We shifted the probe around during pilot studies and found that one point near
the inlet of the aerator provided a consistent and representative sample of average
tank conditions. Observations from the left side of Fig. 2 produced erratic measurements,
as saturated turbulent jets emanating from the paddles created eddies
intertwined with oxygen deficient water from upstream. Oxygen readings were more
consistent at points (A, B, C, and D) on the right side of the tank. Only point D
was used for on-going work because it was closer to the aerator intake and assumed
to produce the best mix of tank water after dissipation of turbulence, yet far
enough away to avoid splash effects.
In another pilot study it was found that increasing depth of water column from
600 to 850 mm resulted in a increasing mass transfer effect of a particular aerator,
but we were constrained by overflowing waves, as the basin was only 900 mm deep.
For on-going tests the water filled to no more than 750 mm, specifically as recorded
in Tables 1–3 Some water escaped with splashing at higher speeds, so the levels
decreased until the tank was refilled. The tank was drained and water was replaced
well before total dissolved solids accumulated to the 2 mg/l maximum concentration
allowed in the standard (ASCE, 1993).