| Summary: | This research explored the use of Oxidation-Reduction
Potential to control two lab-scale sequencing batch reactor
(SBR) wastewater treatment processes. The treatment schemes
investigated were the aerobic-anoxic digestion of activated
sludge (AASD) and the excess biological phosphorus (Bio-P)
removal process. Evaluation of each process consisted of a
consideration of the reactor performances coupled with the
control stability achieved using two different operating
strategies.
The first strategy was known as "Fixed-Time Control" (FT),
since it represents the "classical" management approach; control
is based on conditions externally "fixed" by an operator. For
the AASD set of experiments, the "fixed" variable was the ratio
of air-on to air-off (3 hours each). For the Bio-P experiments,
it was the time of addition of acetate to the reactor (1 hour 25
minutes into the non-aerated sequence).
The second strategy was known as "Real-Time Control" (RT),
since it represents an optimization technique whereby control
conditions are continuously evaluated as time progresses. The
Real-Time aspect of control is derived from the fact that ORP
measurements evaluate the reactor conditions on-line, by
invoking a bacterial vision of the process scheme.
For the AASD experiments, this evaluation took the form of
proportioning the ratio of air-on to air-off, based upon the
bacterial "need" for sufficient time to reduce the nitrates
completely to nitrogen gas (denitrification). Sufficient time is determined by the distinctive breakpoint (correlated to nitrate
disappearance) occurring in the ORP-time profile.
The first experiment (AASD#1) , therefore, had an air-on/airoff
ratio of 3 hours air-on/nitrate-breakpoint-determined airoff.
The second experiment (AASD#2) had the length of aeration
time determined by a match to the previous length of time for
denitrification, as determined by the breakpoint. In the Bio-P
experiments, the ORP breakpoint was used to "trigger" the
addition of acetate to the reactor, thus ensuring the maximum
amount of carbon was available for storage by Bio-P organisms.
Comparisons between the two reactors revealed that for the
AASD strategies, the Real-Time reactor had essentially the same
solids degradation as the Fixed-Time reactor (14% - 21%),
depending upon the strategy considered, the type of solids (TSS
or VSS) and the method of mass balancing used. The RT reactor
was observed to obtain marginally better nitrogen removal (up to
6 % in some cases) over the FT reactor.
Evaluation of the ORP parameter as a "response indicator",
by subjecting the AASD reactors to unsteady process input
conditions, revealed that the Real-Time reactor more readily
accommodated disturbances to the system.
Neither reactor in the Bio-P experiment was particularly
successful in consistently removing phosphorus. A potentially
useful screening protocol was developed for evaluating reactor
performances, based upon the time-of-occurrence of the nitrate
breakpoint, assessed against whether it hindered or aided the
purpose of acetate addition to a Bio-P SBR. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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