Transcript UV Principles

UV Systems;
Alternative Disinfectant to Free Chlorine
Harris County Wastewater Symposium
Wastewater Treatment Plants & Bacteria: Strategies for Compliance
Tim Brodeur, PE
April 26, 2011
Wastewater
Treatment Plants & Bacteria:
Strategies for Compliance
UV Disinfection – Basic Concept
Chemical Disinfection
UV Disinfection
milligrams*min/Liter
millijoules/cm2
(mJ/cm2)
CT
IT
Concentration (mg/L) x
Intensity (mW/cm2) x
Contact Time (minutes)
Contact Time (sec)
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
UV Principles
• Germicidal portion of UV radiation band is
between 220-230 nm in the UV-range (short
wave). Hg vapor lamps are charged by striking
to produce an electric arc. Emitted light is UV
light.
• 3 categories for UV Disinfection Systems:
– Low-pressure low intensity
– Low-pressure high intensity
– Medium-pressure high intensity
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Germicidal Effectiveness of UV
Radiation
• UV light is physical rather than chemical disinfecting agent –
penetrates the cell wall and is absorbed by nucleic acids, which
either kills cells or prevents replication (RNA interference)
• Variables affecting performance:
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–
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Characteristics of UV disinfection system
Overall system hydraulics
Presence of particles
Characteristics of the micro organisms
Chemical characteristics of the wastewater
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Closed Channel & Open Channel Reactors
Drinking Water
Re-Use
WSSC Laurel, MD UV System
(120 mgd )
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Wastewater
Racine, WI UV System
(100 mgd peak)
UV Disinfection System Components &
Configurations
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UV lamps
Quartz sleeves holding UV lamp
Supporting structure for UV lamps/quartz sleeves
Ballasts used to supply regulated power to lamps
(limit current to a lamp)
• Power supply
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Upstream solids removal process has large impact on
UV system sizing and cost
Upstream Solids
Removal Process
Design UVT
(%)
Design UV Dose
(mJ/cm2)
Granular Media
55
100
MF/UF membranes
65
80
RO
90
50
UV transmittance (UVT)
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Increasing UV Cost
Increasing Filtration Cost
Florida/NWRI HLD requirements
UV Open Channel Disinfection Systems
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•
•
Design flow rate is divided
among a number of open
channels with two or more
banks of UV lamps in series.
Need stand-by bank for
system reliability. Includes
flow control or weighted flap
gate to keep lamps
submerged.
Lamps need to be
periodically removed to clean
– prevent fouling.
Mechanical cleaning systems
are used with low-pressure
high-intensity system to
avoid fouling of quartz
sleeves.
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
UV Open Channel Systems
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
UV Closed Systems
•Direction of flow is
perpendicular to
placement of lamps
•Some systems do have
flow parallel to lamps
•Also fixed geometry
arrangements –
medium pressure.
Vendors
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Calgon
Trojan
WEDECO
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Aquionics
Siemens
Atlantium
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
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Ozonia
Severn Trent
Definition of UV Dose (D)
D=IxT
where:
D = dose, mJ/cm2 (= m W.s/cm2)
I = UV intensity (mW/cm2)
T = exposure time, seconds, s
D is analogous to chlorine
disinfection (CRt)
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Problems with UV Applications
• Inaccurate knowledge of the UV intensity.
• Exposure time associated with all of the pathogens passing
through a UV disinfection system.
• Open channels – achieving a uniform velocity field in the
approach and exit channel. Especially difficult is when UV
is retrofitted into existing open channel (converted chlorine
contact chambers).
“UV Disinfection Guidelines for Drinking Water and Wastewater Reclamation” (NWRI and AWWARF,
2000).
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Questions
?
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Tim Brodeur, PE
[email protected]