Hydraulic and earth engineering – Soil remediation – In situ contaminant removal or stabilization
Reexamination Certificate
2003-02-19
2004-06-01
Kreck, John (Department: 3673)
Hydraulic and earth engineering
Soil remediation
In situ contaminant removal or stabilization
C405S128300
Reexamination Certificate
active
06742961
ABSTRACT:
The invention relates to a process for the removal of pollutants from the soil with the aid of a stripping gas which involves
(a) injecting the stripping gas into the soil at the level of and/or under the pollutant,
(b) volatilizing the pollutant with the stripping gas, so that it rises to ground level above the pollutant,
(c) collecting and further treatment of the volatilized pollutant.
Such a process is known from EP-A-429.137.
This publication describes a process for the removal of volatile pollutants from the soil by injecting air into the soil at the location of the pollutant. The injected air rises through the soil up to ground level and on its way up entrains the pollutants. In the known process the soil layer bounding the surface consists of biologically active material in which the pollutants are collected on their way up and degraded in situ.
A problem encountered in this method is that the injected air with the volatilized pollutant does not only move vertically but there is also a certain lateral component in its movement. As a result, part of the pollutant will be able to reach the ground level outside the biologically active layer and will thus be emitted to the ambient air without being degraded in said layer. To prevent this, the patent application referred to mentions the possibility of, for example, digging an infiltration trench around the polluted area or applying air-tight covers at ground level. It is also possible to install sheet piling around the place to be cleaned. Such a trench must be made around the entire circumference of the polluted area and will thus have a relatively large length. The same holds for the area of an air-tight layer to be applied. These measures are costly, hinder the work of third parties on the spot, such as for example production activities, and must furthermore be undone at a later stage, which again entails activities and other costs.
The same problem is encountered in another known process, in which the pollutants rising upward with the injected air are not degraded in a bioactive upper layer but are collected above the ground, for example under a screen, and conveyed to a facility for, for example, incineration or conversion into harmless substances in another manner.
The aim of the invention is to provide a process that prevents, in an economically more attractive way, the diffusion of the pollutant by the injected air flow to outside the area where the pollutants can be collected.
This aim is achieved according to the invention in that a pneumatic screen is installed in the soil outside the circumference of the polluted area.
A pneumatic screen is here understood to be a gas flow rising from a certain depth below ground level that extends continuously around the polluted area. It has been found that it is impossible or virtually impossible for laterally moving pollutants to pass through such a pneumatic screen and that they are removed to ground level within the area surrounded by the pneumatic screen.
In the above-mentioned EP application a reference is incidentally made to the possibility of also injecting stripping gas beside the pollutant but the possibility of doing this in the form of a pneumatic screen and as an alternative to the digging of a trench or the application of air-tight material to prevent diffusion of the pollutant is not divulged therein, nor can it be derived therefrom.
A further advantage of the process according to the invention is that it can also be used when the pollutant is wholly or partially situated under a layer that is impermeable to the volatilized pollutants, for example a concrete or asphalt floor, as often found in a plant or other production environment or around storage tanks. In that case the pneumatic screen is installed outside the non-permeable part and then also encloses a permeable area where volatilized pollutants can reach ground level. By controlling the flow rate and/or the pressure of the injected gas flow along the pneumatic screen the pollutant can be driven in a desired direction, in particular in the direction of the permeable part of the area that is surrounded by the pneumatic screen. The stripping gas, which entrains the volatilized pollutant, can there reach ground level and, as the occasion arises, be treated by a bioactive layer present or applied there. If a bioactive layer is applied its thickness is preferably largest in the non-covered part that is closest to the area of the gas-impermeable layer. This has a favourable influence on the permeability of the soil there, which has a favourable influence on the gas flow in the soil in the direction of the bioactive layer. Further horizontal diffusion is prevented by the part of the pneumatic screen that surrounds the non-covered area. The presence of a pneumatic screen in the process according to the invention thus not only makes it possible to keep the pollutant localized within a chosen area but also to influence the place where the volatilized impurity reaches the surface. The distribution of the gas outflow over the screen is then set so that a resulting gas flow is set in a desired direction in the soil.
In the process according to the invention air can be used as stripping gas, if desired enriched with oxidizing components such as ozone or with other gaseous components. It is also possible to use nitrogen if it is desirable to work under anaerobic conditions. The gas used for the air screen, hereafter referred to as screening gas, can be the same gas as that used for the stripping gas. For price considerations air is generally the most attractive choice as screening gas.
As in the process known from EP-A-429.137, in the process according to the invention use is made of one or more hollow pipes to inject a pressurized stripping gas, preferably air, under the pollutant. The pipes are distributed over the area to be cleaned as uniformly as possible, preferably in a regular pattern of preferably equilateral triangles because it has been found that this gives the most uniform cleaning. The position of the outlet of the pipes is chosen so that the largest possible part of the stripping gas passes through the polluted soil to ground level. The outlet is situated in or, for at least a number of the pipes, under the pollutant. When the stripping gas flows through the polluted soil, the pollutant volatilizes upon which it is entrained to ground level. If the pollutant consists of volatile substances with a sufficiently high vapour pressure (for example acrylonitrile, benzene, toluene, xylene, acetone, styrene, phenol, perchloroethylene or trichloroethyelene), this volatilization takes place through evaporation. The pollutant is entrained in vapour form with the stripping gas passing through. Pollutants can also be volatilized by reaction with a stripping gas component, for example by an oxidizing agent such as a stripping gas enriched with ozone obtained by ozonization of air. The resulting reaction products volatilize while passing through the soil with the stripping gas and rise to ground level. Without further measures uncontrolled escape of the volatilized pollutant would take place at ground level and the pollutant would enter the atmosphere. To prevent this, the volatilized pollutant is collected above ground level for further processing or is passed, before escaping, through a biologically active layer in which the volatilized pollutant is wholly or partially degraded. The biologically active layer extends at least over the area where the pollutant will reach the surface and preferably over some distance outside it. In principle use can be made of the biological activity of the soil layer above the pollutant. Generally, however, it is necessary to upgrade the soil using additives so as to obtain a preconditioned biologically active layer and to maintain its activity at the required level. If upgrading of the soil at or just under ground level to obtain a biologically active layer is not or scarcely possible, use can be made of a layer of a few decimetres of preconditioned bioactive material applied at groun
DSM IP Assets B.V.
Kreck John
Pillsbury & Winthrop LLP
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