hydrograv proload.cfd: Comprehensive support in design of secondary settling tanks and optimisation by CFD-simulations. Our software hydrograv proload.cfd is an expert system for the design and optimisation of secondary settling tanks. It distinguishes itself by taking the interaction with the aeration tanks into account. Tanks to be newly built are primarily designed according to the regulations of the DWA (German Water Association) based on the knowledge of a few loading data. This basic geometry can subsequently be improved with own simulation results. Similarly an operator of an existing tank is able to optimise its hydraulics with a number of simulations on his own. Since sludge displacement within the system of secondary treatment is accounted for in the computations it is possible to examine operating strategies at different transient loading situations.