Part 4 - UltraViolet (UV) Method of Water Disinfection

To remove or nullify the harmful bacteria in water, two types of techniques can be applied.  First, is chemical, wherein a chemical such as Chlorine is added to the water, and the chemical reacts with the bacteria and neutralises its potent.  The second technique is physical, such as boiling, filtration or Ultraviolet disinfection.

Historically, Chlorine was the disinfectant of choice to combat water borne diseases such as Typhoid or Dysentery.  In 1887, Downes and  Blunt discovered the germicidal properties of sunlight.  The development of mercury lamps as artificial UV source, and the use of Quartz as a transmitting medium accelerated the develoment of UV based disinfection techniques.   Although European countries began using UV based disinfection by the 1960s, by the late 1980s the US EPA began considering UV disinfection as a primary disinfectant for municipal drinking water.

The layman is confused with disinfection, sanitization and sterlization  -

Sanitization : 2 log reduction : 50 - 99% reduction
Disinfection: 4 log reduction : 99.99% reduction
Sterilization: 6 log reduction : 99.9999% reduction

Standards: In the US, NSF International, a non-government non-profit company has set benchmark standards for water purification. For ultraviolet technique of disinfection, NSF Standard 55 discusses the standards required for ultraviolet disinfection.  Class A systems (40,000 uwsec/cm2) are designed to disinfect and/or remove microorganisms from contaminated water, including bacteria and viruses, to a safe level. Class B systems (16,000 uw-sec/cm2) are designed for supplemental bactericidal treatment of public drinking water or other drinking water, which has been deemed acceptable by a local health agency.

For most of us in Urban India, assuming that the water is already treated prior to supply to the home, the minimum requirement would be Class B, 16,000 uJ/cm2 or 16 mJ/cm2.  So what does this number mean?

In Ultraviolet technique of disinfection, central to the technique is a lamp that emits Ultra violet light, or light with wavelength less than 400nm, that is not visible to the human eye.  The following precautions should be taken -

• If the UV lamp is glowing, it does not mean it is emitting UV light.  UV light is invisible. What you see is not UV light, but some indicative lamp. You got to ensure it is emitting UV light at the stated power.
• UV light, once emitted, passes through the surroundings and water to reach the target microorganisms. 
• UV dosage is the intensity of the UV lamp multiplied by the residence time in seconds.
• Once the microorganisms are exposed to UV light for the critical time, they are rendered inactive, and cannot multiply anymore.
• UV light can damage the skin and the eyes, so keep away from the UV lamp.
• Regularly service the UV lamp and check for emittance.  UV lamp has a certain life, and although the light glows, it may not emit uV anymore.
• Dosage = UV Intensity at target x dose time.  The key is UV intensity not emitted, but what reaches or hits the target micro-organisms.  UV has to transmit through the Quartz sleeve surrounding the UV lamp, and through the turbid water before reaching its target. Also, as most home based UV Filter systems are not batch UV reactors, but continuous, not the same intensity hits all the micro-organisms. 

 
In industrial water treatment systems, a batch reactor is used with a fixed residence time so UV rays are impinged upon the micro-organisms for a certain duration of time until they are rendered ineffective. There are sensors in place to measure the real time intensity.  Daily and periodic tests are conducted to measure the effectiveness of the treatment.

In comparison, home based systems are a lot smaller in scope.  Most are based on continuous flow of water.  Depending on the flow rate of water, the dosage intensity varies.  There are no online sensors that display the intensity of the UV.  It is difficult to actually test the effectiveness of the system. 

Manufacturers hide much of the factual data when they sell such systems.  Users think that from the moment they start using a UV based filter, they have access to pure, disease free water, which may not be true. 

To be fool proof, we continue to advise people to use a fool proof method such as boiling the water to kill all harmful germs and micro-organsisms.