Offgas analysis is an *in situ* method that provides the most accurate real-time data. The details of the method are very well described in literature (Leu, 2009), but the concept of offgas analysis is illustrated in Fig. 1 below. More practical pictures are found elsewhere. It estimates the oxygen transfer efficiency by analyzing the offgas compositions and comparing them with air composition.

Fig. 1. Schematic of off gas analysis method.

The offgas flow rate can be slightly different from the inlet air flow rate because a portion of O_{2} in the inlet air is consumed while a portion of biologically produced CO_{2} is stripped out from mixed liquor. However, the error from the flow rate is generally not significant. If OTE is 10%, the maximum error from the air flow rate can be only up to 2.1% (10% of 21% O_{2} in the air). In practical situation, the error is much lower than 2.1% because the CO_{2} produced from the bioreaction can compensate a portion of the O_{2} loss. Moisture contents in the air and in the offgas also affect the offgas flow rate, but the error is only up to 2.3% and 4.2% at a water temperature of 20^{o}C and 30^{o}C, respectively, before calibration. Considering the errors in O_{2} demand estimation, these potential errors are not significant. As a result, *OTE* can be calculated using a simplified equation below assuming *Q _{Out}* equals to

*Q*(Redmon, 1983). If all the gas flow rates are normalized against 20

_{in}^{o}C, 1 atm, and 0% humidity, the accuracy of equation (1) increases.

where

*Q _{In }* Air flow to aeration tank (m

^{3}/min at 20

^{o}C, 1 atm)

*Q*Off-gas flow rate from aeration tank (m

_{Out}^{3}/hr at 20

^{o}C, 1 atm)

P

_{O2,In}Partial O

_{2}pressure in fresh dry air (bar)

P

_{O2,Out}Partial O

_{2}pressure in dry off-gas (bar)

*OTR* (kg O_{2}/hr) can be estimated assuming local offgas flow are uniform across the aeration tank surface as equation (2) (Leu, 2009). Test can be performed at multiple areas and the result can be averaged out to obtain a more accurate average OTR.

—————————-(2)

where

*A _{Tank}* Surface area of aeration tank (m

^{2})

*A*The area covered by hood for offgas collection (m

_{Hood}^{3})

Q

_{out}Offgas flow rate from the floating hood (m

^{3}/hr at 20

^{o}C, 1 atm)

© Seong Hoon Yoon