Impact of microporous aeration flow and aeration tube length on oxygen transfer performance in water

Abstract: Microporous aeration systems have become one of the most popular aeration devices used in lakes, reservoirs and aquacultures where there is a need to increase dissolved oxygen concentration. The biggest feature of the microporous aeration aerator is that the initial bubble diameters vary with changes in the aeration tube length, the air flow rate, the submerged water depth, and the input pressure. There are many factors to affect the oxygenation performance, such as the oxygen mass transfer coefficient, oxygen utilization efficiency, and power efficiency. Many researchers consider only the impacts of a single factor or two factors on oxygen transfer performance, without considering the most important keys to affect the oxygenation performance. For this study, the diffused air flow rate and the diffuser tube length were considered the key factors of oxygenation performance to determine the aeration tube length that maximized the oxygenation performance under a given air flow rate. The oxygen mass transfer coefficient, oxygenation rate, and oxygen utilization efficiency were considered the major parameters of the oxygenation performance in this study. To explore the effects of the diffused air flow rate and the diffuser tube length on oxygen transfer performance, after placing a disc made of a coiled, microporous diffuser tube in the middle bottom of an experimental pond, a series of re-oxygenation experiments were conducted under different aeration flow rates and aeration tube lengths. The impact of the aeration flow rate and the diffuser tube length on the oxygen volume mass transfer coefficient, the oxygenation amount and the oxygenation efficiency was analyzed and discussed. The results show that when the aeration air discharge was changed from 0.27 to 0.55 m3/s and the diffuser tube length was changed from 0.9 to 1.5 m, the oxygen mass transfer coefficient increased from 0.63 to 1.1 h-1, the oxygenation rate increased from 6.8 to 12.9 g/h, and the oxygen utilization efficiency increased from 6.87% to 9.28%. Under a given aeration tube length, the oxygen mass transfer coefficient and the oxygenation rate were varied directly with the aeration flow rate; however, the oxygenation efficiency was inversely proportional to the diffused airflow rate. Under a given aeration flow rate, the effects of diffuser tube length on the volumetric mass transfer coefficient produced a trend: the first low to high and then low to high. The oxygen mass transfer coefficient and the saturated dissolved oxygen concentration, which should be the main factors affecting oxygenation efficiency, mainly depend on the aeration tube length. The oxygenation efficiency is more sensitive to the diffused airflow rate than the diffuser tube length. The values of the oxygenation efficiency ranged from 6.87%-9.28%. Some studies have shown that there is an aeration tube length that optimizes the oxygen transfer performance. The simulation was deduced to provide a reference for determining the aeration tube length in actual application. The aeration flow rate and the aeration tube length both have significant impacts on the oxygenation performance. It mainly can be seen that the higher air flow rate, the better oxygenation performance is; in addition, oxygenation performance is very sensitive to aeration tube length. Finally, determining the optimal diffuser tube length not only helps to improve the oxygenation performance but also provides a theoretical basis for the design and operation of microporous aeration systems in a low-carbon economy.