WO1997036825A1

WO1997036825A1 - Method and apparatus for purifying water

Info

Publication number: WO1997036825A1
Application number: PCT/NO1997/000028
Authority: WO
Inventors: Yngvar S. Owesen
Original assignee: Klean A/S
Priority date: 1996-04-03
Filing date: 1997-02-03
Publication date: 1997-10-09
Also published as: NO961393D0; AU2106297A

Abstract

The present invention relates to a method and an apparatus for purifying water, especially a method including combined pre-and post-purification, and for the purpose of providing a finally purified water which is O3-influenced, free from microorganisms, free from colorants and also having been subjected to a very intense and time-optimized UVC-irradiation, it is according to the invention suggested a) passing the contaminated water (A1) under an appropriate pressure (Δp) to a pre-chamber (1) comprising one or more UVC-sources (1a, 1b, 1c) for the production and dosage of O3-(ozone), b) passing on the O3-containing water (A2) to one or more particle filters (3a, 3b) adapted for removing particles, microorganisms and colorants from the water (A3) and, c) passing the basically processed water (A4) along a plurality of guiding means (6a-6n) having intermediate further UVC-sources (7a-7n) for prolongation of the flowpath of the water (A4) and further UVC-irradiation.

Description

Method and apparatus for purifying water

Field of the invention

The present invention relates to a method for purifying water, especially a method including combined pre- and post-purification.

The present invention also relates to an apparatus for purifying water, especially an apparatus including combi¬ ned pre- and post-purification.

Prior art

There are previously known UV-aggregates based on so- called positive radiation geometry, wherein the liquid to be disinfected flows through a substantially cylindrical quartz tube reactor-chamber. Outside the cylindrical quartz tube, there is mounted a ring of UV-lamps includ- ing reflectors. A powerful radiation field is intended to extend all the way to the centre of the reactor-chamber, it being understood that the generated radiation profile is substantially equal to the flow profile of the water. Such a structure devises that the positioning of the UV- lamps around the cylindrical reactor-chamber will also let water having low UV-transmission be influenced by the intensive UV-radiation from all directions.

Such UV-aggregates are based on the utilization of the energy potential in that range of the ultraviolet spectre having a biocide effect, as well as on the principle of positive radiation geometry.

However, the radiation doses being achieved in such a prior art UV-aggregate will be strongly reduced for the water flowing along the centre-axis of the aggregate, the distance to the centre-axis being very large if it is a token of aggregates having a relatively large capacity.

Such a prior art UV-aggregate will otherwise not right away be able to provide a particle or microbe free water, let alone a clear final water having colorants removed therefrom.

Neither will such a prior art UV-aggregate be able to provide 0₃-containing water which is also purified of microbes, clarified and inactivated.

Background of the invention

The object of the present invention is to give instruc- tions for a method and an apparatus for purifying water, which secures that the contaminated water being subject to a purifying process not only will be 0₃ inactivated, but also purified as regards microbes, colorants and further unwanted matters, such that the final water can be used for industrial purposes having various require¬ ments as regards purity and inactivity.

Brief disclosure of the invention

The object is achieved according to the invention by a method as stated in the preamble, which is characterized by the features as stated in the appended patent claim 1. An apparatus according to the invention is characterized by the features as stated in the appended patent claim 14.

Further features and advantages of the present invention will appear from the following description taken in con¬ nection with the drawings, as well as from the mentioned and further appended patent claims. Brief disclosure of the drawings

Fig. 1 is a schematical section through a first embodi¬ ment of an apparatus for purifying water, according to the invention, wherein the method according to the inven¬ tion can be realized.

Fig. 2 is schematical cutout of a detail of the apparatus illustrated in Fig. 1.

Fig. 3 is a graphical representation illustrating the relation between light intensity in percent as ordinate and light path as abscissa.

Fig. 4 is a graphical representation wherein the radiant efficiency is illustrated as ordinate, whereas operating time is illustrated as abscissa.

Description of embodiments

In Fig. 1 there is illustrated a schematical section through an apparatus for purifying water, designed in accordance with the teachings of the present invention, especially an apparatus which is generally designated by reference numeral 1A and which is specifically favourable for combined pre- and post-purification.

The apparatus 1A itself can be designed as a cylindrical, parallel epipedic or similar structure, all of which being adapted to capacity and field of use.

In the embodiment of the apparatus 1A illustrated in Fig. 1, this may be cylindrical or parallel epipedical, for example having a height of approximately 1 m and a dia- meter of approximately 80 cm, and having a capacity of for example 4000 1 water/hour at a water pressure of for example 4,5 kg. In Fig. 1 it is depicted that contaminated water A1 hav¬ ing an appropriate pressure Δp is introduced into a lower pre-chamber 1 comprising one or more UV-sources, here for example three sources, 1a, 1b and 1c, respectively, espe- cially for the production and dosing of 0₃ (ozone).

The 0₃-containing water A2 emerging from the pre-chamber 1 is passed on through one or more particle filters, here for example two particle filters 3a, 3b, serving for the removal of particles, microorganisms and colorants from the entering water A2, which means that the particle free and 0₃-containing water A3 emerges from the latter filter 3b for thereafter being passed through a subsequent UVC- chamber 4 comprising a plurality of UVC-sources 5a-5n, for example 10 units arranged substantially horizontally and parallelly with a suitable spacing 5x therebetween.

It is to be understood that the contaminated water A1 which is introduced into the pre-chamber 1 wherein the UVC-sources 1a, 1b and 1c generate appropriate quantities of 0₃, can be supplied with a pressure Δp of a magnitude range corresponding to usual water supply pressure, for example approximately 3-6 kg.

Further, it is to be understood that in the pre-chamber 1 there may be used UVC-sources 1a, 1b and 1c having a wavelength of 253,7 nm, preferably without uvisil glass, i.e. preferably for letting through 185 nm shortwave mercury lines.

In Fig. 2 there is illustrated an embodiment wherein UVC- sources 1a, 1b and 1c can be mounted in the pre-chamber 1, namely by being mounted in waterproof glass tubes 1aa, 1bb, 1cc preferably made of the quartz glass, and then with appropriate gaskets towards the surroundings, such that the generated 0₃ will not be released to the sur¬ roundings, but, on the contrary, to said pre-chamber 1. The quantity of 0₃ which is generated, i.e. mg 0₃/m³, can be regulated by means of a control system being influenc¬ ed by an 0₃-meter arranged at the exit of the apparatus, and which will be further discussed in the following.

The originally contaminated water A1 which has become 0₃- containing and 0₃-inactivated in the pre-chamber 1 and having been purified as regards colorants and micro¬ organisms in the filters 3a and 3b, will consequently on the one hand leave behind 0₃-destructed microbes in the filters 3a, 3b, which prevents the fouling of the filt¬ ers, and on the other hand the removal of the colorants will mean that the transmission ability for the UV-radia- tion is optimized, which will be favourable when the 0₃- containing and purified water A3 enters said UV-chamber 4 housing the UVC-sources 5a-5n. Also here, there may be used UVC-sources having a wavelength of 253,7 nm, but not necessarily for further 0₃-production here. Preferably, said UVC-sources 5a-5n may be arranged in a tube of quartz glass, which are water-proof but which by their mutual positioning and narrow intermediate spaces 5x will result in that the by-passing water A4, which is also forced to flow through a subjacent perforated grate 5y, will be distributed very evenly in said intermediate spaces 5x, at the same time as none of the water partic¬ les will pass the UVC-sources at a larger distance than for example approximately 1 cm, resulting in an intense irradiation of all of the by-passing water A4.

The water A4 being discharged from the UVC-chamber 4 and still containing 0₃, but which now has been subjected to an even and thorough basic UVC-irradiation, will thereaf¬ ter be passed along a plurality of guiding means 6a-6n, here for example five plates, each of the guiding plates alternatively protruding substantially horizontally out from opposite walls or wall portions in the apparatus 1A itself, and each guiding plate at its free edge portion 6a defining a slit 6y between itself and a wall portion. In each of said slits 6y there is mounted a UVC-source, here five sources 7a-7n along which the water A4 from the UVC-chamber 4 must pass along on both sides of the res- pective UVC-source, and also in meander- or slalom-form along said guiding plates 6a-6n.

Each of said UVC-sources 7a-7n which also here can be mounted in water-proof individual glass tubes preferably of quartz glass, will consequently on both sides be passed by the water A4, for example in a capacity of 1,4 1/s on each side of the UVC-source 7a-7n, respectively.

The mentioned guiding plates 6a-6n can also be designated as delay plates, since they also aid in giving the water which is to be processed, surplus time to be UVC-irradi- ated, and then with an appropriate by-pass flow velocity along the UVC-sources in question, for thereby optimizing the UVC-exposure.

After the final guiding plate 6n and throughflow through the final slit 6y, the further UVC-exposed water A5 will be pressed through an upper perforated plate 8a which distributes the water A5 to an upper set of UVC-sources 8, for example ten units, which are housed in a terminal chamber 4b.

Microbe free and 0 -inactivated water A6 will thereafter be tapped off through an appropriate outlet 15a, 15b, but only after having passed the previously mentioned 0₅- meter 2 which is positioned in the terminal chamber 4b, as well as an intensity meter 14, which 0₃-meter 2 and intensity meter 14 are included in a control system which in turn influences the intensity of the 0₃-production in the pre-chamber 1 housing said 0₃-producing UVC-sources la, 1b and 1c. The control system can for example be switchable to vari¬ ous operational modes, i.e. with large 0₃-generation and large UVC-intensity, which means the switching-in of more or a larger number of 0 -generators and UVC-sources.

Microbe free and 0₃-inactivated water A6 can thus be discharged through the outlet 15b to a tapping cock 9 and be used for example for washing and cleaning, i.e. wash¬ ing floors and machines and still wanting an 0 -effect on the surfaces to be cleaned. In dependence of the program¬ med generation, the water A3 being tapped off through the cock 9 can retain its 0 -inactivation on the cleaned surfaces for many hours, for example for six hours if the foundation to be washed is relatively 0₃-resistant, for example hard metals as stone, ceramics, etc. By cleaning softer surfaces, for example plastic webs, there may be used 0₃-inactivated water of a lower intensity, but still having a time favourable effect for the field of applica¬ tion in question.

From Fig. 1 it appears that the microbe free and 0₃- inactivated water A4 can also be discharged from the terminal chamber 4b through a second outlet 5a in order to be pressed through one or more carbon filters, here two carbon filters 10a, 10b for removal of 0₃, whereafter the water can be tapped off through a second cock 11 as 0₃-free water, especially for use in nutrients or in connection with food processes.

Possibly, the water A7 coming out from the carbon filters 10a, 10b can be passed through an inverse osmose device 12a for thereafter being tapped off through a third cock 12 as oppositely osmose cleaned 0₃-free water, for example for use in laboratories wherein extremely partic- le free water is to be used for testing purposes.

Since the osmose membrane has a pore size of approximat- ely 7-8 A, even the smallest microbes or protein similar elements will not be let through the device 12a.

It is to be understood that the method and the apparatus according to the invention, are specifically favourable in relation to prior art, especially because the water to be purified will have a relatively low velocity when passing said UVC-sources, i.e. the water is subjected to long exposure time, and not to forget a short distance from the respective sources. It has been mentioned that the individual water particles will not have a larger distance than 1 cm from the UVC-sources in question, but in specific reactors or apparatuses this dimension can of course be reduced further.

In addition to the water in this manner being subjected to a very even, intense and long-lasting UVC-exposure, the water will also be inactivated due to the preceding 0₃-generation, which in total gives a final product which excels in purity, clarity and inactivation.

In Fig. 3 there is illustrated the relation between light intensity in percent and the light path in water in cm, and at the discussed distance of approximately 1 cm it can be seen from the graph of Fig. 3 that the by-passing water will be subjected to a light intensity of approxi¬ mately 75% or more.

Due to the number of UVC-sources and the location thereof in the apparatus or reactor, no microbe will be subjected to a lower radiation dose than 1100 J/m², even after 7000 operative hours of the sources, see Fig. 4, from which it appears that at 7000 hours of operation the radiant-effi¬ ciency will be approximately 65%, which by an appropriate re-calculation corresponds to said 1100 J/m .

It is to be informed that at a radiation dose of 300 J/m the microbes will be subjected to a destruction efficien¬ cy of 99,99% in mostly all of the batteries.

Since there is generated 0₃ in the pre-chamber 1 , the 0₃- inactivation will aside from preventing fouling in the subsequent filter 3, also contribute in the destruction of possible, remaining microorganisms in the subsequent chambers, i.e., the UVC-chamber 4a, the intermediate "delay chamber" 6a-6n and the terminal chamber 4b.

In order to secure a reliable operation and safe mainten¬ ance in connection with the method and apparatus accord¬ ing to the invention, it will upon power failure be ordered a stop of the throughflow in the apparatus, for example by closing an appropriate magnet valve 13 at the input to the apparatus 1A.

Upon failure in one or more UVC-lamps appropriately arranged lamp sensors will give signal regarding stopping the throughflow, by closing said magnet-valve 13. This magnet valve 13 can also be controlled by said previously mentioned intensity meter 14 and bring the magnet valve 13 to a closed position when the irradiation intensity from said UVC-sources falls below a certain limit, for example after a certain time of operation.

It is also to be understood that manual shut down of the apparatus can be used, for example by closing the magnet valve 13 after the plant has been in operation for a certain number of hours of operation, which requires a timely inspection of a counter unit which reflects time for replacing UVC-sources.

It is further to be understood that the apparatus 1A can also be shut off for cleansing at regular intervals, for example by means of an appropriate shut off valve 16, as well as appropriate flush supply conduits 17 for through- flushing of surfaces and quartz glass, which cleaning process of course can be arranged so as to be carried out automatically.

It is to be understood that upon power failure or by too low intensity from the UVC-sources, there may be provided optical and acoustic alarm, it then being given the pos¬ sibility of re-setting the acoustic alarm, but not for the optical one before the failure has been corrected or the maintenance has been concluded.

Further, it is to be understood that the reactor 1A itself can be constructed in a plurality of ways, and be given various dimensions. Cylindrical, parallel epipedi- cal and similar shapes have been mentioned, but also the combinations thereof can be contemplated.

The discussed intermediate UVC-chambers can thus be made not only with a stage by stage meander or slalom-like flowpath for the water. For example, there may be used a plurality of subjacent round plates which in radial tracks are provided with UVC-sources, which plates can have their slits displaced in relation to each other, respectively have different numbers and sizes of the slits for thereby prolonging and varying the water irra¬ diation path.

Claims

P a t e n t c l a i m s

1. Method for purifying water, especially a method including combined pre- and post-purification, c h a r a c t e r i z e d i n that the purifying comp¬ rises the following steps: a) passing the contaminated water (A1 ) under an appro¬ priate pressure (Δp) to a pre-chamber (1) comprising one or more UVC-sources (1a, 1b, 1c) for the produc- tion and dosage of 0₃-(ozone), b) passing on the 0 -containing water (A2) to one or more particle filters (3a, 3b) adapted for removing particles, microorganisms and colorants from the water (A3), and c) passing the basically processed water (A4) along a plurality of guiding means (6a-6n) having intermedi¬ ate further UVC-sources (7a-7n) for prolongation of the flowpath of the water (A4) and further UVC-irra¬ diation.

2. Method as claimed in claim 1 , c h a r a c t e r i z e d i n that the particle free and 0₃-containing water (A3) is passed into and through a subsequent UVC-chamber (4) comprising one or more UVC- sources (5a-5n) serving for intensive basic irradiation of the water (A4) before the basically treated and basic¬ ally irradiated water (A4) is discharged into the pro¬ longed flowpath (6a-6n).

3. Method as claimed in claim 1 or 2, c h a r a c t e r i z e d i n that the basically and further irradiated water (A5) is passed by a terminal chamber (4b) including still another set of UVC-sources (8), and that the microbe free, 0₃-inactivated and UVC- irradiated water (A6) is passed out for direct use or via further purifying means (10a, 10b; 12a) for special use (A7, A8).

4. Method as claimed in any of the claims 1-3, c h a r a c t e r i z e d i n that the contaminated water (A1 ) is passed into the pre-chamber (1) at a pres¬ sure (Δp) of a magnitude range corresponding to usual water supply pressure, for example approximately 3-6 kg.

5. Method as claimed in any of the claims 1-4, c h a r a c t e r i z e d i n that for the control of the dosage of 0₃-quantities in the pre-chamber (1), there is used an 0₃-meter (2) which preferably is located in the terminal chamber ( b), and which by mean of a control valve regulates mg 0₃-πr of the water (A2) in said pre- chamber (1 ) .

6. Method as claimed in any of the claims 1-5, c h a r a c t e r i z e d i n that in the pre-chamber (1) there are used UVC-sources having a wavelength of 253,7 nm, preferably without uvisil glass, i.e. preferab¬ ly for letting through 185 nm short-wave mercury lines.

7. Method as claimed in claim 6, c h a r a c t e r i z e d i n that there are used UVC- sources (1a, 1b, 1c) which are mounted in water-proof glass tubes preferably of quartz glass, and having appro- priate gaskets towards the surroundings.

8. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that the particle free and 0₃-containing water (A3) will have supplied so much ozone (1,2) and be subjected to so fine filtering (3, A3) that the water (A3) will be so clarified and colorant free that on the one hand the 0₃-inactivation prevents fouling in the filters (3a, 3b) and on the other hand optimizes the transmission ability for UVC-radiation.

9. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that in the subsequent UVC-chamber (4) there are used a set of substantially horizontally arranged water-proof UVC-tubes (5a-5n), preferably quartz glass, and that over the set of tubes (5a-5n) there is provided a perforated plate (5y) which contributes to the water (A3) coming from the pre-chamber

( 1 ) flowing evenly distributed along the individual tubes (5a-5n) through mutually narrow spaces (5x) between said tubes.

10. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that above said UVC- chamber (4) the particle free and 0₃-containing and furt¬ her UVC-irradiated water (A4) is passed along two or more guiding plates (6a-6n) which alternately protrude sub- stantially horizontally out from opposite walls or wall portions in the apparatus (1a) and at their free edge portions (6x) define a slit (6y) between themselves and a wall portion, there being in each such slit (6y) pressed water beyond in each slit mounted UVC-source (7a-7n) which the water (A4) from the UVC-chamber (4) must pass on both sides and also in meander or slalom-form along said guiding plates (6a-6n).

11. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that above the upper guiding plates (6n) the basically and further irradiated water (A5) is distributed by means of an upper perforated plate (8a) for distribution and close passing past an upper set of UVC-sources (8) which are housed in a ter- minal chamber (4b).

12. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that from the terminal chamber (4b) the outflowing 0₃-containing water (A6) will be guided past an intensity meter (14) and said 0₃-meter

(2) before the microbe free and 0₃-inactivated and final¬ ly irradiated water (A6) can be tapped off through i) a first cock (9) preferably for flushing and clean¬ ing, or ii) passed on to one or more active carbon filters (10a, 10b) for the removal of 0₃ and tapping through a second cock (11), for example for use in food pro¬ cesses, or iii) from the active carbon filters (10a, 10b) to be guided through an inverted osmose device (12a) for tapping through a third cock (12) as inverted osmose purified 0₃-free water, for example for testing and laboratory purposes.

13. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that there is used such a number of UVC-sources and such a mutual location of said sources that no microbes are subjected to a less irradiation dose than approximately 1100 J/m², even after 7000 hours of operation.

14. Apparatus for purifying water, especially an appara¬ tus including combined pre- and post-purification, c h a r a c t e r i z e d i n that the apparatus (1a) comprises, a) a pre-chamber (1 ) comprising one or more UVC-sources (1a, 1b, 1c) for the production and dosage of 0₃-

(ozone) , b) one or more particle filters (3a, 3b) adapted for removing particles, microorganisms and colorants from the water (A2) coming from the pre-chamber (1), and c) a plurality of guiding means (6a-6n) having inter¬ mediate further UVC-sources (7a-7n) for prolongation of the flowpath of the water (A4) and further UVC- irradiation.

15. Apparatus as claimed in claim 14, c h a r a c t e r i z e d i n that it comprises a subsequent UVC-chamber (4) comprising one or more UVC- sources (5a-5n) serving for intensive basic irradiation of the water (A3) emerging from said particle filters (3a-3b).

16. Apparatus as claimed in claim 14 of 15, c h a r a c t e r i z e d i n that it comprises a terminal chamber (4b) including another set of UVC-sour¬ ces (8), and outlet conduits (15a, 15b) for tapping the microbe free, 0₃-containing, 0₃-inactivated and UVC-irra- diated water (A6) for direct use (9) or further purifica¬ tion (10a, 10b); (12a) for special use (A7, A8) .

17. Apparatus as stated in any of the claims 14-16, c h a r a c t e r i z e d i n that it is supplied with an inlet including a valve (13) adapted for substantiat¬ ing a pressure (Δp) of a magnitude range corresponding to usual water supply pressure, for example approximately 3- 6 kg.

18. Apparatus as claimed in any of the claims 14-17, c h a r a c t e r i z e d i n that the apparatus comp- rises an 0₃-meter (2) for measuring 0₃-generation in said pre-chamber ( 1 ) , said 0₃-meter (2) preferably being pro¬ vided in the terminal chamber (4b) for by means of a con¬ trol system to regulate mg 0₃/m³ of the water (A2) in said pre-chamber (1 ) .

19. Apparatus as claimed in any of the claims 14-18, c h a r a c t e r i z e d i n that in the pre-chamber

(1 ) there are provided UVC-sources having a wavelength of 253,7 nm, preferably without uvisil glass, i.e. preferab- ly for letting through 185 nm short-wave mercury lines.

20. Apparatus as claimed in any of the claims 14-19, c h a r a c t e r i z e d i n that said UVC-sources are provided in water-proof glass tubes (1aa, 1bb, 1cc) preferably of quartz glass and being provided with appro- priate gaskets against the surroundings.

21. Apparatus as claimed in any of the claims 14-20, c h a r a c t e r i z e d i n that it comprises means (2) for regulating the ozone production in said pre-cham- ber (1) related to the final use of the final purified water and preventing the fouling of subsequent filters, and that the filter means (3a, 3b) for removing micro¬ particles remove both microbes and colorants for optimiz¬ ing the transmission quality of subsequent UVC-radiation.

22. Apparatus as claimed in any of the claims 14-21, c h a r a c t e r i z e d i n that in said UVC-chamber (4) there is provided a set of substantially horizontally arranged parallel water-proof UVC-tubes (5a-5n), prefer- ably quartz glass, and that over the set of tubes (5a-5n) there is provided a perforated plate (5y) which contri¬ butes to the water (A3) coming from the pre-chamber (1) flowing evenly distributed along the individual tubes (5a-5n) through mutually narrow spaces (5x) between said tubes.

23. Apparatus as claimed in any of the claims 14-22, c h a r a c t e r i z e d i n that above said UVC- chamber (4) there are provided two or more guiding plates (6a-6n) which alternately protrude substantially hori¬ zontally out from opposite walls or wall portions in the apparatus (1a) and at their free edge portions (6x) define a slit (6y) between themselves and a wall portion, there being in each such slit (6y) pressed water beyond in each slit mounted UVC-source (7a-7n) which the water

(A4) from the UVC-chamber (4) must pass on both sides and also in meander or slalom-form along said guiding plates (6a-6n) .

24. Apparatus as claimed in any of claims 14-23, c h a r a c t e r i z e d i n that above the upper guiding plate (6n) there is provided a further perforated plate (8a) for distribution and close passing past a subjacent upper set of UVC-sources (8) which are housed in a terminal chamber (4b).

25. Apparatus as claimed in any of the claims 14-24, c h a r a c t e r i z e d i n that in the terminal chamber (4b) there is provided an intensity meter (14) and said 0₃-meter (2) which the 0₃-containing and microbe- free and 0₃-inactivated water (A6) must pass before it can be tapped off through i) a first cock (9) preferably for flushing and clean¬ ing, or ii) passed on to one or more active carbon filters (10a, 10b) for the removal of 0₃ and tapping through a second cock (11), for example for use in food pro¬ cesses, or iii) from the active carbon filters (10a, 10b) to be guided through an inverted osmose-device (12a) for tapping through a third cock (12) as inverted osmose purified 0₃-free water, for example for testing and laboratory purposes.

26. Apparatus as claimed in any of the claims 14,25, c h a r a c t e r i z e d i n that the apparatus (1A) comprises such a number of UVC-sources and such a mutual location of said sources that no microbes are subjected to a less irradiation dose than approximately 1100 J/m², even after 7000 hours of operation.