water – the problem of purity

8/14/2019 Water the Problem of Purity

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WATER THE PROBLEM OF PURITYWATER THE PROBLEM OF PURITY

In its pure state, water is one of the most aggressiveIn its pure state, water is one of the most aggressive

solvents known. Called the universal solvent,solvents known. Called the universal solvent,

water, to a certain degree, will dissolve virtuallywater, to a certain degree, will dissolve virtually

everything to which it is exposed. It will dissolve theeverything to which it is exposed. It will dissolve thequantity of material available until the solutionquantity of material available until the solution

reachesreaches saturationsaturation, the point at which no higher, the point at which no higher

level of solids can be dissolved.level of solids can be dissolved. ContaminantsContaminants

found in water include atmospheric gases, minerals,found in water include atmospheric gases, minerals,organic materials (some naturally occurring,othersorganic materials (some naturally occurring,others

man-made) plus any materials used to transport orman-made) plus any materials used to transport or

store water.store water.


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TheThe hydrologic cyclehydrologic cycle illustrates the process ofillustrates the process of

contamination and natural purification.contamination and natural purification.


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Natural Contamination andNatural Contamination and

PurificationPurification WaterWaterevaporatesevaporates from surface supplies andfrom surface supplies and transpirestranspires fromfrom

vegetation directly into the atmosphere.vegetation directly into the atmosphere.

The evaporated water then condenses in the cooler air on nucleiThe evaporated water then condenses in the cooler air on nuclei

such as dust particles and eventually returns to the earths surface assuch as dust particles and eventually returns to the earths surface asrain, snow, or otherrain, snow, or otherprecipitationprecipitation. It dissolves gases such as carbon. It dissolves gases such as carbon

dioxide, oxygen, and natural and industrial emissions such as nitricdioxide, oxygen, and natural and industrial emissions such as nitric

and sulfuric oxides, as well as carbon monoxide. Typical rain waterand sulfuric oxides, as well as carbon monoxide. Typical rain water

has ahas a pHpH of 5 to 6. The result of contact with higher levels of theseof 5 to 6. The result of contact with higher levels of these

dissolved gases is usually a mildly acidic condition what is todaydissolved gases is usually a mildly acidic condition what is today

called called acidacid rain that may have a pH as low as 4.0. rain that may have a pH as low as 4.0.


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As the precipitation nears the ground, it picks up many additionalAs the precipitation nears the ground, it picks up many additional

contaminants - airbornecontaminants - airborne particulatesparticulates, spores,, spores, bacteriabacteria, and emissions, and emissions

from countless other sources.from countless other sources.Most precipitation falls into the ocean, and some evaporates beforeMost precipitation falls into the ocean, and some evaporates before

reaching the earths surface. The precipitation that reaches landreaching the earths surface. The precipitation that reaches land

replenishesreplenishes groundwater aquifersgroundwater aquifers and surface water supplies.and surface water supplies.

The water that percolates down through theThe water that percolates down through the porousporous upper crust ofupper crust of

the earth is substantially filtered by that process. Most of thethe earth is substantially filtered by that process. Most of the

particulate matter is removed, much of the organic contaminationparticulate matter is removed, much of the organic contamination

is consumed by bacterial activity in the soil, and a relatively clean,is consumed by bacterial activity in the soil, and a relatively clean,

mildly acidic solution results. This acidic condition allows the watermildly acidic solution results. This acidic condition allows the water

to dissolve many minerals, especially limestone, which contributesto dissolve many minerals, especially limestone, which contributescalcium. Other geologic formations contribute minerals, such ascalcium. Other geologic formations contribute minerals, such as

magnesium, iron, sulfates and chlorides. The addition of thesemagnesium, iron, sulfates and chlorides. The addition of these

minerals usually raises groundwater pH to a range of 7 to 8.5.minerals usually raises groundwater pH to a range of 7 to 8.5.


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This mineral-bearing water is stored in natural undergroundThis mineral-bearing water is stored in natural underground

formations called aquifers. These are the source of the wellformations called aquifers. These are the source of the well

water used by homes, industries and municipalities.water used by homes, industries and municipalities.

Surface waters such as rivers, lakes and reservoirsSurface waters such as rivers, lakes and reservoirs

typically contain less mineral contamination because thattypically contain less mineral contamination because that

water did not pass through the earths soils. Surface waterswater did not pass through the earths soils. Surface waters

will, however, hold higher levels of organics andwill, however, hold higher levels of organics and

undissolved particles because the water has contactedundissolved particles because the water has contactedvegetation and caused runoff to pick up surface debris.vegetation and caused runoff to pick up surface debris.


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Bacterial Contamination

One difficulty of water purity is bacterial contamination

and control of bacterial growth. Water is essential for all

life. It is a necessary medium for bacterial growthbecause it carries nutrients. It is an essential component

of living cells. Its thermal stability provides a controlled

environment. Water will support bacterial growth with

even the most minute nutrient sources available.


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IDENTIFYING IMPURITIES

The impact of the various impuritiesThe impact of the various impurities

generated during the hydrologic cyclegenerated during the hydrologic cycle

and/or bacterial colonization dependsand/or bacterial colonization dependsupon the water users particularupon the water users particular

requirements. In order to assess the needrequirements. In order to assess the need

for treatment and the appropriatefor treatment and the appropriatetechnology, the specific contaminantstechnology, the specific contaminants

must be identified and measured.must be identified and measured.


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Specific Impurities

Many individual impurities can be quantified

through water analysis techniques. Below is adiscussion of most ionic individual contaminants.

Common Ions

A number of terms are used to express the level ofmineral contamination in a water supply.


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Common Ions

Iron Manganese Sulfate Chloride

Alkalinity ((CO3)2,(HCO3-) and (OH-)). Nitrate/ Nitrite Silica Aluminum Phosphate Sodium

Potassium


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Dissolved Gases

Carbon DioxideCarbon DioxideDissolved carbon dioxide (CODissolved carbon dioxide (CO22) associates with water molecules to form carbonic acid) associates with water molecules to form carbonic acid

(H(H22COCO33), reducing the pH and contributing to corrosion in water lines.), reducing the pH and contributing to corrosion in water lines.

OxygenOxygen

Dissolved oxygen (02) can corrode water lines, boilers and heat exchangers, but is

only soluble to about 14 ppm at atmospheric pressure.Hydrogen SulfideHydrogen Sulfide

The infamous rotten egg odor, hydrogen sulfide (H2S) can contribute to corrosion.

It is found primarily in well water supplies or other anaerobic sources.


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Heavy Metals

Heavy metals such as lead, arsenic, cadmium,selenium and chromium when present abovecertain levels can have harmful effects onhuman health. In addition, minuteconcentrations may interfere with themanufacture and effectiveness ofpharmaceutical products, as well as laboratoryand industrial processes of a sensitive nature.


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Dissolved Organic Compounds

Dissolved organic materials occur in water both as the product of

material decomposition and as pollution from synthetic compoundssuch as pesticides.

Naturally-OccurringNaturally-Occurring

Tannins, humic acid and fulvic acids are common natural

contaminants. They cause color in the water. They have no known health

consequences in normal concentrations. In the presence of free halogencompounds (principally chlorine or bromine), they form chlorinated

hydrocarbons and trihalomethanes (THMs), which are suspected

carcinogens.

Synthetic Organic Compounds (SOCs)Synthetic Organic Compounds (SOCs)

A wide variety of synthetic compounds which are potential health hazardsare present in water systems due to the use of industrial and agricultural

chemicals. These compounds are not readily biodegradable and leach

from soil or are carried by runoff into water sources.


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Volatile Organic Compounds (VOC)Volatile Organic Compounds (VOC)

Due to relatively low molecular weight, many synthetic organiccompounds such as carbon tetrachloride, chloroform and

methylene chloride will easily volatilize. Volatility is the tendency of

a compound to pass into the vapor state. Most are introduced into

the water supply in their liquid phase. If ingested they may be

absorbed into the bloodstream.

Radioactive ConstituentsRadioactive Constituents

Water in itself is not radioactive but may contain radionuclides. They

are introduced either as naturally-occurring isotopes (very rare) or

refined nuclear products from industrial or medical processes,

radioactive fallout or nuclear power plants.


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Microbiological Contamination

Microbiological contamination can be classified as viableand nonviable. Viable organisms are those that have

the ability to reproduce and proliferate. Nonviable

organisms cannot reproduce or multiply.

Bacterial ContaminationBacterial contamination is quantified as Colony Forming

Units (CFU), a measure of the total viable bacterial

population. CFUs are typically determined by

incubating a sample on a nutritional medium andcounting the number of bacterial colonies that grow.


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Pyrogenic Contamination

Pyrogens are substances that can induce a fever inPyrogens are substances that can induce a fever in

a warm-blooded animal. The most common pyrogenica warm-blooded animal. The most common pyrogenicsubstance is the bacterial endotoxin. Pyrogens aresubstance is the bacterial endotoxin. Pyrogens are

quantified as Endotoxin Units per milliliter (EU/mL).quantified as Endotoxin Units per milliliter (EU/mL).

Total Organic Carbon (TOC)

TOC is a direct measure of the organic, oxidizable,TOC is a direct measure of the organic, oxidizable,carbon-based material in water. TOC is a vitalcarbon-based material in water. TOC is a vital

measurement used in sophisticated water treatmentmeasurement used in sophisticated water treatment

systems such as electronics grade where anysystems such as electronics grade where any

amount of contamination can adversely affect productamount of contamination can adversely affect productquality and yield.quality and yield.


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Biological Oxygen Demand (BOD)

BOD is a measure of organic material contamination in water,

specified in mg/L. BOD is the amount of dissolved oxygen required forthe biochemical decomposition of organic compounds and the

oxidation of certain inorganic materials (e.g., iron, sulfites). Typically

the test for BOD is conducted over a five-day period.

Chemical Oxygen Demand (COD)

COD is the amount of dissolved oxygen required to cause chemical

oxidation of the organic material in water. Both BOD and COD are

key indicators of the environmental health of a surface water supply.They are commonly used in waste water treatment but rarely in

general water treatment.


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General Qualitative Identification

Qualitative identification is usually used to

describe the visible or aesthetic characteristics

of water. Among others these include: turbidity (clarity)

taste

color odor


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Turbidity

Turbidity consists of suspended material in water, causing a

cloudy appearance. This cloudy appearance is caused by the

scattering and absorption of light by these particles. The

suspended matter may be inorganic or organic. Generally thesmall size of the particles prevents rapid settling of the material

and the water must be treated to reduce its turbidity.

Turbidity can be measured by different optical systems. Such

measurements simply show the relative resistance to light

transmittance, not an absolute level of contamination.


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A candle turbidimeteris a very basic visual method

used to measure highly turbid water. Its results areexpressed in Jackson Turbidity Units (JTU). A

nephelometeris more useful in low turbidity water, with

results expressed in Nephelometric Turbidity Units

(NTU).

Suspended matter can also be expressed

quantitatively in parts per million (ppm) by weight or

milligrams per liter (mg/L). This is accomplished by

gravimetric analysis, typically filtering the sample through

a 0.45-micron membrane disc, then drying and weighing

the residue.


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Taste

The taste sense is moderately accurate and able

to detect concentrations from a few tenths to

several hundred ppm. However, taste oftencannot identify particular contaminants. A bad

taste may be an indication of harmful

contamination in drinking water, but certainly

cannot be relied on to detect all harmfulcontaminants.


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Color

Color is contributed primarily by organic material,

although some metal ions may also tint water. While not

typically a health concern, color does indicate a certain

level of impurities, and can be an aesthetic concern.True color refers to the color of a sample with its

turbidity removed. Turbidity contributes to apparent

color. Color can be measured by visual comparison of

samples with known concentrations of colored solutions.Color can also be measured using a spectrophotometer.


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Odor

The human nose is the most sensitive

odor-detecting device available. It can

detect odors in low concentrations downto parts per billion (ppb). Smell is useful

because it provides an early indication of

contamination which could be hazardous


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General Quantitative IdentificationGeneral Quantitative Identification

The relative acidic or basic level of a solution is measured by pH. ThepH is a measure of hydrogen ion concentration in water. Since pH isexpressed in log form, a pH of 6.0 is 10 times more acidic than apH of 7.0, and a pH of 5.0 is 100 times more acidic than a pH of7.0. The pH level can be determined by various means such ascolor indicators, pH paper or pH meters. A pH meter is the mostcommon and accurate means used to measure pH.

pH


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Total Solids

Total Solids (TS) (Table 1) is the sum of Total

Dissolved Solids (TDS) and Total Suspended Solids

(TSS). In water analysis these quantities are determined

gravimetrically by drying a sample and weighing the

residue.


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Conductivity/Resistivity

Ions conduct electricity. Because pure water

contains few ions, it has a high resistance to

electrical current. The measurement of waters

electrical conductivity, or resistivity, can provide an

assessment of total ionic concentration.

Conductivity is described in microSiemens/cm (S)

and is measured by a conductivity meter. Resistivityis described in megohm-cm, is the inverse of

conductivity and is measured by a resistivity meter.


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Water PurificationWater Purification

Water purification is the removal of contaminants fromWater purification is the removal of contaminants from

raw water to produce drinking water that is pure enoughraw water to produce drinking water that is pure enough

for human consumption or for industrial use. Substancesfor human consumption or for industrial use. Substances

that are removed during the process include bacteria,that are removed during the process include bacteria,minerals and man-made chemical pollutants. A smallminerals and man-made chemical pollutants. A small

amount of disinfectant is usually intentionally left in theamount of disinfectant is usually intentionally left in the

water at the end of the treatment process to reduce thewater at the end of the treatment process to reduce the

risk of re-contamination in the distribution system.risk of re-contamination in the distribution system.


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Sources of drinking waterSources of drinking water

1.1. Deep groundwaterDeep groundwater

2.2. Shallow groundwater (wells)Shallow groundwater (wells)

3.3. Upland lakes and reservoirsUpland lakes and reservoirs

4.4. Rivers, canals and low land reservoirsRivers, canals and low land reservoirs

5.5. Atmospheric water generationAtmospheric water generation

6.6. Rainwater harvesting or fog collectionRainwater harvesting or fog collection


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Municipal Water TreatmentMunicipal Water Treatment

There are three principal stages in waterThere are three principal stages in water

purification:-purification:-

1.1. Primary treatmentPrimary treatment

2.2. Secondary treatmentSecondary treatment

3.3. Tertiary treatmentTertiary treatment


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Primary TreatmentPrimary Treatment

Pumping and containmentPumping and containment - The majority of water must be pumped from its- The majority of water must be pumped from its

source and directed into pipes or holding tanks.source and directed into pipes or holding tanks.

ScreeningScreening - The first step in purifying surface water is to remove large debris- The first step in purifying surface water is to remove large debris

such as sticks, leaves, trash and other large particles which may interferesuch as sticks, leaves, trash and other large particles which may interferewith subsequent purification steps.with subsequent purification steps.

StorageStorage - Water from rivers may also be stored in bankside reservoirs for- Water from rivers may also be stored in bankside reservoirs for

periods between a few days and many months to allow natural biologicalperiods between a few days and many months to allow natural biological

purification to take place.purification to take place.

Pre-conditioningPre-conditioning - Many waters rich in hardness salts are treated with soda-- Many waters rich in hardness salts are treated with soda-

ash (Sodium carbonate) to precipitate calcium carbonate.ash (Sodium carbonate) to precipitate calcium carbonate.


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Secondary TreatmentSecondary TreatmentThis includes the techniques that can be used to removeThis includes the techniques that can be used to remove

the fine solids, micro-organisms and some dissolved inorganicthe fine solids, micro-organisms and some dissolved inorganic

and organic materials. The choice of method will depend onand organic materials. The choice of method will depend on

the quality of the water being treated, the cost of the treatmentthe quality of the water being treated, the cost of the treatment

process and the quality standards expected of the processedprocess and the quality standards expected of the processed

water.water.

pH AdjustmentpH Adjustment

If the water is acidic, lime or soda ash is added to raiseIf the water is acidic, lime or soda ash is added to raisethe pH. Lime is the more common of the two additivesthe pH. Lime is the more common of the two additives

because it is cheaper, but it also adds to the resulting waterbecause it is cheaper, but it also adds to the resulting water

hardness.hardness.


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Together, coagulation and flocculation are purification methods that work by using chemicalsTogether, coagulation and flocculation are purification methods that work by using chemicals

which effectively "glue" small suspended particles together, so that they settle out of thewhich effectively "glue" small suspended particles together, so that they settle out of the

water. Many of the suspended water particles have a negative electrical charge. The chargewater. Many of the suspended water particles have a negative electrical charge. The charge

keeps particles suspended because they repel similar particles. Coagulation works bykeeps particles suspended because they repel similar particles. Coagulation works by

eliminating the natural electrical charge of the suspended particles so they attract and stick toeliminating the natural electrical charge of the suspended particles so they attract and stick to

each other. The joining of the particles so that they will form larger settleable particles is calledeach other. The joining of the particles so that they will form larger settleable particles is calledflocculation. The larger formed particles are called flocs.flocculation. The larger formed particles are called flocs.

Coagulation and FlocculationCoagulation and Flocculation


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The coagulants include Fe and Al saltsThe coagulants include Fe and Al salts

Alum K2(SO4)3. 24H2OAlum K2(SO4)3. 24H2O

Ferrous Suphate Fe2SO4.7H2OFerrous Suphate Fe2SO4.7H2O

Sodium Aluminate NaALO2Sodium Aluminate NaALO2

The chosen coagulant and the raw water is slowlyThe chosen coagulant and the raw water is slowly

mixed in a large tank called a flocculation basin. The flocculationmixed in a large tank called a flocculation basin. The flocculation

paddles turn very slowly to minimize turbulence. The principlepaddles turn very slowly to minimize turbulence. The principle

involved is to allow as many particles to contact other particles asinvolved is to allow as many particles to contact other particles as

possible generating large and robust floc particles.possible generating large and robust floc particles.


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SedimentationSedimentationWater exiting the flocculation basin enters theWater exiting the flocculation basin enters the

sedimentation basin, also called a clarifier or settling basin. Itsedimentation basin, also called a clarifier or settling basin. It

is a large tank with slow flow, allowing floc to settle to theis a large tank with slow flow, allowing floc to settle to the

bottom. The outflow is typically over a weir so only a thin topbottom. The outflow is typically over a weir so only a thin toplayer-furthest from the sediment-exits. The amount of floc thatlayer-furthest from the sediment-exits. The amount of floc that

settles out of the water is dependent on thesettles out of the water is dependent on the timetime the waterthe water

spends in the basin and thespends in the basin and the depth of the basindepth of the basin..

As particles settle to the bottom of the basin aAs particles settle to the bottom of the basin a

layer of sludge is formed on the floor of the tank. This layer oflayer of sludge is formed on the floor of the tank. This layer of

sludge must be removed and treated.sludge must be removed and treated.


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FiltrationFiltration

After separating most floc, the water is filtered as the final step to removeAfter separating most floc, the water is filtered as the final step to remove

remaining suspended particles and unsettled floc. The most commonremaining suspended particles and unsettled floc. The most common

type of filter is a rapid sand filter. Water moves vertically through sandtype of filter is a rapid sand filter. Water moves vertically through sand

which often has a layer of activated carbon above the sand. The top layerwhich often has a layer of activated carbon above the sand. The top layer

removes organic compounds affecting taste and odour. Most particlesremoves organic compounds affecting taste and odour. Most particles

pass through surface layers but are trapped in pore spaces or adhere topass through surface layers but are trapped in pore spaces or adhere to

sand particles. This property of the filter is key to its operation: if the topsand particles. This property of the filter is key to its operation: if the top

layer of sand were to block all the particles, the filter would quickly clog.layer of sand were to block all the particles, the filter would quickly clog.


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Ultrafiltration MembranesUltrafiltration Membranes

Ultrafiltration membranes are a relatively newUltrafiltration membranes are a relatively newdevelopment; they use polymer film with chemicallydevelopment; they use polymer film with chemically

formed microscopic pores that can be used in place offormed microscopic pores that can be used in place of

granular media to filter water effectively withoutgranular media to filter water effectively without

coagulants. The type of membrane media determinescoagulants. The type of membrane media determines

how much pressure is needed to drive the water throughhow much pressure is needed to drive the water through

and what sizes of micro-organisms can be filtered out.and what sizes of micro-organisms can be filtered out.


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Tertiary TreatmentTertiary Treatment

Disinfection is normally the last step in purifyingDisinfection is normally the last step in purifying

drinking water. Water is disinfected to destroy anydrinking water. Water is disinfected to destroy any

pathogens which passed through the filters. Followingpathogens which passed through the filters. Following

the introduction of any chemical disinfecting agent, thethe introduction of any chemical disinfecting agent, the

water is usually held in temporary storage - often calledwater is usually held in temporary storage - often called

a contact tank or clear well to allow the disinfectinga contact tank or clear well to allow the disinfecting

action to complete.action to complete.


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ChlorineChlorine

The most common disinfection method is some form of chlorine which is a strongThe most common disinfection method is some form of chlorine which is a strong

oxidant that kills many micro-organisms.oxidant that kills many micro-organisms.

Because chlorine is a toxic gas, there is a danger of a release associated with its use.Because chlorine is a toxic gas, there is a danger of a release associated with its use.

This problem is avoided by the use of bleaching powder which is a relativelyThis problem is avoided by the use of bleaching powder which is a relatively

inexpensive solid that releases free chlorine when dissolved in water.inexpensive solid that releases free chlorine when dissolved in water.

CaOCl2 + H2O ---------------- Ca(OH)2 + Cl2CaOCl2 + H2O ---------------- Ca(OH)2 + Cl2

Cl2 + H2O --------------- 2HCL +[O]Cl2 + H2O --------------- 2HCL +[O]

Germ + [O] --------------- oxidized germGerm + [O] --------------- oxidized germ

Although chlorine is effective in killing bacteria, it has limited effectiveness againstAlthough chlorine is effective in killing bacteria, it has limited effectiveness against

protozoans that form cysts in water. Another drawback of chlorine gas is that it reactsprotozoans that form cysts in water. Another drawback of chlorine gas is that it reacts

with organic ompounds in the water to form potentially harmful levels of the chemical by-with organic ompounds in the water to form potentially harmful levels of the chemical by-products trihalomethanes (THMs). The formation of THMs is minimised by effectiveproducts trihalomethanes (THMs). The formation of THMs is minimised by effective

removal of as many organics from the water as possible before disinfection.removal of as many organics from the water as possible before disinfection.


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ChloraminesChloramines

Chloramines are another chlorine-based disinfectant.Chloramines are another chlorine-based disinfectant.

Although chloramines are not as effective asAlthough chloramines are not as effective as

disinfectants, compared to chlorine gas or sodiumdisinfectants, compared to chlorine gas or sodium

hypochlorite, they are less prone to form THMs.hypochlorite, they are less prone to form THMs.

ClNH2 + H2O --------- HOCL +NH3ClNH2 + H2O --------- HOCL +NH3

HOCl ---------- HCl + [O]HOCl ---------- HCl + [O]


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Ozone (O3)Ozone (O3)

Ozone is made by passing oxygen through ultraviolet light or aOzone is made by passing oxygen through ultraviolet light or a

electrical discharge. To use ozone as a disinfectant, it must beelectrical discharge. To use ozone as a disinfectant, it must be

created on site and added to the water by bubble contact. Some ofcreated on site and added to the water by bubble contact. Some of

the advantages of ozone include the production of relatively fewerthe advantages of ozone include the production of relatively fewer

dangerous by-products (in comparison to chlorination) and the lackdangerous by-products (in comparison to chlorination) and the lack

of taste and odor produced by ozonation. But disadvantages includeof taste and odor produced by ozonation. But disadvantages include

high cost and no residual disinfectant in the water.high cost and no residual disinfectant in the water.

O3 ----------- O2 + [O]O3 ----------- O2 + [O]


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UV RadiationUV Radiation

UV radiation is very effective at inactivatingUV radiation is very effective at inactivating

cysts, as long as the water has a low level ofcysts, as long as the water has a low level of

colour so the UV can pass through without beingcolour so the UV can pass through without beingabsorbed.absorbed.


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Because neither ozone nor UV radiation leaves aBecause neither ozone nor UV radiation leaves a

residual disinfectant in the water, it is sometimesresidual disinfectant in the water, it is sometimes

necessary to add a residual disinfectant after they arenecessary to add a residual disinfectant after they are

used. This is often done through the addition ofused. This is often done through the addition of

chloramines, discussed above as a primary disinfectant.chloramines, discussed above as a primary disinfectant.

When used in this manner, chloramines provide anWhen used in this manner, chloramines provide an

effective residual disinfectant with very little of theeffective residual disinfectant with very little of the

negative aspects of chlorination.negative aspects of chlorination.

water – the problem of purity

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