The wastewater treatment
facility for Princeton is designed as a conventional A/O process. The three
(3) stage anaerobic selector consist of a RAS denitrification stage followed
by two (2) anaerobic stages. The anaerobic selector utilizes the Block &
Hong process which is an improvement to the conventional anaerobic selector
design. The Block & Hong process is described in detail under the section
titled Biochemical Principles of Phosphorus Removal on the Phosphorus
Removal page in this section.
Biochemical
Principles of Phosphorus
Removal
Biological phosphorus removal in the A/O system is accomplished by the
proliferation of A/O micro organisms capable of accumulating large amount of
phosphorus. Normally, in the absence of oxygen, the conventional activated
sludge organisms would be unable to assimilate the SBOD. However, the A/O
organisms can absorb SBOD through the use of chemical energy liberated by
the hydrolysis of polyphosphate to orthophosphate. Therefore, the anaerobic
selector proliferates the growth of A/O organisms. During the aerobic phase,
the A/O organisms oxidize the absorbed BOD to generate energy to
re-synthesize the polyphosphate from soluble orthophosphates and to form new
cells, resulting in a net uptake of phosphorus. The orthophosphate is
removed from the wastewater and becomes incorporated in the waste sludge.
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End View of
Anaerobic Selector |
Energy utilization by the A/O organisms
for the purpose of BOD absorption during the anaerobic phase is provided by
the breakdown of stored polyphosphate. The structure of polyphosphate is a
linear molecule formed by linking together orthophosphate molecules.
As the polyphosphate breaks down in the anaerobic phase, soluble
orthophosphate is formed inside the cell. This orthophosphate then diffuses
to the liquid outside the cell causing an increase in soluble phosphate in
the mixed liquor of the anaerobic zone.
As the available soluble BOD is used up, the rate of soluble phosphate
release decreases and the phosphate concentration reaches a plateau. The
amount of phosphate released is related to the amount of soluble BOD5
contained in the influent wastewater. The exact relationship between BOD
absorbed and phosphate released varies with the composition of the waste,
but in laboratory studies using glucose it has been shown to be one mole of
glucose absorbed for two moles of phosphate released. The rate of phosphate
uptake in the aerobic zones is proportional to the soluble phosphorus
concentration. For this reason, phosphate uptake is initially very rapid and
phosphate concentration is expected to decrease before discharging from the
oxidation ditch.
In wastewater treatment plants which
nitrify the influent, ammonia is converted into nitrates. Subsequently, the
recycling of nitrates to the anaerobic zone through the return activated
sludge (RAS) line is unavoidable, even in plants which practice simultaneous
nitrification/denitrification (SBND). In a conventional selector, RAS and
influent waste streams are mixed in the first stage of the anaerobic zone.
Small concentrations of nitrates, introduced by the RAS stream, in the
anaerobic zone can significantly reduce the efficiency of the A/O process in
two ways. First, in the presence of nitrates, the anaerobic zone will become
anoxic. Instead of hydrolysing polyphosphate stored energy to absorb the
SBOD, the mixed liquor will utilize the chemically bound oxygen in nitrates
to oxidize the SBOD. This will delay the onset of the enhanced biological
phosphorus uptake mechanism and reduce the efficiency of the process.
Secondly, denitrification in the anaerobic zone will also diminish the
readily available SBOD in the influent. This decrease in the BOD:P ratio in
the anaerobic selector may deprive the system of sufficient carbon to
efficiently fuel the process.
However, by utilizing the Block and Hong process, true anaerobic conditions
can be maintained in the anaerobic selector. In this system, the return
activated sludge (RAS) is returned to the head of the anaerobic zone in a
separate stage. Any chemically bound oxygen, such as nitrates, that are
present in the return stream will be removed prior to its mixing with the
influent wastewater. As such, a suitable concentration gradient of available
BOD, as well as a true anaerobic conditions, are maintained in the second
and third stage of the selector. As a result, optimal biological phosphorus
removal is realized without the construction of elaborate or additional
reactors or the dosing of costly chemicals or alternative carbon sources.
Additionally, sludge bulking in the secondary clarifier is eliminated.
Superior sludge-settling characteristics result from the inhibition of
filamentous bacteria and the increased density of the mixed liquor caused by
the biological phosphorus removal mechanism. |
