Advanced Environmental Chemistry 3

8/12/2019 Advanced Environmental Chemistry 3

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CHAPTER3

DRINKING WATER TREATMENT

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Clean drinking water is the most important public

health factor.

But well over 2 billion people worldwide do not haveadequate supplies of safe drinking water.

Worldwide, between 15 to 20 million babies die

every year from water-borne diarrheal diseases such

as typhoid fever and cholera. Contaminated water supplies and poor sanitation

cause 80% of the diseases that afflict people in the

poorest countries.

The development of municipal water purification inthe last century has allowed cities in the developed

countries to be essentially free of water-carried

diseases. 2


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In 1974, discovered that water disinfectants react withorganic compounds form unintended disinfectionbyproducts (DBPs) causing health risks

(Trihalomethane DBPs were regulated by the EPA in1979)

Since then, several DBPs (bromodichloromethane,bromoform, chloroform, dichloroacetic acid, and bromate)

have been shown to be carcinogenic in laboratoryanimals at high doses..

EPA published guidelines for minimizing theirformation and established standards in 1998 fordrinking water concentrations of DBPs and disinfectant

residuals. Goal of EPA disinfectant and disinfection byproduct

regulations is to balance the health risks of pathogencontamination, normally controlled by waterdisinfection, against DPB formation.

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WATER TREATMENT

Major changes are occurring in the water treatment field

driven by increasingly tighter water quality standards, asteady increase in the number of regulated drinking watercontaminants (from about 5 in 1940 to around 100 in 1999),and new regulations affecting disinfection and disinfectionbyproducts.

Municipalities are constantly seeking to refine their water

treatment and provide higher quality water by moreeconomical means.

A recent development in water treatment is the application ofmembrane filtration to drinking water treatment. Membranefilters have been refined to the point where, in certain cases,they are suitable as stand-alone treatment for small systems.

More often, they are used in conjunction with other treatmentmethods to economically improve the overall quality offinished drinking water.

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BASIC DRINKING WATER TREATMENT

The purpose of water treatment:

(1) to make water safe to drink by ensuring that it is

free of pathogens and toxic substances,

(2) to make it a desirable drink by removing offensive

turbidity, tastes, colors, and odors.

4 steps in conventional drinking water treatment:

1) Primary settling

2) Aeration

3) Coagulation and filtration4) Disinfection

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PRIMARYSETTLING

Water, which has been coarsely screened to

remove large particulate matter, is brought into a

large holding basin to allow finer particulates to

settle.

Chemical coagulants are added to form floc.

Lime is added at this point to help clarification if pH

< 6.5.

The floc settles by gravity, removing solids larger

than about 25 microns.

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AERATION

The clarified water is agitated with air to promote oxidation

of any easily oxidizable substances

If chlorine were added at this point and reducing agents

were still in the water, they would reduce the chlorine and

make it ineffective as a disinfectant.

Fe2+is a particularly troublesome reducing agent. water passing through iron pyrite (FeS2) or iron carbonate

(FeCO3) minerals.

with dissolved oxygen present, Fe2+is oxidized to Fe3+

(precipitates as Fe(OH)3, at any pH greater than 3.5).

Fe(OH)3gives a metallic taste to the water and causes theugly red-brown stain commonly found in sinks and toilets in

iron-rich regions.

stain is easily removed with weak acid solutions, such as

vinegar.7


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COAGULATIONANDFILTRATION

The finest sediments (pollen, spores, bacteria, andcolloidal minerals) dont settle out in the primary settling

step.

Hydrated aluminum sulfate (Al2(SO4)318 H2O) = alum or

filter alum, applied with lime (Ca(OH)2):

Al2(SO4)3+ Ca(OH)2-> Al(OH)3(s) + CaSO4

At pH = 68, Al(OH)3(s) is near its minimum solubility and

formed as a light, fluffy, gelatinous flocculant having an

extremely large surface area that attracts and traps small

suspended particles, carrying them to the bottom of thetank as the precipitate slowly settles.

Additonal filtration with sand beds or membranes may be

used in a final polishing step before disinfection.8


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DISINFECTION

Killing bacteria and viruses is the most important part of

water treatment. Proper disinfection provides a residual disinfectant level

that persists throughout the distribution system (not onlykills organisms that pass through filtration andcoagulation at the treatment plant, preventing reinfectionduring the time the water is in the distribution system.

In a large city, water remain in the system for 5 days ormore before it is used. Five days is plenty of time for anymissed microorganisms to multiply.

Leaks and breaks in water mains can permitrecontamination, especially at the extremities of the

system where the pressure is low. High pressure causes the flow at leaks to always be

from the inside to the outside.

At low pressure, bacteria can seep in.9


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Concerns about DBPs, the EPA and the water

treatment industry are placing more emphasis on

the use of disinfectants other than chlorine, which

at present is the most commonly used water

disinfectant.

Another approach to reducing the probability of

DBP formation is by removing DBP precursors(naturally occurring organic matter) from water

before disinfection.

However, use of alternative disinfectants has also

been found to produce DBPs. Current regulations try to balance the risks between

microbial pathogens and DBPs10


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Halogenated organic compounds: trihalomethanes

(THMs), haloacetic acids, haloketones, and otherhalogenated compounds (formed primarily when chlorineor ozone (in the presence of bromide ion) used fordisinfection).

Organic oxidation byproducts: aldehydes, ketones,

assimilable organic carbon (AOC), and biodegradableorganic carbon (BDOC). AOC and BDOC (large organicmolecules) being oxidized to smaller molecules, which aremore available to microbes, plant, and aquatic life as anutrient source. Oxidized organics are formed when strongoxidizing agents (ozone, permanganate, chlorine dioxide, orhydroxyl radical) used.

Inorganic compounds: chlorate, chlorite, and bromate ionsare formed when chlorine dioxide and ozone disinfectantsare used. 11

DBPs


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DISINFECTIONPROCEDURES

Most disinfectants are strong oxidizing agents that react

with organic and inorganic oxidizable compounds in water. In some cases, the oxidant is produced as a reaction

byproduct (hydroxyl radical)

To destroy pathogens, disinfectants are also used forremoving disagreeable tastes, odors, and colors. They also

can assist in the oxidation of dissolved iron andmanganese, prevention of algal growth, improvement ofcoagulation and filtration efficiency, and control of nuisancewater organisms (Asiatic clams and zebra mussels).

Chlorine is commonly used in water treatment disinfectant

(first used in Belgium in the early 1900s) Other disinfectants sometimes used are ozone, chlorine

dioxide, and ultraviolet radiation.

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DISINFECTIONPROCEDURES

Only chlorine and chlorine dioxide have residual

disinfectant capability.Adding a small excess of disinfectant maintains protection

of the drinking water throughout the distribution system(residual chlorine or chlorine dioxide concentration of about0.2 to 0.5 mg/L).

Disinfectants (not provide residual protection) are normallyfollowed by a low dose of chlorine to preserve a disinfectioncapability throughout the distribution system.

Part of the disinfection procedure involves removing DBPprecursors, mainly total organic carbon (TOC), by

coagulation, water softening, or filtration.A high TOC concentration (greater than 2.0 mg/L) indicates

a high potential for DBP formation.

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Required Percentage Removal of Total Organic Carbon by

Enhanced Coagulation(a)for Conventional Water treatment Systems

Source Water TOC

(mg/L)

Source Water Alkalinity

(mg/L as CaCO3)

0 to 60 >60 to120 >120

>2.0 to 4.0

>4.0 to 8.0>8.0

35.0%

45.0%50.0%

25.0%

35.0%40.0%

15.0%

25.0%30.0%

(a)

Enhanced coagulation: the coagulant dose where an incrementaladdition of 10 mg/L of alum (or an equivalent amount of ferric salt)

results in a TOC removal to below 0.3 mg/L.


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DISINFECTION BYPRODUCTS AND

DISINFECTION RESIDUALS

Principal precursor of organic DBPs is naturally occurringorganic matter (NOM).

Halogenated organic byproducts: NOM reacts with freechlorine (Cl2) or free bromine (Br2).

Nonhalogenated DBPs: Reactions of strong oxidants(nonchlorine oxidants: ozone and peroxone) with NOM. Common nonhalogenated DBPs: aldehydes, ketones, organic acids,

ammonia, and hydrogen peroxide.

Br is present, especially where geothermal waters impact

surface and groundwaters, and in coastal areas (saltwaterincursion is occurring).

Ozone or free chlorine oxidizes Brto form brominatedDBPs: bromate ion, bromoform, cyanogen bromide,bromopicrin, and brominated acetic acid. 15


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STRATEGIES FOR CONTROLLING DISINFECTION

BYPRODUCTS

DBPs are difficult to remove from a water supply.

DBP control focused on preventing their formation:

Lowering NOM concentrations in source water by

coagulation, settling, filtering, and oxidation

Using sorption on granulated activated carbon (GAC) to

remove DOC Moving the disinfection step later in the treatment train,

so that it comes after all processes that decrease NOM

Limiting chlorine to providing residual disinfection,

following primary disinfection with ozone, chlorine dioxide,

chloramines, or ultraviolet radiation

Protection of source water from bromide ion

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The EPA classifications for carcinogenic potential of

chemicals (January 1999):

A: Human carcinogen; sufficient evidence in epidemiologicstudies to support causal association between exposure and

cancer.

B: Probable human carcinogen; limited evidence in

epidemiologic studies (B1) and/or sufficient evidence from

animal studies (B2).C: Possible human carcinogen; limited evidence from animal

studies and inadequate or no data in humans.

D: Not classifiable; inadequate or no animal and human

evidence of carcinogenicity.

E: No evidence of carcinogenicity for humans; no evidence ofcarcinogenicity in at least two adequate animal tests or in

adequate epidemiologic and animal studies.

Note: Not all of the EPA cancer classifications are found among the

listed disinfectants and DBPs. The EPA is in the process of revising

these cancerguidelines.


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CHLORINE DISINFECTION TREATMENT

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At room temperature, chlorine is a corrosive and toxic yellow-green gas with a strong, irritating odor, stored and shipped as aliquefied gas.

Chlorine is the most widely used water treatment disinfectantbecause of its many attractive features:

Effectively against a wide range of pathogens commonlyfound in water, particularly bacteria and viruses.

Leave a residual that stabilizes water in distribution systemsagainst reinfection.

Be economical and easily measured and controlled.

Used for a long time and represents a well-understoodtreatment technology.

Maintain an excellent safety record despite the hazards of

handling chlorine gas. Be available from sodium and calcium hypochlorite salts, as

well as from chlorine gas. For small treatment system,hypochlorite solutions are more economical and convenientthan chlorine gas.


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Besidedisinfection, chlorination used for:

Taste and odor control, including destruction ofhydrogen sulfide.

Color bleaching.

Controlling algal growth.

Precipitation of soluble iron and manganese.

Sterilizing and maintaining wells, water mains,

distribution pipelines, and filter systems.

Improving some coagulation processes.

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Problems with chlorine usage:

Not effective against Cryptosporidium and limitedeffectiveness against Giardia lambliaprotozoa.

Formation of undesirable DBPs when react with

NOM

Require special equipment and safety programsbecause of hazards of handling chlorine gas.

Arise taste and odor problems in site conditions

with high chlorine doses.

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Chlorine dissolves in water by the following

equilibrium reactions:

Cl2(g)Cl2(aq)

Cl2(aq) + H2OH+(aq) + Cl(aq) + HOCl(aq)

HOCl(aq)H+(aq) + OCl(aq)

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- pH < 7.5, hypochlorous acid (HOCl) is the dominant.

- pH > 7.5, chlorite anion (OCl) is dominant.

- Cl2exist only below about pH = 2

- Formation of H+chlorination reduces total alkalinity


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Definitions

Chlorine dose: the amount of chlorine originally

used. Chlorine residual: the amount remaining at time of

analysis.

Chlorine demand: the amount used up in oxidizing

organic substances and pathogens in the water,for example the difference between the chlorine

dose and the chlorine residual.

Free available chlorine: the total amount of HOCl

and ClOin solution. (Cl2is not present abovepH= 2.)

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All these species are oxidizing agents, but chloride

ion (Cl) is not.

HOCl is about 100 times more effective as adisinfectant than OClthe amount of chlorine

required for a given level of disinfection depends on

the pH.

Higher doses are needed at a higher pH. At pH 8.5,7.6 times as much chlorine must be used as at pH

7.0, for the same amount of disinfection.

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Relations among chlorine dose, chlorine

demand, and chlorine residual


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DRAWBACKS TO USE OF CHLORINE: DISINFECTION

BYPRODUCTS (DBPS)

Trihalomethanes (THMs)

Greatest problem concerns with the use of chlorine is the

formation of chlorination byproducts,particularly

trihalomethanes (CHCl3, CHBrCl2, CHBr2Cl, CHBr3, CHCl2I,

CHBrClI) and CCl4as possible carcinogens. THMs were formed by chlorination of dissolved methane

(reaction of HOCl with acetyl groups in NOM, chiefly

humic acids).

There is no evidence that chlorine itself is carcinogenic.

Addition of ammonia with chlorination forms chloramines -

weaker oxidants than chlorine and are useful for providing

a residual disinfectant capability with a lower potential for

forming DBPs. 27


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Chlorinated Phenols

Phenol and derivatives from industrial activitiesare in the waterproblem of taste and color.

Phenols are easily chlorinated, formingcompounds with very penetrating antisepticodors.

At the ppm level, chlorinated phenols can makewater completely unfit for drinking or cooking.

If phenol is present in the intake water, treatmentchoices are to employ additional nonchlorine

oxidation for removing phenol, to remove phenolwith activated charcoal, or to use a differentdisinfectant.

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CHLORAMINES

Many utilities use chlorine for disinfection and

chloramines for residual maintenance. Chloramines are formed in the reaction of ammonia

with HOCl (inexpensive and easy to control).

Reaction of chlorine with ammonia can be used for the

purpose of destroying ammonia and also serves togenerate chloramines (usefulness are more stable and

longer lasting in a water distribution system than is free

chlorine)

Chloramines are effective for controlling bacterialregrowth in water systems although they are not very

effective against viruses and protozoa.

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CHLORAMINES

Chloramines are not useful for oxidizing iron and

manganese. When chloramine disinfection is the goal, ammonia is added

in the final chlorination step (Chloramines are always

generated on site).

Optimal chloramine disinfection occurs when weight ratio of

chlorine:ammonia(N) around 4 before the chlorinationbreakpoint occurs. Monochloramine (NH2Cl) and

dichloramine (NHCl2) are the main reaction products and the

effective disinfectant species.

The normal dose of chloramines is between 1 and 4 mg/L. Residual concentrations are usually maintained between 0.5

and 1 mg/L.

The maximum residual disinfection level (MRDL) mandated

by the EPA is 4.0 mg/L.32


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Chlorine dioxide (ClO2) is a gas at T > 12C with high

water solubility. ClO2was first used as a municipal water disinfectant

in Niagara Falls, NY in 1944. In 1977, about 100municipalities in the U.S. and thousands in Europewere using it.

Unlike chlorine, it reacts quite slowly with water,remaining mostly dissolved as a neutral molecule.

It is a very good disinfectant, about twice as effectiveas HOCl from Cl2but also about twice as expensive.

Main drawback to its use is that it is unstable andcannot be stored. It must be made and used on site,whereas chlorine can be delivered in tank cars.

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CHLORINE DIOXIDE


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5 NaClO2+ 4 HCl4 ClO2(g) + 5 NaCl + 2 H2O.

2 NaClO2+ Cl2(g)2 ClO2(g) + NaCl.

2 NaClO2+ HOCl2 ClO2(g) + NaCl + NaOH.

Sodium chlorite is extremely reactive, especially in the

dry form, and it must be handled with care to prevent

potentially explosive conditions. If chlorine dioxide generator conditions are not

carefully controlled (pH, feedstock ratios, low feedstock

concentrations, etc.), the undesirable byproducts

chlorite (ClO2) and chlorate (ClO3) may be formed.

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Insufficiently high vapor pressures with chlorine dioxidesolutions < 10 g/L to create an explosive hazard under normalenvironmental conditions of temperature and pressure.

For drinking water treatment, ClO2solutions are generally lessthan 4 g/L and treatment levels generally are between 0.07 to2.0 mg/L.

ClO2is an oxidizer but not a chlorinating agenttrihalomethanes or chlorinated phenolsnot unformednoproblem in taste or odor.

Common applications for ClO2are to control taste and odorproblems associated with algae and decaying vegetation, toreduce the concentrations of phenolic compounds, and tooxidize iron and manganese to insoluble forms.

Chlorine dioxide can maintain a residual disinfection

concentration in distribution systems. The toxicity of ClO2restricts the maximum dose. At 50 ppm,

ClO2can cause breakdown of red corpuscles with the releaseof hemoglobin (dose of ClO2is limited to 1 ppm).

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Ozone use for water disinfection started in 1893 in the

Netherlands and in 1901 in Germany. Significant usein the U.S. did not occur until the 1980s. Ozone is oneof the most potent disinfectants used in watertreatment today.

Ozone (O3) is a colorless, highly corrosive gas at

room temperature, with a pungent odor that is easilydetectable at concentrations as low as 0.02 ppmv well below a hazardous level.

Strongest chemical oxidizing agents available, secondonly to hydroxyl free radical (HO), among

disinfectants commonly used in water treatment...

Ozone disinfection is effective against bacteria,viruses, and protozoan cysts, includingCryptosporidium and Giardi lamblia. 36

OZONE DISINFECTION TREATMENT


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Ozone is unstable, it cannot be stored and shippedefficiently.must be generated at the point of application.

Ozone gas is transferred to water through bubble diffusers,injectors, or turbine mixers. Once dissolved in water, ozonereacts with pathogens and oxidizable organic andinorganic compounds.

Undissolved gas is released to the surroundings as off-gas

and must be collected and destroyed by conversion backto oxygen before release to the atmosphere.,

Ozone is readily converted to oxygen by heating it toabove 350C or by passing it through a catalyst held above100C.

OSHA currently requires released gases to contain no

more than 0.1 ppmv of ozone for worker exposure. Typical dissolved ozone concentrations in water near an

ozonator are around 1 mg/L.

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OZONE DISINFECTION TREATMENT


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3 O2(g) + energy 2 O3(g)

high voltage

electric

discharge

(20,000 V)

(dry, pressurized air)

Ozonator off-gascontains as much as

3000 ppmv of ozone


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O3

+OH-

+ H2O2/HO2-

+UV

OH.

+M

Mox

+M

Off gas ozone

Direct Oxidation

Radical Oxidation

(indirect Oxidation)

ScavangersKHCO3-, KCO3

2-

Applied Ozone


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Cyclo addition (Criegee mechanism)

Consequentially to is dipolar structure, an ozone molecule can

undergo a 1-3 dipolar cyclo addition with saturizedcompounds (double or tripple bonds). This leads to theformation of a compound called ozonide (I):

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disintegration of ozonide


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Electrophilic reactions

Electrophilic reactions occur in molecular solutions that havea high electronic density and mainly in solutions that contain ahigh level of aromatic compounds.

Aromatic compounds that are substituted by electron donors(such as OH and NH2), have a high electronic density on thecarbon compounds in ortho and para position.Consequentially, in these positions aromatic compounds reactactively with ozone.

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Indirect reactions

- Contrary to those of ozone, OH-radical reactionsare largely selective.

- Indirect reactions in an ozone oxidation process

can be very complex.

1. Initiation

2. Radical chain-reaction

3. Termination

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1. Initiation

The first reaction that takes place is accelerated

ozone decomposition by a type of initiator. This can

be an OH-molecule:

1: O3+ OH-O2 - + HO2

This radical has an acid/ base equilibrium of pKa=

4,8. Above this value, this radical no longer splits,

because it forms a superoxide radical:

2: HO2O2-+ H+(pKa= 4,8)

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Radical chain-reaction

Now, a radical chain-reaction takes place, duringwhich OH-radicals are formed. The reactionmechanism is as follows:

3: O3

+ O2

-O3

-+ O2

4: O3-+ H+HO3 (PH < 8)

The OH-radicals that have formed react with ozone

according to the following reaction mechanism:

5: OH + O3HO4

6: HO4O2 + HO244


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Initiator Promotor Radical catcher(inhibitor)

OH- Humic acid HCO3-/CO3

2-

H2O2 Aryl-R PO34-

Fe2+ Primary and secondaryalcohols

Humic acids

Aryl-R

Tert-butyl alcohol (TBA)

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Advanced Environmental Chemistry 3