WO2000061260A1

WO2000061260A1 - Apparatus for cleaning of liquids comprising a granulated filter material

Info

Publication number: WO2000061260A1
Application number: PCT/NO2000/000120
Authority: WO
Inventors: Bjørnar Eikebrokk
Original assignee: As Norsk Leca
Priority date: 1999-04-13
Filing date: 2000-04-13
Publication date: 2000-10-19
Also published as: EP1194209A1; NO314291B1; CA2367516A1; NO991750D0; AU3846100A; NO991750L

Abstract

The present invention concerns a granulated material and the use of this in depth filters for the cleaning of liquid. The material consists of grains of one single material with varying density and diameter. The grain density increases when the grain diameter decreases in such a way that an ideal distribution of the grain size from coarse to fine in the flowing direction of the liquid down through the filter bed is achieved as a result of the sedimentation of the grains after ended backwash and corresponding expansion of the bed. The filter bed's design and properties can be adapted to various filtration purposes and the entire filter bed can be supplied as one total package from the same supplier. The properties of the grains can be manipulated and an ideal filter bed can be put together to optimise cleaning processes and the use of the filter, without the need of using several filter media.

Description

Apparatus for cleaning of liquids comprising a granulated filter material.

The present application concerns an apparatus for the cleaning of liquids, comprising a filter bed for downstream filtration of the liquid, said filter bed comprising one or more granulated materials. The invention also describes the use of the filter material in an apparatus for the cleaning of liquid comprising a filter bed for downstream filtration of the liquid.

When removing particles from liquids by means of deep bed filters, it is known that filter beds operates at its optimum when the efficiency of filtration, or the filter coefficient for the filter, increases in the direction of flow of the liquid, and this can be achieved by letting the filter grain size decrease in the direction of flow of the liquid through the filter bed. Today this ideal situation is attempted achieved by using upstream filters or two- or multimedia downstream filters. Up- stream filtration has certain disadvantages relating to its operation, and will not be described further. Downstream filterbeds are based on a combination of two- or several different filter materials with different grain size and density, such as anthrasite, silica sand and garnet sand. Normally, are one or several relatively thin support layers of coarse material situated underneath the main filter bed. The support layers are not a part of the filter bed and have no function except from keeping the filter bed in place, and to contribute to an even distribution of the backwash water, etc. and will not be further dealt with.

To be able to keep the desired stratification and grain distribution from coarse to fine down through such filter beds, that are regularly cleaned by back- washing and expansion, followed by the sedimentation of the grains back into the filter bed, is it necessary that coarse grains have a lower sinking rate than the smaller grains. This is achieved by letting the filter materials in the different layers of the filter bed, besides being of a dissimilar grain size, have different densitites, e.g. coarsely grained anthrasites with low density in the upper filter layers and finely grained sand with higher density in the lower layers.

Two- or multimedia filters of such type are well described in the related literature, such as Ives, K.J.: Basic Concepts of Filtration, Degremont: Water Treatment Handbook, B. Eikebrokk: Dr. Ing. avhandling, NTH, 1982. Such tradi- tional filters are described in the patent literature, for instance EP 0163 531 B1 , US 3,814,247 and US 3,343,680.

The advantages of two- or multimedia filters as compared to single medium filters are also thoroughly described and there are many descriptions of how two- or multimedia filters should be designed in terms of materials that can give favourable combinations of density and grain size. An example of this is shown on fig. 1 (Ives, K.J., Zeitschrift fϋr Wasser- und Abwasser-Forschung, 12, No. 3/4/79, pp. 108). With this figure as reference, two- or multimedia filters can be put together and comprise two- or several dissimilar materials (filter layers) where density and grain size are chosen to fulfil two essential criteria of function: firstly, that filter layers must be composed of materials with such a grain size and density that liquid on its way through the filter bed first encounters a filter layer with the largest filter grains, and thereafter layers with reduced filter grain size, and, secondly, that materials (the filter layers) must have dissimilar sinking rates in order to make the materials (layers) maintain their respective place in the filter bed even after many sequences with backwash, expansion and sedimentation of the filter grains. As given on the figures, the selection of materials for various layers of the filter bed must take place along a line falling towards the right (multilayer), so that grain size and sinking rate differ from layer to layer. The optimal and ideal, continuously decreasing grain size from coarse to fine in the direction of flow of the liquid, has previously been attempted achieved by using upstream filters or downstream filters with two- or several types of materials (filter media), where the ideal situation is approached by using filter beds comprising two- or several layers with various media. The latter filter types are called two- or multimedia filters respectively. In these filters coarse to fine grain distribution is approached from filter layer to filter layer in the direction of flow of the liquid by using filter materials or filter media of various types, grain size and density, and where each material forms a separate layer. The filter bed is designed with an upper of the largest grains with the lowest density, followed by the next layer with somewhat smaller grains and somewhat higher density, and so on. The sum of these layers forms the total depth of the filter bed.

Typical examples of commonly used filters of this type, for the treatment of drinking water, are two-media filter beds with the coarsely grained anthracite (1- 2 mm) with low density (1 ,4 kg/dm³), over finely grained sand (0,5-1 ,0 mm) with higher density (2,7 kg/dm³), or three-media filters where in addition, a layer of coarser and lighter plastic materials is above the anthrasite and the sand. Alternatively, a three-media filter may comprise a layer of finely grained garnet sand with high density, placed under the anthrasite and sand layers. Multimedia filter- beds comprising more than three different materials can also be found, but they are still put together according to the same principles.

By using the above mentioned traditional filter materials according to the state of the art, the designer of the filters is restricted to the given properties of the materials and must know the relative properties of the materials. In some cases, the selection of the materials and the composition of the filter bed may not be optimal, and this may for instance affect the efficiency of the cleaning, the functionality and capasity of storage, and may necessitate frequent backwash with high consumption of backwash water. An unfavourable choice of materials in the filter bed may also result in the backwash having to be performed with larger amounts of liquids to fluidise the filter bed than what is ideally necessary.

At the same time, some filter media have high density and weight, even as dry, and this can be a disadvantage in terms of freight and handling. Additionally, today's filter materials are manufactured and delivered by several different manufacturers and suppliers, and it is therefore necessary for the designers of the filter plants (consultants, plant owners, etc.) to provide necessary data from several places concerning the various filter materials, in order to select the right material type and combination to design an optimal filter bed, to provide the various materials that are used in the right amount, etc. This poses a challenge to the consultants, the designers, the plant owners and users of this type of filters. It is therefore a need for a better adapted filter medium where the filter designer to a greater extent can choose the properties of the materials regardless of the inherent properties of the natural filter materials such as anthrasite and sand, a filter that gives a better optimised filtration and backwash process, which is easy to handle and that results in less responsibility on the plant owners and operators in connection with handling, ordering and composition of the filter bed. The present invention therefore comprises, according to the independent claim, an apparatus for the cleaning of liquids, comprising a filter bed for downstream filtration of said liquid where the filter bed comprises one or several granulated materials. At least one of the granulated materials comprises grains with varying density and size.

This material may be haydite or light clinker of expanded clay or plastic material with properties such as density and a grain size of the filter material can be adapted during production so that the density of the filter grains is reduced with increasing grain size. In this way a filter bed can be designed with one single raw material with substantially continuous or stepwise (stratified) gradation or size from large to smaller grains in the direction of flow of the liquid. This gives according to the filter theory and experience, an optimal filtration process as the particles that are removed from the liquid, are stored as evenly over the depth of the filter bed as possible. This results in less pressure loss, optimal length of the filter cycles and storage capacities, the minimum amount of backwash water, etc. By reducing the backwash frequency, the wear on the filter media and related filter equipment is reduced. This will lead to an increase of the useful lifespan of the filter medium and the rest of the filter components.

The invention is intended to encompass filter beds with the above material alone or in combination with other filter media such as sand or anthrasite.

The filter material can be supplied from one single manufacturer, it can be adapted to various filtration purposes, and the filter designer or plant owner can design and compose of the filter bed. The filter bed can be delivered mixed prepacked as a unit from one supplier, with grain sizes, densities and quantity specially adapted for various types of use, such as cleaning of water, sewage water or other liquids.

The filter medium can be backwashed and then sedimented in place in the filter bed by the filter operator.

The backwash (cleaning) of the filter bed can be performed with lower flushing velocities and less backwash water than what is normally used, and this is achieved by letting the filter bed comprise easily fluidised material with low density and low a sinking rate. The invention will now be described by means of an embodiment and with reference to the figures where:

Fig. 1 shows the sinking rate of grains of various filter media, with an example of a composition of multimedia (Mehrschicht/multilayer) filters, where the sinking rate of the filter grains is given as a function of grain size and density. Fig. 2 shows a typical deep bed filter with a granulated material according to the invention.

Table 1 shows an example of the results achieved with direct filtration of coagulated drinking water in partial filters comprising a filter medium according to the invention and with a traditional two-layer anthrasite sand filter as reference. Fig. 1 shows how it is possible to assemble multimedia filters in terms of grain size and type of material (density) to achieve proper sedimentation of the filter material after backwash, so that coarse grains stratifiy over (i.e. as a lower sinking rate) the finer grains. On fig. 2, is a two-media filter 1 according to the invention shown with an inlet 4 and an outlet 5. The liquid 6 that is to be filtered is on top of the filter. The filter comprises a filter medium with coarse grains with low density 2 over finer grains with higher density 3. The grains are made of one single material, such as expanded clay or plastic. The filter has two thin support layers 7 in the bottom. The plastic material can be expanded to foam with various densities.

Table 1 shows an example of results after direct filtration of drinking water containing humus in two partial filters with filter beds comprising the granulated grain material of expanded clay (LK) according to the invention in two fractions of grain sizes and densitites, and a traditional two-media filter bed with anthrasite over sand (A/S) respectively. The table shows that the LK filter gives results that fulfil the quality standards according to the Norwegian drinking water regulations with a good margin, and furthermore that results from the LK filter can be compared with a parallelly run traditional anthrasite sand filter in otherwise similar conditions. The filter beds have the same total depth whereas the grain sizes are as follows:

Two layers of material according to the invention, where the first has a grain diameter of 1 to 2 mm above a layer with 0,8 to 1 mm grain diameter, and where the grains in the two layers have different densitites, such as 1 ,2 and 2,0 kg/dm³. The filter medium is made of expanded clay. As a comparison, the used two-media filter according to the state of the art has a layer of anthrasite with a codiameter of 0,8 to 1 ,6 mm over a layer of sand with a grain size of 0,4 to 0,8 mm. Table 1

Ut2-A/S 8.5 0.05 1 90 0.73 72 Ut1-LK 4.2 0.13 2 89 0.86 67

Claims

1. Apparatus for cleaning liquids, comprising a filter bed for downstream filtration of the liquid where the filter bed comprises one or several granulated ma-

5 terials, wherein at least one of the granulated materials consists of grains with varying density and size.

2. Apparatus according to claim 1 , wherein the granulated material with varying density and grain size is expanded clay or light clinkers or haydite.

10

3. Apparatus according to claim 2, wherein the light clinkers are produced of expanded clay, burned at 1100-1200°C.

4. Apparatus according to claim 1 , wherein the granulated material with vary- i5 ing density and grain size is a plastic material.

5. Apparatus according to any preceding claim, wherein the granulated material with varying density and grain size has a grain diameter within the range ot 0.5 mm to 0.8 mm.

20

6. Apparatus according to any of the preceding claim, wherein the granulated material with varying density and grain size has a grain density in wet condition in the order from 1.2 kg/dm³ to 2.6 kg/dm³.

25 7. Apparatus according to any of the preceding claim, wherein the granulated material with varying density and grain size has a grain density that increases when the grain diameter decreases.

8. Apparatus according to any of the preceding claim, wherein the granulated 30 material's grains, after the backwash and accompanying expansion of the filter bed, is sedimented at such a rate that the smallest grains end up at the bottom of the filter bed and the largest grains end up on uppermost.

9. Apparatus according to claim 1 , wherein the liquid is water or sewage water.

10. Use of filter material of grains with varying density and grain diameter, where this is made of one single material, in an apparatus for the cleaning of liquid comprising a filter bed for downstream filtration of the liquid.