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. Organic waste composting equipment for bacterially decomposing organic waste material to composted material, the organic waste composting equipment comprising: an elongated tank into which organic waste material can be charged, the elongated tank comprising an inlet for receiving organic waste material, an outlet for discharging composted material, and a tank interior comprising a first cylindrical mixing zone and a second cylindrical mixing zone, wherein the first and second cylindrical mixing zones are at least partially defined by first and second elongated heated semi-tubular surfaces, respectively, of a lower inner wall of the elongated tank; a first rotating mixing shaft and a second rotating mixing shaft rotatably mounted within the tank interior, the first and second rotating mixing shafts comprising helical mixing blades connected thereto by one or more supports radially extending from the first and second rotating mixing shafts, wherein each of the helical mixing blades comprises a radially-outwardly facing surface spaced apart from and facing the semi-tubular surfaces, the radially-outwardly facing surfaces each defining a major surface area having a width transverse to the helical direction, and wherein each of the helical mixing blades further comprises a minor surface area having a thickness in the radial dimension, wherein said width is substantially greater than said thickness; and a screw feeder rotatably mounted in a tubular housing at the lower inner wall between the first and second elongated heated semi-tubular surfaces, the tubular housing comprising an upper opening open to the tank interior, wherein the first and second rotating mixing shafts are positioned substantially parallel with respect to each other in the elongated direction of the tank, wherein the first rotating mixing shaft is positioned concentrically with the first elongated heated semi-tubular surface, and wherein the second rotating mixing shaft is positioned concentrically with the second elongated heated semi-tubular surface.

2. Organic waste composting equipment according to claim 1, wherein the helical mixing blades comprise exterior surfaces and wherein a constant distance is maintained between the exterior surfaces and the elongated heated semi-tubular surfaces.

3. Organic waste composting equipment according to claim 2, wherein the constant distance is less than 0.5 cm.

4. Organic waste composting equipment according to claim 2, wherein the constant distance is less than 1 cm.

5. Composting equipment for bacterially decomposing organic waste material to composted material, the composting equipment comprising: an elongated tank defining an interior into which organic waste material can be loaded, the elongated tank comprising an inlet opening for receiving organic waste material and an outlet opening for expelling composted material therethrough, and wherein the tank is provided with rotating mixing shafts provided with helical mixing elements connected to the mixing shafts by supports radially extending from the mixing shafts, wherein the mixing shafts are positioned substantially parallel with respect to each other in an elongated direction of the elongated tank thereby defining cylindrical mixing zones in the tank, wherein the cylindrical mixing zones are at least partially defined by elongated heated semi-tubular surfaces along a lower inner wall of the elongated tank, wherein a screw feeder is positioned in a tubular housing between the semi-tubular surfaces, the tubular housing comprising an opening towards the interior of the tank, wherein the screw feeder expels composted material through the outlet, and wherein the helical mixing elements comprise a radially-outwardly facing surface spaced apart from and maintained at a constant distance from the elongated heated semi-tubular surfaces, the radially-outwardly facing surface defining a major surface area having a width transverse to the helical direction, and wherein the helical mixing elements further comprises a minor surface area having a thickness in the radial dimension, wherein said width is substantially greater than said thickness.

6. Composting equipment according to claim 5, wherein the constant distance is less than 1 cm.

7. Composting equipment according to claim 6, wherein the helical mixing elements are helical blades.

Description

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

(2) 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),

(3) 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.

(4) FIG. 3 shows the tank (2) as of FIG. 1 from above.

(5) 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 (110, 111) which runs at a certain distance from the semi-tubular surfaces (10,11). The blade (110, 111) is suitably shaped such that the entire radially-outwardly facing surface (110a, 111a) 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). The radially-outwardly facing surfaces (110a, 111a) of the blades (110, 111) define a major surface area having a width (W) transverse to the blade's helical direction. The blades (110, 111) further define a minor surface area having a thickness (T) in the radial dimension. The width (W) is substantially greater than the thickness (T).

(6) 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.

(7) 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.

(8) The invention shall be illustrated by the following example.

EXAMPLE 1

(9) 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.

(10) 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.

(11) 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.

(12) 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

(13) 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

(14) 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.