1. Field of the Invention
The present invention concerns a method for the anaerobic decomposition of degradable organic waste and for the extraction of biogas from the latter in a reactor, according to which method the waste is put in a reactor which contains an active, anaerobic, methanogene biomass and which is exposed to anaerobic fermentation without any mixing in the reactor.
By degradable organic waste is meant in particular the organic fraction of domestic or biological waste or similar organic fractions such as sludge, industrial organic waste, etc.
2. Description of Related Art
According to the known methods for the decomposition of domestic solid waste, or similar solid waste, water is added to the organic fraction of the solid waste, such that pulp is obtained containing 10 to 12% solid matter which is decomposed in an anaerobic manner. The waste is regularly supplied to a reactor for anaerobic decomposition in which the concentration of solid matter amounts to 4 to 8% and in which the waste is decomposed for 10 to 30 days at a temperature of about 35 to 50 degrees Celsius. The content of the reactor is regularly mixed, such that the freshly supplied waste makes contact with the already decomposed residue in the reactor.
With such methods, thorough mixing is important, such that the supplied waste is evenly distributed in the reactor and the methane bacteria make contact with their nutrients. Thus, the formation of inactive zones in the reactor can be avoided.
The concentration of dry matter, however, is limited in these entirely mixed, anaerobic reactors to about 8% at the most. This is mainly due to crustation, as a result of which the mixing is ineffective, and consequently inactive zones are formed or the reactor acidifies. Consequently, the gas production is limited to 1 to 1.5 m.sup.3 biogas per m.sup.3 waste per day.
Mixing and crustation problems in the treatment of the organic part of domestic waste, similar solid waste or semi-solid substrates and the ensuing low load level of the reactor and restricted gas production can be avoided by making use of dry or liquid two-phase, highly-efficient decomposition devices.
Dry anaerobic, highly efficient decomposition devices work with a solid concentration whereby no crustation or phase-separation takes place, and whereby the mixing is done by removing the content of the reactor from this reactor and by mixing it in an appropriate mixing device with supplied nutrient substrate and by subsequently pumping the mixture back in the reactor. By using high concentrations of solid matter, phase separation, and consequently crustation, is avoided and the production of biogas with a high output of 6 to 8 m.sup.3 biogas per m.sup.3 waste per day is possible. In fact, one could say that these devices work with the crust itself.
The dry anaerobic composting method, as described in EP-A-O 131 319 and EP-A-O 205 721 B1, whereby the organic fraction of the domestic waste is decomposed with a concentration of totally solid matter of 25 to 45%, is for example based on a thorough re-circulation of two thirds of the matter which was taken out of the reactor. This recycled material is then mixed as inoculum with an amount of fresh organic material equal to less than half of this recycled amount. However, this method is especially suited for dry, solid substrates, such that a high concentration of solid matter can be maintained in the reactor.
Liquid, two-phase, highly efficient decomposition devices, as opposed to the above-mentioned dry decomposition devices, work with a very low concentration of totally solid matter in the methanogene phase, and make use of a sludge bed reactor or other types of anaerobic reactors with a high water level for the treatment of waste waters with a low concentration of suspended solid matter.
In these devices, the organic fraction of the domestic or organic waste is preliminary treated in a shredder and hydrolysis tank, such that the biologically degradable fraction is made as liquid as possible. This preliminary treated fraction is separated by means of a press or other dehydrating means from the remaining solid matter, and the liquid containing less than 2 to 3% totally solid matter and whose solid matter is preferably dissolvable for more than 80%, is subsequently decomposed in an anaerobic upward sludge bed or similar reactor. This method is more suited for the easily hydrolysable and biologically degradable fraction of domestic waste, which is also the most humid fraction of the domestic waste.