EP3634916A1

EP3634916A1 - Water purification

Info

Publication number: EP3634916A1
Application number: EP18729207.3A
Authority: EP
Inventors: Dionysius Cornelius Antonius Maria VAN OIRSCHOT
Original assignee: Rietland bvba
Current assignee: Rietland bvba
Priority date: 2017-05-22
Filing date: 2018-05-16
Publication date: 2020-04-15
Also published as: WO2018215888A1; BE1025249B1; BE1025249A1

Abstract

Device for treating wastewater, comprising: a substantially watertight tub; - an inlet for supplying wastewater to the tub; an outlet for discharging wastewater from the tub; granular substrate which substantially fills the tub, wherein the granular substrate comprises at least 90% granules with a granule size greater than 1 mm and smaller than 120 mm; - an air feed provided at the position of a bottom of the tub; wherein a pressure-distributing layer with a substantially flat finishing layer is provided on the granular substrate at the position of an upper segment of the tub, such that the device can be walked on.

Description

Water purification

The present invention relates to a device for treating wastewater, comprising:

a substantially watertight tub;

an inlet for supplying wastewater to the tub;

an outlet for discharging wastewater from the tub;

a granular substrate which substantially fills the tub, wherein the granular substrate comprises at least 90% granules with a granule size greater than 1 mm and smaller than 120 mm;

an air feed provided at the position of a bottom of the tub.

The invention further relates to a method for constructing such a device.

US 2003/0024874 describes a system and method for removing impurities from water. The principle of purifying water is here based on natural water purification using plants, particularly reed. Such systems are known as so-called wetlands. This device describes a surface which is provided with plants. The plants are placed on a granular substrate and are configured such that water can be supplied, can flow through the device and can then be discharged. When the water flows through the device, the water flows along the roots of the plants and along the granules of the granular substrate. The roots of the plants in particular are known to add oxygen to the water, so that aerobic bacterial growth is stimulated. At locations where no or less oxygen is present anaerobic bacterial growth will occur. The granules of the granular substrate have a surface to which the bacteria can adhere. The aerobic and anaerobic bacteria break down the contaminants from the water, so that the water is purified. The granules of the granular substrate provide here an adhesion surface for the bacteria, and at the same time provide for a water-permeability so that the water is able to flow between the granules along the bacteria.

The device of US 2003/0024874 describes a so-called wetland wherein air is supplied on the underside of the device. This improves the purification process of the water. Reed is provided on an upper side of the wetland.

A drawback of such devices is that a large surface area has to be available and is taken up by the device. Particularly in industrial areas, when a notably large flow rate of water must be purified, there is often insufficient space to construct such a device.

It is an object of the invention to provide a device for purifying water which fits more efficiently into the surrounding area.

The device for treating wastewater according to the invention is characterized for this purpose by a pressure-distributing layer with substantially flat finishing layer being provided on the granular substrate at the position of the upper segment of the tub, such that the device can be walked on. The invention is based on the insight that when air is supplied at the bottom of the device and moves through the granules of the granular substrate toward the water surface, oxygen can be added so as to stimulate bacterial growth without plants having to be provided for this purpose. Plants may be provided, but need not necessarily be provided in order to enable the water purification. The device according to the invention is provided here with a pressure-distributing layer with a finishing layer. Providing the pressure-distributing layer enables the granular substrate to be walked on. Because the top side of the device can be walked on, the space taken up by the device is multifunctional. In this respect it is noted that a traditional wetland is not multifunctional and functions only as water purification, wherein the top of the wetland is typically taken up by reed. In the device according to the invention the top side can be walked on, such that it can function as a path or seating area, or other surface which can be walked on.

The pressure-distributing layer and finishing layer can be formed in different ways, integrated or separately of each other. The pressure-distributing layer can also be formed by grass grids, wherein grass is provided as finishing layer. The grass grids can be walked on and the grass gives the device a natural appearance. Alternatively, the pressure-distributing layer and finishing layer can be embodied as a hardened surface, for instance tiles. The top layer of the tiles can here form the finishing layer, while the structure of the tile provides for the pressure distribution.

Because the device is multifunctional, the impact on the surrounding area of the device being arranged is considerably smaller than in the case of a wetland. The device according to the invention will hereby be considerably more widely applicable, and can be arranged more efficiently in the surrounding area.

The substantially watertight tub is preferably embodied as a combination of an excavation in the ground with a substantially watertight layer. The device according to the invention operates optimally when the water flows through the device slowly. A tub with a large volume is necessary in order to purify large flow rates of water with such a device. Such a tub with a large volume can be formed very cost-effectively by digging an excavation in the ground and providing the walls and the bottom of the excavation with a watertight layer. By digging an excavation in the ground and providing a watertight layer it is not necessary to provide a preformed tub. A preformed tub, particularly with large dimensions, is difficult to transport and is typically heavy and difficult to handle. The watertight tub embodied as a combination of an excavation in the ground with a watertight layer can be realized inexpensively and be made to size in simple manner. This further facilitates setting up of the device in the surrounding area, taking into consideration ambient factors such as available surface area and the form of this available surface area.

The finishing layer preferably extends at the height of the edges of the excavation so that the excavation is covered by the device. By providing the finishing layer at the height of the edges, the ground surface adjacently of the excavation will continue into the finishing layer of the device. Because the finishing layer can be walked on, a user will thus be able to walk from a position adjacently of the device to a position on the device without notable obstacles. Having the finishing layer extend at the height of the edges allows the device to be fully incorporated into the surrounding area. The visual impact of the device within the surrounding area will be zero particularly when the finishing layer of the device is chosen to at least locally correspond to or be at least partially the same as a finishing layer of the ground around the excavation.

The upper side of the pressure-distributing layer is preferably embodied as finishing layer. This simplifies installation of the device. Placing the pressure-distributing layer simultaneously provides the finishing layer.

The pressure-distributing layer with the substantially flat finishing layer is preferably configured to be passable by car. The skilled person is deemed to be familiar with structural requirements for making a surface passable for a car. When plastic grass grids are provided as pressure-distributing layer, DIN standard 53454 will for instance apply and the grass grids will preferably be able to withstand a load of at least 150 tons/m², preferably at least 250 tons/m². Tests performed in accordance with DIN 53454. Other standards, which are deemed known by the skilled person, will be relevant for other structures and/or other materials as pressure-distributing layer. When the pressure-distributing layer with the finishing layer is configured to be passable by car, the device can be provided under a car park. The upper side of the device can serve as ground surface for the car park. This further increases the possible applications of the device according to the invention. When the pressure-distributing layer is water-permeable, rainwater, which may or may not be contaminated with oil residues and so on of parked vehicles, will find its way directly into the device and be treated.

The pressure-distributing layer is preferably constructed with a plurality of rigid elements which are placed in a grid in order to cover substantially the whole upper side of the device. The grid here preferably has a plurality of rows and a plurality of columns, and the plurality of rigid elements can be connected to each other. Constructing the pressure-distributing layer as a grid provides increased flexibility in determining of the width and length of the device. Transporting of such rigid elements is also simple. Tests have shown that a pressure-distributing layer constructed with such multiple rigid elements provides sufficient stability to make it passable by foot and/or by car.

The plurality of rigid elements are preferably formed by grass grids. Grass can be provided on the grass grids in order to create an natural appearance. Grass grids are already known for their ability to be walked on and also to be driven on by cars. Grass grids are further air-permeable, such that the air supplied at the bottom of the device is able to escape through the grass grids. In this context it is noted that when a pressure-distributing layer and/or finishing layer is provided which is not air-permeable, air discharge elements can be provided at the position of an upper side of the device, under the pressure-distributing layer and/or finishing layer, in order to discharge air via for instance a flue.

The pressure-distributing layer preferably has a lower segment with crushed granules. Crushed granules are known to be water and air-permeable and to offer a greater resistance against sliding/displacing relative to each other. A more stable layer is hereby obtained.

The pressure-distributing layer is preferably air-permeable such that air, supplied into the tub via the air feed, can leave the device via the pressure-distributing layer. When the pressure- distributing layer is air-permeable, the air supplied at the bottom of the device can move upward in natural manner and leave the device at the top, via the pressure-distributing layer.

The watertight tub preferably has a surface area of at least 35 m², more preferably at least

50 m², most preferably at least 70 m². The device according to the invention is particularly suitable for purifying large flow rates and/or volumes of water.

The granule size is preferably greater than 4 mm, more preferably greater than 6 mm and the granule size is preferably smaller than 80 mm, more preferably smaller than 50 mm, most preferably smaller than 25 mm. Tests have shown that the granule size has an effect on the throughflow of water and air on the one hand and on the facilitating of the bacterial growth on the other hand.. The granule size ideally lies between 8 and 16 mm.

The outlet is preferably embodied as overflow to keep a water level in the device substantially constant. The volume of water in the device will hereby remain substantially constant.

The invention further relates to a car wash wherein at least a part of a washing zone is provided above a device according to the invention, wherein the pressure-distributing layer with the substantially flat finishing layer is water-permeable such that the inlet is at least partially formed by the pressure-distributing layer with finishing layer. A car wash has been found to be an ideal use for the device according to the invention. Cars can be washed on the device, wherein the dirty washing water falls directly onto the upper side of the device and finds its way via the upper side, through the water-permeable pressure-distributing layer, into the device. There, the washing water can be purified.

The invention further relates to a method for constructing a device according to the invention, wherein the method comprises of:

- digging an excavation in the ground;

providing a substantially watertight layer at the position of the inner surfaces of the excavation so that a substantially watertight tub is obtained;

providing an air feed on a bottom of the substantially watertight tub; filling the excavation with a granular substrate, wherein the granular substrate comprises at least 90% granules with a granule size greater than 1 mm and smaller than 120 mm; providing an inlet for supplying wastewater to the tub and an outlet for discharging wastewater from the tub;

characterized in that the method further comprises of providing a pressure-distributing layer with a substantially flat finishing layer on the granular substrate, such that the device can be walked on.

The method allows a device with the above described advantages and effects to be provided in cost-effective manner.

The invention will now be further described on the basis of an exemplary embodiment shown in the drawing.

In the drawing:

Figure 1 shows an installation for purifying water, embodied as prior art wetland 1 ;

Figure 2 shows a schematic cross-section of an installation for purifying water according to a first embodiment of the invention; and

Figure 3 shows a schematic cross-section of an installation according to a second embodiment of the invention.

The same or similar elements are designated in the drawing with the same reference numerals. .

Figure 1 shows a prior art wetland 1. Wetland 1 comprises a watertight tub 2, for instance formed by digging an excavation and placing a PE film against the walls and bottom of the excavation and thus making the walls of the excavation substantially watertight. The walls and bottom of the excavation, including the bottom, together form the watertight tub 2.

Provided at the bottom of watertight tub 2 is an air feed 3 for blowing air into the tub. Further provided at the bottom of the tub is a water discharge 4, embodied as drainage pipe, which is connected to an overflow 5 for discharging water from the tub. By making use of an overflow 5 for discharging water a substantially constant water level can be maintained in the tub. Provided at the top is a water supply 7, embodied as pipe system in order to distribute the water over the surface of wetland 1.

The tub is substantially wholly filled with gravel 1 1. Plants 6 are provided in the gravel at the top of the tub. Reed is a known plant for such installations for purifying water. The reed covers the installation substantially wholly and grows over the installation. For the sake of clarity the reed is only shown on the left-hand side in figure 1 , the whole installation will be overgrown in practice. The roots of the reed extend from the upper segment of the granular substrate 1 1 , this being the top layer of the granular substrate 1 1 , into a central segment of granular substrate 1 1 . The central segment of granular substrate 1 1 is the segment lying between the upper segment of granular substrate 1 1 and the lower segment of granular substrate 1 1. The lower segment of granular substrate 1 1 is defined here as the segment in which air feed 3 and/or the drainage pipe is provided. Figure 2 shows a cross-section according to a first embodiment of the invention, in this first embodiment the water for purifying is guided vertically through the device. The upper side of the device, preferably substantially the whole upper surface of watertight tub 2, is for this purpose configured as an inlet for water. This can be configured in different ways. According to a first embodiment, the pressure-distributing layer 9 with finishing layer 10 is water-permeable, such that the water can be added to watertight tub 2 via finishing layer 10. This is illustrated in figure 2 with arrows 8. This allows a user of the device to distribute the water over finishing layer 10 as they see fit so that it can enter in accordance with this distribution.

Alternatively, a water supply 7, and preferably a water manifold, can be provided under pressure-distributing layer 9 and/or under finishing layer 10, wherein water can be added. A pipe system can for instance be provided in an upper segment of granular substrate 1 1. This pipe system can be provided with holes, such that the pipe system can be pumped full of water for purifying, wherein the water flows via the holes into the device. A pretreatment system can be provided here before the water is transferred to the pipe system, so that impurities of a size which could block the holes are stopped by the pretreatment system. Examples of pretreatment systems are a septic tank, particulate filter, primary settling tank, lamella separator, grease trap or other type of pretreatment system. Figure 2 is based on a water-permeable pressure-distributing layer 9 with finishing layer 10.

Provided at the bottom of the structure is a water discharge 4. The water discharge 4 at the bottom is preferably formed by providing a set of drainage pipes at the position of a lower segment of granular substrate 1 1 and connecting this drainage pipe system to an overflow 5 for discharging water. As an alternative to a drainage pipe assembly, discharge openings can also be provided in a lower segment of granular substrate 1 1. These discharge openings can then be connected to water pumps, which preferably discharge water in controlled manner.

Further provided in the lower segment of granular substrate 1 1 is an air feed 3. Air feed 3 is preferably connected to a compressor or blower for supplying compressed air to air feed 3. In a preferred embodiment air feed 3 is embodied as a pipe system wherein holes (not shown) through which air can escape are provided in the pipes at regular distances. Alternatively, as shown in figure 2, air feed 3 is formed by a plurality of air supply elements. The compressor preferably supplies a predetermined flow rate of air. The air will typically move upward in bubbles when the air is added in the lower segment. In this context it is noted that the lower and middle segment of granular substrate 1 1 are typically submerged. As an alternative to a pipe system, air injectors can be provided in the device at predetermined positions.

Air feed 3 can optionally be placed higher than water discharge 4, and preferably higher than 20 cm, more preferably higher than 30 cm relative to water discharge 4. By providing air feed 3 above water discharge 4 in a device wherein the water is provided to move vertically through the device, the water above air feed 3 will mainly come into contact with aerobic bacteria, while the water below air feed 3 will mainly come into contact with anaerobic bacteria. The water can hereby be purified by a combination of aerobic and anaerobic bacteria. Air feed 3 can further be distributed and positioned in the device such that it is possible to identify the zones in the device in which more air is added than in other zones. Alternatively, the air can be supplied intermittently by alternately switching the compressor or blower on and off. This can also be used to optimize the difference in water purification by aerobic and anaerobic bacteria in relation to the water flow through the device and/or in relation to the time. The distributing of the air supply in space and/or time can further be used to adjust the aeration to a possibly variable supply of wastewater. This can save energy. The intensity of aeration can further be varied by providing a plurality of

independently controllable aerating networks which may or may not overlap.

At the position of the top segment of the device, the device comprises a pressure- distributing layer 9 with a finishing layer 10. In the embodiment shown in figure 2 the pressure- distributing layer 9 is formed by grass grids. Grass grids are plastic open structures of modular construction which can be coupled to each other in order to cover a surface. The grass grids are configured here to distribute pressure. Because the grass grids are open structures, water can enter through the grass grids in simple manner. The air which is supplied at the bottom of the device via air supply 3 can further easily escape from the device via the grass grids.

In figure 2 finishing layer 10 is formed by grass sown in and/or on the grass grids. It is optionally possible here to fill the grass grids with a soil substrate. The soil substrate thereby forms together with the grass the finishing layer 10 and ensures that the grass is able to take root.

Pressure-distributing layer 9 is optionally provided at the bottom with crushed granules 12.

Crushed granules 12 typically have a higher resistance to sliding, whereby a more stable layer can be obtained with crushed granules 12 than with whole granules 12. A layer of crushed granules 12 typically has a higher density and greater strength, whereby it is suitable to be placed at the bottom of a pressure-distributing layer 9.

Granular substrate 1 1 forms the bulk material for filling watertight tub 2. In the broadest sense, granular substrate 1 1 can be formed by any substrate which allows water and air to pass and with which an excavation can be filled. Granular substrate 1 1 can preferably be formed by gravel, by plastic granules or by another type of granular material. The granules serve on the one hand as a passage for water and air and on the other hand as adhering surface for bacteria.

The watertight tub 2 of figure 2 is formed by a combination of an excavation in ground 13 with a watertight layer 14 at the position of the walls of the excavation. The walls are here deemed to comprise the bottom. Examples of watertight layers which can be used for sealing walls of an excavation are PE, preferably PE film. Alternatively, PVC or EPDM film can be applied. Tests have been performed with flexible polypropylene (FPP) as watertight layer. As further alternative, a concrete floor and concrete walls can be poured. As further alternative, clay mats having a high degree of watertightness can be provided. The above stated examples are not limitative and the skilled person will appreciate that many options can be envisaged for making walls and bottom of an excavation substantially watertight. It is noted here that it is not essential for the walls to be 100% watertight, but the walls do have to be able to fulfil a function of retaining sufficient water in the tub and of admitting a controlled inflow and outflow of water into and out of the tub. Minimal leakage through the walls of the device does not detract from the operation of the device.

Finishing layer 10 of the device of figure 2 is provided at the height of ground level 15 of the surrounding area. In the case of a flat surrounding area the ground level 15 of the surrounding area will therefore preferably continue substantially without interruption into the finishing layer 10 of the device. In the case of a sloping surrounding area it will be apparent that only a portion of the surrounding area will come to lie at the height of finishing layer 10 of the device. One edge or a segment of one or more edges can thus for instance be provided continuing into the ground surface.

Figure 3 shows an alternative embodiment of a device according to the invention. The device according to figure 3 is embodied as horizontal flow-through installation. Water supply 7 is here arranged on one side (in figure 3 on the left-hand side) or at the position of one edge of the device, while water discharge 4 is arranged on another side (in figure 3 the right-hand side) or opposite edge of the device. This allows water supply 7 and water discharge 4 to be arranged differently than described above. Water supply 7, also referred to as inlet, can thus be embodied as one or more water supply pipes or channels which are connected to the device above the water surface of the device or below the water surface of the device so that water can be supplied to the device at the position of one side or edge. Water discharge 4, also referred to as outlet, will in such a configuration not extend at the position of a lower side of the device, or do so to lesser extent, but will extend at the position of an opposite side, wherein the discharge can for instance be provided directly as overflow 5.

In the device of figure 3 pressure-distributing layer 9 with finishing layer 10 is provided as tile paving. A series of tiles is laid on granular substrate 1 1 , optionally with a layer of crushed granules 12 between granular substrate 1 1 and the tiles, wherein the tiles have both a pressure- distributing function and a finishing function. The tiles can be provided with water-permeable and air-permeable joins. This allows the device of figure 3 to operate according to the principles as described above with reference to figure 2. Alternatively, pressure-distributing layer 9 and finishing layer 10 are not water-permeable and not air-permeable, or negligibly so. Because air is supplied at the bottom of the device, an air discharge 16 will have to be provided under pressure- distributing layer 9 and/or finishing layer 10 for the purpose of discharging the air. Collectors or collecting points in which air is collected to be discharged can for instance be provided at the top of granular substrate 1 1. As further alternative, the device can be embodied such that the water level comes to lie several centimetres below the underside of pressure-distributing layer 9 and/or finishing layer 10 by placing overflow 5 at a predetermined height, such that air is collected at the top of the device and can there be discharged from the upper side via a horizontal displacement on a predetermined side or at a predetermined location. This allows the device to be arranged under for instance an underground car park or under a building. In a device of figure 3 zones in which air is supplied at the bottom of the device, as seen in the flow direction of the water, can be alternated with zones in which no or less air is supplied. Zones with aerobic bacteria and zones with anaerobic bacteria are hereby obtained.

A device as shown in figures 2 and 3 is preferably constructed in situ. The device is here preferably constructed from the bottom up, starting with digging an excavation and forming watertight tub 2 by placing the watertight material on the walls of the excavation, thus forming watertight tub 2. Water discharge 4 or air feed 3 is then placed depending on the type of device, horizontal or vertical throughflow of water. Each of the water discharge 4 and air feed 3 can be placed on a layer of granular substrate 1 1 such that a distance is created between the bottom of watertight tub 2 and air feed 3 and/or water discharge 4 by the layer of granular substrate 1 1. The middle segment of watertight tub 2 is then typically filled with the granular substrate 1 1.

Depending on whether an air discharge 16 and/or water supply 7 has to be provided, these are placed. A layer of crushed granular substrate 1 1 is then optionally put down, on which a pressure- distributing layer 9 with finishing layer 10 is placed. Water discharge 4 is typically connected to an overflow 5 in order to achieve a substantially constant water level in the device. It may further be necessary when starting up the device to add one or more predetermined bacterial cultures to the device, so that they can adhere to the surfaces of the granules in the device. The bacteria feed on the impurities in the water, whereby the bacteria multiply and whereby the water is purified.

The method for forming the device, and the manner in which the water is purified in the device, is very environmentally friendly and inexpensive. Because the device can be built into a surrounding area in flexible manner as a result of its pressure-distributing layer 9 with finishing layer 10, the device has considerable advantages relative to existing water purification installations.

The skilled person will appreciate on the basis of the above description that the invention can be embodied in different ways and on the basis of different principles. The invention is not limited here to the above described embodiments. The above described embodiments and the figures are purely illustrative and serve only to increase understanding of the invention. The invention is not therefore limited to the embodiments described herein, but is defined in the claims.

Claims

1. Device for treating wastewater, comprising:

a substantially watertight tub (2);

- an inlet for supplying wastewater to the tub;

an outlet for discharging wastewater from the tub;

granular substrate (1 1 ) which substantially fills the tub, wherein the granular substrate (1 1) comprises at least 90% granules with a granule size greater than 1 mm and smaller than 120 mm;

- an air feed (3) provided at the position of a bottom of the tub;

characterized in that a pressure-distributing layer (9) with a substantially flat finishing layer (10) is provided on the granular substrate ( 1 1) at the position of an upper segment of the tub, such that the device can be walked on.

2. Device for treating wastewater according to claim 1, wherein the substantially watertight tub (2) is embodied as a combination of an excavation in the ground (13) with a substantially watertight layer (14).

3. Device for treating wastewater according to claim 2, wherein the finishing layer ( 10) extends at the height of edges of the excavation so that the excavation is covered by the device.

4. Device for treating wastewater according to any of the foregoing claims, wherein the upper side of the pressure-distributing layer (9) is embodied as finishing layer (10).

5. Device for treating wastewater according to any of the foregoing claims, wherein the pressure-distributing layer (9) with the substantially flat finishing layer (10) is configured to be passable by car.

6. Device for treating wastewater according to any of the foregoing claims, wherein the pressure-distributing layer (9) is constructed with a plurality of rigid elements which are placed in a grid in order to cover substantially the whole upper side of the device.

7. Device for treating wastewater according to claim 6, wherein the grid has a plurality of rows and columns and wherein the plurality of rigid elements can be connected to each other.

8. Device for treating wastewater according to claim 6 or 7, wherein the plurality of rigid elements are formed by grass grids.

9. Device for treating wastewater according to any of the foregoing claims, wherein the pressure-distributing layer (9) has a lower segment with crushed granules ( 12).

10. Device for treating wastewater according to any of the foregoing claims, wherein the pressure-distributing layer (9) is air-permeable such that air, supplied into the tub via the air feed (3), can leave the device via the pressure-distributing layer (9).

1 1. Device for treating wastewater according to any of the foregoing claims, wherein the watertight tub (2) has a surface area of at least 35 square metres.

12. Device for treating wastewater according to any of the foregoing claims, wherein the granule size is greater than 4 mm, preferably greater than 6 mm, and wherein the granule size is smaller than 80 mm, preferably smaller than 50 mm, more preferably smaller than 25 mm.

13. Device for treating wastewater according to any of the foregoing claims, wherein the outlet is embodied as overflow (5) to keep a water level in the device substantially constant.

14. Car wash, wherein at least a part of a washing zone is provided above a device according to any of the foregoing claims, wherein the pressure-distributing layer (9) with the substantially flat finishing layer (10) is water-permeable (8) such that the inlet is at least partially formed by the pressure-distributing layer (9) with finishing layer (10).

15. Method for constructing a device according to any of the foregoing claims, wherein the method comprises of:

digging an excavation in the ground (13);

providing a substantially watertight layer (14) at the position of the inner surfaces of the excavation so that a substantially watertight tub (2) is obtained;

providing an air feed (3) on a bottom of the substantially watertight tub (2);

filling the excavation with a granular substrate (1 1), wherein the granular substrate ( 1) comprises at least 90% granules with a granule size greater than 1 mm and smaller than 120 mm;

providing an inlet for supplying wastewater to the tub and an outlet for discharging wastewater from the tub;

characterized in that the method further comprises of providing a pressure-distributing layer (9) with a substantially flat finishing layer (10) on the granular substrate (1 1), such that the device can be walked on.