Description of a digestion plant for biowaste and kitchen waste

Digestion plants for biowaste and kitchen waste are usually implemented on a large industrial scale. Biowaste is separately collected organic waste generated by households. The contents of organic waste collection bins, into which kitchen and garden waste from private households is placed, varies depending on the time of year. Kitchen waste mainly originates from restaurants, hotels, canteens and other facilities with commercial kitchens. Thanks to increased and stringent hygiene regulations, this waste, which in the past was primarily used as pig feed, is now utilised in biogas plants.

Prior to the actual fermentation process, biowaste and kitchen waste must be pretreated. Removal of impurities, screening, grinding or chaffing and homogenisation form an integral part of pretreatment. If your substrates contain impurities, we develop a customised process chain for processing your substrate. To achieve this we use wet or dry processing techniques. If hygienisation is required, this is usually carried out at a temperature of 70° C and for a duration of 1 hour. Input material with a high dry-matter content is diluted with process water in order to guarantee that there are no problems with the process technology. Depending on the quantity of kitchen waste within the biowaste, a two-stage digestion process may be required; i.e. the input material first goes into the hydrolysis tank and subsequently into the digester. The initial biological process is hydrolysis. With this type of acidification, the biogas produced is primarily made up of CO2 and does not contribute to energy generation. The actual production of biogas then takes place in the digester.

Digesters are built according to industrial standards and are made of concrete or coated steel, and they are sometimes agitated centrally from above, or sometimes laterally. These methods of mixing are intended to prevent the formation of any floating or sedimentation layers.

Gas engines are usually employed to utilise the resultant gases, and these may well have capacities in the MW range. The gas is normally stored temporarily in large gas tanks. The plant is equipped with an emergency flare to burn off the excess biogas. Other system components are: cooler, condensate shaft, desulphurisation system, gas measuring device, etc.. Alternatively, the biogas can be processed to meet natural gas specifications and fed into the natural gas grid in the form of biomethane.

Following fermentation, the digestate is often dewatered. The solid part can be sold as compost to private households, tree nurseries, garden centres, wine-growing enterprises or landscape gardeners. The liquid part is pumped back to the start of the process, in order to liquefy the input material. One potential problem with this process water recirculation system is the fact that the liquid contains a lot of dissolved salts, and after several cycles high concentrations build up which may inhibit bacterial growth. Appropriate expertise and experience are essential for the process engineering design.

For a digestion plant to be able to operate effectively and efficiently, it is monitored by a comprehensive control system featuring a variety of measuring and control devices.

The fermentation of biowaste is relatively complex. The Krieg & Fischer engineers have many years of experience planning these kind of plants, and can therefore offer profound and competent expertise.

Here is a list of some of the digestion plants we have implemented:

  • Huntstown (Ireland): 2018/2019 ;2 x 2,4 MWel; thermal pressurised brown bin and food waste
  • Quinhuangdao (China): 2013/2014; biogas upgrading, biomethane used for vehicle fuel; kitchen waste
  •  im Brahm (Germany): 2005, 2011; 760 kWel; pig manure, horse manure, kitchen waste   
  • Mc Donnell (Ireland): 2009; 250 kWel; liquid cattle manure, chicken manure, kitchen waste, animal by-products
  • Noyon (France): 2008; 716 kWel; sewage sludge, fat, process water, waste from the food industry, kitchen waste
  • Goettingen (Germany): 2007; 254 kWel; percolate from the composting of biowaste
  • Bluemel (Germany): 1995; 320 kWel, biowaste