Presentation on theme: "UV Systems; Alternative Disinfectant to Free Chlorine"— Presentation transcript:
1UV Systems; Alternative Disinfectant to Free Chlorine Harris County Wastewater SymposiumWastewater Treatment Plants & Bacteria: Strategies for ComplianceTim Brodeur, PEApril 26, 2011
2UV Disinfection – Basic Concept Chemical Disinfection UV Disinfectionmilligrams*min/Liter millijoules/cm (mJ/cm2)CT ITConcentration (mg/L) x Intensity (mW/cm2) x Contact Time (minutes) Contact Time (sec)
3UV PrinciplesGermicidal 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 intensityLow-pressure high intensityMedium-pressure high intensity
4Germicidal 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 systemOverall system hydraulicsPresence of particlesCharacteristics of the micro organismsChemical characteristics of the wastewater
5Closed Channel & Open Channel Reactors Drinking WaterRe-UseWastewaterWSSC Laurel, MD UV System(120 mgd )Racine, WI UV System(100 mgd peak)
6UV Disinfection System Components & Configurations UV lampsQuartz sleeves holding UV lampSupporting structure for UV lamps/quartz sleevesBallasts used to supply regulated power to lamps (limit current to a lamp)Power supply
7Upstream Solids Removal Process Increasing Filtration CostIncreasing UV CostUpstream solids removal process has large impact on UV system sizing and costFlorida/NWRI HLD requirementsUpstream Solids Removal ProcessDesign UVT(%)Design UV Dose(mJ/cm2)Granular Media55100MF/UF membranes6580RO9050UV transmittance (UVT)
8UV 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.
10UV Closed SystemsDirection of flow is perpendicular to placement of lampsSome systems do have flow parallel to lampsAlso fixed geometry arrangements – medium pressure.VendorsCalgonTrojanWEDECOAquionicsSiemensAtlantiumOzoniaSevern Trent
11Definition of UV Dose (D) D = I x Twhere:D = dose, mJ/cm2 (= m W.s/cm2)I = UV intensity (mW/cm2)T = exposure time, seconds, sD is analogous to chlorine disinfection (CRt)
12Problems 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).