E NVIRONMENTAL E NGINEERING 343 Lecture 8: Water Treatment (3) Filtration & Disinfection Philadelphia University Faculty of Engineering Department of Civil.

ENVIRONMENTAL ENGINEERING

343

Lecture 8:

Water Treatment (3)

Filtration & Disinfection

Philadelphia UniversityFaculty of Engineering

Department of Civil EngineeringFirst Semester, 2013/2014

FILTRATIONFiltration is a process for separating suspended

or colloidal impurities from water by passing through a porous medium, usually a bed of sand or other medium.

Settled water (Sedimentation effluent) turbidity range 1-10 TU – due to residue of flocs particles. So turbidity need to be reduce to less than 0.3

Common materials for granular bed filters: Sand (slow, rapid or high) Anthracite coal Dual media (Coal plus sand) Mixed media (coal, sand & garnet)

FILTRATION- FILTER MEDIA

Loading rate

Where, Vo = water velocity (m/s) Q = water flow (m3/s)Ac= cross surface area (m2)

Installing a Sand filter after the sedimentation tank . The design loading rate to the filter is 200m3/d.m2 How much filter surface area should be provided for the design flow rate of 0.5 m3/s

EXAMPLE

FILTRATION

Purpose to reduce the turbidity from 1- 10 TU to less than 0.3 TU

Slow sand filters Low filtration rate with the use of smaller sand Filter sand is less uniform Particles are removed on the surface of the filter

(forming a mat of materials , called schmultzdecke)

Schmultzdecke forms a complex of biological community that degrade some organic compounds.

Pretreatment is not important Require large area of land and are operator

intensive Loading rate 2.9-7.6 m3/d.m2

FILTRATION

Rapid sand filters ( most common) Graded (layered) within the bed to optimize

the passage of water while minimizing the passage of particulate mater

Cleaned in place by backwashing process Pretreatment to destabilize particles is

essential Loading rate 120 up to 235 m3/d.m2

For larger loading rate, a minimum of 4 filters is suggested

FILTRATION

FILTRATION

Dual-media Filters• Constructed of silica sand and anthracite coal. • Depth of sand is about 0.3m and coal 0.45 m. Size

and uniformity is selected to produce a distinct separation after backwashing

• Disadvantage is that filtered materials are held loosely in the anthracite layer and can dislodge with sudden changes in hydraulic loading. The material can then bind to the sand layer

• Loading rate up to 250 m3/d.m2 or more

FILTRATION

Mixed Media Filters• The perfect filter is composed of a grading of

large media at the top to small at the base. This is best achieved by the use of 3 or more media with ranging size, density and uniformity coefficient

• Typical installation – Overall bed depth 0.75m; 60% anthracite; 30% silica sand; 10% garnet sand

• Size range from 1.0mm anthracite to 0.15 garnet sand

• Filtration rates range from 10 to 20m/hr

FILTRATION

Terminal head loss: As the filter clogs, it become very hard to force water through the bed and the filter bed must be cleaned for backwashing process

Head loss: (pressure drops) that occurs when clean water flows through a bed of clean filter media. It is important in design value to determine the overall head require to operate the filter

Backwashing process; allowing large flow of water (clean water) to be pumped to enter the filter bed. This forces the sand bed to release the colloidal particles that trapped in the pores and released and escaped with the washwater.

FILTRATION

FILTRATION DESIGN

Key Elements

Hydraulics

Parameters to be measure during operation The head loss across the filter The turbidity of the effluent

DISINFECTION

Disinfection is used in water treatment to reduce pathogens (diseases-producing microorganisms) to an acceptable level

DISINFECTION- INTRODUCTION

Treatment of water with chemicals to kill bacteria”

Two objectives: Primary disinfection : Kill any pathogen in water Secondary (residual) disinfection : Prevent

pathogen re growth in the water. Method use :

Should be harmless and unobjectionable to the consumer

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

DISINFECTION- PROPERTIES

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

4. Not change water properties5. Have residual in a sufficient concentration

to provide protection against recontamination

DISINFECTION- PROPERTIES-CON’D

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

7) Dispensable at reasonable cost8) Safe and easy to store, transport, handle

and supply10) Not form toxic by-products due to their

reactions with any naturally occurring materials in water.

DISINFECTION- METHODS

1. Chlorination- chlorine2. Ultra violet Radiation3. Ozonation

DISINFECTION METHODS-CHLORINATION

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 –WATER REACTION

H2O+Cl2 (g) H+ + HOCl + Cl-

HOCl H++ OCl- (pH > 8) H++ OCl- HOCl (pH < 7)

HOCl= hypochlorous acidOCl- = hypochlorite ionHOCl is more effective than OCl-

Chlorine gas will be injected for water pH less than 3

CHLORINATION- AMMONIA REACTION

Ammonia chlorine HOCl+NH3 H2O+NH2Cl (Monochloramine)

HOCl+NH2Cl H2O+NHCl2 (Dichloramine*)

HOCl+NHCl2 H2O+NCl3 (Trichloramine)

*These reactions depend on : Low pH Low Temp Time high Cl2 : NH3 initial concentration

In favor of dicloramine

AMMONIA REACTION CON’D

The proportion of chloramines depends on 1. pH.2. Temperature3. Time4. Initial chlorine to ammonia ratio

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

CHLORINATION- ADVANTAGE 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- DISADVANTAGE

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

2) Formation of halogenated disinfectant by-products (DBPs). Includes trihalomethanes (THMs) such as 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.

CHLORINE EFFECTIVENESS -FACTORS

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.

DISINFECTION - OZONE Sweet smelling, unstable gas-Form of oxygen in which

three atoms of oxygen combined to form the molecule O3 O+O2 O3

Air in generating equipment contain up to 13% ozone.  Most powerful disinfectant and widely used in water More effective against cysts and viruses than free

chlorine. Leaving no taste or odor problems. Faster contact time No residual remain Not forming THM Unaffected by the pH or the ammonia content of the

water  High cost – Construction, Operation and Maintenance

DISINFECTION - UV Employed by submerged UV lamps into the water to

be treated Energy is absorbed by genetic material in the

microorganism, interfering with their ability to reproduce and survive.

Multiple lamps used to provide greater coverage Ability of UV lights to pass through the water to get

the target organism. Much higher energy level than visible light Potential to inactivate pathogens. Performs well against both bacteria and viruses. Leaves no residual protection for distribution system. High cost