Process for the anaerobic fermentation of biogenic waste materials and plant for carrying out this process

Abstract

A process for the anaerobic fermentation of biogenic waste materials including the steps that a fermentation material containing the waste materials is mixed with liquid and the fermentation material suspension obtained in this manner is anaerobically fermented in a fermenter and the digest produced in the fermentation of the fermentation material suspension is dewatered of a dewatering device, wherein, with separation of press water, a press cake is obtained that has a dry matter proportion increased with respect to the digest; the press water is subjected to a solids separation, wherein a residual liquid having a dry matter proportion reduced with respect to the press water of at most 15% by weight is obtained and the residual liquid is used at least in part for the mixing according to the fermentation material step.

Claims

1. A process for the anaerobic fermentation of biogenic waste materials, comprising: a) mixing a liquid with a fermentation material containing the waste materials to obtain a fermentation material suspension, and anaerobically fermenting the fermentation material suspension in a fermenter to produce a digest; b) dewatering the digest by means of a dewatering device to obtain a press water and a press cake having a dry matter proportion increased with respect to the digest; and c) subjecting the press water to a solids separation to obtain a residual liquid having a dry matter proportion reduced with respect to the press water, the dry matter proportion of the residual liquid being at most 15% by weight, wherein the residual liquid is used at least in part for the mixing according to step a).

2. The process as claimed in claim 1, performed continuously or semi-continuously.

3. The process as claimed in claim 1, wherein the solids separation according to step c) is performed by means of filtration and/or by means of screening, and the residual liquid is formed by the filtrate of the filtration or by the screen underflow of the screening.

4. The process as claimed in claim 1, wherein the press water obtained in step b) and/or the residual liquid obtained in step c) is temporarily stored before the respective following step.

5. The process as claimed in claim 1, wherein step a) comprises mixing the fermentation material in a mixing device with the residual liquid and optionally further water to obtain the fermentation material suspension.

6. The process as claimed in claim 1, wherein the dry matter proportion in the fermentation material suspension in step a) is between 25 and 35% by weight.

7. The process as claimed in claim 1, wherein a part of the digest, the press cake obtained during the dewatering according to step b), the residual solids obtained in the solids separation in step c), or a mixture thereof is added to the fermentation material or the fermentation material suspension as inoculum material.

8. The process as claimed in claim 1, further comprising determining and setting to a desired value one or more parameters of the residual liquid, said parameters being selected from the group consisting of: pH, buffer capacity, H.sub.2S content, ammonium ion content or ammonia content, content of fatty acids, and content of short-chain carboxylic acids.

9. The process as claimed in claim 1, further comprising adding at least one of the following auxiliaries or additives to the residual liquid: acid, lye, buffer, micronutrient elements or trace elements, and macronutrient elements.

10. The process as claimed in claim 3, wherein the solids separation according to step c) is performed by means of screening having two or more screening stages, wherein, in a first screening stage c1), a coarse fraction of solids components is separated off from the press water and from the suspension remaining from the stage c1) as screen underflow, in a subsequent second screening stage c2), a fine fraction is separated off in order to obtain the residual liquid.

11. The process as claimed in claim 1, wherein the solids separation according to step c) is performed using a vibrating screen and/or a shaking screen.

12. The process as claimed in claim 10, wherein the solids separation according to step c) is performed using a vibrating screen and/or a shaking screen.

Description

(1) The invention is illustrated further with reference to the figures. Of these:

(2) FIG. 1 shows a flow diagram of the process according to the invention;

(3) FIG. 2 shows a schematic representation of a plant according to the invention;

(4) FIG. 3 shows a solids separator particularly suitable for the purposes of the present invention, in perspective view;

(5) FIG. 4 shows the solids separator shown in FIG. 3 in side view; and

(6) FIG. 5 shows a section through the solids separator shown in FIGS. 3 and 4, through the section planes A-A according to FIG. 4.

(7) The specific process shown in FIG. 1 according to the present invention comprises the steps that the biogenic waste materials that are to be fermented anaerobically are fed as fermentation material B into a mixing device 2.

(8) Liquid is introduced into the mixing device 2 in order to set the desired homogeneity and the desired dry matter proportion of the fermentation material suspension BS that is to be fermented. At least a part of the liquid that is fed is fed to the mixing device in the form of a filtrate F, the provision of which is described in detail further below. A further part of the liquid can be fed to the mixing device 2 in the form of fresh water W.

(9) The mixing device can be designed, for instance, in an analogous manner, as described in EP-A-1 076 051 in paragraphs [0020] and [0021], the contents of which are hereby incorporated by reference.

(10) The fermentation material suspension BS generated in the mixing device 2 has a dry matter proportion in the range from 25 to 35% by weight and is fed via a feed line 12 to a fermenter 1. The feed proceeds here continuously or semi-continuously and determines the time of passage through the fermenter of the fermentation material suspension BS. Alternatively to the embodiment shown, in which the mixing device 2 is a device separate to the fermenter 1, the mixing of the fermentation material B with additional liquid can also be carried out in the fermenter.

(11) The digest D which is produced in the fermentation of the fermentation material suspension BS and is at least virtually completely fermented, and the dry matter proportion of which in this specific case is still in the range from 20 to 30% by weight, is finally fed via a digest feed line 5 to a dewatering device 13 and dewatered by means of said dewatering device, wherein, with separation of press water PW, a press cake PK is obtained which has a dry matter proportion which is increased with respect to the digest D. Specifically, the dry matter proportion of the press cake PK is in the range from 30 to 40% by weight, whereas the dry matter proportion of the press water PW in the specific case is between 15 and 20% by weight.

(12) The press water PW obtained in this manner is then fed to a solids separator 20 in the form of a filtration device 200 or a screen 201, wherein, as residual liquid RF, a filtrate F or a screen underflow SD having a dry matter proportion reduced in comparison with the press water PW of at most 15% by weight, specifically a dry matter proportion in the range from 5 to 15% by weight, is obtained, whereas the retentate R or the screen oversize SU produced as residual solid RS of the filtration has a dry matter proportion in the range from 35 to 45% by weight.

(13) As previously mentioned, finally, the filtrate F and/or the screen underflow SD is recirculated to the mixing device 2, where it is used for homogenizing the fermentation material B and for setting the desired dry matter proportion.

(14) In the embodiment specifically shown, the press water PW obtained in the dewatering is fed via a first press water feed line section 21 to a press water tank 15, in which it is stored temporarily before it is passed via a second press water feed line section 22 to the filtration device 200. In addition, the filtrate F obtained in the filtration is also stored temporarily in a filtrate tank 34 before it is recirculated via a residual liquid feed line 16 in the form of a filtrate feed line 160 to the mixing device 2. Of course, it is also conceivable that the press water PW and/or the filtrate F or the screen underflow SD is fed directly, that is to say without temporary storage, to the subsequent filtration or mixing device 200 or 2, respectively.

(15) The plant according to the invention is described further hereinafter with reference to FIG. 2.

(16) In the embodiment shown specifically in FIG. 2, the fermenter 1 comprises a horizontal fermenter tank 10.

(17) The fermenter tank 10 can either be configured as a steel structure or as a lined concrete structure and extends in the axial direction from a first inlet-side end 101 to a second outlet-side end 102. An agitator passes axially through the fermenter tank 10, which agitator is intended to mix thoroughly fermentation material introduced via a fermenter inlet 3 while said fermentation material passes through the fermenter tank 10 in the direction towards a fermenter outlet 14. An agitator drive 11 is assigned to the agitator, which agitator drive is arranged at the inlet-side end of the fermenter tank 10.

(18) The fermentation material is fed in via a preferably heatable feed line 12 which at one end is connected to the fermenter inlet 3 and which at the other end is coupled to a mixing device 2 that is described in connection with FIG. 1, and in which the fermentation material suspension that is to be fed in is prepared.

(19) As mentioned in connection with FIG. 1, the feed-in is continuous or semi-continuous. Likewise, the passage through the fermenter 1 proceeds continuously or semi-continuously.

(20) Via the fermenter outlet 14, the at least virtually completely fermented digest arrives into a dewatering device 13 for dewatering the digest. While the press cake PK obtained via the dewatering is fed to a secondary digestion, the press water PW is fed via a first press water feed line section 21 to a press water tank 15 and there stored temporarily. Via a second press water feed line section 22, press water is fed from the press water tank 15 to a solids separator 20 in the form of a filter device 200 or a screen 201, in which the press water is filtered, such that the filtrate F or screen underflow SD obtained as residual liquid has a dry matter proportion of at most 15% by weight.

(21) From the filter device 23, the filtrate F or the screen underflow SD is fed via a residual liquid feed line 16 in the form of a filtrate feed line 160 or screen underflow feed line 161, respectively, to the mixing device 2, which in addition is charged with fresh fermentation material that is to be treated.

(22) The mixing device 2 is here charged, not as is customary from an intermediate bunker inside the building, but rather the fresh fermentation material passes directly from the delivery to a metering conveyor 17, from which the fermentation material passes via a discharge unit 18 to a conveyor belt 19 opening out into the mixing device.

(23) Customarily, waste materials that are to be fermented are comminuted before they are fed to the fermenter. For this purpose, comminuters can be used that are not shown here. Instead of comminuting the waste materials to a suitable size, the fermentation material can also be freed from excessively large components by screening, for example in a drum screen.

(24) The mixing device 2 is connected via a connection line 33 to a fermenter return line 30 and secondly to the feed line 12, wherein the fermenter return line 30 may be opened and closed by means of a slider 31. Also, a slider 32 is assigned to the feed line 12 for opening and closing. Therefore, when the slider 31 is open digest from the fermenter 1 can be introduced into the connection line 33. This digest thus arrives back at the inlet-side end of the fermenter tank, where it is used as inoculum material for the fermentation process.

(25) A solids separator that is particularly highly suitable for the purposes of the present invention is the vibrating screen 201 shown in FIGS. 3 to 5.

(26) The vibrating screen 2010 comprises a screen unit 36 containing two screen containers 38a, 38b, that are stacked one above the other, and which are arranged on a screen underflow container 40. The screen containers 38a, 38b and the screen underflow container 40 have a substantially circular, identical cross section and are delimited towards the outside via a shell wall 42 running in the peripheral direction.

(27) As proceeds, in particular, from FIG. 5, the first screen container 38a, considered from the top, that is to say considered in the direction of screening, has a first screen cloth 44a which has a larger mesh width than the second screen cloth 44b of the subsequent second screen container 38b. Not only the screen containers 38a, 38b, but also the screen underflow container 40 has an outlet 46 radially projecting from the respective shell wall 42.

(28) The screen unit 36 is supported via corresponding spring elements 48 on a pedestal 50. In addition, a drive housing 52 having a drive 54 is arranged on the screen unit 36, which drive has a drive shaft 56, on the end regions 58a, 58b of which in each case an eccentric flywheel mass 60 sits.

(29) During operation, the screen unit 36 is therefore put into a circular vibration movement. As a result, the press water applied to the first screen contains 38a is constantly agitated, wherein, in particular, the solids components of the press water that are settling on the surface of the first screen cloth 44a are also constantly agitated, and blockage of the screen cloth is effectively prevented.

(30) The screen oversize containing a coarse fraction of solids components is then removed via the coarse fraction outlet 46a, while the screen underflow still containing a fine fraction of solids components is collected in a dished tank 62 arranged beneath the first screen cloth 44a and fed to the second screen cloth 44b.

(31) A screen oversize containing the fine fraction, which screen oversize is removed via the fine fraction outlet 46b, and a residual liquid having a greatly reduced dry matter proportion of at most 15% by weight result from the screening in the second screen container 38b. This residual liquid flows via an upwardly dished base of the screen underflow container at the edge thereof, from where it is ultimately removed from the screen unit 36 via the residual liquid outlet 46c and is recirculated to the fermenter.

LIST OF REFERENCE SIGNS

(32) 1 Fermenter

(33) 2 Mixing device

(34) 3 Fermenter inlet

(35) 5 Digest feed line

(36) 10 Fermenter tank

(37) 11 Agitator drive

(38) 12 Feed line

(39) 13 Dewatering device

(40) 14 Fermenter outlet

(41) 15 Press water tank

(42) 16; Residual fluid feed line;

(43) 160; 161 Filtrate feed line; screen underflow feed line

(44) 17 Metering conveyor

(45) 18 Discharge unit

(46) 19 Conveyor belt

(47) 20; Solids separator;

(48) 200; Filter device;

(49) 201; 2010 Screen; vibrating screen

(50) 21 First press water feed line section

(51) 22 Second press water feed line section

(52) 23 Filter device

(53) 24 Filtrate feed line

(54) 30 Fermenter return line

(55) 31 Slider in fermenter return line

(56) 32 Slider in feed line

(57) 33 Connection line

(58) 34 Filtrate tank

(59) 36 Screen unit

(60) 38a, 38b Screen containers

(61) 40 Screen underflow container

(62) 42 Shell wall

(63) 44a, 44b Screen cloth

(64) 46a, 46b, 46c Outlet

(65) 48 Spring elements

(66) 50 Pedestal

(67) 54 Drive

(68) 56 Drive shaft

(69) 58a, 58b End regions of the drive shaft

(70) 60 Eccentric flywheel mass

(71) 62 Tank

(72) 64 Base of the screen underflow container

(73) 101 Inlet-side end of the fermenter tank

(74) 102 Outlet-side end of the fermenter tank

(75) B Fermentation material

(76) BS Fermentation material suspension

(77) D Digest

(78) PW Press water

(79) PK Press cake

(80) RF; F, SD Residual liquid; filtrate, screen underflow

(81) RS; R, SU Residual solids; retentate, screen oversize