Flow aeration in oxidation ditch

An oxidation ditch, commonly with an oval channel or ring shaped configuration and equipped with aeration and mechanical agitation devices, is a common structure in activated sludge plant. The idea is to promote the existence of the dissolved oxygen and its transport in the residual water, inducing the microbial growth whose contact with the residual water re-moves the biodegradable organic compounds. When the flow in the oxidation ditch is ade-quately aerated, the process is efficient, simple, stable and capable of treating different toxic loads without affecting the quality of the effluent and of production a well stabilised sludge. The design of such structures for treatment processes and its operation, which is currently based on the general empirical guidelines and operator experience, often dominated by the general rule, can be improved and involves significant costs in the management of a WWTP.

Several CFD models have already been applied to flows in specific structures, considering successful multiphase flows and including bio-kinetic reactions, but their use remains a chal-lenge. This work describes an OpenFOAM model to simulate the operation of aerators com-posed by several turbines rotating on the effluent free surface in an oxidation ditch. As aera-tion is a fundamental issue in the treatment process of waste water in an oxidation ditch, sev-eral rotational velocities of the aerator, are analysed within a modified interFoam solver and dynamic mesh to understand the relation between velocity, free surface perturbation and air-entrainment. The work is a contribution to understand the process of air entrainment caused by aerators.