Inhibition of activated sludge respiration by sodium azide addition: Effect on rheology and oxygen transfer

Abstract

Although microorganism respiration inhibition by sodium azide (NaN 3) is used in some studies to identify activated sludge adsorption capacity, little is known about the effect of this compound on the suspension properties. In this study we have investigated the effect of NaN 3 addition on both volumetric oxygen mass transfer coefficient and rheology of activated sludge (AS) suspensions in a 1.9 L bioreactor. The rheological properties (shear thinning one) of AS suspensions with and without NaN 3 addition are measured in situ (triphasic conditions). It appears that NaN 3 addition leads to a deflocculation of AS suspensions and thus a decrease in apparent viscosity. A small amount of suspended solids was added in order to obtain identical apparent viscosities (under 1.2 or 46.3 s −1) for AS suspensions with and without NaN 3 addition. K L a values were then measured in both respiring and non-respiring suspensions for different air flow rates (2, 3 or 4 L/min) and under low or high mechanical shear rate (1.2 or 46.3 s −1). Results show that under high mechanical shear rate, the respiration state for a given air flow rate does not impact the K L a values. On the contrary, under low mechanical shear rate, NaN 3 addition induces an increase of K L a values in comparison with those obtained with the respiring biomass. This effect, for a same apparent viscosity, is attributed to the deflocculation observed in the presence of NaN 3. Indeed, AS with and without NaN 3 addition used for the K L a measurements induce a modification of the floc internal structure, corresponding to smaller floc size in the case of NaN 3 addition.