HOW DISINFECTION WORKS

Disinfection kills or inactivates living organisms that cause disease Oxidation destroys the physical structure of the organism (cell walls, cellular organs etc.) Heat also destroys the physical structure of the organism Ultraviolet radiation destroys DNA so that the organism is unable to grow and reproduce Chlorine, hypochlorites, chlorine dioxide, chloramines and ozone are oxidizers Boiling water is a method of heat disinfection. Disinfection with ultraviolet light.

Chlorine-based Oxidizers are the disinfectants normally used in water treatment Oxidation is a chemical reaction, so we will have to talk about chemistry for just a moment: Cl 2 + H 2 0 HOCl + HCl Chlorine plus water yields hypochlorous acid and hydrochloric acid

Unfortunately, HOCl is not very stable …. hypochlorous acid HOCl Hydrogen ion Hypochlorite ion

The HOCl is continually breaking apart and re-forming H OCl H O H O H O H O H O

HOCL is 100 times stronger as a disinfectant than OCL.. H OCl WEAK DISINFECTANT STRONG DISINFECTANT SO WE WANT TO MAXIMIZE THE AMOUNT OF HOCL AND MINIMIZE THE AMOUNT OF OCL

HOW CAN WE DO THIS? The factor with the biggest influence on HOCL versus OCL concentration in a chlorine solution is…….. pH

So, which gives you more of the strong HOCL disinfectant: A LOW pH Or A HIGH pH ?

FACTORS AFFECTING DISINFECTION pH we just talked about this one! Dose Demand Contact time Temperature

CHLORINE DOSE CHLORINE DEMAND Dose is the chlorine added to water in ppm or mg/L Demand is the amount of chlorine used up in reacting with all of the impurities in the water Residual is the amount of chlorine left over after all demand has been met

I like to think of this as a checking account….. DOSE = DEPOSIT DEMAND = CHECKS Pay to the Order of:_______ RESIDUAL= BALANCE Zero or Negative Is BAD

So let’s check our Chlorine account balance… DOSE = 3 ppm DEPOSIT DEMAND = 2.1 ppm Pay to the Order of:_______ RESIDUAL= BALANCE ?.9 ppm

And one more time DOSE = 4.2 ppm DEPOSIT DEMAND = 2.9 ppm Pay to the Order of:_______ RESIDUAL= BALANCE ? 1.3 ppm

OK, you can work this another way, also.. DOSE = 3 ppm DEPOSIT DEMAND = ? ppm Pay to the Order of:_______ RESIDUAL= BALANCE.5 ppm 2.5 ppm

Calculating dose, given demand and the residual you want.. DOSE = ? ppm DEPOSIT DEMAND = 2.1 ppm Pay to the Order of:_______ RESIDUAL= BALANCE.2 ppm 2.3 ppm

What is “Chlorine Demand”? When chlorine is added to water, some of it is immediately used up in reacting with impurities in the water. The amount of chlorine that is used up is the “demand” of this particular water. DEMAND can be caused by iron, manganese, harmful organics (like pathogens), harmless organics, ammonia… anything that chlorine will react with.

FACTORS AFFECTING DISINFECTION pH Dose Demand Contact time Temperature More Is Better!

TEMPERATURE Higher temperatures tend to INCREASE the rate of chemical reactions: Chlorine kills pathogens faster in warm water Particles settle out faster in warm water Corrosion happens faster in warm water Unfortunately, this also means chlorine residuals disappear faster in warm water (tanks and distribution system)

POINT OF CHLORINE FEED COAGULATION Polymer fed to raw water here FLOCCULATIONSEDIMENTATION FILTRATION Fluoride fed here CLEARWELL RAW WATER PRO: Lots of Contact Time CON: Lots of Organics Still in The Water here PRO: Good Contact Time CON: Organics Have not Settled Out yet PRO: Organics removed CON: Less contact time PRO: Organics removed CON: Very little contact time

WHAT’S ALL THIS ABOUT ORGANICS? WHEN MOST CONSUMERS THINK ABOUT ‘ORGANIC’ THEY THINK IT’S A GOOD THING! FOR WATER TREATMENT PROFESSIONALS, ‘ORGANICS’ ARE BAD NEWS

ORGANICS Organic chemicals are compounds produced by living things. Some living things (grass, leaves, etc.) produce compounds that are harmless to humans Other living things (particularly warm-blooded animals) can be carriers of bacteria, viruses, and protozoa that cause infection. ANY ORGANIC chemical has the potential to combine with chlorine to produce harmful DISINFECTION BY PRODUCTS (DBPs)

TRIHALOMETHANES (THMs) THM STANDS FOR: Tri Halo Methane Trihalomethanes are formed when free chlorine bonds with organic matter. (Chloramines and Chlorine dioxide don’t bond to form THMs) THM’s are known carcinogens and suspected of other health effects

FORMATION OF THMs ORGANIC CHLORINE

REVIEW OF CHLORINE REACTIONS CHLORINE WILL REACT WITH: –PATHOGENS –OTHER ORGANICS LIKE TASTE/ODOR COMPOUNDS –AMMONIA –INORGANICS LIKE IRON AND MANGANESE DEMAND FROM THESE MUST BE MET TO CREATE FREE Cl RESIDUAL

BREAKPOINT.5 ppm 1.5 ppm 1 ppm 1.5 ppm.5 ppm TOTAL CL 2 ppm FREE CL.5 ppm COMBINED 1.5 ppm Combined chlorine Free chlorine

Aren’t chloramines a disinfectant, too?.5 ppm 1.5 ppm 1 ppm 1.5 ppm.5 ppm Combined chlorine Free chlorine Chloramine systems Free chlorine systems

HOW DOES AN OPERATOR KNOW WHEN BREAKPOINT HAS BEEN REACHED? IF you have a certain residual (say 2.0) and a certain residual (say.5)….. And you increase the dose (say, to 2.3) – You’d expect the residual to go up to.8, wouldn’t you? If it does, you have reached breakpoint. If not, (say it only increased to.6) there was still some demand left in the water and you haven’t reached breakpoint yet.

To go back to our checking account analogy… RESIDUAL= BALANCE.5 ppm If you had.5 ppm in the bank… You’d expect your new balance to be.8 ppm, wouldn’t you? DOSE =.3 ppm DEPOSIT And you made a deposit of.3 ppm If it was only.6 ppm, you’d have to say “WHOOPS”….. DEMAND =.2 ppm Pay to the Order of:_______ I must have had an outstanding Check for.2 ppm that I didn’t know about!