TVM 4145 Vannrenseprosesser / unit processes Disinfection Prof. TorOve Leiknes torove.leiknes@ntnu.no

Disinfection What do the directives / regulations state? Demand: “Drinking water shall not have pathogenic organisms” All drinking water requires disinfection Principle of muliti-barriers

Disinfection process: penetration of cell walls and destruction of internal structures attacks cell membranes and changes properties attacks microorganisms enzyme system 1. chemical - halogens - oxidizing substances - metals - acids/bases - surfactants 2. physical - heat - UV-exposure 3. mechanical - sedimentation - filtration / membranes - adsorption 4. radiation - gamma and x-ray Desinfeksjons metoder:

Disinfection kinetics: Chick; neutralizing microbiological activity ca be compared to (1908) a chemical reaction. A 1st order reaction. Chick’s law: Assumptions; one culture media constant temp., pH, water quality constant disinfection dose 2 1 1. Chick’s law 2. Takes time for effect to start 3. “Easiest” are destroyed first ln(N/No) 3 time

“destruction is a function of disinfection concentration” Watson: “destruction is a function of disinfection concentration” Watson’s law: (n is a function of disinfection agent) n < 1, contact time t is more important than concentration C n > 1, efficiency decreases with dilution Combining Chick’s law and Watson’s law:

Parameters that affect the disinfection efficiency: 1. Temperature: Effect follows Arrhenius relationship disinfection temperature 2. pH: pH < 3 and pH > 11 Die out or inactivation 3. Water quality: Consumption of oxidation agent inorganic material which is oxidized organic material which is oxidized - adsorption to the cell which hinders disinfection - complex reactions which are less effective - oxidation which removes disinfection properties

Halogens Fluorine F Chlorine Cl Bromide Br Iodine I Of these chlorine is mostly used for disinfection systems History: 1774, chlorine gas first prepared by Scheele 1825, first used in wastewater treatment, France 1831, used as prophylactic against cholera epidemics 1900s used as disinfection of drinking water 1917, chloramines taken into use, Canada og USA Characteristic: Yellow-green gas, very strong characteristic smell. Very strong oxidizing agent, can be explosive Poisonous, forms several undesired chlorinated substances

Chlorine reactions with water vann: (dissociation pH dependent) HOCl more effective than OCl- defined as “free chlorine” will react with other substances

Chloramines: Reactions - Mono-chloramine Di-chloramine Tri-chloramine Formation of chloramines is pH dependent Referred to as “bound chlorine” Break-point chlorination Disinfection efficiency: HOCl > OCl- > NH2CL > NHCl2 > NCl3

Advantages/disadvantages with chlorination: Well known technology Works quickly Easy to measure Good residual effect on the distribution system Overall al economics relatively low Can cause taste and odour problems Can cause harmful byproducts (THM) Chlorine gas is poisonous, health / safety issues for operation etc.

Oxidizing substances Ozone, O3 H2O2 Permanganate Of these ozone (O3) is mostly used for disinfection History: 1783, discovered by Van Marum 1857, first ozone generator constructed by Siemens 1893, first commercial use of the equipment 1893, used for disinfection of drinking water, Netherlands 1906, installed in a water treatment process, France Characteristics: Blue gas med strong characteristic smell. Very strong oxidation agent. Poisonous; max. conc. 0,2 mgO3/m3 air Production: Generated from dry air or oxygen in a high voltage electric discharge.

Advantages/disadvantages with ozonation: reduces odours, taste, colour oxidizes organic pollutants works very quickly no residual chemicals limited dangerous byproducts no danger of “over dosing” oxygenation of the water effective against chlorine resistant pathogens no residual effect relatively high investment costs must be generated on site can be difficult to operate difficult to control the effect can require pretreatment oxidizes other material under specific conditions may form harmful byproducts

Thermal disinfection History: cooking is the oldest disinfection method known Characteristics: can reach near sterilization by boiling Spores are thermal resistant, few water borne diseases are from spore forming bacteria freezing will not disinfect water Application: Not used at treatment plants due practical and economic reasons, mainly a recommendation during emergencies.

History: Characteristics: UV - radiation Exposure to direct sun light to improve water quality is well known in several historic documents. 1940s a proposal was made for guidelines on how to dimension and design UV-systems Has not traditionally been used in larger plants but in recent years has had a lot of focus and interest as an alternative solution Characteristics: UV light is classified in three groups; 1. UV-A: 315-400nm, long wave lengths 2. UV-B: 280-315nm, medium wave lengths 3. UV-C: <280nm, short wave lengths

Disinfection effect between 200-300 nm. Highest effect at 256nm Low-pressure lamps: monochromatic radiation quarts mercury lamps wave length; 253,7 nm high intensity High-pressure lamps: polychromatic radiation several wave lengths in radiation somewhat lower intensity

UV-disinfection process: Photo chemical reaction: UV wave length breaks down the structure in the DNA molecule prevents the bacteria’s capacity to reproduce NB! All microorganisms have a certain capacity to repair their DNA structures by photo reactivation when exposed to light in the wave length area of 300-550 nm. Some minor dark reactivation may also be possible.

Dimensioning of UV disinfection: Efficiency is a function of radiation intensity and retention time where: D = dose in mWatt-sek/cm2 (ultrad) I = radiation in mWatt-sek t = radiation time in sec General demands: - dimensioned for Qmax - poorest raw water quality - min. 16000 ultrad - should always be the final step in a treatment scheme - alternative reserve disinfection system necessary

Advantages/disadvantages with UV disinfection: works very quickly no effect on odours and taste no residual chemicals minimal harmful byproducts no danger of “over dosing” no change in the waters natural composition easy operation and maintenance no residual effect on distribution network traditionally viewed as system only suitable for efficiency dependent on raw water quality

Mechanical Adsorption: most microorganisms are not de independent or free suspended entities in the water phase and are commonly found in aggregates and on particles in the water. Unit processes that remove particles will to a certain degree therefore also remove microorganisms. Processes which will remove microorganisms from water: 1. Chemical precipitation 2. Sedimentation / flotation 3. Filtration / membrane filtration In general, mechanical removal of microorganisms with the exception of some membrane filtration processes will not be accredited as a hygienic barrier and recognized as a disinfection process.