If we want to measure OTE, SOTR and E for a deep tank in operation, we must use the off-gas analysis method (Fig. 5.11).
An off-gas apparatus consists of an off-gas hood with instruments for the measurement of flow rates and oxygen concentration. The floating hood may have a cross-sectional area of 2-4 m2 (Schuchardt 2005). Air is drawn off by a fan passing first through a rotameter for flow rate measurement of the off-gas. A side-stream is passed through H2O and CO2 adsorbers and is then analyzed to measure O2 consumption.
The results were obtained from measurements at the aerobic activated sludge basin of the WWTP Waßmannsdorf, near Berlin, equipped with ceramic tubes (see Fig. 5.12). For low and high flow rates of compressed air and specific power demands (P/V), the efficiency E decreases only slightly. Maximum efficiencies (E = 2.5-2.7 kg O2 (kWh)-1) are obtained only in a narrow band of 11 < P/V < 20 (W m-3) with values of 31 < SOTR < 52 (g O2 (m3 h)-1) (Schuchardt et al. 2002; Schuchardt 2005). For lower and higher values of P/V, the efficiency decreases slightly, down to E = 2.1-2.2 kg O2 (kWh)-1. With optimized aeration equipment and optimized automatic process control, great amounts of energy can be saved.
If we compare these results with those presented in Fig. 5.7 for a small Simcar aerator (d = 40 cm) we establish a quite good correspondence. We should not misinterpret these relative small derivatives: We need more studies for the optimization of aeration systems of large WWTP's in a time of arrising costs for energy!
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