PROJECT 3A. WATER DISINFECTION

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Maximilian Taylor
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1 PROJECT 3A. WATER DISINFECTION 1 1. Disinfection Review Basic concepts - Disinfection: selective kill of disease-causing organisms Sterilization: kill of all organisms disinfection sterilization - Common disinfectants Primary disinfectants» those used directly for the inactivation of diseasecausing organisms» including Cl 2, ClO 2, O 3, and UV Secondary disinfectants:» those used to suppress biological regrowth in the water distribution systems» including Cl 2, ClO 2 and various chloramines Factors Affecting the Disinfections - Characteristics of disinfectants, generally, O 3 >HOCl>ClO 2 >OCl - >NHCl 2 ~NH 2 Cl - Microorganisms characteristics Resistance Spores > protozoan cysts > viruses > vegetative bacteria Physical and physiological conditions - Water quality Suspended solids Disinfectant demand materials ph: affect the forms of disinfectants Temperature - Concentration of disinfectants - Contact time

2 Disinfection Kinetics - Types of kinetics first-order decay shoulder effect curve tailing effect curve combinations thereof - Chick model dn = kn At C = constant, integrating the equation leads to: 2 N ln = kt No Plotting log(n/n o ) vs. t yields a straight line - Chick-Watson model Assume k = k C n dn n = kc N where: n = coefficient of dilution» n > 1, C is more important than t» n < 1, t is more important than C At C = constant, integrating the equation leads to: N n ln = kc t No Plotting log(n/n o ) vs. t again yields a straight line - Other kinetic models Hom model Zhou and Smith model Disinfection Design - Dose-response approach log(n/n o ) = f (disinfectant dose, water quality)

3 - CT approach Assume Chick s model with n =1 log(n/n o ) = f(ct) To account the decay of disinfectants in water and backmixing, the CT values are calculated as: CT = C ave x T 10 where: C ave = average conc. of disinfectant, mg/l T 10 = effective contact time (T 10 ), min - Mathematical Models Axial Dispersion Model Back Flow Cell Model CFD - Surface water disinfection Giardia: 3-log removal Viruses: 4-log removal Cryptosporidium: 2-log removal Conventional treatment with filtration: 2.5-log credits Direct filtration: 2.0-log credits - Groundwater disinfection Use chlorination Maintain adequate Cl 2 residual in distribution systems - Wastewater effluent disinfection Usually not required due to the formation of DBPs Design based on contact time and chlorine residual to achieve fecal coliform < CFU/dL Normally dechlorinated prior to discharge 3

4 4 Disinfection By-Products (DBPs) Control Strategies - Source control - Precursor removal: enhanced coagulation, GAC, membrane filtration - Alternative disinfectants: combined chlorine, ozone, chlorine dioxide, multiple disinfectants (e.g., ozone + chloramines) - DBPs removal: GAC, air stripping

5 2. Chlorine Feed System - Direct solution or dry chemical feed systems, e.g., NaOCl: 12 to 15 % Cl 2 Ca(OCl) 2 : 70 % Cl 2 - Gas-to-solution system - Direct gas injection - On-site generation and feeding 5 Gas-to-solution system - Storage:» 10 to 1000 kg/d feed rate: 45 to 900kg (2000lb) cylinders» > 1000 kg/d feed rate: tank trucks or railroad cars» Minimum storage: 15 to 30 days» Minimum storage cylinders: 2 - Evaporator (for some cases only) Usually use the electric resistance heat in a hot water bath Must install an automatic shutoff valve - Chlorinator - Injector - Diffuser - Chlorine contact basin

6 3. UV Generation and Measurement - UV Generation Low pressure lamps Medium pressure lamps - UV Measurement Radiometer UV Sensors Actinometer (ferrioxalate, persulfate, or iodide/iodate) Bioassay (Bacillus subtilis) 6 UV Disinfection Kinetics - Chick-Watson Model dn = kin where: k = inactivation rate constant (cm 2 /mw s) I = fluence rate (mw/cm 2 ) - Series-in-Events Model Basic Assumptions» An event represents a discrete unit of damage» Inactivation occurs when the number of events reaches a threshold level, n» Constant threshold level In a batch reactor, the rate of change in the concentration of viable organisms is: N dn = i = ki( Ni N i 1 ) r i i >1 = kin o i =1 Total concentration of viable organisms is equal to the summation of above equation as: N = n 1 N i= o i = N T e kit n 1 i= o ( kit) i! i

7 7 - Other models: Multi-targets, Collins-Selleck, etc Just replace the concentration of chemical disinfectant by UV dose (I t) Important Factors Affecting UV Disinfection - UV Dose - Non-ideal flow pattern - Water quality (turbidity/ss, UV absorbance, Fe, Mn, etc) Note: non-uniform distribution for both I and v field! System Design - Configurations of UV Systems Open channel: horizontal or vertical in staggered pattern Closed system - Current Design Approaches Collimated beam tests for determine required UV dose Pilot tests CFD approach

Disinfection. Disinfection is used to treat both domestic water and wastewater.

Disinfection. Disinfection is used to treat both domestic water and wastewater. Disinfection Disinfection is the selective destruction of disease causing organisms (viruses, bacteria, protozoans). It destroys most recognized pathogenic microorganisms, but not necessarily all microbial