The power of sunlight to destroy microbial life has long been known and appreciated. Effective disinfection in air, on surfaces and in water has been accomplished by exposure to the direct rays of the sun. Sunlight is an important factor in the self-purification of water in streams, lakes and reservoirs. The effect of sunlight in destroying bacteria, particularly intestinal bacteria, has been documented many times. The visible rays of sunlight play little part in this bactericidal action. The results are caused by the ultraviolet rays. Man-made sources of high intensity ultraviolet light have been developed which can be used to disinfect water, wastewater, air, etc.

The term "ultraviolet light" or simply "ultraviolet (UV)" is applied to electromagnetic radiation emitted from the region of the spectrum lying beyond the visible light and before x-rays (Figure 1). The upper wavelength limit is 400 nanometers (1 nm =10-g meter) and the lower wavelength limit is 100 nm, below which radiation ionizes virtually all molecules. The region between 400 and 190 nm has been divided into three regions: NEAR-ultraviolet radiation or UV-A can be considered to lie in the wavelength range 320-400 nm. The long wavelength limit represents the beginning of the visible spectrum, while the short wavelength limit corresponds roughly to the point below which proteins and genetic material begin to absorb significantly. Below this region is the MID-UV region or UV-B (290320 nm), where proteins and genetic material begin to absorb and where sunburn and skin cancer are most effectively produced. (UV radiation present in sunlight at the surface of the earth at noon in clear weather includes both the NEAR-UV and the MID-UV regions.}

FAR-UV (UV-C) wavelengths range from 200-290 nm and because of their strong absorption by genetic material are highly destructive to biological matter. These wavelengths are almost all absorbed by the ozone in the stratosphere. The wavelength of ultraviolet light produced by the UV lamps which are used for the disinfection of water is 254 nm, which is in the FAR-UV or UV-C range.


The narrow band of UV light lying between the wavelengths of 200 and 300 nm has often been called the germicidal region because UV light in this region is lethal to microorganisms including: bacteria, protozoa, viruses, molds, yeasts, fungi, nematode eggs and algae. The most destructive wavelength is 260 nm which is very close to the wavelength of 254 nm produced by germicidal lamps. UV light's ability to kill the fecal coliform bacteria, Escherichia coil , is directly related to the ability of its genetic material (i.e. nucleic acid) to absorb UV light. UV light causes molecular rearrangements in the genetic material of microorganisms and this prevents them from reproducing. Most microorganisms have relatively short life cycles and therefore depend on rapid reproduction to sustain and grow their population. Therefore, if a microorganism cannot reproduce then it is considered to be dead. Normally when DNA replicates , the Thymine (T) must join the Adenine (A) , and the Cytosine (C) must join with Guanine (G). When DNA is exposed to Ultraviolet Light at a wavelength of 254 nm, an error occurs in the replication process. The Thymine forms a dimer, that is, a double bond between the Thymine molecules. This error prevents the pathogen from reproducing properly and so eventually it dies off.

Fluorescent Lamp and Germicidal Lamps

The sun is one important source of UV light, but much of its transmitted energy does not extend below the wavelength of 296 nm. For practical applications, UV energy must be generated electrically with special germicidal lamps. The low pressure UV radiating mercury lamp consists of a transparent tube with an electrode at each end. The tube may be of pure transparent vitreous silica for maximum transmission of UV radiation, or may be of a UV-transmitting glass. The filling is not mercury alone, but a mixture of mercury and an inert gas, usually argon, at a pressure of a few torr. The mercury is introduced as a single drop and almost all the mercury remains in liquid form during the operation of the lamp. The lamp pressure is just the vapour pressure of mercury at the operating temperature, which is in the region of 40°C.

The vapour pressure of mercury at this temperature is 7 x 10-3 torr, which is very conveniently the optimum for the most efficient production of resonance radiation. The inert gas is therefore at a much higher pressure than the mercury, but contributes almost nothing to the spectral output. Germicidal lamps operate electrically on the same principle as fluorescent lamps. UV light is emitted as a result of an electron flow through the ionized vapour between the electrodes of the lamps. The glass of the germicidal lamp is made of quartz which transmits UV light and the glass of a fluorescent lamp is made of soft glass which absorbs all of the UV light at a wavelength of 254 nm. The bulb of the fluorescent lamp is coated with a phosphor compound which converts UV to visible light. A germicidal lamp produces about 86% of its total radiant intensity at a wavelength of 254 nm and about 1% at other germicidal wavelengths.

Germicidal lamps with high quality quartz also produce UV light at a wavelength of 185 nm. This wavelength produces ozone which is corrosive to the UV equipment and the ends of the lamps. The UV lamps in Ironbrook Partners' equipment are carefully manufactured so that they do not produce ozone. Slimline Instant Start Lamp This is the lamp of choice for 100 percent of the large UV wastewater systems. It is an instant start lamp that produces 26.7 watts of UV-C from 75 watts of power at 0.18 watts of UV-C per centimeter of arc length. Its optimum operating surface temperature is 40°C. It is called instant start because a high voltage is applied to the cathodes to instantly strike an arc. According to the lamp manufacturers, these lamps have a rated average useful life of 13,000 hours at which time the UV-C output has dropped by 40 percent. Experience with full scale UV systems at wastewater treatment plants has confirmed this and shown that the lamps can be reliable for as long as 40 months on some application.


The major parameters which must be taken into consideration when a UV disinfection system is being designed for wastewater are as follows: (The customer or the consultant must provide this information to the UV system manufacturer since each UV system is designed on an individual basis. Ironbrook Partners uses this information for the proper sizing and design of UV Disinfection Systems.)

  • UV Transmission or Absorbance of the Wastewater
  • Suspended Solids in the Wastewater
  • Flow Rate and Hydraulics of the UV System
  • Level Control for the UV System
  • Iron Content of the Wastewater
  • Hardness of the Wastewater
  • Sources of the Wastewater
  • Required Disinfection Performance
  • UV Lamp Life
  • UV System Configuration and Redundancy