Experimental Analysis of Sedimentary Oxygen Demand in Lakes; Dependence on Near-Bottom Flow Velocities and Implications for Aerator Design

Aeration technology is applied in hundreds of Minnesota lakes and reservoirs for at least three purposes: (a) to prevent winterkill of fish in shallow lakes under ice cover, (b) to reduce nutrient release rates from the sediments and (c) in aquaculture to provide aerated water to high-density fish populations. A major uncertainty in the design, selection and application of aeration systems is the often observed increase in oxygen demand after aeration systems are installed and operated. As a result, the improvement in dissolved oxygen is often less than anticipated, even zero. In this study we have investigated this problem through a series of carefully designed experiments. We have shown that sedimentary oxygen demand (SOD), frequently the major oxygen consumer in lakes, increases proportionally to the velocity with which the water above the sediments moves. Aeration devices often and intentionally increase water velocity above the sediments and thereby increase oxygen consumption in the lake. The results given in this report allow a more realistic estimation of oxygen demand in lakes for aerator selection. Recommendations for aerator placement are also given.