1. UV Systems; Alternative Disinfectant to Free Chlorine
Harris County Wastewater Symposium
Wastewater Treatment Plants & Bacteria: Strategies for Compliance
Tim Brodeur, PE
April 26, 2011

2. UV Disinfection – Basic Concept
Chemical Disinfection UV Disinfection
milligrams*min/Liter millijoules/cm (mJ/cm2)
CT IT
Concentration (mg/L) x Intensity (mW/cm2) x Contact Time (minutes) Contact Time (sec)

3. UV Principles
Germicidal portion of UV radiation band is between 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

4. 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:
Characteristics of UV disinfection system
Overall system hydraulics
Presence of particles
Characteristics of the micro organisms
Chemical characteristics of the wastewater

5. Closed Channel & Open Channel Reactors
Drinking Water
Re-Use
Wastewater
WSSC Laurel, MD UV System
(120 mgd )
Racine, WI UV System
(100 mgd peak)

6. UV Disinfection System Components & Configurations
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

7. Upstream Solids Removal Process
Increasing Filtration Cost
Increasing UV Cost
Upstream solids removal process has large impact on UV system sizing and cost
Florida/NWRI HLD requirements
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)

8. UV Open Channel Disinfection Systems
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.

9. UV Open Channel Systems

10. 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
Calgon
Trojan
WEDECO
Aquionics
Siemens
Atlantium
Ozonia
Severn Trent

11. Definition of UV Dose (D)
D = I x T
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)

12. 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).

13. Questions ?
Tim Brodeur, PE

14. UV offers simple operations and small footprint
Atlanta UV System (180 mgd)
Boston UV System (450 mgd)
Vendors
Calgon
Trojan
WEDECO
Ozonia
Aquionics
Siemens
Atlantium
Severn Trent
Rahway, NJ (85 mgd)