Relationship between oxygen transfer rate and airflow for fine-pore aeration under process conditions

Although feedback systems that control the air supply to aeration tanks inh erently incorporate some assumption about oxygen transfer response to chang es in airflow, it is rare to measure this relationship under process condit ions. This paper reports measurements of oxygen mass-transfer curves (MTCs) for a tank at the Tillman Water Reclamation Plant in Los Angeles, Californ ia. The curves were obtained by measuring the oxygen transfer efficiency (O TE) at selected points for several set values of airflow while the plant wa s operating. They approximate inverted parabolas because increasing the air flow increases the amount of oxygen supplied by the blowers, but decreases the OTE, which is the fraction of the supplied oxygen that actually enters the water. Data were recorded from both recently cleaned diffusers and ones that were moderately to severely fouled. The peaks in the curves from the fouled diffusers are at or below the midpo ints of the observed ranges of airflows. Hence, there is only a narrow rang e of usable airflows between the lower limit, determined by the manufacture r of the diffusers, and the peak of the MTC, which is the maximum amount of oxygen that can be supplied. The peaks for the cleaned diffusers are highe r, which allows more ability to adjust to changing biological loads. These results show that existing dissolved oxygen control systems may not b e adequate and that fouling may reduce not only the overall efficiency of a n aeration system but its ability to respond to changes in the biological l oad. The measurements also provide some insight to the limitations of using sparsely distributed dissolved oxygen sensors to control the aeration proc ess and the excess costs that are incurred by the consequent need to compen sate for uncertainty with extra air. However, additional testing is needed to determine whether the present results are aberrant or typical of tanks w ith fouled or cleaned diffusers.