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New Information Regarding Intermittent Aeration in Liquid Swine ManureJun Zhu, Minnesota Extension EngineerThe goal of intermittent aeration of swine manure is to save energy but still provide enough oxygen to decompose organic compounds. This usually involves techniques to control the level of dissolved oxygen in the liquid. With intermittent aeration, as oxygen concentrations fall outside the preset upper and lower limits, the aeration system is switched off or on. However, this method may not work as expected in the real world since the performance of any aeration system is largely dependent upon the oxygen uptake rate by the aerobic bacteria. As the available soluble substrate is oxidized, significant changes in the oxygen uptake rate may be observed at different aeration stages. Because little information has been available about such changes during intermittent aeration, it has been difficult to design an optimal intermittent aeration scheme to save energy while accomplishing aerobic treatment. To examine this phenomena, recent research was conducted to examine the changes in the oxygen uptake rate of a venturi air injector system during an intermittent aeration process and to learn how those changes affect the performance of such an aeration system. To test the performance of an intermittent aeration system, swine manure in a 500-gallon tank was circulated with a pump through a venturi air injector at a flow rate typical of what would be used in an intermittent aeration system of this scale. During circulation, the oxygenated liquid was always dispersed into the tank from the bottom up, with the deoxygenated liquid at the top being sucked into the venturi system for air entrapment. Dissolved oxygen (DO) levels were monitored every five seconds, and a computer controlled the pump switch based on the measured DO levels. The aeration process started every three hours and stopped when the DO in liquid reached steady state. Once liquid DO levels reached steady state, the pump was switched off by the computer. To test this system, 31 runs were conducted over a 93-hour time period. The results indicated that the time to reach the steady state for dissolved oxygen levels increased from around 15 seconds during the first experimental run to about 270 seconds in the last run. The increase in time appeared linear with a coefficient of correlation of 0.9567, but the curve bent upwards during the last few runs, implying that there might be a substantial increase in time to reach steady DO levels in liquid thereafter. This increase is mainly due to the increasing growth and metabolic activity of aerobic bacteria, which accelerates the consumption of oxygen in liquid. When aeration stopped, the oxygen was depleted faster in the later runs as compared to early runs, indicating a growing demand of oxygen. These research observations potentially challenge the concept of using intermittent aeration to treat swine manure. In this experiment, the pump-off time declined from about 11 minutes to about 3 minutes during the last of the runs in the 93-hour period. This implies that the aeration time must be increased run by run and, at some point, will become continuous to meet the preset oxygen concentration if an automatic controller is used. Therefore, it can be inferred that intermittent aeration systems, if solely controlled by DO levels in liquid, may eventually end up in a continuous operating mode, without any energy savings. Based on the information observed from this study, animal producers should think twice when planning to adopt an intermittent aeration system to treat animal wastewater. |
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