The present invention relates to a method of purifying water i an open, solid filter bed in which, after the addition of chemicals, the water to be purified is passed through a filter bed, said filter bed being regenerated by counter-flow flushing. It also relates to plant for performing the method.

Conventional water purification using open sand filters com¬ prises the steps of rough separation, mixing in chemicals, floc- culation, sludge separation and then filtration through the sand bed. The bed is counter-flow flushed at certain intervals de¬ pending on the degree of pollution of the water entering. There are certain drawbacks to this process, one being that the water entering the sand bed must be relatively pure since the resis ^¬ tance caused by pollution build-up will otherwise quickly become too great for the pressure from the available water column tc being able to force the water through the sand bed. Further ^¬ more, the capacity is low, primarily because the net production, i.e. the quantity of pure water produced reduced by the quantity of flushing water, is low. The septic tanks also mean that the whole plant will require considerable space.

According to another technique pressure filters are utilized. In this case flocculation takes place after the rough separa ^¬ tion, and the flocculated water is pumped into a pressure con- tainer in which the water is forced through the filter bed. The drawback here is that poor flocculation is obtained and that no control over the process is possible in the closed tank.

The use of continuous filters offers considerable advantages over the above methods. An example of such a filter is the

CAREX filter (reg. trademark) , where water is pumped into the bottom of a sand filter and forced up through the sand bed while clean sand is simultaneously added at the top and removed ^' from the bottom of the filter. However, this type of filter is rela- tively complicated and is best suited to moderate volumes of wa¬ ter. It is far too expensive for small volumes of water, e.g. for individual households, greenhouses and the like.

The object of the present invention is to achieve a method cf purifying water with the aid of an open, solid filter bed which offers high capacity and is also suitable for small quantities of water, and also to achieve plant for performing the methoc, which is both reliable and economical.

This is achieved by the method according to the present inven ^¬ tion which is characterised in that the water to be purified in the filter bed is caused by force to flow through the bed at a constant rate by means of a partial vacuum created by a pump means arranged downstream of the bed, the flow being kept con¬ stant with the aid of a control valve arranged en the pressure side of the pump. The production of water is thus nor depenάen: on a water column above the bed tc force the water through the filter bed. The volume above the filter bed shall only ensure uniform distribution over the bed and that the surface cf the filter bed never becomes dry.

According to a preferred embodiment of the method according tc the invention, the purified water is used to flush the filter bed, the supply of water being cut off when a certain predeter¬ mined partial vacuum is sensed en the suction sice cf the pum , and the bed is then flushed counter tc the direction cf flew cf the water during the water purification phase.

According to another embodiment cf the method according tc the invention purified water is collected in separate flush and re¬ servoir tanks or the like, control means being arranged tc throttle the supply of untreated water when the tank is full, and initiate flushing of the filter bee, the flow of purified water to the flush-water tank is regulated depending on the cer ^■ ree of pollution cf the untreated water εc that when the tank i: full it is time to flush the filter b i. The use cf a less s ^~ - hi icated co ⁿ trol t^"? increases 0 sz3. ^~ - _. ~ ^~ reliabilitv ~- ^~ - ving a plant which is particularly suitable for projects m ά -

tank to the degree of pollution in the untreated water entering ensures regular flushing of the filter bed without the risk of clogging and without risk of reduced capacity due to the bed be ^¬ ing flushed more often than necessary.

The method according to the invention is performed in a plant comprising means for supplying untreated water, adding chemicals, performing vigorous mixing, producing floe and filte ^¬ ring, as well as pipes and valves for automatically conducting the water through the plant, consisting of a first pump means arranged downstream the filter bed to cause the polluted water, by means of partial vacuum, to flow through the filter bed, and is characterised by means for maintaining a constant flow through the bed, said means consisting of a float-controlled valve arranged on the pressure side of the first pump means, the float being arranged above the filter bed, whereby a set flow and a desired volume above the filter bed can be guaranteed ir¬ respective of the degree of pollution of the filter bed. If the level rises the valve opens or throttles the out-flow if the le- vel falls, i.e. the capacity and suction capacity cf the pump is regulated depending on the degree of pollution in the bed until the set partial vacuum, i.e. the stipulated maximum degree cf pollution is reached.

According to a preferred embodiment of the invention a pressure gauge is arranged downstream the filter bed to sense the resis¬ tance above the bed and is arranged, at a certain predetermined partial vacuum level, to stop the supply of untreated water ana initiate flushinσ of the filter bed.

According to another embodiment, the means according tc the in¬ vention comprises a separate flush-water tank having contrcl means for water entering the tank and control means which, wher. the tank is full, are arranged tc throttle the supply cf untrea¬ ted water and initiate fluεhinα cf the filter bed.

Additional objectives, characteristics and advantages cf the present invention will be revealed in the following description of an embodiment exemplifying the invention, with reference tc the accompanying drawing in which

the figure shows schematically a plant for performing the method according to the invention.

The drawing thus presents the flowchart for a plant for pεrfor- ing the method according to the invention. The parts of signi¬ ficance to an understanding of the invention will be described in detail below whereas conventional elements and functions will be touched upon only briefly.

The water to be purified, termed raw or untreated water, is supplied to the plant through a raw-water pump 1. Pipeε 3 and 4 are connected to the raw-water pipe 2 to enable the addition cf chemicals to the untreated water. The chemicals are not added until the pressure gauge 38 confirms that the water has reached it. The chemicals may be aluminium sulphate and lye tc produce flocculation and adjust the pH value. These are stored in tanks 5, 6 and are dosed by means of pumps 7, 8 and valves 9, 1C . Tne untreated water passes a control valve 11, a flow gauge 12 and continues to the mixing tank 13.

The untreated water with chemicals added is introduced at the bottom of the mixing tank by way of a vigorous mixer, consisting in the example shown of a pipe 14 filled with glass bails, fcr instance, so-called turbulation material, to ensure thorough mixing of the chemicals. The stirrer 15 is designed to supplv energy corrresponding to the shearing strength of the flee, i.e. considerable agitation at the bottom, gradually decreasing ccr- responding to the size cf the flee. The water flows over frc.τ the top of the mixinσ tank tc the filter vessel 15.

The filter vessel 16 contains a filter bed 17. The c -omll.p.--'osC_: . and structure cf this bed shall be suited tc the εoecific use

which the purified water is to be put. The filter bed may con ^¬ sist, for instance, of fine sand at the top and coarse sand at the bottom or, if the water is to be used in greenhouses, or in the case of very dirty water, an uppermost layer of hydroanthra- cite. However, the precise composition and structure of the bee has no bearing on the actual invention.

Substantially horizontal, perforated pipes 18 are arranged at the bottom of the filter vessel 16, through which the water which has passed the filter bed is withdrawn and flows on along the pipe 19 due to the partial vacuum generated by the pump 20. A pressure gauge is arranged in the pipe 19 on the suction side of the pump 20. In the embodiment shown this gauge is a vacuu- meter 21 sensing the partial vacuum in the pipe 19. The pump 2 ^" . pumps the purified water through the pipe 22 which is provided with a float-controlled valve 23 and a spring-loaded check valve 24, to the reservoir and flush-water tank 25. The spring-loaded valve 24 has the task of preventing flush-water from passing the pump 20 while the filter bed is being flushed, when the pump 20 is not in operation.

The valve 23 with float 34 has the task, irrespective cf hew dirty the bed is, of limiting the flow through the bed tc that stipulated, and maintaining a constant zone of liquid phase above the filter bed 17.

An cutlet pipe 26 at the bottom of the tank 25 communicates via a pump 27 and check valve 28 with the system for the water lea ^¬ ving, in the example illustrated by a pressure tank 29. The pressure tank allows water to be withdrawn by a consumer curing the flushing phase, as well as closing the check valve 28 during flushing. A feedback pipe 30 with solenoid valve 31 is arranged between said pump 27 and the check valve 28. This feedback p± - 29, constituting the flush-water pipe for counter-flow flushinπ of the filter bed 17, opens into the cutlet pj filter vesεel 16 en the suction side cf the pipe 2C

An overflow outlet 32 is arranged at the top of the filter ves ^¬ sel 16, through which flush-water, together with contaminants collected in the filter bed, flows out and through the flush-w= ter pipe 33.

Chemicals required for disinfection and adjusting the pH value are preferably added in pipe 22 after the spring-loaded valve 24. Control means 35, 36 and 37 are arranged in the reservoir and flush-water tank 25, their function being described in mcrt detail below.

The method according to the invention is performed as fellows : the plant illustrated in the drawing:

The raw-water pump 1 is started, the pressure gauge confirms that the water has reached it, chemicals are added, mixed in ar the mixture allowed to remain in the mixing tank 13 long enougr for flocculation to occur. It then flows over into the filter vessel 16 where a desired water level is maintained above the filter bed 17 with the aid cf the float 34. Water flows, or rather is sucked through the filter bed 17 due to the partial vacuum created by the pump 20, and is carried further through the pipe 22. E influencing the valve 23, the float 34 control the flow supplied by the pump 20.

The vacuumeter 21 senses the partial pressure in the pipe 19 leaving the filter bed. When the partial vacuum reaches a cer¬ tain predetermined level, depending on the contaminants collec¬ ted in the filter bed, the pump 20 will be stopped, the solenc: valve 31 opened and the pump 27 will pump water through the feedback pipe 30 and pipe 19, in through the perforated pipes I and up through the filter bed 17. The water is prevented by t: spring-loaded valve 23 from flowing up through the pump 20 and back through the pipe 22.

The flush-water takes with it the contaminants collected in the filter bed, and flows out through the overflow outlet 32 to the flush-water outlet pipe 33.

The control means 35, 36, 37 in the reservoir and flush-water tank 25 ensure that if the water tank is filled, the raw-water pump is stopped and that it is started again when the level in the water tank has fallen to a certain level. They also prevent flushing if the water volume in the tank is insufficient to carry out flushing.

The filter bed must not be permitted to become dry on the sur¬ face since a filter head would then be formed which would drama¬ tically reduce the flow through the filter bed as well as making it difficult to flush the bed clean.

Of course elements having a similar function can be exchanged for various elements in the plant and some elements may be omit¬ ted without essentially altering the function of the plant, without departing from the inventive concept as defined in the following claims.