A new tool for the disinfection toolbox? UV Disinfection
Ultra-Violet Light UVA (315 – 400 nm): sun tans UVB (280 – 315 nm): sun burns UVC* (200 – 280 nm): disinfection (*Nearly 100% filtered by the atmosphere.) Interferes with DNA Replication/ Reproduction
History of UV Drinking Water Disinfection • 1910 Marseilles, France:UV is first used to treat drinking water. Abandoned for free chlorine (by-product of soda production) • 1970’s:Disinfection by-products discovered. UV disinfection of drinking water common in Europe. • 1990’s:Outbreaks in the US lead to concern that chlorine is not effective against some organisms. • March 1993: a cryptosporidium outbreak in Milwaukee leads to 400,000 illnesses and 100+ deaths.
Poly-alum added Sedimentation Lake Ontario Gravel, sand, and GAC filters Chlorine residual added Storage UV History of UV Water Disinfection • 1999: Town of Ontario, NY becomes the first community in North America to disinfect surface source drinking water with UV light. • Currently: there are over 1500 wastewater treatment plants using UV in the US.
UV System Components • UV Bulbs and Ballast (AC or DC) • Mercury-argon lamps (an electrical arc ignites the mercury vapor which emits UV light) • Hopefor LEDs in the future… • Chamber • Quartz Sleeve • passes UV • maintains bulb temperatures • Sensor • to monitor bulb output • Cleaning Mechanism
Mercury Vapor Bulbs • Like florescent bulbs except: • Quartz not glass • No phosphor coating • 88% of the output at 253.7 nm • Lamp is susceptible to cooling by the effluent • Limited Lamp Life—1 year continuous operation
Determining Dose (Fluence) Dose (Fluence) = Intensity * Exposure Time (J/m2) (W/m2) (s) Function of bulb and water characteristics Function of hydrodynamics & Geometry of reactor
Simple Microbial Response 0% inactivated 90% inactivated, 10% surviving 99% inactivated, 1% surviving 99.9% inactivated, 0.1% surviving 99.99% inactivated, 0.01% surviving 99.999% inactivated, 0.001% surviving
? and imbedded bacteria A More Complicated Response • Explanation of Shoulder • Threshold of DNA “hits” to inactivate? • Reactivation? • Explanation of Tailing • “Shielding” depends on particle absorbance • Imbedded bacteria • UV resistant bacteria
Water Composition Turbidity • Can lead to scattering, reflection, blocking of microorganisms by particles • Considered negligible up to 5 NTU • Theoretically, overall intensity is not reduced, but, energy is lost through interactions with particulates • Embedded microorganisms present the real limitation • Precede by filtration or flocculation/sedimentation Absorption Coefficient • Certain particles absorb at germicidal wavelengths • Fe, sulfites, aromatic organics, humic acid, dechlorination with Sodium Thiosulfate • Presence can greatly increase attenuation
UV Regulatory Standards US • 1966: Dept. Health, Education & Welfare – 160 J/m2 • 1999: NY State still required 160 J/m2 when I started working with UV • ~2000: WHO Recommends 380 J/m2 • 2002: ANSI/NSF Standard 55 set at 400 J/m2
Bulb Holders UV Bulb Aluminum Plate Collimating Box Concentric Apertures Quasi Collimated UV rays Manual UV Shutter Petri Dish with sample and stir bar Magnetic Stir Plate Estimation of Dose (Fluence) • Biological Assays with Virus • Develop dose (fluence) inactivation relationship with Quasi Collimated Beam Apparatus • Measure Log inactivation in UV device
Estimation of Dose • Chemical Actinometry • Photochemical reaction rate • Molecules react with UV photos • Products of the reaction used to determine quantity of photons absorbed (dose)
p R H O i , j I N T E N S I T Y : = · e x p – ( s · R · w i , j 2 R p 4 · n · · R H O i , j x R RHO σw= abs of water Intensity y Dose Estimation: Point Source Summation ) • Bulb is considered a line of point sources distributing light equally in all directions • Intensity at each point is calculated as the sum of all point sources
Benefits of UV • No known byproducts • Short contact time (sec, instead of min) • No danger of overdosing • Ability to inactivate cyst forming organisms (e.g. Giardia, Cryptosporidium) at doses used for water treatment • No transportation of hazardous chemicals
Limitations Add chlorine residual • No residual disinfectant • Photoreactivation and dark repair possible • Bulb fouling • Organic constituents, hardness, algae and biofilm on quartz sleeve • Requires electricity • Shield from visible light for 1-2 hrs Overdose • Clean sleeve • Suspend bulb
Side View (transparent) 65 cm water outlet G-8 germicidal UV bulb water level Design: ss-PVC
Figure 1: Cross Section and Side View of the Ferro Cement UV-Tube Cable Ballast Ferro cement cover Nut and Bolt Metal Cover Bulb Ferro cement trough Inlet Water Outlet Ferro Cement UV-Tube
Constant Head Tank Flowmeter UV-Tube Mixing Tank with Pump Laboratory Testing All UV-Tube Designs • Microbial Testing • Hydrodynamic Tracer Tests • Materials Degradation Testing Bulb Studies • Cycling • Warm-Up Time
MS2 Microbial Testing MS2 Fluence Response Curve Avg: 4.38 95% CI: 2.98 – 5.77 643 967 1292
Rhodamine Tracer Testing • Pulse input with syringe • Samples every 3 seconds • Measure on spectrophotometer 555 nm
Materials Degradation Interactions of UV-Tube Materials with UV in the presence of water… • Flow through, minimal flow. • Overnight test, 16 hours. • Vacation test, 8 days. • Total Evaporation, 35 days. Test for volatile organic compounds and metals.
Materials Degradation Results • PVC alone carcinogenic volatile organics • ABS alone Benzene! • Galvanized steel High zinc levels (taste) • Lined PVC low levels of vocs, acceptable • Stainless with PVC endcaps low levels, acceptable • Copper and aluminum have not been tested.
Bulb Covers Cycling Timers Bulb Studies Cycling Study • One cycle daily: 12 hours on, 12 hours off • Four cycles daily: 3 hours on, 3 hours off • Twelve cycles daily: 1 hour on, 1 hour off Warm up Study
UV Issues/Challenges • Electricity requirements • Safety: UV exposure and electric shock • Material interactions with UV • Water depth and hydraulics (UV dose) • Water Characteristics • Embedded Bacteria • Fe, sulfites, aromatic organics, humic acid, & dechlorination with sodium thiosulfate absorb UV • Safe storage (no residual disinfectant) • Bulb life • *****Dissemination *****
Important Facts: • UV is bad for your eyes, skin • Turbidity, chemicals affect UV transmittance • Bulb life is limited • Water depth and hydraulics are important • No residual disinfectant
Materials and UV-C • Reflects UV: • Aluminum • Reacts with UV: • PVC • ABS • Other plastics? • Blocks UV: • Glass • Plexiglass • Transmits UV: • Quartz • Teflon
Design Features: • Effective • Eliminate microorganisms • Portable or built in place • Size, weight • Construction • Simple • Local materials • Simple tools
Design Features: • Reliable • Easy operation • Little maintenance required • Long term use • Cheap! • Fast (2 lpm or more) • Safe • NO UV Exposure • Visual cue telling if light is on or not
Challenges • Compatible with 15 W (G15T8) or 30 W (G30T8) UV bulb • 12 V DC or 110 V AC ballast • Flow regulation • Filtration • Uniform and optimal UV exposure
The Biggest Challenge: Final production price: < $100 US