Sludge reduction by ozone: Insights and modeling of the dose-response effects

Applying ozone to the return flow in an activated sludge (AS) process is a way for reducing the residual solids production. To be able to extend the activated sludge models to the ozone-AS process, adequate prediction of the tri-atoms effects on the particulate COD fractions is needed. In this study, the biomass inactivation, COD mineralization, and solids dissolution were quantified in batch tests and dose-response models were developed as a function of the reacted ozone doses (ROD). Three kinds of model-sludge were used. S1 was a lab-cultivated synthetic sludge with two components (heterotrophs X_H and X_P). S2 was a digestate of S1 almost made by the endogenous residues, X_P. S3 was from a municipal activated sludge plant. The specific ozone uptake rate (SO₃UR, mgO₃/gCOD.h) was determined as a tool for characterizing the reactivity of the sludges. SO₃UR increased with the X_H fraction and decreased with more X_P. Biomass inactivation was exponential (e^-β.ROD) as a function of the ROD doses. The percentage of solids reduction was predictable through a linear model (C_Miner + Y_sol ROD), with a fixed part due to mineralization (C_Miner) and a variable part from the solubilization process. The parameters of the models, i.e. the inactivation and the dissolution yields (β, 0.008-0.029 (mgO₃/mgCOD_ini)^-1 vs Y_sol, 0.5-2.8 mg COD_sol/mgO₃) varied in magnitude, depending on the intensity of the scavenging reactions and potentially the compactness of the flocs for each sludge.