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Ultraviolet Disinfection
What is disinfection?
Human exposure to wastewater discharged into the environment has
increased in the last 20 years with the rise in population and the
greater demand for water resources for recreation and other purposes.
Wastewater is disinfected to prevent the transmission of infectious
diseases and to ensure that water is safe for human contact and
the environment. There is not a perfect disinfectant, However, there
are certain characteristics to look for when choosing the most suitable
disinfectant:
- ability to penetrate and destroy infectious agents under normal operating conditions
- lack of characteristics that could be harmful to people and
the environment
- Safe and easy handling, shipping and storage
- absence of toxic residuals, such as cancer causing compounds,
after disinfection
- affordable operation and maintenance costs.
What is UV disinfection?
One way to disinfect wastewater is through ultraviolet (UV) radiation,
which inactivates diseases causing bacteria by electromagnetic radiation.
Wastewater flows through or around a tube with UV light penetrating
it from all directions. Radiation is transferred to the cell walls
of the bacteria rendering the organisms sterile.
A UV disinfection system consists of mercury arc lamps, a reactor,
and ballasts. The lamps are the source of UV radiation, the ballasts
provide power to the system, and wastewater flows through tubes
in the reactor. Two types of mercury lamps can be used: Low pressure
and medium pressure.
There are two types of UV disinfection reactor configurations;
contact and noncontact. In both of these types, wastewater can flow
either perpendicular or parallel to the lamps. The graphic shown
shows two UV contact reactors with submerged lamps placed parallel
and perpendicular to the direction of the wastewater flow. In the
noncontact reactor, the UV lamps are suspended outside of a clear
conduit. Flap gates or weirs are used to control the level of the
wastewater.
What determines the performance of UV
disinfection systems?
A UV disinfection system must be designed to reach the most bacteria
with the strongest UV dose for the longest time possible. The success
of UV disinfection depends on the amount of time the wastewater
is exposed to UV radiation, the intensity of UV radiation, and the
characteristics of the particular wastewater at the time of disinfection. The amount and type of microorganisms vary with different wastewater. The concentration of total suspended solids and of particle associated microorganisms determines how much UV radiation ultimately reaches the target organisms. The higher these concentrations are, the lower the UV radiation absorbed by the organisms, and thus, the less effective disinfection can be.
Are UV disinfection systems easy to operate
and maintain?
Proper operation and maintenance is needed to keep a UV system functioning
at maximum performance. This requires that all surfaces between
the UV radiation and the target organisms be kept clean - mainly
the tubes lamps, and reactor Inadequate cleaning is one of the most
common causes for a UV system's failure to perform.
Operation and maintenance also involves replacing the tubes, lamps,
or quartz sleeves regularly, according to manufacturer's instructions
lamps are generally replaced after 12,000 hours of use, quartz sleeves
after 5-8 years, and ballasts every 10 -15 years.
What is the cost of UV disinfection?
The cost of UV disinfection systems depends on the manufacturer,
the site, the capacity of the plant, and the characteristics of
the wastewater to be disinfected. The main operating costs are power
consumption, equipment replacement and repairs and cleaning supplies.
What are the advantages and disadvantages
of using UV disinfection?
Advantages:
- is effective at inactivating most viruses, spores and cysts.
- is a physical process rather than a chemical disinfectant; thus eliminating the need to generate, handle, transport, or store toxic/hazardous
or corrosive chemicals.
- There are no toxic residuals that could be harmful to humans or aquatic life.
- user-friendly for trained operators
- The wastewater needs to be in contact with UV light for only a short time to be adequately disinfected ( approximately 20 - 30 seconds with low pressure lamps.)
- equipment requires less space than other methods
Disadvantages:
- Low dosages may not effectively inactivate some viruses, spores,
and cysts
- Organisms can sometimes repair themselves and "undo"
the effects of UV disinfection.
This phenomenon is known as photoreactivation.
- The tubes used to carry the wastewater can develop a buildup
of slime, or fouling, which may require regular cleaning for preventive
maintenance.
- It is more difficult to penetrate microorganisms in wastewater
that is not clear (containing high amount of solids in suspension).
- In some cases, UV can be more expensive than other disinfection
methods.
- There is no measurable residual to indicate the effectiveness
of UV disinfection.
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