EP0981500A1

EP0981500A1 - Method and apparatus for biological removal of nitrogen compounds from water

Info

Publication number: EP0981500A1
Application number: EP98916800A
Authority: EP
Inventors: Svatopluk Mackrle; Vladimir Mackrle
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-05-14
Filing date: 1998-05-13
Publication date: 2000-03-01
Also published as: AU7028098A; WO1998051626A1; CZ147997A3; PL336931A1; CA2288642A1; SK151299A3

Abstract

Method of removing nitrogen and its compounds from water using biological process in activated sludge suspension. The nitrogen in the form of nitrates and nitrites is removed by means of a denitrification proceeding in a fluidized layer of sludge blanket, the layer being formed by a suspension of biological sludge and being kept in a fluidized condition. Apparatus for performing the method comprises an upwards widening denitrification zone (5), into the lower part of which an admission for the water containing nitrates, an admission for the activated sludge and an admission for the substrate, which has been biologically oxidized during the denitrification, mouth.

Description

METHOD AND APPARATUS FOR BIOLOGICAL REMOVAL OF NITROGEN

COMPOUNDS FROM WATER

The present invention relates to a method of removing nitrogen and its compounds from water by means of a biological process in an activated sludge suspension, as well to an apparatus for realizing said method.

BACKGROUND OF THE INVENTION At biological removal of nitrogen compounds from water a denitrification process is used for the removal of nitrates and nitrites . The nitrates and the nitrites serve as a source of oxygen for microorganisms and they are reduced by its metabolism down to a gaseous nitrogen. At the same time, the microorganisms consume a suitable biodegradable substrate and oxidize the same using the received oxygen. The microorganisms themselves may serve as a substrate for the denitrification in the process of a so called endogenic respiration, e.g. in case the supplying of the biodegradable substrate is irregular.

To make the denitrification run, anoxide conditions are necessary, i.e. there must be a lack of molecular oxygen in the medium. In case there is any molecular oxygen available in the medium, the microorganisms orientate on the consumption of the same and they change to the denitrification only when there is no more oxygen. The anoxide conditions are usually reached by preventing any access of oxygen when the concentration of microorganisms is sufficient, and said microorganisms themselves create anoxide conditions by consuming the present oxygen.

Generally, the biological processes may be employed either with microorganisms attached to a solid basis or with microorganisms being in an activated sludge suspension

("attached - growth process" and "suspended - growth process" - see Metcalf and Eddy, Wastewater Engineering, McGraw-Hill, New York, 1985) . On grounds of several practical reasons, processes with microorganisms being in an activated sludge suspension are used more frequently.

During the denitrification process in an activated sludge suspension the suspension is led away along with the purified water, from which it must be separated and returned back into the denitrification process. However, the separation of the suspension is complicated by the bubbles of separated nitrogen, which stick at the same. Removal of said bubbles is realized by an intense mechanical movement in the suspension, which is done usually by aeration, in the known methods .

When purifying waste water the aeration serves mostly the purpose of an aerobic activating water purification and at the same time to create conditions for a nitrification of ammonia and organic nitrogen, i.e. to transform any remaining nitrogenous contamination to nitrates . The result of connection of the denitrification and the nitrification is the biological removal of all nitrogen compounds from water. The activated sludge, separated from the purified water flowing away, is usually also returned to the denitrification.

However, said denitrification has a number of disadvantages . For the biological processes in an activated sludge it is necessary that the activated sludge is in the state of suspension. During the aerobic activation the suspension of the sludge is ensured automatically due to the aeration. During the denitrification the suspension of the sludge is secured usually by mechanical mixing. It is disadvantageous that an agitator and a source of movement - e.g. an electric motor with a gearing - is necessary for the mechanical mixing. Said mechanical parts increase the investment costs and in case of any defect the repair is complicated due to their arrangment in the denitrification zone. Another disadvantage of the above denitrification is represented by a limitation for the concentration of the activated sludge in the denitrification only to the values given by mixing of the components entering the denitrification, which limits the rate of the denitrification related to a volume unit of the denitrification zone.

SUMMARY OF THE INVENTION The above disadvantages are eliminated by the method according to the invention, the essence of which is that the nitrogen in the form of nitrates and nitrites is removed by means of a denitrification proceeding in a fluidized layer od sludge blanket, the layer being formed by a suspension of biological sludge and being kept in a fluidized condition.

It is also essential that the activated biological sludge is led into the denitrification zone by streaming from below upwards, wherein the streaming rate in the direction from below upwards is slowed down at least in the lower part of the denitrification zone.

For the employment of the method according to the invention it is significant that the superfluous activated sludge in the form of flocks is led away from the upper part of the denitrification zone and then, it is subjected to an aeration, separated form water and returned into the lower part of the denitrification zone.

It is also essential that the streaming rate in the upwards direction is slowed down at least in the lower part of the fluidized layer, wherein the water containing nitrates, the activated sludge, and the substrate, which is biologically oxidized during the denitrification, are led into the lower part of the fluidized layer. It is advantageous that after the denitrification, the water and the superfluous activated sludge are led away from the upper part of the fluidized layer and the activated sludge, which has been led away, is freed from the sticking nitrogen bubbles by means of an aeration, it is separated from the purified water and it is returned into the lower part . It is a contribution to a better removal of nitrogen from the water to be purified that during the aeration, nitrogen in the form of ammonia and organic nitrogen are biologically oxidized to nitrates and the water containing these nitrates is returned into the denitrification process along with the activated sludge .

The essence of the apparatus for performing the method according to the invention consist in that the apparatus comprises an upwards widening denitrification zone, into the lower part of which an admission for the water containing nitrates, an admission for the activated sludge and an admission for the substrate, which has been biologically oxidized during the denitrification, mouth.

It is significant for the reliability of the method that the denitrification zone is interconnected in its upper part with the activation zone, which comprises aeration elements and is interconnected with the separation zone, in the upper part of which an outlet for the purified water is arranged, the separation zone being provided with a withdrawal for separated sludge which is provided with a means for a forced movement and mouths into the lower part of the denitrification zone.

It is also important that the withdrawal for separated sludge is positioned in the lower part of the separation zone near to its interconnection with the activation zone, the outlet for the water with nitrates from the activation zone designed to return it into the denitrification zone being realized by means of identical structural members, and that the admission for the water containing nitrates, the admission for the activated sludge and the admission for the substrate, which has been biologically oxidized, are realized as a common admission.

BRIEF DESCRIPTION OF THE DRAWINGS The apparatus according to the invention meant for purifying the home sewage is presented at the drawings, wherein Fig. 1 depicts a cross section of the apparatus and Fig . 2 the ground plan of the apparatus .

PREFERRED EMBODIMENT OF THE INVENTION The apparatus consists of a vertical cylindrical tank with a casing 1 and a bottom 2 , wherein an upwards funneling denitrification zone 5. is formed in said tank by a vertical partition wall 2_. a first semi-conical wall 4. and a part 2__ of the bottom 2 between the lower edges of the partition wall 2 and of the first semi-conical wall 4. Further, there is a separation zone 2 formed by the vertical partition wall 2 and a second semi-conical wall £ in the other part of the cylindrical tank. An activation zone 2 is formed between the casing 1 and both semi-conical walls 4 and _.. An inlet _. for purified water with an outlet iQ_ is arranged in the upper part of the separation zone 2- The water level 11 is determined by the position of the inlet __ for purified water. There is an interconnection 12 between the denitrification zone _. and the activation zone 2 realized in the upper part of the first semi-conical wall 4.. Said interconnection 12. is formed by two circular apertures, the upper edges of which are positioned above the water level 11 and the lower edges below the water level li. However, the interconnection 12 may be formed by other means, e.g. by recessions in the upper edge of the first semi-conical wall 4. or by another embodiment of the overflow formed by lowering the upper edge of the first semi-conical wall £• An opening 12 is prepared in the lower part of the second semi-conical wall £, said opening 12 providing the interconnection between the separation zone 2 and the activation zone £. There is a vertical semi-circular piping 14 arranged in the denitrification zone 2 near to the vertical partition wall 2, said piping 2A forming the common admission for the biodegradable substrate, activated sludge and nitrates into the lower part of the denitrification zone 5_. A basket 15. is positioned in the upper part of the semi-circular piping 14, wherein a sewage water admission

16 is situated above said basket 12- There is a drain mouth

17 for the separated activated sludge provided in the partition wall 2 opposite the opening 12, in the lower part of the separation zone 1_, said mouth 12 leading into an air-lift pump 12- The air-lift pump 12 is substantially inserted into the semi-circular piping ___ and leads below the basket 12 (Fig.l) . A pressurized air supply 1 is provided in the lower part of the air-lift pump 12. Aeration elements 2Q_ are positioned in the lower part of the activation zone 2 near to the bottom 2 . A large bubble auxiliary aeration element 21 is positioned in the lower part of the denitrification zone 2 near to the part 21 of the bottom 2 . The method according to the invention and the function of the above apparatus are described in the following part .

The sewage water containing nitrogen compounds, e.g. urea, ammonia, nitrogen in organic materials, and further various biodegradable materials, which will be further designated as the substrate, flows through the sewage water admission !£_ into the basket 12. Larger pieces, e.g. paper, are retained in the basket 12. These pieces are constantly held up by the air coming from the air-lift pump 12 below the basket 12, which prevents the basket 12 from clogging. If said pieces are biodegradable, their biodegradation and thereby their destruction occurs effected by the air and activated sludge coming from the air-lift pump 12. Pieces, which are not biodegradable, e.g. polyethylene bags and so on, remain in the basket 12 and they are removed therefrom during a periodical inspection of the apparatus.

The sewage water, i.e. the water with biodegradable substrate, after flowing through the basket 12, is mixed with water, which contains nitrates and returned activated sludge and which is led by air-lift pump 12 below the basket 12- The mixture of water, biodegradable substrate, nitrates, and activated sludge is led by the semi-circular piping H into the lower part of the denitrification zone 2, where the direction of its movement is changed so that said mixture in the denitrification zone 2 flows upwards . As the denitrification zone 2 becomes wider in the upward direction, the flow rate becomes lower in the upward direction. This causes the flocks of the activated sludge to be held in the denitrification zone 2 by the gravitational force and to form therein a fluidized layer of a sludge blanket, through which water with nitrates and biodegradable substrate flows . The oxygen which is present in the water is quickly consumed by the microorganisms already in the lower part of the denitrification zone 2, which circumstances create anoxide conditions in the whole denitrification zone 5, or at least in its prevailing part. As a result of the anoxide conditions the denitrification occurs in the denitrification zone 2» during which the nitrates present in the water are reduced to gaseous nitrogen. At the same time, a part of the biodegradable substrate is consumed. Another part of the substrate is retained by the activated sludge and consumed later in the period, when no substrate is supplied from outside. The water from which nitrates and most of the biodegradable substrate has been removed, overflows from the denitrification zone 2 through the interconnection 12 into the activation zone 2- Particles of the activated sludge, at which the bubbles of separated nitrogen produced by denitrification stick, are raised upwards from the fluidized layer of the sludge blanket in the denitrification zone 2 as they are held up by said bubbles, and they are entrained by the flowing streaming water through the interconnection 12 into the activation zone 8. Due to the operation of the air-lift pump 12, further and further activated sludge is supplied through the semi-circular piping 14 into the lower part of the denitrification zone 2, and therefore the surplus activated sludge also overflows through the interconnection 12 into the activation zone after the whole denitrification zone 2 has been filled by the fluidized layer of the sludge blanket . The pressurized air is led into the aeration elements 22, bubbles through the activation zone 2 upwards, and creates a turbulence, which removes the sticking bubbles of nitrogen from the particles of the activated sludge suspension. At the same time, the water in the activation zone is saturated with oxygen and a streaming is formed, which keeps the activated sludge in suspension and mixes the same evenly in the whole activation zone 2- The water, which has got from the denitrification zone through the interconnection 12 into the activation zone 2, is mixed gradually in the latter, as well. At the presence of oxygen, the mixed activated sludge oxidizes all available nitrogen compounds to nitrates and removes the rest of the dissolved biodegradable materials from the water. At the same time, an aerobic stabilization of the activated sludge occurs.

The pressurized air is blown into the air-lift pump 12 by the supply 12, gets through the air-lift pump 12 upwards and creates there the known air-lift pumping effect. As a result, the activating mixture consisting of water with nitrates and mixed activated sludge is sucked from the adjacent part of the activation zone 2 by means of the mouth 17 through the opening 12 and is led below the basket 12 and through the semi-circular piping 14 into the lower part of the denitrification zone 2, as described above. So the air-lift pump 12 serves as the source of positive movement, by means of which a closed circulation circuit is provided, said circuit serving the purpose of returning the nitrates formed in the activation zone 2 back into the denitrification zone 2-

The sewage water flows through the admission 12 in shocks and irregularly. At the period, when no sewage water is flowing in, the nitrates are led through the described circulation circuit from the activation zone 2 into the denitrification zone 2, where they are removed by the denitrification. As a biodegradable substrate for the denitrification serves first of all the substrate retained before in the activated sludge in the fluidized layer of the sludge blanket in the denitrification zone 2- In case, the retained substrate has been exhausted, further denitrification proceeds on the endogenic respiration of the activated sludge. The concentration of the nitrates in the activation zone 2 is lowered constantly down to a very low value .

When a certain volume shock of sewage water flows in through the admission 12, said water forces out the same volume of water from the denitrification zone through the interconnection 12 into the adjacent part of the activation zone. Than, the water drives out the same volume of the activating mixture through the opening 12 from the adjacent part of the activation zone 2 into the lower part of the separation zone 2- The activating mixture transferred in said way forces out clear water from the upper part of the separation zone 2 into the inlet 2 for purified water, wherefrom the water flows away through the outlet 12- The activated sludge in the expelled activating mixture forms during the flow a fluid layer of sludge blanket in the separation zone 2 and it is separated from the water.

If the flowing of the sewage water through the admission 12 takes a longer time, the activating mixture flows through the formed fluid layer of sludge blanket and the activated sludge is retained and attaches to the flocks in the sludge blanket . The formed large flocks fall in the separation zone 2 down and they are drained off by means of the mouth 12 through the air-lift pump 12 into the semi-circular piping 2A, through which they get into the lower part of the denitrification zone 2- So the separated sludge returns into the denitrification zone 2. After the described separation of the activated sludge the purified water flows away from the separation zone 2 through the inlet 2 for purified water into the outlet 10.

After the flowing of sewage water through the admission 16 is stopped, the leading of the purified water away through the outlet 12 is also stopped. Then, the fluidized layer in the separation zone 2 falls and its flocks are drained off by means of the mouth 12 along with the activating mixture drained off through the opening 12 from the activation zone 2. So, all the activated sludge, which has been separated from the water of the activating mixture forced out into the separation zone 2, is drained off along with the drained activating mixture. Only the purified water remains in the separation zone 2 above the opening 12, and said water is forced out through the inlet 2 for purified water, next time, when the sewage water fitfully flows in through the admission 12, as described above. The purified water remaining in the separation zone 2 is then the water of the activating mixture in that part of the activation zone 2, which is adjacent to the opening 12, and it has got through the denitrification zone 2 and activation zone 2 before and from which biodegradable materials and nitrogen compounds have been removed in that way. Through the described operation of the exemplifying embodiment of the apparatus the sewage water flowing into the apparatus through the admission 12 is striped of biodegradable materials and nitrogen compounds and the water flows out from the apparatus through the outlet 12 as a purified water.

Due to the fact that the activated sludge, which is constantly led in due to the operation of the air-lift pump 18. accumulates in the fluid layer of the sludge blanket in the denitrification zone 2, and only the sludge in suspension goes over into the activation zone 2, and the superfluous sludge comes into the activation zone 2 only after the sludge blanket has filled the whole denitrification zone 2, it could happen in case of a smaller amount of the activated sludge in the whole apparatus, e.g. in case of initiating the operation that practically all sludge would accumulate in the denitrification zone 2 and a permanent lack of activated sludge would arrive in the activation zone 2- This would result in a disruption of the nitrification in the activation zone 2 and thereby in a disruption of the operation of the apparatus. Also in case of a fall-out loss of electrical energy, which results in stopping the operation of the air-lift pump 12 and in stopping the flow in the denitrification zone 2, a disruption of the operation of the apparatus could occur, as the fluidized layer in the denitrification zone 2 would settle and would form an sediment, which would form a gel in the course of time, and said gel would not be brought in suspension after the flow restoration. An auxiliary aeration element 21 is provided to prevent such cases and it is installed in the denitrification zone 2 near to the part 21 of the bottom 2- At starting the auxiliary aeration element 21 the air going out of it breaks all incidental accumulations, gets them from the part 21 of the bottom 2 in suspension and mixes the fluidized layer in the denitrification zone 2 so that the activated sludge goes over from the denitrification zone 2 into the activation zone 2 along with the flowing water. So a short-time, sporadic switching-on of the auxiliary aeration element 21 may serve as a prevention against the above mentioned failures . To document the function of the exemplifying apparatus the results of its test operation are provided in the Table below, wherein the results have been won in a series of withdrawals from the operation of ten test units used in family houses, where the number of inhabitant ranged from 2 to 7 persons. The functional spaces of said test unit had the following volumes: the denitrification zone 2 - 0,25 m³, the separation zone 2 - 0,4 m³, the activation zone 2 - 1,1 m³. TABLE

In the Table means COD chemical oxygen demand

BOD_s biological oxygen demand

SS suspended solids

N-NH ^* nitrogen in the form of ammonia

N-NO nitrogen in the form of nitrates

All stated values are in mg/1.

If we take the known tabular data for the daily water consumption and production of pollution for an equivalent inhabitant as the input values (150 1, 60 g BOD , 12 g N) , the achieved average values correspond to the removal of 91,5% of nitrogen and 98,5% of B0D_s .

The method and apparatus according to the invention have numerous advantages . The concentration of the activated sludge in the fluidized layer of the sludge blanket in the denitrification zone 2 is higher than the concentration of the activated sludge in the activation zone 2, which results in the fact that the denitrification rate in a volume unit is higher than in a mechanically mixed denitrification zone. The substrate, which is brought into the denitrification zone 2 and serves the denitrification, is retained there in the fluidized layer of the sludge blanket so that its utilization for denitrification is higher than in a mechanically mixed denitrification zone. There is no mechanical apparatus necessary to keep the activated sludge in suspension inside the denitrification zone 2- All these facts lower the investment costs of the apparatus and raise the effectiveness of the purifying processes. INDUSTRIAL APPLICABILITY The method and apparatus according to the invention is not limited to the waste water purification and to the described exemplifying apparatus only. It is possible to use it for other purposes, e.g. to remove nitrates from a potable water, where a hygienically suitable organic compound, e.g. alcohol, sugar, and so on, will be added as the substrate. Also the apparatus for realizing the method according to the invention may be of various internal arrangment, wherein the basic principle of the invention is essential, i.e. that the apparatus comprises an upwards extended denitrification zone, in the lower part of which an admission for water containing nitrates, activated sludge and a substrate for denitrification is provided. It is not essential whether the admission for the water containing nitrates, the activated sludge and the substrate for denitrification is common, as it is in the described exemplifying apparatus, or there are independent admissions for the individual components into the lower part of the denitrification zone 2- All kinds of utilization of the basic principle of the above described inventive idea do not digress from the essence of the invention.

List of parts

1 casing 2 bottom

2 ' part of bottom

3 partition wall

4 first semi-conical wall

5 denitrification zone 6 second semi-conical wall

7 separation zone

8 activation zone

9 inlet for purified water

10 outlet 11 water level

12 interconnection

13 opening

14 piping

15 basket 16 sewage water admission

17 drain mouth

18 air-lift pump

19 pressurized air supply

20 aeration elements 21 auxiliary aeration elements

Claims

1. Method of removing nitrogen and its compounds from water using biological process in activated sludge suspension, characterized in that the nitrogen in the form of nitrates and nitrites is removed by means of a denitrification proceeding in a fluidized layer od sludge blanket, the layer being formed by a suspension of biological sludge and being kept in a fluidized condition.

2. Method according to claim 1, characterized in that the activated biological sludge is led into the denitrification zone by streaming from below upwards, wherein the streaming rate in the direction from below upwards is slowed down at least in the lower part of the denitrification zone.

3. Method of claim 2 , characterized in that the superfluous activated sludge in the form of flocks is led away from the upper part of the denitrification zone and then, it is subjected to an aeration, separated form water and returned into the lower part of the denitrification zone.

4. Method of claim 2 or 3 , characterized in that the streaming rate in the upwards direction is slowed down at least in the lower part of the fluidized layer, wherein the water containing nitrates, the activated sludge, and the substrate, which is biologically oxidized during the denitrification, are led into the lower part of the fluidized layer.

5. Method of claim l, 2, 3, or 4, characterized in that after the denitrification, the water and the superfluous activated sludge are led away from the upper part of the fluidized layer and the activated sludge, which has been led away, is freed from the sticking nitrogen bubbles by means of an aeration, it is separated from the purified water and it is returned into the lower part .

6. Method of claim 3, 4, or 5, characterized in that during the aeration, nitrogen in the form of ammonia and organic nitrogen are biologically oxidized to nitrates and the water containing these nitrates is returned into the denitrification process along with the activated sludge.

7. Apparatus for performing the method according to any of claims 1 through 6, characterized in that the apparatus comprises an upwards widening denitrification zone (5) , into the lower part of which an admission for the water containing nitrates, an admission for the activated sludge and an admission for the substrate, which has been biologically oxidized during the denitrification, mouth.

8. Apparatus according to claim 7, characterized in that the denitrification zone (5) is interconnected in its upper part with the activation zone (8) , which comprises aeration elements (20) and is interconnected with the separation zone

(7) , in the upper part of which an outlet (10) for the purified water is arranged, the separation zone (7) being provided with a withdrawal for separated sludge which is provided with a means for a forced movement and mouths into the lower part of the denitrification zone.

9. Apparatus according to claim 7 or 8, characterized in that the withdrawal for separated sludge is positioned in the lower part of the separation zone (7) near to its interconnection with the activation zone (8) , the outlet for the water with nitrates from the activation zone (8) designed to return it into the denitrification zone (5) being realized by means of identical structural members.

10. Apparatus according to claim 7, characterized in that the admission for the water containing nitrates, the admission for the activated sludge and the admission for the substrate, which has been biologically oxidized, are realized as a common admission.