Standard Methods for Examination of Water & Wastewater_16 doc

For example, when using 2.0 mg/L of applied chlorine dosage, for bacterial cultures of about 10 days old, it takes 30 min of contact time to produce the same reduction as for young cultu

17.1 METHODS OF DISINFECTION AND DISINFECTANT AGENTS USED

Generally, two methods of disinfection are used: chemical and physical The ical methods, of course, use chemical agents, and the physical methods use physicalagents Historically, the most widely used chemical agent is chlorine Other chemicalagents that have been used include ozone, ClO2, the halogens bromine and iodine andbromine chloride, the metals copper and silver, KMnO4, phenol and phenolic com-pounds, alcohols, soaps and detergents, quaternary ammonium salts, hydrogen per-oxide, and various alkalis and acids

chem-As a strong oxidant, ClO2 is similar to ozone (Ozone will be discussed ically later in this chapter.) It does not form trihalomethanes that are disinfectionby-products and suspected to be carcinogens Also, ClO2 is particularly effective indestroying phenolic compounds that often cause severe taste and odor problemswhen reacted with chlorine Similar to the use of chlorine, it produces measurableresidual disinfectants ClO2 is a gas and its contact with light causes it to photoox-idize, however Thus, it must be generated on-site Although its principal applicationhas been in wastewater disinfection, chlorine dioxide has been used in potable watertreatment for oxidizing manganese and iron and for the removal of taste and odor.Its probable conversion to chlorate, a substance toxic to humans, makes its use forpotable water treatment questionable

specif-The physical agents of disinfection that have been used include ultraviolet light(UV), electron beam, gamma-ray irradiation, sonification, and heat (Bryan, 1990;Kawakami et al., 1978; Hashimoto et al., 1980) Gamma rays are emitted fromradioisotopes, such as cobalt-60, which, because of their penetrating power, havebeen used to disinfect water and wastewater The electron beam uses an electrongenerator A beam of these electrons is then directed into a flowing water or wastewater

to be disinfected For the method to be effective, the liquid must flow in thin layers

17

Several theories have been proposed as to its mechanics of disinfection, includingthe production of intermediates and free radicals as the beam hits the water Theseintermediates and free radicals are very reactive and are thought to possess the disin-fecting power In sonification, high-frequency ultrasonic sound waves are produced

by a vibrating-disk generator These waves rattle microorganisms and break them intosmall pieces Ultraviolet light will be addressed specifically later in this chapter

In general, the effect of disinfectants is thought to occur as a result of damage

to the cell wall, alteration of cell permeability, alteration of the protoplasm, andinhibition of enzymatic activities Damage to the cell wall results in cell lysis anddeath Some agents such as phenolic compounds and detergents alter the permeabil-ity of the cytoplasmic membrane This causes the membrane to lose selectivity tosubstances and allow important nutrients such as phosphorus and nitrogen to escapethe cell Heat will coagulate protoplasm and acids and alkali will denature it causingalteration of the structure and producing a lethal effect on the microorganism Finally,oxidants, such as chlorine, can cause the rearrangement of the structure of enzymes.This rearrangement will inhibit enzymatic activities

17.2 FACTORS AFFECTING DISINFECTION

The effectivity of disinfectants are affected by the following factors: time of contactbetween disinfectant and the microorganism and the intensity of the disinfectant,age of the microorganism, nature of the suspending liquid, and temperature Each

of these factors are discussed next

17.2.1 T IME OF C ONTACT AND I NTENSITY OF D ISINFECTANT

In the context of how we use the term, intensity refers to the intensive property of thedisinfectant Intensive properties, in turn, are those properties that are independent ofthe total mass or volume of the disinfectant For example, concentrations are expressed

as mass per unit volume; the phrase “per unit volume” makes concentration dent of the total volume Hence, concentration is an intensive property and it expressesthe intensity of the disinfectant Another intensive property is radiation from an ultra-violet light This radiation is measured as power impinging upon a square unit of area.The “per unit area” here is analogous to the “per unit volume.” Thus, radiation isindependent of total area and is, therefore, an intensive property that expresses theintensity of the radiation, which, in this case, is the intensity of radiation of theultraviolet light

indepen-It is a universal fact that the time needed to kill a given percentage of ganisms decreases as the intensity of the disinfectant increases, and the time needed

microor-to kill the same percentage of microorganisms increases as the intensity of thedisinfectant decreases, therefore, the time to kill and the intensity are in inverse ratio

to each other Let the time be t and the intensity be I Thus, mathematically,

(17.1)

t∞ 1

I m

-Disinfection 741

relationship more general

Letting k be the proportionality constant, the equation becomes

(17.2)

In this equation, if I m is multiplied by t, and if I is expressed as the concentration ofthe disinfectant C in mg/L, the equation is the famous Ct concept with m equal to 1and t in minutes Ct values at given temperatures and pH are tabulated in Ct tablesused by regulating authorities and by the U.S Environmental Protection Agency Thetime to kill t is synonymous with the time of inactivation of the microorganisms.The constants may be obtained from experimental data by converting the aboveequation first into an equation of a straight line Taking the logarithms of both sides,

(17.3)This equation is the equation of the straight line with y-intercept ln k and slope m.The constants may then be solved using experimental data

Assume n experimental data points, and divide them into two groups Let there

be l data points in the first group; the second group would have m−l data points.From analytic geometry,

of dose is lethal when administered in a sufficient amount of contact time as calculatedfrom the equation We call Equation (17.2) the Universal Law of Disinfection.

used to disinfect a secondary-treated sewage discharge To determine the contacttime, an experiment was conducted producing the following results:

Contact Time (min/residual fecal coliforms) (No./100 mL)

=

Determine the contact time to be used in design, if it is desired to have a log 2

removal efficiency for fecal coliforms Calculate the Ct value The original

concen-tration of fecal coliforms is 40,000 per/100 mL

first assuming any original value of the concentration of the microorganisms x1,

computing the next value x2 based on the given log removal, and computing the

corresponding percentage Thus, let x1= 8888888 Then,

be 99 Thus, log 2 removal is equal to 99% removal or 99% inactivation

From the previous table, we produce the following table for the time to effect

60 – 30 - 0.99–0.985

0.985 – 0.90 -

0.98975 – 0.995 -

=

The effectiveness of a disinfectant also depends upon the age of the microorganism.

For example, young bacteria can easily be killed, while old bacteria are resistant

As the bacterium ages, a polysaccharide sheath is developed around the cell wall;

this contributes to the resistance against disinfectants For example, when using

2.0 mg/L of applied chlorine dosage, for bacterial cultures of about 10 days old, it

takes 30 min of contact time to produce the same reduction as for young cultures of

about one day old dosed with one minute of contact time In the extreme case are

the bacterial spores; they are, indeed, very resistant and many of the chemical

disin-fectants normally used have little or no effect on them

BrCl Dosage (mg/L)

–

– -;

-–

t1ln l

- m ∑1l

I i ln l

- 1.28

1 -–

-–

6.3381 2 -–4.12336.558

2 -–1.28 -

1 -+0.477 1.28{ }

17.2.3 N ATURE OF THE S USPENDING F LUID

In addition to the time of contact and age of the microorganism, the nature of thesuspending fluid also affects the effectiveness of a given disinfectant For example,extraneous materials such ferrous, manganous, hydrogen sulfide, and nitrates reactwith applied chlorine before the chlorine can do its job of disinfecting Also, theturbidities of the water reduces disinfectant effectiveness by shielding the microor-ganism Hence, for most effective kills, the fluid should be free of turbidities

17.2.4 E FFECT OF T EMPERATURE

We have learned from previous chapters that equilibrium and reaction constants areaffected by temperature The length of time that a disinfection process proceeds is afunction of the constants of the underlying reaction between the microorganism andthe disinfectant; thus, it must also be a function of temperature The variation of thecontact time to effect a given percentage kill with respect to temperature can therefore

be modeled by means of the Van’t Hoff equation This equation was derived for

the equilibrium constants in Chapter 11, which is reproduced next:

(17.6)

K T1 and K T2 are the equilibrium constants at temperatures T1 and T2, respectively

is the standard enthalpy change of the reaction and R is the universal gas constant If K T1 is replaced by contact time t T1 at temperature T1 and K T2 is replaced

by contact time t T2 at temperature T2, the resulting equation would show that as the

temperature increases, the contact time to kill the same percentage of isms also increases Of course, this is not true Thus, the replacement should be theother way around Doing this is the same as interchanging the places in the difference

microorgan-term between T1 and T2 inside the exp function Thus, doing the interchanging,

(17.7)

Table 17.1 shows the standard enthalpy change as a function of pH for both aqueouschlorine and chloramines, and Table 17.2 shows the various possible values of theuniversal gas constant

7.0 and a temperature of 25°C is 30 min What would be the contact time to effectthe same percentage kill if the process is conducted at a temperature 18°C?

Solution:

K T 2=K T 1 ∆H298

o

RT1T2 - T( 2–T1)

8.5 58,615 9.5 83,736

gmmole K⋅ ° - gmmoles

8.315 298( ) 291( ) - 298( –291)

17.3 OTHER DISINFECTION FORMULAS

The literature reveals other disinfection formulas These include Chick’s law for contact

time, modifications of Chick’s law, and relationship between concentration of tant and concentration of microorganisms reduced in a given percentage kill Chick’s

disinfec-law and its modification called the Chick–Watson model, however, are not useful

for-mulas, because they do not incorporate either the concentration of the disinfectant that

is needed to kill the microorganisms or the incorporation of the concentration is incorrect.The relationship of the concentration of disinfectant and the concentration of the micro-organisms is also not a useful formula, since it does not incorporate the contact timerequired to kill the microorganisms It must be noted that for a formula to be useful, itmust incorporate both the concentration (intensity) of the disinfectant and the contacttime corresponding to this concentration effecting a given percentage kill For thesereasons, these other disinfection formulas are not discussed in this book

The Chick–Watson model needs to be addressed further Watson explicitly

expressed the constant k in Chick’s law in terms of the concentration of disinfectant

C as αC n

, where α is an activation constant and n is another constant termed the constant of dilution Chick’s Law, thus, became dN /dt = −αC n

dt, where N is the concentration of microorganisms and t is time Note that C is a function of time.

When this equation was integrated, however, it was assumed constant, thus producingthe famous Chick–Watson model,

where N o is the initial concentration of microorganisms Because the concentration

C was assumed constant with time during integration, this equation is incorrect and,

therefore, not used in this book

17.4 CHLORINE DISINFECTANTS

The first use of chlorine as a disinfectant in America was in New Jersey in the year

1908 (Leal, 1909) At that time George A Johnson and John L Leal chlorinated thewater supply of Jersey City, NJ

The principal compounds of chlorine that are used in water and wastewatertreatment are the molecular chlorine (Cl2), calcium hypochlorite [Ca(OCL)2], andsodium hypochlorite [NaOCl] Sodium hypochlorite is ordinary bleach Chlorine is

a pale-green gas, which turns into a yellow-green liquid when pressurized Both theaqueous and liquid chlorine react with water to form hydrated chlorine Below 9.4°C,liquid chlorine forms the compound Cl2 · 8H2O

Chlorine gas is supplied from liquid chlorine that is shipped in pressurized steelcylinders ranging in size from 45 kg and 68 kg to one tonne containers It is alsoshipped in multiunit tank cars that can contain fifteen 1-tonne containers and tankcars having capacities of 15, 27, and 50 tonnes

In handling chlorine gas, the following points are important to consider:

• Chlorine gas is very poisonous and corrosive Therefore, adequate lation should be provided In the construction of the ventilation system,

the capturing hood vents should be placed at floor level, because the gas

is heavier than air

• The storage area for chlorine should be walled off from the rest of the plant.There should be appropriate signs posted in front of the door and back

of the building Gas masks should be provided at all doors and exits should

be provided with clearly visible signs

• Chlorine solutions are very corrosive and should therefore be transported

in plastic pipes

• The use of calcium hypochlorite or sodium hypochlorite as opposed tochlorine gas should be carefully considered when using chlorination inplants located near residential areas Accidental release of the gas couldendanger the community Normally, small plants that usually lack well-trained personnel, should not use gaseous chlorine for disinfection

Calcium hypochlorite is available in powder or granular forms and compressedtablets or pellets Depending upon the source of the chemical, a wide variety ofcontainer sizes and shapes are available Because it can oxidize other materials, calciumhypochlorite should be stored in a cool dry place and in corrosion-resistant containers.High-test calcium hypochlorite, HTH, contains about 70% chlorine (Available chlo-rine will be defined later.)

Sodium hypochlorite is available in solution form in strengths of 1.5 to 15%with 3% the usual maximum strength The solution decomposes readily at highconcentrations Because it is also affected by heat and light, it must be stored in acool dry place and in corrosion-resistant containers The solution should be trans-ported in plastic pipes Sodium hypochlorite can contain 5 to 15% available chlorine

17.4.1 C HLORINE C HEMISTRY

The chemistry of chlorine discussed in this section includes hydrolysis and optimum

pH range of chlorination, expression of chlorine disinfectant concentration, reactionmediated by sunlight, reactions with inorganics, reactions with ammonia, reactionswith organic nitrogen, breakpoint reaction, reactions with phenols, formation oftrihalomethanes, acid generation, and available chlorine

Chlorine has the electronic configuration of [Ne]3s23p5 and is located in GroupVIIA of the Periodic Table in the third period [Ne] means that this element has the

electronic configuration of the noble gas neon The letters p and s refer to the p and

s orbitals; the superscripts indicate the number of electrons that the orbitals contain Thus, the p orbital contains 5 electrons and the s orbital contains two electrons, making

a total of seven electrons in its valence shell This means that the chlorine atom needs

to acquire only one more electron to attain the neon configuration for stability Thismakes chlorine a very good oxidizer In fact, it is a characteristic of Group VIIA toattain a charge of −1 when the members of this group oxidizes other substances Themembers of this group starting from the strongest oxidizer to the least are fluorine,chlorine, bromine, iodine, and astatine This group forms the family of elements called

the halogen family.

All the chlorine disinfectants reduce to the chloride ion (Cl−) when they oxidizeother substances, which must, of course, be reducing substances The chlorine starts

with an oxidation state of zero and ends up with a −1; it only needs one reduction step.One the other hand, the hypochlorites start with oxidation states of +1 and end up withalso a −1; thus, they need two reduction steps Because the chlorine atom only needsone reduction step, while the hypochlorites need two, the chlorine atom is a strongeroxidizer than the hypochlorites As a stronger oxidizer, it is also a stronger disinfectant.

Hydrolysis and optimum pH range of chlorination As previously mentioned,

chlorine is supplied in the form of liquefied chlorine The liquid must then be rated into a gas As the gas, Cl2(g), is applied into the water or wastewater, it dissolvesinto aqueous chlorine, Cl2(aq), as follows:

evapo-(17.8)

Cl2(aq) then hydrolyzes, one of the chlorine atoms being oxidized to +1 and the otherreduced to −1 This reaction is called disproportionation The reaction is as follows:

(17.9)From Equation (17.9), the hypochlorous acid, HOCl, is formed, which is one ofthe chlorine disinfectants If its formula is analyzed, it will be found that the chlorinehas an oxidation state of +1, as we mentioned before Note also that hydrochloric acid

is formed This is a characteristic in the use of the chlorine gas as a disinfectant Thewater becomes acidic Also, as we have mentioned, the chlorine molecule is a muchstronger oxidizer than the hypochlorite ion and, hence, a stronger disinfectant FromEquation (17.9), if the water is intentionally made acidic, the reaction will be driven

to the left, producing more of the chlorine molecule This condition will then producemore disinfecting power As will be shown later, however, this condition, where thechlorine molecule will exist, is at a very low pH hovering around zero This makesthe chlorine molecule useless as a disinfectant

HOCl further reacts to produce the following dissociation reaction:

(17.10)Using Equation (17.9), let us calculate the distribution of Cl2(aq) and HOCl.Expressing in the form of equilibrium equation,

(17.11)

Taking logarithms, rearranging, and simplifying,

(17.12)

pKH is the negative logarithm to the base 10 of KH

Table 17.3 shows the ratios of [Cl2(aq)]/[HOCl] and [HOCl]/[Cl2(aq)] as functions

of pH and the chloride concentrations, using Equation (17.12) The concentration of1.0 gmmole/L of chloride is 35,500 mg/L This will never be encountered in thenormal treatment of water and wastewater Disregarding this entry in the table, the

concentration of Cl2(aq) is already practically nonexistent at around pH 4.0 and above.

In fact, it is even practically nonexistent at pH’s less than 4, except when the pH isclose to zero and chloride concentration of 0.1 gmmol/L; but, 0.1 gmmol/L is equal

to 3,500 mg/L, which is already a very high chloride concentration and will not beencountered in the treatment of water and wastewater Practically, then, for conditionsencountered in practice, at pH’s greater than 4.0, [HOCl] predominates over Cl2(aq).Now, using Equation (17.10), let us calculate the distribution of HOCl and OCl−.Note that from the previous result, HOCl predominates over Cl2(aq) above pH 4.0,

Cl2(aq) being practically zero Thus, above this pH, the distribution of the chlorinedisinfectant species will simply be for HOCl and OCl− Expressing Equation (17.10)

in the form of the equilibrium equation,

(17.13)

Taking logarithms, rearranging, and simplifying,

(17.14)

pK a is the negative logarithm to the base 10 of K a

Table 17.4 shows the ratios of [OCl−]/[HOCl] and [HOCl]/[OCl−] as functions

of pH using Equation (17.14) This table shows that HOCl predominates over OCl−

at pH’s less than 7.5 Also considering Table 17.3, we make the conclusion thatfor all practical purposes, HOCl predominates over all chlorine disinfectant species

- [HOCl]

[Cl2(aq)]

-[Cl2(aq)] HOCl

in all pH ranges up to less than 7.5 At exactly pH 7.5, the concentrations of HOCland OCl− are equal and above this pH, OCl− predominates over all chlorine disinfectantspecies This reality is more than just a theoretical interest, because HOCl is 80 to100% more effective than OCl− as a disinfectant (Snoeyink and Jenkins, 1980) Wenow conclude that the optimum pH range for chlorination is up to 7.0 Beyond thisrange, OCl− predominates and the disinfection becomes less effective.

The three species Cl2(aq), HOCl, and OCl− are called free chlorine Although

Cl2(aq) is a stronger oxidizer than the other two, it is not really of much use, unlessthe chlorination is done under a very acidic condition

Example 17.3 At pH 6.0, calculate the mole fraction of HOCl.

Table 17.4, the mole ratio of HOCl to OCl− at this pH is 31.62

Therefore,

Expression of chlorine disinfectant concentration Now that we have detailed

the various reactions of the chlorine disinfectants, it is time to unify the tions of the chlorine species By convention, the concentrations of the three speciesare expressed in terms of the molecular chlorine, Cl2 The pertinent reactions arewritten as follows:

concentra-(17.15)

(17.16)(17.17)

- [HOCl]

-[OCl−−−−] [HOCl]

=31.6231.62+ +1 0 - 0.97 Ans

Cl2 aq( )+H2O
HOCl+H++Cl−1NaOCl
Na++OCl−

Ca OCl( )2
Ca2++2OCl−

From these reactions,

equa-is Cl2

Reaction mediated by sunlight Aqueous chlorine is not stable in the presence

of sunlight Sunlight contains ultraviolet light This radiation provides the energythat drives the chemical reaction for breaking up the molecule of hypochlorous acid.The water molecule breaks up, first releasing electrons that are then needed to reducethe chlorine atom in HOCl to chloride The overall reaction is as follows:

(17.21)

The O2 comes from the break up of the water molecule, oxidizing its oxygen atom

to the molecular oxygen

The previous reaction in the presence of sunlight is very significant If the fectant is to be stored in plastic containers, then this container must be made opaque;otherwise, the chlorine gas will be converted to hydrochloric acid, and the hypochlo-rites will be converted into the corresponding salts of calcium and sodium

is stored in a transparent plastic container It had been stored outdoors for quite sometime and then used to disinfect a swimming pool How effective is the disinfection?What material is used instead for the disinfection? And how many kilograms is it?

con-tainer, the following reaction occurs:

From this reaction, no disinfectant exists in the container and the disinfection is not

effective Ans

The material used instead for disinfection is the salt NaCl Ans

The amount of sodium chloride used to disinfect is

HOCl - 2 35.5( )

1.008+16 +35.5 - 1.35

NaOCl - 2 35.5( )

23+16 +35.5 - 0.95

2HOCl→2H++2Cl−+O2

2NaOCl→2Na++2Cl−+O2

NaClNaOCl - 50( ) 23+35.5

23+16+35.5 - 50( ) 39.26

Reactions with inorganics Reducing substances that could be present in the raw

water and raw wastewater and treated water and treated wastewater are ferrous, ganous, nitrites, and hydrogen sulfide Thus, these are the major substances that caninterfere with the effectiveness of chlorine as a disinfectant The interfering reactionsare written as follows:

man-with ferrous:

(17.22)(17.23)––––––––––––––––––––––––––––––––––

(17.24)

with manganous:

(17.25)(17.26)–––––––––––––––––––––––––––––––––

(17.27)

with nitrites:

(17.28)(17.29)–––––––––––––––––––––––––––

(17.30)

with hydrogen sulfide:

(17.31)(17.32)–––––––––––––––––––––––––––––––

(17.33)

For activated sludge plants that produce only partial nitrification, it is a commoncomplaint of operators that a residual chlorine cannot be obtained at the effluent.The reason for this is the reaction of nitrites with the chlorine disinfectant producingnitrates as shown in Reaction (17.30)

order, and a decision has been made following approval from a state agency todischarge raw sewage to a river The effluent was found to contain 8 mg/L of ferrous,

3 mg/L manganous, 20 mg/L nitrite as nitrogen, and 4 mg/L hydrogen sulfide.Calculate the mg/L of HTH needed to be dosed before actual disinfection is realized.What is the chlorine concentration?

2Fe2+→2Fe3++2e−HOCl+H++2e−→Cl−+H2O2Fe2++HOCl+H+→2Fe3++Cl−+H2O

Mn2+→Mn4++2e−HOCl+H++2e−→Cl−+H2O

Mn2++HOCl+H+→Mn4++Cl−+H2O

NO2−+H2O→NO3−+2H++2e−HOCl+H++2e−→Cl−+H2O

NO2−+HOCl→NO3−+Cl−+H+

H2S+4H2O→SO42−+10H++8e−4HOCl+4H++8e−→4Cl−+4H2O

H2S+4HOCl→SO42−+4Cl−+6H+

Solution: Examining Reactions (17.22) to (17.33) reveals that the number ofreference species is equal to two moles of electrons except Reaction (17.33), whichhas eight moles of electrons By considering all the other reactions, the number ofmilliequivalents of HOCl needed

Therefore,

From Equation (17.20), the chlorine concentration = 73.69(0.99) = 72.95 mg/L

Ans Reactions with ammonia and optimum pH range for chloramine formation.

Effluents from sewage treatment plants can contain significant amounts of ammoniathat when disinfected, instead of finding free chlorine, substitution products of ammonia

called chloramines are found In addition, in water treatment plants, ammonia are

often purposely added to chlorine This, again, also forms the chloramines Chloraminesare disinfectants like chlorine, but they are slow reacting, and it is this slow-reactingproperty that is the reason why ammonia is used The purpose is to provide residualdisinfectant in the distribution system In other words, the formation of chloraminesassures that when the water arrives at the tap of the consumer, a certain amount ofdisinfectant still exists

The formation of chloramines is a stepwise reaction sequence When ammoniaand chlorine are injected into the water that is to be disinfected, the following reactionsoccur, one after the other in a stepwise manner

(17.34)(17.35)(17.36)

2 -

2 1.008( +16+35.5) -

First, it is to be noted that the reaction is expressed in terms of HOCl By theequivalence of reactions, however, the above reactions can be manipulated if theequivalent amount of the other two species is desired to be known In monochloram-ines and dichloramines, therefore, the chlorine is combined in ammonia; they are

called combined chlorines As will be shown in subsequent discussions, the

concen-tration of trichloramine is practically zero during disinfection; thus, it is not included

in the definition of combined chlorine

Reaction (17.34) indicates that at the time when one mole of HOCl is added toone mole of NH3, the conversion into monochloramine is essentially complete Inview of the relationship of HOCl and OCl− as a function of pH, however, thisstatement is not exactly correct From previous discussions, at pH 7.5, hypochlorousacid and the hypochlorite ion exist in equal mole concentrations, but beyond pH 7.5,the hypochlorite ion predominates OCl− does not directly react with NH3 to formthe monochloramine, but must first hydrolyze to produce the HOCl before Reaction(17.34) proceeds Thus, when the pH is above 7.5, addition of one mole of HOCl

to one mole of ammonia does not guarantee complete conversion into NH2Cl Atthese pH values, the one mole of HOCl added becomes lesser, because of thepredominance of the hypochlorite ion HOCl, however, exists at practically 100concentrations at pH’s below 7.0; hence, at this range, a mole for mole additionwould essentially guarantee the aforementioned conversion into monochloramine.Now, Reactions (17.35) and (17.36) indicate that by adding two moles of HOCland three moles of HOCl, the conversion into dichloramine and the trichloramine are,respectively, essentially complete For the same reason as in the case of the conversioninto monochloramine, these two and three moles are not really these values, becausethe resulting concentrations depend upon the pH of the solution Above pH 7.5, theconversions are not complete

Let us have more discussion regarding the formation of dichloramine The tion state of nitrogen in NH2Cl from where the dichloramine comes from is −1 Theoxidation state of the nitrogen in NHCl2, itself, is +1 This means that in order to formthe dichloramine, two electrons must be abstracted from the nitrogen atom Now, theother substances that have been observed to occur, as the amount of hypochlorous acidadded is increased, are the nitrogen gas and nitrates Take the case of the nitrogen gas.The oxidation state of the N atom in the N2 molecule is zero This means that in order

oxida-to form the nitrogen gas from NH2Cl only one electron needs to be abstracted fromthe nitrogen atom; this is an easier process than abstracting two electrons It must then

be concluded that before the dichloramine is formed, the gas must have already beenforming, and that for the dichloramine to be formed, more HOCl is needed than isneeded for the formation of the gas

The reaction for the formation of the nitrogen gas may be written as follows:

(17.37)(17.38)––––––––––––––––––––––––––––––––––––––

(17.39)

2NH2Cl→N2 g( )+2Cl−+4H++2e−HOCl H+ 2e−→ Cl−

H2O

2NHCl2+HOCl→N2 g( )+3Cl−+3H++H2O

Let us discuss the formation of the monochloramine versus the formation of thenitrogen gas The oxidation state of the nitrogen atom in ammonia is −3 And, again,its oxidation state in NH2 Cl is −1 Thus, forming the monochloramine from ammonianeeds the abstraction of two electrons from the nitrogen atom Now, again, theoxidation state of nitrogen in the nitrogen gas is zero, which means that to form thenitrogen gas from ammonia needs the abstraction of three electrons; this is harderthan abstracting two electrons Thus, in the reaction of HOCl and NH3, themonochloramine is formed rather than the nitrogen gas, and the gas is formed onlywhen the conversion into monochloramine is complete by more additions of HOCl.Consider the formation of the nitrate ion The oxidation state of nitrogen in thenitrate ion is +5 Thus, this ion would not be formed from ammonia, because thiswould need the abstraction of eight electrons If it is formed from the monochloram-ine, it would need the abstraction of six electrons, and if formed from the dichloram-ine, it would need the abstraction of four electrons Thus, in the chloramine reactionswith HOCl, the nitrate is formed from the dichloramine We will, however, comparewhich formation forms first from the dichloramine: trichloramine or the nitrate ion.The oxidation state of the nitrogen atom in trichloramine is +3 Thus, to form thetrichloramine, two electrons need to be abstracted from the nitrogen atom Thismay be compared to the abstraction of four electrons from the nitrogen atom toform the nitrate ion Therefore, the trichloramine forms first before the nitrate iondoes.

The reaction for the formation of the nitrate ion may be written as follows:

(17.40)(17.41)––––––––––––––––––––––––––––––––––––––

(17.42)

Now, let us discuss the final fate of trichloramine during disinfection In dance with the chloramine reactions [Reactions (17.34) to (17.36)], by the time threemoles of HOCl have been added, a mole of trichloramine would have been formed.This, however, is not the case As mentioned, while the monochloramine decomposes

accor-in a stepwise fashion to convert accor-into the dichloramaccor-ine, its destruction accor-into the nitrogengas intervenes Thus, the eventual formation of the dichloramine would be less; infact, much, much less, since, as we have found, formation of the gas is favored overthe formation of the dichloramine In addition, monochloramine and dichloramine,themselves, react with each other along with HOCl to form another gas N2O [NH2Cl +NHCl2+ HOCl → N2O + 4H++ 4Cl−

] Also, there may be more other side reactionsthat could occur before the eventual formation of the dichloramine from mono-chloramine Overall, as soon as the step for the conversion of the dichloramine to thetrichloramine is reached, the concentration of dichloramine is already very low andthe amount of trichloramine produced is also very low Thus, if, indeed, trichloraminehas a disinfecting power, this disinfectant property is useless, since the concentra-tion is already very low in the first place This is the reason why combined chlorine

is only composed of the monochloramine and the dichloramine Also, it follows

NHCl2+3H2O → NO3−+2Cl−+7H++4e−2HOCl 2H+ 4e− → 2Cl−

2H2O

NHCl2+2HOCl+H2O→NO3−+2Cl−+5H+

that since dichloramine is, itself, simply decomposed, it is not the important combinedchlorine disinfectant; the monochloramine is If the objective is the formation of thedisinfecting chloramines, it is only necessary to add chlorine to a level of a littlemore than one mole of chlorine to one mole of ammonia in order to simply formmonochloramine Beyond this is a waste of chlorine.

Now, let us determine the optimum pH range for the formation of the chloramine The key to the determination of this range is the predominance of HOCl.Hypochlorous acid predominates over the pH range of less the 7.0; therefore, theoptimum pH range for the formation of monochloramine is also less than 7.0

system, chloramination is applied to the treated water If two moles of HOCl havebeen added per mole of ammonia, calculate the moles of nitrogen gas produced

chlorami-nation that it is not possible to determine exactly the amount of resulting species.Experimentally, a sample may be put in a closed jar and chloramination performed.The liberated nitrogen gas may then be measured; but in this problem the moles ofnitrogen produced simply cannot be calculated Ans

Reactions with organic nitrogen Chlorine reacts with organic amines to form

organic chloramines Examples of the organic amines are those with the groups–NH2, −NH−, and −N = Parallel to its reaction with ammonia, HOCl also reactswith organic amines to form organic monochloramines and organic dichloramines

by the chloride atom simply attaching to the nitrogen atom in the organic molecule.For example, methyl amine reacts with HOCl as follows:

so organic amides as well as organic amines are important in chloramination Althoughthe organic chloramides and organic chloramines have some disinfecting power, theyare not as potent as the ammonia chloramines; thus, their formation is not beneficial.Organic chloramides and organic chloramines are also combined chlorines

Solution: A characteristic property of chlorine disinfectant is the conversion of

the chlorine atom in the disinfectant into the chloride ion Thus, in portraying the chemicalreaction, the formation of the chloride should always be shown Let CH3NHCl representthe organic chloramines Therefore, its half reaction as a disinfectant is as follows:

As this half reaction shows, the disinfectant grabs four mole electrons from the

organism disinfected per mole of the disinfectant, disabling the organism Ans

function of chlorine dosage From zero chlorine applied at the beginning to point A,the applied chlorine is immediately consumed This consumption is caused by reduc-ing species such as Fe2+, Mn2+, H2S, and The reactions of these substances onHOCl have been discussed previously As shown, no chlorine residual is producedbefore point A

In waters and wastewaters, organic amines and their decomposition products such

as ammonia may be present In addition, ammonia may be purposely added forchloramine formation to produce chlorine residuals in distribution systems Also, otherorganic substances such as organic amides may be present as well Thus, from point

A to B, chloro-organic compounds and organic chloramines are formed Ammonia will

be converted to monochloramine at this range of chlorine dosage

Beyond point B, the chloro-organic compounds and organic chloramines breakdown Also, at this range of chlorine dosage, the monochloramine starts to convert

to the dichloramine, but, at the same time, it also decomposes into the nitrogen gas and,possibly, other gases as well These decomposition reactions were addressed previously

FIGURE 17.1 Chlorine residual versus applied chlorine.

CH3NHCl+4H++4e− from organism disinfected( )→Cl−+NH4++CH4

NO2−

Destruction of chlorine residual

by reducing compounds

Formation of chloro-organic compounds and chloramines

Chloramines and chloro-organic compounds

Presence of chloro-organic compounds not destroyed

Combined residual

Free and combined residual

Free residual

Combined residual Breakpoint

As the curve continues to go “downhill” from point B, the dichloramine converts tothe trichloramine, the conversion being complete at the lowest point indicated by

“breakpoint.” As shown, this lowest point is called the breakpoint In addition,

nitrates will also be formed from the dichloramine before reaching the breakpoint

In fact, other substances would have been formed as decomposition products frommonochloramine and dichloramine, as well as other substances would have beenformed as decomposition products from the chloro-organic compounds and organicchloramines

As shown by the downward swing of the curve, the reactions that occur betweenpoint B and the breakpoint are all breakdown reactions Substances that have beenformed before reaching point B are destroyed in this range of dosage of chlorine

In other words, the chloro-organics that have been formed, the organic chloraminesthat have been formed, the ammonia chloramines that have formed, and all othersubstances that have been formed from reactions with compounds such as phenolsand fulvic acids are all broken down within this range These breakdown reactions

have been collectively called breakpoint reactions.

The breakpoint reactions only break down the decomposable fractions of therespective substances All the nondecomposables will remain after the breakpoint.This will include, among other nondecomposables, the residual organic chloramines,residual chloro-organic compounds, and residual ammonia chloramines As we havelearned, the trichloramine fraction that comes from ammonia chloramines has to bevery small at this point to be of value as a disinfectant All the substances that couldinterfere with disinfection and all decomposables would have already been destroyed,therefore, any amount of chlorine applied beyond the breakpoint will appear as freechlorine residual

Important knowledge is gained from this “chlorine residual versus applied chlorine”curve We have learned that all the ammonia chloramines practically disappear at thebreakpoint We have also learned that the organic chloramines are not good disinfec-tants Therefore, as far as providing residual disinfectant in the distribution system isconcerned, chlorination up to the breakpoint should not be practiced Since the maxi-mum point corresponds to the maximum formation of the ammonia monochloramine,the ideal practice would be to chlorinate with a dosage at this point Note that, in

Figure 17.1, appreciable amounts of combined residuals still exists beyond the point; however, these combined residuals mainly consist of combined chloro-organics,which have little or no disinfecting properties, and combined organic chloramines,which have, again, little or no disinfecting properties Trichloramine, as we havementioned, will be present at a very minuscule concentration

break-The practice of chlorinating up to and beyond the breakpoint is called chlorination Superchlorination ensures complete disinfection; however, it will only

super-leave free chlorine residuals in the distribution system, which can simply disappearvery quickly

Note: If superchlorination is to be practiced to ensure complete disinfection and

it is also desired to have long-lasting chlorine residuals, then ammoniashould be added after superchlorination to bring back the chlorine dosage

to the point of maximum monochloramine formation

Example 17.8 Referring to Figure 17.1, if a dosage of 1.8 mg/L is administered,determine the amount free chlorine residual that results, the amount of combinedresidual that results, and the amount of combined ammonia chloramine residual thatresults Also, determine the amount of organic chloramine residual that results.

Solution: From the figure, the concentration of residual chlorine at a dosage of

1.8 mg/L = 0.38 mg/L The concentration of the residual at the breakpoint = 0.20 mg/L.Therefore,

free chlorine residual = 0.38 − 0.20 = 0.18 mg/L Ans

amount of combined residual = 0.20 mg/L Ans

amount of combined ammonia chloramine  0 Ans

amount of organic chloramine cannot be determined Ans

Reactions with phenols Chlorine reacts readily with phenol and organic

com-pounds containing the phenol group by substituting the hydrogen atom in the phenolring with the chlorine atom These chloride substitution products are extremely odorous.Because phenols and phenolic groups of compounds can be present in raw watersupplies as a result of discharges from industries and from natural decay of organicmaterials, the formation of these odorous substances is a major concern of watertreatment plant operators

Figure 17.2 shows the threshold odor as a function of pH and the concentration

of chlorine dosage Figure 17.2a uses a concentration of 0.2 mg/L and, at a pH of9.0, the maximum threshold odor concentration is around 28 µg/L When the pH isreduced to 8.0 this threshold worsens to around 20 µg/L, and when the pH is furtherreduced to 7.0, the threshold concentration becomes worst at around 13 µg/L Thus,chlorination at acidic conditions would produce very bad odors compared to chlo-rination at high pH values This is very unfortunate, because HOCl predominates atthe lower pH range, which is the effective range of disinfection

In Figure 17.2b the concentration of chlorine has been increased to 1.0 mg/L.For the same adjustments of pH, the maximum threshold concentrations are aboutthe same as in Figure 17.2a; however, in the cases of pH’s 7.0 and 8.0, the thresholdodors practically vanish at approximately 3 to 5 h after contact as opposed to greaterthan 60 h when the dosage was only 0.2 mg/L Thus, increasing the dosage producesthe worst nightmare for odor production

Figure 17.3 shows the reaction scheme for the breakdown of phenol to odorlesslow molecular weight decomposition products using HOCl The threshold odorconcentrations of the various chloride substituted phenolic compounds are alsoindicated in brackets Note that the worst offenders are 2-monochlorophenol and2,4-dichlorophenol, which have an odor threshold of 2.0 µg/L In order to effectthese breakdown reactions, superchlorination would be necessary, which would alsomean that the odor had increased before it disappeared

displaced in ortho chlorophenol by the chlorine atom to form 2,6-dichlorophenol?

chlorine Ans

Formation of trihalomethanes Reactions of chlorine with organic compounds

such as fulvic and humic acids and humin produce undesirable by-products These

by-products are known as disinfection by-products, DBPs Examples of DBPs are

chloroform and bromochloromethane; these DBPs are suspected carcinogens Snoeyinkand Jenkins (1980) wrote a series of reactions that demonstrate the basic steps by whichchloroform may be formed from an acetyl-group containing organic compounds.These reactions are shown in Figure 17.4

Note that the initial reaction involves the splitting of the hydrogen atom from themethyl group using the hydroxyl ion The hydroxyl ion is again used in (3), (5), and(7) Because the hydroxyl is involved, this would mean that chloroform formation isenchanced at high pH To prevent formation of the chloroform, all that is necessary

FIGURE 17.2 Threshold odor from chlorination of phenol: (a) chlorine 0.2 mg/L, initial

phenol 5.0 mg/L; (b) chlorine 1.0 mg/L, initial phenol 5.0 mg/L; all at 25 °C and threshold odors are concentrations in µg/L (From Lee, G F (1967) Principles and Applications of

Water Chemistry S D Faust and J V Hunters (Eds.) John Wiley & Sons, New York With

then is simply to chlorinate at low pH, and this in fact, would be fortunate, since HOClpredominates at low pH values instead of at high pH values Based on this fact, ifsuperchlorination is to be conducted, it should be done at low pH values Furtherresearch, however, should be performed to establish the accuracy of this assumption.

FIGURE 17.3 Chlorination for the breakdown of phenol; numbers in brackets are odor

threshold concentrations in µg/L.

FIGURE 17.4 Proposed scheme for chloroform formation

oxidation low molecular

weight product:

Chlorinated waters and wastewaters can contain not only chloroform and mochloromethane but also other brominated compounds In addition, iodinated com-pounds may also be produced; that is, this is the case if iodine (or bromine in thecase of brominated compounds) is present, in the first place In general, the productsformed from the halogen family to produce the derivative products of methane are

bro-called trihalomethanes The formula is normally represented by CHX3, where X can

be Cl, Br, or I Examples of other brominated species are bromodichloromethane,chlorodibromomethane, and bromoform The most commonly observed iodinatedtrihalomethane is iododichloromethane The reason why brominated and iodinatedtrihalomethanes can be formed is that bromine and iodine are below chlorine in thehalogen family of the periodic table It is an observed fact in chemistry that strongeracids drive the weaker acids The acid precursor of stronger acids (Cl in HOCl) arehigher in the series than those of the precursor of the weaker acids (Br and I inHOBr and HOI, respectively) For this reason, HOCl drives the weaker acids HOBrand HOI These two acids then react in the same way as HOCl when it producesthe brominated and iodinated trihalomethanes

group, what happens to the double between carbon and oxygen?

neg-ative and single bonded The double bond switches to become a carbon-to-carbondouble bond As indicated in subsequent reactions, this flip-flopping of the doublebond continues until the formation of chloroform Ans

Acid generation Whether or not acid will be produced depends upon the form of

chlorine disinfectant used Using chlorine gas will definitely produce hydrochloric acid.Sodium hypochlorite and calcium hypochlorite will not produce any acid; on thecontrary, it can result in the production of alkalinity Superchlorination using HOClwill definitely produce acids

As shown in Equation (17.9), a mole of hydrochloric acid is produced per mole

of chlorine gas that reacts Chlorination uses up the disinfectant, so this reaction would

be driven to the right and any mole of chlorine gas added will be consumed Thus, if

a mmol/L of the gas is dosed, this will produce a mmol/L of HCl This is equivalent

to one mgeq of the acid, which must also be equivalent to a mgeq of alkalinity Theanalytical equivalent mass of alkalinity in terms of CaCO3 is 50 mg CaCO3 per mgeq.Thus, the mmol/L of hydrochloric acid produced will need 50 mg/L of alkalinityexpressed as CaCO3 for its neutralization Or, simply, one mmol of hydrochloric acidrequires 50 mg of alkalinity expressed as CaCO3 for its neutralization

In superchlorination, breakpoint reactions like Eqs (17.37) to (17.42) willtranspire A host of other reactions may also occur such that all these, as shown

by the preceding reactions, produce acids The number of reactions are many,therefore, it is not possible to predict the amount of acids produced by using simplestoichiometry The only way to determine this amount is to run a jar test as is done

in the determination of the optimum alum dose Metcalf & Eddy, Inc wrote that

in practice it is found that 15.0 mg/ L of alkalinity is needed per mg/ L of ammonianitrogen (Metcalf & Eddy, Inc., 1972) But, again, the best method would be torun the jar test