Impact of different aeration approaches on dissolved oxygen for intensive culture ponds

Aerators are essential in semi-intensive and intensive aquaculture to maintain an environment congenial to the physiological requirements of the cultured organisms. In an intensive aquaculture, the full oxygen demand of the cultured species cannot be met through natural aeration only. Therefore, artificial aeration is essential. In past years, various types of aerators have been developed to maintain the desired concentration of dissolved oxygen (DO) in the pond water to improve the energy efficiency of the oxygen mass-transfer process. In the present study, it is intended to compare the dynamical efficiency and economic performance of three different aeration approaches, including paddle wheel aerators, diffused-air systems, and water cultivators at different initial DO concentrations of ponds and operating durations of aerators. The study involved a typical Chinese grass carp culture with commonly practiced stocking density and feeding. The comparison included two parts: daytime and nighttime experiments with a repeated measurement design for each one. In the daytime experiment, the paddle wheel aerators and diffused-air systems were operated between 12:00 and 16:00 and the water cultivators between 8: 00 and 18:00. In the nighttime experiment, the three aeration systems were all operated between 0:00 and 6:00. Based on the investigation, phytoplankton photosynthesis on sunny days was still the main source for DO replenishment in such intensive culture ponds since aerators were not operated all day. The operation of aerators enhanced water exchange between the upper and lower layers of water, decreased the escape of DO in the daytime due to over-saturation, and increased the total stock of DO. Accordingly, aerators reduced the risks of fish raising their heads above water to breathe in the night/dawn and were beneficial for fish feeding. The diurnal variation of DO in the upper layer of water was greater than that in the lower layer and displayed dis-synchronization which could be attenuated by mechanical mixing/aeration. The mixing/aeration capacity of paddle wheel aerators and diffused-air systems were significantly higher than that of cultivators, but the paddle wheel aerators also increased the oxygen consumption rates during the night in comparison to the cultivators, possibly due to the enhanced decomposition of organic matter promoted by water circulation. In the current case, the mechanical aeration capacity of cultivators, diffused-air systems, and paddle wheel aerators were 0.22, 3.2, and 4.53 kg/h, corresponding to a decrease rate of DO 0.58 mg/h for the cultivators, 0.43 mg/h for the diffused-air systems, and 0.27 mg/h for the paddle wheel aerators during nighttime. Energy consumption in the daytime was in the following order: paddle wheel aerators (12.0 k W?h) > diffused-air systems (6.4 k W?h) > cultivators (0.8 k W?h). Based on the dynamic efficiency and energy savings, the cultivator is more suitable for mixing/aeration in the daytime, while the paddle wheel more effectively mitigates oxygen depletion during the night.