WO2003014030A1

WO2003014030A1 - Method for killing of microorganisms in the water by uv-tio2 photocatalytic reaction and reactor for killling of microorganisms

Info

Publication number: WO2003014030A1
Application number: PCT/KR2002/001495
Authority: WO
Inventors: Si Wouk Kim; Jung Kon Kim; Yong-Ho Kim; Young-Sang Lee
Original assignee: Chosun University
Priority date: 2001-08-06
Filing date: 2002-08-06
Publication date: 2003-02-20
Also published as: KR20030012956A; CN1301917C; CN1538939A; KR100439195B1

Abstract

There are disclosed a method for killing of microorganisms in the water by UV-TiO2 photocatalytic reaction and a reactor for killing of microorganisms.

Description

METHOD FOR KILLING OF MICROORGANISMS IN THE WATER BY UV-Ti0₂ PHOTOCATALYTIC REACTION AND REACTOR FOR KILLING OF

MICROORGANISMS

TECHNICAL FIELD The present invention relates generally to water disinfection by photocatalytic reaction. More particularly, the present invention relates to an apparatus for water disinfection and a method of inactivating or destroying microorganisms and organic materials in water using such an apparatus, which comprises the steps of adding hydrogen peroxide into contaminated water, introducing contaminated water added with hydrogen peroxide into a photocatalytic reactor containing photocatalyst- immobilized porous beads, injecting air thereinto, and applying UV radiation thereinto.

BACKGROUND ART

Typically, regions with low levels of precipitation are faced with shortages of drinking water as well as water for irrigation. Especially in Korea, the annual rainfall has recently decreased, resulting in lack of fresh water. Most farmers in Korea are using conventional irrigation systems that are inefficient due to economic problems in introducing modern rill irrigation, which efficiently supplies water to the desired area, but requires high investment costs for its establishment. Therefore, to solve such difficulties in irrigating fields for the cultivation of crops, water, especially wastewater, is essentially recycled. In addition, the recycling of wastewater is necessary for continuous use of seawater or fresh water in large-sized aquariums. In order to recycle wastewater, first of all, it is most important to kill microorganisms in wastewater.

To control the growth of bacteria, viruses and algae, chlorine has been widely used for water disinfection in the conventional wastewater-recycling systems. However, such water disinfection is disadvantageous in that carcinogenic trihalomethane (THM) is produced and chloride remains. Especially, the residual chloride is problematic in production of pure water or ultra-pure water. Furthermore, the high technology purification process for disinfecting water using ozone is uneconomical in terms of requiring high investment cost for its establishment and management.

Recently, disinfection using a photocatalyst, titanium dioxide (Ti0₂) , has been tried, and such a disinfection system is inexpensive in its establishment and maintenance.

Disinfection using titanium dioxide (Ti0₂) photocatalyst is based on the following Schemes .

[Scheme 1] ^τi°z + hv e-_{CB + h}+_γB

[Scheme 2] h⁺ _VB + OH- OH-

⁺ _VB + H₂0 OH- + H⁺

OH- + Organic materials ► Organic materials

h⁺ _VB + Organic materials Organic materials

[ Scheme 3 ]

θ CB ⁺ ^2 o₂--

20₂~- + 2H₂0 20H- + 20H~ + 0₂

H₂0₂ ► v 20H-

H₂0₂ + θ _CB OH- + OH-

H₂0₂+ 0₂ OH- + OH- ⁺ 0,

OH- + Organic materials ► Organic materials e~cβ ⁺ Organic materials ► Organic materials

According to Scheme 1, when Ti0₂ is irradiated with photons of sufficient band gap energy or greater, a photon

(hv) excites an electron from the valance band, overcoming the energy band-gap to the conduction band, and leaves an electronic vacancy, a hole (h⁺) in the valance band. The conduction band electron (e^'cβ) and the valance band hole

(h⁺ _B) then diffuse and move over the surface of the Ti0₂.

The excited electrons and the resulting holes may participate in redox processes with H₂0, OH^", organic materials and O2 in water. Referring to Scheme 2, the diffused hole (h⁺ _VB) reacts with OH^" in water to produce an OH radical (OH-) , or with water molecules (H₂0) to generate OH- and H⁺, as well as directly oxidizing organic materials. Referring to Scheme 3, the electron (e^~ _CB) reacts with oxygen in water, producing a superoxide radical (0₂ ^~-) . The superoxide radical reacts with water molecules to generate OH-, OH- and oxygen molecules. When water contains hydrogen peroxide, hydrogen peroxide absorbs UV energy, producing OH-, or reacts with e^~ _CB or dissolved oxygen to produce OH- . The produced OH radicals (OH-) participate in oxidation of organic materials .

However, the disinfection method employing the photocatalyst Ti0₂, and following the reaction mechanism as described above, has several drawbacks as follows. When titanium dioxide in powder form is applied to water containing contaminants, continuous treatment of water with Ti0₂ powder leads to consumption of a great deal of electrical energy for resuspension of Ti0₂ powder, and in addition, the photocatalyst should be recovered from the treated water. When Ti0₂ is directly coated on UV lamps, the Ti0₂ must be undesirably thrown away along with the lamps at the end of the lifespan of the lamps. In addition, when coating the inside of a reactor with Ti0₂, the coating operation is very difficult if the reactor is large in its size. Moreover, after the coating with Ti0₂, the reactor should be heat-treated at over 500 °C, thus limiting materials capable of being used in manufacturing the reactor. Also, when Ti0₂ is immobilized to glass beads, a Ti0₂ film layer is gradually eroded from the surface of glass beads by the continuous water flow.

DISCLOSURE OF THE INVENTION

Leading to the present invention, the intensive and thorough research into a reactor for water disinfection by photocatalytic reaction, and a method of inactivating or destroying microorganisms and organic materials contained in water using such an apparatus, with the aim to solve the problems encountered in the background art, resulted in the finding that an apparatus comprising a photocatalytic reactor and a method for water disinfection using the apparatus, where contaminated water is disinfected through a procedure of adding hydrogen peroxide to water to be treated, introducing the water added with hydrogen peroxide into the photocatalytic reactor containing photocatalyst-immobilized porous beads, injecting air into the photocatalytic reactor and applying UV radiation into the photocatalytic reactor, make it possible to shorten disinfection running time and increase disinfection efficiency thanks to the introduction of air and hydrogen peroxide, as well as the reactor being capable of being constructed in a small size-, thus allowing its installation in a narrow place, and its being dismantled, thus facilitating its cleaning. It is, therefore, an object of the present invention to provide an apparatus for water disinfection, comprising a photocatalytic reactor which contains photocatalyst- immobilized porous beads and is equipped with UV lamps for irradiation of UV and an inlet for injection of air thereinto.

It is another object of the present invention to provide a method of effectively inactivating microorganisms in contaminated water, using the apparatus for water disinfection.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sectional view showing the construction of an apparatus for water disinfection according to a primary embodiment of the present invention;

FIG. 2 is a side view showing a photocatalytic reactor included in the apparatus of the present invention;

FIG. 3 is a view showing an apparatus for water disinfection with several photocatalytic reactors arranged in parallel in accordance with another embodiment of the present invention;

FIG. 4 is a photograph showing a Ti0₂-immobilized bead ;

FIGS . 5a and 5b are graphs showing bactericidal activities of Ti0₂-immobilized porous beads and glass beads, in which viable cell numbers and bactericidal rates (%) versus E. col± are plotted against time in FIG.

5a and FIG. 5b, respectively;

FIGS . 6a and 6b are graphs showing bactericidal effect per UV lamp according to the diameter of the photocatalytic reactor viability, in which viable cell numbers and bactericidal rates (%) versus E. coli are plotted against time in FIG. 6a and FIG. 6b, respectively;

FIGS. 7a and 7b are graphs in which viable cell numbers and bactericidal rates (%) versus E. coli are plotted against time in FIG. 7a and FIG. 7b, respectively, when air is injected into the photocatalytic reactor;

FIG. 8 is a graph showing an effect of concentration of hydrogen peroxide on disinfection efficiency versus E. coli when the photocatalytic reaction occurs, in which bactericidal rates (%) at various concentrations of hydrogen peroxide are plotted against time;

FIG. 9 is a graph showing an effect of concentration of hydrogen peroxide on disinfection efficiency versus E . coli when the photocatalytic reaction is not induced, in which bactericidal rates (%) at various concentrations of hydrogen peroxide are plotted against time;

FIG. 10 is a graph showing an effect of concentration of hydrogen peroxide on the sprouting of beans;

FIGS. 11a and lib are graphs in which viable cell numbers of E. coli and bactericidal rates (%) are plotted against time in FIG. 11a and FIG. lib, respectively, when volume of treated water is doubled and photocatalytic reaction is induced;

FIGS. 12a and 12b are graphs in which viable cell numbers and bactericidal rates (%) versus bacteria in water used in sprouting of beans are plotted against time; and

FIGS. 13a and 13b are graphs in which viable cell numbers and fungicidal rates (%) versus fungi in water used in sprouting of beans are plotted against time. *Brief description of the reference number

100: apparatus 10: photocatalytic reactor

11: photocatalyst-immobilized porous beads

12: UV lamp 13: water outlet

14 : water inlet 15 : inner frame

16: outer frame 17: O-ring

18: locking means 20: first water resirvoir

21: water supply tube 22: water supply pump

30: air inlet tube 31: air pump

40: UV-stabilizing means

41: electric circuit 50: filter

51: second water resirvoir BEST MODES FOR CARRYING OUT THE INVENTION

The present invention relates to an apparatus for water disinfection, as will be described in detail with reference to the accompanying drawings. FIG. 1 shows the construction of an apparatus for water disinfection according to a preferred embodiment of the present invention.

As shown in FIG. 1, the apparatus 100 for water disinfection, in which water is disinfected by a photocatalytic reaction, comprises a photocatalytic reactor 10 containing porous beads 11 to which a photocatalyst is immobilized. UV lamps 12 are axially installed in the photocatalytic reactor 10 at regular intervals, such that the lamps 12 are positioned at the center and ends of a cross as best seen in FIG. 2, when being cross-sectioned, whereby OH radicals are generated when the photocatalyst- immobilized porous beads 11 are exposed to UV radiation from the UV lamps 12. An air inlet tube 30 is axially installed at a lower inside portion of the photocatalytic reactor 10, through which air from an air pump 31 is injected to the inside of the photocatalytic reactor 10. A water inlet 14 and a water outlet 13 are provided at a lower part and an upper part of the photocatalytic reactor 10, respectively, and are diagonally located on the housing of the reactor 10 when being sectioned along a longitudinal axis, thus increasing the residence time of water in the photocatalytic reactor 10. A filter 50 for preventing an undesired discharge of the photocatalyst-immobilized porous beads 11 from the reactor 10 is installed in each of the water inlet 14 and the water outlet 13. The housing of the reactor 10 has a sealed structure with inner and outer frames 15 and 16. The inner frame 15 and the outer frame 16 are assembled with each other by a locking means 18 with an O-ring 17 made of rubber sandwiched between the two frames 15 and 16. The inner and outer frames 15 and 16 of the photocatalytic reactor 10 are made of transparent acryl, thus allowing a user to observe the inside of the photocatalytic reactor 10 as well as check for abnormalities and replacement time of the UV lamps 12 with the naked eye.

The porous beads 11 can be formed by using photocatalyst-immobilized beads such as muscovite beads or bentonite beads. In detail, as shown in FIG. 4, the porous beads 11 Porous bead made of muscovite is prepared by baking muscovite having high porosity at 800 °C, coating the surface thereof with a photocatalyst, and then heat-treating at 1400 °C . The photocatalyst is Ti0₂. By having a large surface area, the photocatalyst- immobilized porous beads 11 are capable of effectively inactivating or destroying microorganisms and organic materials. The porous beads are preferably 6-10 mm in diameter. When being below this range in diameter, the porous beads pack densely together, thus interrupting flow of water in the photocatalytic reactor. In contrast, when the diameter is above the range, the porous beads become diminished in their total surface area, thus reducing disinfection efficiency.

Especially, when exposed to UV light, the photocatalyst-immobilized porous beads 11 serve as a catalyst in a process for disinfecting water. After the photocatalyst-immobilized porous beads 11 are used several times, microorganisms and organic materials absorbed in the pores of the porous beads 11 can be easily removed through heat treatment, thus allowing their semipermanent use.

The photocatalyst-immobilized porous beads 11, which fill the inside of the photocatalytic reactor, are advantageous in comparison with the conventional glass beads coated with a photocatalyst, as follows. The porous beads have excellent absorptivity to microorganisms and organic materials thanks to their porosity, and improved disinfection efficiency due to their large surface area, as well as being semipermanently used. In contrast, the glass beads have a short life span owing to easy desorption of the photocatalyst, which is caused by continuous water flow. In accordance with an example of the present invention, when water disinfection is performed in the photocatalytic reactor using the photocatalyst-immobilized porous beads and the glass beads, with reference to FIGS. 5a and 5b, the Ti0₂-immobilized beads have greater disinfection ability than the Ti0₂-coated glass beads.

The UV lamps 12 are installed in the photocatalytic reactor 10 at regular intervals, where the intervals between the UV lamps affect disinfection efficiency in contaminated water. Since OH radicals (OH') serving as a powerful oxidant are generated when UV light from the UV lamps 12 is irradiated onto the photocatalyst-immobilized porous beads 11, the UV lamps 12 should be installed in an array allowing all of the porous beads 11 to be exposed to UV radiation. The intervals between the UV lamps 12 may be changed according to the size of the photocatalytic reactor 10. In accordance with an example of the present invention, when water disinfection is performed in the cylinder-shaped photocatalytic reactor 10 of 20 cm in diameter and 72 cm in length in which the UV lamps 12 are arranged at regular intervals, the most efficient disinfection is achieved when the UV lamps 12 are arranged at intervals of 55-80 mm. However, when disinfecting a large amount of contaminated water, the intervals between the UV lamps 12 may be modified according to the size of the photocatalytic reactor 10.

According to the prior art, the electrodes of the UV lamps were disposed in the photocatalytic-reactor, thus UV lamps needed to be covered with expensive quartz, in order to prevent the electrodes of the UV lamps from contacting the water. However, according to the present invention, the UV lamps 12 can be installed without the quartz tube since the electrodes of the UV lamps 12 are projected to the outside of the photocatalytic reactor 10, and allow a complete sealing effect of the reactor 10, thus reducing the cost of materials. Since the inner and outer frames 15 and 16 are made of transparent acryl, the inside of the photocatalytic reactor 10 is visible, thus allowing a user to investigate with the naked eye any abnormality of the lamps 12 and determine time to replace the existing lamps 12 with new ones. Also, the inner and outer frames 15 and 16 are assembled by the locking means 18 to maintain air tightness of the photocatalytic reactor 10. The locking means 18 is loosened as desired, thus facilitating replacement of the UV lamps 12 with new ones only by loosing the locking means 18. The locking means 18 may be selected from among conventional nuts and bolts, and other types of separable locking devices. To inject air into the photocatalytic reactor 10, the air inlet tube 30 is installed at the lower part of the photocatalytic reactor 10. The air inlet tube 30 is finely perforated to prevent its clogging by the porous beads 11 coated with the photocatalyst, and is connected to the air pump 31, which may have a regulator or not, in order to control air pressure. Air from the air pump 31 is introduced into the photocatalytic reactor 10 through the air inlet tube 30, thus increasing the amount of dissolved oxygen in water. Moreover, since the influx of a large volume of air generates a warm current in water and thus increases reaction efficiency of water, it is preferable that the air pump 31 is powerful.

To prevent the efflux of the photocatalyst- immobilized porous beads 11 to a water supply tube 21, the filter 50 made of plastic material is installed in the water inlet 14 and the water outlet 13. The pore size of the filter 50 may vary depending on the size of the porous beads .

In accordance with the present invention, the apparatus 100 for water disinfection further comprises the water supply tube 21 through which water is injected into the photocatalytic reactor 10, and a UV stabilizing means 40 controlling the operation of the UV lamps 12.

As described above, the apparatus 100 for water disinfection according to the present invention comprises the photocatalytic reactor 10 including the photocatalyst-immobilized porous beads 11, the UV lamps 12 and the air inlet tube 30. To disinfect a large volume of contaminated water, a plurality of photocatalytic reactors 10 may be fabricated in large sizes, or fabricated in small sizes and then arranged in parallel, as shown in FIG. 3. Such fabrications of the photocatalytic reactors in terms of size and array may be modified by those skilled in the art, and the apparatus 100 for water disinfection may be used in disinfecting and then recycling all kinds of water including agricultural water and seawater or fresh water used in aquariums . In accordance with another aspect of the present invention, there is provided a method of inactivating or destroying microorganisms and organic materials in contaminated water using the above-mentioned apparatus 100. The method for water disinfection by photocatalytic reaction comprises the steps of: adding hydrogen peroxide to water to be treated; introducing the water added with hydrogen peroxide into the photocatalytic reactor 10 containing the photocatalyst-immobilized porous beads 11; and injecting air into the photocatalytic reactor 10 through the air inlet tube 30, and applying UV radiation into the phptpcatalytic reactor 10 so as to induce the photocatalytic reaction.

Water from a first water reservoir 20 flows into the photocatalytic reactor 10 through the water inlet 14 installed at the lower part thereof, circulates therein, and then flows out through the water outlet 13, which is stimulated by a water supply pump 22. Water in the photocatalytic reactor 10 is disinfected by OH radicals, which are produced when UV light is irradiated onto the porous beads 11, and the disinfected water is stored at a second water reservoir 51. In addition, with aim of enhancing disinfection efficiency, a small amount of hydrogen peroxide is added to water in the first water reservoir 20. The concentration of hydrogen peroxide may vary depending on the volume of water, and is preferably 25-50 mg/L, where a minimum amount of hydrogen peroxide is preferably used owing to possible problems in aspects of economy and stability when hydrogen peroxide is applied at a high concentration. As shown in FIG. 8, when water is treated with various concentrations of hydrogen peroxide, disinfection efficiency in water is increased with concentration of hydrogen peroxide, thus shortening time required for water disinfection.

As shown in FIG. 9, in which disinfection efficiency by addition of hydrogen peroxide without exposure to UV is compared with that with exposure to UV, it was demonstrated that hydrogen peroxide alone can not effectively inactivate microorganisms in water, whereas high disinfection efficiency can be achieved by treatment with hydrogen peroxide along with exposure to UV radiation.

In addition, much higher disinfection efficiency is attained by increasing the amount of dissolved oxygen in water thanks to the use of the air inlet tube 30 and the air pump 31. When air is introduced into the photocatalytic reactor 10 via the air inlet tube 30 from the air pump 31, the amount of dissolved oxygen in water is increased, resulting in the formation of a warm current and thus leading to increased contact between contaminants in water and OH radicals. The volume of air injected into the photocatalytic reactor used in the present invention is 30 L/min, where the air pump is preferably as powerful as possible while not negatively affecting to operation of the photocatalytic reactor.

As shown in FIG. 7, when air is introduced into the photocatalytic reactor, disinfection efficiency of water at the initial stage is higher than that when disinfection is performed with no use of air. In accordance with an example of the present invention, during the first 1 min, bactericidal efficiency of 95 % or 90.6 % were observed when air was injected or not, respectively. This result is due to the fact that the introduced air raises a warm current and thus increases the level of dissolved oxygen, leading to increased production of OH radicals and thus more effective disinfection. As apparent in Table 1, below, and FIG. 10, water treated with hydrogen peroxide is stable. Table 1 shows an effect of hydrogen peroxide on sprouting of beans .

rTable 1]

As apparent in Table 1, sprouting rate is slightly reduced in a concentration of hydrogen peroxide of 50 mg/L, but recovered with the passage of time. As apparent in FIG. 10, total length of bean sprouts is increased with concentration of hydrogen peroxide. Total length of bean sprouts is significantly increased at 500 mg/L of hydrogen peroxide, where a significance level is 5 %. There is no significant difference in length of hypocotyls between bean sprouts being grown in water not treated with hydrogen peroxide or treated with hydrogen peroxide at 100 mg/L, but the length of hypocotyls is remarkably increased at 500 mg/L of hydrogen peroxide. Also, length of roots of bean sprouts is increased with addition of hydrogen peroxide. There is no difference in thickness of hypocotyls between bean sprouts treated with hydrogen peroxide or not. These results indicate that water treated with hydrogen peroxide is stable.

When the volume of treated water is doubled, the apparatus 100 for water disinfection comprising the photocatalytic reactor 10 containing the photocatalyst- immoblized porous beads 11, as shown in FIGS. 11a and lib, also shows excellent disinfection efficiency, where initial disinfection efficiency is slightly lower, but recovered in a short period. Besides, treatment of a large amount of water can be achieved using several photocatalytic reactors 10 arranged in parallel. The present invention will be explained in more detail with reference to the following examples in conjunction with the accompanying drawings. However, the following examples are provided only to illustrate the present invention, and the present invention is not limited to them.

<EXAMPLE 1> Method for water disinfection using a photocatalytic reactor

Optimum conditions for water disinfection were determined using various reactors, and the optimum conditions are as follows. Water disinfection was performed using the photocatalytic reactor 10 having a length of 720 mm and a diameter of 200 mm, Ti0₂- immobili-zed porous beads 11 of 8 mm in diameter, and the UV lamps 12 emitting the maximum UV light of 39 W at 254 nm, with addition of hydrogen peroxide of 0-75 mg/L and injection of air through the air inlet tube 30 at a rate of 30 L/min. In all of EXPERIMENTAL and COMPARATIVE EXAMPLES of the present invention, disinfention efficiency was evaluated by collecting samples at regular intervals of time, serially diluting the samples, smearing 100 μl of the finally diluted samples on solid culture media to allow the growth of bacteria and fungi, and then counting colonies grown on the media. EXPERIMENTAL EXAMPLE 1> Assay for viability of E. coli per UV lamp according to diameter of the photocatalytic reactor

To determine the optimum size of the photocatalytic reactor for obtaining the maximum disinfection efficiency per UV lamp, inhibition level of the growth of E. coli was investigated in the photocatalytic reactors 10 of 55, 80 or 110 mm in diameter, in which one UV lamp 12 was installed.

After the Ti0₂-immobilized porous beads were added into each photocatalytic reactor 10, a buffer solution containing E . coli was circulated in the photocatalytic reactor 10 under exposure to UV radiation for 15 min. The resulting disinfection efficiencies are given in Table 2, below, and FIGS. 6a and 6b.

;Table 2]

As shown in Table 2 and FIGS. 6a and 6b, in the photocatalytic reactor 10 of 55 mm in diameter, the initial cell number of E . coli (7.1xl0³ cells/ml) was reduced to 375 cells/ml after 1 min, and to 13 cells/ml after 15 min, showing bactericidal efficiency 95 % and 99.8 %, respectively. In the photocatalytic reactor 10 of 80 mm in diameter, the initial cell number of E. coli (7.3xl0³ cells/ml) was reduced to 823 cells/ml after 1 min, and 21 cells/ml after 15 min, showing bactericidal efficiency of 88.6 % and 99.7 %, respectively, where the initial bactericidal activity is lower than that upon using the photocatalytic reactor of 55 mm in diameter, but it was recovered to a level similar to that of the photocatalytic reactor of diameter of 55 mm after 15 min. However, upon using the photocatalytic reactor of 110 mm in diameter, the initial cell number of E. coli (7.1xl0³ cells/ml) were reduced to 3.1xl0³ cells/ml after 1 min, and to 201 cells/ml after 15 min, showing bactericidal efficiency of 57.5 % and 97.2 %, respectively, which are much lower than those upon using the photocatalytic reactor of 55 mm and 80 mm in diameter. These results shows that the photocatalytic reactors of 55 mm and 80 mm in diameter supply high disinfection efficiencies, whereas the photocatalytic reactor of 110 mm in diameter gives remarkably low efficiency in disinfection. Thus, the most suitable size of the photocatalytic reactor per UV lamp 12 is between 55 mm and 80 mm in diameter.

EXPERIMENTAL EXAMPLE 2> Effect of injection of air on inactivation of E. coli

Air was injected into the photocatalytic reactor 10 at a rate of 30 L/min via the air inlet tube 30 using the air pump 31, and viability of E. coli was investigated for 15 min in comparison with that in the case of not injecting air. The results are given in Table 3, below, and FIGS. 7a and 7b.

[Table 3]

As shown in Table 3 and FIGS. 7a and 7b, when air was not injected, the initial cell number of E. coli

(7.1xl0³ cells/ml) was reduced to 668 cells/ml after 1 min, showing bactericidal efficiency of 90.6 %, and to 70 cells/ml after 15 min. In contrast, when air was injected, the initial cell number of of E. coli (7.1xl0³ cells/ml) was reduced to 357 cells/ml after 1 min, showing bactericidal efficiency of 95 %, and to 13 cells/ml after 15 min. Disinfection efficiency upon introducing air was found to be higher than that upon not introducing air. This result is believed to be due to the fact that injected air generates a warm current in water in the photocatalytic reactor 10 where the photocatalytic reaction occurs, thus allowing OH radicals to react more effectively, as well as increasing the amount of dissolved oxygen and thus production of OH radicals.

EXPERIMENTAL EXAMPLE 3> Effect of injection of H₂0₂ on the growth of E. coli Hydrogen peroxide was added to water in the photocatalytic reactor 10 at amounts of 10, 15, 20 and 25 mg/L, with addition of air at a rate of 30 L/min using the air pump 31, and viability of E. coli was evaluated for 15 min. The results are given in Table 4, below, and FIG. 8.

[Table 4]

As shown in Table 4 and FIG. 8, when hydrogen peroxide was not added, the initial cell number of E . coli (7.3xl0³ cells/ml) was reduced to 267 cells/ml after

1 min, and to 13 cells/ml after 15 min, showing bactericidal efficiency of 96 % and 99.8 %, respectively.

When hydrogen peroxide was added at an amount of 10 mg/L, the initial cell number of E . coli (7.5xl0³ cells/ml) was reduced to 242 cells/ml after 1 min, showing bactericidal efficiency of 96.7 %, slightly higher than that upon not adding hydrogen peroxide, and after 15 min, showing bactericidal efficiency of' 99.8 %, similar to that upon not adding hydrogen peroxide. When hydrogen peroxide was added at an amount of 15 mg/L, the initial cell number of

E . coli (9.2xl0³ cells/ml) was reduced to 203 cells/ml after 1 min, showing bactericidal efficiency of 97.8 %. When hydrogen peroxide was added at an amount of 20 mg/L, the initial cell number of E . coli (8.5xl0³ cells/ml) was reduced to 157 cells/ml after 1 min, showing bactericidal efficiency of 98 %, and showing complete bactericidal efficiency after 15 min. When hydrogen peroxide was added at an amount of 25 mg/L, the initial cell number of E . coli (8.5xl0³ cells/ml) was reduced to 82 cells/ml after 1 min, showing bactericidal efficiency of 99 %, and showing complete bactericidal efficiency after 10 min. These results indicate that high disinfection efficiency is achieved through addition of a small amount of hydrogen peroxide .

EXPERIMENTAL EXAMPLE 4> Assay for disinfection efficiency when the volume of water to be treated is doubled

The amount of water to be treated in the photocatalytic reactor 10 was doubled, along with addition of hydrogen peroxide at amounts of 20, 25, 30 and 50 mg/L as well as injection of air at a rate of 30 L/min using the air pump 31, and viability of E. coli was evaluated for 15 min. The results are given in Table 5, below, and FIG. 11.

[Table 5]

No . of viable cells 0 min 1 min 2 min 5 min 15 min (cells /ml )

As shown in Table 5 and FIG. 11, when hydrogen peroxide was not added, the initial cell number of E. coli (3.2xl0⁴ cells/ml) was reduced to 1.5xl0⁴ cells/ml after 1 min, 1.2xl0⁴ cells/ml after 2 min, 1.8xl0³ cells/ml after 5 min, and 37 cells/ml after 15 min, showing bactericidal efficiency of 51 %, 62.3 %, 94.3 % and 99.8 %, respectively. When hydrogen peroxide was added at an amount of 20 mg/L, the initial cell number of E. coli (3.8xl0⁴ cells/ml) was reduced to l.lxlO⁴ cells/ml after 1 min, 7.0xl0³ cells/ml after 2 min, 1.8xl0³ cells/ml after 5 min, and 15 cells/ml after 15 min, inactivating 69.3 %, 81.8 %, 95.3 % and 99.9 % of E . coli cells, respectively. When hydrogen peroxide was added at an amount of 25 mg/L, the initial cell number of E . coli (3.1xl0⁴ cells/ml) was reduced to 2.2xl0⁴ cells/ml after 1 min, 1.5xl0³ cells/ml after 2 min, and 4 cells/ml after 15 min, showing bactericidal efficiency of 92.8 %, 95.2 % and 99.98 %, respectively. When hydrogen peroxide was added at an amount of 30 mg/L, the initial cell number of E . coli (3.5xl0⁴ cells/ml) was reduced to 1.9xl0⁴ cells/ml after 1 min, 82 cells/ml after 5 min, and 2 cells/ml after 15 min, showing bactericidal efficiency of 94.4 %, 99.7 % and 99.99 %, respectively. In addition, when hydrogen peroxide was added at an amount of 50 mg/L, the initial cell number of E. coli (3.4xl0⁴ cells/ml) was reduced to l.lxlO⁴ cells/ml after 1 min, 13 cells/ml after 5 min, and 1 cell/ml after 15 min, showing bactericidal efficiency of 96.6 %, 99.96 % and 99.99 %, respectively.

EXPERIMENTAL EXAMPLE 5> Assay for disinfection efficiency versus bacteria in water used in sprouting of beans

In this test, instead of the buffer solution containing E . coli , water used in sprouting of beans, which is more practical, was used, and bactericidal efficiency in water was evaluated. Air was injected into the photocatalytic reactor 10 at a rate of 30 L/min, and hydrogen peroxide was added at various amounts of 25, 50 and 75 mg/ml. Viability of bacteria in water was evaluated for 90 min. The results are given in Table 6, below, and FIGS. 12a and 12b.

[Table 6]

Water used in sprouting of beans for 4 hours contained bacteria of 4.0xl0⁴ cells/ml. As shown in Table 6 and FIGS. 12a and 12b, when hydrogen peroxide was not added, the initial number of viable bacteria were reduced to 6.8xl0³ cells/ml after 1 min and 123 cells/ml after 15 min, showing bactericidal efficiency of 82.9 % and 99.7 % , respectively. The number of viable bacteria was increased to 343 cells/ml after 30 min, but reduced again to 220 cells/ml after 90 min, showing bactericidal efficiency of 99.5 %. When hydrogen peroxide was added at an amount of 25 mg/L, bactericidal efficiency was found to be 82.6 %, 99.4 %, 99.5 % and 99.9 % after 1, 15, 30 and 90 min, respectively, where the bactericidal efficiency at the initial stages was slightly lower than that upon not adding hydrogen peroxide, but after 90 min, it was increased to a level higher than those upon not adding hydrogen peroxide. Also, no decrease in disinfection efficiency was observed during the disinfection process, although such a phenomenon was observed when not adding hydrogen peroxide.

When hydrogen peroxide was added at an amount of 50 mg/L, bactericidal efficiency was found to be 88.2 %, 99.8 % and 99.99 % after 1, 15 and 90 min, respectively. When being added at an amount of 75 mg/L, hydrogen peroxide showed bactericidal activity of 91.2 %, 99.8 % and 99.99 % after 1, 15 and 30 min, respectively, and complete inactivation versus bacteria was observed after 90 min.

It was found that disinfection efficiency versus all bacteria is slightly lower than that versus E . coli alone, and the total number of viable bacteria is slightly higher than that of viable E. coli alone. However, these results are due to the following reason. That is, there is a limit to the growth of E. coli in the buffer solution containing no nutrients, whereas continuous proliferation of bacteria was possible owing to existence in water of organic acids from beans. In addition, when hydrogen peroxide was added at an amount of over 50 mg/L, much higher disinfection efficiency versus bacteria was observed.

EXPERIMENTAL EXAMPLE 6> Assay for disinfection efficiency versus fungi in water used in sprouting of beans

Fungicidal activity against fungi in water used in sprouting of beans was evaluated using the same method as in Experimental Example 5. The results are given in Table 7, below, and FIGS. 13a and 13b.

[Table 7^

Water used in sprouting of beans for 4 hours contained fungi at l.OxlO⁴ cells/ml. As shown in Table 7 and FIG. 13, when hydrogen peroxide was not added, fungicidal efficiency was 69.3 %, 99.6 % and 99.7 %, after 1, 15 and 90 min, respectively. When added at an amount of 25 mg/L, hydrogen peroxide exhibited fungicidal efficiency of 71.1 %, 99.5 % and 99.88 % after 1, 15 and 90 min, respectively. When added at an amount of 50 mg/L, hydrogen peroxide displayed fungicidal efficiency of 90 % and 99.8 % after 1 and 10 min, respectively, and fungi in water were completely inactivated after 15 min. When hydrogen peroxide was added at an amount of 75 mg/L, fungicidal efficiency was found to be 93.7 % after 1 min, and no viable fungi were observed after 4 min.

<COMPARATIVE EXAMPLE 1> Assay for disinfection efficiency upon using Ti0₂-immobilized porous beads or glass beads

In this test, high porosity muscovite and glass beads were used as support materials for Ti0₂ immobilization. In order to compare disinfection efficiency of Ti0₂- im obilized porous beads, namely, Ti0₂-immobilized muscovite beads, with that of Ti0₂-coated glass beads, in which Ti0₂ served as a photocatalyst, after the photocatalytic reactor 10 was filled with the Ti0₂- im obilized porous beads or glass beads, disinfection versus E. coli was performed with addition of air at a rate of 30 L/min under exposure to UV radiation for 15 min, and viability of E. coli was evaluated. The results are given in Table 8, below, and FIGS. 5a and 5b. [Table 8;

As shown in Table 8 and FIGS. 5a and 5b, in which disinfection efficiency of Ti0₂-immobilized muscovite beads was compared to that of Ti0₂-coated glass beads, when using Ti0₂-coated glass beads, the initial cell number of E . coli (7.1xl0³ cells/ml) was reduced to 357 cells/ml after 1 min and 13 cells/ml after 15 min, showing bactericidal efficiency of 95 % and 99.8 %, respectively . Upon using Ti0₂-immobilized muscovite beads, the number of viable bacteria was reduced to 370 cells/ml from 7.2xl0³ cells/ml after 1 min and 1 cell/ml after 15 min, showing bactericidal efficiency of 95 % and 99.9 %, respectively, indicating that disinfection efficiency of Ti0₂-immobilized muscovite beads is slightly higher than that of Ti0₂-coated glass beads.

In addition, Ti0₂ on the glass beads was found to be gradually desorbed from the surface of glass beads by continuous water flow, thus requiring a new coating step of used glass beads with Ti0₂. In contrast, the Ti0₂ photocatalyst is more easily impregnated into the porous beads, such as muscovite powder having high porosity, than the glass beads. Moreover, desorption of Ti0₂ from the porous beads was not observed thanks to the treatment of the Ti0₂-immobilized porous beads at high temperature. <COMPARATIVE EXAMPLE 2> Assay for disinfection efficiency upon adding H₂0₂ with no exposure to UV radiation

In order to compare disinfection efficiencies upon addition of hydrogen peroxide (H₂0₂) with or without exposure to UV radiation, hydrogen peroxide was added to water at an amount of 25 mg/L, and viability of E . coli was evaluated. The results are given in FIG. 9.

As shown in FIG. 9, no viable E. coli was detected after 5 min when disinfection was performed with addition of H₂0₂ of 25 mg/L under simultaneous exposure to UV radiation. In contrast, when UV light was not supplied, bactericidal efficiency was found to be only 51 % even after 15 min. These results indicate that excellent disinfection efficiency is not achieved only by hydrogen peroxide, but by addition of hydrogen peroxide under simultaneous exposure to UV radiation.

INDUSTRIAL APPLICABILITY

As described hereinbefore, the apparatus for water disinfection according to the present invention makes it possible to shorten time required for water disinfection, and improve disinfection efficiency with addition of hydrogen peroxide and air. In addition, the apparatus may be prepared in a small size as desired, thus allowing its installation in a narrow place. Further, the apparatus is easily dismantled, thereby facilitating its cleaning. Therefore, the apparatus for water disinfection is very useful in inactivating or destroying microorganisms and organic contaminants in water.

Claims

What is claimed is:

1. An apparatus for water disinfection by a photocatalytic reaction, comprising: a first water reservoir storing water to be treated; a photocatalytic reactor containing photocatalyst- immobilized porous beads and equipped with a water inlet, a water outlet, UV lamps and an air inlet tube; a water supply pump connected between the first water reservoir and the photocatalytic reactor; an air pump introducing air into the photocatalytic reactor via the air inlet tube; and a second water reservoir storing disinfected water treated from the photocatalytic reactor.

2. The apparatus according to claim 1, wherein each of the water inlet and the water outlet is equipped with a filter to prevent the photocatalyst-immobilized porous beads from flowing out the photocatalytic reactor.

3. The apparatus according to claim 1, wherein the water inlet and the water outlet are installed at a lower part and an upper part of the photocatalytic reactor, respectively, and are located diagonally to each other to increase the residence time of water in the inside of the photocatalytic reactor.

4. The apparatus according to claim 1, wherein the photocatalyst is Ti0₂.

5. The apparatus according to claim 1, wherein the porous beads are made of muscovite and are 6-10 mm in diameter .

5 6. The apparatus according to claim 1, wherein the .

UV lamps are axially arranged in the photocatalytic reactor at regular intervals.

7. The apparatus according to claim 1, wherein the UV lamps are installed in a manner such that electrodes

.0 of the UV lamps are projected to the outside of the photocatalytic reactor.

8. The apparatus according to claim 1, wherein the air inlet tube is installed at a lower part of the photocatalytic reactor and is finely perforated at

.5 regular intervals.

9. The apparatus according to claim 1, wherein the photocatalytic reactor has a sealed structure by sandwiching an O-ring between an inner frame and an outer frame .

!0 10. The apparatus according to claim 1, wherein the photocatalytic reactor is provided with a sealed transparent acryl window to allow a user to observe the inside of the reactor with the naked eye.

11. The apparatus according to claim 1, wherein the photocatalytic reactor is designed such that two or more

5 reactors are arranged in parallel.

12. A method for water disinfection by photocatalytic reaction using the apparatus of claim 1, comprising the steps of adding hydrogen peroxide into contaminated water:

LO introducing contaminated water added with hydrogen peroxide into a photocatalytic reactor containing a photocatalyst-immobilized porous beads; injecting air into the photocatalytic reactor; and applying UV radiation into the photocatalytic ,5 reactor.

13. The method according to claim 12, wherein the hydrogen peroxide is added to water at an amount of 25-50 mg/L .