How is two film theory in waste water treatment applicable?.
In this post I will explain this concept by considering a natural phenomenon Re-aeration.If we consider Oxygen to be the element/gas of our interest, it's equilibrium is maintained in the atmosphere as follows:
In the above shown figure let us assume that the dark line is the interface between water and air be it the surface of a lake.The lighter lines represents two thin films.
This concept assumes that thin films exists towards liquid and gas phases.This is the area where the mass transfer takes place.How does mass transfer occur?
To know this lets see how equilibrium is maintained....
The equilibrium is generally maintained in accordance to Henry's constant, which is defined as:
H = Concentration in the gaseous phase/Concentration in the liquid phase
H has different units.*
As we know at equilibrium, concentration of Oxygen in water is 9.1 mg/l and concentration in gaseous phase be 0.21(expressed as partial pressure).
Let us assume that the lake water got contaminated with discharge of waste water.Therefore it acts as a sink to DO content in lake.As a result,the equilibrium concentration shifts back(Shown s dotted straight line).So a concentration gradient develops between gaseous and liquid phases.This is when the mass transfer from gas to liquid phase starts(shown in the inter facial area as inclined dotted line).It continues until the equilibrium is achieved.This is how the Re-aeration in the atmosphere takes place.
In the above shown figure let us assume that the dark line is the interface between water and air be it the surface of a lake.The lighter lines represents two thin films.
This concept assumes that thin films exists towards liquid and gas phases.This is the area where the mass transfer takes place.How does mass transfer occur?
To know this lets see how equilibrium is maintained....
The equilibrium is generally maintained in accordance to Henry's constant, which is defined as:
H = Concentration in the gaseous phase/Concentration in the liquid phase
H has different units.*
As we know at equilibrium, concentration of Oxygen in water is 9.1 mg/l and concentration in gaseous phase be 0.21(expressed as partial pressure).
Let us assume that the lake water got contaminated with discharge of waste water.Therefore it acts as a sink to DO content in lake.As a result,the equilibrium concentration shifts back(Shown s dotted straight line).So a concentration gradient develops between gaseous and liquid phases.This is when the mass transfer from gas to liquid phase starts(shown in the inter facial area as inclined dotted line).It continues until the equilibrium is achieved.This is how the Re-aeration in the atmosphere takes place.
Assumptions:
It is assumed that the area under films is bulk(i.e concentration remains constant==completely mixed).So whatever transfer/change that takes place, was assumed to be with in the inter facial area.
Therefore the rate of change of mass is directly proportional to the concentration gradient, inter facial area.
(dM/dt)=KL *A*(Cs-C)
where
dM/dt=rate of change of mass of oxygen
KL = mass velocity transfer rate
A=Inter facial area
Cs=Saturation concentration
C=Concentration at a given time
K defines how fast the mass transfer takes place from one phase to other.
Applicability in waste water treatment:
In Activated Sludge Treatment, this phenomenon is applied in aerator.
In an aerator to get the desired DO, a diffuser can be selected by considering oxygen transfer rate.
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