Optimising operations for Final Settling Tanks (2005)

Transport processes in Final Settling Tanks (FST) are decisive to predict the operation of the entire activated sludge system and finally the quality of the effluent. The recycled sludge between FST and the reactor, the sludge properties and the sludge transport system are the main parameters for the sludge management in a FST.

The goal of the actual study is to optimise the sludge transport system for different stresses of the FST. By physical and numerical modelling, parameters of the sludge transport system, like scraper height, scraper velocity, distance between two scrapers and mainly the scraping direction will be varied.

Main energy intake in FST is caused by the inflow jet and the scraper movement. Energy assumption by kinetic and potential energy of the inflow shows, that the minimum energy intake occurs while positioning the inlet on the level of the sludge blanket. If the inlet lies under the sludge blanket a buoyant jet through the sludge bed takes place. The energy intake by the scraper movement depends on the overflow of the sludge over the scraper blade, which is a function of the density gradient in the sludge bed and of the scraper operation.
The subsequent analysis of the investigated data will lead to different correlations of various parameters and give the base to important criterions for an optimised FST operation.