Since twenty century, ultraviolet disinfection technology has been widely applied into water treatment industry.

Mechanism of UV disinfection:

Ultraviolet disinfection system can sterilize water in an effective and efficient way. Heating and addition of harmful chemicals do not needed. There is no abnormal flavor in the water after treatment.

RNA and DNA in the microorganisms can absorb ultraviolet energy. Ultraviolet energy ( 254nm wavelength) can polymerize the base pair of DNA, disable reproduction by preventing the biosynthesis of protein, and finally destroy the microorganisms. Different from Chlorine, Chlorine Dioxide, Ozone, and other chemical disinfection, ultraviolet disinfection is a physical process. During the disinfection process, ultraviolet disinfection system does not produce any byproduct.

The advantages of the ultraviolet:

1.Ultraviolet disinfection technology is very effective in destroying bacteria

Ultraviolet energy can destroy bacteria and virus in about 1-5 seconds. In order to reach the same disinfection effect, it costs other chemical methods (Chlorine, Chlorine Dioxide, Ozone) about 30 minutes or more contact time.

2.Efficient disinfection

Ultraviolet disinfection technology has been wildely applied for efficiently destroying pathogenic microorganisms, bacteria, and virus.

3.No secondary pollution

Since no chemical is added, ultraviolet disinfection system will not cause secondary pollution for water and environment.

4.Low operational costs, simple operation and maintenance, and low costs.

5.Occupied small space

Occupied small space and easy assembly enable ultraviolet equipment to be assembled in the place where is close to water and power source. The ultraviolet equipment starts working after connecting the inlet and outlet of the ultraviolet equipment and powering up. For equipment maintenance, quartz sleeves should be cleaned every three months, and ultraviolet lamps should be changed every year. ( The suggested replacement period of ultraviolet lamp should be 8000 hours of continuous service, about 12 months.) Ballasts should be replaced every three years.

The disadvantages of using Ultraviolet equipment:

1.No continuous disinfection capacity.

For chemical disinfection (Chlorine, Chlorine Dioxide, Ozone), the continuous disinfection can be maintained because there are some residual disinfectant in the water. However, ultraviolet disinfection is a physical process and can not provide continuous disinfection, so using ultraviolet equipment alone for water treatment can not solve the problems (microorganisms secondary pollution and reproduction) during long distance transportation.

2.Incoming water quality restriction (water source quality)

Water quality ( light transmittance, turbidity, and suspended substance in water) has some effect on ultraviolet disinfection. To use ultraviolet disinfection, there are strict requirements for incoming water

quality:

① Bacteria index: The quantity of Escherichia coli in the water should be less than 1000/L and the total quantity of bacteria should be less than 2000/mL

② The chroma should be less than 15 degree.

③ The total iron content should be less than 0.3 mg/L

④  Turbidity should be less than 5 degree.

3.Microorganisms photoreactivation

With enough ultraviolet dosage, the dead virus and bacteria can not photoreactivate. But, when ultraviolet dosage is not enough, the dead virus and bacteria that were killed by ultraviolet energy can reactivate with the help of light to repair their destroyed tissue (light repair), or even by self repair( dark repair).

4.Working condition: water temperature affects the ultraviolet transmittance

The relationship between the surface temperature of UV lamp and ultraviolet transmittance is as follows:

The environment temperature will affect the radiation intensity of ultraviolet lamp and disinfection effect. Output of ultraviolet lamp will decrease as the temperature decrease between 50C to 400C. The ultraviolet lamp may have difficulty in starting up when the temperature is below 50C.

Ultraviolet Energy

Light is electromagnetic radiation, or radiant energy traveling in the form of waves. UV energy is found in the electromagnetic spectrum between visible light and x-rays and can best be described as invisible radiation. The energy employed for UV water treatment is further categorized into two primary levels measured as wavelengths- 254nm and 185nm. (1nm=0.01 miron)

The regular wavelength in the photochemistry is 100-1000nm (100,000-10,000-1). The photon energy whose wavelength is larger than 1000nm is very low, so it does not cause chemical change when it is absorbed. However, The photon energy whose wavelength is less than 100nm is very strong and it causes ionization and molecular rupture, so it is mainly used in radiation chemistry. The wavelength spectrum of radiation chemistry can be separated into several wave ranges with different names.

In near-infrared range, seldom light reaction happens, but photosynthetic bacteria can store solar energy beyond 980nm wavelength. In visible light range, it is an active area for green plants and algae's photosynthesis and many dyestuffs have photochemical transformation and photoresponse. Photochemical research and application focus on ultraviolet range, and the ultraviolet spectrum can be separated into three ranges according to people's sensitivity to UV: UVA range makes skin black; UVB range hurt skin and may cause cutaneum carcinoma; UVC range is very dangerous because it is absorbed by protein, DNA, RNA, and causes cells change, cancer, or cells death. UVC range is also called sterilization range because it destroys bacteria and virus. VUV can be absorbed by every materials ( including air and water), so it just transmits in vacuum. The VUV absorption causes bonds to be broken.

UV dosage

The specific amount of energy required for effective microbial, TOC or ozone destruction is referred to as UV dosage and is expressed in μW-Sec/cm2 or mW-Sec/cm2 (1μW-Sec/cm2 = 0.001 mW-Sec/cm2 =0.000001W-Sec/cm2)

UV Dosage = UV Intensity * Retention time

Where: UV intensity is the measurement of UV light energy radiated per unit area falling on a given surface.

Retention time is the length of time the treated fluid is exposed to specific UV energy.The scientific community has documented the minimum UV dosage requirements(D10) for destruction of different microorganisms. The dosage of UV radiation required to achieve 90% kill of specific microorganism is called D10- the dosage that which only 10% of the microorganisms survive. Doubling the dosage achieves an overall 99% reduction. Taking E. Coli (D10= 6600μW-Sec/cm2) as an example, the dosage needed to achieve a 99.999% kill can be calculated as shown as follows:

UV Life

Generally, UV lamps do not burn out as normal fluorescent lamps do. Instead, the special quartz envelop material undergoes a photochemical ageing process know as solarization. This chance reduces the amount of UV energy available to the water stream. A typical UV intensity decreasing vs burning hours is shown as bellows:

UV Transmission:

UV energy will be absorbed when transmit through media. Good quartz sleeves transmit over 95% of UV energy. UV transmission in water varies depending on the type and amount of dissolved and suspended solids in the water. Through sampling and testing, the measurement of a liquid's ability to absorb UV energy of 254 nm as know as Coefficient of Absorption ( C of A) can be determined.

Other factors:

Flow pattern: To assure maximum UV performance, UV systems should be designed without "dead" corners and to obtain maximum retention time. Turbulent flow design is much preferable than laminar flow design.

UV chamber: UV chamber material should be stain and UV resistant. At specific ultra-pure water systems applications, the internal wet metal of UV Chamber will be in 316L stainless steel with electropolished.

Temperature: Lamp working temperature will affect the lamp efficiency. The water temperature will affect the UV transmission.