Evaluation of Activated Sludge Oxygen Uptake Rate Test Procedures

Abstract

A series of bench‐ and pilot‐scale experiments was conducted to evaluate the ability of biochemical oxygen demand (BOD) bottle‐based oxygen uptake rate (OUR) analyses to represent accurately in situ OUR in complete mix‐activated sludge systems. Aeration basin off‐gas analyses indicated that, depending on system operating conditions, BOD bottle‐based analyses could either underestimate in situ OUR rates by as much as 58% or overestimate in situ rates by up to 285%. The use of a continuous flow respirometer system was used to verify the off‐gas analysis observations and assess better the rate of change in OUR after mixed liquor samples were suddenly isolated from their normally continuous source of feed. OUR rates for sludge samples maintained in the completely mixed bench‐scale respirometer decreased by as much as 42% in less than two minuts after feeding was stopped. Based on these results, BOD bottle‐based OUR results should not be used in any complete mix‐activated sludge process operational control strategy, process mass balance, or system evaluation procedure requiring absolute accuracy of OUR values.