Computational fluid dynamics (CFD) was used to simulate the mixing characteristics of a small circular anaerobic digester tank (diameter 6 m) equipped sequentially with 13 different plunger type vertical linear motion mixers and two different type internal draft-tube mixers. Rates of mixing of step injection of tracers were calculated from which active volume (AV) and hydraulic retention time (HRT) could be calculated. Washout characteristics were compared to analytic formulae to estimate any presence of partial mixing, dead volume, short-circuiting, or piston flow. Active volumes were also estimated based on tank regions that exceeded minimum velocity criteria. The mixers were ranked based on an ad hoc criteria related to the ratio of AV to unit power (UP) or AV/ UP. The best plunger mixers were found to behave about the same as the conventional draft-tube mixers of similar UP. Water Environ. Res., 86, 816 (2014).

The effects of anoxic and oxic fill strategies on SBR performance under nitrogen (NH4Cl as the nitrogen source) deficiency and rich conditions are evaluated using glucose as the sole substrate. The performance is evaluated according to substrate removal, sludge settleability, supernatant suspended solids (SS), and reactor biomass concentration. Substrate removal efficiencies are found to be independent of the fill strategies adopted under all conditions tested. The incorporation of anoxic selector environment fails to prevent the development of bulking sludge under nitrogen deficiency conditions thereby resulting in a gradual depletion of reactor biomass. Under nitrogen rich conditions, on the other hand, the sludge settleability improves significantly by adopting the anoxic fill strategy. Furthermore, SS readings taken at the end of settling period are greater with anoxic fill than with oxic fill, indicating that the latter discourages the growth of dispersed bacteria.

Computational Fluid Dynamics (CFD) was used to simulate the grit and sand separation effectiveness of a typical hydrodynamic vortex separator (HDVS) system. The analysis examined the influences on the separator efficiency of: flow rate, fluid viscosities, total suspended solids (TSS), and particle size and distribution. It was found that separator efficiency for a wide range of these independent variables could be consolidated into a few curves based on the particle fall velocity to separator inflow velocity ratio, Ws/Vin. Based on CFD analysis it was also determined that systems of different sizes with length scale ratios ranging from 1 to 10 performed similarly when Ws/Vin and TSS were held constant. The CFD results have also been compared to a limited range of experimental data.