COMPARISON OF 3 MIXING DEVICES IN EARTHEN CULTURE PONDS OF 4 DIFFERENT SURFACE-AREAS

Mechanical mixing of culture ponds with low-powered devices can conser ve photosynthetically-produced dissolved oxygen, and so reduce the nee d for more expensive aeration. This work aimed to test inexpensive, ea sily obtained devices and to establish the utility of quantifying stra tification and mixing processes in power units to facilitate compariso ns and projection of requirements to new situations. Three mixing devi ces of power consumption less than 0.25 hp (63.5-173.6 W) were compare d in tropical earthen ponds of surface areas ranging from 200 to 1400 m(2). Mixers were operated during the time of maximum stratification i n control ponds (13:00-16:00 h), to standardize test conditions. Perfo rmance was assessed as reduction in a pond's stratification energy (SE ), contained in the uneven vertical distribution of mass. Neighboring unmixed ponds showed very similar diel cycles of SE; unmixed ponds als o showed similar patterns on successive days, but varied more than nei ghboring ponds assessed simultaneously. The mixing device of greatest power consumption, a fan-blade aerator-mixer (AM) operated below water surface, reduced stratification energy more quickly than a submersibl e impeller pump (SP) and an air-lift (AL). The AM and AL were more eff icient than the SP, but all were of low efficiency (less than 0.1%). E fficiencies were related to pond size, with perimeter/area ratio being significant but surface/volume not so. Mixing effects propagated rapi dly horizontally. The AM applied sufficient power to exceed the observ ed daytime rate of increase in stratification energy, ie. to prevent s tratification, in ponds of all sizes except the largest. The AL and SP did not apply power at sufficient rates, and the AM would have been i nadequate at other times. If is not necessary. however to prevent stra tification completely for all mixing applications.