HK1104009B - Process for removing a contaminant from contaminated groundwater - Google Patents

Description

Method for removing contaminants from contaminated groundwater

Technical Field

The present invention relates to the purification of contaminated groundwater.

Background

Methods for removing contaminants from groundwater are well known. An example is the so-called "pump-treat" process, in which contaminated groundwater is pumped out of the soil and the sewage is then purified on the ground.

The disadvantage of this method is that the sucked-up groundwater is treated ex-situ. This results in a relatively high cost and difficult control of the system. In addition, waste is generated and air pollution may occur. Further possible disadvantages are: groundwater extraction is not always possible, for example, due to soil drying or sinking, or in extracting groundwater, the groundwater level is lowered, which causes the so-called smear zone (smearzone) to expand. A smear zone is an area of soil that has been in contact with contaminated groundwater or supernatant in groundwater, but that area no longer has a groundwater or supernatant layer. The formation of smear zones is due to groundwater level fluctuations. If the soil in the smear zone has been in contact with groundwater or supernatant, the smear zone will also contain contaminants if the groundwater is contaminated. However, because the groundwater is no longer present, the smear zone cannot be purified by techniques that pump contaminated groundwater up to the surface. When the smear zone is contacted with pure water, for example due to fluctuations in groundwater level or infiltration (e.g. by rain water), a new equilibrium of dissolution will be established, allowing a proportion of the contaminants present in the smear zone to enter the groundwater. This prolongs the purification operation.

Another disadvantage of the pumped-treatment technique is that with this method, clogging often occurs due to, for example, oxidation and precipitation of dissolved iron. Biological clogging also often occurs or fine particles accumulate in the aspiration tube.

Disclosure of Invention

It is an object of the present invention to provide a method for removing contaminants from groundwater in which the groundwater does not have to be sucked out of the soil.

This object is achieved by the method according to the invention.

The method according to the present invention is a method for removing contaminants from contaminated groundwater, the method comprising the steps of:

a) applying the bioactive layer to or in soil;

b) contacting the contaminated groundwater with a biologically active layer.

The contaminants are converted within the biologically active layer.

Drawings

FIG. 1 schematically illustrates a schematic diagram of a process for removing a contaminant from contaminated groundwater according to one embodiment of the invention.

FIG. 2 schematically illustrates a schematic diagram of a process for removing a contaminant from groundwater containing a nitrogen contaminant according to one embodiment of the invention.

Detailed Description

In the context of the present invention, a biologically active layer means a layer comprising a biologically active material. By bioactive material is meant a material comprising microorganisms by which a contaminant can be decomposed or converted. The bioactive layer may be applied continuously or discontinuously. Discontinuous application means that the biologically active layer is applied discretely, i.e. in the form of discrete portions which together form the biologically active layer. In one embodiment, the bioactive layer is applied discontinuously. An example of a discontinuous layer is a layer comprising a plurality of trenches, which may be the same or different in shape and size.

The combination of number, length, width and depth of the trenches is typically selected to constitute an optimal process.

Good contact between the contaminated groundwater and the biologically active layer is important, but the conditions of the area and the surface containing the contaminated groundwater also affect the shape and size of the biologically active layer. When at least a portion of the biologically active layer is introduced into the contaminated groundwater, the contaminated groundwater is then brought into contact with the biologically active layer and only needs to be moved to a limited extent.

In an embodiment of the invention, the biologically active layer is placed so as to achieve direct contact between the biologically active layer and the groundwater. In embodiments, this is accomplished by creating a biologically active layer that is deep enough that the bottom of the layer is located in the groundwater.

However, it is also possible to leave the biologically active layer not in direct contact with the groundwater. In this case, it is necessary to actively bring the contaminated groundwater into contact with the biologically active layer one or more times. In an embodiment of the method according to the invention, the contact between the contaminated groundwater and the biologically active layer is achieved by letting the contaminated groundwater enter into or on top of the biologically active layer.

In a preferred embodiment, the contaminated groundwater is contacted with the biologically active layer one or more times with the aid of a gas. In a more preferred embodiment, the contaminated groundwater is repeatedly contacted with the biologically active layer with the aid of a gas. The gas is typically injected into the soil through one or more pipes into or below the contaminated groundwater. The excavation depth and the lying distance of one or more canals are determined by the nature of the subsoil and the level and verticality of the pollution. The pipe may be perforated over part of its length, for example over a length of 0.5-1.5. As a result of the pipe, voids may form along the outer surface of the pipe in the soil. To prevent the injected gas in the pipe from immediately rising to the surface through this gap, a gas impermeable material such as bentonite is injected around the pipe. However, the entire lower portion of the tube and the perforated portion of the tube (if present) need not be surrounded by a gas impermeable material. In the case of pouring such a pipe bottom and perforated part, an aeration material such as sand should be used.

A continuous or discontinuous bioactive layer is or has been prepared at the surface, between and beside the injection pipes, the length and width of which is determined by the degree and concentration of contamination, depth of groundwater and presence or absence of supernatant. Groundwater is moved by the air lift principle (airlift principle) to flow through the trench containing the biologically active layer by supplying air to the pipe at a sufficiently high pressure. The microorganisms in the biologically active layer ensure that the desired transformation takes place.

The principle of this process is schematically represented in fig. 1. In fig. 1, a broken line (1) indicates the water level of the existing groundwater. The positions where the biologically active layer is present are indicated by (2). The pipe through which the gas is injected into the soil is indicated by (3). These pipes are also known as lances. The curved arrows (4) schematically indicate how the groundwater circulates due to the gas injected into the soil.

Aerobic conversion processes in the biologically active layer are promoted by contacting the biologically active layer with contaminated groundwater with the aid of an oxygen-containing gas, or preferably an oxygen-enriched gas.

The anaerobic conversion process may be facilitated by the use of a low-oxygen or anaerobic gas.

The anaerobic and aerobic conversion processes can be achieved in the biologically active layer by alternating the use of a low-oxygen or anaerobic gas and an oxygen-containing or preferably oxygen-enriched gas. This is beneficial when, for example, the contaminant is first anaerobically converted to a compound (which, although different from the contaminant, is still considered to be a contaminant), and the resulting anaerobic conversion product is then aerobically converted to a compound that is not considered to be a contaminant. Air is an example of an oxygen-enriched gas.

In another embodiment, the contaminated groundwater is contacted with the biologically active layer without the aid of a gas by pumping the contaminated groundwater into the biologically active layer. The injected gas may also be combined with pumping the contaminated groundwater to bring the contaminated groundwater into contact with the biologically active layer.

Each time the contaminated groundwater comes into contact with the biologically active layer, at least a portion of the contaminants are converted or decomposed. Depending on the concentration of the contaminant in the groundwater, the activity of the biologically active layer and the size of the biologically active layer, the contaminated groundwater needs to be contacted with the biologically active layer one or more times to obtain groundwater of the desired purity.

Preferably, the contaminated groundwater is contacted with the biologically active layer more than once.

In a preferred embodiment, the contaminated groundwater is pumped out of the lower part of the contaminated area with the aid of a pump, after which the contaminated groundwater is injected into or onto the biologically active layer. The contaminated groundwater then descends through the biologically active layer. By continuously pumping water from the contaminated area below the biologically active layer and injecting it into or onto the biologically active layer, a circulating water system is created, which is purified again each time it passes through the biologically active layer, as far as the groundwater contains also contaminants. This cycle does not involve large scale extraction of groundwater from the soil. Any water present at the surface will be immediately injected into the biologically active layer at or below the surface. Preferably, one or more substances are added to the withdrawn effluent, which substances can be utilized by the microorganisms in the biologically active layer to convert the contaminants. For aerobic conversion, an example of such a substance is oxygen. By supplying these substances sufficiently, the conversion of the contaminants optimally takes place continuously and the process according to the invention usually takes less time to remove the desired amount of contaminants from the groundwater than would be required if the substances were not added to the pumped-off water. The choice of the substances to be added is determined by the desired conversion. In the case where it is desired to remove, for example, nitrates from groundwater using microorganisms in the biologically active layer that result in anaerobic conversion, an organic carbon source may be added to the pumped groundwater before the groundwater passes through the biologically active layer.

In a preferred embodiment, the contaminated groundwater is contacted with the biologically active layer more than once with the aid of a gas. Due to the gas lift principle, the injected gas will circulate the contaminated groundwater. This process is illustrated in fig. 1. The latter embodiment has the advantage that the gas to be injected contains the substances required for the microorganisms in the biologically active layer to convert the contaminants, or such substances can be added to the gas to be injected. For aerobic conversion, an example of such a substance is oxygen. By supplying these substances sufficiently, the conversion of the contaminants optimally takes place continuously and the process according to the invention usually takes less time to remove the desired amount of contaminants from the groundwater than would be required if the substances were not added to the pumped-off water. Another advantage of using a gas for contacting the contaminated groundwater with the biologically active layer is that any volatiles present can flow along the gas into the biologically active layer. Thus, in a preferred embodiment, in which the contaminated groundwater is contacted with the biologically active layer with the aid of a gas, both volatile contaminants and water-soluble contaminants can be removed simultaneously.

The process according to the invention is suitable for removing any contaminants dissolved in water. It is preferred for the contaminants to be removed by the process according to the invention to be readily soluble.

If the contaminants are readily soluble, the process results in the desired removal of contaminants faster than if the contaminants were less soluble. Unlike known pump-down processes, the method according to the invention is also suitable for removing poorly soluble contaminants, although it usually takes more time to reduce the poorly soluble contaminants to the desired level, because the poorly soluble contaminants are less soluble in the groundwater and therefore only a small amount of the contaminants are in contact with the biologically active layer. However, by frequently contacting contaminated groundwater containing poorly soluble contaminants with the biologically active layer, such contaminants can be adequately removed by the method of the present invention. In the context of the present invention, a poorly soluble compound is defined as a compound having a solubility of 1 molecule/m³Underground water-10 g/m³Compounds between groundwater. All solubilities are higher than 10 g/m³Compounds of groundwater are defined as readily soluble.

If a pump-down process (in which contaminated water is pumped out and drained) is used to remove poorly soluble contaminants, a large volume of water needs to be extracted from the soil and drained or re-filtered, with all the attendant disadvantages.

In an embodiment according to the invention, a scavenger is added to the groundwater. In the context of the present invention, a scavenger means any substance that promotes the decomposition of the contaminant to be removed. Preferably, a biodegradable decontaminant is used. Preferably, the decontaminant is used with less adhesion to the soil to be decontaminated than to the contaminant to be removed. More preferably, the decontaminant does not adhere or hardly adheres to the soil to be decontaminated. In a preferred embodiment according to the invention, cyclodextrins are used as scavengers. In another preferred embodiment, an electron acceptor is used during the process.

The process according to the invention is preferably used for removing nitrogen-containing contaminants (e.g. NH)₃). In a preferred embodiment, the method according to the invention is characterized in that ammonia is nitrated to nitrate, and subsequently the nitrate is converted to N by adding a carbon-containing component₂。

Wherein the contaminant is NH₃A method of may comprise the steps of:

a) applying the bioactive layer to or in soil;

b) the contaminated groundwater is contacted with a biologically active layer under aerobic conditions, wherein in the biologically active layer NH is present₃Conversion to NO₃ ^-；

c) In the reaction of NH₃Repeating step b) during the time required for the concentration to drop to the desired level;

d) then, NH is added₃Groundwater whose concentration has been reduced to a desired level is contacted with the biologically active layer under anaerobic conditions;

e) in the presence of NO₃ ^-Repeating step d) during the time required for the concentration to drop to the desired level.

For the removal of nitrogen-containing contaminants, the process comprises a nitrification step followed by a denitrification step. In an embodiment, the trench in which the biologically active layer is applied is first excavated, and the pipe is then placed in position(air is injected into the soil below the contaminated groundwater using the pipe), whereby the contaminated groundwater is moved due to the air-lift principle and is thus brought into contact with the biologically active layer. In the biologically active layer, nitrogen-containing pollutants are converted aerobically into nitrates, for example from NH, in the presence of air under the influence of microorganisms₃Conversion to NO₃-。

With the aid of the injected air, the sewage is recirculated through the biologically active layer until the concentration of the nitrogen-containing contaminants has fallen to the desired level. This nitrification step is followed by a denitrification step. In the denitrification step, an electron acceptor (e.g. in the form of an organic carbon source) is added to the nitrogen-containing groundwater, and nitrate is converted to N in the biologically active layer in the presence of the electron acceptor₂. Preferred organic carbon sources for use in the process according to the invention are methanol, acetic acid, lactate or molasses.

In an embodiment, the denitrification process is performed by two steps: pumping the nitrate-containing groundwater through a pipe through which air is injected during the nitrification stage, and adding an electron acceptor to the nitrate-containing water on the ground, whereupon the nitrate-containing water is pumped onto the biologically active layer, wherein the nitrate is converted to N by the microorganisms in the presence of the electron acceptor₂. This embodiment is illustrated by figure 2. In fig. 2, a broken line (1) indicates the water level of the existing groundwater. The positions where the biologically active layer is present are indicated by (2). Groundwater to which an electron acceptor is added is injected through a pipe (3). The nitrate-containing groundwater is extracted from the soil through a pipe (4). The pump is shown schematically at (5) and (6) indicates the point at which the electron acceptor is added to the groundwater.

The advantage of adding an electron acceptor to the pumped-out groundwater is that the concentration of nitrate can be easily determined by pumping out the groundwater, whereupon the feed rate of the electron acceptor can be adjusted accordingly. However, it is also possible to bring the nitrate-containing groundwater into contact again with the biologically active layer with the aid of a gas by the gas lift principle, and to add volatile electron acceptors to the gas so that the entire process can be carried out in the soil. In both embodiments, the removal of the nitrogen-loaded contaminants is performed in situ, since the conversion of the contaminants is effected in a biologically active layer applied to or on the soil.

Claims (12)

1. A method for removing a contaminant from contaminated groundwater, the method comprising the steps of:

a) applying the bioactive layer to or in soil;

b) the contaminated groundwater is contacted with the biologically active layer.

2. The method of claim 1, wherein the bioactive layer is applied discontinuously.

3. The method of any one of claims 1-2, wherein the depth of the biologically active layer is such that the bottom of the layer is located in the groundwater.

4. A method according to any of claims 1-2, characterized in that the contaminated groundwater is brought into or on top of the biologically active layer.

5. The method according to any of claims 1-2, wherein the contaminated groundwater is repeatedly contacted with the biologically active layer with the aid of a gas.

6. The method of any of claims 1-2, wherein the contaminated groundwater is contacted with the biologically active layer by pumping.

7. The method of any of claims 1-2, wherein the contaminated groundwater is contacted with the biologically active layer more than once.

8. The method of any one of claims 1-2, wherein the contaminant is freely soluble.

9. The method of any one of claims 1-2, wherein an electron acceptor is added during the method.

10. The method according to any one of claims 1-2, characterized in that ammonia is nitrated to nitrate, followed by conversion of nitrate to N by addition of a carbon-containing component₂。

11. Such as rightThe method of any one of claims 1-2, wherein the contaminant is NH₃The method comprises the following steps:

a) applying the bioactive layer to or in soil;

b) the contaminated groundwater is contacted with the biologically active layer under aerobic conditions, wherein in the biologically active layer NH is present₃Conversion to NO₃ ^-；

c) In the reaction of NH₃Repeating step b) during the time required for the concentration to drop to the desired level;

d) then, NH is added₃The groundwater, the concentration of which has been reduced to a desired level, is contacted with the biologically active layer under anaerobic conditions;

e) in the presence of NO₃ ^-Repeating step d) during the time required for the concentration to drop to the desired level.

12. A method according to any one of claims 1-2, wherein a scavenger is added.