WO1993006951A1

WO1993006951A1 - Installation for decontaminating soil

Info

Publication number: WO1993006951A1
Application number: PCT/NL1992/000175
Authority: WO
Inventors: Gerardus Bernardus Johannus Borggreve; Hendrikus Van Den Beld; Klaas Roelof Westerterp; Gerrit Herman Willem Geesink
Original assignee: Te Pas Milieutechniek B.V.
Priority date: 1991-10-04
Filing date: 1992-10-02
Publication date: 1993-04-15
Also published as: NL9101678A; EP0609308A1

Abstract

The invention relates to an installation for decontaminating soil, comprising: i) at least one basin shielded from the environment for receiving the contaminated soil and provided with liquid supply means, liquid drainage means, air supply means, air drainage means and means for adding microorganisms; ii) at least one bioreactor connected to the liquid drainage means; and iii) at least one membrane filter unit which forms a circuit with the bioreactor, wherein the membrane filter unit preferably comprises a microfilter in the form of a fibre bundle and the filter pore size is smaller than 0.5 νm, preferably smaller than 0.3 νm.

Description

Installation for decontaminating soil

The present invention relates to an installation for decontaminating soil wherein micro-organisms are used. The invention relates more particularly to an installation for decontaminating soil wherein at least a portion of the conta minants is added to water and subsequently broken down in a bioreactor using micro-organisms, which bioreactor is prefer ably equipped with a membrane filter unit in order to sepa¬ rate purified watery permeate.

An installation is known for decontaminating soil wherein the soil is percolated with water and optionally aerated so that micro-organisms present in the soil break down the contamination or make it leachable. The percolation liquid is separated off and the decontaminated soil is in principle suitable for re-use. The present invention has for its object to provide an installation of the above described type wherein the removal or breaking down of the contaminants present in the soil gets under way more rapidly and/or takes place more rapidly. Used liquid and air are preferably re-used or after-treated such that discharge into the environment is acceptable.

This is achieved in accordance with the invention with the so-called "BIOPAS" method, an installation for which comprises: i) at least one basin shielded from the environment for receiving the contaminated soil and provided with liquid supply means, liquid drainage means, air supply means, air drainage means and means for adding micro¬ organisms,* ii) at least one bioreactor connected onto the liquid drainage means; and iii) at least one membrane filter unit which forms a circui with the bioreactor and which is connected to the liquid supply means with a filtrate conduit. The use of a bioreactor achieves on the one hand that the contaminants coming from the soil and present in the liquid are broken down in optimal conditions, while the therein formed surplus biomass which has adapted itself optimally to the contaminants can be used at least partially as micro-organisms that are to be added. Because the bioreac¬ tor forms a circuit with the membrane filter unit, in which liquid freed of micro-organisms is separated off, a maximum quantity of filtrate can be separated in efficient manner. An optimal filtrate separation by means of crossflow filtration is obtained when more preferably the membrane filter unit comprises a microfilter in the form of a fibre bundle.

In order to hold back the micro-organisms it is furthe recommended that the pore size of the microfilter is smaller than 0.5 μm, more preferably smaller than 0.3 μm. In practical conditions the pore size usually amounts to 0.1-0.2 μm. The circuit ratio of the liquid flow rate in the circuit relative to the filtrate flow rate is herein preferably 10- 100:1. More preferably the circuit ratio amounts to 10-90:1 and in optimum conditions 20-50:1. This means that for in¬ stance 20-50 m³ liquid from the bioreactor passes through membrane filter unit and only 1 ³ filtrate is separated out of that volume. For optimum break-down of contaminants in the bioreac¬ tor and for optimal formation of biomass it is further recom¬ mended that the dry solid content of biomass in the liquid in the bioreactor is smaller than 4% by weight. In general this is smaller than 2% by weight and in practical conditions the dry solid content usually amounts to 0.1-1% by weight.

The bioreactor is preferably provided with an outlet for discharge of a surplus of formed biomass, this in order to ensure the continuous availability of and the option of adding biomass to the basin for the break-down continuity and to ensure the process speed. This biomass can either be used as occulent in starting a new decontaminating cycle or sold commercially. In this latter case the outlet for micro-orga¬ nisms is connected to the means for adding micro-organisms to the basin. In order to enable adjustment of an optimum temperatur in the soil which is present in the basin and which is being decontaminated by micro-organisms present therein, it is recommended that a heat exchanger is incorporated in the liquid supply conduit to the bioreactor and/or to the basin. The temperature of the liquid lies in general below 40°C and is a temperature dependent on the ambient temperature so that the temperature of the soil in the basin lies in a tempera¬ ture range of 20-30°C, more preferably 23-27°C, for instance 25°C. Another important aspect is that the heat developed by the means for maintaining the cycle is available to the bioreactor and the basin.

In order to sustain an optimally functioning biomass i the basin it is recommended that nutrients are added to the biomass such as nitrogen compounds and phosphate compounds. The nutrient requirement is subject to the C/N ratio of the contaminants in the basin as well as in the bioreactor. The nutrients can be added to the contaminated soil or to the water added to the contaminated soil. Surface-active agents can further be added in order to make the contaminants more easily accessible to microbic degradation or to leaching with the percolation water.

If in addition the air drainage means are connected to a preferably biological air filter it is ensured that air for discharging is freed of compounds having an unfavourable impact on the environment. Such a biological air filter may for instance consist of a so-called compost filter which can for instance consist of a number of beds connected in series. If more preferably the installation comprises two or more basins, soil can be decontaminated in these basins wherein the phase of the soil decontamination between the basins mutually differs such that an optimal use is made of the biomembrane reactor. Contaminated percolation water may for instance come from the one basin and be decontaminated with a biomass present in the bioreactor which has initially broken down contaminants from another basin. Formed surplus biomass can be used in a third basin for an adequate starting therein of the soil decontamination using the biomass which acts as occulent. For an optimal operation of the installation in a temperate climate for a major part of the year it is recom¬ mended if necessary to add additional heat in homogeneous manner to the bed of contaminated soil received in the basin. The installation is therefore preferably provided with means for homogeneous supply of heat. According to a first embodi¬ ment these comprise heat supply means which comprise a slop¬ ing roof construction with a sunlight-transparent outer laye and a sunlight-absorbing inner layer separated therefrom by an air layer, which air layer communicates with the outside air via an air inlet gap and with the basin via an air outlet gap. Use can thus be made of solar energy. The air layer thickness amounts generally to 10-50 cm, more preferably 20- 40 cm and often in practice to 25-30 cm. If the quantity of heat supplied via sunlight becomes too great the supplied quantity of heat can be decreased by limiting the quantity of absorbed solar energy and it is therefore recommended that the inner layer comprises a remo¬ vable sheet, and preferably one that can be rolled up. In the colder seasons it may be useful to screen off the air layer above the basin as much as possible. It is recommended for this purpose that the inner layer is provided with a thermal insulation layer. Loss of heat by radiation from the bed of contaminated soil received in the basin can be further avoided if more preferably a reflection screen is suspended above the basin in order to reflect heat to the basin.

According to another embodiment the heat supply means comprise electrokinetic means. This means that by means of an alternating voltage applied over the bed of contaminated soil heat is developed homogeneously in the soil. To this end these electrokinetic means preferably comprise a grid or interwoven network of metal elements arranged in the bottom of the bed of contaminated soil and another grid of electri- cally conducting elements arranged in the top of the bed of contaminated soil.

The treatment of contaminated soil in the installation according to the invention can be further shortened if conta¬ minated soil suited to the purpose is subjected to a pre- cleaning in which volatile, particularly volatile organic, substances and/or washable contaminants are separated before hand from the contaminated soil. Additives such as detergent and the like can optionally be added for this purpose. It is recommended for this purpose that the installation comprises a pre-cleaning unit wherein the contaminated soil is sub¬ jected to a pre-cleaning, wherein in the case volatile sub¬ stances are being removed the pre-cleaning unit preferably comprises an aerating unit. In the case of washing the pre- cleaning unit preferably comprises a washing unit. During pre-cleaning the same additives can be added as described above in the case of direct decontamination, such as soil improvers, nutrients, detergents and the like, all of which are preferably bio-degradable. In the subsequent decontamina- tion less percolation thus takes place and precisely the moisture content and the temperature in the soil bed can be controlled to a greater extent.

Mentioned and other features of the installation according to the invention will be elucidated hereinafter on the basis of an embodiment given by way of example, while reference is made to the annexed drawing. In the drawing: figure 1 is a flow diagram of an installation accordin to the invention; figure 2 shows a perspective, partly broken away view of an installation according to the invention equipped with two basins; figure 3 shows on a larger scale a section along the line III-III of figure 2; figure 4 shows on a larger scale a section through an air discharging conduit present in the basin of figure 3; figure 5 is a diagram of another installation accordin to the invention; figure 6 shows on a larger scale detail VI of figure 5 figures 7 and 8 each show views corresponding with figure 6 of variants for the building construction; and figure 9 shows on a larger scale detail IX of figure 6 Figure 1 shows an installation 1 according to the invention for decontaminating soil 2 received in a basin 3 that is shielded from the environment. This shielding can be realized by means of a building 4 as shown in figure 2.

The basin 3 is provided with liquid supply means 5 which comprise a large number of sprayers 6 with which liquid 7 can be sprayed on the soil 2 for decontaminating. The spray volume amounts in general to 1-30 cm bed height per day, more preferably 5-10 cm bed height per day. Alternatively the soil can be inundated temporarily from below.

The basin 3 is further provided with liquid drainage means 8 with which the excess water added to the contaminated soil is drained. The drainage water comprises contaminants leached out of the contaminated soil and contaminants or degradation products thereof which are released from the soil using micro-organisms. These micro-organisms are added via the line 9. In practice this can consist of adding biomass which is subsequently mixed with the contaminated soil 2.

Finally, the basin 3 is provided with an air drainage conduit 10 with which volatile components are discharged from the space and out of the soil. In this manner oxygen is added to the micro-organisms in the soil and the emission of vola¬ tile components is avoided.

The contaminants and possible micro-organism-containing liquid which is drained via the liquid drainage 8 passes through a heat exchanger 11 wherewith the drainage liquid is brought to a temperature (by heating or cooling depending on the ambient conditions) which is optimal for breaking down the contaminants present in the liquid in the bioreactor 12. Additional water, nutrients and the like can optionally be added via a feed line 13. If necessary the liquid can be guided, prior to reach¬ ing bioreactor 12 via inlet 14, through a sand separator and/or oil-petrol separator so that substantially only an aqueous phase reaches bioreactor 12.

The bioreactor 12 provided with an intensive aeration forms with a membrane filter unit 15 a circuit 16 provided with a pump 38. Further incorporated into the circuit 16 are a pump 17 and a short circuit line 37 so that liquid is guided through the membrane filter unit at a very great flow rate (40-80 ³ per hour) , while depending on the permeate flow rate liquid is fed from the bioreactor 12. The circuit ratio of the liquid flow in the circuit to the filtrate flow that is discharged via the filtrate conduit 18 provided with a heat exchanger 16 amounts to 40-80. The membrane filter unit used comprises a fibre bundle, the filter pore size of which amounts to 0.2 μm. A filtrate is thus formed which is sub¬ stantially free of contaminants and of micro-organisms. In this way an optimum temperature adjustment becomes possible in the bioreactor and in the basin. The filtrate conduit 18 connects onto the liquid supply means 5.

Surplus biomass formed in the bioreactor 12 can be discharged via an outlet 19 and at least a portion thereof can be fed via the recirculation line 20 to the occulent inlet 9 of basin 3. The air coming out of basin 3 which is discharged via discharge 10 is purified in a biological air filter 21, for instance a compost filter having two filter beds connected in series. An outlet 22 of air filter 21 preferably comprises a sensor 23 which measures the contamination level in the outlet. If this level is acceptably low a valve 24 is then actuated such that the air is released via a sluice 25 into the environment. Should the soil contain a high level of volatile contamination the valve 24 is then actuated such that this air is returned via a conduit 26 to the basin for new decontamination by the soil 2, whereafter the air once again leaves the basin 3 via the air outlet 10.

Figure 2 shows an installation 27 according to the invention, which comprises two adjacently disposed basins 3 each in a different phase of the decontamination process so that the bioreactor (not shown) is subjected to a contamina¬ tion load that changes to a lesser extent in time. Occulent material formed during decontaminating of the soil from the one basin can also be used to start up the decontamination process in the other basin. Figure 3 shows on a larger scale a basin 3. This basin is formed by a channel 29 which is formed in the ground 28 and in which is arranged liquid-impermeable foil 30. In the bottom of the channel is situated a water-permeable sand layer 31 in which liquid drainage conduits 32 are arranged at a lower level and air drainage conduits 33 at a level locate thereabove.

In order to avoid too much liquid disappearing via th air drainage conduit an air conduit 33 is only provided on its lower portion 34 with air inlets 35.

In preference however, the liquid and air drainage conduits are integrated in a common conduit (see figure 9) .

Figure 5 shows an installation 40 according to the invention which comprises a pre-cleaning unit 41 for pre- cleaning the contaminated soil and a cleaning unit 42 for th pre-cleaned soil 43, which cleaning unit 42 substantially corresponds with the cleaning device 1 and 27 of figure 1 an 2.

The pre-cleaning unit comprises a rotating drum 45 disposed at an incline and having an inlet 46 for contamina¬ ted soil and an inlet 47 for air, whereby the contaminated soil 2 in the drum 45 is aerated in counterflow. Air charged with volatile, particularly organic volatile, compounds is drained via the conduit 48 to the air filter 21. The pre-cleaning unit 41 further comprises a washing unit 49 with a washing tank 50 in which soil coming from drum 45 via a conduit 51 is washed with water which is supplied via conduit 52 and which may originate from the filtrate pipe 18. The soil and the washing water are fed via a conduit 53 to a filter tank 54 in which water is separated from the contaminated soil and fed via a conduit 55 to the bioreactor 12. The separating of water is realized substantially without separation of particles to size. The filter tank is provided for this purpose with a short circuit line 57 which is provi- ded with a pump 56 and with which filtrate and fine soil particles possibly incorporated therein can be returned to the top of the filter layer 58. The pre-cleaned soil 43 is carried via transporting means 59 to the decontamination unit 42. The decontamination unit 42 comprises a building 60 with a basin 61 for the pre-cleaned soil 43.

The soil 43 is provided with water via sprayers 6 connected to the liquid supply means 5. Occulent material is optionally supplied via the conduit 9. Liquid and air are drained via a common conduit 62 and after passing through a pump 63 are separated in a liquid-air separator 64 into air which is discharged via a conduit 65 to the filter 21 and liquid which is carried via a conduit 96 back to the bioreac¬ tor 12. Insufficiently decontaminated air can if required be returned via the conduit 26 to the building 60.

The bioreactor 12 is once again incorporated with the membrane filter unit 15 in a circuit 16. The circuit ratio amounts in this case to 20-50.

For homogeneous supply of heat to the bed of contami- nated soil arranged in the basin the building 60 is provided with a typical roof construction 66 which comprises a sun¬ light-transparent outer layer 67, a sunlight-absorbing inner- layer 68 and an air layer 69 located therebetween with an inlet gap 70 and an outlet gap 71. Since the building 60 stands under a small underpressure as a result of air being drawn out via the conduit 62, air will be admitted to the basin 61 via the inlet gap 70 and will heat up during the passage through the air layer 69. The roof construction 66 is shown in more detail in figure 6. The outer layer 67 consists of corrugated plates of light-transparent material such as glass or fibre-reinforced plastic supported by rails 73 fixed to purlins 72.

The inner layer 68 is formed by a removable sheet 75, in this case windable onto a drum 74, which is suspended at its free end on a member 76. The sheet consists of an LTV resistant black, preferably plastic sheet which may optional¬ ly be provided on the underside with an insulation layer (not shown) .

Further incorporated into the building 60 are electro- kinetic means 77 which can be used separately or in combina¬ tion with the roof construction 66 for homogeneous supply of heat to the soil 43. The electrokinetic means comprise two grids 78 and 79 of electrically conducting elements, for example galvanized metal rods, which are respectively ar- ranged at the bottom and top of the basin 61. Via a unit 80 an alternating voltage is applied over both grids 78 and 79, which alternating voltage is subject to the temperature measured in basin 61 with a sensor 81. The side walls 82 and 83 can consist of construction elements or of optionally insulated sheet material.

In the building 84 shown in figure 7 the inner layer has been temporarily removed and replaced by two reflection foils 85 and 86 suspended from the ridge, whereby loss of heat by radiation out of the basin 61 is decreased.

Figure 8 shows a building 87 wherein the roof con¬ struction 88 comprises an outer layer 67 formed from plates 89 and an inner layer 68 consisting of plates 90 provided with insulation material. The plates 89 and 90 are suspended from a common roof framework 91.

Finally, figure 9 shows in more detail a drain line 62 which comprises an inner pipe 92 provided with perforations 93 and a porous layer 94 arranged therearound and consisting of a mixture of sand grains 95 and cured plastic binder. It is noted that the diverse means for homogeneous supply of heat to the contaminated soil, namely by means of the various described roof constructions and by electrokine¬ tics, as well as the pre-cleaning unit 41 can in principle be applied without the circuit used of bioreactor 12 and mem¬ brane filter unit 15.

With the devices according to the invention it is possible to decontaminate contaminated soil thoroughly and more rapidly. The contaminants which can be removed with the installation according to the invention mainly comprise biodegradable, possibly physically/chemically pre-treated material which originates for instance from motor fuel ins¬ tallations and therefore contains oils, aromates, hydrocar¬ bons and the like.

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Claims

1. Installation for decontaminating soil, comprising: i) at least one basin shielded from the environment for receiving the contaminated soil and provided with liquid supply means, liquid drainage means, air supply means, air drainage means and means for adding micro¬ organisms; ii) at least one bioreactor connected to the liquid drain¬ age means; and iii) at least one membrane filter unit which forms a circuit with the bioreactor and which is connected to the liquid supply means with a filtrate conduit.

2. Installation as claimed in claim 1, wherein the membrane filter unit comprises a microfilter in the form of a fibre bundle. 3. Installation as claimed in claim 1 or 2, wherein the filter pore size is smaller than 0.5 μm, preferably smaller than 0.

3 μm.

4. Installation as claimed in claims 1-3, wherein the circuit ratio of the liquid flow rate in the circuit relative to the filtrate flow rate amounts to 10-100:1, preferably 10- 90:1 and more preferably 20-50:1.

5. Installation as claimed in claims 1-4, wherein the dry solid content of biomass in the liquid in the bioreactor is smaller than 4% by weight, preferably smaller than 2% by weight, such as 0.1-1% by weight.

6. Installation as claimed in claims 1-5, wherein the bioreactor is provided with an outlet for micro-organisms.

7. Installation as claimed in claim 6, wherein the outlet for micro-organisms is connected onto the means for adding micro-organisms to the basin.

8. Installation as claimed in claims 1-7, wherein a heat exchanger is incorporated in the liquid supply conduit to the bioreactor and/or to the basin.

9. Installation as claimed in claims 1-8, wherein the air drainage means are connected to a biological air filter.

10. Installation as claimed in claims 1-9, wherein th basin is provided with means for homogeneously supplying hea to the bed of contaminated soil received in the basin.

11. Installation as claimed in claim 10, wherein the heat supply means comprise a sloping roof construction with sunlight-transparent outer layer and a sunlight-absorbing inner layer separated therefrom by an air layer, which air layer communicates with the outside air via an air inlet gap and with the basin via an air outlet gap.

12. Installation as claimed in claim 11, wherein the inner layer comprises a removable sheet which can preferably be rolled up.

13. Installation as claimed in claim 11 or 12, wherein the inner layer is provided with a thermal insulation layer.

14. Installation as claimed in claims 10-13, wherein reflection screen is suspended above the basin in order to reflect heat to the basin.

15. Installation as claimed in claims 10-14, wherein the heat supply means comprise electrokinetic means.

16. Installation as claimed in claims 1-15 provided with a pre-cleaning unit wherein the contaminated soil is subjected to a pre-cleaning.

17. Installation as claimed in claim 16, wherein the pre-cleaning unit comprises an aerating unit.

18. Installation as claimed in claim 16 or 17, wherein the pre-cleaning unit comprises a washing unit.

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