Influence of Temperature and Turbulence on Oxygen Transfer in Water
25
2. INFLUENCE OF TURBULENCE
The energy consumption for the oxygen supply and mixing in the different aeration
systems may vary considerably. Furthermore, the degree of mixing is highly dependent
on the aeration method. In recent times the possibility of a separation of the oxygen
supply and mixing has therefore once more received attention in the literature [15]. An
example of such a system was described by Imhoff [16] in the 1920s.
Investigations by Born [17] with pneumatic aeration devices have shown that flow
conditions become so stable with increasing air supply that the necessary lateral mixing
does no longer occur. Kalinske [18] mentions the inherent limits of pneumatic aeration
units with respect to turbulence regulation. The retention investigations by Dahlem [19]
have also indicated that the flow process in pneumatically aerated spiral-flow aeration
tanks implies strong elements of plug flow. In this procedure macro-turbulence mainly
prevails. According to the experiences of Kalinske [18] and Born [17] complete
diffusion can only be achieved with mechanical aeration units (rotary aerator, stirring
devices, turbines, impellers etc.). Only these devices can produce the required number of
shearing surfaces and thus the necessary micro-turbulence. With pneumatic aerators a
further stabilization of the main flow is achieved with increasing oxygen supply and
energy input (watt/m
3 ), whereas with mechanical aerators an ideal mixing can be
achieved with a sufficient energy supply. According to recent (1968) measurements by
the Ruhrverband [20] in aeration tanks with surface aerators, a distributed supply of
waste water is therefore not necessary.
These correlations show that with pneumatic aerators the oxygen supply must be
dependent to a much higher degree on the diffusion processes than with mechanical
devices. This automatically results in a different temperature dependency of the systems.
Pasveer [6] showed that irrespective of the oxygen deficits the molecular boundary
layer of the water is spontaneously saturated with oxygen when subjected to an air
interface. In this first phase of gas transfer the temperature is only important in so far as
it determines the saturation value. During the second phase the absorbed oxygen has to
be transferred from the boundary to the interior of the body of water. In systems with
low turbulence this distribution is mainly effected by diffusion. Here an increase of the
oxygen transfer rate with rising temperature can be observed since the diffusion processes
are accelerated.
In systems with high micro-turbulence the diffusion is less important. Here the oxygen
transfer is mainly effected by an intensive mixing of the water particles saturated at the
boundary layers with the other oxygen deficient water. The temperature changes
occurring in practice do not indicate an influence on this mixing process.
3. DIFFERENT AERATION TESTS
According to the present (1971) view the compensating influence between the oxygen
absorption rate and the oxygen saturation value with changing temperature conditions
applies only to pneumatically aerated systems as well as low-turbulence mechanically
aerated systems. When the Ruhrverband in 1964 designed the devices for artificial
aeration of Lake Baldeney, the above mentioned dependency was also applied to the
high-turbulence mechanical aerators. After commissioning it was observed that the
oxygen transfer rate decreased at elevated water temperatures.
25
2. INFLUENCE OF TURBULENCE
The energy consumption for the oxygen supply and mixing in the different aeration
systems may vary considerably. Furthermore, the degree of mixing is highly dependent
on the aeration method. In recent times the possibility of a separation of the oxygen
supply and mixing has therefore once more received attention in the literature [15]. An
example of such a system was described by Imhoff [16] in the 1920s.
Investigations by Born [17] with pneumatic aeration devices have shown that flow
conditions become so stable with increasing air supply that the necessary lateral mixing
does no longer occur. Kalinske [18] mentions the inherent limits of pneumatic aeration
units with respect to turbulence regulation. The retention investigations by Dahlem [19]
have also indicated that the flow process in pneumatically aerated spiral-flow aeration
tanks implies strong elements of plug flow. In this procedure macro-turbulence mainly
prevails. According to the experiences of Kalinske [18] and Born [17] complete
diffusion can only be achieved with mechanical aeration units (rotary aerator, stirring
devices, turbines, impellers etc.). Only these devices can produce the required number of
shearing surfaces and thus the necessary micro-turbulence. With pneumatic aerators a
further stabilization of the main flow is achieved with increasing oxygen supply and
energy input (watt/m
3 ), whereas with mechanical aerators an ideal mixing can be
achieved with a sufficient energy supply. According to recent (1968) measurements by
the Ruhrverband [20] in aeration tanks with surface aerators, a distributed supply of
waste water is therefore not necessary.
These correlations show that with pneumatic aerators the oxygen supply must be
dependent to a much higher degree on the diffusion processes than with mechanical
devices. This automatically results in a different temperature dependency of the systems.
Pasveer [6] showed that irrespective of the oxygen deficits the molecular boundary
layer of the water is spontaneously saturated with oxygen when subjected to an air
interface. In this first phase of gas transfer the temperature is only important in so far as
it determines the saturation value. During the second phase the absorbed oxygen has to
be transferred from the boundary to the interior of the body of water. In systems with
low turbulence this distribution is mainly effected by diffusion. Here an increase of the
oxygen transfer rate with rising temperature can be observed since the diffusion processes
are accelerated.
In systems with high micro-turbulence the diffusion is less important. Here the oxygen
transfer is mainly effected by an intensive mixing of the water particles saturated at the
boundary layers with the other oxygen deficient water. The temperature changes
occurring in practice do not indicate an influence on this mixing process.
3. DIFFERENT AERATION TESTS
According to the present (1971) view the compensating influence between the oxygen
absorption rate and the oxygen saturation value with changing temperature conditions
applies only to pneumatically aerated systems as well as low-turbulence mechanically
aerated systems. When the Ruhrverband in 1964 designed the devices for artificial
aeration of Lake Baldeney, the above mentioned dependency was also applied to the
high-turbulence mechanical aerators. After commissioning it was observed that the
oxygen transfer rate decreased at elevated water temperatures.