Non-steady state desorption method

In this method, oxygen uptake rate (OUR) is measured first using the non-steady state desorption method. Assuming OUR is identical to the oxygen dissolution rate, oxygen transfer efficiency (OTE) can be calculated by dividing the OUR by oxygen supplying rate through aeration.

OUR can be measured from the DO declining curve as shown in Fig. 1 assuming it is not a function of dissolved oxygen level. This is true especially for the systems running at high DO such as 2 mg/L or above since the half reaction constant (Ks) of dissolved oxygen concentration for BOD oxidation is low at below 0.5 mg/L. According to the Monod equation, the reaction constant is affected by DO denoted by C  and calculated by the following equation (1). If C equals to Ks, the reaction constant, k, becomes a half of the maximum reaction constant, kmax. As C increases, reaction constant, k, approaches to the maximum. If C is substantially higher than Ks, C no longer affects reaction constant, k.

1               ————————- (1)

Fig. 1 shows an experimental DO curve when intermittent oxygenation is performed, where oxygenation stops when DO reaches 4 mg/L and resumes at 2.6 mg/L. This graph clearly shows DO drops linearly, which indicates oxygen consumption rate is not a function of DO in the range.  From the slope of the curve, OUR can be calculated at 88 mg/L/hr. The slope of DO decline in the second cycle is found nearly identical.

In-situ desortioon methodFig. 1. Time curve of DO when intermittent oxygenation is performed in aeration basin (Irizar, 2009).

Following is the procedure to estimate OUR as shown in Fig. 1.

  • Increase oxygenation (or aeration) rate to raise DO to at least 4 mg/L. The higher the peak DO is, the higher the accuracy of OUR is.
  • Once the DO reaches the target level, stop oxygenation.
  • Continuous mechanical mixing is required to obtain high accuracy in OUR. The amount of oxygen dissolved through the water surface can be neglected.
  • Monitor DO declining rate.
  • The slope obtained from the graph equals to OUR

Non-steady state desorption method is simple and relatively accurate, but biological flocs can be broken down when aeration rate is raised during the surge period, which artificially raises OUR (Krause, 2003). But, this method is not commonly used for air-based biological treatment system because of following reasons.

  • Typically there are no enough redundancies in aeration capacity to raise DO above 4 mg/L
  • Typically there are no mechanical mixing mechanisms available during desorption period

If mechanical mixers are used with pure oxygen to oxygenate aeration basin, DO can be readily raised close or above 10 mg/L and mechanical mixings can be continued during desorption period (Irizar, 2009). Since biological flocs are exposed to a constant shear field in this condition regardless of the oxygen supply, floc breakages can hardly affect OUR and high accuracy can be obtained.

Oxygen consumption rate in aeration basin can be estimated by multiplying OUR by the basin volume, which equals to oxygen dissolution rate. Oxygen transfer efficiency (OTE) can be calculated by dividing the oxygen dissolution rate by the amount of oxygen supplied to the water as equation (2), where Q is air flow rate at 20 oC and 1 atm.

1                 ———————- (2)

As equation (3) here, apparent mass transfer coefficient, kLa can be also calculated as equation (3).

1                    ———————- (3)

By definition, α-factor can be calculated as follow from the ratio between kLa and kLa0. It represents the relative ratio of oxygen dissolution in process water against that in clean water.

1                              ———————–(4)


© Seong Hoon Yoon