GB2064350A - Regenerating sand filters - Google Patents

Abstract

A sand filter can be regenerated by expanding (but not fluidizing) the sand with air and simultaneously passing liquid through the sand in the same direction as the air.

Description

SPECIFICATION Method of regenerating a sand filter This invention relates to a method of regenerating a sand filter and, more particularly but not exclusively, is concerned with a method of regenerating a sand filter which has been used for clarifying sewage effluent.

Traditionally, sand filters for clarifying sewage effluent comprise a vertical cylindrical vessel the lower and upper portions of which are empty and the middle of which is filled with a bed of sand. In use, sewage effluent is introduced into the lower portion of the vertical cylindrical vessel and is pumped upwardly through the bed of sand. The sand traps the suspended particles in the sewage effluent and the clarified effluent leaves the upper portion of the sand filter. After a certain period of time the sand becomes clogged with the particles and it is necessary to remove the particles from the sand. This operation is referred to as regeneration.

Originally, regeneration was achieved by pumping a stream of effluent downwardly through the sand filter. A typical plant operating on this principle is shown schematically in Figure 1. In particular, the plant comprises a sand filter 1 which comprises a vertical cylindrical vessel 2 having empty upper and lower portions 3 and 4 separated by a bed 5 of sand.

Two pipes 6 and 7 are connected to the bottom of the sand filter 1. Pipe 6 is provided with a pump 8 and a valve 9 whilst pipe 7 is provided with a valve 10. Two pipes 11 and 12 are connected to the top of the sand filter 1. Pipe 11 is provided with a valve 13 and a pump 14. Pipe 12 is provided with a valve 15.

During clarification valves 10 and 13 are closed whilst valves 9 and 15 are open. Pump 8 displaces sewage effluent through pipe 6 and sand filter 1 which the clarified effluent leaves through pipe 12. Once the sand is clogged with particles valves 9 and 15 are closed and valves 10 and 13 opened. Clarified effluent is then pumped by pump 14 from a reservoir 16 through pipe 11, sand filter 1 and pipe 7, until the sand 5 is ready for use again.

It should be noticed that in the arrangement shown in Figure lit is necessary to stop the flow of sewage effluent whilst regeneration is being effected and for that reason two or more said filters are normally used, one being regenerated whilst the other(s) is on stream.

The plant shown in Figure 1 was generally succeeded by that shown in Figure 2. In this figure, parts having similar functions to those shown in Figure 1 have been identified by the same reference numerals.

Referring to Figure 2, sewage to be clarified was pumped through line 6 by pump 8 and the clarified effluent left the sand filter 1 through pipe 12. When the bed 5 was ready for regeneration, valves 9 and 15 were closed and valves 20 and 25 opened. Valve 20 was connected to an air compressor 21 which blew air upwardly to fluidize the sand 5. This displaced the suspended material to the top of the sand 5. Valve 20 was then closed and valve 22 opened to permit a supply of clarified effluent from tank 23 to be displaced through the sand filter 1 through pump 24 at a flow rate of approximately double the flow rate of sewage effluent normally passing through pump 8. This system of cleaning was far more effective than that shown with regard to Figure 1.

In order to improve the arrangement shown in Figure 2, it has been proposed to use clarified water and air simultaneously. However, those concerned with such proposals have used air and water flow rates comparable to those previously used for regeneration with the result that initially, large amounts of sand were discharged through pipe 26. This problem was somewhat reduced by the insertion of a sheet of mesh over sand 5 but this solution has not been altogether satisfactory.

We have found that regeneration comparable in quality with the plant shown in Figure 2 and frequently taking less time and energy can be achieved using a simpler layour and method of operation.

Our invention is based on the discovery that instead of blasting vast quantities of air and water through the sand all that is needed is sufficient air to open up the particles of sand. Once the particles of sand are open the whole bed can be regenerated by washing with either clarified water or simply the same effluent which is normally being clarified. A further advantage which transpires is that the flow rate of the clarified water or sewage effluent can be the same as that used during the clarifying operation rather than the high rates normally used.

According to the present invention there is provided a method of regenerating a sand filter containing a bed of sand contaminated with particles which method comprises the steps of applying sufficient gas to the sand to expand, but not fluidize, the bed and passing liquid through the bed in the same direction as the gas to remove the particles from said bed.

Although the gas will normally be air it is envisaged that oxygen or oxygen enriched air could also be used.

Preferably the air will be supplied to the bed at the rate of from 25 to 85 Nm3/m2 cross-sectional area of bed per hour (Nm3/m2/hr) and advantageously between 45 and 70 Nm3/m2/hr.

Advantageously, the liquid will be passed through the bed at between 2 and 12 m3/m2/hr.

As an approximate guide the height of the bed during regeneration should not exceed 1.2 times its height at rest, i.e. when there is no flow through the sand filter. In this connection we would recommend that the height during regeneration should be between 1.05 and 1.10 times its height at rest.

One embodiment of an apparatus for carrying out a method in accordance with the present invention is shown, by way of example, in Figure 3 of the accompanying drawings.

Referring to Figure 3, sewage effluent is pumped through pipe 6 into sand filter 1 at a rate of 6.25 m3/m2/hr. The suspended particles in the sewage effluent are trapped in the bed 5 of sand and the clarified effluent leaves the sand filter 1 via pipe 12. The bed 5 of sand comprises an upper layer of 2 mm to 4 mm sand and a lower layer of 5 mm to 10 mm sand supported by a gravel layer of 30-50 mm particle size. When the bed 5 of sand becomes clogged, valve 1 5 is closed and valves 25 and 20 are opened. Blower 21 supplies air through nozzles 27 mounted in the lower portion 4 of the sand filter 5. The air is supplied at the rate of 65 Nm3/m2/hr. As the air bubbles rise they expand the bed and the effluent leaving the sand filter 1 through pipe 26 becomes loaded with entrapped debris.

Regeneration is rapidly achieved and, on completion, valves 20 and 25 are closed and valve 15 is opened.

Various tests were carried out to determine whether there was an optimum relationship between the condition of the bed 5 and the efficiency of regeneration. Firstly, it was discovered that the sand bed should not be fluidized as this led to a certain amount of sand being entrained with the particles leaving the sand filter 1. Furthermore, such high amounts of air and the associated energy needed to supply them were unnecessary. Secondly, sufficient air should be supplied evenly to maintain the height of the expanded bed during regeneration between 1.05 and 1.10 times the height of the bed at rest. In this connection the bed which we were using fluidized when its height was increased by 25% of its height at rest.

It will be seen that the operation of the plant described with reference to Figure 3 has several advantages from the prior art. In particular, the pump 8 ran continuously at constant displacement.

Secondly, regeneration was accomplished using sewage effluent rather than clarified effluent. Thirdly, no provision had to be made for the storage of clarified effluent for use in regeneration. This last mentioned feature is particularly important when it is considered that sand filters are frequently sold as part of a package complete with their ancillary equipment so that the user simply has to connect his sewage effluent pipe to the inlet of the package, one outlet of the package to the local river, and the other to the primary treatment unit of his sewage works.

For the avoidance of doubt the term "sand" as used in the introduction and claims embraces both natural and synthetic particles capable of acting as a mechanical filter.

Turning now to a related topic it was found that whereas the sewage effluent entering the sand filter 1 during filtration contained 1 1.6 mg/l N03-N, the clarified effluent contained only 4.8 mg/l N03-N. This is particularly surprising since heretofore it has always been found necessary to add a carbon source such as methanol to the sewage effluent to achieve such a reduction.An analysis of the sewage effluent and clarified effluent is as follows: Sewage Clarified Effluent Effluent pH 7.5 7.9 suspended solids (mg/l) 31 22 OXN (mg/l) 11.6 4.8 BODS (mg/l) 9.0 6.0 We attribute this reduction to the combination of two factors, viz: a relatively thick bed (1.5 to 3.0 metres (preferably 2.0 to 2.5 metres) and a relatively low flow rate of effluent (2 to 12 (preferably 5 to 10) M3/m2/hr). We believe that in these conditions the bed will support some form of biological activity which will be forced to turn to nitrates for oxygen. This should be contrasted against prior art systems where the maximum thickness of the bed was about 0.3 metres for a gravity bed or where, in the case of a deep pressure filter, the flow rate was much higher than indicated above. In the former conditions sufficient oxygen for the bacteria is more readily available from the air whilst in the latter conditions the high intrinsic velocities may not enhance this form of biological activity in the pressure filter.

Claims (8)

1. A method of regenerating a sand filter containing a bed of sand contaminated with particles by the use of liquid and gas characterized in that sufficient gas is applied to the bed to expand but not fluidize the bed and simultaneously, liquid is passed through said bed in the same direction as said gas to remove the solid particles from said bed.

2. A method according to Claim 1, characterized in that said gas is air.

3. A method according to Claim 1 or 2, characterized in that said gas is supplied to said bed at the rate of from 25 to 85 Nm3/m2/hr.

4. A method according to Claim 3, characterized in that said rate is from 45 to 70 Nm3/m2/hr.

5. A method according to any preceding Claim, characterized in that said liquid is sewage effluent.

6. A method according to any preceding Claim, characterized in that said liquid is passed through said filter at from 2 to 12 m3/m2/hr.

7. A method according to any preceding Claim, characterized in that during regeneration said flow of gas is adjusted to maintain the height of said bed between 1.05 and 1.10 times its height at rest.

8. A method of regenerating a sand filter substantially as herein before described with reference to Figure 3 of the accompanying drawings.