DIFFUSED AERATOR TESTING UNDER PROCESS CONDITIONS
James A. Mueller, Associate Professor
Jeffrey J. Rysinger, Graduate Student
Environmental Engineering and Science Program
Manhattan College
Bronx, New York 10471
Design of aeration equipment for wastewater treatment plants is normally based on
manufacturer's clean water testing results which are modified to account for temperature
differences and the effect of wastewater characteristics. Two of the modification factors,
a(K, a waste/K, a water) and 6 (temperature correction factor), appear to be dependent on
type of aeration device utilized. The a factor is also significantly affected by the method of
testing employed and may range from 0.4 to 1.5 for a wastewater using different testing
techniques and devices. Thus the reliability of oxygen transfer rates for aeration equipment
under process conditions estimated using the above techniques is relatively low. Accurate
aerator testing under process conditions in full scale plants is required to attain actual process transfer rates. This data could then be combined with manufacturer's clean water
results to obtain reliable a. values or else aerator performance could be specificed under process conditions.
The ASCE subcommittee on Oxygen Transfer Standards recently evaluated the capability
of various testing techniques to measure the oxygen transfer rates under process conditions
[ 1 ]. Many of the techniques require special equipment and chemical addition to aeration
tanks which becomes expensive and/or relatively difficult to employ. The field has historically used steady state testing for this evaluation which requires 02 uptake measurement.
However when a sample is withdrawn from an aeration tank for an 02 uptake analysis, the
value measured in the analysis may be significantly lower than that occurring in the tank due
to BOD depletion after withdrawal from the tank. Batch conditions are therefore recommended with this technique where a relatively constant uptake rate in the aeration tank is
maintained.
This paper presents two techniques to evaluate oxygen transfer rates under process
conditions: (1) batch steady-nonsteady state analysis; and (2) batch dual nonsteady state
analysis. Both techniques require no chemical addition and no specialized monitoring equipment except for dissolved oxygen probes. Field testing results at the Tallmans Island Water
Pollution Control Plant in New York City are presented for the steady-nonsteady state
analysis along with laboratory testing to confirm the viability of the dual nonsteady state
technique under controlled conditions.
BATCH STEADY-NONSTEADY STATE ANALYSIS
Mathematical Development
The nonsteady state dissolved oxygen equation for a field aeration tank with no flow
at constant oxygen uptake rate is given by:
dc
— = KLa,a;C*_ - C) - R (1)
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