Existence of biocarriers (or floating media)
The existence of biocarrier certainly affects OTE, but it is not intuitive to draw a firm conclusion whether it improves OTE or decreases it. In one hand, the biocarrier may increase the contact time of bubbles with water by interfering the bubble rise. On the other hand, biocarriers may promote bubble coalescence and reduce the total surface area. There are not many publications based on scientific methodologies exist on this topic, but following are the some of the outcomes of recent studies.
Based on the available data in public domain, biocarriers significantly hamper oxygen transfer efficiency perhaps up to by 50%. Details are discussed here.
Aeration tank type and the location in the tank
-factor is not uniform even in a same aeration tank depending on the location in the tank. Different F/M ratio depending on the location in aeration tank is responsible for this phenomenon. Especially when aeration tank is designed as a plug-flow reactor, substrates availability gradually decreases from upstream to downstream. -factors were observed at 0.3 and 0.55 in both ends of the 300 meter plug-flow tank in a full-scale CAS process (Rosso, 2007). Since -factor is the lowest where oxygen is most required, tapered aeration plug-flow reactor requires a great deal of aeration tapering.
In addition, -factor is not constant even in a same location in an aeration tank. When -factor was monitored real-time in a full-scale MBR plant using an offgas analysis method, -factor varied between 0.4 and 0.6 in a fixed location depending on the time of the day (Cornel, 2002).
It is well known that surfactants diminish oxygen transfer by screening gas-liquid interface on bubble surface. When bubbles are rising, surfactants tend to move to the gas-liquid interface where hydrophobic moieties of the surfactant orient to air while hydrophilic moieties extend into water phase. Biopolymers such as proteins, polysaccharides, long-chain volatile acids, etc. also act like surfactant in bubble column. The molecules enriched in gas-liquid interface act as a physical barrier for oxygen dissolution. Depending on the level of surfactant-like materials, factor varies.
© Seong Hoon Yoon