High-intensity Low-pressure Electrodeless Mercury-Argon Lamp for UV Disinfection of Wastewater

Recently, ultraviolet (UV) radiation has been applied as a disinfection method for wastewater treatment plants. A high-intensity and high-power UV light source is required in the field of bacilli sterilization. The electrodeless UV lamp turns on by delivering RF-frequency power from the power supply through the matching circuit into the induction coil around the lamp. The electric current passing through the induction coil generates a magnetic field. The gas sealed inside the lamp is excited and ionized by the magnetic field, and so discharge of the gas occurs and light is emitted. Thus the discharge produces UV emission. Using RF frequency power circuit technology and optimizing UV lamp shape, we have made it possible to input power of 500W to a 500mm-length luminescent lamp. Moreover, an over 30% rate of UV radiating efficiency has been obtained. It realizes a high-intensity and high-power ultraviolet light source. Optimization of water current path in the sewage effluent disinfection has realized a colon bacillus removal ratio of 99.9%.

High-intensity Low-pressure Electrodeless Mercury-Argon Lamp for UV

Disinfection of Wastewater

Shinji Kobayashi and Akinori Hatano

Toshiba Corporation,

Environmental System Engineering and Electromagnetic Application R & D Group

1,Toshiba-cyo, Fuchu-shi, Tokyo,183-8511, JAPAN(e-mail:Shinji.kobayashi@toshiba.co.jp)

ABSTRACT

Recently, ultraviolet (UV) radiation has been applied as a disinfection method for wastewater treatment plants

A high-intensity and high-power UV light source is required in the field of bacilli sterilization

The electrodeless UV lamp turns on by delivering RF-frequency power from the power supply through the matching circuit into the induction coil around the lamp The electric current passing through the induction coil generates a magnetic field The gas sealed inside the lamp is excited and ionized by the magnetic field, and so discharge of the gas occurs and light is emitted Thus the discharge produces UV emission

Using RF frequency power circuit technology and optimizing UV lamp shape, we have made it possible to input power of 500W to a 500mm-length luminescent lamp Moreover, an over 30% rate of UV radiating efficiency has been obtained It realizes a high-intensity and high-power ultraviolet light source Optimization of water current path in the sewage effluent disinfection has realized a colon bacillus removal ratio of 99.9%

KEYWORDS

Electrodeless UV lamp, Inductive coupled plasma, Low-pressure Mercury-Argon Lamp, RF frequency

INTRODUCTION

The mercury-argon lamp is usually applied in wastewater treatment plants as a UV light source The lifetime of a UV lamp is determined by two factors: the exhaustion of electrode and the reduction of UV transmittance of a quartz glass Use of electrodeless lamp eliminates the problem

of absolute life resulting from the electrode life

The wall temperature of the lamp increases due to the subsequent increase in the input power, thereby increasing the vapor pressure of mercury Due to the increase in the vapor pressure of mercury, the electron temperature decreases, leading to a decrease in the number of electrons that excite the mercury atom to the upper level of 254nm Then the excited mercury atoms lose emission energy due to collisions with low-energy electrons, which decreases the quantum efficiency of the absorption and re-radiation by a resonance line of mercury to less than 1.0 Finally, the UV emission

is saturated [1]

The lifetime of a UV lamp is determined by two factors: the exhaustion of electrode and the reduction of UV transmittance of a quartz glass Use of electrodeless lamp eliminates the problem

of absolute life resulting from the electrode life

In this paper, in order to obtain high-intensity UV emission, we present the results obtained by controlling the plasma condition of an electrodeless mercury-argon lamp with a RF (13.56MHz) power supply

Lighting principal of electrodeless UV lamp

Lighting principle of electrodeless UV lamp is shown in Fig 1 The electrodeless mercury-argon lamp used is 500mm in length and 86mm in diameter The electrodeless UV lamp turns on by delivering RF-frequency power from the power supply through the matching circuit into the three parallel induction coils around the lamp The electric current passing through the induction coils generates a magnetic field The gas sealed inside the lamp is excited and ionized by the magnetic field, and so discharge of the gas occurs and light is emitted Thus the discharge produces UV emission

Fig.1 Lighting Principle of Electrodeless UV lamp Fig 2 Photograph: Case of three parallel induction coils

The photographs of the discharge are shown in Fig.2 To keep uniform discharge, using RF frequency power circuit technology and optimizing UV lamp shape So, we have made it possible to input power of 500W to a 500mm-length luminescent lamp

Features and specifications of UV Disinfection system

The lamp module consists of three lamps, lamp flame, matching box, and automatic cleaning mechanism for protective quartz sleeves (See fig 3) Structurally, each current path is composed of

UV lamps, a submerged weir and an overflow weir to pass the sewage around the lamps uniformly Therefore, the disinfection performance is unaffected even when a flow rate changes

In particular, the problem of charge in disinfection performance due to divagation at a small flow

rate does not arise Distributed installation of the sewage effluent disaffection system for wastewater treatment is shown in Fig 4

The disinfection system is a standard product incorporating a set of electrodeless UV lamps

By combining and installing several UV modules, it is possible to implement a system suitable for any size sewage treatment plant without increasing a water head loss in a water channel

The rated water amount of treatment of a lamp module is 60 (m3/hour) Under the conditions of

UV transmittance of effluent of 70%, SS of 15mg/l or less and a colon bacillus removal ratio of 99.9% are achieved

Fig 3 Lamp Module

(a) Distributed installation (b) A photograph of the installation

Fig 4 The Sewage Effluent Disinfection System

Experimental Setup

The experimental setup is shown in Fig 5 A pump circulates the water and the water temperature is controlled by a heat exchanger UV intensity is measured by the illuminometer (UV-MO2:ORC Corp.) Distance between a lamp and the illuminometer is 2.2 (m)

Fig 5 Experimental setup

Results and Discussions

(1) Water temperature dependence of Electrodeless UV Lamp

The temperature of the coldest spot of the lamp determines the Hg vapor pressure One can therefore use that temperature as a parameter representing Hg vapor density in the discharge

The dependence of the 254nm emission on the water temperature is shown in Fig 6 Over 30% rate of UV radiating efficiency has been obtained Optimizing UV lamp shape, we have made it possible to input power of 500W Controlling the temperature of the coldest spot of the lamp realizes efficient UV emission at the water temperature from 15℃ to 35℃

Fig 6 The dependence of the 254nm emission n the water temperature

(2) Spectrum of Electrodeless UV Lamp

The main UV emission of the mercury lamp comes from the resonance lines of 184.95nm (1P1-1S0) and 253.65nm (3P1-1S0) These lines are referred to as being 185 and 254nm emissions, respectively, in the following text Energy level diagram for mercury is shown in Fig.7

Spectrum of the UV lamps depends on Water temperature is shown in Fig 8 It increases the water temperature, and the vapor pressure increases Due to the increase in the vapor pressure of mercury, the electron temperature decreases, leading to a decrease in the number of electrons that excite the mercury atom to the upper level of 254nm

So, it increases the vapor pressure of the mercury over the optimal value, and the emission efficiency decreases The heat loss estimated from the lamp temperature was about 50% of the input power The other reason is that the input power, except for heat loss, is converts to the power of the other emission lines such as185nm, 546nm etc

Fig.7 Energy level diagram for mercury (All wavelengths given in nanometers.)

Fig 8 Dependence of UV lamp spectrums on water temperature (3) Estimate of the lifetime of electrodeless UV lamp

The lifetime of a UV lamp is determined by the two factors: the exhaustion of electrode and the

reduction of UV transmittance of a quartz glass The absolute life is time arising from electrode wear, i.e., and the time until one of lamps is exhausted

Use of electrodeless lamp eliminates the problem of absolute life resulting from the electrode life Also, whereas in conventional UV lamps, evaporation of electrodes reduces transmittance of UV

in the quartz glass tube, the electrodeless UV lamp is not subject to that drawback

Total lighting time is defined as the time until a lamp cannot be turned on or until luminous flux drops to 70% of initial flux (See fig 9) Consequently, lamp life of 50,000 hours was expected

Fig 9 Secular Deterioration Curve of Electrodeless UV lamps

Conclusions

We developed low-pressure electrodeless UV lamps for sewage effluent disinfection systems The key points regarding the development are as follows:

(1)Use of electrodeless lamp eliminates the problem of absolute life resulting from the electrode life

(2)Use of RF frequency power circuit technology has made it possible to input power of 500W (3)Optimization of UV lamp shape with the coldest spot of the lamp has realized efficient UV emission at the water temperature from 15℃ to 35℃

(4)Our sewage effluent disinfection system realizes a colon bacillus removal ratio of 99.9% This performance is advantageous for the system applied in sewage treatment works

References

1) J.H.Waymouth: H Electric Discharge Lamps ( M.I.T Press, Cambridge, 1971)