GB2264653A - Filter bed - Google Patents

Abstract

A downward flow filter bed 2 is supported on an inclined floor 5, fitted with nozzles 6 for draining filtrate and delivering backwash water and air. The space below floor 5 is divided by upright walls into compartments 7 which communicate only at their lower parts by openings 11, which also accommodate a manifold 9 with outlets 10 for compressed air. Openings 11 offer resistance to water flow which cancels out the different resistances presented by relatively thick and thin sections of bed 2 over respective compartments when filtrate is drained or backwash water supplied through duct 8. The stems of nozzles 6 in respective compartments 7 extend down to stepped levels as shown. An outlet (not shown) above the lowest part of floor 5 may allow discharge of bed material. <IMAGE>

Description

Filter with Inclined Floor This invention relates to the construction of filters which employ a granular medium such as activated carbon.

Granular media filters in the context of this invention consist of tanks or vessels provided with a perforated or porous floor connected to a collecting system. The tank or vessel contains a granular medium through which water passes in the course of filtration or adsorption.

Sand filtration of water is one very common example. In this case the flow must be reversed from time to time to flush out the solids which have been intercepted by the sand. It is also common practice to distribute air through the floor to agitate the grains during the reversed flow (or backwash) to assist the cleaning action.

When such a device is used with granular activated carbon or other adsorbent materials to adsorb impurities from the water, the design must be such that the adsorption grains may be removed easily when exhausted, and that the backwash water and air can be distributed uniformly at other times, because such granular adsorbent will also act as a filtering medium.

During the backwashing process the flow is designed to fluidise the grains within the tank. Under this condition, the grains flow freely and the bed is self-levelling and most of it will flow out of any low level aperture. However, to remove the last residue, it is helpful to slope the perforated floor so that the granular of material may be extracted through a pipe or sump at the lowest point.

A sloping floor in such a system creates a number of problems. Because the resistance to flow through the bed of grains increases with its depth, there will tend to be a higher flow through the shallower parts and the porous floor must be capable of controlling the flow across the whole area, so that flow variations are within acceptable limits . Secondly and more importantly, if air is to be distributed into the bed at any point during backwash, conventional filter strainers (such as those described in our British Patent No.

2,111,400) set in such an inclined floor would be incapable of distributing the flow evenly. Indeed it is a feature of this invention that the stems of the strainers may be adjusted to ensure that they are level regardless of errors in the concrete structure so that uniform air distribution is obtained.

During air injection or air scour, orifices in the nozzles and slots in the nozzle stems provide resistance so that the air/water interface is depressed, leaving an air space that allows the air free access to all parts of the floor. The air flow through each stem depends upon the height of the orifice above the air/water interface, and the area of the slot exposed to the air.

This invention comprises a filter for use with a granular filter medium, including a tank for holding the medium, the tank having a floor provided with openings serving as outlets for filtered liquid and inlets for backwash fluids, in particular water and air, the floor being situated over an underfloor drain area or plenum provided with means for draining off filtered liquid and means for providing a supply of backwash fluids; in which filter the floor is sloped to one or more low level sumps and one or more exits are provided above the floor, but at a low level, for removal of the medium; and in which the underfloor area is divided by walls into a horizontal series of separate compartments which may interconnect only at a low level, each compartment having at least one said opening in the floor at the top of the compartment, and where a compartment has two or more said openings, they are arranged so that their lowest extents are at a common level, the level of the lowest extent of an opening in any compartment being, depending on the slope of the flow, higher or lower than that of openings in other compartments.

The walls may have low level apertures between them to permit water to flow between the compartments of the floor, but not at the level where the air is present.

Air, when required, is delivered into each compartment from orifices in a manifold pipe running through the compartments or from orifices in an end wall. Such orifices will normally be level with each other and designed to provide an equal flow into each compartment or if the latter are not of equal area, then the flow will be proportional to the area.

The strainers and outer bushes of the floor nozzles will normally follow the sloping profile of the upper surface of the floor, the nozzle stems will be adjusted to a common level in each compartment.

The general concept of the floor may be extended in two dimensions, for example with two side slopes falling to a central gutter which may itself slope away at right angles.

For moderate variations in depth of bed, the water flow across the bed can be controlled by choosing a suitable pressure loss across the nozzles. The losses in any typical nozzle vary with the square of the flow rate, being turbulent whereas the loss through the bed is laminar and varies linearly with the depth. It is a further feature of this invention that should the variations in depth in the bed be such that the loss of head through the nozzle is incapable of maintaining the range of flow within an acceptable band, then the backwash flow to the filter may be admitted to the lowest compartment only, allowing the other compartments to be fed via each other, through an orifice (or orifices) to provide an incremental pressure loss corresponding to the incremental pressure loss through the areas of fluidised bed above each compartment.

In the forward filtering mode conditions are governed primarily by the resistance of the granular bed, the floor merely provides a low resistance underdrain.

The invention is illustrated with reference to the Figure which is a cross-section through a filter tank charged with a filter medium and with water. A tank (1) contains a granular medium (2) with clear water (3) above which can overspill into washout launders (4).

The granular medium is supported on a sloping floor (5) provided with strainers (6) which include an upper perforated strainer and a stem which projects through the floor (5). This floor is supported upon dwarf walls which define compartments (7) which have varying depth.

A connecting pipe (8) allows filtrate to be discharged or backwash water to be injected in the reverse direction. If air is to be distributed into the filter this maybe achieved by a manifold pipe (9) provided with a discharge orifice (10) in each compartment. There is also a step in the operating level between each compartment. During backwash with such an arrangement, the backwash water will enter the shallowest compartment (7a) and flow progressively to compartments (7b), (7c), and (7d) to supply the outer compartments. The headloss through the orifice (11) in each case should be sufficient to offset the progressively lower headlosses through the shallower bed.

During air scour, which may be simultaneous or separate, the airflow is controlled by the orifices (10). The air will form a space at the top of each compartment and the nozzle stems will normally be adjusted so that their bottom ends level within each compartment. In such a way uniform air scour will be achieved across the entire bed.

If so desired, the lowest compartment (7a) may be furthermore subdivided and slope away in the third dimension to a bottom outlet to enable the granular media to be discharged when required.

Thus the invention comprises a floor system for a granular media filter that can be constructed in an inclined or shallow trough form which will still distribute air and water together or separately with accuracy similar to that of a level floor. This is achieved by forming the plenum as a set of cells each with an individual supply of air and either an individual supply of water or a supply which flows in series through the compartment as described.

Claims (1)

1. Claims

   1. A filter for use with a granular filter medium, including a tank for holding the medium, the tank having a floor provided with openings serving as outlets for filtered liquid and inlets for backwash fluids, in particular water and air, the floor being situated over an underfloor drain area or plenum provided with means for draining off filtered liquid and means for providing a supply of backwash fluids; in which filter the floor is sloped to one or more low level sumps and one or more exits are provided above the floor, but at a low level, for removal of the medium; and in which the underfloor area is divided by walls into a horizontal series of separate compartments which may interconnect only at a low level, each compartment having at least one said opening in the floor at the top of the compartment, and where a compartment has two or more said openings, they are arranged to that their lowest extents are at a common level, the level of the lowest extent of an opening in any compartment being, depending on the slope of the flow, higher or lower than that of openings in other compartments.