Yield determination by respirometry : the possible influence of storage under aerobic conditions in activated sludge

In this study we found that the oxygen uptake rate (OUR) response from activated sludge to the addition of a single organic substrate can be divided into two phases. The first phase reflects the primary metabolism of the added substrate, while the second presumably originates from metabolism of stored polymers like polyhydroxyalkanoate (PHA) and/or glycogen produced during the metabolism of the exogenous substrate. This shift is due to the depletion of exogenous substrate. The obtained yield on acetate of 0.71 g COD/g COD correlates very well with experimental results in literature where addition of excess acetate to a pure culture led to extensive formation of polyhydroxybutyrate (PHB). Furthermore the highest obtained yield on glucose of 0.91 g COD/g COD is very close to the theoretical yield for formation of glycogen from glucose (0.96 g COD/g COD). This indicates that the response of activated sludge to a substrate addition is to store the substrate instead of an immediate growth response.