In the last class we discussed about the wastewater
treatment that is we discussed about primary
treatment, secondary treatment and tertiary
treatment. In fact we also discussed further
on the various units that
are present in the primary treatment and certain
design aspects. The systems that are units
or the systems that are present in the primary
treatment are screens, grid chamber and sedimentation
tank. These are very
important units. These units are designed
and some of the design aspects we have seen.
In today's class we will discuss about the
secondary treatment.
Secondary treatment is very very important
because the objective of the secondary 
treatment is; number one, to remove 
biodegradable organic matter and this biodegradable
organic matter is in terms of two
things; one is in colloidal form as well as
dissolved form. So the secondary treatment
is aimed at removing the organic matter which
is biodegradable and that organic matter which
is biodegradable may be
present in colloidal form or in dissolved
form. So this organic matter if not removed
from the wastewater and if we discharge the
wastewater with this organic matter into the
river then there will be oxygen
depletion.
So discharge of wastewater containing organic
matter is not permitted we have to remove
it. In order to remove it what we do is we
employ biological waste treatment systems
or biological waste treatment
plants. The purpose of biological treatment
plant is to remove the organic matter that
is the colloidal form as well as in the dissolved
form. So the biological waste treatment plants
obviously employ the
microorganisms. Microorganisms 
are important or most important components
of a biological wastewater treatment system.
So, as the microorganisms do the work for
us that is cleaning up the environment we
should know something more about the microorganisms
or the microbes. In fact the microbes are
used to clean up the
environment. We are using the microorganisms
to clean up the environment. I will write
'env' for environment so the environment could
be three types. One is what is called it could
be lithosphere, it could be
hydrosphere, it could be atmosphere.
Lithosphere means land. These three components
of the environment are; one is lithosphere
that is the land environment where we live
in then the hydrosphere which is the water
environment where we use water
for various beneficial purposes and the third
component of the environment is the atmosphere
that is air by which we breathe and so on.
That means if the environment is polluted
with biodegradable organic
matter what we try to do is we clean up the
environment using the microorganisms.
If I clean up the lithosphere land, clean
up of land if I call it using the microorganisms,
again everywhere I am using the microorganisms
for cleaning up then it is called bioremediation.
Bioremediation 
is a process
by means of which I can clean up the pollutants
present in the land using the microorganisms.
In the river water if I want to clean up the
river I employ what is called wastewater treatment
plants. These are
designed to clean up the waste before it enters
into the rivers. So cleaning up the river
is by employing the wastewater treatment plant,
treat the wastewater and discharge into the
rivers that are the clean up of the
environment as far as the hydrosphere is concerned,
the river is the hydrosphere.
The third type of thing is atmosphere. So
air pollutants that are 
present in the atmosphere can be removed using
the microorganisms. In such a case we use
what is called bio trickling filters. Using
bio trickling
filters we can sort of clean up the air. The
air may contain oxides of sulphur, oxides
of nitrogen SOx and NOx. So SOx and NOx can
be removed using the biological reactors.
In other words the microorganisms are very
versatile, they can be used to remove the
pollutants from the land, they can be used
to remove the pollutant from the river, from
the water, they can be used pollutant
from the air, all three things are present.
So now it is the basic requirement for us
to know about the microorganisms since we
employ the microorganisms to do work for us
to clean up the environment. So to
know about the microorganisms first we should
know something about the microbiology to some
extent.
I will give a brief introduction for microbiology.
We will discuss only that part of microbiology
that is required for the waste treatment or
clean up of the environment. So, microbiology
means in this particular
thing what I will be discussing is the types
of microorganisms we like to employ in the
wastewater treatment or in the treatment of
the land or in the treatment of the air.
So the type of microorganisms we will discuss
and then their nutritional requirements, nutritional
requirements of microorganisms which is important
and we will also discuss about environmental
conditions
which affect the growth of microorganisms
and we will also discuss about what is called
bio kinetic parameters for the microorganisms.
The bio kinetic parameters are required for
the design of the treatment
system, so for the design purposes this is
required.
So, for the design purposes the bio kinetic
parameters that are required are the following.
for example, I would like to know the growth
rate of microorganisms that is again writing
bio kinetic parameters, the rate
at which the microorganisms are growing of
microbes in a given organic matter microbial
growth rate, BOD removal rate we will study
and decay rate of microorganisms. We will
study all these things in the bio
kinetics parameters.
All these parameters are to be found out in
order to effectively design the treatment
system. In a series of lectures we are going
to discuss these things and finally I also
would like to discuss 
types of bio reactors
that are employed to treat the wastewater
and also the oxygen requirements 
by the microbes will also be discussed in
this particular thing. Finally I will discuss
the treatment units like activated sludge,
tickling filter
and thirdly anaerobic system. We will discuss
the design of these things and at the end
of the series of lectures you should be in
a position to design given the wastewater
and it characteristics and the quantity of
wastewater or the flow rate of wastewater.
I should be able to choose which of the treatment
systems I should employ and how do design
the particular treatment system. That is what
we are going to do in series
of lectures from today onwards.
Now coming back to the 
type of microorganisms there are many ways
by means of which the microorganism can be
classified. I will use a method of classification
which is based on the nutritional requirement,
nutrient requirements. For that matter any
living system including us would require the
following things: Number one, we require an
energy source. In the food we eat we require
energy source. Energy source is
required in order to produce the energy for
the microorganisms so we also require what
is called a carbon source. That means the
food we eat should contain carbon compounds,
the food we eat should contain
a compound which produces energy. So this
energy that is produced from the energy source
will be utilized for two purposes; one is
for the 
maintenance and the other thing is for biosynthesis.
Maintenance energy that is the energy required
for the cell to move around, cell to conduct
certain activities so that is the maintenance
energy and this is the biosynthesis for producing
more biomass. So this is
the carbon source, carbon source is required
for the growth of microorganisms, growth of
microbes.
For the growth of microorganisms we require
the carbon source. That means the food we
eat or the organic matter I am providing to
the microorganisms should contain a carbon
source, it should contain an
energy source. The microorganisms can utilize
different components different compounds for
the energy source. For example, for energy
if microorganisms use or the microbes use
organic carbon 
compounds
for energy then it 
is called heterotrophic. It will use organic
compound for energy. And this heterotrophic
organism uses organic compounds for energy,
it also uses organic compound for biosynthesis
as carbon
source. So the same organic compound can be
used as a carbon source as well as the energy
source by heterotrophic organisms. These are
the majority of organisms we encounter in
nature; the majority of
microbes we encounter in nature are heterotrophic
in nature.
In fact heterotrophic organisms are those
organisms or the organisms we employ mostly
in the wastewater treatment so these are employed
in the wastewater treatment plant. Suppose
if the microorganisms
instead of organic compounds for energy purpose
if they utilize inorganic compounds for energy
purpose they are known as chemo-autotrophic
organisms. so the second group of organisms
are
chemo-autotrophs, autotrophs is self dependent
so they are independent organisms, they are
depending on self only, self depended organisms
and they depend on the chemo, chemo means
chemical, they
depend on the chemicals that is the chemicals
are inorganic chemicals.
For example, energy source for the chemo-autotrophs
is inorganic 
compound like ammonia. So this inorganic compound
like ammonia is oxidized 
to nitrite and nitrate to produce energy so
this produces energy.
This energy that is produced is in the form
of ATP adenosine triphosphate. This ATP is
adenosine triphosphate. Adenosine triphosphate
is an energetic compound and it gives out
the energy whenever energy is
required so ATPs are produced. These organisms
which utilize this oxidized ammonia to nitrite
to nitrate in the presence of oxygen where
oxygen is required for this are called nitrosomonas
and this organism
here 
is called nitrobactor. These are the two organisms
which utilize the inorganic compounds for
energy purpose.
The second one is what is the carbon source
for them? The carbon source could be inorganic
carbon compound. So these organisms which
utilize the inorganic compounds for energy
and inorganic carbons as
carbon source, and the carbon source is required
for the growth of microorganisms, this is
used for the growth, this is the energy source
and this is the carbon source these are known
as autotrophic organisms.
These are special organisms, we use them specifically
for the nitrification to occur in tertiary
treatment because I discussed about the nitrification
when I discussed about the tertiary treatment.
So in the tertiary
treatment the nitrification occurs and I want
nitrification to take place so this will occur
because of these microorganisms.
The third type of organisms is what is called
photo autotrophic organisms. Photo autotrophic
organisms are those which utilize sunlight,
the energy source in this case is sunlight,
they get energy from sunlight
and utilize the carbon dioxide as a carbon
source. The carbon source is carbon dioxide
here. The simple reaction I can write here
is; they utilize carbon dioxide plus water
and this is the sunlight in the form of h
nu, h nu is the sunlight plant constant and
this is the energy sunlight energy and these
are the photosynthetic 
plants and an example of microscopic photosynthetic
plant is algal cell.
So with this reaction what would happen is
that, the resultant of this reaction is H2On
plus oxygen. H2On is nothing but a carbohydrate
this is the carbohydrate they prefer carbohydrate
and oxygen is produced
by the photosynthetic cell. This is the photosynthetic
reaction what we are writing. So we utilize
photo autotrophic organisms like algae in
the wastewater treatment especially in oxidation
ponds. The example
where we use this particular thing is oxidation
ponds. Example of photosynthetic or photo
autotrophic organism in oxidation ponds: that
is what we use. We use the photosynthetic
autotrophic organisms. That
means we use heterotrophic organisms, we use
chemo autotrophic organisms and we also use
photoautotrophic organisms for the wastewater
treatment and depending upon the type of reactors
we are designing.
So basically most commonly used wastewater
treatment or commonly used microorganisms
in wastewater treatment as I already indicated
are heterotrophic organisms. Again I emphasize
that the classification of
these organisms are based on the nutritional
requirements that is the carbon source and
then the energy source that is what I have
been discussing particularly. There is certain
other classification; people have so
many other classifications. Another classification
I can tell you is based on the temperature.
We have what is called as cycrophilic microorganisms.
Cycrophilic microorganisms are those microorganisms
which grow under low temperatures. For example,
these organisms grow in the refrigerators
also; they spoil the food stored in the refrigerator.
So the
organism that grow around four degrees or
to ten degrees Celsius they are cycrophilic
organisms. The second type of organisms is
mesophilic organisms.
Mesophilic organisms are those organisms which
grow in the medium temperature that is about
say 27 to 40 degree Celsius, then we have
got thermophilic organisms. Thermophilic organisms
are those
organisms which grow at higher temperature
that is about 40, 45 degrees to 60 degree
Celsius. This is yet another classification
based on the temperature, the tolerance of
the microorganisms to the temperature.
However, that classification is one classification,
this is another classification based on the
nutritional requirements. We will move forward
with the heterotrophic organisms because these
heterotrophic organisms
are employed very widely in the wastewater
treatment. So these are widely employed 
in wastewater treatment and most of the treatment
plants are based on the activity of heterotrophic
organisms and hence I
should try to find out what is called growth
curve for heterotrophic organisms. For these
organisms I should know what the growth curve
is.
What we mean by growth curve?
Growth curve shows, with respect time how
the growth of microorganisms take place in
a reactor.
Let me consider a batch reactor. What is a
batch reactor?
Batch reactor is a reactor where there is
no inflow or outflow or output from the reactor
that is batch. Now let me consider a batch
reactor. This is sort of a batch reactor where
I have the liquid and this batch
reactor is completely mixed; I am mixing it
completely, a completely mixed reactor that
means the contents are mixed thoroughly. That
means wherever I take the sample, if I take
the sample over here or over
here wherever I take the sample the concentration
of the contents will be the same that is what
is called the batch reactor.
Let us say in this batch reactor I have a
time t = 0 to start with. I have what is called
a concentration of microorganisms, these microorganisms
are heterotrophic microorganisms, this concentration
microorganism is X0, that means X0 is the
concentration of microorganisms in the reactor
and then there is also food. What is the food?
Food is the energy source and carbon source.
That is, for the
heterotrophic organisms organic compounds
recalling these organic compounds are both
energy source and carbon source.
Organic compounds provide energy as well as
the carbon to the microorganisms. So such
organic compounds which give energy as well
as carbon for biosynthesis for the growth
of microorganisms is known
as the substrate. Let us call this as substrate.
Substrate is also biodegradable organic matter
so substrate also can be called BOD biochemical
oxygen demand, oxygen that is required to
decompose or to oxidize
organic matter organic matter, substrate under
aerobic conditions where oxygen required is
nothing but the BOD.
We can say the BOD is same thing as the substrate.
So let us take time t = 0, concentration of
microorganism is X0 and concentration of substrate
I will go back here substrate is equal to
S0. So in this reactor I
have S0 and at time t = 0 the conditions are;
X equal to x0 and S = S0. X is the biomass
microbes concentration or biomass concentration
it is also called as biomass and S is the
substrate concentration.
What I do is that I now bring in contact the
microorganisms and the substrate in the reactor
time t = 0 and I am mixing completely and
during this mixing what is happening is that
I am also providing what is
called aeration oxygen supply. I am also providing
aeration or oxygen supply is also there in
this reactor. So because of this particular
thing what would happen is the organisms utilize
the organic matter present
in the wastewater or present in this reactor.
So, for example, if I say that organic compounds
can be called as glucose I will take a simple
example C6H12O6 is the glucose this is a representative
of organic
matter in wastewater or this is nothing but
the substrate for me now, substrate plus oxygen,
this is mixing or aeration and aeration is
providing the oxygen and then I have microorganisms.
And what type of microorganisms I have?
We can say that it had heterotrophic microorganisms.
The microorganisms use organic compounds for
energy purpose as well as for the carbon source
or for biosynthesis. So when these things
happen what
will happen is that the reaction will take
place with the production of carbon dioxide
plus water, that is the carbon is converted
into carbon dioxide, hydrogen to water plus
we will have energy. This energy is in
the form of ATP adenosine triphosphate as
already indicated. So this energy is used
by the microorganisms to produce more cells
so that is what it is going to happen. That
means now what is happening is I am
giving food to the microorganisms I am giving
substrate to the microorganisms, the microorganisms
utilize the organic matter and increase in
the number or increase in the BOD weight.
So as a result as the time
progresses, as the time increases X is going
to increase, the biomass concentration increases
and S is going to decrease. There is decrease
in the organic matter because microorganisms
are utilizing organic
matter and they increase in number that means
X is increasing as the time progresses.
So, if I were to plot a graph of this particular
thing then the time is on X axis, and micro
milli concentration on the y axis then at
time t = 0 I have certain amount of microorganisms
that is x0 to start with, in fact
time t = 0 whatever organisms are presented
or introduced in the system they are called
seed microorganisms present in the seed and
if you try to find out the concentration of
microorganisms as a function of
time then initially you may have something
like this and then afterwards it will move
up and then it will go and then it will be
some sort of stationary and then over a long
period of time may be I will break it here
over a long period of time then it would be
declining. So there are four distinct segments
of the curve. This curve is known as bacterial
growth curve when I am trying to grow the
organisms in a batch process.
Batch process is the process where we do not
have in flow and out flow, the organisms are
growing within a closed environment, so closed
eco system the microorganisms are growing.
So let me call it as
number one segment, this is number two segment
number three segment, number four segment.
There are four segments in the bacterial growth
curve. This bacterial growth curve indicates
how the micro
organisms are utilizing the food and growing
in numbers.
Therefore in segment number one what is happening
is, if you see there is no increase in the
number of microorganisms, there are almost
same microorganisms up to this time. This
particular period is called lag
period. The lag period is there because the
microorganisms are put into a new environment
I am putting the microorganisms in the new
environment and may be the substrate, the
food is new to them as a result
of which they take sometime to get acclimatized
to the new environment and also to the new
substrate. So during this particular process
even though there is no net growth of microorganisms
lots of activities are
taking place so the organisms are physiologically
very active, they are very very active physiologically.
This means that they are producing new enzymes
in their BOD in the cells so as to act on
the substrate and
also to adjust to the new environmental conditions.
So this is a physiologically active condition,
physiologically active state. Lots of activities
are taking place. Enzymes are produced; they
know how to degrade
the substrate the food so that they can increase
in the number. Having done that segment number
two is called log growth phase.
Log growth phase is where the organisms are
multiplying or increasing exponentially. They
are increasing and the growth rate is exponential.
That is the reason why we have got a sudden
increase here of
microorganisms. As the microorganisms are
increasing very rapidly or as the microbial
concentration is increasing rapidly what is
happening to the substrate concentration,
if the microorganisms are increasing the
substrate should be decreasing, so there is
decrease in the substrate concentration number
one. Number two is, because of this there
are more microorganisms and less food so in
this particular case what is
happening is there are more microbes and less
food that means there is more competition
for food.
I told you there is a closed environment,
a closed eco system, in this closed eco system
there is no new food coming in as a result
of which the food is limited and the micro
organisms are full as the result of
which there is competition for food and because
of that competition the growth rate starts
decreasing.
For example, if you take, there is a point
of inflection here if you go straight like
this if it is exponential it should have been
like this but there is a point of inflection
from this point onwards there is a change
in the
slope of this particular curve, the slope
of the curve starts decreasing so it goes
like this. So the point of inflection is occurring
because of the depletion of food. This is
depletion of substrate or food and again I
tell you that it is a closed environment.
So the toxic end product, metabolic end product
starts accumulating in the reactor. So accumulation
of toxic end product becomes harmful to the
microorganisms. We are
not removing any material from this so toxic
end products accumulate and these end products
are a result of metabolites. The 
toxic metabolites 
accumulate as a result of which the growth
rate decreases, so as a
result of these two things their net result
is decrease in the growth rate.
This is number three segment and this decrease
in growth rate is called 
stationary phase. This is the stationary phase
where there is no increase in the microbial
concentration. That means this microbial
concentration remains constant here, it has
reached a plateau it will not increase further
so this is called the stationary phase.
In stationary phase there is no net growth
of microbes. Now, if you reach number four
segment, what is happening number four segment?
The fourth segment is the segment where the
microbial concentration is
decreasing with time. As the time passes this
is decreasing. That means there is no growth.
In fact there is destruction of microbes;
this is the result of destruction of microorganisms.
Microorganisms are
decreasing with time and that is what it is
happening in this particular thing.
Why it is happening?
The food is almost exhausted. number one,
the reason for this is the substrate 
is exhausted I will put it here number one
and number two reason for this particular
thing is that accumulation of toxic metabolize
toxic end product. So these are the reasons
which are responsible for the declining growth
phase.
If you look at the microbial growth rate or
bacterial growth rate curve we have got these
four segments and these four segments are
depending upon the availability of the substrate
and growth of microorganisms.
So these are the things, this particular curve
is very very important for us in understanding
what exactly is happening in the wastewater
treatment plant.
Now let us move forward. After understanding
the growth curve we will try to follow what
exactly happens to the reactor. We will try
to put mathematical equations for the growth
curve. In fact there are several
mathematical models to describe the rate growth
rate of microorganisms.
Number one is a very simple model based on
Monod's model 
and people also have applied another model
which is what is called a logistic growth
model to describe the growth curve or the
behavior of growth
of microorganisms.
Now we will try to use both the models and
then see how exactly we can formulate some
of the mathematical models. So now I will
try to look at the fundamentals of the mathematical
models. Now, again I go
back to the reactor this reactor at time t
= 0 X = X0 S = S0. Mind you, both S and X
are measured in the same units so the milligrams
per liter of biomass and milligrams per liter
of substrate both are measured
in terms of milligrams per liter X and S are
units measured in the same units.
Now here I can write down; dx by dt rate 
of growth rate of microbes 
proportional to the microorganisms present
at that time. This is the one simple equation
we are taking about, the growth rate is proportional
to X, X is the microorganisms present at that
time. In other words this can be dx by dt
is equal to mu into X removing the proportional
to constant with the mu.
Here mu is known as the specific 
growth rate and the units of mu you take it,
the units of X here and X here are the same
they cancel out and mu will take a unit of
t inverse time inverse, mu has the unit of
time
inverse, mu is the specific growth rate. Now,
this specific growth rate mu is not a constant.
So, what is mu?
mu is the specific growth rate of microorganisms
that should depend on the concentration of
the substrate. In other words mu is not a
constant but mu is a function of substrate
concentration S that is what I will
write. It is a function of concentration and
also it is a function of environmental conditions.
Let us say that environmental conditions are
kept very conducive for the growth of microorganisms
so we are not going
to change that particular thing.
So now what is happening is that the relation
between mu and S is given by mu is equal to
mu max multiplied by S over Ks plus S. So
suppose I want to plot a graph between S that
is the substrate concentration
and then mu here mu is the specific growth
rate and then the substrate concentration
on this particular thing when the substrate
concentration is zero there is no substrate
concentration then what is going to
happen to the growth rate is the growth rate
is also zero, no food, no growth rate. Then
as the substrate concentration increases growth
rate also increases like this and then finally
it would go and then it will
become sort of asymptotic or it could be stationary,
it becomes stationary to this.
As the S increases this particular thing is
almost constant. So this constant thing is
called as 
mu max. Maximum growth rate has occurred at
this particular thing. now again going back;
another substrate
concentration is increasing mu also increases
and this follows this trace of this curve,
this is the curve and then beyond this particular
concentration of the substrate whether the
substrate is this or substrate is this
whatever be the substrate the mu value is
constant. That is, the mu value when it takes
the mu max value then that is independent
of substrate concentration. Up to this particular
period up to this particular point
probably the two are a function of substrate
concentration and beyond this particular thing
it is independent of substrate concentration
and that is what we can see in this particular
thing so that is the mu max.
The mu max is the 
maximum growth rate constant.
But only now I am including the word 'constant'
because growth rate is a constant. Here it
is only specific growth rate and here it is
a constant because that is the only one unique
value for a particular
microorganism for a particular substrate.
For a particular wastewater as well as for
particular group of microorganisms mu max
will be the only one value that is why it
is a constant and S is the concentration of
the substrate 
and milligram per liter so this also can be
called as BOD whatever BOD I have in the wastewater.
Now the Ks is the term which I have to say
Ks is nothing but the value of S this is Ks
what is Ks? Ks is the value of S when mu is
equal to mu max by 2 that is half of mu max.
Now I know the mu max value,
half of mu max value I take and corresponding
S value is equal to Ks. So Ks is a saturation
substrate 
concentration when mu is equal to half of
mu max that is what it is. These are all terms
indicated here. In
other words mu is the function of the substrate.
So, substituting back in this equation I have;
dx by dt the growth rate of microorganisms
is equal to mu into, mu is given by mu max
multiplied by over Ks plus
multiplied by the X. So you can see here that
the growth rate of microorganisms is a function
that depends upon a constant mu max, another
constant Ks, Ks is also a constant so I can
write it here this is also a
constant, Ks is also a constant it depends
upon the substrate concentration and it depends
upon the microbial concentration so this is
the actual equation which governs the growth
of microorganisms and then
growth rate curve.
Now let us take case one. I will try to simplify
this particular equation with certain conditions.
Let us take case one. Case one is that when
substrate S is unlimited.
What does it mean?
When the substrate is unlimited means if I
go to this particular graph somewhere here
I am there is very high substrate concentration,
substrate unlimited means very high concentration
of substrate. So when is
the high substrate concentration present in
the reactor is during the initial periods
of the batch reactor. In the batch reactor
during the initial periods we have this particular
thing. So, that S is very very high
compared to Ks; Ks is small, S is very high
and hence I can say Ks plus S is approximated
to S. In the denominator Ks plus S can be
approximated to S because S is very great
compared to Ks. That means if I
take that equation now and substitute this
condition that the substrate is unlimited
dx by dt is equal to mu max multiplied by
S over S because Ks plus S is S multiplied
by X. So this S and S cancels out dx by dt
is equal to mu max into X.
Can I solve this equation now? Yes I can solve
this equation. I could not have solved the
equation dx by dt equal to mu into X; mu is
the specific growth rate and mu max is the
maximum specific growth rate
constant. This is a constant that is why I
can differentiate dx by X = mu max into dt.
I can solve this equation when t = 0, X = X0
and when t = t; X = X. That means the solution
of this particular thing is nothing
but applying X is equal to X0 into e to the
power of mu max multiplied by t. So this is
the solution of this. So this particular equation
is telling me that X equal to X0 into e to
the power of mu max into t. The mu
max is a positive value so all these are positive
value, X is a positive value multiplied by
X0 and hence it is a positive value.
In other words this particular thing describes
the growth of microorganisms at exponential
growth rate. That means this equation describes
the exponential growth rate or this is the
log growth rate of the curve.
Suppose if I plot this curve again, t versus
X so I have this, this is the curve what we
have so this particular thing is represented
by dx by dt is equal to 
mu max into X. This is the curve, exponentially
it goes that
means this is exponential curve. That means
this exponentiality is valid only up to certain
level, afterwards it is not valid. We will
continue in the next class when the substrate
is limited. In this, unlimited substrate
we have taken but when the substrate is limited
what is going to happen is what we will discuss
in the next class.
