Steady state method (Offgas analysis)

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.

Steady10Fig. 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 O2 in the inlet air is consumed while a portion of biologically produced CO2 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% O2 in the air). In practical situation, the error is much lower than 2.1% because the CO2 produced from the bioreaction can compensate a portion of the O2 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 20oC and 30oC, respectively, before calibration. Considering the errors in O2 demand estimation, these potential errors are not significant. As a result, OTE can be calculated using a simplified equation below assuming QOut equals to Qin (Redmon, 1983). If all the gas flow rates are normalized against 20oC, 1 atm, and 0% humidity, the accuracy of equation (1) increases.

1               —————————- (1)


QIn                     Air flow to aeration tank (m3/min at 20 oC, 1 atm)
QOut                 Off-gas flow rate from aeration tank (m3/hr at 20 oC, 1 atm)
PO2,In               Partial O2 pressure in fresh dry air (bar)
PO2,Out             Partial O2 pressure in dry off-gas (bar)

OTR (kg O2/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.

1                       —————————-(2)
ATank      Surface area of aeration tank (m2)
AHood     The area covered by hood for offgas collection  (m3)
Qout        Offgas flow rate from the floating hood (m3/hr at 20 oC, 1 atm)


© Seong Hoon Yoon