Physico - Chemical Processes for Wastewater Treatment

Professor V. C. Srivastava
Department of Chemical Engineering
Indian Institute of Technology, Roorkee
Lecture - 06
Water Quality Monitoring: Chemical Parameters - II
So, good day everyone, welcome to this lecture on Understanding the Chemical Parameters
while doing the water quality monitoring in the course on Physico Chemical Processes for
Wastewater Treatment. So, in the last class, we were discussing regarding hardness and
various other elements in particular, like sodium, which has important parameter and for
understanding the usage of various types of water, in particular, for irrigation.

Now, we will be continuing the chemical parameters in this lecture as well. And there are
important parameters like alkalinity which are not well understood. So, in particular, so,
alkalinity is one of the important parameters along with pH. And that actually not only helps
us in determining which method has to be used, also many times it is inherently we must have
sometimes alkalinity in the water for any treatment process to work sufficiently well.

So, many times we have to use the alkalinity, add the alkalinity from outside. And that way
when a particular alkalinity value is not present, then the treatment process may not work. So,
for this reason, alkalinity is one of the important parameters and it has to be within certain
limits to how to find out the alkalinity, what is its importance, this is the first parameter that
we are going to study in this lecture.

(Refer Slide Time: 2:05)

So, if I ask anybody sometimes to very common students like if we have 2 water and one
water is having, one water sample is having pH 7 and another having pH 7.5. So, is it always
necessary that pH 7.5 water will have always higher alkalinity? So, it is not correct, any
higher pH water may have lower alkalinity also. So, this is a possibility.

So, alkalinity is not directly related to pH. It is related but it may still be changed their
alkalinity value may change or differ with respect to waters having same pH or otherwise. So,
what is the alkalinity? It is a measure of the ability of a solution to neutralize acids to the
equivalence points of carbonate or bicarbonate. So, here alkalinity is like a buffering
capacity.

So, if you add acids actually it will neutralize the acid. So, it is the waters ability to absorb
hydrogen ions without significant pH change. So, if the alkalinity value is high, so, that
means that water has more buffering capacity. And if any acid rain or anything happens, it
will not change the pH of that water.

So, this is very important parameter and alkalinity in general is it is equal to the
stoichiometric sum of all the bases present in the solution. So, that base may not be only
hydroxyl radical, that base may be other types of anions also. So, this is possible in natural
environment which is commonly there, if any water body is there and which is open to
atmosphere.

So, it will always contain some carbonate alkalinity, this is so, because some amount of CO2
will always get dissolved into the water up to its solubility limit depending upon the pH of
the water. So, alkalinity, natural alkalinity is always present in any water which is under the
atmospheric condition because the CO2 will always get dissolved. So, this is always present.

Other natural components also may contribute to the alkalinity and that may be like hydroxyl
hydroxide ions, borate, phosphate, silicate, nitrate, dissolved ammonia, conjugate basis of
some of the organic basic, organic acids and sulphide.

So, all these things add to the alkalinity. And these alkalinity is very, very important and how
to determine this alkalinity we are going to learn in this particular lecture. Now, higher
alkalinity of water protects the faeces and aquatic animals from any rapid change in pH due
to acid rain.
Because any acid rain which will occur, so, the hydroxyl ions will be neutralized by this
alkalinity. So, pH change will not happen. So, it will protect the fish and aquatic animals.
Large amount of alkalinity present in more than the required amount. So, it will impart some
bitter taste to the water.

Also precipitates may form because of the reaction between cations in the water and
alkalinity if more amount of alkalinity is there. But it is in some other treatment like
coagulation, this is what is that is why if alkalinity is not present, we have to add alkalinity
because we want precipitate to form and if alkalinity is not there precipitate will not form.

So, in natural water precipitation may not be good. In the treatment process we want
precipitation to occur. So, we had to add alkalinity from outside. Alkalinity present in the
water can corrode pipes in water distribution systems also. So, this is an undesirable thing
with respect to usage of water, which is having higher amount of alkalinity in industrial
cases, so this is there.

(Refer Slide Time: 6:27)

Now how to determine the alkalinity and how it is defined? So, alkalinity is usually expressed
in milliequivalent per liter as it has been done and it is like all the anions are added together.
So, here we can see the bicarbonate, carbonate and then we have OH and finally, subtracting
the H plus. In general we can put it like this also all the anions together in form of adding
them together minus all the cations but then only we consider H plus only, so this is there.
Alkalinity is usually expressed in meq/L (milliequivalent per liter).

Alkalinity ( mol L ) =  HCO3−  + 2 CO3−2  + OH −  −  H + 

where, the quantities in parenthesis are concentrations in meq/L or mg/L as CaCO3.

Concentration of X ( mg L ) ×50 mg CaCO3 /meq

( mg L ) of X as CaCO3 =
Equivalent weight of X ( mg meq )

And what we do is that we always report try to find out in milliequivalent per liter, but for
water treatment uses as well as for our common knowledge by so that we can understand that
how much amount of alkalinity is present, we always convert this milliequivalent per liter
into milligram per liter as CaCO3.

So, what we do is that we use this formula. So, initially what we do is that we calculate the
concentration in milliequivalent per liter or if it is in milligram per liter also we can calculate
and then we can convert it into milligram per liter CaCO3. You will understand it better when
we solve the problem, so, in today itself.

(Refer Slide Time: 7:56)

Now, determination of alkalinity, so, in natural environment already we have added this.
Now, if a water sample, what we do is that for determining the alkalinity, we take the water
sample. We go on adding some amount of indicators and then we add H2SO4 or any other
acid can also be used but generally H2SO4 is used. And we go on adding the H2SO4 and
measuring the amount of H2SO4 that has been used.
So, and it will depend upon the molarity of H2SO4 which is being used for titration, so, this
is there. And we have two important pH up to which we determine that how much amount of
H2SO4 has been used and those pH are like pH 8.3 and pH 4.5. So if any wastewater is there,
suppose it is having pH 10. So, what we do is that? That water sample we will take.

We will add some indicators, and then we will determine that how much of H2SO4 of
particular molarity or normality has been used to lower the pH up to 8.3. Similarly, we will
measure up to 4.5. So, two important volume of a particular molar H2SO4 or normal H2SO4
which has been used to lower the pH of that wastewater up to 8.3 and 4.5 it is determined.

And from that we can calculate the all different types of alkalinities. Now, if a, what are the
basis why we choose 4.5 and 8.3 it is written in this. And if a water sample is having pH
greater than 4.5 and it is titrated with any acid to lower the pH up to 4.5. At 4.5, all the
natural anions which are present like hydroxyl, carbonate, bicarbonate, they get neutralize.

So, what does it mean? If a sample of water is having pH lower than 4.5 it will have no
alkalinity et al. So, if any H plus is added further, it will lower the pH very quickly, there will
be no buffering capacity of that water which is having a pH less than 4.5. Above 4.5. It will
be having some buffering capacity.

Now, if a water sample is having pH more than 8.3 it is titrated with any acid to lower the pH
up to 8.3. So, at 8.3 equivalence point it is known that all the OH ions and half of the
carbonate ions get neutralized. So, these two important knowing’s are there through which
we calculate the different types of alkalinity as well as the total alkalinity. Now, how the
titration works?

(Refer Slide Time: 11:19)

So, in any water sample, what we do is that, suppose pH is having it is having pH more than
8.3. So, we add phenolphthalein indicator and as soon as we add the phenolphthalein
indicator, it will change the color of the water to pink. This pink color is because of the
presence of hydroxyl ion. So, phenolphthalein with hydroxyl ions gives pink color. Now, if
we add sulfuric acid to this sample, so, at around 8.3 the disappearance of the pink color will
be there and that will be because of the neutralization of all the OH minus ions.

Now, that tells that we have reached the 8.3. Here half of the carbonate alkalinity also gets
removed. Now, addition of, at this point now, we add another indicator, mix indicator and as
soon as we add that, it will change the color to blue indicating still the carbonate, carbonate
ions are present as well as bicarbonate ions are present.

Now, we again go on adding the sulfuric acid to that and it will change the color from blue to
red, add 4.5 equivalents point and add that point all the other alkalinities because of carbonate
and bicarbonate everything will get neutralized. So, this is the end point which is pH 4.5. So,
this is how we go on.

So, we have to note the value of acid, volume of acid used, sulfuric acid used till 8.3 pH point
as well as that 4.5 equivalence points. So, 2 equivalence points are important, one is 8.3,
another is 4.5. Now, one example, we will solve today itself.

(Refer Slide Time: 13:18)

So, a 50 ml sample of water is having an initial pH of 11.2, determine the species and the
quantity of each species of alkalinity present. If the 8.3 equivalence point is reached at by
using 8 ml of 0.01 normal H2SO4 and 4.5 equivalence point is reached using 18 ml of 0.01
normal H2SO4. So, this is the problem and we have to determine the total alkalinity as well
as the individual alkalinities also. The hydroxyl alkalinity, the carbonate alkalinity and the
bicarbonate alkalinity. So, all three we have to determined. Now, here some calculations are
shown.

(Refer Slide Time: 14:12)

So if suppose one normal H2SO4 is there, so how we can write this? So, this is we want to
convert one normal H2SO4 into milligram per liter of CaCO3. Because alkalinity is always
reported in milligram per liter. Now, we can easily calculate this using the tricks which are
given here.

So, one this is one normal, so that means 1 equivalent per liter of H2SO4, and this is one
normal. And one normal H2SO4 is equivalent to 1 equivalent and for one CaCO3, 1
equivalent is equal to 5000 milligram of CaCO3.

Because we can easily calculate for calcium carbonate. It is having atomic weight of calcium
is 40 plus 12 for carbon and plus 48 for oxygen, so, it is 100 in total. Now 100 divided by 2
because we have 2 valency, so that is why it is 50. And for this 1 equivalence point is equal to
5000 milligram CaCO3.

So, one normal H2SO4 is actually equal to 5000 milligram per liter of CaCO3 in terms of
calculation. Now, if the normality of H2SO4 used for titration is n, so, for the present
question, it is we have taken as 0.01, but here the normal calculation is given any other value
may be there.

Now, if n normal H2SO4 is used. So, it will be having we have to multiply by the normality
only to this, so this is there. Now, so, up till here this was a general idea is given. Now, if
volume of the n normal H2SO4 which is required to lower the pH to up to 4.5 is total volume
which is required is V.

So, the total alkalinity in milligram per liter will be equal to using the formula which is given
here. So, 5000 into volume of, we are using the trick of V1, N1 is equal to V2N2. So, through
that we are finding out the total alkalinity. And this is the volume of sample which was taken.

(Refer Slide Time: 16:46)

 a
So, in the previous example, the volume of sample which was taken was 50 ml. So, this
volume was 50 ml and 5000 is the conversion ratio and V is the volume of the H2SO4 up to
4.5. N is the normality of H2SO4 and through that we can calculate the total alkalinity. So,
now, for OH alkalinity what we do is that the pH is already given. So, from pH we can find
out the pOH.

So, like and from that we can directly calculate and what we have to do is that we had to
multiply by this only to convert into corresponding calcium carbonate milligram per liter
value. Similarly, with respect to if same normal H2SO4 used and volume required up to 8.3
equivalence point this is, this is sorry this is 8.3, so up to 8.3 whatever is the value required
from that we calculate the total hydroxyl alkalinity as well as half of the carbonate alkalinity.

So, we can from here we can determine this and through that we can perform the calculation.
So, we can we know this total value and then we can find out the carbonate alkalinity by
subtraction. So, we will solve the problem now.
(Refer Slide Time: 18:20)

So, in this present case what is given is that normality is 0.1, volume is 50 divided by 1000
because we have to take the value in liter. pH is 11.2, volume up to H2SO4 equivalence point
of 4.5 is 18. So, we again divide by 1000 to convert it into ml and for 8.3 equivalence point
the volume required was 8 ml, so, we convert into liter. So, total alkalinity will be using this
18 value. So, from 18 we use a normality, the volume of sample is 50.

So, we use the formula total alkalinity and we can directly find out that 180 milligram per
liter of CaCO3 as alkalinity is the total alkalinity. Now, next step what we find out is that
what is OH alkalinity because up to 8 pH from pH also we can get the OH alkalinity and now
for pH is for this case it was 11.2. So, 14 minus pH will give pOH and from pOH we can
easily determine that 79.245 is the OH alkalinity in this water.

Now, once this is determined, then what we do is that we determine the alkalinity with
respect to OH plus half of the CO3 2 minus. And that can be obtained using this this formula
itself, but in place of 18 it will be 8, which will be there. So, this will be like 50,000 into 8
into 0.01 divided by 50. So, this will whatever is the value, this will give total OH alkalinity
minus, plus half of this.

So, we can directly determine the carbonate alkalinity via subtracting, so this is what is
shown, so total value is already known to us, this OH alkalinity is already known to us, so,
we can write in this formula like 79.245 plus 0.5 of the carbonate alkalinity is this will be
equivalent to this particular value, what is, whatever is given here, so this will be 50,000 into
8 into 0.01 divided by 50.
And from this we can directly determine the carbonate alkalinity what is given here and this
is what has been done. So, here only in terms of formula it is written that directly also we can
find. So, this value is 1.511. And once OH alkalinity and carbonate alkalinity are known, we
can subtract these from total alkalinity and we can get the value of bicarbonate alkalinity.

So, this way, we can determine all the 3 types of alkalinity values which are given in this
case. So, this is that total alkalinity and all the species alkalinity have been determined and
they will be used further also. So, alkalinity is one of the important parameter.

Now, there are many dissolved gases may be present in the water, already we learned about
carbon dioxide. So, carbon dioxide will always be dissolved in the natural water. Also
oxygen will also be dissolved. So, depending upon the solubility limit, so, how to calculate
that how much amount of any gas can get dissolved in the water at any temperature that we
will learn later on using the Henry's constant values. But we will try to understand the what
are the importance of dissolved gases in the water. Already for CO2 we learned.
(Refer Slide Time: 22:39)

Water may also contain some other amount of gases in addition to oxygen and carbon dioxide
and those are dangerous gases generally hydrogen sulphide, ammonia depending upon the
pH, anaerobic or aerobic condition of the water and what are the various organic species
present in the water. So, that will depend upon that.

The surface water always absorbs oxygen from the atmosphere and algae and other tiny
plants and other life species which are there in the water they use that water for their
purposes. But if dissolved gases, dissolved oxygen is necessary for sustenance of life that is
there.

So, and the water also absorbs carbon dioxide from the atmosphere and the calcium and
magnesium salts if they are there in the water, they will get converted into bicarbonate, if
carbon dioxide is present and they cause hardness.

So, they are very interrelated thing that which gases are present, how much concentration
whether other types of ions and minerals are present. So, they are weathering their
precipitation, everything will be dependent upon pH upon how much amount of gases are
present and how they react with each other. So, they are very, very interesting topics to
understand. We will not be going further.

(Refer Slide Time: 24:05)

Similarly, chlorine may also be present sometimes generally it is not there. So, dissolved free
chlorine is never found in the natural water. But in the water which is like discharged from
the industrial premises they may contain chlorine. Also a disinfection we do in the water that
we take out from the ground before actually using the water for drinking.

So, some amount of chlorine may be there because hypochlorite, etc., is used. So, if chlorine
is there residual chlorine is there in the treated water. So, it is not good, up to a certain value
it will kill the pathogenic bacteria. But if it is used beyond the desired value, then it will cause
problem.

So, residual chlorine has to be determined in the water and that is determined by using the
starch iodide test. And in this test actually we use potassium iodide along with the starch
solution and if blue color is found we say that okay residual chlorine is present in the water
and that should be determined and so, what we have to do?

We have to remove this blue color. So, what we do is that we titrate by the sodium thiosulfate
solution and via using the calculations we can determine the quantity of chloride which is
present in this particular water. And there are intensity of the color actually is compared with
the standard colors to determine the residual chlorine. There is a limit that the residual
chlorine should always be less than 0.5 milligram per liter. So, point it is between 0.05 to
0.02 milligram per liter in the water and so that it remains in the safe limit against pathogenic
bacteria.
(Refer Slide Time: 26:18)

There are other types of metals and other chemical substances may be present in the water
and different types of metals such as iron, manganese, copper, lead, barium, cadmium,
selenium, fluoride, arsenic, etc., may be present. So, and all these metals, most of these
metals are highly toxic and poisonous and they must be removed in totally.

So, and the treatment process should work to remove these toxic elements of the water. So,
one thing is that how to determine the presence of these metals and many other organic
compounds in the water. So, there are many sophisticated instruments which are there which
can be used for determining the quantity of metals and other organic substances present in the
water and one of the important element which is there is like fluoride.
So, if fluoride is present it will cause excessive cavities in the teeth, etc. So, we have to
remove the fluoride. So, that testing has to be done. There are many approaches for
determining all these metals and organic substances also. But, these lab based methods have
limits and they can determine the concentration if the concentration amount of the substances
present in the water is high. However, the standards which have been set for drinking or
otherwise are very, very low.

So, these titration methods and other methods cannot work. So, we have to use lots of
sophisticated instruments for determining the concentration of these in the water. Lead and
arsenic is one of the element which is very common. And in particular arsenic is present in
many places in our country in particular in Bengal and nearby places. And they are usually
not found in natural water, but because of the weathering of the rocks that happens and this is
so, because our water tables are going down.

So, more amount of CO2 and other gases, they go inside the soil and they do the weathering
of these rocks. And because of that they cause problem. And these arsenic and other things or
other metals like selenium, lead, etc., are highly toxic and they must be detected and they
must be removed before drinking. So, how to remove these things we will discuss later. But
how to determine, so we have to use a lot of sophisticated instruments for determining the
concentration of these things.

(Refer Slide Time: 29:17)

So, there are some of the instruments I have listed here, which can be helpful in quantifying
the metals inorganics in water sample at very low concentration. So, the simplest one is like
UV visible spectrophotometer for determining metals and inorganics what we use is called a
colorimetric method.

So, what we do is that we use some chemicals, which after combination with the metals and
other thing they impart color. And based upon that color, we try to determine the
concentration. So, we have to make standards and a calibration graph and from that we can
determine. But again, this will be higher having a lot of limitations and the concentration up
to which UV visible spectrophotometer can be used are very less.

Then we have atomic absorption spectrophotometer which is commonly called there is

double AAS, then ion chromatograph can be used for determining all types of cations-anions
in the water. Then we have microwave plasma atomic emission spectrometer. Then we have
inductively coupled plasma optical emission spectrometer, then inductively coupled plasma
mass spectrometer.

So, there are many instruments which are there. As we go down a concentration we can
determine is low enough and but the cost will also be high. So, as we go down the cost goes
up and the concentration which can be determined easily maybe up to PPT levels also using
these instruments.

(Refer Slide Time: 31:01)

Similarly, for quantifying organics in the water, many types of organic compounds are
coming and these are coming from cosmetic industry, these are coming from pharmaceutical
industry, then antibiotics, all those things can also be determined in the water sample using
many sophisticated instruments like UV visible spectrophotometer, it can easily work in the
UV range.

Many of these organics may not be having color, but they still they will be having some
absorbance in the UV range. So, it is possible to determine them, then we have gas
chromatograph then high performance liquid chromatograph, ultra performance liquid
chromatograph, liquid chromatograph in combination with mass spectrometry.

Then quadrupole time of flight mass spectrometer. So, as again as we go down, the cost is
increasing of each of them and it is increasing exponentially. Right now, the for example, we
can buy a good UV visible spectrometer in rupees 5 lakh or INR value of rupees 5 lakhs, but
quadrupole will take not more money around 5, around 2.5 to 3 crore rupees. So, this is the
cost increases exponentially, but the concentration limit also goes down exponentially, so, we
can determine the concentration at much lower values also.

(Refer Slide Time: 32:40)

So, this is it. Thank you very much and we will continue with other water quality parameters
in particular the biological or biochemical parameters in the next lecture. Thank you.