WO1997022559A1

WO1997022559A1 - Method for biological purification of water

Info

Publication number: WO1997022559A1
Application number: PCT/SE1996/001700
Authority: WO
Inventors: Kurt Jönsson
Original assignee: Nordic Water Products Ab
Priority date: 1995-12-19
Filing date: 1996-12-18
Publication date: 1997-06-26
Also published as: AU1218597A; SE9504544L; SE9504544D0

Abstract

The invention concerns a method for biological treatment of water in a filter bed including a granular filter medium which in a dirty state is taken out at the lower part of the bed and after transport upwards and cleaning, is brought back to the upper part of the bed, whereby water to be purified passes from below and upwards through the filter bed. The invention is distinguished by the thickness of a bacteria layer around the granules of the granular filter material being brought to be limited to a maximum of 50 νm by said transportation upwards and/or washing such that a stationary water layer which is present outwardly around the granules comprises the main resistance against diffusion into the bacteria layer of substances intended for reaction in said layer.

Description

METHOD FOR BIOLOGICAL PURIFICATION OF WATER

This invention concerns a method for biological purification of water in accordance with the preamble of claim 1.

When carrying out biological processes such as nitrification and COD-reduction in filters including granular filter medium the advantage is obtained that a comparatively small reactor volume is required in order to nitrify the contents of ammonium respectively reduce the COD-contents in a certain water flow. The same applies with respect to denitrification.

In carrying out such processes in a reactor in accordance with the Swedish patent 7602999-0 it is previously known, in order to give priority to a large amount of bacteria and this way try to achieve a high reactivity, to use bed material consisting of porous granules . This gives a large surface for the bacteria to grow on but also a thick layer containing a large amount of dead bacteria.

The function of the biological purification process is based on that the substances participating in the reaction diffuse through the stationary layer of water which is present around each granule, the so called water skin, and into and at least partly through the layer of bacteria. In order to try to achieve an increased reduction of the impurity levels, the water velocity over the granules of the bed material has conventionally been lowered in order to allow increased diffusing time for the reacting substances. In order to compensate for the low water velocity and thus basically a totally low efficiency, several parallel filters have typically been installed so as to obtain a desired purification of incoming water. Taken together, this has resulted in an ineffective and expensive solution.

It is an aim of this invention to avoid the problems of known processes of the above kind and to provide a method which in a decisive way allows the possibility of increased efficiency and thus improved economy of plants where the method is applied. This aim is achieved in a method as above which is characterized by the features of the characterizing portion of claim 1.

By limiting the thickness of the bacteria layer to at the most 50 μm, such that the stationary water layer around the granules of the bed material comprises the main part of the resistance against diffusion it is achieved that the reactivity for for instance NH₄-N is almost proportional to the surface load. This is of great practical importance since an increase of the surface load (cubic meter water per hour and square meter bed surface) gives an almost unchanged value of (g NH₄-N/m³ _ιn - g NH₄-N/m³ _0ut:) . Plants using the method according to the invention may thus, within certain limits, be operated with a chosen surface load and still obtain a desired reduction of the ammonium content.

A certain desired reduction of the ammonium content may thereby simply be achieved by dimensioning the height of the bed. If the method is thus carried out in a reactor having a high bed, the desired reduction corresponding to this height will be obtained and it is also possible to operate the reactor with high surface load. High reactivity is thereby achieved which results in a smaller plant to be constructed.

Alternatively two or more reactors may be operated in series, e.g. with high surface load, whereby the result will be a step¬ wise reduction of the concentration of the reacting substances to be reduced. In this case the combined bed heights of the reactors will decide the magnitude of the reduction.

In a further alternative a certain part of the treated water may be recirculated so that it is mixed into the water to be purified before the intake of the reactor, whereby a reduction of the impurity content in the incoming water is simply obtained. This way a resulting great reduction of the impurity content in the purified water may be achieved according to what is desired depending on the amount recirculated.

A higher surface load will be obtained by the recirculation and thereby an increased reactivity. As a matter of fact, by this aspect of the invention the amount of recirculated cleaned water may be controlled in such a way that the surface load and thereby the biological reactivity is optimized for a certain amount of incoming dirty water to be purified.

By, in accordance with claim 11, carrying out the method with a bed material consisting of non-porous granules having a rough surface, the thin resulting bacteria layer is easily obtained after the treatment in a gas lift pump or the like, which in turn assures that an active bacteria layer with a reduced amount of dead bacteria is obtained on the separate granules .

The treatment in a gas lift pump results in strong turbulence of a mixture of bed material, water and air, which tears off lose particles. Further, the granules of the bed material intensively and repetetively collide with each other. The treatment results in that the bacteria layer which is present on the bed material granules is reduced to a thin but active bacteria layer. This because the dead bacteria are washed away in the mammoth pump.

The remaining bacteria layer is according to the invention held thinner than 50 μm and is preferably as thin as about 10 - 20 μm. The dimensions are based on an imaginary layer seen evenly spread over the granules. The factors that influence the thickness of the bacteria layer and which can be adjusted for achieving the desired limitation, are firstly the diameter of the pump and to a certain degree the amount of air in the pump. Corresponding factors are applicable if any other kind of lifting device is used.

With the aid of an immune detection method based on the fact that monoclonal anti-bodies specifically recognize Nitrobacter, the number of Nitrobacter has been determined to 2-10ⁿ/kg sand, which is 10 times as much as has been found in other applications. Previously it has been recognized that the amount of bacteria is about 750 mg/kg sand. Assuming that the number of Nitrosomonas is as big as the number of Nitrobacter, this would mean that the number of living bacteria is about 15% of the total bacteria amount which is a very high content. By using basalt in stead of conventional filter sand as the bed material a still better result is obtained.

The influence of the surface load on the reactivity has thus proved to be surprisingly great. This particularly in nitrification when a gas containing oxygen, such as air, is brought into the bed. As a general rule, added gas partly fills the cavities between the granules resulting in that the pressure drop over the bed radically increases, typically is doubled. This way the water will have less flowing space, wherefore the flow velocity closest to the surfaces of the granules is very high which according to the above results in a correspondingly very thin stationary water layer and advantageous conditions for the desired diffusion. When the method is carried out in a filter medium having a high relative density, it is avoided that the bed will fluidize in spite of the high surface load which would otherwise ruin the filtering function.

Also in case of denitrification it is achieved, to a certain degree, that the reactivity increases with increased surface load. The effect is however, not as prominent as according to what is described above which should depend on the fact that nitrogen gas being formed does not participate in the process. All taken together, in most cases a certain reduction of the NH₃- N-content may be obtained by reducing the surface load.

What has been said above concerning nitrification is also to a certain degree applicable for COD reduction (Chemical Oxygen Demand, a collective name for organic material, requiring oxygen for its transformation to i.a. carbon dioxide) . It is true that COD is not reacting homogenously since it is not the question of a unitary compound, but in principle the reactions with respect to this material are the same as with respect to nitrification. It may also be added that in most cases it is not the question of a pure simple nitrification since also COD is present. This invention therefore also includes methods including different kinds of reactions. The invention will now be described in more detail by way of embodiments and with reference to the annexed drawings, wherein:

Fig. 1 shows a section through a reactor for carrying out the method according to the invention, and

Fig. 2 shows a reactor with means for recirculating a portion of the purified water.

The reactor shown in Fig. 1 comprises a type of filter consisting of a tank containing filter material. In the centre of the filter there is arranged a gas lift pump 2 which is connected to an air supply via the conduits 5 and 4, said pump under turbulence transporting the dirty bed material from the bottom of the filter to its upper part where the material is cleaned in a cleaning device 3 and thereafter brought back to the upper part of the bed. This principle is described in the above mentioned Swedish patent document.

Incoming water to be purified is taken in via conduits 6 and the horizontal roof-edge shaped arms 7 to the bed, where it is filtered through the bed during simultaneous biological treatment, whereafter purified water over an overflow discharge (not shown) runs out from the filter. (Neither the water level is shown in the Figure) . Gas is supplied under the roof-edge means 9 via conduits 10 just above the intake for the water to be purified. The construction of the roof-edge means 9 and the roof- edge shaped arms 7 prevents the bed material from coming into direct contact with the mouths of the conduits. A conical element 8 distributes the sand (the filter medium) in its movement downwards in order to assure even distribution. Removed dirt is discharged separately from the cleaning device 3.

Fig. 2 shows diagrammatically a plant according to the invention having a recirculating conduit R including a pump P for a portion of the purified water. Preferably the branch at 14 is arranged such that a chosen portion of the purified water may be recirculated. The recirculated water is at 15 mixed into the stream of incoming water to be purified. As an example of the effect of the recirculation may be mentioned that a supplied water flow having a NH₄-N-content of 70 mg/1 after mixing (at 15) with recirculated purified water resulted in a NH₄-N-content of 29 mg/1, which after the treatment in a filter functioning according to a method according to the invention resulted in a treated water (B) having a NH₄-N-content of 2 mg/1.

By carrying out the method using a bed material having a relatively high specific gravity and in particularly a relatively high bulk density, it is possible to use fine grain material and still operate with high surface load without fluidization occurring. This is particularly noticeable in processes where gas is present in the bed which is the case in denitrification as well as in nitrification. According to the above, the presence of gas in the bed will result in an increase of the pressure drop over the bed and thus increased risk of fluidization. The conventional methods using porous bed material as carrier for the bacteria makes it necessary to have a low surface load in order to avoid fluidization.

So far methods including nitrification have not been possible to carry out with a higher reactivity than 500 g/ (m³ -d) NH₄-N at 6°C. A plant using a method according to the invention can however be carried out at such a high water surface load as at least about 20 and as example 25 m/h, whereby a reactivity of 2000 g/ (m³ -d) NH₄-N may be obtained at 6°C. This is a very high reactivity and provides a very surprising result. The example concerns nitrification of municipal sewage or ammonium containing ground water.

Compared to conventional processes the invention makes it possible to connect in series two or more filters and to operate these with essentially higher, e.g. three times as high a surface load. This way the reactivity expressed as g/ (m^{3 -}d) NH⁴-N is about three times as high as with the parallel connection. This means that only a third of the filter volume is needed at the same time as it is possible to obtain a low content of NH₄-N in the outgoing water. Compared to conventional plants wherein, at low temperatures of as an example 6°C, very large plants are needed the method according to the invention further provides the possibility of considerable space saving.

The invention may be modified within the scope of the following claims. As has been mentioned above the upward transportation of the dirty bed material is preferably carried out by a gas lift pump, but it is not excluded to use another suitable lifting device, for example a device which is located outside the filter container. As an example of alternative lifting devices may be mentioned water jet pumps. As to the bed material, it is suitable to use basalt but also other materials may be used provided they have the desired properties as in particular: high bulk density but also non-porous structure, rough adhesion-friendly surface and high specific gravity.

Claims

C L A I M S

1. Method for biological treatment of water in a filter bed including a granular filter medium which m a dirty state is taken out at the lower part of the bed and after transport upwards and cleaning is brought back to the upper part of the bed, whereby water to be puπfleded passes from below and upwards through the filter bed, c h a r a c t e r i z e d in that the thickness of a bacteria layer around the granules of the granular filter material is brought to be limited to a maximum of 50 μm by said transportation upwards and/or washing such that a stationary water layer which is present outwardly around the granules comprises the mam resistance against diffusion into the bacteria layer of substances intended for reaction said layer.

2. Method according to claim 1, c h a r a c t e r i z e d in that said thickness is brought to be about 10 - 20 μm

3. Method according to claim 1 or 2, c h a r a c t e r i z e d in that it is carried out m a reactor having a bed height over

2 meters

4. Method according to any of the claims 1 - 3, c h a r a c t e r i z e d m that it is carried out in at least two reactors connected series.

5. Method according to claim 4, c h a r a c t e r i z e d that nitrification is carried out in at least two reactors which are connected in series.

6. Method according to any of the claims 1 - 5 c h a r a c t e r i z e d in that a portion of the treated water is recirculated to the filter bed.

7. Method according to claim 6, c h a r a c t e r i z e d that the amount of the recirculated water is controlled so that the surface load and thus the biological reactivity is optimized.

8. Method according to any of the claims 1 - 7, c h a r a c t e r i z e d in that gas is added to the bed for increasing the flow rate of the water over the granules of the filter bed.

9. Method according to any of the claims 1 - 8, c h a r a c t e r i z e d in that gas is added to the bed for influencing the biological process.

10. Method according to claim 9, c h a r a c t e r i z e d in that the gas contains oxygen for nitrification.

11. Method according to any of the claims 1 - 10, c h a r a c t e r i z e d in that the treatment is carried out in a filter bed including non-porous granules having a rough surface.

12. Method according to claim 11, c h a r a c t e r i z e d in that it is carried out in a filter bed containing basalt as the most essential filter medium.

13. Method according to any of the previous claims, c h a r a c t e r i z e d in that it is carried out using a high surface load of at least 10 m/h.