PROCESS TO BACTERIALLY DECOMPOSE ORGANIC WASTE

Abstract

The invention is directed to a process and equipment for use in a process to bacterially decompose organic waste to a dry composted end material wherein organic waste is composted in a composting tank in the presence of oxygen and aerobic bacteria to obtain a wet composted material. The wet composted material is partly removed from the composting tank and dried such to lower the water content of the composted material to obtain dry composted end material.

Claims

1. Process to bacterially decompose organic waste material to a composted material in a series of two or more composting reactor spaces of one or more composting tanks in the presence of oxygen and aerobic bacteria, wherein in a batch operation waste material is added continuously or intermittedly to the first reactor space of two or more composting reactor spaces which reactor space comprises composted material from an earlier batch and aerobic bacteria and wherein the content of the first reactor space is partly removed from the first reactor space as intermediate wet composted material comprising aerobic bacteria at the end of the batch operation and wherein part of the composted material and aerobic bacteria remain in the first reactor space for performing a next batch operation and wherein the batch operation in the first reactor space has a duration of between 10 and 30 hours as calculated from the start of the addition of waste material in the batch operation to the start of removing wet intermediate composted material from the first reactor space.

2. Process according to claim 1, wherein the composted material has a water content of between 20 and 90 wt. %.

3. Process according to claim 2, wherein the composted material has a water content of between 50 and 70 wt. %.

4. Process according to claim 1, wherein the composted material is dried such to lower the water content of the composted material to obtain dry composted end material.

5. Process according to claim 4, wherein the water content of the dry composted material is below 30 wt. %.

6. Process according to claim 5, wherein the wet composted material is dried at a temperature of between 30 and 70 C.

7. Process according to claim 4, wherein the wet composted material is dried using a conveyor-belt drier.

8. Process according to claim 1, wherein at least 80 wt. % of the organic waste material is added to the first reactor space in a period between 0 and 24 hours from the start of the addition of waste material in the batch operation.

9. Process according to claim 1, wherein the temperature in the first reactor space is between 50 C and 70 C.

10. Process according to claim 1, wherein between 10 and 40 wt. % of the content of the first reactor space remains in the first reactor space at the end of the batch operation when the content of the first reactor space is partly removed from the first reactor space as intermediate wet composted material.

11. Process according to claim 1, wherein the intermediate wet composted material obtained from the first reactor space is added to a second reactor space wherein the intermediate wet composted material is kept for between 8 and 48 hours in a batch operation to obtain composted material.

12. Process according to claim 11, wherein the intermediate wet composted material is kept for at most 100 hours in a batch operation in the second reactor space.

13. Process according to claim 1, wherein part of the content of the first reaction space is removed from the first reactor space as wet intermediate composted material comprising aerobic bacteria by means of a screw feeder which is orientated such to transport the wet intermediate composted material in a horizontal direction in the first reactor space to an outflow opening of the first reactor space.

14. Process according to claim 1, wherein the compositing reactor spaces of the series of two or more composting reactor spaces comprise of an elongated tank into which organic waste can be charged, wherein the tank comprises an inlet for organic waste, an outlet for composted material and wherein the tank is provided with two rotating mixing shafts provided with a helical mixing element connected to the shafts by supports which radially extend from the shaft, wherein the two shafts are positioned substantially parallel with respect to each other in the elongated direction of the tank thereby defining two cylindrical mixing zones in the tank, wherein below each of the two cylindrical mixing zones an elongated semi-tubular surface is provided as the lower inner wall of the tank, and wherein a screw feeder is positioned at the lower inner wall of the tank between the two semi-tubular surfaces and positioned in a tubular housing which tubular housing is open at its upper end facing the interior of the tank and wherein in use composted material may be moved to the outlet for composted material.

15. Process according to claim 14, wherein the distance between the helical mixing element and the semi-tubular surfaces is smaller than 0.5 cm.

16. Organic waste composting equipment comprising an elongated tank into which organic waste can be charged, wherein the tank comprises an inlet for organic waste, an outlet for composted material and wherein the tank is provided with two rotating mixing shafts provided with a helical mixing element connected to the shafts by supports which radially extend from the shaft, wherein the two shafts are positioned substantially parallel with respect to each other in the elongated direction of the tank thereby defining two cylindrical mixing zones in the tank, wherein below each of the two cylindrical mixing zones an elongated semi-tubular surface is provided as the lower inner wall of the tank, and wherein a screw feeder is positioned at the lower inner wall of the tank between the two semi-tubular surfaces and positioned in a tubular housing which tubular housing is open at its upper end facing the interior of the tank and wherein in use composted material may be moved to the outlet for composted material.

17. Organic waste composting equipment according to claim 16, wherein the distance between the helical mixing element and the semi-tubular surfaces is smaller than 0.5 cm.

18. Use of a organic waste composting equipment according to claim 16 in a process to bacterially decompose organic waste material to a composted material.

19. Use of a organic waste composting equipment according to claim 17 in a process to bacterially decompose organic waste material to a composted material.

Description

[0024] Reference is made to FIGS. 1-4 when describing the novel organic waste composting equipment according this invention.

[0025] FIG. 1 shows the organic waste composting equipment (1) according to the invention from above and without a cover. The equipment (1) is provided with an elongated tank (2) into which organic waste can be charged. An outlet (4) for composted material is shown. The tank (2) is provided with two rotating mixing shafts (5,6) provided with radially extending agitating blades (7) fixed to the shaft (5,6), wherein the two shafts (5,6) of which in FIG. 1 only one is visible. An elongated semi-tubular surface (11) is visible as the lower inner wall (12) of the tank (2),

[0026] FIG. 2 shows a cross-sectional view of the tank (2) of FIG. 1. Both rotating mixing shafts (5,6) are shown provided with agitating blades (7). Two cylindrical mixing zones (8,9) in the tank (2) are shown positioned substantially parallel with respect to each other in the elongated direction of the tank (2). Two cylindrical mixing zones (8,9) are shown and a screw feeder (13) is positioned at the lower inner wall (12) of the tank (2) between two, suitably heated, semi-tubular surfaces (10,11) and positioned in a tubular housing (14) which tubular housing (14) is open at its upper end facing the interior (15) of the tank (2) and wherein in use composted material may be moved to the outlet (4) for composted material. Each semi-tubular surface (10,11) may have a radius (16) running from the rotating shaft (5,6) of the cylindrical mixing zone as shown in this Figure.

[0027] FIG. 3 shows the tank (2) as of FIG. 1 from above.

[0028] FIG. 3a shows the tank as in FIGS. 2 and 3 except that a helical mixing element (7a) is connected to the shafts (5,6) by supports (7) which radially extend from the shaft (5,6). The helical mixing element is suitably a blade which runs at a certain distance from the semi-tubular surfaces (10,11). The blade is suitably shaped such that entire surface of the blade is spaced apart from the two semi-tubular surfaces (10,11) at a constant distance. It has been found that this distance is preferably minimal such that the blades may remove any deposits formed on the semi-tubular surfaces (10,11). Preferably this distance is smaller than 1 cm and more preferably smaller than 0.5 cm. The minimal distance will be determined by the requirement that the blades are spaced apart from the semi-tubular surfaces such that the mixing element may rotate within the tank (2).

[0029] FIG. 4 shows the tank (2) of FIG. 1 with a cover (16) and an inlet (3) for organic waste and an inlet (17) and outlet (18) for of air.

[0030] FIG. 5 shows 4 sequential stages how the process may be performed in a semi-batch type of operation. In stage A organic waste (21) is intermittedly added to first reactor space (20). Valve (22) is closed. In second reactor space (23) intermediate wet composted material from earlier batches of first reactor space (20) is further composted. Valve (24) is closed and no material is dried on belt dryer (25). In stage B part of the content of first reactor space (20) is transported via open valve (26) to second reactor space (23). Valve (24) remains closed and no material is dried on belt dryer (25). In a next stage C the operation mode of organic waste (21) is added to first reactor space (21) and composting takes place in second reactor space (23). Stages A-C are repeated until the content of wet composted material in the second reactor space (23) reaches a predetermined level and wherein the average residence time of the organic waste in the second reactor space (23) is sufficiently high that almost all of the organic waste has been composted. For example, stages A-C may be repeated 3 to 5 times before performing stage D. In stage D the content of second reactor space (23) is transported to belt dryer (25) via open valve (27) where evaporated water (28) is separated from the composted material to obtain dry composted end material (29). The maximum rate of emptying of the second reactor space (23) may be determined by the speed at which the composted material dries on the belt dryer (25). Once all the wet composted material is dried the system may return to stage A after closing valve (27). The scheme of stages shows that water (28) is only obtained in stage D. Thus the process according this invention does not continuously produce water, as part of gaseous effluents, but only in stage D. The collection of water in stage D can furthermore be more efficient because the evaporated water content of the gas as it leaves the dryer (28) can be significantly higher than the water content in the prior art processes leaving the composting tank.

[0031] The invention shall be illustrated by the following example.

EXAMPLE 1

[0032] A composting reactor as show in FIG. 3a and having a distance between the helical mixing element (7a) and the semi-tubular surfaces (10,11) of a few millimeters is used in this example. To this reactor 5000 kg of a biological waste, consisting of vegetables, fruit excluding citrus fruit shells, meat and fish in about equal proportions as obtained in a Dutch hospital as leftovers or as outdated food products was added. 100 kg composted material was already present in the reactor from a previous batch. This material consisted also of the aerobic composting bacteria. 5 kg of fresh bacteria on a Lithothamnium Calcareum support was added after 8 hours and another 5 kg of fresh bacteria was added after 16 hours. The mass in the reactor was stirred at a temperature of 55 C. After 24 hours 80 wt % of the content of the reactor was discharged using the screw feeder.

[0033] The discharged intermediate wet composted material comprising the aerobic bacteria was added to a second reactor. The second reactor was a copy of the first reactor. The content of the second reactor was stirred for 24 hours at a temperature of between 45 and 50 C.

[0034] The thus obtained composted material contained 23 wt % water. The material was dried using a belt dryer using air having a temperature of 110 C. A composted and dried matter was obtained having the following properties as listed in Table 1.

TABLE-US-00001 TABLE 1 Example Unit 1 2 Dry matter kg/kg fresh waste 0.824 0.813 Total nitrogen (N) kg/kg fresh waste 0.0299 0.0252 Phopshate (P.sub.2O.sub.5) kg/kg fresh waste 0.0068 0.0066 Pottasium (K.sub.2O) kg/kg fresh waste 0.0104 0.0098 Magnesium (MgO) kg/kg fresh waste 0.001 0.001 Sulphur (S) kg/kg dry matter 0.003 0.0042 Chloride (Cl) kg/kg dry matter 0.0064 0.0062 Sodium (Na) kg/kg fresh waste 0.0099 0.0098 Organic matter wt % of dry matter 94.9 94.7 pH 4.4 4.2 Hg mg/kg dry matter <0.050 <0.050 Pd mg/kg dry matter <5.0 <5.0 As mg/kg dry matter <3.0 <3.0

EXAMPLE 2

[0035] Example 1 was repeated except that a the waste also contained orange shells. The composition of the dry matter as obtained in listed in Table 1. The dry matter as obtained in Examples 1 and 2 had properties which make it suitable as compost for plants not sensitive for chloride. A suitable dose would be between 0.5 and 0.75 kg/m2, preferably before the seeds are planted and propagated.

EXAMPLE 3

[0036] Example 1 was repeated using a reactor as shown in FIG. 3 (without the helix strips) for first and second reactor. This resulted in that the mixing in the reactors was not optimal and that 45 wt % of the material was not fully composted.