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UV Systems; Alternative Disinfectant to Free Chlorine

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Presentation on theme: "UV Systems; Alternative Disinfectant to Free Chlorine"— Presentation transcript:

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)

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