analysis of factors influencing the action of disinfectants

7/29/2019 analysis of factors influencing the action of disinfectants

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Analysis of Factors Influencing the Action of Disinfectants

In applying the disinfection agents or means that have been described, the following factors

must be considered: (1) contact time, (2) concentration and type of chemical agent, (3) intensity and

nature of physical agent, (4) temperature, (5) number of organisms, (6) types of organisms, and (7)nature of suspending liquid.

Contact TimeIt is one of the most important variables in the disinfection process. In general, it has been

observed that for a given concentration of disinfectant, the longer the contact time, the greater the kill.

This observation was first formalized in the literature by Chick. In differential form, Chicks law is

Where:

Nt= number of organisms at time t

t= time

k= constant, time -1

IfNo is the number of organisms when t equals 0, the equation can be integrated to

Or

Departures from this rate law are common. Rates of kill have been found to increase with time

in some cases and to decrease with time in other cases. To formulate a valid relationship for the kill of

organisms under a variety of conditions, an assumption often made is that


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where m is a constant. If m is less than 1, the rate of kill decreases with time, and, if m is greater than 1,

the rate of kill increases with time. The constants can be obtained by plotting ln (N/No) versus the

contact time ton log- log paper. The straight- line form of the equation is

Another formulation that has been used to describe the observed effects of contact time is

This equation results from the analysis of chlorination data that have been found to plot as

straight lines on log- log paper.

Concentration and Type of Chemical AgentDepending on the type of chemical agent, it has been observed that, within this limits,

disinfection effectiveness is related to concentration. The effect of concentration has been formulated

empirically:

Where:

C= concentration of disinfectant

n= constant

tp= time required to effect a constant percentage kill


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Intensity and Nature of Physical AgentHeat and light are physical agents that have been used from time to time in the disinfection of

wastewater. It has been found that their effectiveness is a function of intensity. For example, if the

decay of organisms can be described with a first order reaction such as

Where:

N= number of organisms

t= time

k= reaction velocity of constant, 1/ min

then the effect of the intensity of the physical disinfectant is reflected in the constant k through some

functional relationship.

TemperatureThe effect of temperature on the rate of kill can be represented by a form of the vant Hoff-

Arrhenius relationship. Increasing the temperature results in a more rapid kill. In terms of the time t

required to effect a given percentage kill, the relationship is

Where:

t1, t2 = time for given percentage kill at temperatures T1 and T2, K, respectively


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E= activation energy, J/mol (cal/mol)

R= gas constant, 8.314 J/mol-K (1.99 cal/ K- mol)

Since typical values for the activation energy for various chlorine compounds at different pH

values are reported below.

Activation Energies for Aqueous Chlorine and Chloramines at Normal Temperatures

Compound pH E, cal/mol

Aqueous chlorine 7.0 8,200

8.5 6,400

9.8 12,000

10.7 15,000

Chloramines 7.0 12,000

8.5 14,000

9.5 20,000

Number of OrganismsThe effectiveness of various disinfectants will be influenced by the nature and condition of the

microorganisms. For example, viable growing bacteria cells are killed easily. In contrast, bacterial spores

are extremely resistant, and many of the chemical disinfectants normally used will have little or no

effect. Other disinfecting agents, such as heat, may have to be used.


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Nature of Suspending LiquidIn addition to the foregoing factors, the nature of the suspending liquid must be evaluated

carefully. For example, extraneous organic material will react with most oxidizing disinfectants and

reduce their effectiveness.

Turbidity will reduce the effectiveness of disinfectants by absorption and by protecting

entrapped bacteria.

CHLORINE DISINFECTION

The most common form of disinfection regarding wastewaters is the use of Chlorine compounds

since chlorine is the only substance able to capture all the requirements and properties of a disinfectant.

Widely used chlorinated compounds include chlorine (Cl2), sodium hypochlorite (NaOCl), calcium

hypochlorite (Ca(OCl)2), and chlorine dioxide (ClO2).

Properties of Chlorinated Compounds for Disinfection

A. Chlorine (Cl2)

This substance may be present in gaseous or liquid form. Gaseous form is greenish

yellow in color and is 2.48 times heavier than air while liquid chlorine is amber colored and is

1.44 times heavier than water. Unconfined liquid chlorine rapidly vaporizes to a gas at standard

temperature and pressure with 1 liter of liquid yielding 450 liters of gas.

Chlorine is moderately soluble in water with a maximum solubility of about 1 percent at

10C or 50F. Chlorine is supplied as a liquefied gas under high pressure in containers varying in

size from 45 kg (100 lb) and 68 kg (150 lb) cylinders, 908 kg (1 ton) containers, multiunit railcars

containing fifteen 908 kg (1 ton) containers, and railcars with capacities of 14.5, 27.2, and 49.9

Mg (16, 30, and 55 tons).


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Properties of Chlorine, Chlorine Dioxide, and Sulfur Dioxide

Important considerations for the continued use of Chlorine Compounds

1. Chlorine is a highly toxic substance that is transported by accident-prone means like trucksand rails.

2. Accident release of the substance may pose health risk exposure to the plant operator andgeneral public.

3. Due to toxicity, proper implementation of the stringent requirements and for containmentand neutralization according to Uniform Fire Code (UFC) should be done.

4. Upon reaction to organic compounds, chlorine produces odorous substance.


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5. Upon reaction to organic compounds, it forms by-products known to be carcinogenic and/ormutagenic.

6. Residual chlorine in treated wastewater effluent is toxic to aquatic life.7. Concerns exist pertaining to the discharge of organo-chloride compounds to the

environment which long-term effects remain unknown.

B. Sodium Hypochlorite

Many of the safety concern regarding handling and containment of chlorine is

eliminated by the use of sodium hypochlorite and calcium hypochlorite. Sodium hypochlorite is

also known as liquid bleach with the chemical formula NaOCl and has the following distinctions:

Only available in liquid

Contains 12.5 to 17% available chlorine upon manufacture In bulk purchasing, it can have 12-15% available chlorine

However, sodium hypochlorite decomposes more readily in high concentrations and is

highly affected by light and heat. For example, a 16.7% solution contained at 26.7C (80F) will

lose in 10 days, 10% of its strength, 20% in 25 days, and 30% in the next 45 days therefore

storage in a cool and corrosive-resistant tank is a must. Other disadvantages include:

High chemical cost (NaOCl costs 150-200% more than liquid chlorine) Design considerations due to corrosiveness and chlorine fumes formation

C. Calcium HypochloriteThis substance, with chemical formula Ca(OCl)2, is commercially available in dry or wet

form. High-test calcium hypochlorite contains at least 70% available chlorine. In dry form it is

available in off-white powder, or as granules, compressed tablets and pellets. Granular or


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pelleted forms are readily soluble in water from about 21.5g/100ml at 0C (32F) to 23.4g/100ml

at 40C (104F).

Due to high oxidative potential, these should be stored in cool, dry location away from

other chemicals in corrosion-resistant containers but with proper storage conditions, the

granular Ca(OCl)2 is relatively stable. This chlorinated compound is used most commonly at

small installations.

On the other hand, hypochlorite is more expensive than liquid chlorine, loses its

available strength in storage, and maybe difficult to handle. Calcium hypochlorite tends to

crystallize thereby clogging metering pumps, piping, and valves.

Chemical Reactions of Chlorinated Compounds

A. Chemistry of Chlorine in WaterUpon application of either liquid or gaseous form of chlorine to water, two chemical reactions

take place: hydrolysis and ionization.

Hydrolysis is defined as the reaction between chlorine and water producinghypochlorous acid (HOCl)

Cl2 + H2O HOCl + H+

+ Cl-

This reaction has an equilibrium constant KH equivalent to:

KH= [HOCl] [H+] [Cl-] / [Cl2] = 4.5 x 104 (mol/L)2 at 25C

An equilibrium constant of this magnitude means that large amount of Cl2 can be

dissolved in water.

Ionization is defined as the formation of hydrogen (H+) and hypochlorite (OCl- )ions fromhypochlorous acid (HOCl)

HOCl H+ + OCl-

Ionization constant, KI, is equivalent to:


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KI= [H+] [OCl-] / [HOCl] = 3 x 10-8 mol/L at 25C

The total HOCl and OCl-

present in water is known in the collective term free available

chlorine. It is very important that HOCl amount be monitored as compared to OCl- since the previous

has a killing efficiency of around 40-80 times that of the latter.

The percentage distribution of HOCl can be computed together with the values ofKIand the pH

of the sample:

[HOCl] / [HOCl] + [OCl-] = 1/ 1 + KI10pH

Values of KI at different temperatures

B. Hypochlorite Reactions with WaterFree available chlorine can also be added to water in the form of hypochlorite salts.

Both sodium and calcium hypochlorite form hypochlorous acid in the following mechanism

which would then be disassociated further in the earlier discussed reactions:

C. Chlorine Reactions with AmmoniaUntreated wastewater contains nitrogen in the form of ammonia and various combined

organic forms. The effluent from most treatment plants also contains significant amount of


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ammonia or nitrate if the plant is designed to achieve nitrification. Due to the very oxidative nature

of hypochlorous acid, it will readily form 3 types of chloramines.

The reactions and their products are highly dependent of the pH, temperature, contact time,

and ratio of ammonia to chlorine. Only two components are predominant in the reactions namely

monochloramine and dichloramine while trichloramine is negligible up to the chlorine-nitrogen ratio of

2.0.

Chlorine contents of these compounds are termed combined available chlorine. Chloramines

are also considered disinfectants however these are relatively slower than those with free available

chlorines. When these chloramines are solely used as the disinfectant, the measured residual chlorine,

is then called combined chlorine residual while for hypochlorous acid and hypochlorite it is called

free chlorine residual.

DECHLORINATION

Chlorination is one of the most commonly used methods for the destruction of pathogenic and

other harmful organisms that may endanger human health. However, certain organic constituents and

compounds may react with chlorine to form toxic compounds that can have long term adverse effects

on the beneficial uses of the waters to which they are discharged. To minimize the effects of these

potentially toxic chlorine residuals on the environment, dechlorination of the wastewater is necessary.

Dechlorination is a practice used to reduce or remove the chlorine discharge levels. Free and

combined chlorine residuals are reduced by sulfur dioxide, sulfites and other dechlorinating agents. The

most cost effective dechlorinating agent is sulfur dioxide. Stoichiometrically, 0.9 parts of sulfur dioxide


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are required to remove one part chlorine. In actual practice, at least 10% excess may be required for

complete dechlorination.

1. Dechlorination with Sulfur Dioxide

Sulfur dioxide is handled in equipment very similar to standard chlorine systems. When added to

water, sulfur dioxide reacts to form sulfurous acid (H2SO3-), a strong reducing agent. In turn, the

sulfurous dissociates to form HSO3- that will react with free and combined chlorine, resulting in

formation of chloride and sulfate ions. Sulfur dioxide successively removes free-chlorine,

monochloramine, dichloramine, nitrogen trichloride, and poly-n-chlor compounds.

Reactions between sulfur dioxide and free chlorine:

SO2 + H2O HSO3- + H+

HOCl + HSO3-

Cl-+SO4

2-+ 2H

+

SO2 + HOCl + H2O Cl- + SO4

2- + 3H+

Sulfur dioxide is the most common dechlorinating agent for the following reasons:

1. Removes free or combined chlorine residual

2. Cost effective

3. Similar to chlorine feeding apparatus design

4. Simple to control

Sulfur dioxide dechlorination is a very reliable unit process in wastewater treatment, provided

that the precision of the combined chlorine residual monitoring service is adequate. Excess sulfur

dioxide dosages should be avoided, not only because of the chemical wastage but also because of the

oxygen demand exreted by the excess sulfur dioxide.


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A. Disinfection with Ozone

Ozone (O3), or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an

allotrope of oxygen that is much less stable than the diatomicallotrope (O2). Ozone in the lower

atmosphere is an air pollutant with harmful effects on the respiratory systems of animals and will burn

sensitive plants; however, the ozone layer in the upper atmosphere is beneficial, preventing potentially

damaging ultraviolet light from reaching the Earth's surface. Ozone is present in low concentrations

throughout the Earth's atmosphere. It has many industrial and consumer applications.

Although historically used primarily for the disinfection of water, recent advances in ozone

generation and solution technology have made the use of ozone economically more competitive for

wastewater disinfection. Ozone can also be used in wastewater treatment for odor control and in

advanced wastewater treatment for the removal of soluble refractory organics, in lieu of the carbon-

adsorption process.

Steps in Ozone Disinfection

1 Computer animation of a bacterial cell

2 Close-up of an ozone molecule on the bacterial cell wall

3 Ozone penetrates the cell wall and causes corrosion

4 Close-up of the effect of ozone on the cell wall

5 Bacterial cell after it has come in contact with a number of ozone molecules

6 Cell destruction (lysis)

Ozone as Disinfectant


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Ozone functions as both an oxidant and disinfectant in the treatment of drinking (potable) water

and wastewater. This is similar to chlorine. Chlorine and Ozone, however, operate by different

mechanisms when disinfecting water. As a result, ozone and chlorine can act synergistically.

Ozone's germicidal properties are associated with its high oxidation potential. Disinfection byozone is a direct result of bacterial cell wall disintegration, also known as cell lysis. This mechanism is

different than that by chlorine. Although the exact chemical action of chlorine is not clear, it is believed

that the chlorine residual in aqueous solution diffuses through the cell wall of the microorganisms and

attacks the enzyme group which results in the destruction of the microorganism.

Advantages and Disadvantages of Ozone:

Pros Cons

- Extremely powerful disinfectant - Expensive option

- Does not form trihalomethanes- Can form other hazardous

disinfection-by-products such as


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bromated

- Requires relatively short

contact time

- Requires high level of

technology

- Reduces taste, odor, and color in

water by oxidizing the algea and

humic material which causes these

problems

- Requires another disinfectant to

achieve residual disinfection levels

- Forms microfloc upon contact

therefore improving coagulation

and reducing the required

coagulant dose

- Unstable - must be generated

on-site

- Can improve filtration rates. With

improved coagulation, more

material settles in the

sedimentation basin. Hence, less

material reaches the filters and

the filters can be run longer

before backwashing.

- Climate control needed to

maintain solubility

Ozone vs. Chlorine

ACTION IN WATER CHLORINE OZONE

Oxidation Potential (Volts)- 1.36 2.07

Disinfection:
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Bacteria

Viruses

Moderate

Moderate

Excellent

Excellent

Environmentally Friendly No Yes

Color Removal Good Excellent

Carcinogen Formation Likely Unlikely

Organics Oxidation Moderate High

Micro flocculation None Moderate

pH Effect Variable Lowers

Water Half-Life 2-3 hours 20 min.

Operation Hazards:

Skin Toxicity

Inhalation Toxicity

High

High

Moderate

High

Complexity Low High

Capital Cost Low High

Monthly Use Cost Moderate-High Low

Air Pre-treatment None Filer and dehumidify air
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