WO1995023767A1

WO1995023767A1 - Processing water in a biologically activated and continuously operating granular filter bed

Info

Publication number: WO1995023767A1
Application number: PCT/SE1995/000214
Authority: WO
Inventors: Hans F. Larsson; Kurt Jönsson
Original assignee: Nordic Water Products Ab
Priority date: 1994-03-03
Filing date: 1995-03-01
Publication date: 1995-09-08
Also published as: SE504447C2; AU1906895A; SE9400734L; EP0748297A1; JPH09509613A; SE9400734D0

Abstract

The present method provides a solution on problems in connection with nitrogen generation in or gas supply to a continuously operating granular filter in a biological process during processing water. The solution is that the biological process is carried out in the filter bed and that the filter medium (1) is transported from the bottom of the filter bed to the upper surface thereof, however with the essential difference that the filter medium (1) is not subjected to any washing in connection with this transport (2) and altogether not at all. The water from the transport is instead mixed with the water passed through and processed in the filter bed and are discharged together, eventually to a succeeding treatment step.

Description

PROCESSING WATER IN A BIOLOGICALLY ACTIVATED AND CONTINUOUSLY OPERATING GRANULAR FILTER BED

The invention relates to processing water, preferably waste water, in a biologicly activated and continuously operating granular filter bed in which process gas is generated or supplied. As gas generating process means a denitrification process and as a process, in which gas is supplied, means a nitrification process or a BOD-reduction process.

A continuously operating granular filter means in the following a filter containing a filter bed, in which the filter medium during operation is in movement downwards while the suspension or solution to be processed is caused to pass through the filter bed mainly upwards. The flow of the filter medium downwards is accomplished owing to the fact that the filter medium in the lower portion of the filter bed successively is removed for transport to the upper portion of the filter bed. According to prior art a washing of the filter medium is carried out associated with this transport and suspended material is removed from the filter medium prior to its return to the upper portion of the filter bed. Such a filter is disclosed in SE patent No. 7602999-0.

Also, it is previously known to carry out denitrification in continuously operating granular filter. Also non-continuously operating filter beds, i.e. static filter beds, are used for denitrification. The disadvantage of the static filter beds are that they faster than normally clog owing to the accumulation of nitrogen. This is particularly troublesome in those filters where the waste water passes the filter from above downwards. The operation must be shut off several times per day for backwashing in order to remove nitrogen bubbles, a technique called "bumping". During the operation of a continuously operating granular filter for the denitrification of waste water the nitrogen is removed in connection with the transport of the filter medium to a wash device. The nitrogen is generally completely separated from the filter medium before the filter medium passes the final washing for removing suspended particles as well as a part of the bacterial stock.

An article published in Research Journal PCF, Vol. 62, No. 3, 1990, Ben Koopman et al: "Denitrification in a moving bed upflow sand filter", page 239-245, discloses pilot tests of the denitrification of a waste water containing nitrates and nitrites having a total nitrogen content of up to 22g/m³. Compressed air is supplied to a mammoth pump with a rate up to l,lm³/h for the transport of the filter medium up to the wash device. The maximum load of the filter bed during these tests was 11,0 m/h (m³ suspension/hour/m² filter area) . Furthermore, Ben Koopman et al. establish in this article that they did not achieve any upper limit for the load of the filter bed.

It is, however, an upper limit for the amount of nitrogen that can be separated in a continuously operating filter bed even if this limit can not be exactly stated. However, an estimation can be made, that, if the nitrogen concentration should be nearly 50 g/m³, such a huge filter medium transport should be required up to a washing in order to manage to accomplish a removal of the nitrogen that the reject water amount obtained in doing so should be too large in relation to the obtained filtrate amount, thereby unacceptable both practically and on an economic basis.

In a nitrification process of waste water or in a BOD-reduction air is usually supplied into the filter bed. This gas amount causes difficulties during the filtration through the filter. Air bubbles follow the water up through the filter bed and whirl the filtrate. In doing so problems arise on one hand in a sand discharge at the outlet, on the other in that air bubbles follow the wash water into the wash device if this is positioned in direct contact with the filtrate. The washing phase is disturbed or can even be blocked. A method for over¬ coming this problem is disclosed in SE patent No. 9102179-0.

The present invention offers a solution to all problems

instance mammoth pumps. By this measure is in each transport unit obtained flow conditions which do not cause too large loss of the bacterial stock and thereby do not effect the reactivity of the filter bed. Such a method and arrangement is more in detail described in SE patent application 9302184-8.

Additional problems can arise both in the denitrification process and in a nitrification process. The desired high resetting of filter medium results in that the mammoth pump must be operated hard. The mammoth pump transports besides the filter medium also water. Because this water shall have sufficient time to be treated biologically the inlet is positioned higher up in the bed. It can in existing cases be appropriate to arrange inlets to the filter bed on several alternative levels to choose between according to the type of waste water to be processed. The air supply required upon nitrification and BOD-reduction is suitably positioned above the inlet for the waste water. Owing to the movement of the filter bed downwards also the filter medium below the inlet will be oxygenated.

In the following the invention is described more in detail with reference to the accompanying drawings.

Fig. 1 illustrates schematically in a perspective view a section of an apparatus for carrying out the method according to the invention.

Fig. 2 illustrates a longitudinal section of an apparatus according to the invention.

Fig. 3 illustrates a cross section of the apparatus illustrated in fig. 2.

Fig. 1 is a schematic view illustrating the principle how a continuously operating filter bed only intended for denitrification can be built up. It is in this embodiment a question of a conventional continuously operating filter bed, where the wash device with its outlet for the wash water reject has been removed. A tank having a conical bottom encloses the filter medium 1, for instance sand, which during a running-in period has been activated with bacterial stock adequate for denitrification. The waste water to be processed is supplied to a centrally positioned tube 2, as arrow A shows, and flows out into the filter bed from a number of distribution arms 3. A conical shell 4 is designed to give the filter medium an appropriate flow profile downwards as the arrows B illustrate. This flow is effected by the filter medium transport of the mammoth pump 5 up to the top surface of the filter bed. As transport medium is used compressed air, which by means of an outer tube 7 is guided down to inlet valves 6 of the transport tube 5 of the mammoth pump. At the upper mouth 8 of the transport pipe 5 the nitrogen is discharged and the filter medium flows downwardly to the top surface of the filter bed.

An outlet 9 for reject water, partly comprising water flowing upwards through the filter bed as arrows C illustrate and partly water following upwards in the mammoth pump, leads to next processing step for the waste water. A filter bed intended for nitrification or BOD-reduction may have the same principle design as described above, however with the addition of inlet means for the air supply, preferably positioned just above the inlet for the waste water.

The subsequent processing step after a nitrification process or a BOD-reduction carried out according to the present invention is a denitrification. This denitrification can of course as described above be carried out according to the present invention, however also otherwise according to known technique. The subsequent processing step after a denitrification process is substantially a filtration. It is, however, inevitable and also to be desired that this subsequent filter bed is biologically activated. Residues of nitrates and nitrites which not have had sufficient time to be treated in the preceeding process, can in this processing step be finally treated at the same time as the water is filtrated. This filtration removes

instance, bottom outlet means can be provided for the filter medium to some type of mechanical transport means, which outside the reactor tank transports the filter medium and the liquid up to the top surface, wherein also during this transport nitrogen is separated in a denitrification process.

The inlet openings of the transport devices should be positioned in or just above tunnel-shaped cavities on the bottom of the container for governing the filter medium to the inlet openings.

Finally, it shall be emphasized that with the designation waste water is here intended to be any water which for some reason can require to be processed according to the invention, even if it actually is not waste water.

Claims

1. A method for processing water in a biologically activated and continuously operating granular filter bed, in which process gas is generated in or supplied to the filter bed, c h a r a c t e r i z e d i n that the water only is processed within the filter bed, that the filter medium is transported from the filter bed bottom to the upper surface thereof without washing, that the accompanying gas is separated, and that the water processed during the passage through the filter bed to the upper surface thereof is discharged together with water and sludge accompanying the transport.

2. A method according to claim 1, c h a r a c t e r i z e d i that the treatment in the filter bed consists of a nitrification process.

3. A method according to claim 1, c h a r a c t e r i z e d i n that the treatment in the filter bed consists of a denitrification process.

4. A method according to claims 1-3, c h a r a c t e ¬ r i z e d i n that the transport of the filter medium from the filter bed bottom to the upper surface thereof takes place by means of several transport devices uniformly distributed horizontally.

5. A method according to any of the preceeding claims, c h a r a c t e r i z e d i n that an increased agitation of the water above the upper surface of the filter bed is carried out.

6. An apparatus for processing water in biologically activated granular filter bed in accordance with claim 1, in which filter bed gas is generated or to which gas is supplied, said apparatus including a filter bed positioned in a container, said filter bed operating continuously by transporting during the process the filter medium of the bed from the lower portion 10

of the filter bed to the upper surface therof, c h a r a c t e r i z e d i n a common outlet from the container for the water processed during the passage through the filter bed as well as water and sludge accompanying the transport.

7. An apparatus according to claim 6, c h a r a c t e ¬ r i z e d i n several transport devices having inlet openings for the filter medium uniformly distributed horizontally in the filter bed.

8. An apparatus according to claim 7, c h a r a c t e ¬ r i z e d i n that said inlet openings of the transport devices are positioned in or just above tunnel-shaped cavities arranged on the bottom of the container.

9. An apparatus according to claim 7 or 8, c h a r a c t e ¬ r i z e d i n agitation means for the water above the upper surface of the filter bed, preferably by means of air.