Strategies for dissolved oxygen control in the river Arno

One of the main concern with water quality control is the level of dissolved oxygen (DO). Organic substances degrading in the aquatic environment, consume dissolved oxygen. When all the oxygen in the water is depleted, anaerobic conditions are created. Most substances can be degraded also under anaerobic conditions, but the process is generally slow and foul odours are often generated. Furthermore, the level of DO determines aquatic species that can survive in the stream. Therefore, controlling river quality, and DO level in particular, is necessary to preserve the ecosystem and to avoid the arising of nuisance from the river. Degradation of organic wastes, algal photosyntetic activity, atmospheric reaeration and water temperature are the main responsible for changes in the DO balance. Low DO levels can be easily found during the summer period, when organic waste concentration is increased by reduced flow rate and temperature of the water rises, decreasing solubility of oxygen in water. Conditions may become critical especially during the night hours when also algae respiration contributes to oxygen depletion. River systems can tolerate low DO levels for limited time periods. However, to avoid low DO levels lasting for a too long time, planning and realization of proper control actions are mandatory. In this work, we use a numerical model to simulate the arising of low DO levels in the river Arno and to evaluate the effect of different strategies for low DO levels remediation. We use a non-steady Advection-Dispersion-Reaction Equation (ADRE) fit on experimental data collected on the site. DO, algae and biochemical oxygen demand (BOD) were monitored to model DO balance dynamics. Experimental data collection was made during the summer of 1996. In that summer DO level never fell below the threshold value thanks to limited increase of water temperature. We considered the following strategies for low DO control: Flow rate variation; Local oxygenation in the critical points; Dynamic modification of wastewater load.