Ultraviolet Water Treatment – Disinfection, TOC Reduction, Ozone Destruction and Dechlorination
While many of us are familiar with the use of ultraviolet light for disinfection of
high purity water streams, this technology has become increasingly popular for
Using sizing criteria such as flow rate, UV wavelength, water
quality, UV dosage requirements and transmission quality, ultraviolet technology
is also used for total organic carbon (TOC) reduction, ozone destruction, and
more recently has been applied for dechlorination.
In simple terms, ultraviolet light is an energy source which has the ability to
penetrate the cell wall of an organism and alter its DNA to prevent it from
reproducing. This energy source at 254 nm wavelength is referred to as
germicidal wavelength light. UV light applied at this wavelength and appropriate
flow rates will provide 99.9% or better destruction of microorganisms. The typical
industry standard UV dosage for disinfection is 30,000 µW sec/cm2 at the end of
the lamp life.
For ozone destruction, the UV light catalyzes the decomposition of ozone to
oxygen. Ozone is rapidly and completely destroyed at dosage levels as low as
90,000 µW sec/cm2. The dosage for complete destruction is dependent on the
amount of ozone in the feedwater and the treated water requirements of the
Total Organic Carbon Reduction
For TOC reduction, the UV energy generates hydroxyl radicals within the
water by photo-oxidation; these hydroxyl radicals break the TOC bonds to form
simpler ionic constituents. An ultraviolet system designed for TOC reduction
uses 185 nm wavelength lamps; and the dosage requirement can vary
depending on the organics in the stream and their concentration(s). As a rule of
thumb, the system will be sized for a minimum of five (5) times the disinfection
The UV oxidation of chlorine within water treatment processes is governed by
many of the same parameters which influence TOC reduction. The UV dosage
required for dechlorination depends on adsorption of the UV energy in the water,
the total amount of chlorine present, background level of organics, ratio of free
chlorine vs. combined (chloramines) and the targeted effluent chlorine level. The
typical UV dosage required for removal of free chlorine can be as much as 30-50
times the dosage required for disinfection; therefore, the systems can become
quite large. Still, this technology is gaining wider acceptance for dechlorination,
as it offers the following advantages over conventional methods:
- Instantaneous process which does not require mixing, contact tanks,
or chemical addition.
- Adds nothing to the water and does not have a residual.
- Does not change water chemistry, pH or TDS.
- Relatively inexpensive from both a capital and operating cost
- Eliminates risk of introducing microorganisms or biological
- Unlike chemical injection, it cannot be overdosed.
- Requires minimal operator attention or maintenance.
For further information or an evaluation as to whether UV technology has an
application for your process, consult your Process Solutions, Inc. sales