogas plants have large gas holders and emergency flares in case the engine(s) is (are) not in operation and biogas has to be burnt. The gas system may include a blower, condensate trap, desulphurisation unit and so on. Everything is controlled by a gas system control unit. top
       
  The digested substrate is dewatered. The compost is either spread onto the fields for agricultural use or composted (cured). After this standard composting people can buy high quality compost for their own use or the compost is used in nurseries, gardening, viniculture and landscape-gardening. After dewatering the liquid phase is pumped to the beginning of the process and recycled for slurrying of the dry input substrate. This is done to avoid spoiling fresh water all the time. Mechanically it is fairly simple but there are several microbiological problems. As there are lots of dissolved salts within the liquid phase high concentrations may develop there after several cycles. This may hamper the growth of microorganisms - depending on the input substrate.  
       
  All the large-scale digestion plants are controlled by an overall process controll system. There are lots of devices for measurement and safety reasons. At night everything is run automatically; during day there are operators on site - especially for pretreatment, repair and maintanance reasons and for accepting the biowaste and kitchen waste.  
       
  Large-scale digestion plants are constructed for one reason only: to make a profit. Therefore the plant must operate day and night. The investment costs may be up to several million Euros. Depending on the input substrate, the pretreatment aggregates have to be engineered and constructed. The hydraulic retention time may vary as a function of the input substrate and the pretreatment. This has a direct impact on the digester volume. Therefore a great deal of knowhow is needed to construct a large-scale digestion plant.  
       
  Biowaste digestion is not simple. After many years of engineering such kinds of plants Krieg & Fischer is able to offer competent knowhow. Kitchen waste digestion is even more difficult. Souring, difficult two-stage processes, high nitrogen contents and much more are matters for discussion here. Krieg & Fischer not only engineered several biogas plants for cofermentation of kitchen waste but have also engineered even most difficult of all thermophilic kitchen waste digestion plants. top
       
       
  Please take a look at some Krieg & Fischer digestion plant references  
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