Sand filtering device DESCRIPTION Field of the invention

The present invention relates to a sand filtering device used in Water Treatment Plants, or WTPs, specifically in primary, secondary and cooling water treatment plants.

State of the art

Various sand filtering devices used in water treatment systems are known. Such devices however, display various disadvantages, such as: - the forming of surface crusts with frequent filter obstruction phenomena and consequent frequent back washing operations;

- they do not allow to reach high filtering speeds and consequently the systems require a great number of filters;

- they require a layer of anthracite over the layer of sand to stop the oils and greases contained in the dirty water;

- they imply high investment and maintenance costs.

The need to make a sand filtering device system which allows to overcome the aforesaid drawbacks is thus felt. Summary of the invention It is a main object of the present invention to make a sand filtering device, for the treatment of water in primary, secondary and cooling water treatment plants, comprising a single layer filtering bed, or simply filter, which allows to obtain perfectly clean water and a high operative working life at high filtering speeds. A further object of device of the invention is to obtain a very efficient filter back washing operation capable of ensuring filter efficiency in time.

The present invention thus suggests to reach the aforementioned objects by making a water treatment sand filtering device which, in accordance with claim 1 , comprises a substantially cylindrical container, defining a longitudinal axis, provided with a plate arranged internally so as to divide the container itself into an upper zone and a lower zone, in which the upper zone contains a filtering bed comprising a single layer of sand having a height H determined by the formula

H=a ^* e b-v

where parameter "a" has a value from 500 to 800, parameter "b" has a value from 0,020 to 0,045 and filtering speed "v" has a value in the 40-70 m/h range. Another aspect of the invention relates to a back washing method of the abovementioned filtering device which, in accordance with claim 10, comprises the following steps:

- drainage of the water to be filtered contained in the upper zone of the container until the water level is taken near the upper surface of the filtering bed;

- introduction of a back flow of air to break the head and form air bubbles in the filtering bed for a time of approximately 1 minute;

- introduction of a back flow of air-water to move away the dirt which is released, for the time required to exchange the water contained in the volume of sand at least 10 times,

- introduction of a back flow of water only for a time from 2 to 5 minutes to make the bed deposit in a homogenous, uniform manner;

- final rinsing in parallel flow to prepare the bed for the filtering operation and to eliminate the dirt not removed during the back washing step. Advantageously, said correlation between the height of the filtering layer, the grain size of the used sand and the filtering speed allows a dimensioning of the filtering bed such as to ensure optimal filtering conditions at high speeds.

The filtering device object of the present invention, being suitable to efficiently work at filtering speeds which may also reach 70m/h, allows to obtain the following advantages:

- it allows to suggest filtering systems which employ approximately half of the conventional filters;

- it allows to make compacter and more rational layouts with a significant reduction of investment and maintenance costs.

Advantageously, the device of the invention uses a high height of the single layer filtering bed, so as to increase the filtering path, and a relatively coarse grain size of the sand thus allowing to exploit the greater filtering volume and distribute the dirt contained in the water to be purified over said volume, avoiding the formation of surface crusts and consequently frequent obstructions of the filter. Thanks to such feature, the device of the invention does not require a layer of

anthracite over the layer of sand, as occurs in the known filters, to stop the oils and the greases contained in the dirty water, with a considerable saving of investment costs and maintenance.

The dependent claims describe preferred embodiments of the invention. Brief description of the Figures

Further features and advantages of the invention will be more apparent in the light of the detailed description of a preferred, but not exclusive, embodiment, of a filtering device illustrated by way of non-limitative example, with the aid of the accompanying drawings, in which: Figure 1 shows a schematic section view of a filtering device according to the invention;

Figures 2a and 2b shows section views along reciprocally perpendicular planes of a component of the device of Fig. 1 ; Fig. 3 shows a top view of a part of a second component of the device of the invention;

Fig. 4 shows a side view of a third component of the device of the invention;

Figure 5 shows an enlargement of a detail of the view of Fig. 1.

Detailed description of a preferred embodiment of the invention

With reference to the Figures, it is shown a filtering device, indicated as a whole by reference numeral 1 , contained within a substantially cylindrical steel tank or container 2, provided with vertical walls 3, a bottom 4 and a lid 5. Tank 2 is divided internally into two zones by a plate 7. The upper zone is provided with a filtering bed 6 comprising a single layer of suitably selected sand. Normally, the filtering bed 6 rests on a thin layer of rather coarse gravel 30, not performing filtering action, in turn resting on plate 7.

The water to be purified is fed to the filtering bed 6 through a central conduit 8 arranged along the longitudinal axis of tank 2. Such conduit 8 is provided with an inlet section 10 of the water to be filtered near the tank bottom 4 and an outlet section of said water near lid 5. Advantageously, the fact of contemplating a single layer of filtering sand allows a more homogenous exploitation of the filtering bed. The grain size of the sand, relatively coarse and homogenous, allows to reach high filtering speeds, to exploit the filtering layer in depth and thus distribute the dirt over the entire layer, avoiding

the formation of surface crusts and thus early filter obstruction phenomena. A further advantage is represented by the fact that filtering bed 6 has a predetermined height such to define a greater filtering volume and a longer filtering path with respect to that of conventional filters; this also allows to extend the time of use of the filter before needing to perform the back washing because it does not get obstructed.

For the correct dimensioning of the filtering bed or single layer filter 6, accounting for the grain size used, the following mathematical correlation which provides the optimal height of the layer has been advantageously worked out:

where "a" and "b" are parameters which depend on the sand grain size and "v" is the filtering speed. Such equation shows the logarithmic curve of the layer height against filtering speed. Parameter "a" has a value from 500 to 800 and parameter "b" has a value from 0,020 to 0,045.

The best performing grain size appears that comprised in the 1 ,0-2,0 mm range, to which a value of "a" equal to approximately 580 and a value of "b" equal to approximately 0,029 corresponds, which allows to obtain a filtering speed of at least 50 m/h with a height of filtering bed 6 equal to approximately 2,5 metres. In such conditions, starting from a concentration of particles suspended in the input sludge equal to 80ppm, it is thus possible to obtain a maximum output concentration of only 10ppm.

It has been found that a smaller grain size, equal to 0,7-1 ,0 mm, requires a thinner filtering layer, but is excessively fine to allow the dirt to be deposited over the entire filtering bed, advantageous condition for allowing prolonged working times, before needing to perform back washing.

It has further been found that coarser grain sizes, greater than 2mm, would instead require filtering layers too thick to be applied in filters of acceptable dimensions at faster filtering speeds, equal to or higher than 50 m/h. The filtering bed 6 is supported by plate 7 on which nozzles 9 of the type, for example, described in patent application DE202004006883, are uniformly distributed. Said nozzles, preferably formed by polypropylene or glass fibre reinforced polypropylene, are inserted at holes made in plate 7 and are fastened

thereto by means of a lock nut. Heads 13 of the nozzles are immersed in the small layer of coarse gravel 3 which forms the base for the filtering bed 6. The function of nozzles 9 is to ensure both the water drainage during the filtering step and to distribute a back flow of air and/or water during the back washing step. During the filtering operation, each nozzle 9 allows the input of filtered water through very narrow slots 12, obtained on the surface of its head 13, and then drains it from the bottom through tubular stem 14, having a circular-crown-shaped section, into the lower zone of tank 2. Slots 12 of nozzles 9 are constructed so as to minimise the risk of soiling the head. The lower zone of tank 2, suitable to collect the filtered water output from tubular stems 14, is provided on bottom 4 of a stub pipe or pipe 8' which is used to evacuate the filtered water, but also to drain the filter and to introduce back washing water.

Two vertical slots 15, 15', preferably facing in the axial direction, which have a precise function of letting the air through during the filter back washing step, are advantageously contemplated in the lower part of each of stems 14; the input of back washing water in the nozzles occurs preferably through the hole on the lower end of stem 14.

Air and water are mixed already within the stem and then be mixed more closely in the passage through slots 12 of head 13. The air-water interface, also called 'meniscus', in the lower zone of tank 2 is advantageously maintained at a height comprised in the segment between slots 15, 15' and the lower end of stem 14, so as to always leave an introduction gap for the air through at least one of said slots. The meniscus is advantageously flat and stabile during the entire back washing step thus ensuring that there are no rushes in the flow rate of air and water to the nozzles and therefore that there is back washing uniformity.

The introduction of back washing air occurs by means of a jacket 16 external and coaxial to central water input pipe 8. The air reaches jacket 16 through tube 22. Said jacket 16 is connected to a plurality of air distributors 17, advantageously arranged as a dial, preferably in number of six, underneath plate 7 as shown in Fig. 3. Nozzles 9 are preferably distributed on said plate at circular sectors defined by the dial of distributors 17.

Advantageously, each distributor 17 has a rectangular section and is provided with slotted holes 18, with vertical longitudinal axis, on two sides and reciprocally

arranged at a closer distance on the outermost part of the distributor, with respect to the longitudinal axis of the tank, because the surface to be covered is greater. The air is uniformly distributed under nozzle holder plate 7 thus forming an air pocket. This back washing air distribution configuration is particularly advantageously, improving the back washing efficiency itself.

The introduction of back washing water occurs, instead, by means of a tube connected to stub pipe 8' of the tank.

The back washing water, which has dragged away the substances withheld by filtering bed 6 during its passage through the granules of sand, is then conveyed to be drained into central conduit 8, previously used for the introduction of the water to be purified into the filter. For this purpose, an upward flared funnel 1 1 , which has the feature of being relatively large and squashed, is contemplated in the terminal part of said pipe 8. The particular shape of funnel 1 1 , provided with an essentially conical-shaped edge 31 and flared downwards, allows an optimal overflow and promotes the necessary separation of the air from the back washing water: such effect is facilitated by the concurrent slowing of the speed and the formation of a thin film in the funnel which facilitates the release of the less dense air. The air which is released from the water is relieved through an appropriate relief valve 20 placed on the upper part of tank 2. Funnel 1 1 also allows to feed the water well during introduction for filtering.

The filtering process and subsequent back washing processes, periodically needed to clean the filtering bed, are described below.

The filtering process allows to remove from the water to be purified all the suspended substances which cause turbidity of the water itself and simply allows in the passage of the water to be purified, or simply sludge, through the single filtering layer of selected sand. The water crosses filtering bed 6 from the top downwards and the filtering occurs following the gradual slowing of the solid particles due to loss of kinetic energy in the rebounds/collisions with the sand particles. Advantageously, the filtering speed is in the 40-70 m/h range. After a certain operating cycle of the filter, the filtering bed needs to be cleaned or regenerated. The regeneration is performed by means of a back washing operation which consists in flowing water and/or air in the direction counter to the filtering direction, i.e. from the bottom upwards, so as to perform a back washing

which allows to remove the dirt accumulated in the filtering bed, then draining the dirty water into the aforesaid central pipe.

Such operation must be very effective because filtering bed 6, having a high thickness preferably from 2 to 4 m and a grain size preferably from 1 to 2 mm, implies that dirt is deposited very deeply in the filtering bed.

Advantageously, the back washing operation contemplates the steps of:

- draining;

- introducing back washing air only to break the head;

- introducing back washing air-water for an efficient back washing; - introducing back washing water only;

- final rinsing in parallel flow.

During the draining operation, the water contained in tank 2 is drained through stub pipe 8' in order to reduce the weight in the water column which weighs on plate 7. The drainage is performed to take the level of the water near the upper surface of the filtering bed, approximately a few centimetres above it. This operation is performed to allow a faster and above all smoother start up of the subsequent bubbling of the air in the filtering bed.

The introduction step of back washing air only leads to the formation of air bubbles in the sand layer and to the consequent bubbling of the air in the filtering bed which raises and expands. This operation lasts for approximately 1 minute and allows to efficiently detach the dirt from the sand granules. Advantageously, an introduction step of a mixture of back washing air and water follows. Such step occurs immediately after the previous one to promptly remove the dirt which has been released, thus preventing it from penetrating even deeper in the layer. Therefore, the back washing operation itself is performed with air and water, where the air serves again to detach the dirt from the sand granules while the water serves to move the removed dirt away. This step is the most important and indicatively lasts for the time needed to exchange the water contained in the expanded volume of sand at least 10 times. The turbulence generated by the air- water mixture within the granular material allows to perform a very efficient washing.

The subsequent step provides the introduction of back washing water only. Such step lasts for 2-5 minutes and allows the bed to be deposited in homogenous and

uniform manner without leaving free spaces which could reduce the filtering efficacy.

The back washing operation ends with a parallel current rinsing step which serves to prepare the filter for the filtering operation, further compacting the deposited layer, and to allow the elimination of the dirt not removed during the back washing step thus ensuring the production of perfectly clean water as soon as the filter is restarted. Such rinsing step lasts for approximately 5 minutes. Advantageously, the air and water speed parameters are respectively from 60 to 1 10 m/h and from 1 1 to 14 m/h during back washing.