Wastewater Treatment Plants & Bacteria: Strategies for Compliance UV Systems ; Alternative Disinfectant to Free Chlorine Harris County Wastewater Symposium.

Wastewater Treatment Plants & Bacteria: Strategies for Compliance

UV Systems;

Alternative Disinfectant to Free ChlorineHarris County Wastewater SymposiumWastewater Treatment Plants & Bacteria: Strategies for ComplianceTim Brodeur, PEApril 26, 2011


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)


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


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


Closed Channel & Open Channel Reactors Drinking Water Re-Use Wastewater

Racine, WI UV System (100 mgd peak)

WSSC Laurel, MD UV System (120 mgd )


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


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 100MF/UF membranes 65 80

RO 90 50

Florida/NWRI HLD requirements

Incr

easi

ng F

iltra

tion

Cost

Incr

easi

ng U

V Co

st

UV transmittance (UVT)


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.


UV Open Channel Systems


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


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)


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).


Questions

?Tim Brodeur, PE

[email protected]

Wastewater Treatment Plants & Bacteria: Strategies for Compliance UV Systems ; Alternative Disinfectant to Free Chlorine Harris County Wastewater Symposium.

Documents

uv disinfection systems

wastewater slide

wi uv system

uv system sizing

md uv system

banks of uv lamps

design uv dose mjcm

mgd slide