METHOD AND DEVICE FOR BIOLOGICAL SOIL DECONTAMINATION BY AERATION

Technical field The invention relates to the sanitation of contaminated soils.

Background of the invention

The present invention is based on a need to purify biochemically contaminated soils which, unless treated, may cause impurities and toxic substances to penetrate into groundwater or to act deleteriously on the surrounding environment in some other way. The soil may, for instance, have been contaminated as a result of leakages from gas and oil pipes, fuel leakages, industrial effluents and chemical pesticides, weed killers and the like.

According to the Swedish Environmental Protection Agency contaminated soil is found in a large number of places solely in Sweden alone. The contaminants are often residues of earlier industrial activities or from, e.g., gasoline stations. It is estimated that in Sweden there are about 6000 older gasoline stations that have been closed down and disbanded, where contaminated soil needs to be treated in an environmentally efficient way. In addition, there are many gasworks and energy plants and mining industries that have been closed down while leaving contaminated soil masses untreated. The Swedish Environmental Protection Agency has estimated that the costs for treating contaminated soil in Sweden would be more than 20 billion Swedish kronor and take more than 20 years to carry out effectively. Soil that is contaminated by petroleum, oil sludge and certain PAH- impurities can be treated by the so-called BioSan treatment process, which is a process with which organic compounds present in contaminated soil can be quickly degraded biologically in a controlled manner. Biological treatment can be used to break down certain contaminants such that the soil will be sufficiently pure for re-use, for instance as a covering material or top layer in depots or in plant construction work.

The BioSan method was originally developed in the USA where this technique was used to treat successfully more than 5 million tons of petroleum- contaminated soil masses. The method has been further developed by SITA

Sverige AB to enable the treatment of oil sludge and certain PAH-contaminants. More than 100 000 tons of soil have been treated in Sweden with this further development of the method.

The BioSan method involves utilizing bacteria which are effective in break- ing down petroleum hydrocarbons and certain polyaromatic hydrocarbons (PAH), and which are controlled so as not to cause disease. The bacteria are applied to the contaminated soil. This is done initially together with nutrients, such as nitrogen and phosphorus, so as to quickly achieve a high degradation rate. Factors that control the degradation rate are humidity, temperature, pH, metal content, the content of organic material, the access to oxygen and the access to inorganic nutrients. As the petroleum hydrocarbons are "consumed" by the bacteria, carbon dioxide and water are produced in concert with the consumption of oxygen. Bacteria are provided with ideal oxygen conditions by effective aeration. The amount of gasses in the circulating air is measured regularly and indicates the rate of degra- dation on the one hand and termination of the degradation process on the other hand.

As a result of this so-called BioSan treatment, the content of the contaminants in the soil with respect to volatile hydrocarbons and water-soluble substances is greatly reduced. Experiences obtained with the treatment in Sweden have shown a reduction of volatile hydrocarbons by more than 90 %.

The process involves placing soil masses that have been prepared, technically processed, for biochemical degradation in a bio-treatment pile, a so-called bio pile. During the initial stage of the process, the soil is handled openly although after the soil has been placed in the bio pile, it is covered so as to reduce emis- sions to the surrounding air. A synthetic fabric or the like is used to prevent emissions to the surrounding atmosphere and to allow precipitation to run off and to prevent the soil from drying out as a result of evaporation of its water content. Perforated pipes for aerating and draining purposes have been installed in the pile. The pile is constructed with a trapezoidal cross-section. Air is caused to circulate through the bio pile with the aid of a fan via the perforated pipes and also by a droplet separator. A minimal loss of volatile hydrocarbons occurs during this process. Any leaching water that occurs is led to the leaching water system of the plant via an oil separator. Experiences have shown that only a very small amount of leaching water is formed during the BioSan treatment process.

The US patent specification 6,109,358 teaches a valve arrangement and a method associated therewith for, among other things, ventilating a soil mass in the form of a pile of soil with the aid of a wind vane that functions to drive an air pump which is adapted to pump air into a pipe matrix located in the soil pile for the pur- pose of decontaminating the soil in the pile microbiologically. The specification of this prior US patent document also illustrates that further pipe matrices may be added to the soil pile as it increases in height.

EPA 0429137 teaches a process for removing contaminants from beneath the surface of the ground in situ, by pumping air down through the contaminated area with the aid of a pipe system. The pumped air then rises up to the surface of the ground through the contaminated area, carrying the contaminants with it up to the surface of the ground, where these contaminants are collected in a biometric filter for further treatment.

EPA 0832699 teaches a process for the decontamination of contaminated soil in which the soil is heated dielectrically to a soil temperature that lies in range of 10° C - 50 ° C. This patent document also describes an installation for such a process.

WO 93/05896 teaches a system for decontaminating a contaminated pile of soil with the aid of hot air. Decontamination is effected by recycling hot air from a combustion chamber through the soil pile with the aid of pipelines.

Object of the invention

The object of the present invention is to provide an improved soil decontamination device by increasing the speed of the decontamination process, partly by improving aeration of the soil and partly by using more effectively the process heat in the biological degradation of processed soil quantities.

A further object is to improve the cleansing of the processed soil quantities.

A further object is to improve soil decontamination by providing an eco- nomically beneficial decontamination device and also a method associated with said device.

Summary of the invention

These objects are achieved by means of the present invention as set forth in the independent claims. Suitable embodiments of the invention will be apparent from the dependent claims. The invention relates to a soil decontamination device for decontaminating a pile of soil that has been processed for biological degradation. The lower part of the pile of soil includes a perforated pipeline connected to a main line. A medium directing element that has a suction side and a pressure side and is connected to the main line at the pressure side of said element. The medium directing element includes an air transportation element that includes at least one fan or at least one pump or at least one compressor. Some embodiments of the invention that lie within the scope of the invention have a medium directing element which includes two fans. This element may also include one or more adjustable valves and/or also one or more check valves. The medium directing element is adapted to aerate the pile of soil via the main pipeline and the perforated pipes such as to hasten the biological degradation process. In addition, a second processed soil pile is separated from the first soil pile for biological degradation and connected to the suction side of the medium directing element. This second soil pile is also provided with perforated pipes in its lower part through the medium of a second main line. The medium directing element is herewith designed so as to enable its pressure side and its suction side to change places, so that alternating aeration and therewith an alternating supply of heat can be applied to both soil piles.

The soil piles can be provided with covering material in the form of synthetic carpets or rubber carpets or carpets comprised of some other suitable mate- rial for increasing the process temperature and therewith the rate of decontamination, and to prevent emissions to atmosphere and to prevent the run-off of precipitates and for preventing the soil from drying out as a result of the evaporation of its water content.

According to one embodiment of the invention, a valve system is con- nected between the first and the second main lines. The system includes a first and a second line coupling valve which form a first valve pair and a first and a second cross-coupling valve which form a second valve pair. The valve pairs are adapted so that when both line coupling valves are closed while both cross- coupling valves are open, the air transporting element will function to transport air

from the second soil pile to the first soil pile. On the other hand, when both line coupling valves are open while both cross-coupling valves are closed, the air transportation element will function to transport air from the first soil pile to the second soil pile. There is preferably also included a control element which is adapted to change simultaneously the operating mode of the first valve pair on the one hand and the operating mode of the second valve pair on the other hand.

In one embodiment, the control element is connected to a temperature sensitive control circuit which is adapted to change all valve positions when an air temperature threshold value is reached in a main line.

In a further embodiment, the control element is connected to a timing circuit which is adapted to change all valve positions when a predetermined time interval has been reached from the nearest earlier valve position adjustment.

In yet another embodiment, the medium directing element is connected to a motor control which is adapted to reverse the direction of rotation of the air transportation element so as to change the flow direction either fully manually or automatically in response to reaching either a threshold value of the air temperature in a main line or in a soil pile or in response to having reached a time interval from the nearest preceding change in the direction of rotation. In a further embodiment a second air transportation element which counteracts the first air transportation element is connected between the first soil pile and the second soil pile parallel with the first air transportation element, such that each air transportation element will be connected to a respective main line via a transportation line, both on the suction and the pressure side of the air transport- ation element, wherein an electrically controlled switch is connected to an electromechanical coupling for activating an air transportation element from a driving motor either purely manually or in response to either reaching an air temperature threshold value in a main line or in a soil pile, or in response to having reached a given time interval from the nearest preceding coupling change. In other embodiments each transportation line is provided with a check valve, in order to eliminate return flow through the inactive air transportation element.

According to a further embodiment of the invention, a second transportation element that counteracts the air transportation element is connected between the first soil pile and the second soil pile parallel with the first transportation element, wherewith each of the air transportation elements is driven by its respective motor, wherein a motor switching means is connected between the two motors and adapted either for manual activation of the first drive motor or the second drive motor, or, alternatively, to activate the motors automatically solely by having reached an air temperature threshold value in a main line or in a soil pile or as a result of having reached a given time interval from the earliest preceding switch- over.

The invention also relates to a method for decontaminating a soil pile processed for biological degradation, comprising a medium directing element by means of which air is delivered to and transported through the soil pile so as to accelerate the biological degradation process, wherein said element alternatively also supplies air to a second soil pile which has been processed for biological degradation and which is separated from the first soil pile, wherein the element changes the direction of air flow in accordance with external factors so as to provide alternating aeration and therewith an alternating supply of heat to both soil piles. One of these external factors involves manual adjustment of the flow direction.

Another of said external factors involves automatic adjustments of the flow direction when the temperature in a main line in a soil pile has reached a given threshold value. Another of said external factors involves automatic adjustment of the flow direction when a given time interval from an earlier adjustment has reached a threshold value.

The supply of air to the soil piles is effected through said perforated pipes which are placed at the bottom of the soil masses to be decontaminated. When further quantities of soil are added, additional perforated pipes can be disposed in another level or in several other levels in the soil pile.

Brief description of the drawings

The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings, in which Fig. 1 is a schematic illustration of a first embodiment of the invention;

Fig. 2a illustrates schematically a part of a second embodiment of the invention;

Fig. 2b illustrates schematically a part of a third embodiment of the invention; and

Fig. 2c illustrates schematically a part of a fourth embodiment of the invention;

Description of the invention Figure 1 illustrates schematically a first embodiment of a soil sanitation device which includes a first soil pile H1 and second soil pile H2. Each pile has a height of about 3m, a width about of about 20m and a length of about 50 m and has a trapezoidal cross-section. A matrix of pipes 1 which includes upwardly facing holes 2 is placed in the lower part of each pile. Each pipe in the matrix is con- nected to a first main pipe 10 for connecting the matrix in the first soil pile to a first central pipe 11 and to a second main pipe 20 for connecting to the matrix of the second soil pile to a second central pipe 22. Between the first central pipe 11 and the second central pipe 22 there is placed a medium directing element 30 which includes an air transportation element in the form of a fan 31 which is driven by a motor 51 and which is adapted for the transportation of air from one of the soil piles to the other. As will be seen from the figure, the two central pipes are connected to a cross-coupled pipe system such that the first central pipe 11 will be connected to the suction side 32 of the fan 31 via a first valve 33 on the one hand and connected to the pressure side 35 of the fan 31 via a second valve 34 on the other hand. Moreover, the second central pipe 22 is connected to the suction side 32 of the fan 31 via a third valve 36 and to the pressure side 35 of the fan 31 via a fourth valve 37. This cross-coupled arrangement of the air transportation element 31 enables the fan to transport air from the first soil pile H1 to the second soil pile H2 through the medium of the first valve 33 and the fourth valve 37, whereas the

second valve 34 and the third valve 36 are both closed. Conversely, if the first valve 33 and the fourth valve 37 are both closed and the second valve 34 and the third valve 36 are both open, air will be transported from the second soil pile H2 to the first soil pile H1 as illustrated by the arrows in the figure. Thus, the transporta- tion of air between the two soil piles can change direction by virtue of the pressure side 41 and the suction side 42 of the medium directing element 30 changing places, so as to provide alternating aeration and therewith the supply of heat to both soil piles. As will also be seen from the figure, a control element 38 is connected to the medium direction element 30 for manoeuvring the four valves 33, 34, 36, 37 automatically in accordance with the temperatures in the central pipes 11 , 22 or in the soil piles, or in accordance with the time that has passed since the nearest preceding change in the direction of the airflow.

The direction of airflow through the soil sanitation device with the aid of the medium directing element 30 is denoted in figure 1 with a group of flow arrows in towards the perforated pipes in the second soil pile H2, via the direction arrows through the medium directing element 30 and a group of flow arrows out from the perforated pipes in the first soil pile H1 as a result of the first valve 33 and the fourth valve 37 both being closed while the second valve 34 and the third valves 36 are both open. The modus operandi of the device in this case is that the control element

38 registers the temperature in the first soil pile H1 or in a main pipe 10, 20 by means of a temperature sensor (not shown), wherein the control element 38 functions to control the valves electrically when the temperature has reached a predetermined temperature in the range of 50-70 ⁰ C, such that the control opens the first and the fourth valve 33, 37 and closes the second and third valve 34, 36. Despite the fact that the fan still operates in the same direction, the airflow will change direction, wherewith the suction side 42 and the pressure side 41 of the medium directing element 30 will change places.

Figure 2a illustrates an alternative embodiment of a medium directing element 30 that includes a pressure side 41 and a suction side 42 and that includes a fan 31 driven by a motor 51 , which in turn is controlled by a time- dependent or temperature-dependent motor control 52 that is adapted to change the direction of rotation of the motor such that the fan will work in the opposite direction. In this regard, the fan is constructed to operate with the same power irre-

spective of its direction of rotation. The reverse direction of the rotation the fan is marked with the reference numeral 311. Correspondingly to the figure 1 embodiment, the suction side 42 and the pressure side 41 of the medium directing element 30 will change places, although in the case of this embodiment not until the working direction of the fan has been reversed.

Figure 2b illustrate a third embodiment of a medium directing element 30 which has a pressure side 41 and a suction side 42 and which includes a first fan 61 and a second fan 62 which operate in parallel but in opposite directions to one another and which are connected to a respective main line via transportation lines Tr. Both of the fans are driven by a motor 63 via a coupling 64 which either activates the first fan 61 or the second fan 62. The coupling mode of the coupling 64 is controlled by a time-dependent or a temperature-dependent coupling control 65, which is adapted to shift the coupling mode of the coupling from the first fan 61 to the second fan 62 such as to allow the suction side 42 and the pressure side 41 of the medium directing element 30 to change places.

Figure 2c illustrates a fourth embodiment of a medium directing element 30 which has a pressure side 41 and a suction side 42 and which includes a first fan 61 and a second fan 62 which operate in parallel but in opposite directions to one another, although only one at a time, and are connected to respective main pipes via transportation lines Tr. Each of the fans are driven by a respective motor 71 and 72, these two motors being controlled via a motor switch 73 in response to a time-dependent or temperature-dependent sensor such as to cause the first motor, and therewith the first fan, to stop or to cause the second motor, and therewith the second fan to start, and vice versa. As a result, the suction side 42 of the me- dium directing element 30 and its pressure side 41 will change places, depending on the preset time or preset temperature at which the suction side and the pressure side of the medium directing element 30 change place in the manner described above.

The third or the fourth embodiment may also include check valves for eliminating the backflow of air through the fan that is inactive at that moment. The fan or the fans described in these embodiments may alternatively comprise a pump, compressor or some corresponding air transportation device.