Oxygen transfer efficiency (OTE)

Standardized oxygen transfer efficiency (SOTE)

SOTE represents the oxygen transfer efficiency (OTE) at 20 oC, zero dissolved oxygen, and zero salinity. SOTE tends to decline if air flow rates increase. The increased bubble coalescence and the increased bubble rising velocity cause diminished bubble surface area and contact time, which in turn causes lower SOTE. Therefore the SOTE for a specific diffuser is usually reported as a function of air flow rate to the diffuser.

Since SOTE is only valid only for the water depth that the diffuser was tested, SOTE is often reported as specific SOTE (SOTES) by dividing the diffuse submergence by the depth assuming SOTES is constant regardless of the water depth. The SOTES is used to estimate the SOTE in the tanks with various depths. Although oxygen transfer from bubbles to water decreases gradually while the bubble rises due to the depleting oxygen content and the decreasing static pressure. However, the errors caused by neglecting these effects are relatively not significant comparing to the uncertainties caused by the fluctuation of biological condition, unique diffuser placement, etc.

Table 1 summarizes the typical SOTE of various diffusers, but more accurate values can be obtained from diffuser manufacturers.

Table 1. Comparison of clean water oxygen transfer efficiency of selected diffusers (reproduced from Shammas, 2007)

Diffuser type place in grid  SOTE at 4.5m submergence ( – )   SOTE(/m)
Ceramic discs 0.26-0.33 0.058-0.073
Ceramic domes 0.27-0.29 0.06-0.064
Porous plastic discs 0.28-0.32 0.062-0.071
Nonrigid porous plastic tubes 0.26-0.36 0.058-0.08
Single spiral roll 0.19-0.37 0.042-0.082
Perforated membrane tubes 0.22-0.29 0.049-0.064
Coarse bubble diffusers 0.09-0.13 0.02-0.029

 

OTE estimation in process water/mixed liquor

SOTE in process water is symbolized as SOTE, which is a normalized OTE against 20oC, zero salinity, and zero DO just like SOTE. The relative SOTE  in process water to the SOTE in clean water is defined as α-factor (Stenstrom, 2006).

image016                      —————————-(1)

Diffuser fouling by inorganic and organic foulants not only causes OTE loss by increasing the bubble size (see Fig. 2 in Diffuser Fouling), but also causes energy efficiency loss by increasing the head pressure. The OTE loss is factored into the equation by a fouling factor (), which is 1.0 for new diffusers αSOTE.  can be also written as αFSOTE including the fouling factor.

The relative OTE in the process water (or mixed liquor) with soluble ions to that in clean water is defined as β-factor. The temperature effect is considered using θ-factor. In typical municipal MBR conditions, β- and θ-factors are often neglected since α-factor dominates . The equation for field oxygen transfer efficiency, , is as follow based on all the rationales above. Typical ranges of the factors are summarized in Table 2.

2                                —————————-(2)

where

α             = alpha factor (-)
β             = beta factor (-)
θ              = theta factor (-)
f               = fouling factor of diffuser (-)
h              = diffuser submergence (m H2O)
C              = average DO in aeration tank at a given condition (mg/L or ppm)
1-1            = average saturated DO at a given temperature and zero salinity (mg/L or ppm)
1-2         = average saturated DO at 20oC, 1 atm, and zero salinity (mg/L or ppm)
SOTEs     = specific SOTE at 20 oC, 1 atm, and zero salinity (/m H2O)
T              = water temperature (oC)

In equation (2),  β  and  1-1 are calculated using the following equations based on experimental data taken from Weiss (1970).

 

3                                                                                                —————–(3)

4        —————–(4)

where

T                     : water temperature (oC)
h                     : diffuser submergence (m H2O)
S                    : conductivity of wastewater at 25 oC (μS/cm)
AL                  : altitude of the plant (m)

1-2 is calculated as equation (5).

5              ————————(5)

 

Table 2. Typical ranges of factors affecting oxygen transfer efficiency
Table2

 

Oxygen Transfer Efficiency Calculator

 

© Seong Hoon Yoon