WATER TREATMENT TECHNOLOGY (TAS 3010) LECTURE NOTES 9e - Disinfection

Izan Jaafar, Engineering Science, FST, UMTIzan Jaafar, Engineering Science, FST, UMT

© SHAHRUL ISMAIL, DESc.University College of Science and Technology Malaysia

CHAPTER 3:Environmental Microbiology

CHAPTER 9e : CHAPTER 9e : CHAPTER 9e : CHAPTER 9e :

TAS 3101 : WATER TREATMENT TECHNOLOGY

Water Water

Treatment Treatment

Process :Process :

DisinfectionDisinfection

Water Water

Treatment Treatment

Process :Process :

DisinfectionDisinfection


Menu

1) Introduction

2) mg/L Unit

3) meq/L unit

4) Hardness & Alkalinity

Measurement Unit


- “Treatment of water with chemicals to kill bacteria”

- Two(2) objectives :

i) Primary disinfection : Kill any pathogen in water

ii) Seconday (residual) disinfection : Prevent pathogen re growth in the water.

- Method use :

i. Should be harmless and unobjectionable to the consumer

ii. Should be able to retain a residual disinfecting effect for a long period

Introduction


1) Destroy bacteria / pathogens within a practicable period of time, over an expected range of water temperature.

2) Effective at variable compositions, concentration and conditions of water treated.

3) Neither toxic to humans and domestic animals nor unpalatable or otherwise objectional in required concentration.

4) Not change water properties

5) Have residual in a sufficient concentration to provide protection against recontamination

Properties


6) Can be determined easily, quickly, and preferably automatically.

7) Dispensable at reasonable cost

8) Safe and easy to store, transport, handle and supply

9) Persist within disinfected water in a sufficient concentration to provide reasonable residual protection against its possible recontamination before use.

10) Not form toxic by-products due to their reactions with any naturally occuring materials in water.

Properties – Con’d


METHODSMETHODS

HEATHEAT CHLORINATIONCHLORINATION

OZONATIONOZONATIONULTRAVIOLET ULTRAVIOLET

RADIATIONRADIATION

Methods


- Free chlorine disinfection

- Effective and the most common application

- Available in granular, powdered, liquid or gases form

- Developed by using chlorine gas (Cl2), sodium hypochlorite (NaOCl) or calcium hypochlorite (Ca(OCl)2).

- Reacts in water to produce dissolved chlorine gas (Cl2(aq)), hypochlorous acid (HOCl) and hypochlorite (OCl-).

-

Disinfection - Chlorination


H2O + Cl2 HCl + HOCl- + Cl-

HOCl H+ + OCl- (pH > 8)

H+ + OCl- HOCl (pH < 7)

• HOCl = hypochlorous acid

• OCl- = hypochlorite ion

• HOCl is more effective than OCl-

• Chlorine gas will be injected for water pH less than 3.

Water Reaction


- Provide chlorine residual for secondary disinfection.

- Chlorine residue must be maintained in the treated water to the end user. This secondary disinfection functioned to control pathogen distribution during water distribution.

- Increase lifetime of chlorine residual by adding ammonia to treated water.

- Ammonia reacts with free chlorine to form chloramines (NH2Cl, NHCl2 and NCl3) which are termed combined chlorine.

- Chloramines less effective as oxidants than HOCl, seldom used as primary disinfectant. However, more persistent than free chlorine and maintain secondary disinfectant.

Chlorination – Advantage


HOCl + NH3 H2O + NH2Cl (monochloramine)

HOCl + NH2Cl H2O + NHCl2 (dichloramine)

HOCl + NHCl2 H2O + NCl3 (trichloramine)

• NH2Cl = monochloramine

• NHCl2 = dichloramine

• NCl3 = trichloramine

Ammonia Reaction


The proportion of chloramines depends on :

1. pH

2. Temperature

3. Time

4. Initial chlorine to ammonia ratio

* Combined chlorine : Combination of chlorine with ammonia nitrogen or organic nitrogen compounds. Combined chlorine is less reactive.

Ammonia Reaction – Con’d


1) Effectiveness is less with protozoan cysts, mostly etc. Giardia lamblia and Crytosporidium and virus.

2) Formation of halogenated disinfectant byproducts (DBPs). Includes trihalomethanes (THMs) such as carcinogen, chloroform (CHCl3) and haloacetic acids (HAAs).

- THMs and HAAs created when free chlorine combines with natural organic substances that may still be present in the water.

-

Chlorination – Disadvantage


1. Dosage – Sufficient high concentration to inactive pathogen

2. Contact time – Physical contact with pathogen for sufficient time to achieve inactivation.

3. Turbidity – Present particles (turbidity) hides pathogen from the disinfectant.

4. Other reactive species – Consume disinfectant, reduce concentration available for inactivation.

5. pH – Most effective at pH values less than 7.5.

6. Water temperature – Disinfection increases, however chlorine become less stable.

Chlorine Effectiveness - Factors


1) Disinfectant, kg = Chlorine dosage, kg

% Available Chlorine

2) Dosage of chlorine, kg = Flow, L x Chlorine concentration, mg/L

Chlorination - Calculation


- Sweet smelling, unstable gas

- Form of oxygen in which three atoms of oxygen combined to form the molecule O3.Air in generating equipment contain up to 13% ozone.

- Most powerful disinfectant

- More effective against cysts and viruses than free chlorine.

- Leaving no taste or odor problems.

- Faster contact time

- No residual remain

- Unaffected by the pH or the ammonia content of the water

- High cost – Construction, Operation and Maintenance

- Unstable and cannot be stored

-

Disinfection - Ozone


-Much higher energy level than visible light

-Potential to inactivate pathogens.

-Employed by submerged UV lamps into the water to be treated.

- Multiple lamps used to provide greater coverage.

- Ability of UV lights to pass through the water to get the target organism. Energy is absorbed by genetic material in the microorganism, interfering with their ability to reproduce and survive.

- Performs well against both bacteria and viruses.

- Leaves no residual protection for distribution system.

- High cost.

Disinfection - UV


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WATER TREATMENT TECHNOLOGY (TAS 3010) LECTURE NOTES 9e - Disinfection

Documents

engineering science

combined chlorine

chlorine dosage

dosage of chlorine

available chlorine

water ph

umt water reactionh

combination of chlorine