A ONE-DIMENSIONAL MODEL FOR A SECONDARY SETTLING-TANK INCLUDING DENSITY-CURRENT AND SHORT-CIRCUITING

This paper presents a dynamic one-dimensional flux model for the secon dary settling tank which is suitable for use with the latest innovatio ns in models for activated sludge tanks, and which takes into account observed effects of density current and short-circuiting. The componen ts of the influent to the settling tank are divided into three fractio ns. Soluble components, non-settleable particulate components (primary particles), and settleable particulate components. (macroflocs). Solu ble components and primary particles are considered to follow the hydr aulic flow in the settling tank. The transport of macroflocs in the se ttling tank is modelled according to the traditional flux theory on a layer model of the settling tank extended with a model for density cur rent and short-circuiting. For modelling of the density current in the inlet region of the settler a dynamic inlet height is introduced. The short-circuiting is modelled by the introduction of a factor which ac counts for the dilution in the suspended solids concentration at the b ottom of the settling tank down to the concentration in the return slu dge flow. Settling velocities of the macroflocs for both free and hind ered sedimentation are measured and a new model for the settling veloc ity is proposed. The model is validated with data from the wastewater treatment plant Lynetten, Copenhagen, Denmark. It was found that the s uspended sludge concentration profile and the suspended sludge concent ration in the return sludge were predicted well with the model.