Lecture 15 Disinfection


Last few classes we were discussing about
various processes in water treatment to remove the solids present in the water and we have
seen plain sedimentation coagulation flocculation and even softening to remove hardness and we discussed in detail
about filtration. Today we will discuss about disinfection.
because even if we remove the solids the physical and chemical characteristics of water is very
good, the water is not fit for drinking unless the bacteriological
quality is safe or bacteriologically the water safe. We have already discussed that bacteriological
quality is the most important water quality parameter. The reason is, if a single pathogen is present
in the water and if it enters in our body there is a chance of getting diseases. So it is very very important that the water
whatever we are supplying or whatever we are drinking is bacteriologically safe. So, for that purpose only we are going for
disinfection. So we will see what this disinfection is. Disinfection is the process in which pathogen
organisms are destroyed or inactivated. Disinfection is only destroying the pathogenic
organism but it is different from sterilization. Sterilization means killing all type of organisms whether it is pathogen or non-harmful
microorganism we are killing everything in sterilization so there is a distinct difference
between disinfection and sterilization. But though we tell that in disinfection all
the pathogens are killed but most of the time it is nothing because we don’t have any measures
to see whether all the viruses are getting removed from the water. So now we will discuss the different methods
of disinfection. We can remove the microorganism by various
processes because we have seen various unit operations and processes in water treatment. The physico-chemical treatments: in each and every
unit removal of bacteriological or microbial cells will be taking place but we won’t be
able to achieve hundred percent removal in any of these processes. That’s why after a series of treatment processes
we have to go for disinfection separately. Then another method of killing the microorganism
is direct application of thermal energy. This process was being used in age old days. Another one is ultraviolet, gamma, X-rays
and microwave radiations. You know that if you open the water to the sunlight
and keep in for long time it will be getting disinfected. The reason is, the ultraviolet rays whatever
is coming from the sun is responsible for the killing of the microorganism. Another method is ultrasonic disruption. These ultrasonic rays will be going and disrupting
the cells in the microorganism. The other method is addition of chemicals. This is the one most commonly practiced in water treatment plants. Then there are other things. If you keep the water for long time in some
storage tanks then the microorganisms will not be having contusive environment for their growth so natural death will be taking place. This we will discuss in detail when we come
to wastewater treatment. There we have some polishing units especially
to kill the e-Coli and the pathogenic organisms. What we do there is we allow the water to
stand for long time under sunlight for example in oxidation ponds. So what will happen is the oxidation pond
will be getting exposed to sun light and sun light you know it is having UV radiation so because of that
one the microorganisms are getting killed. I have already told that there is a distinct
difference between disinfection and sterilization. Sterilization is the process in which complete
destruction or inactivation of micro organisms are taking place. And disinfection, only pathogenic organisms are
getting killed. Now we will see, there are various methods
for disinfection or killing the microorganism from the water. What is the property that makes a good disinfectant? These are the properties. The disinfectant should be able to kill all
pathogenic organisms and it should be effective in a wide rage of pH and mineral compositions
because we will be coming across various types of water with various degrees of pollution and various pH conditions
and mineralogical conditions. So if the disinfectant or the chemical or
the method whatever we are selecting if it is applicable only for a narrow range of values
of pH and mineral compositions then it is not
a good disinfectant because if we want to have hundred percent efficiency we have to
bring the entire water to that specified composition. So a good disinfectant is the one which is able to remove or kill the microorganisms
for a wide range of conditions. The next point is, there should not be any
toxic byproduct produced during the disinfection process because mostly chemical agents are
there so when it acts with the water or when it reacts with the microorganism and inactivates that one during
that time it should not form any byproduct which is toxic. And the fourth point is there should be some
residual affect of disinfectant after the disinfection process is over. This is required because after the disinfection
or after the complete treatment what we do is we usually put the water in the distribution
system so there are chances of bacterial entrance or pathogenic entrance into the distribution system so we
have to keep the water free of microorganisms or pathogens till it reaches the receiver
or the consumer so, for that there should be some residual disinfectant property left over. Therefore, these are the four important properties
to select a good disinfectant. Now we will see what is the mechanism of disinfection. Though the mechanism of disinfection is not
completely clear one thing is very very sure. The disinfectant whatever we are adding either
it is chemical, terminal or any other radiation it will be acting on cell protein to inactivate the critical enzyme
system essential for microbial life. So once it is inactivating the microbial system
the microorganisms will not able no more able to survey or support the life so naturally they will be dying, so, that is the mechanism. Or in detail we can tell like this; it damages
the cell wall and it alters the cell permeability. Thus, when the cell permeability is altered
the cell material whatever is present inside the cell wall will be coming out of the cell so that will be destructing the cell
availability. The third one is changing the colloidal nature
of the protoplasm because these chemicals will be acting with the protoplasm material
and it will be changing the colloidal nature of that one or it will be
inactivating the cell material and the last and most important one is that the disinfectant
will be inactivating the critical enzymes. The critical enzymes are those enzymes which are responsible for all life supporting
reactions or all life supporting functions of the cell. Now we will see the entire steps involved. If these are the mechanism either they will
be altering the cell permeability or attacking the cell or reacting with the critical enzymes. We know that one chemical is present and the
microorganisms are their in the same water so that first the chemical has
to come and reach the cell wall and it has to penetrate through the cell. Unless it enters in the microbial cell it
cannot deactivate the cell component. So the first step is the chemical coming close to the cell and entering
through the cell wall and second step is, once the chemical has entered in the cell
it has to react with the cell components and inactivate them. These are the two major steps of disinfection. From this one it is very clear that the disinfectant
or the material whatever we are using if it is not having any charge then it will be having
high chance of getting penetrated to the cell. Whenever the bacterial cells are present in medium it will be having a
negative charge. So if you are disinfectant is a negatively
charged iron there will be a repulsive pore between the cell and the disinfectant so that
iron will not be able to penetrate through the cell wall. Unless it is penetrated through the cell wall
the chemical will not be able to deactivate the cell. That is the reason; either cations or the
irons without any charge are the best disinfectants. So mostly neutral irons are more active compared
to anions that is why chlorine, ozone, chlorine dioxide etc or being used as good disinfectants. The ozone and chlorine dioxide when they enter
into the cell they have a very high oxidizing capacity so they oxidize the cell matter beyond
inactivating the critical enzymes. Now we will see the most commonly used chemical disinfectants. We can use halogens. the most commonly used halogens is chlorine
because in India we practice chlorination as the most commonly used disinfection method
and bromine and iodine are being used in some places and ozone is a very good disinfectant
but the problem is it is expensive and it also destroys color and odour effectively. But another disadvantage of this ozone is,
it will not be having any residual effect. Other oxidizing agents like KMnO4 and H2O2
are also being used as chemical disinfectant. Then metal irons for example silver irons
it is acting as a bactericidal and copper can act as algicidal and fungicidal. Copper sulfite is being used in swimming pools
to control algal growth. In India nowadays most of the households use
water purifiers, in most of the water purifiers the adsorbent or the activated carbon which
is used for removing the turbidity and color
and odour it will be coated with these silver irons to remove the pathogens. So silver is being commonly used in such domestic
water treatment units. Another method is using alkali and acids. You know that the microorganisms are able
to service under a particular pH value. If you reduce the pH to less than three or
if you increase the pH above eleven then the environment will not be contusive for the
growth of microorganism. So this one we can use for the disinfection
purpose. That means if you can decrease the pH les
than 3 by adding from strong acids or if we can increase the pH by adding from strong
alkali the medium will be or the water will be free of microorganisms. Now we will see what are all the factors influencing
chemical disinfection. We have seen what all are the mechanisms of
disinfection and what all are the various chemicals we can use for disinfection. So it is very very important to know what all are the factors
that effect the disinfection. Unless we know that one, we will not be able
to achieve hundred percent disinfection. Therefore, the first and most important one
is nature of the organisms and concentration and distribution of the microorganism in the
system. the nature of microorganism means if it is
in colloidal form or if it is present as discrete particles in the water
then it is very very easy to destruct them because the chemical can attack them directly. But if the microorganisms are in the form
of flocs the microorganism which are present in the outer surface of the floc will
be getting exposed to the chemical but whatever is there sitting inside they will not be getting
exposed to the chemical that much. Hence, unless the chemical dose is very very high the microorganism whatever
is there in the center of the floc will not be destroyed that is one reason. Another one is, each cell will be forming
a cyst, cysts are highly resistive to this chemical so if the microorganisms are of cyst
forming nature then naturally it will not be getting destroyed or destructed using any of these chemicals. And if you have very high concentration of
the microorganism then also it is very difficult to achieve hundred percent disinfection unless
we add very high dose of disinfectant. So the nature, concentration and distribution of microorganisms
are very very important. Second one is nature, distribution and concentration
of the disinfecting substance. Hence, what is more important is how well
we are distributing the chemical, what is the concentration we are providing and what
is the nature of the disinfectant. if you are giving the chemical dose lower than required for killing the microorganism then
naturally the microorganisms are not going to get destroyed. Similarly, if the chemical or the agent whatever
we are adding for disinfectant is not very stable it will get disassociated immediately
and will fail be to be an effective disinfectant. Third one is the temperature. We know that for any chemical reaction or biochemical
reaction temperature is very very important because as the temperature increases the activation
energy of the reaction will be increasing so definitely high temperature will be preferable for high rate
of disinfection. The next one is the rate and condition of
water to be disinfected. Because, if the water is having lot of turbidity
what will happen is this turbid particle will be acting as a protection for the microorganisms. What will happen is the microorganism will go and
get attached to these turbid particles and when the chemical is applied these microorganisms
will not be getting exposed to the chemical completely so destruction of the microorganism becomes very very difficult. This is one of the reasons why there is a
standard for or there is a limit for turbidity present in drinking water. We know that it is one NTU. If the turbidity is more, disinfection will
not be effective. Another one is if the water is highly contaminated
with organic matter or some inorganic compounds like ferrous, manganese etc the chemical reagents whatever we are
adding……… we have seen that most of the chemical reagents whatever we use for
disinfection are all having high oxidizing capacity so when we add these chemicals to the water the chemical will be used by
the organic matter or the inorganic compounds present in the water so the chemical will
be completely utilized for the oxidation of these compounds so the microorganism will not be getting exposed
to high concentration of these chemicals so naturally the disinfection will not be proper. Hence, if you want to have a high efficiency
of disinfection the water whatever we are subjecting for disinfection should be clear
of turbidity organic matter and other pollutants. The last thing is the time of contact. If the microorganisms are getting exposed
to a toxic compound for longer time definitely the rate of killing will be more so time of
contact is also very very important. Now we will see how we can find out the rate
of disinfection. The most commonly used law or the most famous
law of disinfection is this Chick’s law of disinfection. How can we relate the contact time and the
rate of disinfection? This is the relationship Chick has given;
minus dN by dt is equal to K into N or rate of kill is proportional to the number of microorganisms present. So here dN by dt is nothing but rate of destruction
and K is the rate constant and it is the characteristics of the organism. So this K is not a constant it will be varying
with respect to the reagent or the disinfectant as well as the microorganism. So if you integrate the expression then we
will get; ln N by N0 is equal to minus K into t where N0 is the initial number of microorganism
present in the system and N is the number of microbes present after a time t or we can find out N whatever is the remaining
number of microorganism that is equal to N0 into e raised to minus K into t or Chick’s
law of disinfection is telling; rate if disinfection is proportional to number of microorganisms present. It is not giving any relationship between
the concentration of the disinfectant or the nature of the organism, everything is taken
care by this K. But this statement is true only if all the microorganisms present in the system are having
uniform susceptibility to the chemical. That means all the microorganisms are having
the same resistance or same toxicity for the chemical. For example, if we take water we know that
pathogens are present and escherichia coli which we use as indicator organism will also
be present in the water. But if you consider these two types of microorganisms or bacteria the pathogens are having less
resistance to chlorine compared to E.coli so naturally if you apply a particular amount
of chorine to that water and if you see the number of microorganisms separately naturally the pathogenic organisms number
will be getting reduce at a faster ray compared to E.coli. Therefore, Chick’s law is valid only if uniform
susceptibility of all species present to the particular disinfectant and the second important
point is constant concentration of disinfectant. But we know that when we add the disinfectant to the water with respect
to time the concentration of the disinfectant or concentration of the chemical will be getting
reduced. So at a reduced concentration the effect on
the microorganism will be less so Chick’s law is not considering
that effect also. But that law is considering whether the concentration
of the disinfectant is remaining the same or concentration of the disinfectant is not
having any effect on the disinfection or rate of kill
but that is not true. The third one is absence of any interfering
components. For example, if organic matter present or
iron or manganese is present in the water certain amount of the disinfectant will be
used for the destruction of those materials and the remaining only will
be available for disinfection so that is also not being considered in Chick’s law of disinfection. Another one is well defined conditions; example
pH temperature and ionic strength. These parameters are also not considered in
Chick’s law because if pH is different then the effectiveness of the disinfectant will be different. Similarly, temperature; for high temperature
the activity will be more compared to low temperature and ironic strength also is having
a comparable effect on disinfection. Till now we have seen that rate is a constant
but rate can increase or rate can decrease. When the rate can increase? For example, initially the concentration of
the disinfectant is less with respect to time the microorganisms are there in the same water
and the disinfectant is there in the water so with respect to time the chemical or the disinfectant will be getting accumulated
in the cell wall. So it will keep on be getting accumulated
and at a certain stage the concentration of that chemical will be exceeding the maximum
permissible concentration which the body of the microorganism
can withstand. Once that concentration is reached the microbial
destruction rate will be increasing. That’s why rate can increase when accumulation
of lethal dose takes place. When can the rate decrease? The rate can decrease when the cells increase
the resistance to that particular chemical. We all are aware that if you keep on giving
certain medicine the microorganism will not be responding to that medicine some bacteria or some microorganism will be developing
resistance for certain disinfectant or certain chemicals. So if the microorganism can develop the resistance
in a short period of time what will happen, initially it will be getting killed at a faster rate but
with respect to time as they have already developed a resistance for that chemical so
the rate of kill will be decreasing so we can modify the Chick’s equation like this; minus dN by dt is equal to K into N plus K2
into N into N0 minus N. So this is the velocity of acceleration. Or we can write it in another way; minus dN
by dt is equal to K by 1 plus alpha t into N or N is equal to N0 into 1 plus t raised
to minus K by alpha. So here alpha is the retardation coefficient. That means if the cell already develops some resistance what
will happen to this K? This alpha is a number which is more than
zero then what will happen is this K value will be getting reduced. But if the cell is not developing any resistance then what will happen is this value
will be zero so you will be getting the same equation; minus dN by dt is equal to minus
K. So this term will be taking care of retardation
and this will be taking care of the velocity acceleration because we know
that depending upon whatever value N is, that one will be varying. This is the one which takes care of the velocity
acceleration and this is the one which takes care of the retardation coefficient. So the rate of kill may not be or need not
be a constant always. We have seen that Chick’s law is taking care
of only the number of organisms present. But the concentration of disinfectant is also
very very important. That we can represent using this formula;
C raised to n into t is a constant. So this n can have any value between zero
to one or above that one. If n is equal to 1 that means concentration
and time are very very important for disinfection, if n is having
a low value less than 1 then concentration is less important compared to the contact
time and if n is greater than 1 then concentration is more important than the contact time. We will discuss about this in detail and about
other chemicals whatever we are using as disinfectants and how their behavior is. Now we will see how the temperature effect
is coming into disinfection. We know that as a temperature increases the
activation energy of the reaction also will increase so the rate of disinfection will
be changing or increasing with increase in temperature. So this equation shows that one. Temperature ln N by N0 this is the number
of microorganisms present at any time t and this is the number of microorganisms present
when t is equal to 0 that means before the disinfection. That we can write like this K1 into t1, K1
is the constant and t1 is the time. This is a time at a temperature T1 degree
Kelvin and same is written here for a temperature T2. So this rate constant is changing and contact time is changing. But we know that in both the cases this term
is same so we can equate K1t1 equal to K2t2 or ln K2 by K1 where K1 and K2 are the rate
constants at different temperatures that is equal to ln t1 by t2 that means the
contact time which is equal to E into T2 minus T1 by R into T2T1 where R is the universal
gas constant so this is very very important. Hence, the rate of kill is the rate of the
rate of disinfection varies with respect to temperature. And if you know the rate of disinfection at
a particular temperature We can find out the rate of disinfection at
another temperature using this formula;. This is the famous Van’t Hoff’s Equation. Now we will discuss about the disinfection
by chlorine because chlorine is the most commonly used disinfectant in India. In developed countries chlorine is banned
because of various reasons. The major reason what they are telling is that chlorine can
create carcinogenic compounds with the organic compounds present in the water so most of
the chlorinated organic compounds are carcinogens. If organic matter is present in the water
when we apply the disinfectant what will happen they will be forming chlorinated organic compounds. These compounds will be present throughout
the water so when we drink the water these compounds will be getting
accumulated in our body and most of these compounds are cynabiotic in nature that means
very very difficult to destruct. What will be happen is it will be getting accumulated in our body and it can cause cancer. That is the reason why chlorine is banned
in develop countries but in India it is being used now also. We have seen that the risk involved because
of the carcinogenic nature of the compounds created by chlorine is much negligible compared
to the disinfectant characteristics of the chlorine. That’s why we are using chorine very commonly in India. First we will see the reaction that takes
place when chlorine combines with water, this is what is happening; chlorine reacts with
water to give HOCl plus HCL Hypoclorus acid and hydrochloric acid and HOCl now can
disassociate as HOCl that is H plus and OCl minus, this reaction is very very important. We have seen that when we talk about disinfection
neutral molecules is more effective compared to charged one especially if it is having
a negative charge. Most of the microorganisms will be having
a negative charge. So when HOCl disassociates to OCL, OCl is
having a negative charge that will not be able to penetrate the cell wall as effective
as HOCL. So HOCl is very very important as for as chlorine
disinfection is considered. That’s why the pH is very very important. What will happen is, at low high pH HOCl will
be disassociating to H plus and OCl minus and OCl concentration will be very high whereas
HOCl concentration will be less. But at low pH what will happen is HOCl will
be predominant compare to OCl minus so disinfection will be more effective. That’s what I have written here; the distribution
of HOCl and OCl are pH dependent and the sum of HOCl and
OCl whatever is present in the water is known as free available chlorine. So here again I have given the reactions Cl2
plus H2O gives HOCl plus HCl so we can find out what is the equilibrium constant
for that reaction HOCl into H into Cl minus by Cl2 so this is the equilibrium constant
for that reaction and this is the reaction with HOCl. HOCl disassociate to form H plus and OCl minus
and ka equilibrium is 2.5 into 10 raised minus 8 at 20 degree centigrade and we know that
it is a function of pH because H plus ion is coming here. Or if you know the pH of the water and if you know the
total amount of HOCl present in the system then we will be able to find out the percentage
HOCL. Also we know that HOCl is having hundred times
lesser disinfecting power than OCl minus. This is also very very important. Our aim is to maximize whatever is the HOCl
component present in the water compare to OCl minus. This graph shows the HOCl and OCl distribution
with respect to pH. We can see that. At around pH7 both HOCl and OCl will be almost equal but if the
pH is slightly lower then almost all will be in the in the form of HOCl but if the pH
is higher than 9 then everything will be in the form of OCl minus. So, in water if the pH is in alkaline side
the disinfection efficiency will be very less compared to acidic range and this chlorine
can react with ammonia present in the water also so this reaction is also very very important. Thus the ammonium iron whatever is present
in the water is combined with water to form ammonia and H plus so this is the reaction. Now, whatever ammonia is present that will
react with the chlorine to form chloramines. So first is ammonia, one hydrogen ion will
be replaced by chlorine ion so we will be getting NH2CL which is known as monochloromine. And after that one what will happen is this
monochloromine will be reacting with another molecule or another atom of chlorine to form
NHCL2 or dichloromine, it is having two chlorine and again it will be reacting with three chlorine to form trichloromine. So if you further or chlorine then this will
be disassociating and chlorine will be regenerated and N2 gas will be escaping from the system. So if you have ammonia present in your water
then we have to add more chlorine to remove the ammonia. If you add chlorine to water and if you measure
the residual chlorine and added chlorine and if you plot it we will be getting a typical
curve like this known as the breakpoint curve it is very very important. Initially what is happening is up to here
the residual chlorine is almost at nil. So this is used for the destruction of other
compounds whatever is present. We have seen that if organic matter is present
it will be used for the destruction of organic matter
then if microorganisms are there that also will be getting destructed so some amount
of chlorine will be use for the destruction of the organic compounds present in the system so there will not be any residual chlorine
at all. So, after sometime what will happen is all
those things are destructed then ammonia is present in the system hence further what will
happen is the monochloromine, dichloromine, trichloromine and all those things will be formed in a stepwise manner. So if you find out the residual chlorine you
will be getting a curve as something like this. Residual chlorine will be a sum of HOCl OCl
and monochloromine, dichloromine and trichloromine. Now at this point B what will happen is all
the ammonia whatever is present in the water is already converted to trichloromine that
means NCL3 so if you add some chlorine at the time what will happen this trichloromine will be disassociating and nitrogen
gas will be going out and chlorine will be coming so your combined residual chlorine
will be coming down because NCL3 is getting destructed from the system so this is the breakpoint that means all the
chloramines and chloro organic compounds are getting destructed from the system. Here whatever you see up to this point is
the combined residual chlorine so some more chloramines whatever is present in the system
that we can get from here and this portion from here to here gives you the free residual chlorine and this is the breakpoint
chlorine. This break point is very very important. So if you want to find out the disinfectant
concentration required what we have to do is we have to prepare this breakpoint and we have to add chlorine up
to this breakpoint and add little more to maintain the residual chlorine as prescribed
by the standard. So this is the typical curve for water if
it is having ammonia. Now, if you have only distilled water and
if you add chlorine how the curve will look like? It will be of a line at an angle of 45 degree
because whatever you are adding will be available as chlorine residual so it will be a 45 degree
line. But if you have a solution which contains
some glucose or something like that then initially some chlorine will
be utilized for the destruction of the organic matter or the glucose solution then again
it will be raising at an angle of 45 degree, you will not be getting this type of a curve. This is typical of the presence of ammonia. But sometimes in water treatment system what
people do is, this combined residual chlorine is much more stable compared to the free residual. So, if you want to maintain the residual chlorine
effect for a longer time then after treatment is completely over
little ammonia will be added to the system before chlorination therefore some combined
residual chlorine will be there so that will be remaining in the system for a longer time. What is the mechanism of chlorine disinfection? We have already discussed the mechanism so
we can see that once again. Penetration into the cell wall: Cell wall
is negatively charged so we can see that HOCl can easily enter in the cell whereas OCl will
be having some resistance that is why HOCl is hundred times more active than OCl minus and CL2 affects the sulphydryl enzyme
which is required for the oxidation of glucose. sulphydryl enzyme is the one which generates
the energy. So the chlorine will go and act on that enzymes
so the energy generation system will be getting affected
and it also inhibits the respiratory enzymes and also inhibits the phosphate uptake and
permeability of the cell membrane is altered and leaching of the cell material will be happening. So chlorine is having all these properties
so in all these ways it will be destructing the cell. That means permeability changes, phosphate
uptake is getting affected, prescribly enzymes are getting affected, sulfiteral enzymes are getting affected
so this cell will not be able to survive at all. Now we will see what are the different types
of chlorination. We can classify the chlorination into basically
five categories depending upon the point of application. First one is; • Plain chlorination
• Pre chlorination • Post chlorination
• Break point chlorination and • Dechlorination By the name itself it is clear. What is plain chlorination? Plain chlorination is the one where no other
treatment is there only chlorination is given after treatment to the system. This is practiced mostly in case of emergencies. So the water treatment system is not working
at all so what one has to do is we have to make sure that the water whatever we are supplying
is free from pathogens. Because if the water is not safe chemically
or physical appearance-wise it can be tolerated for a short period of time. But if the water is not safe bacterialogically
it cannot be tolerated even for a single hour because what will happen is if pathogens
enter into our body it can cause diseases. So whatever be the condition we have to make
sure the bacterialogical safety of the water. So in emergency purpose or if your water source is very very clean or it
is unpolluted then we can only go for plain chlorination we don’t have to go for any other
treatment. Thus, plain chlorination we have surface water
with no other treatment. In such cases what will happen is if high
organic content is there high chlorine dose is required and when we go for plain chlorination
we have to make sure that a combined available
chlorine of 0.5 milligram per liter is available after the chlorination and after the contact
time and the turbidity should be less than 5 to 100 NTU or preferably 5 NTU because if more turbidity is there the disinfection
will not be effective and Fe and Mn should be less than 0.3 milligram per liter. So, if the concentration of Fe and Mn is higher
what will happen is this will be utilizing the chlorine which is a strong oxidizing
agent for the oxidation of Fe2 plus 3 Fe3 plus and manganese. So whatever chlorine you are adding mostly
it is used for the oxidation of the other pollutants that is why we tell that plain chlorination can be
applied only to relatively unpolluted sources. The next one is prechlorination. prechlorination is nothing but the application
of chlorine before any other treatment. It is having certain advantages. It improves coagulation. Especially if you are collecting the water from some stagnant water bodies as they
will be having lot of algae so removal of algae by coagulation flocculation is very
very difficult or we have to add excess alum dose to remove the algae. But if you chlorinate the water and kill the alga then
it becomes very easy to remove, thus, improve coagulation. The next one is, if some organic matter is
present and if you chlorinate it chlorine can oxidize the organic matter so it reduces
the taste and odour of water. So, by reducing algae and other organisms
keep the filters and cleaner otherwise these microorganisms
will be growing on the filter sand and filtration will not be effective especially the rapid
sand filtration and its effectiveness depends upon the residual chlorine concentration available. If you have sufficient residual chlorine then
all the treatment units will be protected from bacterial growth and preferred residual
concentration is in the range of 0.1 to 0.5 milligram per liter. So, if the residual concentration is low then it will not be having
the effect so it is always advisable to keep a residual chlorine concentration at 0.1 to
0.5 milligram per liter. Now the other one is post chlorination. That means addition of chlorine after all other treatment. This is the most commonly practiced one. What we do here is, after the filtration or
after the end of all the treatment units we are adding chlorine to the clean water coming
out of the treatment unit and we allow it to stand for 30 minutes because the usual
contact time given in chlorination unit is 30 minutes so whatever microorganisms are
present here in it will be killed. This process is known as post chlorination. This is the most commonly practiced one. In post chlorination if you want to find out
the dose we have to find out the break point and add some dose which is slightly higher
than the break point so that the required chlorine is present. Now we will see what dechlorination is. Dechlorination is practiced when we need quick
disinfection. When the water is highly contaminated we know
that the water is having lot of pathogens present in that so at the time what we have to do is we have to
add heavy dose of disinfectant to that one so that all the pathogens will be getting
killed very fast then take the water for entire treatment but after that if you taste the water it will be having lot of this disinfectant
taste so it will not be acceptable by the public. Therefore it is higher in residue and it is
not advisable to send it into the distribution system. So if high residual is present then we have
to go for dechlorination. That means we are going for high dose of chlorine
and the residual chlorine dose is very very high so we have to remove the excess chlorine whatever is present so we are going for dechlorination. Now we will see what are the methods we can
practice for dechlorination. One is aeration. When we do aeration whatever are the dissolved
gasses it will be going out so we can use this in method for removing the excess chlorine
and another one is addition of sulfur dioxide, sodium bisulfite or sodium sulfite. You know that all these things are reducing
agents. So sulfur dioxide is a reducing agent so it
will be getting oxidize to sulfite; similarly sodium bisulfate and sodium sulfite. And you know chlorine is a very strong oxidizing agent
so chlorine will be getting reduced to chloride and these compounds will be getting oxidized
to the corresponding compounds. So these are the reactions. Another method is adsorption on activated
carbon. So SO2 and chlorine react to give H2SO4 and
hydrochloric acid. Similarly, sodium bisulfate and sodium thiosulfate
all these things can be used for removing chlorine or as dechlorinating agents. Now we will see the disadvantages of chlorine. This I have already mentioned in the initial
slides. One is formation of chlorophenols. If the water is having phenols then this chlorine will be reacting with the phenols
to form chlorophenols; it is having a very bad smell; fishy smell. So if phenol is present in the water and if
you go for disinfection using chlorine you will be getting taste and odour. Another one is formation of carcinogenic compounds
which are chlorinated organics. These are the major problems with chlorine. Now we will see what are the non chemical
methods of disinfection. One is thermal method. This is direct application of heat energy. Naturally if you go for this nearly complete
sterilization takes place. We know that the energy involved in this process
is very very high so it is applicable only for small volumes of water. Thermal resistance varies from cell to cell. So even if you boil the water and keep it
boiling for sometime certain cells will not be getting
destructed. The next method is UV radiation. UV can destruct the cells. UV may kill the cell, retard its growth or
change its heredity by gene mutation. So these are the actions on how the UV radiation
is killing the cells. It will be acting on the nucleic acid and you know that in nucleic acid DNA and
RNA are there so what will happen is this DNA or RNA will be getting affected so UV
will be directly affecting DNA RNA resulting in nutrition of the gene that will take place and it can retard the growth and
when UV radiation is acting on water it can produce H2O2 so H2O2 is a strong oxidizing
agent so this oxidizing property of this H2O2 is also one of the reasons of destruction. The major disadvantage of UV radiation is
there is no residual effect and it is costly compared to chlorination. Another one is gamma and X ray radiation. Any electromagnetic radiation can kill the
cells and it can penetrate and destroy the cells. But again it is very very costly so not usually
practiced in water treatment plants. Now coming to ozone, ozone is a very very
good disinfectant. But here we can see that ozone is having a
peculiar nature; it will not be having any disinfection property up to a certain concentration
and once it reaches that concentration the disinfection
or the efficiency increases drastically. This is the surviving microorganism; this
is the chemical dosage. So it is having a no or complete effect. That is the case of ozone. But chlorine you can see that at low concentration
itself it is having sun disinfection concentration and it will not
be having hundred percentage like ozone. Advantages of ozone:
It is a strong oxidizing agent and immediate bactericidal properties are there and it can
work on any pH. We have seen that chlorine can work only on
that acidic range ozone can work under any pH condition. The mode of action of disinfection is similar
to chlorine and it attacks dehydrogenase and respiratory system and when ozone reacts with
water it produces radicals are HO2 radicals and this plays a significant role in disinfection and hence it is effective
for cysts and even viruses but again it doesn’t have any residual property. How can we add chlorine? Chlorine can be added to the system by addition
of a weak solution prepared from bleaching powder, addition of weak solution by electrolyzing
a solution by brine or by addition of chlorine as gaseous form. So there are different forms by which we can
add chlorine and most of the times the water treatment plants use chlorine gas so we have
to be very very careful when we handle chlorine because it is very toxic. So cylinders should be stored upright and
ready access and easy removal is possible. You know that chlorine is very dense compared
to air so the chlorine storage rooms should have proper ventilation and the ventilation should be placed in the lower
side of the room. The reason is it is heavier compared to the
air so all the chlorine will be getting accumulated in the bottom so if you want to remove that
one effectively the ventilators should be placed in the bottom
of the row. Then this is the commonly used chlorinator. You can see the chlorine; this is the meter
which adjusts the flow rate and all those things. Some water is coming here and chlorine gas
is coming and this is an adsorption tower so what will happen here
is the chlorine will get mixed up with the water and we will be getting a weak solution
of chlorine which will be going to the water. This is another chlorination panel, another
method. Here an injection tray is there and vent hose
is there and this is the water coming so it will be coming here and will be mixing with
the chlorine and whatever mixed chlorine solution is coming out will
be going to the tank which is meant for disinfection. Here, this is the gas whatever is generated
so we can mend through this hose. Now we will see what are the most commonly used chlorine compounds in India. Bleaching powder: Bleaching powder is very
very commonly used and in bleaching powder the available chlorine is only 20 to 30%. So when we make the calculation of the dose
we have to be very very careful because the available chlorine is only 20
to 30%. Another one is hypochlorites, here the available
chlorine is 60 to 70% and another one is chlorine dioxide. Chlorine dioxide is having one advantage;
it is efficient for a pH range of 6 to 10. So, if you have high alkaline in water and
if you want to disinfect that without bringing the pH to 7 or lower then we can go for chlorine
dioxide. We know that we have to maintain chlorine
residual in the distribution system because we have to take care of the microorganism
whatever can enter in the distribution system. So according to the standard we have to maintain 0.3 to 0.4 milligram per
liter of residual chlorine in case of prechlorination and 0.5 milligram per liter in water works
especially when the cysts of E-hystolytica is present that means cyst cannot be destructed by low concentration of chlorine
and 0.5 milligram per liter of prechlorine is present for one hour in which it can inactivate
viruses and 0.4 to 0.5 milligram per liter for 6 hours can inactivate nematodes. Now we see what are the things we have discussed
today. We know that disinfection is a very very important
treatment for water distribution because water should be free from microorganism or bacteriological safety is the most important water quality
parameter because their effect will be immediate compare to chemical parameters and physical
parameters. Disinfection can be done by various methods
like any of the physical chemical method it can remove a certain
amount of microorganism but the removal is not hundred percent. It can also be done by direct application
of heat, UV radiation, ozone and chemicals. Chemicals; we can go for any halogens; chlorine,
bromine, iodine then potassium permanganate H2O2 ozone etc and we have seen the mechanism
of disinfection. What is happening is the chemical will be penetrating into the cell and deactivating
the active enzymes which are responsive for the life supporting reaction. So that is the mechanism of almost all the
disinfectant. And we have seen that the most commonly used
disinfectant in India is chlorine because chlorine addition is cheaper and it makes
hundred percent deactivation of the pathogens and the efficiency of chlorination is depending upon the pH of the water, chlorination
will be effective at acidic pH because when chlorine combines with water we get HOCl and
HOCl is stable under acidic conditions. In alkaline condition OCl will be formed which is one by hundred times
efficient compare to HOCL. Then ozone is another important and good disinfectant
but the disadvantage of ozone and UV is it cannot maintain any residual effect. And we have seen that if you want to find
out the dose of chlorine required we have to find out the break point chlorination and
add chlorine slightly above the breakpoint doors so that we will be having enough residual concentration. So depending upon the condition whether viruses
are present or nematodes are present we can change the residual concentration. So the recommended residual chlorine dose
in the distribution system is 0.2 to 0.5 milligram
per liter and chlorine can be added in different ways either as gas or bleaching powder or
chlorine dioxide.

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