Design and model tests for an efficient mechanical circulator/aerator for lakes and reservoirs

A 1.8 m diameter vertical axis upflow impeller and draft tube arrangement is proposed for circulating water bodies such as reservoirs and lakes so as to maintain oxic, destratified conditions throughout the water column. Tests were conducted on a 1/6 scale model (i) to verify theoretical predictions of flow rate and power consumption and (ii) to compare the performance of the impeller in a draft tube with that of the same impeller without a draft tube as used in recent full-scale tests in reservoirs. The model test results support theoretical predictions that by using very low flow velocities of the order of 0.2–0.4 m/s, a prototype could achieve a flow rate of the order of 1 m3/s with about 200 W of hydraulic power in the presence of a weak thermocline, or 130 W with no thermocline. This is a much lower specific energy consumption than for any mechanical mixer hitherto reported in the literature, and results indicate that in about 12 days, with 200 W input power, a reservoir of 1000 megalitres (ML) capacity could be fully circulated or the water above a certain level in a larger reservoir could be circulated so as to maintain dissolved oxygen above that level. It is argued that oxic conditions are best maintained by circulating water from lower levels to the surface where it will become oxygenated by natural processes of diffusion, photosynthesis and wind and wave action, rather than by mixing, which consumes energy through turbulence and does not per se add oxygen.