NL2001725C2 - Method for removing heavy metals from composted organic waste used as horticultural substrate in potting soil for growing trees, involves sieving composted organic waste through sieve bodies that are provided with apertures - Google Patents

Abstract

The method involves sieving composted organic waste through sieve bodies provided with apertures. The waste particles with average mesh size are subjected to filtrating process for removing heavy metals. Residual waste particles are washed with water (4). The residue water is withdrawn from waste particles after washing. The pH is reduced up to around 1.5 after washing the waste particles. An acid containing solution is added with residue water, where the acid is selected from oxalic acid, formic acid, citric acid, sulphurous acid, sulfuric acid, nitric acid or hydrochloric acid. An independent claim is also included for a device for removing heavy metals from composted organic waste.

Description

- 1 -

Method for removing heavy metals from composted GFT waste to make it suitable for use as a horticultural substrate. The present invention relates to a method in which heavy metals are removed from waste, in particular substantially composted GFT waste.

The present invention also relates to 10 residues obtained by applying the method and to a device for carrying out the method.

Such a method is known from WO 91/06343.

The known method adds to binder, such as soil, compost, sediment or the like, an EDTA (Ethylenediaminetetraacetic acid) binder complex that has a chelating effect and connects to the heavy metal ions from the waste. A product is thereby formed which consists of liquid substance, containing the heavy metal, and of solid substance. An ion exchanger is then used to bind the heavy metal ions from the liquid substance.

A drawback of the known method is that the use of such irritating binders entails health and use risks which necessitate taking considerable precautions. In addition, the binder must be able to actually extract the heavy metals from the compost in order for the extracted metals to end up in the liquid substance. This means that relatively strong binders must be brought into contact with the compost during relatively long exposure times. This makes the known method time-consuming, spatially comprehensive and leads to an increase in the cost price.

- 2 -

The object of the present invention is to provide an improved, more environmentally and cost-friendly method for removing heavy metals from the waste, so that this can be used for several useful purposes.

To that end, the method according to the invention has the characteristics that the heavy metal-containing waste, which is mainly composted stable waste, is sieved with sieve members with openings whose average mesh size has been selected for waste particles smaller than the mesh size and essentially the heavy to be removed containing metals through.

The advantage of the method according to the invention is that by screening the passed smaller filtrate particles, which surprisingly appear to contain relatively many heavy metals, are filtered away. In other words, the heavy metals are removed by sieving the waste, in particular -organic-waste. The larger waste particles that form the residue and are found to contain relatively less heavy metals therefore do not need to be treated at all, or only marginally, to remove residual heavy metals from them. As a result, if an acid is used later in the process to remove the heavy metals from the non-permeable larger residue particles, this advantageously requires less and / or a less aggressive acid solution for often shorter periods of time. For such acid solutions acids, then even biodegradable, environmentally friendly acids can be used.

The more small organic filtrate waste particles that, due to their total outer surface area, contain relatively more heavy metals, are removed from the waste during one or more 3-step filtering steps, the less heavy metals the residue of larger waste particles contains.

When using, for example, compost-containing organic waste from coastal areas in particular, which is sieved one or more times and, if necessary, is washed at least once with the aid of neutral water, it has been found to be advantageous to heat the water to a temperature between 70-90 degrees, in particular 80-90 degrees, very particularly up to around 85 degrees Celsius, in order to be able to easily remove the excess salts present therein.

Elsewhere, if desired, washing can be carried out with unheated, if available, rain or spring water, or suitable water from rivers or canals.

A further embodiment of the method according to the invention is characterized in that after the at least one washing the pH is lowered to between 1.3 and 2.0, in particular to around 1.5, by adding an acid-containing solution, which preferably biodegradable and organic, acids from the group comprising: oxalic acid, formic acid, citric acid, sulfuric acid, sulfuric acid, nitric acid, and hydrochloric acid. For environmental and cost price reasons, the use of citric acid is now particularly preferred for lowering the pH and / or for removing residual heavy metals from the residue.

30

Residue obtained by applying the method, wherein the residue contains particles with a diameter greater than between 0.8 and 1.2 mm, preferably greater than about 1 mm; and also particles with a diameter smaller than between 5 and 7 mm, preferably smaller than about 6 mm, contain - 4 - is used in particular in potting soil, sowing soil, compost, substrate, or as a filler or fertilizer for growing trees, shrubs or crops such as vegetables, plants, flowers or the like. In this way a purified agricultural substrate compost obtained from organic waste compost 5 is created which is suitable for use as a growth medium for, for example, horticulture. Moreover, this residue, which emits less CO2 during incineration due to the lower amount of peat, is also fully recyclable. Substrates with 10 higher compost percentages than the 20% currently achievable because of the high degree of contamination of the earlier compost have now become economically possible by applying the method according to the invention.

The method and device according to the present invention will now be further elucidated with reference to the figures below, in which similar processing members and operations are provided with the same reference numerals. In the drawing: Figure 1 shows a possible embodiment of the system according to the invention, on the basis of which the corresponding method will also be explained; and Figure 2 is a block diagram for explaining the method according to the invention.

25

Figure 1 shows a system 1 for the removal of heavy metals, such as nickel, cadmium, lead, copper and zinc, and - if too much is present therein - of salts and other phytotoxic components from contaminated waste 2 supplied (see also block diagram of figure 2). The waste 2 contains, for example, vegetable waste, fruit waste, garden waste, often summarized as organic waste, but also green waste such as from nurseries, or compost residues. The stable composted waste 2 is supplied in batches, with transport means C, and possibly fed into a container or drum 3, which drum is provided with internal sieve members, hereinafter simply referred to as S, with openings of a certain mesh size therein. The sieving can, if desired, be carried out with external sieve members S1, S2, if not arranged in a drum, as desired, one or more times. The average mesh size of the openings is chosen to allow those waste particles 2-1 to pass which are smaller than the selected mesh size and which, surprisingly, appear to contain essentially the heavy metals to be removed. The openings of the screening members S in practice have a diameter between 0.8 and 1.2 mm, preferably around about 1 mm. All waste particles 2-2 that are larger than say 1 mm remain behind as a residue on the screening members S, while the smaller particles 2-1 which are relatively mixed with more heavy metal particles, than the larger waste particles 2-2 as a filtrate through the sieve organs S fall. Removing the smaller particles 2-1 that are responsible for a large part of the actual weight causes the intermediate 2-2 from which these particles are removed to become light and airy. The smaller contaminated particles 2-1 appear to contain relatively much heavy white sand W which can be removed therefrom after having undergone a cleaning treatment R. This treatment comprises the addition of water, in a so-called 'hydrosand' process, in which sand is optionally separated from the organic and heavy metal-contaminated part of the filtrate with the addition of ozone. This now considerably limits the weight of the remaining filtrate particles 30 P contaminated with the metals, so that if the dumping of this type of waste has to be paid per weight, the price for processing this residual waste contaminated with heavy metals becomes considerably lower. The white sand W, which is not contaminated with metals, does not form residual waste, but a useful product which can advantageously be used for, for example, raising paving or for sandboxes.

The sieving with the possibly vibrating sieve members S, S1, S2 can take place dry, or initially dry, and / or wet, while the drum 3 is rotated during use to obtain a good distribution, mixing and flow during sieving. When a combination of conveying means C and screening members S, S1, S2 is used, the combination is often coupled to vibrating motors for the screening members.

By way of example, in a first step, sieving is dry, after which water 4 is brought from a water circulation system via a drum inlet 5 to the screening members S and the contents are washed simultaneously with the sieving and possibly rotating or vibrating of the screening members S15. with the aid of the water 4. The fine filtrate 2-1 runs with an important part of the metal contamination together with the water 4 through an outlet 6, and possibly via further transport means C, to a treatment tank or storage buffer 7. The water can be unheated. water, such as deionized water. As source of water, it is also possible, for example, to use soft spring water or rainwater available locally

The water 4 that is possibly heated in the circulation system by heating elements 8, if any, is preferably ion-depleted water which has a low Ec electroconductive power. In another possible embodiment, the water is heated to a temperature between 70-90 degrees, in particular 80-90 degrees, very in particular to around 85 degrees Celsius, so that in the case of the 30 in question, often about 15 to 20 minutes, wash in the tank 7 with warm water, in particular the neutral and anionic salts are removed. This is particularly important if the waste comes from areas around the sea and contains relatively more salts. With this method, the microbiological quality of the cleaned compost-7 end product improves.

After one but in practice usually several sieves, and cold and / or warm water washes, an acidity increase follows where the pH is lowered to between 1.3 and 2.0, in particular to around 1.5, by adding an environmentally friendly and biodegradable acid-containing solution. By using such solutions, residual heavy metals are also bonded and can be washed away. Examples of suitable organic acids are: oxalic acid, formic acid, citric acid, sulfuric acid, sulfuric acid, nitric acid, and hydrochloric acid. The acid solution is added from a vessel 9 to the water 4 to the drum 3 and / or to the screen members S, S1, S2 and / or the tank 7. When citric acid is used, one of, possibly several, alternating sieving and washing cycles, of course depending on, inter alia, the type of metal contamination, density of the compost waste and the temperature, takes about 40 minutes. After this, the acidity to be monitored is restored, often with water of ambient temperature and a pH of approximately 5.5, to obtain a final product whose pH is between approximately 5 and 6.

After or after each wash, it is preferable to withdraw water from the residue in the drum 3, which can then serve as a centrifuge, which is indicated by 0. If as much water as possible is extracted, a relatively dry compost cake remains which remains intact in terms of volume and from which the particles of about 1 mm or smaller have not all been washed away. These particles are important for the physical properties of a substrate that is particularly suitable for use in horticulture.

After recovery of the acidity, the residue removed from the tank 7 is dried, for example mechanically, by means of an oven or transport dryer T, whereafter the moisture content is preferably 40%. Optionally, the residue can also be air dried, although there is a risk of pollen and bacteriological germ contamination.

The water circulation system shown in Figure 1 has a flotation unit 10 connected to the buffer 7, known per se, for removing the small particles from the filtrate. The purified water 4 resulting therefrom goes to a buffer tank 11 and is finally pumped around for repeated use.

The end product E, namely the residue, can be used in, for example, potting soil, sowing soil, compost, substrate, or as a filler or fertilizer for growing trees, shrubs or crops such as vegetables, plants, flowers or the like.

The rotating drum 3, or the screening members S, S1, S2 may have larger openings with a diameter that is smaller than between 5 and 7 mm, which is preferably smaller than about 6 mm. The residue that results from both sieving with the small and the aforementioned larger openings yields on average particles with a diameter between 1 and 6 mm. After having undergone the above-described method, that residue is eminently suitable for use as seed or potting soil which, because of the lower amount of peat therein, emits less CO2 during combustion and is also completely recyclable.

Residual particles 2-2 which have a diameter greater than 6 mm and are often no larger than about 14 mm can be passed through the system 1 again after being broken.

In a preferred embodiment, optionally generated on site in an ozone generator, ozone gas is added to the contents of the ozone-resistant drum 3 or tank 7. The ozone advantageously burns and removes the organic matter and the (remaining) heavy metals in the waste. Furthermore, the aggressive ozone reduces the bacteria, in particular the bacillio basidiomyceles, the fungi, and further microorganisms and the trace elements in the waste, and no longer germinates any seeds present therein. Moreover, it decomposes the acids present, and with early application in the described method it appears that further less waste is formed. Ozone can optionally be replaced by or used in addition to peroxide.

Claims (14)

1. Method in which heavy metal-containing waste which is mainly composted stable organic waste is sieved with sieve members with openings whose average mesh size is chosen to allow filtrate to contain waste particles smaller than the mesh size and essentially containing the heavy metals to be removed . Method according to claim 1, characterized in that the waste is sieved one or more times and, where necessary, possibly simultaneously, residue waste particles are washed with the aid of water. Method according to claim 1 or 2, characterized in that after washing at least once, water is withdrawn from the residue. 4. Method as claimed in claim 2 or 3, characterized in that the water, whether heated or not, contains deionized water and / or rain and / or spring water. A method according to claim 4, characterized in that the water is heated to a temperature between 70-90 degrees, in particular 80-90 degrees, very particularly to around 85 degrees Celsius, in order to remove any salts present in the residue, to remove waste particles remaining behind the sieve organs. Method according to one of claims 2 to 5, characterized in that after the at least one washing, the pH is lowered to between 1.3 and 2.0, in particular to around 1.5, by adding an acid-containing solution . Method according to claim 6, characterized in that the acidic solution contains one or more, preferably biodegradable and organic, acids from the group comprising: oxalic acid, formic acid, citric acid, sulfuric acid, sulfuric acid, nitric acid, and hydrochloric acid . 5 Method according to claim 6 or 7, characterized in that the pH of the residue is adjusted to between 5 and 6 after lowering the pH on further washing with water at ambient temperature. 10 9. A method according to any one of claims 2-8, characterized in that after washing the residue is dried, for example mechanically, by means of an oven or in air, whereafter the moisture content is preferably 40%. 10. A method according to any one of claims 1-9, characterized in that residue waste particles with a diameter larger than between 0.8 and 1.2 mm undergo an ozone treatment. 11. Residue obtained by applying the method according to any one of claims 1-10, characterized in that the residue contains both waste particles with a diameter larger than between 0.8 and 1.2 mm, preferably larger than about 1 mm ; if waste particles with a diameter smaller than between 5 and 7 mm, preferably smaller than about 6 mm. Use of the residue according to claim 11 in potting soil, sowing soil, compost, substrate, or as a filler or fertilizer for growing trees, shrubs or crops such as vegetables, plants, flowers or the like. Device for carrying out the method - 12 - according to one of the claims 1-10, characterized in that the device comprises first sieve members with openings with a diameter between 0.8 and 1.2 mm, preferably around approximately 1 mm; and second sieve members having openings with a diameter between 5 and 7 mm, preferably around approximately 6 mm. 14. Device as claimed in claim 13, characterized in that the first and second sieving members are accommodated in rotating means, and the device is optionally provided with an inlet for heavy metal-containing waste, a supply for water, an outlet for stripped of the metals residue with particle diameters between approximately 0.8 mm and 7 mm, in particular between 1 mm and 6 mm, and an outlet for the heavy metal-containing filtrate with particle diameters between 0 mm and 1.2 mm, in particular smaller then 1 mm.