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:

Cl2 + H20 HOCl + HCl

Chlorine plus water yields hypochlorous acid and hydrochloric acid

Unfortunately, HOCl is not very stable ….

hypochlorous acid

H O Cl

Hydrogen ion

Hypochlorite ion

The HOCl is continually breaking apart and re-forming

H O Cl

H O Cl

H

O Cl

HO ClH

O ClH

O Cl

HOCL is 100 times stronger as a disinfectant than OCL..

H O Cl

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 DOSECHLORINE 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 = CHECKSPay 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 ppmPay to the Order of:_______

RESIDUAL=

BALANCE

?

.9 ppm

And one more time

DOSE = 4.2 ppm

DEPOSIT

DEMAND = 2.9 ppmPay to the Order of:_______

RESIDUAL=

BALANCE

?

1.3

ppm

OK, you can work this another way, also..

DOSE = 3 ppm

DEPOSIT

DEMAND = ? ppmPay 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 ppmPay 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

MoreIs 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

FLOCCULATION SEDIMENTATION

FILTRATION

Fluoride fed here

CLEARWELL

RAWWATER

PRO: Lots of

Contact TimeCON:

Lots of Organics

Still inThe

Waterhere

PRO: Good

Contact Time

CON: OrganicsHave notSettledOut yet

PRO: Organics removedCON: Less contact time

PRO: Org

anics removed

CON: Very

little

contact tim

e

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

CHLORINE

CHLORINE

REVIEW OF CHLORINE REACTIONS

• CHLORINE WILL REACT WITH:– PATHOGENS

– OTHER ORGANICS LIKE TASTE/ODOR COMPOUNDS

– AMMONIA

– INORGANICS LIKE IRON AND MANGANESE1

4

3

2

DEMAND FROM THESE MUST BE MET TO CREATE FREE Cl RESIDUAL

BREAKPOINT

.5 ppm1.5 ppm

1 ppm

1.5 ppm

.5 ppm

TOTAL CL 2 ppm

FREECL .5 ppm

COMBINED1.5 ppm

Combined chlorine

Free chlorine

Aren’t chloramines a disinfectant, too?

.5 ppm1.5 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 ppmPay to the Order of:_______

I must have had an outstanding Check for .2 ppm that I didn’t know about!