WO1995009038A1

WO1995009038A1 - Method and device for slow filtration of raw water

Info

Publication number: WO1995009038A1
Application number: PCT/FI1994/000433
Authority: WO
Inventors: Juha Huhta
Original assignee: Paroc Oy AB
Current assignee: Paroc Hold Oy AB
Priority date: 1993-09-27
Filing date: 1994-09-27
Publication date: 1995-04-06
Anticipated expiration: 1996-03-27
Also published as: SE9403262D0; FI934241A0; FI96001B; FI96001C; FI934241L

Abstract

The present invention relates to a method and filter device based on adapting the filtration to take place on essentially vertical filter surfaces. The filter elements are advantageously made of mineral fiber. The footprint of a filter plant will be essentially reduced thus permitting the placement of the filter in a closed space. This approach is capable of avoiding the degrading effects of naturally occuring impurities and sunlight on the raw water quality. Moreover, the maintenance of the filter becomes easier and its intervals between maintenance will be extended.

Description

Method and device for slow filtration of raw water

The present invention relates to a filtration method and apparatus for treatment of raw water based on aeration, pretreatment and slow filtration of raw water pumped from an aquifer. Water treated according to the invention is as such, or in some cases after optional al alization, ready for use.

Slow filtration is understood to refer to such a water treatment process in which the surface loading rate in the actual filtration step is smaller than 0.5 m/h (that is, m³ water/h per 1 m² of filter surface) . Slow filtration can be used for the treatment of both surface water and ground water. The chief goals in slow filtration of surface water include improvement of the hygienic quality of the water, removal of solids and reduction of organic matter and nutrients. By contrast, slow filtration of ground water chiefly aims at removing iron and manganese. Other goals could be removal of ammonia, and in some cases, reduction of organic matter, too.

During the treatment process, iron and manganese contained in the ground water are oxidized by air, whereby these ion species are converted from soluble form into solid form, and hence, can be removed from the water with the help of naturally occurring microbial fauna (biological treatment) and sedimentation mechanisms (using physico-chemical treat¬ ments) . The elevated dissolved oxygen content and redox potential of the raw water required for the function of the iron and manganese bacteria employed in the oxidation process and biofiltration are attained by aeration. Multiple different known arrangements are available for accomplishing the aeration such as ejector, tray or stepped-flow aerator. Aeration of raw water also reduces the carbon dioxide content of the water and removes gases such as methane and hydrogen sulfide that degrade the smell and taste of water.

Pretreatment of raw water conventionally comprises at least a coarse filtration step which prior to the slow filtration step is capable of removing a major portion of iron contained in water. This contributes to a longer operating interval of the slow filter prior to its clogging. Aside the coarse filtration step, the pretreatment process may also include a clarification step.

In the slow filtration step proper, the filter medium accu¬ mulates filtered sediment which acts as a growth substrate for iron and manganese bacteria. With the help of oxygen, the strains of iron and manganese bacteria growing on this kind of substrate precipitate iron and manganese species contained in the water. Simultaneously, solids, microbes and other matter are retained by the filter.

Prior-art slow filters have comprised very large basins with a surface area of 5000 m², and even more, in which pure sand is used as the filter medium. The filter sand should have a grain size (d₁₀) of 0.15-0.75 mm, homogeneity factor (d₆₀/d₁₀) of 1.5-3.4 and iron content less than 0.1 %. Furthermore, the sand should be free of humic matter and other impurities. Availability of such pure and homogeneous iron-free sand is a major problem. In summary, a sand filter is hampered by a large land requirement, rapid clogging of the filter surface, difficult cleaning of the filter and poor availability plus high cost of filter sand. Furthermore, raw water pumped to an open basin is subjected to all impurities occurring in nature, even up to the extent that the quality of the raw water to be purified will degrade down to the quality of surface water during prolonged retention in the basin. It is an object of the present invention to overcome the disadvantages and problems described above. This goal is attained by means of a slow filtration method in which filtration takes place on filter surfaces aligned essen¬ tially parallel to the main flow direction of water to be filtered. As is conventional, the pressure required for filtration is accomplished by the action of gravity, whereby the filter surfaces are appropriately constructed vertical.

The embodiment according to the invention provides a high- performance, easy-to-service slow filter characterized by appreciably reduced headroom requirement need as compared to a conventional horizontal basin filter.

The filter elements are advantageously manufactured from mineral fiber which is stiffened by known binding methods to a desired shape. Then, the properties of the mineral fiber layer can be advantageously utilized including its excellent filtering capacity in a direction normal to the fiber orientation and high hydraulic conductivity in a direction parallel to the fiber orientation.

By virtue of a filter construction having a planar bottom of mineral fiber and easily replaceable mineral-fiber filter elements outdistanced from each other at a distance and placed rising from the bottom vertically upward, a slow filter is attained with a headroom requirement of only a fraction of that of a conventional horizontal pool filter. The filter is also immediately in full operating condition after the replacement of the vertical filter elements, because the space between the elements on the bottom of the filter will contain a sufficient seeding amount of iron- and manganese-oxidizing bacteria to restore the biofiltra- tion process. Owing to the modest headroom requirement of the slow filter according to the invention, the filter can be readily constructed in a closed space, whereby the detrimental effects of naturally occurring impurities and sunlight on the water contained in the filter tank are avoided.

In the following the invention will be examined in greater detail with reference to the attached drawings, in which

Figure 1 is a schematic representation of a conventional slow filtration process, and

Figure 2 is a schematic sectional side view of a lamellar slow filter according to the invention.

With reference to Fig. 1, a conventional slow filtration method comprises pumping raw water from an aquifer to a raw water aerator 1 in order to elevate the dissolved oxygen content of water to a level sufficiently high for starting the biofiltration process. The aerated water is prepurified in a coarse filter 3 which may be a rock-bed filter of crushed stone, for instance. During such coarse filtration, solids will be removed from the raw water, and from ground water in particular, a major portion of iron. This results in an essential lengthening of the clogging interval of the slow filter 4 proper. In some cases it is advantageous that the coarse filtered water further passes a clarification step 5 prior to the slow filtration step proper. The slow filter 4 comprises a water inlet 6, a basin 7, a sand filter layer 8 and discharge means 9 of filtered water. The sediment formed on the filtration surface 10 of the filtering sand layer 8 provides a growth substrate for the bacteria employed in the biofiltration process. As water purification principally occurs on said filter surface, the surface area of the sand filter must be extremely large to assure a reliable long-term operation of the filter.

As the sand filter will be clogged after a certain period of use, the surface sand layer of the slow sand filter is peeled off for a depth of several tens of centimeters with the help of earth moving equipment and replaced by fresh sand. Then, exhausted bacterial populations is lost almost entirely with the removed portion of filter sand bed.

With reference to Fig. 2, the construction of a slow filter 4 according to the invention comprises a modular unit formed by a covered container or tank 7 into which the water to be filtered flows via a separate water inlet 6, or alternatively, through a modular coarse filter unit 3 placed above the slow filter 4. The tank bottom is provided with a mineral fiber bottom layer 11 and discharge means 9 of purified water. Projecting upward from the top of the mineral fiber layer 11 are placed a number of appropriately supported, vertical filter elements 8, advantageously made from mineral fiber into the shape of a slab, pipe or other functional element. The lower parts of the filter elements 8 may be inserted into the bottom layer 11 as shown in the diagram. To promote the discharge of purified water from the core of the filter elements 8 in the direction denoted by arrows in the diagram, fiber orientation in at least the center of the filter elements 8 is made parallel to the filtering surfaces 10 of the filter elements. Water level 12 is controlled to such a level that yields a desired through-flow time for the water passing through the filter.

According to an advantageous embodiment, the vertically aligned filter elements 8 of the slow filter 4 comprise parallel-oriented mineral fiber slabs with a standard size of, e.g., 1200 mm x 650 mm and a thickness ranging from 40 mm to 140 mm. Accordingly, filtration takes place on both sides of the mineral fiber slab.

The filter elements 8 may also have a trapezoidal section, whereby the element thickness can be arranged to increase either upward or downward. To the end of assuring unob¬ structed discharge of the filtered water from the filter element 8 to the discharge means 9 of purified water and of simultaneously permitting the filter element surfaces 10 under the action of gravity to gather sediment which acts as a growth substrate for the iron and manganese bacteria, a downward flaring thickness is advantageously employed. To the contrary end of keeping the filter element surfaces unclogged as long as possible, a filter element with upward increasing thickness may be the most preferable choice.

An extremely advantageous embodiment is achieved by fabri¬ cating the filter elements 8 into essentially vertical tubes made of bonded mineral fiber. Then, the filtration can be arranged either so that both the inner and outer surfaces of the tube act as filter surfaces 10, or alter- natively, so that only the outer surface acts as the filter surface 10, while the filtered water seeps into the core of the tube flowing therefrom along the inner surface of the tube to the collecting space arranged below the bottom layer 11. In the latter case the tubular elements must either extend above the water level 12, or alternatively, have closed upper ends.

At the start of the slow filter 4, the filtration princi¬ pally takes place at those areas of the bottom layer 11 which are disposed between the vertical filter elements 8. As these areas will eventually become clogged, the filtra¬ tion process occurs only on the vertical filter surfaces 10 of the filter elements 8. When the vertical filter surfaces 10 become clogged after a long period of use, the filter elements 8 are replaced, whereby the areas disposed on the bottom layer 11 between the filter elements 8 will contain so much iron- and manganese-precipitating bacteria that both the biofiltration and physico-chemical processes of the slow filter 4 are started immediately after the filter elements 8 have been replaced. No run-in time of several weeks is required for growing a new bacterial population. The coarse filter 3 may advantageously have a construction similar to that of the slow filter 4. The filter elements may also in this embodiment be fabricated from mineral fiber with properties suited for coarse filtering. Both the slow and coarse filter modular units may be connected in parallel and series as required.

As tests performed on the function the mineral fiber slow filter have proven that the filtration efficiency of this filter per unit area is at least equal to that of a slow sand filter, the arrangement according to the invention offers a substantial reduction in the footprint of a slow filtration plant and/or extended maintenance interval of the filter. The filter tanks can be constructed entirely closed, whereby a high water quality is easy to sustain.

Moreover, replacement of elements is simple and the opera¬ tion of the filter is restored immediately after filter element replacement.

It must be noted that water aeration in the method according to the invention is carried out along the same principles as in conventional methods .

Claims

Claims :

1. A filtration method for raw water purification, in which method raw water pumped from an aquifer is aerated, prepurified and slow filtered, c h a r a c t e r i z e d in that, during the slow filtration step, the filtration is carried out on filter surfaces aligned essentially parallel to the main flow direction of the water to be filtered.

2. A slow filter (4) for purification of raw water com¬ prising means for water intake (6) and discharge (9) , a water tank (7) with a filter element (8) adapted to said tank, c h a r a c t e r i z e d in that the filter surface (10) of said filter element (8) is aligned essentially vertical.

3. A slow filter (4) as defined in claim 2, c h a r a c ¬ t e r i z e d in that said filter includes a plurality of parallel filter elements (8) .

4. A slow filter (4) as defined in claim 2 or 3 , c h a r a c t e r i z e d in that said filter elements (8) are principally comprised of mineral fiber.

5. A slow filter (4) as defined in claim 4, c h a r a c ¬ t e r i z e d in that said filter elements (8) are slab- shaped and mounted parallel at a distance from each other so that the narrowest dimension of the slab-shaped element is aligned essentially horizontal.

6. A slow filter (4) as defined in claim 4, c h a r a c ¬ t e r i z e d in that said filter elements (8) are com¬ prised of essentially vertical tubes made of bonded mineral fiber.

7. A slow filter (4) as defined in any of foregoing claims 3 - 6, c h a r a c t e r i z e d in that the discharge of filtered water from said filter element (8) is arranged through the bottom surface of the filter element (8) .

8. A slow filter (4) as defined in claim 6, c h a r a c - t e r i z e d in that the discharge of filtered water is arranged to essentially occur via the core of said tubular filter element.

_^ A slow filter (4) as defined in any foregoing claim, c h a r a c t e r i z e d in that the tank (7) of the filter is provided with a cover.