GB2185901A - Treating groundwater by sparging - Google Patents

Abstract

A process for removing volatile contaminants from flowing groundwater on a substantially continuous basin, comprises placing a tube 3 in the ground in the path of the flowing groundwater, the tube 3 being such that it allows water through its walls, and bubbling compressed gas through a head 7 into the water to carry the contaminants to the surface. A cut- off wall may be provided in the ground to guide the flowing groundwater through gaps at which a number of tubes may be located in series. The gas may be air. <IMAGE>

Description

SPECIFICATION Treating groundwater by sparging This invention relates to the removal of volatile contaminants from groundwater by sparging.

In sparging, a gas-typically air is bubbled through the contaminated water. The bubbles act to draw out those contaminants that are able to evaporate.

Hitherto, sparging has been carried out by pumping the contaminated water out of the ground, by sparging the water using equipment at the surface, and by then returning the clean water back into the ground. This is a slow, cumbersome and expensive process, and is a process which also requires the expenditure of energy to drive the water pumps.

The process of sparging is one that must be carried out efficiently and reliably. If the acceptable level of a particular contaminant in drinking water is, say, five parts per billion, then the sparging must be thorough enough to achieve that level. The standard must be met.

When the sparging is carried out above the ground, there is no difficulty in removing the contaminants to the required degree. The water is sparged by simply spraying it into the air until the contaminants are sufficiently evaporated. And, when the process is carried out above ground, tests can easily be done to determine whether the contaminants are being removed properly, and suitable adjustments can easily be made to the process.

It is recognized in the invention that the high standards to which sparging has been carried out can be met even if the water is not taken out of the ground. In the invention, the sparging is carried out in-situ, directly on the groundwater. It is recognized that sparging can be carried out on continuously flowing groundwater, if the sparging is carried out in the manner of the invention.

In the invention, a sparging cell is placed in the ground, in the flow-path of the contaminated water, and air is bubbled through the cell. This process is carried out entirely below ground: there is no need to remove the water from the ground to sparge it, as has been the need hitherto.

The in-situ sparging cell of the invention comprises a tube or chamber that is inserted deep into the ground, i.e. to a depth below or within the zone of contaminated water. If the contaminated flow is contained as regards its depth by clay, or other impervious soil, the tube should be inserted down to the level of the clay.

The tube is provided with a means for letting the water flow freely into and out of the tube. If the tube is a length of PVC pipe, for example, then the pipe should be provided with many slots or holes to permit the water to enter and leave the pipe through the walls of the pipe.

The tube is provided with an air-supply line, which conveys air down from a compressor at the surface to an air-discharge-port at the bottom of the tube.

Air from the air-discharge-port bubbles up through the water in the tube, and the bubbles act to pick up the volatile contaminants from the water and convey them to the surface.

Naturally, one sparging cell on its own is limited to its ability to treat the water over a large area.

When the groundwater to be cleaned is flowing through a large zone, the sparging cells are preferably used in conjunction with a cut-off wall.

The cut-off wall is provided with a gap, or several gaps, and the cells are placed therein. It is recognized in the invention that it is not necessary for the wall to consist entirely of sparging-cells. And, since it is considerably cheaper to construct a wall down into the ground than to install sparging cells into the ground, the gaps should be kept as small as possible, commensurate with not interfering with the flow of water.

In the invention, it is also recognized that there is a considerable advantage to be gained by passing the water through several sparging cells in series. The effectiveness of a sparging process depends to a large extent on the rate at which the bubbles are discharged from the air-discharge-port into the tube.

The general rule is that more of the volatiles can be removed from the water by bubbling the air through at a faster rate, but it is recognized, in the invention, that a rule or diminishing returns applies.

In addition, if excessive quantities of air are used, the flow of bubbles can become such as to impede the flow of water through the tube, but without further reducing the contaminant level.

It is recognized, in the invention, that air that has already picked up some contaminants from the water is markedly less able to pick up more contaminants. It is recognized, in the invention, that it is better to bubble small quantities of clean air through the water than large quantities of partly contaminated air.

In the invention, the water is therefore passed preferably not through one large cell, but through several smaller cells in series. Each of the cells in the series is provided with its own supply of fresh air.

Thus, there are preferably several small cells, and not one large cell, at the gaps in the wall referred to above.

An example of a preferred manner of constructing and installing typical in-situ sparging cells will now be described with reference to the accompanying drawings.

In the drawings:~ Fig. 1 is a cross-section of a sparging cell installed in the ground; Fig. 2 is a plan showing the layout of a cut-off wall with gaps, and sparging cells in the gaps, in relation to a source of contamination; Fig. 3 is a close-up of the cells in one of the gaps in Fig. 2; Fig. 4 is a close-up corresponding to Fig. 3, showing a different arrangement of the sparging cells.

The sparging cell shown in Fig. 1 comprises a tube 3 and an air-supply-line 4. The air-supply-line terminates in a head 5. The top end of the airsupply-line is connected to an air-compressor 6 at the surface, from which compressed air is discharged via an air-discharge-port 7, which comprises the holes 8 in the head.

The tube 3 is made of PVC piping, which has a diameter of 15 cm. The head 5 also is a piece of PVC piping, with a diameter of 5 cm. The head 5 is 25 cm long.

The tube 3 extends typically many meters deep into the ground. The portion of the tube that lies in the path of the contaminated water is perforated with slots and holes 9. The result is that water can flow through the walls of the tube, substantially without interference.

Fig. 2 shows a source of contamination 10. The flow of water around the source 10 is indicated by the streamlines 17. The contaminated flow has the characteristic plume-shape 30.

Acut-offwall 40 is built in the path of the plume 30. The wall 40 goes down deep enough into the ground to reach impervious sub-clay, or otherwise to ensure that the contaminants cannot pass beneath the wall 40. The wall 40 is provided with partially enclosing wings 48,49 to make sure the plume 30 is contained. The presence of the wall 40 can affect the shape of the plume, and the wings 48, 49 should be conservatively designed for the possibility.

The wall 40 is provided with several gates 50. The wall is itself impervious, and the whole flow of water passes through the gates 50. At each of the gates there is provided a group 50 of sparging cells 70. As shown in Fig. 3 the group 60 comprises twenty-four separate cells 70, each of which is identical to the Fig. 1 cell.

To install the group 50, a caisson 76 is driven down into the wall 40. The spoil is then removed from inside the caisson 76 (by jetting for example).

A rectangular crib 77 is next installed inside the caisson 76. The twenty-four sparging cells 70 in the group 60 can now be installed into the crib 77.

The spaces 79,80~which are in the path of the water flowing through the gate - should be filled with a packing material that will permit the waterto flow through, such as sand or gravel. The spaces 88, 89 on the other hand should be filled with bentonite or some other sealant. This will make sure that water that passes through the gate does so only by passing through the group of sparging cells.

The crib 77 and the caisson 76 should be well perforated to create as little interference as possible to the flow of water. Alternatively, the caisson 76 may be removed if suitably designed, once installation is complete.

Another way of arranging the cells in the group is shown in Fig. 4. Instead of a circular caisson, the gap in the wall is defined by temporary sheet piling 90.

The sparging cells 70 may be located between screens or perforated supporting walls 35. The piling 90 is removed after installation has been completed. Again, sand is used for the front and rear packing 79,80 and bentonite is used for the side sealant 88,89. The caisson 76 shown in Fig. 3 has a diameter of 1.5 meters, which accommodates the twenty-four cells 70 as described. This arrangement would be used when the gates 50 are spaced 10 metres apart or so along the wall 40. If a wider spacing of the gates is to be used, and the gates themselves consequently need to be wider, the Fig.

4 design would be more appropriate.

Many other designs of the gate are possible. In designing the arrangement to be used at a particular site, first a survey is carried out to determine the height, depth, and width of the plume; its volumetric flow rate and speed, and whether these vary-with the time of year for example. Then the survey has to determine what contaminants are present and in what quantities. The contaminants vary as to what are the amounts of sparge air needed to eliminate them. The amount of the contaminant that can legitimately be left in the water also depends, of course, on the nature of the contaminant.

Next, the designer must determine the quantities of sparge-air that will be needed to remove each of the contaminants down to acceptable limits. The designer then determines the size and spacing of the gates in the cut-off wall, making sure that the water-table is kept down. The sparge-air-flow, as a result, may be quite a gentle stream of bubbles, or the air-flow may be of such vigour that the cell is virtually boiling with bubbles.

In the Fig. 3 design of the gate, the water passes through four cells in series. It can be arranged that each cell removes say 90% of the contaminants from the water that passes through the cell.

Providing fresh air is used in each cell, each cell removes not the same quantity of contaminants, but more or less the same fraction or proportion of contaminants as the other cell, so that over the four cells some 99.99% of the contaminants will in that case be removed. Thus it may be regarded that the contaminants are reduced exponentially, as more cells are added to the series. In Fig. 4, the water passes through seven cells in series.

Claims (5)

1. Process for removing volatile contaminants from flowing groundwater on a substantially continuous basis, characterized by comprising the steps: of providing an in-situ sparging cell (70) comprising a tube (3) which is inserted deep down into the groundsoil from the surface, and of locating the tube (3) in the path (17) of the flowing water; where the tube (3) is formed in such a manner, and of such a material, that the groundwater may flow, substantially freely, through the walls of the tube (3) into and out of the tube (3); of providing a gas-discharge-port (7) deep down in the tube (3); of providing a source of (6) compressed gas at the surface; of leading the compressed gas from the source down to the gas-discharge-port (7);; of discharging the compressed gas from the gasdischarge-port (7) into the tube (3), in such a manner that bubbles travel upwards through the water in the tube (3) towards the surface; where the rate of discharge of the gas is great enough that the bubbles pick up a major proportion of the volatile contaminants from the water and convey them up the tube (3) to the surface; and where the rate of discharge of the gas is small enough that the bubbles do not substantially interfere with the flow of water through the tube (3).

2. Process of claim 1, comprising the further steps: of providing several such sparging cells (70); and of locating the cells so that the flowing, contaminated groundwater flows from cell to cell in series.

3. Process of claim 1, comprising the further steps: of providing a cut-off wall (40), which is inserted into the ground in the path (17) of the flowing water; of providing a gap or gate (50) in the wall (40), through which the water can flow; and of providing at least one of the said sparging cells (70) in the gate (50).

4. Process of claim 3, where the gate (50) is provided with several sparging cells (70).

5. Process of claim 4, where the wall (40) is provided with several such gates (50), each of which is provided with several such cells (70).