WATER PURIFYING PLANT

Field of the invention The invention relates to a method for separating ochre in a water purifying plant and a water purifying plant arranged for purifying water.

Background of the invention

Providing clean water is essential for the well-being of humans, animals and various instruments such as medical instruments. Groundwater provides for a large source of water, is less vulnerable to pollution from above-surface activities and is also relatively clean due to the natural filtration as the surface water seeps through the several meters of soil in the earth. However, even though the natural filtration taking placing when the water seeps through the soil removes some of the different contaminants, the water might still be less ideal for humans. Therefore, before using the groundwater for public water supplies, the groundwater is usually treated in a water purifying plant. One of the major issues in such plants is the formation of ochre which contaminates the water so that it is not usable in households (e.g. due to stains in laundry and clogging of water fittings). Therefore, it is known to filter water so that the ochre is separated and whereby cleaner water is obtained. At some point, such filters tend to clog up due to the accumulation of solid sediments and therefore, clean water is used to flush the filtration units. The, now ochre-contaminated water, is then led to large settling ponds where multistage precipitation of the ochre may take place so that the water may be flushed into a stream, river or other.

However, such settling ponds are space consuming and the precipitation takes long time. Furthermore, considerable amount of clean water is wasted in this process.

It is therefore an object of the present invention to address the above-mentioned problems so that removal of ochre from water is performed more efficiently. The invention

The invention provides for a method for separating ochre from water in a water purifying plant. The method comprises the steps of:

• leading impure water through at least one filtration unit in a first direction to provide filtrated water,

• returning a portion of the filtrated water back through the at least one filtration unit in a second direction for flushing the at least one filtration unit whereby flushing water is obtained, wherein the second direction is opposite the first direction,

• leading the flushing water through a centrifuge to separate the flushing water into an ochre slurry and centrifuged water, and

• leading the centrifuged water back through the at least one filtration unit. The process of flushing the filtration unit by directing a portion of the filtered water in a second direction through the filtration unit may take as little as a few minutes while the process of multistage precipitation in settling ponds may take several hours or days. If the amount of water required to clean a certain number of filtration units exceeds the capacity of the settling ponds, the settling ponds may form a “choking point” of which the consequences may be that entire cleaning process is slowed down. This may reduce the efficiency of the entire water cleaning plant in that water cleaning must be performed with a reduced number of filtration units until all units are clean.

Thus, it is advantageous to separate ochre from water according to the above- mentioned method in that such a method provides for a more efficient separation of ochre from the flushing water than the state-of-the-art ochre removal methods. Thereby, large settling ponds are not necessary, which is advantageous in that the building expenses related to construction of such large settling ponds are removed, the process of ochre removal may be performed faster and the amount of wasted water is reduced in that the centrifuged water is sufficiently ochre-free so that it may be led back into the at least one filtration unit instead of being flushed out in the environment.

Furthermore, it is advantageous to use a (or several) centrifuge(s) to separate ochre from the flushing water in that these are faster and easier to adjust to the cleaning requirements (e.g. by adding more centrifuges or adjusting the operating speed).

The term “filtration unit” should be understood as a mesh filter, a sand filter, activated carbon filter or other type of filtration unit which may separate ochre from the impure water due to e.g. its large surface area. Furthermore, it should be possible to clean such a filtration unit by allowing flow of water in a second direction through it.

The first direction through the at least one filtration unit is to be understood as the direction which the impure water can flow through the filtration unit during a normal operation mode of the filtration unit, i.e. the impure water is cleaned during its travel in the first direction through the filtration unit.

Accordingly, the second direction is to be understood as the direction through the filtration unit in which the water travels when the filtration unit is flushed (or “backwashed”). In such a flushing process, ochre and other contaminants are substantially flushed away from the filtration unit.

It should be emphasized that the term “centrifuge” covers conical plate centrifuges, decanter centrifuges, turbo centrifuges or other type of equipment which utilizes the centrifugal acceleration for putting an object/fluid in rotation so that dense substances are positioned differently in the radial direction in relation to the lighter substances.

It should be also emphasized that the term “impure water” is to be understood as water which has not been through a substantial ochre removal process before it flows through the at least one filtration unit. In other words, this term is used to refer to the water which has not been filtered yet, even though it may have undergone different purifying steps in the process of purifying water (such as biological, chemical, mechanical or other kind of purification steps preceding the at least one filtration unit).

And the term “flushing water” should be understood as water which has been led through the at least one filtration unit for cleaning the filtration unit. Thus, “flushing water” typically differs from “impure water” in that the “flushing water” is clean water which is, for the main part, contaminated with ochre, i.e. the ochre contamination makes it un-usable for public water supply.

In an aspect of the invention, the method further comprises the step of leading the flushing water to a buffer tank before leading the flushing water through the centrifuge.

During the flushing of the at least one filtration unit, a substantial amount of water may quickly accumulate, and which may surpass the capacity of the centrifuge. Thus, it is advantageous to lead the flushing water to a buffer tank before leading it trough the centrifuge in that water may be temporarily stored in the buffer tank and the water flow to the centrifuge may be adjusted to the capacity of the centrifuge or to an optimum operating speed in which the cleaning capacity of the centrifuge may be optimal.

In an aspect of the invention, the centrifuged water is lead back through the at least one filtration unit in the first direction.

Depending on the workload and efficiency of the centrifuge, the centrifuged water may contain ochre so that it must be filtered once more before being used a public supply water. Thus, it is advantageous to lead the centrifuged water back through the filtration unit in the first direction in that the centrifuged water may be filtered so that any remaining ochre is removed and thereby clean, drinkable water may be obtained from the flushing water. In an aspect of the invention, the method further comprises the step of leading a gas through the at least one filtration unit in the second direction before the step of returning a portion of the filtrated water back through the at least one filtration unit in the second direction.

It is advantageous to lead a gas through the filtration unit in the second direction before returning a portion of the filtrated water in the second direction in that the gas aids in releasing the ochre and other sediments from the filtration unit so that the subsequent filtrated water flushes the filtration unit more efficiently.

Typically, air is used for this purpose, but other gases may also be used such as substantially pure oxygen, C02, oxygen enriched air or other or any combination thereof.

In an aspect of the invention, the method further comprises the step of feeding the impure water to a chemical treatment unit before the step of leading the impure water through the at least one filtration unit.

It is advantageous to feed the impure water to a chemical treatment unit before leading the impure water through the at least one filtration unit in that adjustment of pH level, adjustment of acidity, removal of pesticides, flocculation and/or coagulation of contaminants and other chemical treatments may take place before the water is fed to the subsequent at least one filtration unit.

In an aspect of the invention, the step of leading impure water through at least one filtration unit in a first direction is stopped while flushing the at least one filtration unit and wherein returning a portion of the filtrated water back through the at least one filtration unit in the second direction is stopped while leading impure water through the at least one filtration unit in the first direction and/or while leading the centrifuged water back through the at least one filtration unit. It is advantageous to stop the filtration of impure water through the filtration unit while the filtration unit is being flushed in that the risk of contaminating the water being cleaned in the filtration unit is reduced or eliminated.

And stopping the return of filtrated water through the filtration unit in the second direction while leading impure water or centrifuged water through the filtration unit in the first direction is advantageous in that the water the filtration may take place without being disturbed by the flow of water being led through the filtration unit in second direction

In an aspect of the invention, the centrifuge is arranged to centrifuge the filtrated water to obtain ochre slurry with an ochre content of between 5 and 80, preferably between 10 and 60 and most preferred between 15 and 50 percent by volume.

If the centrifuge is arranged to centrifuge the filtrated water to obtain an ochre slurry with a too high ochre content, the power consumption required in the centrifuge may be too large and if the centrifuge is arranged so that ochre slurry with a too low ochre content is obtained, the handling of the ochre slurry becomes more comprehensive and too much water is wasted with the ochre slurry since transporting such low-ochre content ochre slurry away may include the unnecessary transport and handling of water. Thus, the above-mentioned ochre content volume percentages represent an advantageous relationship between efficiency and handleability. Furthermore, if the ochre content in the ochre slurry obtained from the centrifuge is too high, it may be too “dry” and/or have too high viscosity (i.e. the proportion of ochre in relation to water is very low) so that the viscosity of the ochre slurry is such that it is difficult to direct the ochre slurry away from the centrifuge. The invention also provides for a water purifying plant. The water purifying plant comprises at least one filtration unit arranged for filtering impure water by flowing through the at least one filtration unit in a first direction to provide filtrated water. Return means are arranged to return a portion of the filtrated water back through the at least one filtration unit in a second direction for flushing the at least one filtration unit whereby flushing water is obtained and wherein the second direction is opposite the first direction. Flushing water conductor is arranged for guiding the flushing water to a centrifuge, which is arranged to separate the flushing water into an ochre slurry and centrifuged water. A centrifuged water conductor is arranged for guiding centrifuged water back through the at least one filtration unit.

It is advantageous to provide a water purifying plant as described above in that by use of centrifuges to clean the flushing water, large settling ponds become redundant so that the required area and thereby costs are reduced. Furthermore, a centrifuge is so efficient at removing ochre from the flushing water that the centrifuged water may be led back through the filtration unit without it clogging the filters.

Even further, the risk of people accidently falling into settling ponds or the risk of accidental leak of water with high ochre content into the environment which may compromise the stability of ecosystems is reduced or eliminated.

It should be noted that the term “return means” should be understood as any type of returner or return device suitable for returning a portion of the filtrated water back through the filtration unit in a second direction. I.e. the term includes any kind of pipes, tubes, ducts, pumps, compressor or other or any combination thereof.

In an aspect of the invention, the centrifuged water conductor is arranged for guiding centrifuged water back through the at least one filtration unit in the first direction.

It is advantageous if the centrifuged water is guided back into the at least one filtration unit in the first direction by means of the centrifuged water conductor in that the water used for flushing the filtration unit is filtrated again so that it may be supplied to households instead of flushed out in the environment.

In an aspect of the invention, the water purifying plant further comprises a buffer tank arranged as part of the flushing water conductor.

It is advantageous if the water cleaning system comprises a buffer tank arranged as part of the flushing water conductor in that the buffer tank may adjust the flow of water to the subsequent centrifuge, which is advantageous in that this flow may be adjusted to the optimum operating speed and/or capacity of the centrifuge.

Such a buffer tank could be a vessel, a container, a reservoir or any other type of partly or completely closed receptacle for containing flushing water and which may be used as a storage to cover variations in flushing water flow towards the centrifuge. More specifically, the buffer tank provides for the possibility of storing excess flushing water, for example if the centrifuge cannot keep up with the flow of flushing water or the buffer tank may be used for providing the centrifuge with flushing water in situations with little flow of flushing water so that the number of starts and stop of the centrifuge is reduced.

In an aspect of the invention, the buffer tank is arranged in a controlled environment.

It is important that the filtration units are arranged in an environment where the impure water being filtrated is not accessible from the outside, i.e. it must be protected from bird excrements, insects, animals, plants (e.g. leaves falling into the units), chemical contamination (un-intentional as well as intentional) or other outside contact which may contaminate the water being filtrated. Thus, such filtration units are usually placed in controlled environments where only authorized personnel is allowed to enter and contact between the water being filtrated and the surroundings is reduced. Accordingly, in this context, the term “controlled environment” should be understood as any room, area, space or other type of environment which is protected and controlled so that contact between the inside of the controlled environment and the surroundings is reduced.

Thus, it is advantageous to also provide the buffer tank in such a controlled environment in that the filtered water therefore is protected from the surroundings so that it may not be contaminated before it enters the centrifuge. If the centrifuge is a closed component, it may be positioned outside of the controlled environment as it may not allow the centrifuged water to be brought into contact with the surroundings.

In an aspect of the invention, the centrifuge is a conical plate centrifuge.

Ochre is a very fine and purely particulate material that is cannot be separated from the water through sieves. It is therefore advantageous if the centrifuge is a conical plate centrifuge in that such a centrifuge is particularly efficient at separating a liquid into different phases so that small, suspended solids may be more efficiently removed. Furthermore, its efficiency may be easily adjusted by controlling the angle and/or the area of the inclined plates and/or the rotating speed of the centrifuge.

A conical plate centrifuge (also known as a disc bowl centrifuge or disc stack separator) is a type of centrifuge that has a series of conical discs which provides a parallel configuration of centrifugation spaces. The conical plate centrifuge removes ochre from water by means of an enormously high centrifugal force. The denser ochre which are subjected to these forces move outwards towards the rotating bowl wall while the less dense water moves towards the centre. Special plates (known as disc stacks) increase the surface settling area which speeds up the separation process. The concentrated denser ochre is then removed continuously, manually or intermittently, depending on the design of the conical plate centrifuge. Thus, a conical plate centrifuge is particularly suited for separating ochre from water in that the water has only small proportion of suspended ochre. In an aspect of the invention, the at least one filtration unit comprises a sand filter.

It is advantageous if the filtration unit is a sand filter in that such a sand filter is effective at trapping pathogens, bad taste and bad smell in the sand filter so that such unwanted contaminants are separated from water in a simple way and without the need for chemicals or other.

It should be noticed that the term “sand” should be understood as zeolite, gravel, rubble or other type of sand or granular material which may be used for filtering purposes. The sand in such a sand filter could be of different types along the height of the filtration unit so that the filtration efficiency changes as the water flows through the sand filter. The flow through the sand filter could be driven by pressure or by gravity alone. For example, the flow of impure water through the sand of the sand filter could be driven upwards by a pressure. Or the flow could be driven downwards by gravity and aided by a pressure above the sand filter to accelerate the water seepage through the sand bed.

In an aspect of the invention, the sand filter comprises a layer of activated carbon. This is advantageous in that such a layer of activated carbon is adapted to remove pesticides, organic contaminants, herbicides and other contaminants in water. In an aspect of the invention, the first direction is downwards.

It is advantageous if the first direction is downwards in that the flow of impure water through the at least one filtration unit is driven, or at least aided, by the force of gravity so that the flow of impure water through the filtration unit is self-driven and therefore more energy-efficient. In an aspect of the invention, the height of the sand bed in the sand filter is between 0.5 and 10 meters, preferably between 1 and 8 meters and most preferred between 1.5 and 6 meters.

If the height of the sand bed in the sand filter is too small the filtering effect may not be sufficient so that ochre is not sufficiently separated from the impure water. And if the height of the sand bed is too large, the travel time through the filter may be unnecessary long, i.e. the water may be sufficiently clean well before it reaches the bottom of the sand bed. Thus, the above-mentioned range represent an advantageous relationship between filtration efficiency, sand consumption and filtration time.

In an aspect of the invention, the water purifying plant further comprises gas leading means for leading a gas from a gas supply through the at least one filtration unit in the second direction.

It is advantageous if the water purifying plant further comprises gas leading means for leading gas in that a more efficient flushing process (i.e. filter cleaning process) hereby is obtained.

In this context, the term “gas leading means” should be understood as any kind of a gas guider or gas conductor suitable for leading a gas from a gas supply through the at least one filtration unit in the second direction. I.e. in the term includes any kind of nozzles, pipes, hoses, tubes, pumps, compressor or other or any combination thereof.

In this context, the term “gas supply” should be understood as a gas container, gas vessel, gas tank, the surrounding air or other type a supply of gas to the gas leading means. In an aspect of the invention, the water purifying plant further comprises a chemical treatment unit arranged to chemically treat the impure water before it flows through the at least one filtration unit in the first direction.

It is advantageous if the water purifying plant further comprises a chemical treatment unit arranged to chemically treat the impure water in that contaminants, which may not be filtered away by the subsequent filtration unit, are thereby removed prior to entering the filtration unit.

In this context, the term “chemical treatment unit” should be understood as a treatment step in the water purifying process where chemical treatment is performed. Chemical treatment comprises oxidation, addition of chlorine, control of pH level or other types of treatment which precede the at least one filtration unit so that the filtration unit may separate remaining contaminants (ochre) and therefore may be considered to be the last step in the purifying process.

In an aspect of the invention, the water purifying plant further comprises a water drilling.

Water from water drillings contains ochre that is particularly difficult to handle and remove in conventionally water purifying plants. Thus, the present invention is particularly advantageous in water purifying plants comprising a water drilling.

In an aspect of the invention, the water purifying plant further comprises pump means arranged for pumping water from the water drilling to the at least one filtration unit.

Providing the water purifying plant with pump means to pump the water from the water drilling is advantageous in that the water hereby can be supplied to the filtration units in a controlled quantity and at a controlled pressure. The invention further provides for use of a water purifying plant according to any of the previously described water purifying plants for separating ochre from water.

Using the discussed water purifying plant for separating ochre from water is advantageous in that ochre separation by means of a centrifuge and the reuse of the centrifuged water ensures an inexpensive, more compact and more efficient water purifying plant.

The invention also provides for use of a water purifying plant according to any of the previously described water purifying plants for purifying water in a water purifying plant by means of a method according to any of the previously described methods.

It is advantageous to use a water purifying plants according any of the described plants for purifying water by means of a method according to any of the previously described methods in that particularly such a plant is adapted to efficiently purify water so that use of settling ponds is redundant and clean water waste is reduced.

Figures

The invention will be described in the following with reference to the figures in which fig. i. illustrates the concept of the water purifying plant, fig. 2 illustrates the active fluid lines during filtration of impure water in the water purifying plant, fig. 3 illustrates the step of leading gas through the filtration unit, and fig. 4 illustrates the step of leading water through the filtration unit in the second direction and centrifuging the flushing water. Detailed description

Fig. 1 illustrates the concept of the water purifying plant 1.

The process of filtering water for use in e.g. households is subject to high security arrangement so that the risk of accidental or deliberate contamination is avoided, or at least minimized. Therefore, the filtration units 2 are typically stored inside a controlled environment 9 so that access to the filtration units is prevented or only permitted for authorized personnel. The controlled environment 9 could be a certain area in a building, the area being shielded or protected for example by a fence, glass panel, wall or other type of measures for avoiding unauthorized access to the filtration units. The controlled environment 9 could also be outside, to the extent that it is protected from unauthorized access. It should be mentioned that “unauthorized access” should be understood as access from unauthorized staff, guests, personnel as well as access from unwanted animals, such as rodents, birds, insects and other animals, which could contaminate the filtration units and/or the water being filtrated and the filtrated water.

In this embodiment the water purifying plant 1 comprises a water drilling 19 from which water is pumped by means of pump means 18 to the filtration units 2. In this embodiment the water drilling is an underground water drilling but in another embodiment the water could also or instead be supplied to the water purifying plant 1 from a river, a lake, a rain water reservoir or other and in another embodiment the water could also or instead be led to the filtration units 2 by means of gravity, injectors or other.

In this embodiment the water purifying plant 1 also comprises a chemical treatment unit 5. In this embodiment the chemical treatment unit 5 is located between the water source and the filtration units 2 but in another embodiment the chemical treatment unit 5 could also or instead be located after the filtration units 2, in the filtration units 2 or elsewhere.

When the impure water has been filtrated through the filtration units 2, it is in this embodiment fed to a water tank 14, which in this embodiment is a water tower comprising an elevated water tank so that the water is pressurized due to the potential energy gained from storing the water at an elevated position. The clean water from the water purifying plant may be fed directly out to the consumer or it may be fed to the water tank 14 for storing the clean water until it is needed (e.g. to provide emergency water storage).

In this embodiment, the water tank 14 is a water tower. However, in another embodiment, the water tank 14 could be a container, vessel, reservoir or other type of water tank. The water tank 14 could also be pressurized so that it is not necessary to provide the pressure by elevating the tank. Such pressurization could be achieved by e.g. a pump.

In a preferred embodiment of the invention, the filtration units 2 are arranged in parallel as shown in fig. 1. The number of filtration units could be one, two, five, ten or more or any number in between. However, it should be noted that some of the filtration units 2 could be arranged in series so that a combination of parallel and series connection may be achieved. This is beneficial for example if multiple filtration steps by leading the impure water through several filtration units 2 in series. In an embodiment, at least one or a plurality of filtration units 2 are arranged in series.

A water purifying plant 1 usually comprises different purifying steps which are specifically adapted for removing certain contaminants. It should be noted that in this figure, the solid lines are used to indicate fluid lines. In the following illustrations (figs. 2-4), solid lines are used to indicate active fluid lines while dotted lines are used to indicate inactive fluid lines.

Fig. 2 illustrates the active fluid lines during filtration of impure water in the water purifying plant 1.

In the following, active fluid lines are indicated by solid lines and in-active fluid lines are indicated by dotted lines.

The impure water is fed to the at least one filtration units 2 and sprayed onto the sand bed from above so that the impure water may seep through the sand bed in the first direction (i.e. downwards) so that the ochre (or at least most of the ochre) may be separated from the water.

In this embodiment, the sand in the sand bed is fine at the upper parts of the sand bed and gradually transitions to coarser sand at the bottom. As the impure water seeps through the sand, contaminants and ochre are removed from the water so that it gradually becomes purer during its travel downwards through the sand. Finally, the filtrated water is extracted from the filtration unit 2 by the filtrated water leading means 13, which in this embodiment comprises tubes and a pump (not shown) for conducting and pumping the water from the filtration unit to the water tank 14.

In this embodiment, the sand filter comprises fine(r) sand on top of coarse(r) sand and wherein the transition from fine to coarse sand is gradual so that contaminants are gradually removed from the water. However, in another embodiment, the transition from fine to coarse sand could be abrupt, depending on the type of sand being used. The coarse sand may also be on top of the fine sand and the transition from the coarse sand to the fine sand may be abrupt or gradual. Fig. 3 illustrates the step of leading gas through the filtration unit 2.

During the filtration of the impure water in the filtration units 2, ochre and different contaminants accumulate in the filtration unit 2 so that it becomes less efficient or, with time, may clog up so that the impure water may not seep through it. Thus, it is necessary to flush the filtration units 2 frequently. The frequency of the flushing process of the filtration units depends on the amount of ochre in the water and the amount of water being filtered in the filtration unit 2. To improve the flushing of the filtration unit, gas from a gas supply 11 may be fed through the gas leading means 10 and into the lower part of the filtration unit 2 so that the sand is at least partially fluidized so that ochre and other contaminants may be released from the sand.

In this embodiment, air is injected into the filtration unit by injecting the air through nozzles (not shown) positioned at the bottom of the filtration unit 2 and upwards through the sand bed in the filtration unit 2. In another embodiment, the air could be fed through holes or slits in the filtration unit or in other way injected into the filtration unit so that gas is forced through the filtration unit in the second direction. In an embodiment, gas may be injected from the side portions of the filtration unit so that gas may be injected at different height in e.g. the sand bed.

In this embodiment, the flow of impure water through the impure water leading line 16 is ceased so that the gas being injected into the sand bed of the filtration unit 2 and impure water seeping downwards do not collide and interfere. The active lines are indicated by the solid lines while dotted lines indicate the inactive lines.

Fig. 4 illustrates the step of leading water through the filtration units 2 and centrifuging the flushing water.

After gas has been fed through the filtration units 2, a portion of the filtrated water is returned to the filtration unit by the return means 6, which in this embodiment comprises the tubes which were used to conduct the filtrated water from the filtration unit to the water tanker 14. As the water is fed through the filtration unit 2 in the second direction, (mostly) ochre and minor contaminants retained by the filter, are carried upwards. The filtration unit is flushed until the water level in the filtration unit reaches a certain limit so that the filtration unit is “overflowed” (indicated by the dashed line indicating a raised water level in the filtration unit 2) and the flushing water may flow into the flushing water container 15. In this embodiment, this is performed by overflowing the filtration unit so that flushing water flows into the flushing water container 15 coupled to the filtration unit.

In this embodiment, the flushing water container 15 comprises a trench which surrounds the filtration unit so that the overflowing water is directly captured by the trench. However, in another embodiment, the flushing water tank 15 could be a separate flushing water tank to which the flushing water is pumped to. In yet another embodiment, a pump could be arranged to suck the flushing water from the surface of the sand bed (the water level is above the sand bed due to the overflow) and lead the water either directly to the centrifuge 3 or to the flushing water container 15.

The driving force for returning a portion of the filtrated water through the return means 6 is, in this embodiment, the pressure in the water tank 14 due to the potential energy stored in the water when it is pumped to the elevated position after the filtration process. However, in another embodiment, the return means 6 may comprise a pump (not shown) so that water may be returned to the filtration unit by the pump, e.g. if the water tank is not elevated or pressurized.

The flushing water in the flushing water container 15 may then be fed to a centrifuge 3 for centrifuging the flushing water so that it is separated into an ochre slurry and centrifuged water. Depending on the type of centrifuge and the water content in the ochre slurry, the ochre slurry may be sufficiently liquid so that it can flow and therefore be pumped to an ochre slurry reservoir (not shown). However, if it is desirable to provide for a very “dry” ochre slurry, i.e. low water content ochre slurry, the ochre slurry may be mechanically scraped off the centrifuge and used for other purposes.

The ochre slurry may also be pumped to an ochre slurry container 12 for temporary storing the ochre slurry. The ochre slurry container 12 may also be mobile (e.g. a lorry) which collects the ochre slurry and may deliver it somewhere else for other uses.

In this embodiment, the water purifying plant comprises a closed buffer tank 4 positioned between the flushing water container 15 and the centrifuge 3 to accommodate the water flow to the capacity of the centrifuge 3. The amount of water flowing to the centrifuge may then be adjusted by controlling the buffer tank valve 4a, which in this embodiment is a gate valve, but may also be a butterfly valve, ball valve, needle valve or any other type of valve suitable for the purpose of controlling the water flow from the buffer tank 4 to the centrifuge 3.

The buffer tank 4 may be closed so that it may be positioned outside of the controlled environment 9 or it may be opened (i.e. an open tank). If the buffer tank 4 is open so that the filtered water may be brought into contact with the surroundings, it is preferable that the buffer tank 4 is provided in the controlled environment 9 so that the filtered water is not contaminated. Depending on the size of the controlled environment and the type of centrifuge, the centrifuge may be provided in the controlled environment 9 so that the centrifuged water is also protected from the surroundings. Providing the buffer tank and the centrifuge in the controlled environment is advantageous in that the risk of contamination is reduced so that the centrifuged water may be fed back into the filtration units without contaminating these with contaminants brought in from the outside and which may not be removed by the filtration unit since the water at this stage is only filtered and not treated by any other treatment process (e.g. chemical) which may be necessary to remove some contaminants being introduced from the outside. In this embodiment, the centrifuge is a conical plate centrifuge. However, in another embodiment, the centrifuge could be a decanter centrifuge, turbo centrifuge or other type of centrifuge.

Also, in this embodiment, the flushing water from the flushing water container 15 is led to the centrifuge by use of a pump (not shown). However, in another embodiment, the flushing water may flow to the centrifuge by force of gravity (i.e. difference in elevation between flushing water container 15 and centrifuge 3).

In this embodiment, a centrifuge water buffer tank 17 is positioned after the centrifuge, i.e. between the centrifuge and the impure water leading line 16 so that the centrifuged water may be stored until it may be conducted to the at least one filtration unit 2 through the impure water leading line 16 (which in this step is not active). This may be beneficial as the flow of impure water in the impure water leading line 16 may be stopped during the flushing and therefore flow of centrifuged water into the impure water leading line 16 would not be possible. Thus, the centrifuged water buffer tank may provide for temporary storage of centrifuged water until flow of impure water into the filtration unit is resumed so that the centrifuged water may flow in the water leading line 16. In another embodiment, the centrifuged water may flow back into the filtration unit by dedicated centrifuge water leading lines (not shown), i.e. independent of the impure water leading line 16. Also, the buffer tank and dedicated centrifuge water leading lines may both be present so that it is possible to both store the centrifuge water as well as leading the centrifuged water through the filters via the dedicated centrifuge water leading lines if the flow through impure water leading line 16 is stopped. In an embodiment, flushing water may be fed and stored in the buffer tank 4 until the filtration units have been flushed. The water in the buffer tank 4 is then centrifuged and fed directly to the impure water leading line 16 once the flow of impure water through lines 16 is recommenced. Depending on the efficiency of the centrifuge 3, i.e. how pure the centrifuged water is, a portion of the centrifuged water may be fed through the filtration unit 2 in the second direction so that the centrifuged water may be used for flushing the filtration unit 2.

In this embodiment, the water purifying plant 1 comprises a single centrifuge. However, in another embodiment, the purifying system 1 may comprise a plurality of centrifuges arranged in series, parallel or a combination hereof. Series connection may increase the ochre separation so that the centrifuged water becomes increasingly cleaner during its travel through the series connected centrifuges and parallel connection may increase the allowable volume flow of filtered water to be centrifuged. However, both series connection and parallel connection may increase the allowable flow of water to be centrifuged.

The invention has been exemplified above with reference to specific examples of centrifuge 3, water tank 14 and filtration unit 2. However, it should be understood that the invention is not limited to the particular examples described above but may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims.

List

1. Water purifying plant

2. Filtration unit

3. Centrifuge

4. Buffer tank

4a. Buffer tank valve

5. Chemical treatment unit

6. Return means

7. Flushing water conductor

8. Centrifuged water conductor

9. Controlled environment 10. Gas leading means

11. Gas supply

12. Ochre slurry container

13. Filtrated water leading means 14. Water tank

15. Flushing water container

16. Impure water leading line

17. Centrifuged water buffer tank

18. Pump means 19. Water drilling