This invention relates to a process for producing biogas by digestion of animal manure under production of methane by a continuous process, in which fresh animal manure is supplied to a digestion tank concurrently with the removal of degassed manure from the tank.

In known biogas plants, the treatment of the supplied manure takes place in a single digestion tank either by a so-called mesophilic process where the diges- tion and the accompanying production of methane take place at a comparatively low temp-rature of about 35°C, or by a thermophilic process in which the digestion with a more complete degassing of the manure takes place at a temperature of about 55°C. The mesophilic process re- quires a comparatively long treatment time if a reason¬ able amount of gas is to be evolved, and a complete de- gassing of the manure is difficult to achieve; at the same time, mesophilic process does not make it possible to remove completely pathogenic organisms from the treat- ed manure which after having passed the biogas plant is to be used for fertilizing purposes.

By the thermophilic process, one achieves a more complete degassing, of the manure, but in order to main¬ tain the comparatively high digestion temperature, it is necessary, especially at low outdoor temperatures, to supply a substantial amount of energy to heat the digestor, which reduces the efficiency in respect of energy economy to such an extent that up till now the thermophilic process has not been extensively used, in spite of the fact that it offers substantial environmen¬ tal advantages owing to the fact that it results in a degassed manure of a very high fertilizing value, practically odourless and also free of pathogenic orga¬ nisms, such as intestinal worms and various pathogenic bacteria, which were contained in the fresh manure.

With a view to overcoming the energetic and _^ en¬ vironmental drawbacks of existing biogas plants, the process according to the invention is characterized in

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that the supplied animal manure is, in two separate, successive digestion tanks, subjected first to a meso¬ philic digestion at a temperature of about 35°C and afterwards to a thermophilic digestion at a temperature of about 55°C.

By subjecting successively the animal manure first to a mesophilic digestion and then to a thermo¬ philic digestion, one obtains in the first place an en¬ vironmentally valuable process making the degassed manure intended for fertilizing purposes both odourless and free of pathogenic organisms, so that the process according to the invention is particularly suitable for use in collec¬ tive plants treating manure from a plurality of farms, as it ensure that the degassed manure returned to each farm for fertilizing purposes is not infected with pathogenic organisms of foreign origin.

Moreover, as a result of the first mesophilic digestion process a more homogeneous manure πaterial is supplied as starting material for the subsequent thermo- philic process, in which starting material plant residue and organic refuse from the vegetable production, such as litter straw, have been decomposed to a substantial ex¬ tent as compared to fresh manure. Consequently, the pro¬ cessing time for the complete degassing of manure by the thermophilic process is substantially shorter than in the case of a thermophilic process carried out from fresh manure. In the process according to the invention the duration of the thermophilic digestion will typically be of about 6-8 days, while a thermophilic process on basis of fresh manure will typically require a processing time of 3 to 4 weeks.

For carrying out the process, a plant according to the invention is characterized in that it con rises an inner digestion tank provided with heating members for the thermophilic digestion of manure at a ten erature of about 55°C, and a surrounding outer digestion tank for the mesophilic digestion of manure at a temperature of about 35°C, which tanks are separated by a common heat-

insulating wall and communicate with each other only through a duct provided adjacent to the said wall for transferring mesophilically digested manure from the bottom of the outer tank to the upper part of the inner tank, an inlet duct for fresh manure opening into the upper part of the outer tank and an outlet duct for de¬ gassed manure passing from the bottom of the inner tank througt the outer tank, while outlet ducts for the methane gas are respectively connected with each of two separate chambers which are placed above the tanks and are open towards their respective tank.

This embodiment of the biogas plant with coaxial cylindrical tanks results in a further improvement of the energetic efficiency since the surrounding of the thermophilic digestion tank by the mesophilic digestion tank will further reduce the total heat requirement for digestion and degassing of a given amount of manure per time unit.

The invention will in the following be explained in more detail with reference to the schematic drawing showing a section of an embodiment of a biogas plant according to the invention.

In the embodiment shown, the biogas plant com¬ prises an inner thermophilic digestion tank 1 surround- ed by an outer mesophilic digestion tank 2, the two tanks being separated by a common heat-insulating wall 3. The outer tank 2 has also a heat-insulating outer wall 4.

An inlet duct 5 for fresh animal manure opens in- to the upper part of the outer tank 2, while in connec¬ tion with the common wall 3,, provision is made of a connection duct 6 for transferring the mesophilically digested manure from the bottom of the outer tank 2 to the top of the inner tank 1. From the bottom of the inner tank 1 , an outlet duct 7 for degassed manure passes through the outer tank 2 and the upper part of the outer wall 4.

At the top,' each of the tanks 1 and 2

closed by its respective heat-insulating roof, 8 and 9 respectively, so that there is no air connection between the two tanks but only connection through the transfer duct 6. Outlet ducts 10 and 11 for methane gas 5 from the inner tank 1 and the outer tank 2, respec¬ tively, are passed through the roofs 8 and 9 , re¬ spectively.

The biogas plant according to the invention is of the so-called displacement type where the digestion pro-

10 cess is continuous, since once the process has been started, the supply of fresh manure through the inlet duct 5 takes place concurrently with the removal of degassed manure through the outlet duct 7 and its conveying to a storage vessel (not shown) . The levels

15 of manure in the inner tank 1 and the outer tank 2 will be determined by the position of the lead-in of the outlet duct 7 through the outer wall 4 and of the opening of the transfer duct 6 into the inner tank 1. For heating the inner tank 1 in order to main-

20 tain the temperature of about 55°C necessary to the thermophilic digestion process, provision is made of a heating coil 12, while a heating pipe 13 is provided for the occasional heating of the outer tank 2 in cold periods, in order to maintain the temperature of about

25 35°C for the mesophilic digestion process.

The tanks 1 and 2 may, moreover, be provided with devices for instance in the form of circulating pumps for stirring the manure and thus preventing the formation of a so-called floating (supernatant) layer

30 at the top of the tanks. Such stirring devices can also in a conventional manner be further designed for comminuting manure, in particular in the outer tank 2.