This invention relates to a water treatment device or plant comprising a treatment tank under atmospheric pressure, a first pump for level-operated supply of raw water to the tank, a means for feeding a mixture of air and raw water to the upper part of the treatment tank, a filter disposed in the tank on a lower level than the feeding means, a collecting magazine for treated water disposed below the filter, a pressure tank connected to the collecting magazine, and a filter-washing device. In a water treatment plant, it is essential that the consumer continuously has access to water, regardless of the current phase of the water treatment process. The treatment process may be divided into four phases:

o replenishment of the treatment tank, o filtering of the thus-supplied and treated water, o transfer of treated water to a pressure tank, and o backwashing of the filter.

In many places, the water from bored wells contains iron and/or manganese compounds that have to be removed before the water can be used in the household. To this end, air is usually admixed to the water, such that the iron and/or manganese compounds form insoluble oxides that can be disposed of by precipitation or filtering.

Where filtering for one reason or other is necessary or desirable, the filter employed have to be regenerated at times, i.e. be backwashed with clean water. However, backwashing involves such a considerable consumption of water as to be impracticable in plants with small water resources. If so, one has to content oneself with preci¬ pitating the oxides and flushing out the resulting preci¬ pitate.'

FI-78143 discloses a water treatment plant for self- contained houses, which enables a regeneration of the

filter. This plant has the disadvantage of a low capacity due to the fact that the replenishment phase and the transfer phase cannot occur at the same time, one and the same pump being used in both phases. Another disadvantage is that the oily film of precipitates which are lighter than water and thus accumulate on the water surface in the tank during operation is not removed by the regenera¬ tion process. These precipitates often contain iron and manganese. When the water level has sunk to the level of the outlet connected to the upper surface of the filter, the outlet begins to draw air and no longer draws any water, leaving the oily film in the tank.

The main object of the invention is to obviate the inconveniences of the prior-art water treatment plant described above.

Another object of the invention is to make the effi¬ ciency of the backwashing phase independent of the amount of water found in the upper part of the tank when the filter is being backwashed. According to the invention, these objects are attained by a water treatment plant being of the type stated by way of introduction and having the distinctive features recited in the characterising clauses of the appended claims. The invention will be described in more detail below with reference to the accompanying drawing, which schema¬ tically illustrates a water treatment plant according to the invention.

Thus, the drawing shows an essentially cylindrical tank 1 provided with a lid 2, which carries an ejector 3 for admixing air to raw water supplied through a conduit 4. The design of the ejector 3 is described in Swedish Patent Application 9303412-2. Water from a source 5 is supplied to the conduit 4 by a first pump 7 driven by a motor 6. Further, the motor 6 is controlled by a relay 8, to which are connected vertical electrodes 9, 10 and 11 of different lengths, which are arranged in the tank 1.

An overflow 31 is arranged in the tank 1 on a level above the end of the shortest electrode 9.

In the tank 1, there is further provided a sand fil¬ ter 13 on a level below the longest electrode 11. The filter 13 contains different-sized particles, the size of which increases in the direction of the lower part of the filter. Also, the filter 13 rests on a horizontal grating 14 mounted in the tank 1.

Beneath the grating 14, there is a space 15 in the tank 1 which serves as magazine for filtered water. A conduit 16 connecting the magazine 15 to a pressure tank 17 is provided with a second pump 18, which is driven by a motor 19 that in known manner is controlled by the pressure in the pressure tank, as well as with a non- return valve 20.

A conduit 21 serves to conduct water from the pres¬ sure tank 17 to a place of consumption.

A conduit 22 branching off from the conduit 16 and comprising a solenoid valve 23 connects the magazine 15 directly to the pressure tank 17.

A control system 24 with a timer is adapted to periodically deactivate the pump 18 while at the same time opening the solenoid valve 23.

In combination with the overflow 31 mentioned above, there is provided a siphon 30, whose open end is located at a distance from the upper surface of the filter which it faces. The siphon, which consists of an inverted U-pipe, has one leg connected to the one horizontal arm of the overflow, which consists of an inverted T-pipe, whose other arm is connected, via a hole in the tank wall, to an outlet tube 32 leading to an outlet 33. The body of the T extends vertically upwards, ending slightly above the curve of the U-pipe.

The legs of the U-pipe have different diameters. The leg whose opening is adjacent to the filter has a larger diameter than the other leg. In a preferred embodiment, the former leg has a diameter of 40 mm, whereas the

latter leg has a diameter of 25 mm. This serves to increase the suction capacity of the siphon.

The water treatment plant shown in the drawing and described above operates as follows. When there is a low water level in the tank 1, i.e. when the electrode 10 is located above the water level, the motor 6 of the first pump 7 is activated by the relay 8. As a result, raw water from the source 5 is supplied through the conduit 4 to the ejector 3, where it is mixed with air drawn into the ejector 3.

This means that the water level in the tank 1 will rise. When the water level rises above the lower end of the electrode 9, the relay 8 deactivates the motor 6. When the pressure in the pressure tank 17 falls below a predetermined value, the motor 19 of the second pump 18 is activated, so that water from the magazine 15 of the tank 1 is transferred to the pressure tank 17.

When water is thus transferred from the tank 1 to the pressure tank 17, the water found above the filter 13 will flow downwards through the filter. If the raw water contains, say, iron or manganese, this will be oxidised by the admixed air, and the resulting oxides are all but insoluble in water. The resulting precipitate is inter¬ cepted by the sand filter 13, and the magazine 15 will thus contain clean, clear water.

When the water level sinks below the lower end of the electrode 10, the relay 8 activates the first pump 7, so that raw water is again supplied to the tank 1 by the ejector 3. The filter 13 has to be backwashed at regular inter¬ vals. This is brought about by having the timer of the control system 24 cause the motor 19 of the second pump 18 to be deactivated at intervals, while at the same time the solenoid valve 23 is opened. Owing to the presetting of the timer of the control system, backwashing of the filter can be performed at any water level whatsoever in the tank.

If the water level is low, there is a risk that the pressure in the pressure tank will not be sufficient to raise the level up to the overflow 31, such that the siphon begins to operate and the oily film can flow out. According to the invention, the control system is there¬ fore so designed that the pump 7 is always activated if the water level is located between the electrodes 9 and 10. The replenishment with raw water contributes to rapidly raising the water level to such an extent that the siphon and the overflow may begin to operate. It should be observed that the pump 7 is deactivated when the water level reaches the electrode 9.

Water from the pressure tank 17 will now flow back¬ wards into the tank 1 through the conduit 22 to the maga- zine 15, thereby generating an upwardly-directed water flow through the filter 13. The precipitate of oxides has accumulated in the upper surface layer of the filter 13, there forming a loose filter cake, which now is broken to pieces by the water flowing upwards. As mentioned in the foregoing, an oily film consist¬ ing of precipitates lighter than water forms on the water surface 34 in the tank during operation. When the back¬ wash water penetrates through the filter, the water level in the tank and in the pipes 30 and 31 rises. When the level reaches the upper edge of the U-pipe, the siphon begins to operate, thus drawing out the filter cake. Soon after, the water level reaches the overflow, and the oily film flows out through the pipe 31. When the supply of backwash water is interrupted, the water level in the tank sinks because the siphon continues to operate until the water reaches the opening of the siphon, which then begins to draw air. The end of the electrode 10 is locat¬ ed immediately below the siphon, and the electrode 10 activates the pump 7 when the water level sinks below this end.