MODELING OF NITRATE DISAPPEARANCE AND SLUDGE RISING IN A SETTLING COLUMN SYSTEM

The determination of different rates of endogenous nitrate respiration due to biomass stratification throughout the depth of a sludge blanke t is essential to assess the factors affecting the sludge rising in se condary clarifiers. In this study, an extension of the solids flux the ory in conjunction with nitrate mass balance was used to model the fat e of nitrate in a sludge settling column. The model-predicted nitrate concentrations agree well with those experimentally determined. A new term, sludge stability index, was introduced to reflect the sludge ris ing time and the corresponding critical nitrate concentration necessar y for causing sludge rising. The model predicted well the observed slu dge rising times occurring under batch settling column experiments wit h initial biomass and nitrate concentration ranging from 2000 to 5000 mg 1(-1) and 10 to 30 mg-N1(-1), respectively. Furthermore, simulation results indicated that both the sludge rising time and CNC are signif icantly affected by k(ENR).