Dynamic mixing models and analysis of the mixing processes for an oxidation ditch and an aeration tank

A one-dimensional longitudinal mixing model for an oxidation ditch with a moving symmetrical boundary and a series model for a symmetric aeration mixing unit of an aeration tank were developed in this study to analyze their mixing processes. The analysis of these models showed that the mixing process in the oxidation ditch tended to that of a continuous stirred tank reactor (CSTR) in a mode of circulation flow and dispersion; however, the mixing in the aeration tank tended to that of a CSTR in a mode of dispersion. The tracer output patterns and complete mixing times of the oxidation ditch and aeration tank with treatment capacities of 1000-50,000 m³/d and hydraulic retention time 12.0 h were evaluated. The complete mixing time increased from 7.5 min to 128.5 min for the aeration tank with an increase in volume; and the oxidation ditch with same complete mixing time as that of the aeration tank was obtained. The complete mixing time for the oxidation ditch can be changed in a wider range, for matching the time scale of biochemical degradation of organic pollutants. PRACTITIONER POINTS: Mixing model of an oxidation ditch and an aeration tank were built. Mixing in oxidation ditch tends to that of CSTR in a mode of circulation flow and dispersion. Mixing in an aeration tank is in accordance with advection diffusion and tends to that of CSTR as its Peclet number is large. Complete mixing time for an oxidation ditch can be changed in a wider range.