Transcript Ultraviolet Disinfection Technology

Ultraviolet Disinfection: A Study of Various Technologies
Overview
 Studies have shown that ultraviolet light can penetrate cells and debilitate
DNA, thereby destroying an organism’s ability to reproduce
 Such a discovery has paved the way for the development of ultraviolet
disinfection technology
 Contaminants removed with use of UV-system: microbial; nevertheless, UV
systems may be coupled with a pre-filter to extricate larger organisms and
particulates with attached bacteria
 UV units are used as individual treatment systems or as parts of other
purification processes
Ultraviolet Light: Background
 Ultraviolet light is characterized by a wavelength shorter than 400
nanometers
 It emanates from the sun and is almost wholly filtered out through the ozone
layer upon entering the earth’s atmosphere
 Nevertheless, some UVA and the UVB (280-400nm) does enter the
atmosphere causing damage to the DNA of the cells it penetrates
 Ultraviolet light engenders photoproducts which alter the DNA structure of
harmful microorganisms, thereby inhibiting their replication
UV Disinfection Systems: Orthodox Design
 An orthodox package constitutes mercury arc lamps (low or high intensity), a
reactor, and ballasts
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Contact types
Non-contact types
a series of mercury
UV lamps suspended outside a
lamps, enclosed in quart
transparent conduit, which carries
sleeves to decrease cooling
wastewater to be purified (not as
effects of wastewater, submerged common as contact reactor)
in water
A ballast, or control box, engenders starting voltage for the lamps and maintains
continuous current
Flap gates, or weirs, control level of water being treated
Water runs perpendicular or parallel to lamps
Note: spare parts- operators in remote locations should maintain and house a set of
spare parts on site
More advanced systems: mechanical cleansers, ultrasonic cleaners, other selfcleaners, and alarm systems that indicate minor and major failures
General UV Disinfection Systems: Diagram
Two UV reactors
with submerged
lamps placed
parallel and
perpendicular to
the direction of
water flow
(contact types)
General Operation and Maintenance
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Ideal operating temperature: 40 degrees Celsius
Because UV treatment does not leave any residual in water, suggested installment at a location
proximate to final distribution system and use as a final step of purification
UV units should be implemented on cold water line before any branch lines
Lamp changes: once every year or if light transmission efficiency has decreased to 70 percent
Filter changes: times vary according to water quality; ordinarily every 3-6 months
All surfaces between UV radiation and water should be clean; chemical treatment: use of citric
acid, mild vinegar solutions, or sodium hydrosulfite (ideal for noncontact reactor systems)
Quart sleeves should be wiped down every 6 months with soapy solution; if residue remains after
cleaning, necessary non-abrasive cleaner (does not scratch surface and created to remove iron
and scale buildup); replaced every 5 to 8 years
There should be no fingerprints left on glass
Note: use of UV system and a pump on same electrical line can mar the life of UV lamp and
ballast
Ballast must be compatible with lamps and should be ventilated to protect it from excessive
heating; usually replaced every 10 years
All UV units must be pilot tested upon implementation to ensure applicability
At a minimum, drinking water systems should implement two UV units (design ensures continuous
disinfection when one unit is being cleaned and operation during low-flow demand periods)
General Operation and Maintenance: Personnel and Technology
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Operators:
Operators must ensure continuous dose measurement (example: accurate intensity
and flow-rate measurement) and proper maintenance (cleaning as well as lamp and
sleeve replacement regimes)
Operators should follow a maintenance schedule which envelops inspecting site
periodically, changing lamps, inspecting and cleaning surfaces and UV chamber
interior (once every six months), and inspecting and replacing ballasts, O-rings,
valves, and switches
Operators should monitor water turbidity (dissolved minerals such as calcium
especially harmful) and color as they constitute natural barriers to UV light
transmission
Operators should make it a point to reduce on/off cycles of lamps, since lamps lose
efficacy as a result of repeated cycles
Technology:
Many advanced systems include mechanical cleaners, ultrasonic cleaners, other
types of self cleaners, and alarm systems that assert a minor or major problem
Gravity systems: should be designed to automatically stop water flow or provide
alternative means of disinfection during power outages
A Specific Technology: UV-Tube
 All UV-tube designs entail a germicidal bulb suspended over water in a
horizontal tube or covered trough
 The water passes through one end by means of an inlet in the top of the
tube, courses along the bottom (underneath germicidal bulb), and exits
through an outlet
 The height of the outlet determines the depth of the water in the tube and
governs the hydraulic detention time
 There are three UV-tube designs: materials- steel lined PVC, concrete, or
pottery
bulb sizes- 36”, 18”, or 15”
UV Tube Design: Diagram
Topics To Consider: Theory of UV Technology
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Fluence (Dose)
Intensity
Absorption Coefficient
Flow Rate
Turbidity
Water Depth (Weir Height)
Bulb Types
Re-activation of bacteria
Materials inherent to wastewater/how affect disinfection
Fluence
 Fluence (dose) is the amount of UV light exuded from the germicidal bulb
 Fluence = product of light intensity and exposure time
depends upon: bulb strength &
geometry of
reactor
geometry & hydrodynamics
of reactor
 Standard doses used by a UV systems: between 15 and 50 mW-sec/cm2
 Note: the EPA has not established an official rule concerning dose
requirement
Intensity (further information)
 Decreases because of attenuation and dissipation
 In other words
distance from the source =
strength of light (intensity)
because the light is spread across a larger area (as a result of dissipation)
and because the light interacts with molecules inherent to the water
(attenuation)
Absorption Coefficient
 The absorption coefficient delineates how much light is lost as it passes
through a medium (measured in inverse centimeters)
 Absorption coefficients are calculated experimentally
 While the absorption coefficient of pure distilled water is near zero; natural
organic matter, iron, nitrate, and manganese imbibe UVC light and
consequently increase the absorption coefficient of a water sample
 Absorption coefficients in drinking water should be approximately 0.01 to
0.2
 Another type of absorption coefficient is the naperian value (using base e)
 Note: The EPA has not finalized a standard for levels of UV absorbing
compounds in drinking water
Flow Rate
 Flow rate =
detention time =
dose acquired by water
 These variances must be taken into consideration when determining an
appropriate flow rate
Turbidity
 Turbidity is the calculation of the amount of particulates in a solution
(measured in NTU)
 Research has shown that turbidity inhibits ultraviolet disinfection only when
organisms are lodged within the particles or when the particles themselves
are UV-absorbers. Otherwise turbidity is not a hindrance to disinfection.
 These lodged organisms can be extricated from water supply through the
use of a pre-filter
Water Depth (Weir Height)
 Water depth determines both the residence time and water thickness of a
water sample in question. Consequently, calculating an appropriate water
depth encompasses a trade-off between the two entities:
 water height = volume of water = average residence time (positive
quality)
Nevertheless,
water height = water thickness = attenuation of light = dose
reaching water at bottom tube (negative quality)
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Because attenuation is proportional to the absorption coefficient, the ideal
water height will rely upon the absorption coefficient
Bulb Types: Comparison of Two Models
 There are two main models
considered in regards to use in a
UV-tube: the low pressure bulb
and the medium pressure bulb
 Small-scale ventures make use of
low pressure mercury vapor arc
lamps (opportune lamp wall
temperature: 95-122 degrees F)
 the peak output of low pressure
lamps (253.7 nm) is adequate to
address the maximum UV
absorbance of DNA (260-265 nm)
 Medium pressure lamps are used
for large facilities; 15-20 times
germicidal intensity of low
pressure lamps
Characteristic:
Low Pressure/
Low Intensity
Medium
Pressure/
Medium Intensity
Typical
Energy Use
60 W
5,000 W
Percentage
Output at 253.7
nm
88%
44%
Ozone
Production
None
Possibly
Susceptibility
to Cooling
Yes
No
Susceptibility
to Cooling
Good
Poor
Benefits
Efficiency (lower
energy
requirements)
Smaller, less
maintenance,
use with poor
quality water
Reactivation of Bacteria
 Because the UV-tube does not offer residual disinfection, some bacteria can
repair its DNA and re-activate after a few days of exposure to visible light
 Reactivation, when occurs, on the order of a 1-log increase in posttreatment concentration
 Related to UV dosage; one study asserted that water dosed with 130,000
uW-s/cm2 showed no reactivation after 24 hours
Materials Within Wastewater
Cost: A Comparison of Various Disinfection Techniques
 In relation to the other kinds of
water disinfection systems
outlined , the UV-tube (as a form
of UV disinfection technology) is
an affordable option
 Although the use of chlorine as a
disinfectant is less expensive, the
UV tube supplies many more
advantages than chlorine.
Chlorine requires an inconvenient
contact time; is less successful at
incapacitating protozoa,
helminthes, and some viruses;
and is difficult to dose.
 In general, the cost of UV
disinfection systems depends on
the manufacturer, the site, the
capacity of the plant, and the
make-up of the incoming water
Options
Start-Up
Cost ($)
Monthly
Cost ($)
Average
Monthly
Cost for
first year
($)
UV-tube
$41
$1
$4
Chlorine
Addition
$1
$0
<<$1
Boil Water
(using LPJ)
$20-30
$10
$11
Purchase
Bottled
Water
$3
$12
$11
commercial
UV System
$300
$1
$26
Disadvantages
 While UV technology is becoming an increasingly viable option for water
purification, its design is still undergoing the process of refinement.
 Because UV light is harmful to microorganisms, it is also dangerous to
humans. In this respect, UV light may cause skin irritation and severe eye
damage if direct exposure takes place
 Providing a power source for UV treatment may be difficult to attain (lack of
electricity faculties in community)
 conventional UV systems are not effective against cysts like
Cryptosporidium; must be filtered out of water before UV disinfection
(nevertheless, some advanced UV systems address this problem by using a
stainless steel screen with 2- um openings to capture the cysts)
 Lack of oxidation capability; some locations require an oxidant for attaining
purification standards
 Lack of chemical residual produced; nevertheless, the use of chlorine or
chloramines may be applied to address this inadequacy
 Disinfection not appropriate for treatment of water with high levels of
suspended solids, turbidity, color, or soluble organic matter (substances can
Advantages
 The UV technology (especially UV tube) is opportune for water purification
in developing nations as it may incorporate local resources in its
construction (concrete, pottery). In this way, the UV-tube design can be
disseminated efficiently through community workshops initiated by local,
non-governmental groups or sold by small-scale business people
 The UV-tube does not require water pressure to operate and therefore can
be attached directly to a faucet or filled with a funnel and bucket
 UV-tube models are becoming an increasingly viable option for communities
as research is being done to both refine these models on a physical and
cultural level
 UV technology offers a simplicity of application, appropriate for small
systems
 UV disinfection is a physical process and not a chemical one; eliminates the
necessity of generating, handling, moving, or storing toxic chemicals
 Rapid disinfection (12 seconds), low electricity use, low maintenance (every
6 months), high flow rate (15 l/min), and ability to work with un-pressurized
water
References
“Basics of Ultraviolet Disinfection Technology”. http://socrates.berkeley.edu/%7
Erael/uvtube/uvdisinfection.htm
Cotruvo, Joseph A, et al. “Providing Safe Drinking Water In Small Systems
Technology, Operations, and Economics”. Lewis Publishers, Washington D.C. 1998
“The UV-tube Project”. http://socrates.berkeley.edu/%7Erael/uvtubeproject.htm
“Ultraviolet Disinfection”: Tech Brief. September 2000
“UV Disinfection-General”. http://www.triangularwave.com/a3a.htm
“Wastewater Technology Fact Sheet: Ultraviolet Disinfection”: United States
Environmental Protection Agency. September 1999