Hydraulic would not get too low as this could

Hydraulic
retention time: is the average amount of time that a substance,
such as water, is stored within a space, like a bioreactor (Melcer et al.,
2003). An influent flow rate was selected so that the retention time would
result in the washout of the nitrifying bacteria, but not so high or low that
the all the microbial mass would be removed. The hydraulic retention time was
calculated by dividing the tank volume by influent flow rate (Melcer et al.,
2003). The retention time was manipulated to distress the bioreactors and to observe
how long it would take for the bioreactors to reestablish an equilibrium. This
was done by increasing or decreasing the influent flow rate and the standpipe
height (thus changing the tank depth and volume). It was later decided to keep
the tank volume at a constant volume so that the standpipe height would not get
too low as this could decrease the rate of oxygen transfer to the aerobic
bioreactors.

The Equilibrium
phase was attained when the mass of the nitrifying bacteria entering and
living within the system was equal to the mass of nitrifying bacteria leaving
or dying in the system (Melcer et al., 2003). It was important to know if the
bioreactors were at a steady state so that the decay rate of the nitrifying
bacteria can be quantified (Melcer et al., 2003). This would become the basis
for the computer modelling program of industrial bioreactors.

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There were two ways to determine if the
bioreactor was at equilibrium. The first was to allow the bioreactors time to
recover after each change by allowing them to run for a time period of about
three times the hydraulic retention time before results were considered to be a
true representation of the bioreactors performance at that retention time. For
example, if the hydraulic retention time was two days, the data for six days
after the change in condition was considered to not be reflective of the bioreactors’
operations when under those conditions. The second way to estimate if the bioreactor
was at steady state was based on the ammonia vs nitrate results obtained in the
lab. This was one of the critical decisions that project leader had to make
based on the test results.

There were a few ways to estimate the nitrifying
growth rate of an activated sludge model. The method used for this pilot was
the washout bioassay method where the nitrifying bacteria growth was estimated
using the nitrite/nitrate results for the duration of the study period (Dold,
et al., 2003). One thing to note is that this test did not give an actual
estimate of nitrifying bacteria growth but rather an estimate of the nitrifying
growth rate minus the nitrifying decay rate (Dold, et al., 2003).

The
Substrate limiter was an important factor in running this study as
it influenced the biomass growth and the optimal nitrification rate. In our
case ammonia played this role, thus it was essential to have a concentration of
50 mg/L ammonia-nitrogen. This was achieved by creating a synthetic feed, based
on Ammonia
Chloride vj1 that
was added to the influent.

Temperature’s
effects on nitrifying bacteria has been largely studied in other tests.
While some studies claimed that temperature had a major impact on the nitrification
rates, other studies claimed the opposite (Andersson, Laurent, Kihn, Prévost,
2001). Temperature was another parameter that was manipulated during this study
to determine its effects on the nitrification process.