Basic Biological Processes
much consideration to the environment which the micro-organisms have to put up
with, we can still be rather sure that the wastewater is biologically treated. The
treatment quality will depend on the environments which are created, but it does
not change the fundamental fact that a certain biological treatment is obtained no
matter - just about - how silly one behaves.
The two main types of biological treatment are rather different in respect of selection
mechanisms. The toughest selection takes place in activated sludge plants, and this
is probably the reason why a pure culture was added to the water in the first tests
with this treatment method, perhaps on the grounds that it would then be the only
type of bacteria which could survive. At that time it was not fully realized that a very
varied microbial population is present in raw wastewater.
Selection in biofilters
The selection of organisms which takes place in biofilters can be divided into the
following parts as the selection is based on the following criteria
a.
adhesion
b.
growth rate (substrate, temperature, pH, oxygen, etc.)
The latter selection mechanism is not constant as both substrate and temperature
normally are variables. The substrate may vary if the municipal wastewater contains industrial wastewater from industries which do not operate round the clock or
throughout the week, while the temperature will vary as a function of the season.
Of the two variables, the temperature in particular will be important for the selection to the effect that throughout the year there will be a temperature-dependent
variation of species.
Selection in activated sludge plants
The selection taking place in activated sludge plants can be divided into:
a.
electron acceptor (for example oxygen or nitrate)
b.
substrate
c.
settling or flocculation characteristics
d.
temperature
e.
growth rate
f.
freely suspended life forms
Fig 3.2 shows a chart of the selection mechanisms in activated sludge plants.
a. electron
acceptor
As the principle in an activated sludge plant is to blow air into a
mixture of sludge and wastewater, anaerobic bacteria would hardly
be able to survive for a longer period of time. Anaerobic conditions
may occur in the middle of sludge floes, but as the floes are frequently broken up and re-formed, an anaerobic bacterium may not expect
to survive. Specifically, facultative bacteria will be able to survive
59
much consideration to the environment which the micro-organisms have to put up
with, we can still be rather sure that the wastewater is biologically treated. The
treatment quality will depend on the environments which are created, but it does
not change the fundamental fact that a certain biological treatment is obtained no
matter - just about - how silly one behaves.
The two main types of biological treatment are rather different in respect of selection
mechanisms. The toughest selection takes place in activated sludge plants, and this
is probably the reason why a pure culture was added to the water in the first tests
with this treatment method, perhaps on the grounds that it would then be the only
type of bacteria which could survive. At that time it was not fully realized that a very
varied microbial population is present in raw wastewater.
Selection in biofilters
The selection of organisms which takes place in biofilters can be divided into the
following parts as the selection is based on the following criteria
a.
adhesion
b.
growth rate (substrate, temperature, pH, oxygen, etc.)
The latter selection mechanism is not constant as both substrate and temperature
normally are variables. The substrate may vary if the municipal wastewater contains industrial wastewater from industries which do not operate round the clock or
throughout the week, while the temperature will vary as a function of the season.
Of the two variables, the temperature in particular will be important for the selection to the effect that throughout the year there will be a temperature-dependent
variation of species.
Selection in activated sludge plants
The selection taking place in activated sludge plants can be divided into:
a.
electron acceptor (for example oxygen or nitrate)
b.
substrate
c.
settling or flocculation characteristics
d.
temperature
e.
growth rate
f.
freely suspended life forms
Fig 3.2 shows a chart of the selection mechanisms in activated sludge plants.
a. electron
acceptor
As the principle in an activated sludge plant is to blow air into a
mixture of sludge and wastewater, anaerobic bacteria would hardly
be able to survive for a longer period of time. Anaerobic conditions
may occur in the middle of sludge floes, but as the floes are frequently broken up and re-formed, an anaerobic bacterium may not expect
to survive. Specifically, facultative bacteria will be able to survive
59