WO2019034632A1

WO2019034632A1 - Apparatus and method for filtering aqueous liquid

Info

Publication number: WO2019034632A1
Application number: PCT/EP2018/071975
Authority: WO
Inventors: Debosree CHATTERJEE; Ganesan RAJENDIRAN
Original assignee: Unilever N.V.; Unilever Plc; Conopco, Inc., D/B/A Unilever
Priority date: 2017-08-15
Filing date: 2018-08-14
Publication date: 2019-02-21
Also published as: CN212214709U

Abstract

The invention relates to an apparatus for the filtration of aqueous liquid that is capable of effectively removing suspended particles from aqueous liquid during a prolonged period of time without clogging of the particulate filter media and associated pressure build-up. This filtration apparatus comprises: - a filtration chamber comprising an entry opening that is located near the bottom of the filtration chamber and that is in fluid communication with the inlet for the stream of aqueous liquid, and an exit opening that is located near the top of the filtration chamber; - a screen covering the exit opening of the filtration chamber; - a particulate filter media that partially fills the interior of the filtration chamber, said particulate filter media having a particle size that exceeds the mesh size of the screen and having a specific gravity of 1.5-5 g/ml; wherein the apparatus is arranged to operate in a filtration mode that is regularly interrupted by a cleaning mode in which retentate is removed from the filter media.

Description

APPARATUS AND METHOD FOR FILTERING AQUEOUS LIQUID Field of the invention

The present invention relates to an apparatus for filtering aqueous liquid, which apparatus comprises a filtration chamber that is partially filled with particulate filter media. The apparatus according to the invention is capable of effectively removing suspended particles from aqueous liquid during a prolonged period of time without clogging of the particulate filter media and associated pressure build-up. The apparatus of the present invention is particularly suitable for clarifying source water before that water is used for washing laundry or dishes.

The invention further relates to a method of filtering aqueous liquid by passing a stream of aqueous liquid through a filtration chamber that is partially filled with particulate filter media.

Background of the invention

Water is one of the necessities of human life. Water is important for drinking, cooking food and for preparing beverages. Water is also very important for cleaning items used in the household like clothes, utensils and surfaces, e.g floors and table tops. Further, a large amount of water is used for personal cleaning in the bathrooms and toilets. The quality of water required for each of the above is different.

There are billions of people, especially in the underdeveloped and developing countries, especially in the rural areas, who do not have purified water piped to their homes.

Modern day urban households receive water from large lakes through a network of pipes, while in some rural areas villagers fetch water directly from sources like wells, lakes, rivers and bore wells. Water directly taken from such surface and underground sources are generally more impure as compared to piped water which has been treated by the municipality. Water from such sources is usually turbid and impure and hampers cleaning performance of detergents and personal care products and hence needs to be clarified before use. Several types of water purification systems are known and available. They can be broadly classified into thermal, radiation, chemical and physical methods of purification or a combination of one or more of these methods.

Filtration is a well-known physical method for removing suspended particles from water. Depth filters, i.e. filters that use a porous filtration medium to retain particles throughout the medium, are widely used to filter water. Depth filters offer the advantage that they are able to retain a high quantity of particles without compromising the separation efficiency and that they can be operated at high filtration rates. US 6,1 10,389 describes an apparatus for purification of water, the apparatus

Comprising:

• a filter chamber;

• a water inlet through which unpurified water enters the chamber;

• a water outlet through which purified water exits the chamber;

· a filter system including:

a first filtration screen disposed within the chamber adjacent the water inlet, a second filtration screen disposed within the chamber adjacent the water outlet, and

filtration media trapped between the first and second screens, the water inlet, water outlet, and first and second filtration screens configured such that unpurified water entering the tank through the water inlet passes through the first filtration screen, the filtration media and the second filtration screen before exiting the tank through the water outlet;

• a source of cleaning liquid connected with the chamber for selectively supplying cleaning liquid to the chamber to clean the filtration media;

• a drain outlet for selectively draining water from the chamber, the drain outlet sized for rapidly draining the tank to clean the filter system; and • at least one nozzle which selectively directs the cleaning liquid into the chamber wherein the at least one nozzle includes a plurality of spaced nozzles.

EP-A 2 070 574 describes a device for filtering liquid, said device comprising a filter chamber with an inlet and an outlet, said filter chamber comprising particulate filter media, wherein the flow of the liquid through the filter chamber is in a direction that is opposite to the direction in which the particulate filter media moves through said liquid due to a density difference. Summary of the invention

The inventors have designed an apparatus for the filtration of aqueous liquid that is capable of effectively removing suspended particles from aqueous liquid during a prolonged period of time without clogging of the particulate filter media and associated pressure build-up.

The apparatus of the present invention employs a depth filter that is made up of particles having a specific gravity of 1.5-5 g/ml. The apparatus comprises a filtration chamber that is only partially filled with the particulate filter media. Aqueous liquid to be filtered flows through the filtration chamber in an upward direction with a high flow rate that causes the formation of a filter bed of particulate filter media against a screen that is located at the upper end of the filtration chamber (filration mode).

The filtration apparatus of the present invention is designed to be alternatingly operated in filtration mode and cleaning mode. When the apparatus switches from filtration mode to cleaning mode, the flow rate of aqueous liquid through the filtration chamber is reduced and the particles of filter media are allowed to sink to the bottom of the filtration chamber. The layer of dirty water that is subsequently formed above the settling/settled filter media is discharged via a waste outlet, after which the apparatus can return to filtration mode. The dirty water that is discharged in this manner contains particulate material that was retained by the filter bed (retentate) during filtration mode. By regularly switching the apparatus from filtration mode to the cleaning mode, retentate is repeatedly removed from the filter bed and clogging of the filter bed is effectively prevented. Accordingly, the invention provides an apparatus for filtering aqueous liquid, said apparatus comprising:

• an inlet for a stream of aqueous liquid;

• a first outlet for a filtered stream of aqueous liquid;

· a second outlet for a waste stream of aqueous liquid

• a filtration chamber comprising an entry opening that is located near the bottom of the filtration chamber and that is in fluid communication with the inlet for the stream of aqueous liquid, and an exit opening that is located near the top of the filtration chamber;

· a screen covering the exit opening of the filtration chamber;

• a particulate filter media that partially fills the interior of the filtration chamber, said particulate filter media having a particle size that exceeds the mesh size of the screen and having a specific gravity of 1 .5-5 g/ml, said particulate filter media being composed of inorganic material; and

· a means that regulates the flow rate of aqueous liquid through the filtration

chamber and that can operate in a high flow rate mode or a reduced flow rate mode;

wherein said high flow rate causes the formation of a filter bed of particulate filter media against the screen; said filter bed taking up 10-80% of the internal volume of the filtration chamber; and said reduced flow rate causes the filter bed of particulate filter media to break up and move from the screen towards the bottom of the filtration chamber.

Furthermore, the invention provides a method for filtering a stream of aqueous liquid, said method comprising passing the stream of aqueous liquid through the aforementioned apparatus, said method comprising an alternating sequence of the following steps a. and b. comprising:

a. passing the stream of aqueous liquid with a high flow rate in upward direction through a filtration chamber that is partially filled with particulate filter media, said stream entering the filtration chamber through opening near the bottom of the filtration chamber and leaving the filtration chamber as a filtered stream near the top of the filtration chamber through an opening that is covered by a screen, said filtered stream being discharged through a first outlet for filtered aqueous liquid; said high flow rate causing the formation of a filter bed of particulate filter media against the screen;

said filter bed taking up 10-80% of the internal volume of the filtration chamber; said particulate filter media having a specific gravity of 1 .5-5 g/ml;

b. interrupting the flow of the stream of aqueous liquid to cause the filter bed of particulate filter media to break up and move from the screen towards the bottom of the filtration chamber, and subsequently reinstating the flow of aqueous liquid and discharging a stream of waste liquid through a secondoutlet for waste liquid; or reducing the flow rate of the stream of aqueous liquid to a low flow rate to cause the filter bed of particulate filter media to break up and move from the screen towards the bottom of the filtration chamber, and discharging a stream of waste liquid through a second outlet for waste liquid.

Figures

Figure 1 provides a cross-sectional view of a filtration apparatus according to the present invention, operated in filtration mode. The filter media forms a filter bed against the screen in the top of the filtration chamber as it is pushed upwards by the incoming flow of aqueous liquid. Filter liquid is leaving the apparatus through an outlet for filtered water.

Figure 2 provides a cross-sectional view of a filtration apparatus according to the present invention, operated in cleaning mode (first stage). The flow of aqueous liquid is interrupted and the filter media has descended towards the bottom of the filtration chamber. Above the filter media a layer of waste water has formed.

Figure 3 provides a cross-sectional view of a filtration apparatus according to the present invention, operated in cleaning mode (second stage). The flow of aqueous has been reinstated. The layer of waste water is exiting the filtration chamber and is discharged via a separate outlet for waste water.

Detailed description of the invention

Accordingly, a first embodiment of the present invention relates to an apparatus (1 ) for filtering aqueous liquid, said apparatus (1 ) comprising: • an inlet for a stream of aqueous liquid (2);

• a first outlet for a filtered stream of aqueous liquid (3);

• a second outlet for a waste stream of aqueous liquid (1 1 );

• a filtration chamber (4) comprising an entry opening (5) that is located near the bottom of the filtration chamber and that is in fluid communication with the inlet for the stream of aqueous liquid (2), and an exit opening (6) that is located near the top of the filtration chamber (4);

• a screen (7) covering the exit opening (6) of the filtration chamber (4);

• a particulate filter media (8) that partially fills the interior of the filtration

chamber, said particulate filter media (8) having a particle size that exceeds the mesh size of the screen (7) and having a specific gravity of 1 .5-5 g/ml, said particulate filter media (8) being composed of inorganic material; and

• a means (9) that regulates the flow rate of aqueous liquid through the filtration chamber (4) and that can operate in a high flow rate mode or a reduced flow rate mode;

wherein said high flow rate causes the formation of a filter bed of particulate filter media (8) against the screen (7); said filter bed taking up 10-80% of the internal volume of the filtration chamber (4); and said reduced flow rate causes the filter bed of particulate filter media (8) to break up and move from the screen (7) towards the bottom of the filtration chamber (4),

wherein the apparatus (1 ) is arranged to alternatingly operate in:

a) a filtration mode in which the aqueous liquid is passed through the apparatus (1 ) at a high flow rate and in which the inlet (2) is connected via the filtration chamber (4) and the screen covered exit opening (6) to the first outlet for the filtered stream of aqueous liquid (3); and

b) a cleaning mode in which either

i. the passage of the aqueous liquid through the apparatus (1 ) is first

interrupted and then the aqueous liquid is passed through the apparatus (1 ) at high flow rate and in which the inlet (2) is connected via the filtration chamber (4) to the second outlet for the waste stream of aqueous liquid (1 1 ) when the aqueous liquid is passed through the apparatus (1 ) at high flow rate; or ii. the aqueous liquid is passed through the apparatus (1 ) at a low flow rate and in which the inlet (2) is connected via the filtration chamber (4) to the second outlet for the waste stream of aqueous liquid (1 1 ). In filtration mode, the aqueous liquid that is to be filtered enters the apparatus (1 ) through the inlet (2), passes through the entry opening (5) to enter the filtration chamber (4) and flows through the filtration chamber (4) in an upwards direction, and passes through a filter bed of filter media (8) that is formed against the screen (7), before exiting the apparatus (1 ) through the outlet (3). During operation in filtration mode, particles contained in the aqueous liquid are retained by the filter bed. The longer the apparatus is operated in filtration mode, the more retentate is collected by the filter bed. Eventually, this accumulation of retentate will impair the filtration capacity of the apparatus (1 ).

In cleaning mode, the filter bed is broken up and allowed to settle down at the bottom of the filtration chamber (4). This is achieved by reducing or interrupting the flow rate of aqueous liquid through the filtration chamber (4). Whilst the particles of filter media (8) are moving downwards through the filtration chamber (4), retained particulate material is released into the surrounding liquid. This released retentate material remains suspended in the aqueous liquid and as a result during settling of the filter media (8), a layer of dirty aqueous liquid (18) is formed in the upper part of the filtration chamber (4). In the cleaning mode, this layer of dirty aqueous liquid (18) is discharged from the apparatus (1 ) by introducing aqueous liquid through the inlet (2) and passing the aqueous liquid through the entry opening (5). Thus, the supernatant layer of retentate containing waste liquid (18) is replaced by the incoming aqueous liquid and leaves the filtration chamber (4) near the top of the chamber, to be discharged through the waste outlet (1 1 ).

The cleaning mode can be operated in two different basic manners, i.e. (i) an "interrupted flow cleaning mode" or (ii) a "reduced flow cleaning mode". In the interrupted flow cleaning mode, the flow of aqueous liquid through the apparatus is interrupted and the filter bed is allowed to break up and to descend to the bottom of the filtration chamber (4). Once the filter media (8) has settled on the bottom of the filtration chamber (4), the flow of aqueous liquid is reinstated at a high flow rate and the supernatant layer of waste liquid (18) is pushed out of the filtration chamber (4) and exits the apparatus through waste outlet (1 1 ). Simultaneously, a filter bed of filter media (8) is re-formed against screen (7). Shortly after the filter bed is back in place, the apparatus switches back to filtration mode.

In the reduced flow cleaning mode, the flow rate of aqueous liquid is reduced sufficiently, to allow the filter bed to break up and to descend to the bottom of the filtration chamber (4) and, simultaneously, the waste water that forms above the descending filter bed is discharged through waste outlet (1 1 ). Once the waste water is discharged through the waste outlet (1 1 ), the apparatus (1 ) is switched back to filtration mode.

The interrupted flow cleaning mode is the preferred cleaning mode. The interruption of the flow of aqueous liquid and the reinstatement of the flow of aqueous liquid that occur in this particular cleaning mode may suitably be achieved by interrupting and reinstating the external supply of aqueous to the apparatus (1 ), e.g. by closing and opening a tap.

In one embodiment of the present apparatus (1 ), both in filtration mode and cleaning mode the aqueous liquid leaves the filtration chamber (4) via the same exit opening (6). In this embodiment, the apparatus preferably comprises a valve located downstream of the exit opening (6), which valve is capable of directing the stream of filtered aqueous liquid to the first outlet (3) when the apparatus (1 ) is operated in filtration mode and to the second outlet (1 1 ) when the apparatus (1 ) is operated in cleaning mode. Examples of valves that may be used include two-port valves and flow selector valves. The two-port valve is positioned downstream of the exit opening (6) and directs the stream of filtered aqueous liquid to the first outlet (3) when the apparatus (1 ) is operated in filtration mode and to the second outlet (1 1 ) when the apparatus (1 ) is operated in cleaning mode. The two-port valve may be positioned in the flow path to the first outlet (3) or in the flow path of the second outlet (1 1 ).

According to a particularly preferred embodiment, the two-part valve is located in the flow path to the second outlet (1 1 ) and said two-port valve preferably is an excess flow valve (20). This excess flow valve (20) controls the flow through the second outlet (1 1 ). The second outlet (1 1 ) preferably has a cross-sectional flow area that is substantially larger than the cross-sectional flow area of the first outlet (3). Thus, if the excess flow valve (20) is opened, the flow of aqueous liquid is effectively diverted from the first outlet (3) to the second outlet (1 1 ). When the apparatus (1 ) is operated in filtration mode, the excess flow valve (20) is closed and the stream of filtered aqueous liquid exits the apparatus (1 ) through the first outlet (3). During the first stage of interrupted flow cleaning mode, the excess flow valve (20) is opened, e.g. triggered by the sudden decrease in pressure or flow rate that is caused by the interruption of flow of aqueous liquid. During the second stage of the cleaning mode, which starts with the reinstatement of the flow of aqueous liquid, the excess flow valve (20) remains open and a stream of waste liquid is discharged through the second outlet (1 1 ). The apparatus returns to filtration mode by closing of the excess flow valve (20) which can be triggered by a sudden increase in flow rate or pressure. In the following table the embodiment that utilizes interrupted flow cleaning mode is schematically described, assuming that a tap is used to control the flow through the filtration apparatus (1 )-

In an alternative embodiment, a flow selector valve is positioned downstream of the exit opening (6), said flow selector valve comprising an entry port that is in fluid communication with the filtration chamber (4); a first exit port that is in fluid communication with the first outlet for a filtered stream of aqueous liquid (3) and a second exit port that is in fluid communication with the second outlet for a waste stream of aqueous liquid (1 1 ); and wherein the entry port of the flow selector valve is in fluid communication with the first exit port when the apparatus (1 ) is operated in filtration mode and wherein the entry port of the flow selector valve is in fluid communication with the second exit port when the apparatus (1 ) is operated in cleaning mode.

In the apparatus designed to employ the reduced flow cleaning mode, preferably the flow selector valve is triggered to establish a fluid communication between the entry port and the second exit port when the flow rate of the aqueous liquid is reduced to low flow rate (cleaning mode) and the flow selector valve is triggered to establish a fluid communication between the entry port and the first exit port when the flow rate of the aqueous liquid is increased to high flow rate (filtration mode).

In the apparatus designed to employ the interrupted flow cleaning mode, preferably the flow selector valve is triggered to establish a fluid communication between the entry port and the second exit port during the period starting with interruption of the flow of the aqueous liquid and ending shortly after the increase of the flow of the aqueous liquid to the high flow rate (cleaning mode), and the flow selector valve is triggered to establish a fluid communication between the entry port and first exit port 0.01 to 5 seconds after said increase of flow to high flow rate (filtration mode). In the following table the embodiment that utilizes a flow selector to operate in interrupted flow cleaning mode is schematically described, assuming that a tap is used to control the flow through the filtration apparatus (1 ).

In an alternative embodiment of the apparatus (1 ), the apparatus is arranged to remove the stream of waste liquid directly from the filtration chamber through a separate exit opening. In accordance with this embodiment the filtration chamber (4) comprises two exit openings (6a, 6b), each located near the top of the chamber. The first exit opening (6a) is in fluid communication with the first outlet for filtered aqueous liquid (3) and the second exit opening (6b) is in fluid communication with the second outlet for waste liquid (1 1 ). The apparatus (1 ) comprises a two-port valve that is positioned in the flow path to the first exit opening (6a) or in the flow path to the second exit opening (6b) and that is capable of controlling the flow of aqueous liquid through exit opening (6a) and/or exit opening (6b). According to a particularly preferred embodiment, the two-part valve is located in the flow path to the second exit opening (6b) and said two-port valve preferably is an excess flow valve. This excess flow valve controls the flow through the second exit opening (6b) and the second outlet (1 1 ). When the apparatus (1 ) is operated in filtration mode, the excess flow valve is closed. During cleaning mode, the excess flow valve is opened and the waste liquid exits the filtration chamber (4) through the second exit opening (6b) and is discharged through the second outlet (1 1 ). Opening of the excess flow valve can be triggered if, for instance, the flow or pressure exceeds a preset limit. Closing of the excess flow valve can be triggered if, for instance, the flow or pressure drops below a preset limit.

It is an essential element of the present invention that the filtration chamber (4) of the apparatus is only partially filled with the particulate filter media (8). Preferably, the particulate filter media (8) fills 10-80%, more preferably 20-70% and most preferably 30- 60% of the internal volume of the filtration chamber (4) when it is allowed to form a bed under the influence of gravity.

The particulate filter media (8) that is employed in accordance with the present invention preferably is composed of relatively large particles as this minimizes undesired clogging. The particulate filter media (8) that is contained in the filtration chamber preferably has a mass weighted average particle size between 100 and 900 microns, more preferably between 150 and 700 microns, most preferably between 200 and 600 microns. The specific gravity of the particulate filter media (8) preferably lies in the range of 2-4 g/ml, more preferably in the range of 2.1-3.2 g/ml and most preferably in the range of 2.2-2.9 g/ml.

The particulate filter media (8) that is employed in accordance with the present invention preferably comprises at least 50 wt.%, more preferably at least 80 wt.% and most preferably at least 90 wt.% of inorganic material. This inorganic material is preferably selected from aluminosilicates, silica, calcium carbonate, magnesium carbonate and combinations thereof. The inventors have found that these inorganic materials aid the formation of calcium carbonate particles, thereby contributing to the effective softening of the aqueous liquid in case a calcium sequestrant such as sodium carbonate is added shortly before the liquid enters the filtration chamber.

In accordance with a particularly preferred embodiment, the particulate filter media (8) contains at least 50 wt.%, more preferably at least 80 wt.% and most preferably at least 90 wt.% aluminosilicates.

The mesh size of the screen (7) should be sufficiently small to retain the particulate filter media (8) inside the filtration chamber (7). At the same time, the mesh size should be sufficiently large to allow the aqueous liquid to pass the screen (7) without difficulty. Preferably, the mesh size of the screen (7) is in the range of 30 to 200 microns, more preferably in the range of 40 to 150 microns and most preferably in the range of 50 to 100 microns. In a preferred embodiment, the filtration chamber (4) contains yet another screen that covers the entry opening (5) of the filtration chamber (4). This additional screen should prevent particulate filter media (8) from leaving the filtration chamber (4), especially in case of backflush (pressure differential between upstream & downstream of the bed). The filtration chamber (4) of the apparatus (1 ) typically contains 0.025-0.5 kg of the particulate filter media (8). More preferably, the amount of filter media (8) in the filtration chamber (4) is 0.05-0.2 kg

The filtration chamber (4) preferably has an internal volume of 0.05-5L, more preferably of 0.075-2.5 L and most preferably of 0.1-1 L.

The present apparatus (1 ) is particularly suited for use in households. The apparatus (1 ) can suitably be used to filter source water as well as greywater. The filtration chamber (4) of the present apparatus (1 ) preferably comprises a drain opening (17) that is located near the bottom of the filtration chamber (4). Flow through this drain opening (17) may be regulated by a drain valve. According to a particularly preferred embodiment, the filtration apparatus (1 ) comprises coupling means (10) for attaching the apparatus (1 ) to a water tap (9) so that tap water can enter the apparatus through the inlet (2). According to a particularly preferred embodiment of the present invention, the apparatus (1 ) comprises a first dosing unit (12) that is located upstream of the filtration chamber (4) and that is adapted to release water-soluble components into the stream of aqueous liquid that flows from the inlet (2) to the filtration chamber (4) . This first dosing unit (12) may suitable be used to introduce a calcium sequestrant into the aqueous stream that rapidly forms particles of water-insoluble calcium complex (e.g. water insoluble calcium salt) that are subsequently removed from the aqueous stream in the filtration chamber (4). Typical examples of such a calcium sequestrants are sodium carbonate, potassium carbonate and combinations thereof. According to another preferred embodiment, the apparatus (1 ) comprises a second dosing unit (15) that is located downstream of the filtration chamber and that is adapted to release water-soluble components into the stream of aqueous liquid that flows from the filtration chamber (4) to the outlet (3). Examples of water-soluble components that may suitably be introduced into the aqueous stream from this second dosing unit (15) include acidulants, fragrances, silicones, bactericide and combinations thereof.

In accordance with a particularly preferred embodiment, the first dosing unit (12) is adapted to release sodium carbonate and/or potassium carbonate and the second dosing unit is adapted to release acidulant (15). This arrangement offers the advantage that the pH of the filtered aqueous liquid can be kept near neutral pH.

The filter apparatus (1 ) of the present invention is particularly suited for use in a filtering method as described hereinafter. Another embodiment of the present invention relates to a method of filtering aqueous liquid, said method comprising an alternating sequence of filtration steps a. and cleaning steps b.:

a. passing a stream of aqueous liquid with a high flow rate in upward direction through a filtration chamber (4) that is partially filled with particulate filter media (8), said stream entering the filtration chamber (4) through entry opening (5) near the bottom of the filtration chamber (4) and leaving the filtration chamber (4) as a filtered stream near the top of the filtration chamber (4) through an exit opening (6) that is covered by a screen (7), said filtered stream being discharged through an outlet (3) for filtered aqueous liquid; said high flow rate causing the formation of a filter bed of particulate filter media (8) against the screen (7); said filter bed taking up 10-80% of the internal volume of the filtration chamber (4); said particulate filter media (8) having a specific gravity of 1.5-5 g/ml;

b. interrupting the flow of the stream of aqueous liquid to cause the filter bed of particulate filter media (8) to break up and move from the screen (7) towards the bottom of the filtration chamber (4), and subsequently reinstating the flow of aqueous liquid and discharging a stream of waste liquid through an outlet (1 1 ) for waste liquid; or

reducing the flow rate of the stream of aqueous liquid to a low flow rate to cause the filter bed of particulate filter media (8) to break up and move from the screen (7) towards the bottom of the filtration chamber (4), and discharging a stream of waste liquid through an outlet (1 1 ) for waste liquid.

In accordance with a particularly preferred embodiment, the present method comprises passing the aqueous liquid through an apparatus (1 ) as defined herein before.

The present method offers the advantage that the cleaning step can be conducted within a short period of time. Typically, the duration of cleaning step b. is less than 5 seconds, more preferably less than 4 seconds and most preferably less than 3 seconds.

The present method further offers the advantage that a relatively small volume of water is used to remove retentate. Typically, the total volume of aqueous liquid that is removed as waste liquid during a single cleaning step b. is less than twice the internal volume of the filtration chamber (4).

The high flow rate that is employed in step a) to maintain a filter bed of filter media (8) typically exceeds 0.3 cm/s. More preferably, this flow rate exceeds 0.5 cm/s and most preferably it exceeds 0.7 cm/s. The reduced flow rate employed in step b) typically does not exceed 50%, more preferably does not exceed 30% of the high flow rate that is employed in step a). Preferably, the reduced flow rate in step b) is in the range of 0-0.9 cm/s, more preferably of 0-0.6 cm/s and most preferably in the range of 0-0.3 cm/s.

The present method preferably comprises the addition of a sequestering agent that forms a water insoluble complex with Ca²⁺ to the stream of aqueous liquid water before the stream reaches the filter bed of particulate filter media (8). This sequestering agent is preferably selected from sodium carbonate, potassium carbonate and combinations thereof. Said sequestering agent is preferably added to the stream of aqueous liquid in a concentration of 0.1 -3 g/L, most preferably in a concentration of 0.5-2 g/L.

The particulate filter media (8) that is employed in the present method preferably is a particulate filter media as defined herein before.

Figure 1 provides a cross-sectional view of a filtration apparatus (1 ) according to the present invention, operated in filtration mode. The apparatus (1 ) is coupled to a tap (9) by means of coupling means (10). The tap water to be filtered contains suspended soil particles (14) and dissolved calcium. The tap water is introduced into the apparatus (1 ) from the tap (9) through inlet (2), enters the filtration chamber (4) near the bottom via entry opening (5) and leaves the filtration chamber (4) via exit opening (6) that is covered by screen (7). The tap water that enters the filtration chamber (4) at a high flow rate pushes the particulate filter media (8) in the filtration chamber (4) against the screen (7), thereby forming a filter bed. Suspended particles that are contained in the tap water are retained by the filter bed of filter media (8). The filtered water that exits the filter chamber (4) via exit opening (6) leaves the apparatus (1 ) via the first outlet (3). The excess flow valve (20) that controls the flow through the second outlet (1 1 ) is closed. Further shown in Figure 1 is a dosing unit (12) that is located upstream of the filter chamber (4). The dosing unit (12) contains an aqueous solution of sodium carbonate. During the filtration mode the dosing unit (12) introduces the aqueous solution of sodium carbonate at a constant rate into the stream of aqueous liquid. Downstream of the dosing unit (12) particles of calcium carbonate (13) are formed. These calcium carbonate particles (13) as well as suspended soil particles (14) are removed by the filter bed of filter media (8). Downstream of the filtration chamber (4) another dosing unit (15) is provided. This dosing unit (15) contains an aqueous solution of acidulant (it may also contain cationic surfactants/polymers, hydrotropes, etc.). During the filtration mode the dosing unit (15) introduces the aqueous solution of acidulant at a constant rate into the stream of filtered and softened tap water before the water leaves the apparatus (1 ) via the first outlet (3).

The opening (17) near the bottom of the filtration chamber (4) is closed off by a cap (19). The cap (19) can be removed so that the filter media (8) may be removed or introduced.

Figure 2 provides a cross-sectional view of the filtration apparatus operated in cleaning mode (first stage). The flow of tap water has been interrupted by closing the tap (9). Furthermore, no sodium carbonate solution is released from the dosing unit (12) and no acidulant solution is released from dosing unit (15). The particulate filter media (8) in the filtration chamber (4) that formed a filter bed against screen (7) when the apparatus was operated in filtration has settled on the bottom of the filtration chamber (4).

Above the layer of settled filter media (8) a layer of waste water (18) has formed. This waste water contains suspended retentate material that has been released from the particles of filter media (8) when these particles travelled towards the bottom of the filtration chamber (4).

Figure 3 provides a cross-sectional view of the filtration apparatus operated in cleaning mode (second stage). The flow of tap water has been reinstated by opening of the tap (9) and the excess flow valve (20) is open. The layer of waste water (18) is pushed out of the filtration chamber (4) by the tap water that is entering the chamber through the entry opening (5) and is discharged through the second outlet (1 1 ). The incoming tap water pushes the filter media (8) upwards towards the screen (7). Shortly after the filter bed of filter media (8) has been (re-)formed against the screen (7), the excess flow valve (20) is closed and the apparatus (1 ) returns to filtration mode.

The invention is further illustrated by the following non-limiting examples. Examples

Example 1

An apparatus as shown in Figure 1 was used to filter and soften source water.

The apparatus was connected to a tap. Hard and dirty water from the tap was introduced into the apparatus and passed through the dosing point where sodium carbonate was dosed in line. The resulting water mixed with sodium carbonate passed through the bed of particles wherein suspended solids (including nucleated/nano sized calcium carbonate crystals) were removed from the water. The water that came out of the apparatus through the main outlet of the device was clean and softened.

When the tap was closed the excess flow valve was opened and waste water was released from the apparatus via the waste outlet during a brief period when the tap was opened. After this brief period the excess flow valve was closed and a stream of clean water was came out of the main outlet.

The source water used had a turbidity of 27 NTU, and a hardness of 70 FH with a molar ratio of Ca: Mg of 2:1 .

During the high flow rate mode, the source water passed through the apparatus at a flow rate of 4 L/min. The aqueous sodium carbonate solution that was used to soften the source water had a sodium carbonate content of 200 g/L. During the high flow rate interval sodium carbonate solution was added to achieve a concentration of 1 g/L in the aqueous stream. The filtration chamber had the shape of a standing cylinder with an internal volume 0.350 L and an inner diameter of 4 cm. The filtration chamber contained 250 g of particulate filter media. This filter media consisted of Feldspar having the following particle size distribution: Dio 150 microns

D₅o 320 microns

D₉o 520 microns During operation, every 4 minutes the flow was interrupted for 4 seconds. During the no flow time interval,! no sodium carbonate was added from the dosing unit. When the flow was reinstated, during 2-3 seconds water was discharged through the waste outlet. The source water was filtered and softened in this manner during a period of 40 minutes. The filtered and softened stream of aqueous liquid that left the apparatus during this period had a turbidity of less than 2 NTU and a calcium content that had been reduced by more than 80%. Pressure drop across the filter increased only slightly during this period.

Comparative Example

The experiment described in Example 1 was repeated, except that the apparatus was continuously operated in high flow rate mode for 40 minutes. A very substantial increase in pressure drop across the filter was observed during this period.

Example 2

The experiment described in Example 1 was repeated, except that this time filter media consisted of dolomite particles. The results obtained was similar to that described in Example 1 .

Example 3

The experiment described in Example 1 was repeated, except that this time the filter media consisted of Feldspar particles having different particle size distributions. The effect of particle size distribution on pressure drop and breaking-up of the filter bed during the cleaning cycle were investigated. The results are shown in Table 1 (+ = good; -/+ = reasonably good). Table 1

Particle size (μηι) Pressure drop (psi) Break-up

100 to 250 1 1 -/+

125 to 300 8 +

300 to 425 6 +

425 to 700 4 +

Claims

1. An apparatus (1 ) for filtering aqueous liquid, said apparatus (1 ) comprising:

• an inlet for a stream of aqueous liquid (2);

• a first outlet for a filtered stream of aqueous liquid (3);

• a second outlet for a waste stream of aqueous liquid (1 1 );

• a filtration chamber (4) comprising an entry opening (5) that is located near the bottom of the filtration chamber and that is in fluid communication with the inlet for the stream of aqueous liquid (2), and an exit opening (6) that is located near the top of the filtration chamber (4);a screen (7) covering the exit opening (6) of the filtration chamber (4);

wherein said high flow rate causes the formation of a filter bed of particulate filter media (8) against the screen (7); said filter bed taking up 10-80% of the internal volume of the filtration chamber (4); and said reduced flow rate causes the filter bed of particulate filter media (8) to break up and move from the screen (7) towards the bottom of the filtration chamber (4).

2. Apparatus according to claim 1 , wherein the particulate filter media (8) has a mass weighted average particle size of 100-900 microns.

3. Apparatus according to any one of the preceding claims, wherein the particulate filter media comprises at least 50 wt.% inorganic matter selected from aluminosilicates, silica, calcium carbonate, magnesium carbonate and combinations thereof.

4. Apparatus according to claim 3, wherein the particulate filter media comprises at least 50 wt.% aluminosilicate.

5. Apparatus according to any one of claims 1 to 4, wherein the filtration chamber (4) has an internal volume of 0.05-5L.

6. Apparatus according to any one of claims 1 to 5, wherein the mesh size of the screen (7) covering the exit opening (6) of the filtration chamber (4) is in the range of 30 to 200 microns.

7. Apparatus according to any one of claims 1 to 6, wherein, both in filtration mode and cleaning mode, the aqueous liquid leaves the filtration chamber (4) via the same exit opening (6), and wherein a valve is located downstream of the exit opening (6), which valve is capable of directing the stream of filtered aqueous liquid to the first outlet (3) when the apparatus (1 ) is operated in filtration mode and to the second outlet (1 1 ) when the apparatus (1 ) is operated in cleaning mode.

8. A method of filtering a stream of aqueous liquid, said method comprising:

passing the stream of aqueous liquid through an apparatus (1 ) according to any one of claims 1 to 7, said method comprising an alternating sequence of the following steps a. and b. comprising:

a. passing a stream of aqueous liquid with a high flow rate in upward direction through a filtration chamber (4) that is partially filled with particulate filter media (8), said stream entering the filtration chamber (4) through entry opening (5) near the bottom of the filtration chamber (4) and leaving the filtration chamber (4) as a filtered stream near the top of the filtration chamber (4) through an exit opening (6) that is covered by a screen (7), said filtered stream being discharged through an outlet (3) for filtered aqueous liquid;

said high flow rate causing the formation of a filter bed of particulate filter media (8) against the screen (7); said filter bed taking up 10-80% of the internal volume of the filtration chamber (4);

said particulate filter media (8) having a specific gravity of 1.5-5 g/ml;

9. Method according to claim 8, wherein the duration of cleaning step b. is less than 5 seconds.

10. Method according to any one of claims 8 or 9, wherein the total volume of aqueous liquid that is removed as waste liquid during a single cleaning step b. is less than twice the internal volume of the filtration chamber (4).

1 1. Method according to any one of claims 8 to 10, wherein the high flow rate employed in step a) exceeds 0.3 cm/s.

12. Method according to any one of claims 8 to 1 1 , wherein the low flow rate employed in step b) does not exceed 50% of the high flow rate employed in step a).