Oxygen transfer rate (OTR) is the rate oxygen is transferred from the air into the bulk liquid of the aeration basin (atmosphere/gas to liquid water).  This is typically measured as lbO2/hr. The aeration technology utilized (mechanical aerator vs blower, course vs fine bubble diffusers, etc) impacts the OTR. Each technology has pros and cons. Proper operation and maintenance of aeration equipment is critical for consistent performance. How well do your operational and maintenance practices align with manufacturer recommendations?