UNIT 14 THEORY AND DESIGN OF DISINFECTION

Structure

Introduction Objectives

Types of Disinfection Disinfection by Chlorine 14.3.1 Chlorine Reactions in Water 14.3.2 Modes of Chlorination 14.3.3 Factors Influencing the Disinfection Efficiency

14.3.4 Breakpoint Chlorination 14.3.5 Superchlorination and Dechlorination 14.3.6 Dosage and Contact Time 14.3.7 Advantages of Using Liquid Chlorine as Disinfectant '

Tests for Residual Chlorine Disinfection Using Chlorine Dioxide Disinfection Using Ozone Other Disinfection Practices 14.7.1 Bleaching Powder 14.7.2 Calcium Hypochlorite Granules 14.7.3 Chlorine Tablets 14.7.4 Sodium Hypochlorite Solution 14.7.5 Electrolytic Production of Chlorine 14.7.6 Ultraviolet Radiation 14.7.7 Silver 14.7.8 Iodine 14.7.9 Potassium Permanganate 14.7.10 Boiling of Water

Prevention of Pollution Summary Key Words Answer to SAQs

4 . 1 INTRODUCTION

Disinfection may be defined as the process of destruction or inactivation of harmful micro-organism in water either by physical process or chemical process. Physical process includes heating, ultraviolet radiation etc. and chemical process includes addition of disinfectants such as chemicals. Disinfection can be distinguished from sterilization. In sterilization process, all organisms are usually killed by a physical phenome'non such as boiling for a long period whereas in disinfection disease or illness causing micro-organism are reduced to such low levels that no infection or disease results when the water is used for domestic purposes including drinking water. Since the most important requirement of drinking water is that it should be free from any micro-organisms that could transmit disease illness to the user, disinfection is, therefore, the most important water treatment process among treatment process of water. In fact sedimentation or filtration can be spared of but not the disinfection. Hence, disinfection is an essential minimum treatment requirement for any drinking water and it is the final process in the chain of water purification.

Objectives

This unit deals in detail the process of disinfection and it is defined as the process of destruction or inactivation of harmful micro-organisms in water. Both the

Water Treatment process-physical and chemical are utilised. After studying this unit, you should be able to

e describe physical and chemical processes involved in disinfection,

e write chemical reactions involved in disinfection,

e justify that chlorine is most important disinfectant which is used in different form for disinfection of drinking water on commercial scale,

e justify that use of chlorine is economical as well as easy also, and

justify that disinfection by chlorination has been found to be most effective.

14.2 TYPES OF DISINFECTION

Physical disinfection such as heating in safe and destroys pathogenic micro-organisms such as viruses, bacteria, cysts and ova. However, it can only be applied as a family size or household treatment unit. Ultraviolet light irradiation is effective only for clear and low salinity water and is generally not applicable to community water supplies in the developing countries.

Chemical disinfection is the most widely used method. The chemicals that have been successfully used for disinfection are-

Chlorine and chlorine compounds, bromine, iodine, ozone, potassium permanganate, hydrogen peroxide, silver and some phenolic compounds.

On a plant scale, the following disinfectants are in common use :

i) Chlorine

ii) Chloromine

iii) Chlorine dioxide

iv) Ozone

Other disinfectants used principally for different situations are given below :

1) Ultraviolet radiation for small public supplies, railway refreshment vehicles, restaurants etc.

2) Silver for in-house "water sterilisers".

3) Iodine for temporary, small scale use in emergencies.

4) Potassium permanganate for pre-disinfection of water before other treatment and also for iron and manganese removal.

5 ) Boiling for domestic drinking supplies usually in an emergency but some times as a routine precaution.

The organisms in water, which it may be necessary to kill by disinfection include bacteria, bacterial spores, viruses, cysts and protozoa and the low forms of animal life such as worms and larvae. The resistance of these organisms to process of disinfection varies according to the type and sub-type of organism present, the type of disinfectant used and the form in which it is used, the number of organisms present, the physical and chemical characteristics of the water (temperature, amount a

of suspended solids, organic matter present and pH etc.), the amount of disinfectant applied and the time period for which the disinfectant is applied. Chlorine and their various compounds are the most widely used disinfectants and following are the reasons for their most wide use :

a) ease of distinction of pathogens

b) cheapness and wide availability

c) ease of application due to availability in various forms and high solubility, production of desirable residual chlorine to ensure bacteriological purity in the distribution network.

d) non-coloration and severe odorization of the treated water

However, the chlorine has some disadvantage, which should be kept in mind when using it. It is a poisonous gas, which requires carehl handling. As regarding the toxicity of chlorine gas, the concentrations producing various physiological reactions in human being are generally accepted as given in Table 14.1.

Table 14.1 : Limits of Chlorine Concentrations for Different Harmful Effects

Rate of killing of bacteria by disinfectants-

Sl. No.

1

-

2

3

4

5

where, No = the number of organisms initially;

N, = the number of organisms at time t ;

Items

Harmless concentration in air breathed during 8 hours working period

Perceptible odour

Causing throat irritation --

Causing throat irritation and cough

Maximum for short period of exposure

t = desired contact time;

Limit

1 mgll

3.5 mgA

15 mgA

30 mgA

40 mgA

u = specific decay rate constant in time t for a particular disinfectant

For chlorine, the rate of kill is given by the equation :

40-60 mgA J

1000 mgA

6

7

and on integration it becomes

Dangerous for short exposure

Quickly fatal

The rate constant u, depending on the particular disinfectant also varies with disinfectant concentration, temperature, pH apd other environmental factors. At pH 7, the value of u (base e) for chlorine is about 3.7 x 10'~ per second for free chlorine residuals and about 3.7 x per second for the combined residuals when applied to colifonn organisms. The corresponding value for ozone is about 6 x per second.

14.3 DISINFECTION BY CHLORINE

I

1

I

14.3.1 Chlorine Reactions in Water

Water Treatment

Chlorine is a powefil oxidising agent, which when dissolved in water yields the following reaction :

pH greater than 2 C1, + H20 4- HCI + HClO

pH less than 5

which is followed by the secondary reaction :

pH greater than 2 HClO fw l3+ + C10- (free residuals)

pH less than 5

Figure 14.1 shows the detail.

Figure 14.1 : Dissociation of Hypochlorous at Various pH

Hypochlorous acid HClO is the more effective disinfectant, the chlorite ion C10' being far less effective. The dissociation of HClO is suppressed at acid pH values, the residual being all HClO at 5 and below, about half HClO at pH 7.5 and all C10- at pH 9 (Figure 14.2).

CHLORINE BY REDUCING I

CHLORINE P D D E D ( P- P a m )

CHLORINE RESIDUALS FOR WATER WITH AMMONIA,

Figure 14.2 : Chlorine Reactions in Water

If ammonia is present in the water, the reaction is :

NH, + HClO + NqCI + H20 (at pH 4.5 - 8.5 and alone at pH above 8.5) 4

M n n n r h l n m r n i n m

NH,C12 + H20 NH2Cl + HClO -+ (at pH 4.5 - 8.5)

dichloroamine

NH, + H20 NH2C1 + HC1o Trichloroamine (alone at pH less than 4.4)

The chloroamines are the combined residuals. They are more stable than the free residuals but less effective as disinfectants. For a given kill with constant residual, the combined form required over hundred times, the contact time required by the free residual. Alternatively for a given contact time, the combined residual concentration must be twentyfive times, the free residual concentration to give the desired killing.

Trace concentrations of ammonia are desirable in water destined for chlorination in order to ensure presence of residuals in the distribution system. In the presence of ammonia, the continued addition of chorine produces the characteristic curve shown in Figure 14.2. Theoretically three moles of chlorine react with two moles of ammonia to give off nitrogen gas to reduce chlorine to the chloride ion as follows :

2NH3 + 3C1, + N2 + 6HC1

Thus the breakpoint, which is the point at which the concentrations of the combined residuals have declined to a minimum should occur at chlorine to ammonia ratio 3:2, but in practice the ratio is nearer to 10:l because of pH and other factors. Past the breakpoint, the free residual (HC10) is p~oportional to the dose. The actual chlorine demand is the difference between the applied chlorine dose and the residual concentration at the breakpoint.

Although the primary objective of chlorination of potable water is the destruction or inactivation of bacteria, there are several other important secondary hc t ions achieved by chlorination. These include :

i) the oxidation of iron, manganese and hydrogen sulphide,

ii) the destruction of some taste and odour producing compounds,

iii) control of algae and sljme organisms in treatment plants, and

1 iv) as an aid to coagulation.

The requirements of effective chlorination are met when the chlorine dose is sufficient to meet excess or residual chlorine for disinfecting purposes in the water distribution system under the prevailing alkalinity and temperature of the water and also the contact time. In the developing countries a free chlorine residual of about 0.5 mg/l and a combined residual chlorine of about 1.5 mg/l is recommended for clear water having a pH of about 7 and a contact time of about 30 minutes is generally used.

14.3.2 Modes of Chlorination d Chlorine is available in the following forms :

a) Liquefied chlorine gas which is clear amber coloured vapour, which when dissolved in water is highly corrosive to all metals except silver and lead. It is highly soluble in water (about 7000 mgA). Liquefied chlorine gas is stored either in pressure steel cylinders or drums which are sized to contain about 30 kg. and 800 kg of liquefied chlorine respectively. Storage area of liquefied chlorine should be cool, well ventilated and protected from corrosive vapours and continuous dampness.

b) Chlorine solution which is essentially sodium hypochlorite (NaOCl) containing 12-50% available chlorine in the commercial product (milton) and 3.5% available in the household bleach. Sodium hypochlorite

Theory a md Design of Disinfectfon

Water Treatment solutions are prone to decomposition and as a result they should be stored in dark bottles and cool places.

' c) Powdered chlorine which may be chlorinated lime'(b1eaching powder) or its more stable variant, high test hypochlorite (HTH). Chlorinated lime is loose combination of slaked lime and chlorine gas with the approximate composition :

In water it decomposes to give hypochlorous acid, HOCI. Fresh chlorinated lime has a chloride content 33% - 37% but this rapidly decreases during storage andlor exposure to air, light and moisture. Storage is preferably in dark, cool and dry places and enclosed in corrosion resistant containers. High test hypochlorite (HTH), which is essentially calcium hypochlorite, Ca(OC1),4&0 contains about 60-70% available chlorine and retains its original strength for over one year under normal storage conditions. Powdered chlorine may be obtained in packages of 2-3 kg, in cans of upto 45 kg and also in granular or tablet form in several developing countries. HTH is generally preferred because of its higher chlorine content and stability .

The form in which chlorine should be applied in water treatment is governed by several factors such as the quantity of water to be treated, the cost and availability of chemicals, the equipment needed for its application and the skill required for operation and control. The equipment used for applying chlorine and its compounds are called chlorinators.

Example 14.1

A new water main is disinfected using a 100 mgfl chlorine dosage by applying 5.0% hypochlorite solution. Calculate :

a) how many kilograms of dry hypochlorite powder containing 72% available chlorine should be dissolved in 100 ml of water to make a 5% (50,000 mgfl) solution.

b) the rate at which this solution Chould be applied to the water entering the main to provide a concentration of 100 mgfl.

c) if 35000 litres of water is used to fill the main at a dosage of 100 mgA how many litres of hypchlorite solution is used.

Solution

100 litre x 1.0 kg Aitre x 0.05 a) Kilograms of hypochlorite powder = 72

= 6.94 kg per 100 litre

b) Feed rate for 100 mg/l

- - 1 x volume of 50% solution 500 x volume of water

100 c) Solution usage for 35,000 litre = 35,000 x - 50,000

= 70 litres

Example 14.2

Calculate the ratio of OCI/HOCI in pure water at a temperature of 20°C and 1

pH of 6.

Solution Theory and Design of Disinfection

Chlorine reacts with water to produce hypochlorous acid which dissociates to give hypochlorite ions and protons thus

HOCl t' H? + OC1-

At 2 0 ' ~ the dissociation constant K is 2 x

where the brackets indicate the concentration in gram ions or gram moleculesA. Further, the percentage distribution of OCl- at a given temperature can be calculated using the relationship.

A pH of 6 means that [H+] = lo4 g ions11

i i.e. (0.0211.02) x 100% or 1.96% of HOCl has dissociated.

1 14.3.3 Factors Influencing the Disinfection Efficiency

It is believed that the chlorine compounds formed when chlorine is added to water interfere with certain enzymes in the bacterial cells, which are vital for the support of life. Further, chlorine is a strong oxidising agent which breaks up or organic matter, restrain algae growth and convert iron and manganese in the water to their oxidised forms.

I When treating water with chlorine following factors should be taken into consideration, which influence the disinfection efficiency of chlorine.

i) Time of Contact

The sterilising effect of free chlorine is not instantaneous. The percentage kill of bacteria depends upon the time of contact between the chlorine and the bacteria. A contact period of less than 10 minutes is not suitable for disinfection of a water supply even if the highest doses are applied. For

I disinfection by free chlorine acting in clear water a theoretical contact time of 30 minutes is suitable but in the design of any tank, proper time of retention or contact should be taken into account taking in to consideration of short-circuiting.

* ii) pH Range of Water

In both free and combined chlorination, the more effective sterilising compounds i.e. hypochlorous acid and dichloromine are formed in greater quantities at low pH values. For pH values upto 6.7 at least 90% of the free available chlorine, when ammonia is not present, comes from the hypochlorous acid HOCl. As the pH value rises, the dissociation favours the production of hypochlorite ion OC1 so that at pH 9 and O°C, only 4.5% of the free available chlorine is present as hypochlorous acid.

iii) Effecting of Turbidity and Organic Matter

The penetration of chlorine and hence destruction of bacteria in particles of suspended matter may be uncertain in the presence of high turbidity in water and hence, in the presence of organic impurities and high turbidity, it is difficult to get residual chlorine. Therefore, the disinfection by chlorine or 109

Water Treatment

110

other agents should be done with clear water i.e. after sedimentation and filtration. Of course, when it becomes necessary, chlorination may be done in two stages, first at pretreatment stage known as pre-chlorination and then after filtration so that some residual chlorine is available at distribution points.

iv) Absorption of Chlorine by Metallic Compounds

A substantial amount of chlorine may be used to convert iron and manganese in solution in the water into their higher states of oxidation, which are insoluble in water. Hence, removal of iron and manganese is desirable.

v) Absorption of Chlorine by Ammonia Compounds

The ammonia compound may exist in organic matter or alternatively atnrnonia may exist separately from organic matter and in either case, they form combined available chlorine, which is not so effective a bactericide as free available chlorine.

vi) Effect of Flow Temperature

The difference in kill rate of bacterial between the temperatures of 2 and 20°C is lloticeable both with free and combined chlorine. A substantial decrease in killing power takes place with lowering temperature. And this should be kept in mind when fixing contact period.

14.3.4 Breakpoint Chlorination

It has been found that if a water naturally contains ammonia or organic matter, the addition of chlorine first causes the formation of chlorarnine until the ratio of chlorine to ammonia compounds reaches around 5:l. At this point, the addition of hrther chlorine causes a reduction of the chloramines because of their oxidation by the excess chlorine. When this reaction has been completed, at a ratio of chlorine to ammonia nearer 10:1, the addition of hrther chlorine results in the formation of free chlorine. The point at which this free chlorine begins to form is called the "breakpoint" for the water. The effect on the ratio between chlorine applied and amount of free chlorine produced in a water is shown in Figure 14.3 (a and b). Quite frequently the breakpoint is shown not so much by a pronounced 'dip' in the curve of residuals as by a flattering of the curve, the chracteristics depending upon the type of water being treated. Where there is no ammonia (or albuminoid ammonia) present in the water, there will be no breakpoint since free chlorine will become available in increasing quantities according to the increase in dose applied. The main advantage of breakpoint chlorination is that it ensures the production of free chlorine which is mueh more effective than chloromine as a disinfectant. Taste and odour problems can be reduced and there may be colour removal as well. The process requires good control to prevent the dosage from dropping below breakpoint level or tastes and odour may suddenly be intensified.

T hemtical curve for

I Free residual chlorine increasing

d proportionately to m dose added m .- bl

2 a, C .- L - L U

Applied c hlorine dose --.c

Figure 14.3 (a) : Typical Chlorine Dose Residual Curves

Theory and Design of Disinfection

Breakpoint F-

Applied chlorlne dose - Figure 14.3 (b) : Typical Chlorine Consumption Curves

14.3.5 Superchlorination and Dechlorination

Superchlorination means dosing of water with a heavy dose of chlorine, often much larger than the usual condition of the water demands. This normally means that free chlorine will be obtained and to that extent the treatment is the same as discussed in case of breakpoint chlorination. The advantage of this method is that the water may be prepared to meet any intermittent pollution due to leakage in distribution of pipe etc. Normally, pure water may require a chlorine dose of 0.2 mgll. To wait for the pollution to occur and then to increase the dose may be impracticable h e n ~ e a continuous high dose rate is applied and an adequate time of contact is given for the worst possible conditions so that supply may be made safe against all odds. After the contact period the water may be dechlorinated by the injection of sulphur dioxide and this sulphur dioxide dose may be so controlled that either the whole of the chlorine is removed or only a part is removed, leaving a residual of the desired order to go into supply. The process has been made sophisticated with automatic methods of control.

14.3.6 Dosage and Contact Time

Chlorination is most widely used disinfection procedure. When used the primary requisites for adequate disinfection are :

a) A clear water free of suspended solids and organic matter.

b) A high enough dosage rate to produce an adequate amount of free chlorine in the water.

c) An adequate contact time. -

In general chlorine dosage rates to final treated water are usually in the range of 0.5 to 2.0 mgfl : the lower rates tending to be those used with clear well water and the higher rate to treated surface water or to well supplies potentially liable to experience sudden pollution where super-chlorination followed by dechlorination after adequate contact time may be given.

Theoretical contact time i.e. volume of stored water divided by throughput rate, should be not less than 30 minutes, and preferably one hour. These normally imply actual contact times in tanks of 10 minutes and 30 minutes respectively. Where protection against viruses in the final water is known or suspected to be necessary, a larger chlorine dose and upto three times the contact time required for E.Coli removal may be required. The WHO International Standards recommends 0.5 mgll of free chlorine for one hour and this is sufficient to inactivate virus even in water that was originally polluted.

- - Water Treatment Chlorine Dosing

The form in which chlorine should be applied in water treatment is governed by the following factors :

i) Quantity of water to be treated

ii) The cost and availability of chemicals

iii) The equipment needed for its application

iv) Skill required for operation and control.

The equipment or set-up used for applying chlorine and its components are known as "Chlorinators". As discussed earlier chlorine can be applied in gaseous form (from liquefied chlorine gas), liquid form (chlorine solution) or (chlorinated lime or bleaching powder).

The liquefied chlorine gas is commonly used in the treatment plants having a capacity more than 500 m3./day because it is more economical for this scale of operation. It is also used in compact packaged water treatment plants even in the developing countries. A set up of liquefied chlorine gas chlorination plant is shown in Figure 14.4.

CHLORINE GAS

II-- WEIGHING LIQUID WATER TO BE MACHINE CHLORINE

/ CYLINDER

LT 0 - i- Q

t Z STRONG LT CHLORINE 0 J

SOLUTION z 0

STRAINER TO DISTRIBUTION MAIN - \

I OVER FLOW TO DRAIN

Figure 14.4 : Line Diagram of a Solution Fed Chlorinator Installation

These are two main methods used for the gaseous chlorination of water :

i) Direct feed method

ii) Solution feed method.

In the direct feed method, the gas is fed directly into the water using a special type of silver of plastic diffiser or perforated tubing which helps in the difhsion of the gas. Direct feed of chorine gas results in considerable operational and safety problems and, therefore, are not widely used.

The solution fed method is much more common. It consists of dissolving the chlorine gas in a small volume of water and the suiting chlorine solution is fed to the main stream of water to be disinfected. The solution feed gas chlorinator consists of pressure reducing valves actuated by metal diaphragms or hydraulically operated floats, orifices for measuring the rate of flow gas after it has been reduced to a uniformly low pressure and devises for injecting the gas into the supply (Figure 14.5).

Theory and Design of Disinfection

AS FLOW METER

WATER FROM PRESSURE GAUGE

REGULATING

NON RETURN

CHL TED

Figure 14.5 : A Gas Chlorinator Setup

Powdered chlorine compounds are preferred for small scale application and they are applied in various ways.

Pot chlorinators (Figure 14.6) are used for small water supply systems such as hand dug wells of a capacity around 1.5 m3/d serving a population of about 50 persons (30 I/c/d).

BL. POWER

PEA GRAVEL

6 t o 7 HOLES

Figure 14.6 : Pot Chlorinator

The pot consists of an earthen pot 7 to 10 litres capacity with 6-8 mm diameter holes at the bottom. The pot is half filled with pebbles and pea gravel of 20 mm to 40 mm size. Top of the gravel layer a mixture of 1:2 of bleaching powder and sand is placed and again the pot is filled upto the neck with pebbles. A pot chlorinator containing 1.5 kg of bleaching powder (having 35% chlorine) can provide adequate chlorination for about 10 m3 of water which withdraws at the rate of 1.5 m3/day over a period of one week.

An another form of small chlorinator is double pot chlorinator. It is used in situations where it is intended to chlorinate water gradually. The inner pot is fitted with a moistened mixture of 1 kg bleaching powder with 2 kg of coarse sand to slightly below the level of the hole and is then placed inside the outer pot. The mouth of the outer pot is tied with a polythene sheet and the unit is lowered into the well with a rope. A double pot chlorinator containing 1 kg of bleaching

. . . . . . " , 1 /n. . , A m \

powder (35% of available chlorine) can provide adequate chlorination for about 5 m of

Water Treatment

P L A S T I C OR- E A R T H E N JAR

HOLE l c m -

-HOLE Icm.

B L E A C H I N G 'POWDER A N 0 SAND

OOURLE POT CHLORINATOR 0

figure 14.7 : Diffuser Hypochlorinator

Concentrated solutions of bleaching powder or calcium hypochlorite are commonly used in urban water supply schemes. The concentrated hypochlorite solution is dosed at a constant rate into the stream of the filtered water flowing into the contact or storage chamber. New tanks, mains, pipes and other components of storage and distribution system should be dosed and flushed with very high concentration of chlorine (upto 50 mgll) over a period of at least 48 hours before they can be brought into routine use.

14.3.7 Advantages of Using Liquid Chlorine as Disinfectant

The liquid chlorine is invariably and universally used for disinfecting public water supplies due to following advantages :

i) It can be easily stored for a long period without any risks of its getting deteriorated.

ii) It is quite cheap and easily available.

iii) It can be easily transported as it takes less space for storage.

iv) The initial cost of installing the chlorination plant is quite low.

v) The chlorine dose to be added can be easily measured and hence, the chances of overdosing or under-dosing are minimised.

vi) It is a very powerful disinfectant and remains in water as residual for sufficient time especially if ammonia also is present in water.

vii) The operation of the Plant does not require much skilled supervision.

viii) No sludge is formed in its application.

Example 14.3

The chlorine consumption in the treatment of 10,000 m3/d of a river water is 6 kg/d. The residual chlorine after 30 minutes contact is 0.40 mgn. Calculate the chlorine dosage in milligram per litre and the chlorine demand of river water.

Solution

Water treated per day = 10,000 m3

= 10,000 x lo3 Iitres

= 10 x lo6 litres

= 10 million litres.

Chlorine consumed per day = 6 kg = 6 million mg

:. Chlorine used per litre of water

- - 6 millionmg = 0.6 mgA 10 million ltrs

': Residual chlorine left = 0.4 mg/l (given)

:. Actual chlorine demand (or chlorine dosage which has reacted in water)

Example 14.4

Result of chlorine demand test on a raw water is as follows :

Residual Chlorine after 10 Minutes Contact

(mgll)

0.19

Solution

2

3

4

5

6

7

8 L

The data has been plotted in graph in Figure 14.8. . .

0.4 I -

0.6

0.8

1:: j 0.48

--

1 .o 0.20

1.2 0.40 -

1.4 0 60 -

1.6 0.80

APPLIED CHLORINE ( m g / l ) -

Draw the chlorine demand curve and find out the "breakpoint dosage". What is the "chlorine demand7' at dosage of 1.3 mgA.

Theory and Design of Disinfection

Water Treatment The breakpoint is represented by the point B.P. and it is 1.0 mgll.

Chlorine demand = Applied chlorine - Residual chlorine

= 1.0 - 0.2 = 0.8 mg/l at the breakpoint.

This chlorine demand becomes constant and thereafter, all added chlorine appears as free chlorine.

:. At any dosage above 1.0 mg/l chlorine demand will remain steady and equal to 0.8 mgA.

Hence, the chlorine demand at a dosage of 1.3 mgA will be equal to 0.8 mg~l only.

:. Chlorine residual = 1.3 - 0.8 = 0.5 mgA

SAQ 1

a) Results of a chlorine demand test on treated water are as follows :

Plot the chlorine demand curve. What is the break-point dosage and what is the chlorine demand at dosage of 1.2 mgfl?

Sample No.

b) A water main is disinfected using a 110 mg/l chlorine dosage by applying 4.0% hypochlorite solution. Calculate how many hypochlorite powder containing 80% available chlorine should be dissolved in 100 mi nf water to make a 4% solution?

14.4 TESTS FOR RESIDUAL CHLORINE

Chlorine Dosage ' .(rng/l)

The amount of residual chlorine left in the chlorinated water after required contact period, is determined experimentally by using any of the following tests :

Residual Chlorine after 30 Minutes of Contact

(mgll)

b) D.P.D. test Theory and Design of Dislnfectlon

c) Chlorotex test

d) Starch-iodide test.

The main test used for residual chlorine for many years has been that using orthotolidine. However, orthotolidine is now recognised as being among those chemical substances capable of causing cancer. 10 ml of chlorinated sample of water is taken in a test tube after the required contact period. To this 0.1 ml of orthotolidine solution is added. The colour formed is noted after 5 minutes. The formation of yellow colour indicates the presence of chlorine in the water. The deeper the yellow colour greater is the chlorine residual. Amount of chlorine is ascertained by comparing the colour in the glass tube with the standard colours already kept in the laboratory.

Due to problem with orthotolidine, the use of orthotolidine for water testing has now been superseded by the plain test using DPD (Diethyl-paraphenylepe diamine). The necessary reagents are supplied as tablets with 'detailed instructions. The red coloun obtained are matched in the usual simple type of comparators with ranges of standard colour discs from as low as 0.02 mg/l to 10 mg/l of chlorine.

The chlorotex test is a simple test and uses BDH (British Drug Housepatented) chlorotex reagent which is provided with standard colour matching card. No comparator is used in this method

In the starch iodine test, one litre of water sample is collected in a heat-proof earthenware vessel known as casserole. To this 10 ml of potassium iodide solution is added. After thorough mixing 5 ml of starch solution is added. After this mixing a blue colour is produced. The blue colour is removed by titrating this water sample against sodium thiosulphate solution of normality NI100.

:. Quantity of chlorine 'in mgn in the sample of water = 0.355/numbe'r of ml of thiosulphate required to remove the blue colour

If a chlorine residual is maintained in the distribution system, it should be checked at least once per week selecting different sampling points from time to time.

14.5 DISINFECTION USING CHLORINE DIOXIDE

Chlorine dioxide is most commonly used as a disinfectant in cases where problems of taste and odour arise. Specially those arising from the presence of phenols. It is also used for the oxidation of organics and the reduction of iron, manganese and colour. It also provides a relatively stable disinfectant residual in distribution mains.

The productlon at site is achieved by adding chlorinated water from a normal chlorinator rn a sodium chlorite solution (NaC102). The product contains both free chlorine and chlorine dioxide (C102). The product contains both free chlorine and chlorine dioxide (C102) which helps in getting excess of chlorine. In some cases, concentrated solutions of sodium chlorite and hydrochloric acid are mixed to produce chlorine dioxide and there is no excess of free chlorine. But the process is more costly than simple chlorination. Taste and odour of water chlorinated by chorine dioxide is superior than water chlorinated by free chlorine. Chlorine dioxide has better disinfecting properties at high pH values. Chlorine dioxide does not react with ammonia hence its use is advantageous for water having a high ammonia content (for this reason, it is mostly used for disinfection of swimming pool water).

Wnter Treatment SAQ 2

1 a) What is residual chlorine?

b) What are different tests to determine the amount of residual chlorine in a chlorinated water afier rcquired contact period? Describe any one of them ?

c) The chlorine consumption in the treatment of 10,000 m3/d of river water is 7 kgld. The resjdual chlorine after 30 minutes contact is 0.3 mgll. Calculate the chlorine dosage in milligram per litre and the chlorine demand d river water.

d) What are super chlorination and dechlorination?

e) What are the factors which influence the disinfection efficiency of chlorine'?

14.6 DISlNFECTION USING OZONE

Ozone is a gas Q3 which has a powefil oxidising effect causing rapid and effective disinfection of a clear water. It is regarded as a natural means of disinfecting water and is not objected as chlorination in some cases. Only precaution is to be taken is that it must be manufactured on site and this is a costly and complicated process. It does not leave persistent residual in a water. It causes precipitation of iron and manganese and is not suitable for turbid water.

Ozone is produced by passing a discharge of high voltage alternating current through dry air. A voltage between 4000 and 20,000 volts is applied to di-electric plates about 6 mm apart or concentric tubes through which the dry air is blown. Although some ozone is produced at normal frequency of 50 cycles per second but in' nbrmal practice, the frequency is stepped to 500 or 1000 cycles per second. High voltages and high frequency give higher efficiency of ozone production. The concentration of ozone produced by modem Plant is of the order of 15 to 20 grams per cubic metre of air. The degree of absorption depends upon the depth of immersion of the injection apparatus below water level in the contact tank and the fineness of the air bubbles introduced and varies from 60% to 90%. This is produced under the chemical reaction :

Under high arc voltage 302 ' 203

(Oxygen nlolecule) .(Ozone molecule)

Because of its instability, ozone readily breaks down into normal oxygen and releases nascent oxygen.

30, -02+o

(Ozone) (Oxygen) (Nascent Oxygen)

During the treatment, ozone gas is manufactured and bubbled through water contained in a sterilising chamber having inlets and outlet for ozone. The ozonised water comes out through the outlet at the top.

The effect of ozone is rapid, the usual contact time being between 5 and 15

minutes. Dosage is of the order of 1 to 2 mgll. Excess ozone after treatment may be removed by giving a further short detention time. Removal of excess ozone protects from corrosion tendency. An adequate dose of ozone rapidly destroy bacteria, bacterial spores and viruses. The ability to destroy spores within the usual contact period is an advantage since many of these are not destroyed by chlorine except when chlorine doses are high. The W.H.O. 1971 International Standards advise that 0.4 mgll of free ozone for 4 minutes has been found to make the virus inactive.

The advantages and disadvantages of using ozone as disinfectant :

Advantages

i) Ozone being unstable, nothing remains in water by the time it reaches the distribution system.

ii) The ozonised water becomes tasty and pleasant unlike the chlorinated water which becomes bitter.

iii) Ozone removes colour, taste and odour from water in addition to removing bacteria from it.

Limitations of Ozone using as Disinfectant

I i) It is costlier than chlorination. Total cost of ozonation is of the order of

I three times the cost of equivalent chlorination.

I ii) Ozone needs electricity for its manufacture and heavy consumption of electricity is there.

iii) No residuals can be maintained as it is highly unstable and its use does not ensure safety against possible future contamination.

iv) The volume of ozonised air to be handled is large.

v) Ozone gas is highly toxic, it is detectable at concentration of around 0.01 ppm but it does not have the distinct warning smell as possessed by chlorine. It may produce discoinfort when inhaled, hence provision has to be provided to be absorbed by carbon absorbers when in excess.

SAQ 3

a) How ozone can be used as an effective disinfectant drinking purpose?

b) How does ozone disinfect water?

c) List any two drawbacks of ozone as a disinfectant.

14.7 OTHER DISINFECTION PRACTICES

On large commercial scale mostly chlorine injection plant is set up for disinfection of a water supply project. But if skilled labour is not available in remote places or supply of chlorine gas and spare parts may not be available regularly then there might be difficulty. For chlorination of small supplies also to set up a chlorination Plant may not be advisable. In such cases, following forms of chlorine and other methods of disinfection are available for use.

Water Treatment Chlorine is available in following for disinfection on a small scale.

i) Bleaching powder i.e. chloride of lime.

ii) Calcium hypochlorite granules.

iii) Chlorine tablets

iv) Sodium hypochlorite solutions

v) Chlorine produced by electrolytic means.

Other Disinfectants and Disinfecting Procedures

In special circun~stances, following disinfectants and methods are used :

i) Use of ultraviolet radiation.

ii) Silver in the form of colloidal

iii) Iodine or compounds based upon iodine

iv) Potassium permanganate

v) Boiling of water.

14.7.1 Bleaching Powder

Bleaching powder or chlo~ide of lime is the cheapest form of chlorine available for the chlorination of small supplies. It contains around 30% to 35% by weight of chlorine and its strength declines slowly when the material is kept in sealed drums. The powder is first mixed in water to make a supsension. In lime, it is allowed to settle and the supernatant chlorine water is drawn off for dosing purpose.

For disinfection of small quantity of water, WHO recommends making of 1% of solution comprising 40g of bleaching powder to one litre of water (this assumes the bleaching power has 25% of available chlorine). It is then recommended that three drops of this solution should be added to one litre of water for drinking.

Strong solutions of chlorine are very unstable and rapidly loses their chlorine content if exposed to the air or sunlight. Therefore, it is necessary to store them in closed dark-walled containers. Even when stored in this condition, 0.1% solution will lose a substantial amount of its chlorine in three to seven days.

For chlorination of wells a pot containing bleach or bleach mixed with sand is suspended in the well. The pot has a small hole drilled in it or a porous pot is taken. By this means, the chlorine slowly diffuses into the well water.

However, the bleaching powder is not a satisfactory medium for application of controlled doses of chlorine on a daily basis. Often it results in overdosing of water with chlorine giving a strong chlorinous taste. There is a heavy sediment of lime when the bleaching powder is mixed with water and this addition sludge becomes a problem for disposal. Chemical equations have already been shown in earlier section.

14.7.2 Calcium Hypochlorite Granules

Calcium hypochlorite granules contain 65% to 68% wlw chlorine. Bulk density of granules being around 0.9 kgll. They are adily soluble and granules can be dosed direct to a supply or a standard strength so 7 ution can be made up for dosing. They give a clearer solution than does bleaching powder but after initial standing, the supernatant of a 0.5% chlorine solution still bontains upto 1% suspended matter. It is preferable to use soft water for making up the solution as the use of hard water results in some precipitation of its hardness. The storage life of the solution is similar to that of solutions of bleaching powder.

I

14.7.3 Chlorine Tablets Theoty and lleslgn of Diqinfection

Chlorine tablets are available in a range of sizes and chlorine content. However, they are expensive to be used on a large scale. The smallest tablets contain about 0.12 g of chlorine and are used by travellers for disinfecting drinking water. The largest tablets contain 3 g or more of chlorine and are used for disinfection of swimming pools and disinfecting water mains. Tablets are available in two forms :

I I rapid release of chlorine and slow release of chlorine.

I 14.7.4 Sodium Hypochlorite Solution

Sodium hypochlorite solution is available under many brand names and are used as household disinfectant. It is a clear solution containing 1% to 15% w/w of available chlorine. It rapidly loses its strength when exposed to the atmosphere or sunlight, hence proper storage containers are necessary. It is quite expensive and, therefore, not so common in developing countries.

14.7.5 Electrolytic Production o f Chlorine

Sodium hypochlorite solution is produced by the electrolysis of a brine solution or sea water. A DC current is passed through the electrolyte, producing chlorine at anode. Hydrogen gas is also evolved during the process and it is vented properly. Solutions containing 8 g or more of chlorine per litre can be produced. Electrical power consumption is 5 to 6 kwh per kg of chlorine produced. Where electricity is available, the unit can be installed for disinfection at small u,nits. Use of a brine solution is more preferable to use than sea water because sea water needs straining before use and the manganese content is not to be too high as high content of manganese reduces the efficiency of the electrodes.

14.7.6 Ultraviolet Radiation

Ultraviolet radiation is suitable for disinfection of small supplies which are free of colour, turbidity and suspended matter. Wave length of ultra violet rays lies between 15 and 400 nm (nm = nanoi~ictric = millimicron). But at 260 nm of wave length peak disinfection is achieved. The UV radiaiton is produced by a low or medium pressure mercury are discharge lamp in a quartz cylinder of about 50 mm diameter, sheathed in an outer metal tube of 100 mm diameter. The water to be sterilised is passed through the annular space between the two tubes. A typical tube is about 0.5 m long, the lamp being 1.0 to 1.2 kW power with an output of 55 to 95,W in the range of 230 to 330 nm. Lamp of this type give an exposure time of 0.74 to 1 . 1 1 seconds of radiation to the flowing water of output 60 to 100 m3/d. However, small water filtration plant with disinfection facility are available in Indian market for domestic use.

14;7.7 Silver

Silyer in the form of colloidal or "katadyll" silver is also having a disinfecting effect. Individual house supplies are treated by passing the water through silver-impregnated candles comprising hollow cylinders through which the flow is fr& outside to inside under small head. Silver content of the treated water is too small to have any toxic effect on the consumers. Another method using silver for disinfection of water is to introduce metallic silver ions into the water by passing it through a tube containing solid silver electrodes, which are connected to a D.C. supply of about 1.5 volts. The so introduced silver ions have a strong germicidal action and acts as a disinfectant. The recommended silver dose varies between 0.05 to O. I mg/l and required contact period varies between 15 minutes to 3 hours.

14.7.8 Iodine 9

Iodine or compounds based upon iodine, usually in tablet form are used as Aicinf~rtant hnnt nnlv in an Pmproenrv nr temnnrarilv fnr Aic;nf~rtinn amall

Water Treatment quantit~es of water for drinking. Iodine has some toxic effect on susceptible individual. Two drops of tincture of iodine of 2% strength is sufficient for one litre of water. A contact time of 20 minutes is sufficient.

14.7.9 Potassium Permanganate

Potassium permanganate is a popular disinfectant for well water supplies in villages which is contaminated with lower amounts of bacteria. Although potassium permanganate is cheap, handy, yet does not guarantee 100% removal of bacteria. It removes 100% organisms causing cholera but is not so effective against other disease organisms. But besides killing bacteria, it also helps in oxidising the taste producing organic matter and removing iron and algae. Hence sometimes, it is added in small dose (0.05 to 0.1 mgA) even to filtered and chlorinated water. For treating a well water small amount of potassium permanganate is dissolved in a bucket of w d c ~ and is mixed with the well water thoroughly which turn the well water pink. If pink colour disappears quickly, it shows the presence of organic matter in water. In such cases, more quantity of potassium permanganate is added until the plnk colour stands. The well is not used at least for 48 hours after the addition of the permanganate. The normal doses of the potassium pennanganate varies between 1 to 2 mg/l and contact period being 4 to 6 hours.

4.7.10 Boiling of Water

Boiling of water is an extremely useful disinfection procedurc whether water is clear or cloudy or contaminated. For complete stcrilisation of a polluted water, it is necessary to bring the water to rolling boll and keep it in this stage at least for 5 minutes. For higher elevation, 1 minute extra is given for every 1000 m above sea level because of lower temperature at which boiling taken place.

Since boiliiig is the most easy and common method of disinfection (of course on very small scale), a wise traveller when offered a choice of drinks prefers tea rather than a cold drink made from the local water supply.

Disinfection of Swimming Pool Water

Swimming pool water poses special problems due to heavy organic loading and continual recirculation of the water. Chlorine, ozone and to a lesser extent of bromine and other chemicals are used for disinfection. RelativeIy high levels of free chlorine residue 1 (1.5 to 2.0 mgll) are maintained and appropriate pH maintenance is also necessary.

'sag 4

a) List d~sinfectants that are used in special circumstances?

b) Give WHO recommendation for preparing chlorine solution.

c) Why do you need to store chlorine solution in a closed dark walled container?

d) Distinguish between the treatments by calcium hypochlorite granules and bleaching powder.

e) What are diffeknt kinds cif chlorine tablets?

f ) What is usually termed as 'Household Disinfectant for Water'?

g) How do you produce chlorine by electrolysis?

h) Under what circumstances would you recommend ultra-violet treatment for water?

i) List the close and contact period for disinfection of water by silver A ; r ; n f o r - t n n t

j) Which disinfectant is used for temporary disinfection of water.

k) Why is potassium permanganate is so freauently recommended to disinfect water in rural areas?

1) Why does extra time is needed to disinfect water by boiling method in hilly areas?

m) Which methods of treatment are used to disinfect water in a swimming pool?

14.8 PREVENTION OF POLLUTION

It is better to prevent pollution of the source than to attempt to remove it at the treatment works. It is said also that "One ounce of prevention is better than one tone of cure". In fact the best source of water is that which is leist polluted.

Following point should kept in mind by an engineer dealing with a water supply scheme.

i) The source should be as remote as possible from pollution.

ii) The catchment to the source should be kept under regular surveillance against pollution.

iii) The quality of raw water should be monitored.

iv) Industrial wastes contain more unknowns than domestic wastes and are more to be avoided.

v) Storage of water gives valuable protection.

vi) The clarification 0f.a water is pre-eminent requirement before it is disinfected.

vii) Traditional methods of disinfection using chlorine or ozone are still the best practicable precaution against the transmission of disease by water.

viii) Detection of E.Coli in water is still the best practicable method for indicating the presence of pollution. I'

SAQ 5

List important points that are to be kept in mind while designing a water supply scheme for disinfection of water.

14.9 SUMMARY

Theory and Desip of Disinfection

After going through this unit, you have learnt about disinfection. This is the process which makes potable water free from micro-organism thus protecting health

Water T r e a h e ~ ~ t of public using water. Although there are different disinfectants which can be used for disinfection of water but chlorine is the most important disinfactent, which is most widely used in our country and other developing countries because cost involved is less and method of chlorination is quite simple.

14.10 KEY WORDS

Active Chlorine That portion of total chlorine (or of a chlorine compound) content, which exerts a germicidal effect when added to raw water, also known as available chlorine.

Break-point Chlorination : It gives an idea of the extent of chlorine added to water. It represents such dose of chlorination beyond which any further addition of chlorine will appear as free residual chlorine.

Chlorination

Disinfection

Dechlorination

Double Chlorination

Flow Rate, Q

Influent

Pre-chlorination

Super-chlorination

The application of chlorine to water, sewage or industrial waste genemlly for the purpose of disinfection.

The process of destruction or inactivation of harmful micro-organism in water either by physical process or chemical process.

Removal of chlorine from water.

The process of chlorination twice. It indicates that the water has been disinfected by chlorination twice.

The amount of water passing a plant per hour or per day usually expressed in m3/d, m3k or l k . Also, called capacity of a plant.

Water, sewage or other liquid raw or partially treated flowing into a reservoir basin or treatment plant. \

Indicates that only the chlorine treatment and no other treatment has been given to the raw water.

Normal standard process of applying chlorine in the end when all other treatments have been completed. While treating normal public water supply, the post chlorination is adopted after filtration and before the water enters the distribution system.

It is the process of applying chlorine to the water before filtration or before sedimentation- coagulation.

It is a process of applying additional amount of chlorine (i.e. 5 to 15 mgA) to the water. This is required in some special cases of highly polluted water or during epidemics of water borne diseases.

Theory and Design of Disinfection

Water Treatment

SAQ 3

a)

SAQ 4

a)

the water. This is done mostly at the time of outbreak of epidemics or water borne diseases. It is also done when organism causing amoebic dysentery (histolytica) is found to be present in water.

Dechlorination means the process of removing chlorine from water. This is done when it is found that super chlorination has been in excess (more than 0.1 to 0.2 mgll).

Refer Section 14.3.3.

Ozone and air mixture is introduced into water through an injector system due to low pressure of water or it can be forced under pressure through perforated pipes or ceramic material to disinfect water.

Ozone readily break and releases nascent oxygen which is a powerful oxidising agent. Nascent oxygen removes organic matter as well as . bacttria from water.

Ozone in disinfection is about two times costlier than chlorination. Manufacture of ozone requires electricity. Further ozone gas is toxic if present in excess.

Used bleaching power, calcium hypochlorite granule, chlorine tablets, iodine compounds, potassium permanganate and boiling of water are some of the methods of disinfection of water used under special circumstances.

As per WHO norms, 1% solution of available chlorine which can be made using 40 g of bleaching power in one litre of water with the assumption that bleaching powder contains 25% of available chlorine.

Concentrated solutions of chlorine are highly unstable and rapidly loses chlorine content if exposed to air or sunlight.

Calcium chlorite granules gives a clearer solution than does bleaching power.

There are two kinds of chlorine tablets. They are rapid chlorine release tablet and slow chlorine release tablet.

Sodium hypochlorite solution.

Sodium hypochlorite solution is produced by electrolysis of brine solution or sea water. Chlorine is produced at anode when direct current is passed through the electrolyte.

Disinfection of small quantities/supplies of water free from colour turbidity and suspended material can be treated using UV radiation.

Silver Dose = 0.05 to 0.1 ppm, contact period = 15 to 180 minutes.

Iodine or its compounds.

Potassium permanganate removes. 100% organisms causing cholera, besides killing bacteria and oxidising organic matters. It also reduces iron content and control algae.

Water boils at low temperature. due lower atmospheric pressure. Hence 1 minute extra boiling time is given for every 1000 m height above MSL.

m) Chlorination, ozonation and also sometimes bromine are used to disinfect Theory and Design of Disinfection

swiming pool water.

SAQ 5

One should keep the following points while dealing with disinfection scheme of water supply scheme :

i) Source of water to be as remote as possible from source of pollution.

ii) Constant surveillance against pollution of catchment area. ,

iii) Monitoring of quality of raw water at a reasonable periodicity.

iv) Industrial wastes are more harmful than domestic waste, hence industrial wastes to be avoided.

v) Proper storage of water is a valuable protection.

vi) Water to be clarified first before its disinfection.

vii) Use of chlorine tablets or ozone are still the best precaution measure against transmissing .of diseask by water.