Introduction to Disinfection: From CT to UV

Transcription

1 Introduction to Disinfection: From CT to UV VA AWWA Plant Operations Committee Senior Operators Forum 1 File location name here.ppt Erik Rosenfeldt, Ph.D., P.E.. October 9, 2014 Charlottesville, VA

2 Agenda 2 Background on Disinfection Chlorine Disinfection UV Disinfection Summary and Conclusions

3 3 BACKGROUND Source of some material: Water Treatment, MWH, 2 nd Edition and Wastewater Engineering, Metcalf and Eddy 4 th Edition

4 Control of Waterborne Disease Essential human need provided by water engineers 20 th Century Control of Waterborne Diseases (Typhoid, Cholera, etc) through Engineering Through Treatment Technologies Role of the Engineer in Public Health Water Supply and Distribution Wastewater treatment

5 Potential Health Issues 5 Microbial Health Concerns Cryptosporidiosis and Giardiasis Vomiting and diarrhea, potentially life threatening for immune compromised, elderly and young 1993 Milwauke outbreak 400,000 people got sick over 2 weeks stomach cramps, fever, diarrhea and dehydration 104 deaths attributed to the outbreak Disinfection By-Product Health Concerns Cancer Bladder, colon and rectal Reproductive Neural tube defects and miscarriages? Brominated compounds are thought to pose a greater health risk than chlorinated compounds Nitrogenated compounds may even be worse

6 Disinfection 6 Goal to destroy or inactivate pathogenic microorganisms including bacteria, protozoan cysts, helminths and viruses. Pathogenic disease causing Problem: all chemical disinfectants form unwanted byproducts

7 Infectious Agents Potentially Present in Untreated Domestic Wastewater 7

8 Emerging Pathogens of Concern - Bacteria 8

9 Emerging Pathogens of Concern - Viruses 9

10 Emerging Pathogens of Concern - Protozoa and Algal Toxins 10

11 Types of Disinfectant Systems 11 Chemical agents chlorine (Cl 2 ) chlorine dioxide (ClO 2 ) ozone (O 3 ) chloramines Physical agents UV irradiation membranes

12 Typical Water Treatment Plant Schematic 12 Raw Water Chemical Addition Flocculation Clarification Cl 2 ClO 2 KMnO 4 O 3 Filtration Cl 2 ClO 2 KMnO 4 O 3 Rapid Mix Corrosion Fluoride Cl 2 NH 3 To System UV NH 3 Clearwell

13 Mechanisms of Disinfection 13 Oxidation or rupture of cell wall Diffusion into cell and interference with cellular activity Chemical Disinfection (Oxidation) Mechanism Therefore, the ability to oxidize biological molecules and the ability to diffuse through the cell walls are the requirements of any effective disinfectant.

14 Oxidizing Power of Various Oxidants Oxidant Formula Oxidation Potential (V) Hydroxyl free radical o OH 2.80 Ozone O Hydrogen peroxide H 2 O Permanganate ion MnO Hypochlorous acid HOCl 1.49 Chlorine Cl Hypobromous acid HOBr 1.33 Bromine Br Chlorine dioxide ClO Iodine I Oxygen O Hypochlorite ion OCl - <0.50 Note: italics denotes oxidants with excellent disinfection capabilities Source: AWWA, Chlorine Dioxide Handbook 14

15 Disinfection Kinetics 15 Principle theory: Chick s Law (1908) (Dr. Harriett Chick) ln(n/n o ) = - kt (first order) ln(n/n o ) k N - number of organisms present at time t N o - number of organisms present at time t=0 k - rate constant, depends on: t t - > disinfectant type and concentration > microorganism > water quality (ph, turbidity, temperature) time

16 Disinfection Kinetics, cont. 16 Later that same year, Watson refined the equation to include changes in the disinfectant concentration: k = k C n ln(n/n o ) = -k C n t Chick-Watson Law n- coefficient of dilution k - experimental constant both n and k are determined experimentally when n>1, disinfecting action dependent on concentration when n<1, disinfecting action depends on contact time

17 Kinetics of Disinfection for Various Organisms 17 Source: Berg, 1964 in JMM Book

18 Kinetics of Disinfection Effect of Disinfectant Type 18 Source: Scarpino et. al.,, 1977 in JMM Book

19 CT Concept 19 C is the disinfectant residual concentration T is contact time. For regulatory purposes, we use the T 10 time. T 10 is determined from tracer study C/Co Time, minutes ln(n/n o ) = -k C n t

20 CT Concept 20 CT Values for Inactivation of Giardia Cysts by Free Chlorine at 0.5C or lower Go to CT tables in the SWTR Guidance Manual to find required CT Calculate residual required to meet CT requirements Function of chlorine dose, ph, and temperature Chlorine ph <=6.0 Concentration Log Inactivations (mg/l) <=

21 21 CHLORINE Source of some material: Water Treatment, MWH, 2 nd Edition and Wastewater Engineering, Metcalf and Eddy 4 th Edition

22 Chlorine 22 Most widely used disinfectant Saved millions of lives and eliminated waterborne diseases such as cholera and typhoid Relatively easy to use Safety concerns for liquid chlorine cylinders or tank cars (chlorine gas) Relatively inexpensive Major disadvantages: production of THMs, HAAs, other chlorinated disinfection byproducts

23 Uses of chlorine (oxidant) in drinking water treatment plants Disinfection Oxidizes color Raw Water Chemical Addition Oxidizes iron and manganese Cl Taste 2 and odor control ClO 2 KMnO 4 Rapid Controls aquatic growth in plants O 3 Mix Aid to filtration (particle removal) Zebra mussel control To System Flocculation Clarification Corrosion Fluoride Cl 2 ClO 2 KMnO 4 O 3 Cl 2 NH 3 Filtration NH 3 Clearwell

24 Forms of chlorine Chlorine gas Hypochlorite 1-ton chlorine cylinders Sodium hypochlorite tanks

25 Chlorine gas Most commonly-used form of chlorine Effective disinfectant Readily available Normally most economical method of disinfection Decreases ph Hazardous; must use care in handling Toxic to aquatic life (same for all forms of chlorine)

26 Hypochlorite Usually in the form of NaOCl; also Ca(OCl) 2 Increases ph Higher cost than gaseous chlorine

27 Chlorine Speciation Percent HOCl HOCl ~ 100x stronger disinfectant than OCl - Disinfection efficiency goes up with lower ph Percent OCl HOCl OCl ph

28 Reactions of chlorine with impurities in water Order of reactions Inorganic compounds Ammonia and certain organic nitrogen compounds Organic compounds Rates vary with compound and other conditions

29 Reactions of chlorine with impurities in water Reactions with inorganic compounds Compounds which react quickly: hydrogen sulfide, ferrous iron, MnII Demand occurs before a chlorine residual occurs; must be satisfied before any disinfection can take place 2Fe 2+ + HOCl + H + 2Fe 3+ + Cl 1- + H 2 O

30 Reactions of chlorine with impurities in water Chloramines are formed when Cl 2 reacts with ammonia (NH 3 ) NH 3 + HOCl NH 2 Cl + H 2 O monochloramine NH 2 Cl + HOCl NHCl 2 + H 2 O dichloramine NHCl 2 + HOCl NCl 3 + H 2 O trichloramine (nitrogen trichloride)

31 Breakpoint Chlorination Curve Chlorine concentration Initial Chlorine Demand Inorganic Compounds Reducing Agents Combined Residual Chlorine Oxidation of Combined Residual Material (Chloramines) Chloramines Chloroorganics Oxidation of chloramines to N 2 or NO 3- ; decreasing combined residual BREAKPOINT Free Chlorine Residual Resiudal on a 1 to 1 Basis Chlorine Dose, mg/l

32 Chloramines for Disinfection 32 Not effective for Cryptosporidium inactivation Used to provide a residual in some distribution systems Halts the formation of THMs or HAAs May control biofilms/regrowth better than free Cl 2 Relatively inexpensive

33 Reactions of chlorine with impurities in water Reactions with organic compounds Occur after reactions with inorganic compounds Produce chlororganic compounds or other combined forms of chlorine Have slight disinfecting action Free chlorine residual Produced after all other above reactions Highest disinfecting capability Rarely exists in wastewater with nitrogenous compounds. Amount of chlorine required would be 25 to 150 mg/l.

34 Chlorine dose Chlorine Dose = Chlorine Demand + Chlorine Residual Where: Chlorine Residual = Combined Chlorine + Free Chlorine Contact time needed for disinfection Contact time must be specified since longer contact times increase chlorine uptake (decrease chlorine residual)

35 35 UV DISINFECTION Source of some material: Water Treatment, MWH, 2 nd Edition and Wastewater Engineering, Metcalf and Eddy 4 th Edition

36 Electromagnetic Spectrum 36

37 Mercury Vapor UV Lamp Spectra 37

38 Ultraviolet Disinfection 38 Uses lamps submerged in water that emit light at 254 nanometer wavelength UV light keeps pathogens from reproducing by affecting their DNA and RNA Killing effectiveness depends on the intensity of light and the time in contact with the microorganisms More effective than chemical oxidants for resistant organisms Cryptosporidium Giardia

39 Comparing UV and Chlorine 39 Disinfection Based on Table 18-1 in AWWA WQ&T Parameter UV Disinfection Chlorine Disinfection Action Mechanism Resulting Cell Structure Reactivation Design Dose Calculated Dose Exposure Time Residual Background Demand Effectiveness Physical DNA Damage Intact Photo/dark Fixed/semi-variable I x T Seconds None Absorbance Attenuation Wavelength Chemical Oxidation Damaged Resuscitation Adjustable C x T Minutes Varies with chlorine demand Organics/inorganics Sunlight ph

40 UV Disinfection: Mechanism 40 of Action Physical Process Light Energy Absorbed by DNA Pyrimidine Dimer Formation (C s and T s) Inhibits Replication Organism that Cannot Replicate, Cannot Infect C G A T A T UV T G A C C G A T DNA

41 UV Disinfection is Dependent 41 on Microorganism

42 Microbial Repair 42 Bacteria can repair the damage caused by UV light Photorepair enzymes energized by exposure to light break the pyrimidine dimers. Dark repair is any repair process that does not require the presence of light. Example: Excision Repair Ex: Light Repair Ex. Dark Repair VIS C G A T A T T A C G G A T C DNA C G A T A T T A C G G A T DNA C

43 Effectiveness of UV - 43 Wavelength DNA Absorbance

44 WATER QUALITY IMPACTS ON UV EFFECTIVENESS 44

45 UV Absorbance (Transmittance) Stuff in water other than microorganism (target) that can absorb UV light UV decays exponentially through a medium I = I o x 10 -αl α = background absorbance l = distance from source Absorbance and Transmittance are related %UVT = 100x10 -A

46 Particles 46 Problem Particles shielding microorganism from UV Solution: Multiple light sources?

47 Particles 47 Reality: particles are not spheres, and microorganism are very small Solution: Remove particles with filtration prior to UV disinfection

48 Lamp Fouling 48 Materials in water can deposit on the quartz sleeve Hardness Organics Iron Affects Intensity of light entering reactor

49 Hydraulics are extremely 49 important

50 Summary 50 Drinking water disinfection is a key component of public health protection. There are various disinfection chemicals and applications available to utilities Selection of disinfection techniques is system specific Disinfection can lead to the formation of DBPs in the system

51 Acknowledgements 51 Dr. James K. Edzwald Bill Becker and Julie Herzner, Hazen and Sawyer

52 Thank You! 52 For additional information please contact: Erik Rosenfeldt: