Optimal Control Strategies for Different Stabilization Pond Systems

Wastewater generation is inevitable and its discharge into surface waters leads to environmental problems and health risks. To avoid this, standards for wastewater discharge have been enforced and are expected to become stricter, this resulting in a growing interest area of study [1]. Wastewater treatment in stabilization ponds mainly results from settling and complex symbiosis of bacteria and algae where the oxidation of organic matter is accomplished by bacteria in presence of dissolved oxygen supplied by algal photosynthesis and surface re-aeration [2].

In this work different configurations of stabilization ponds are considered. Dynamic modeling of each pond was implemented within a dynamic optimization environment and the whole system was simulated during a fixed time horizon. A detailed mechanistic model is constructed, based on first principles of mass conservation, of different types of systems of anaerobic, aerobic and facultative ponds in series.

We address the control problem of wastewater stabilization ponds of the differents systems by formulating an optimal control problem considering electrical motor power for mixers in the aereted pond and nutrient addition rate as control variables (degrees of freedom of the problem)[3]. Constraints are embedded in the DAE model and boundaries on the control variables. As the specification on chemical oxygen demand (COD) in the outlet stream is far from the target one, the objective is to minimize the offset between the desired value and the current one, along a time horizon of a year. As a result of the dynamic optimization problem the optimal time profiles of motor power and nutrient addition rates are obtained for the time horizon. The main objective of the work is to compare the performance of the different configuration pond system under different control strategies, by the amount of organic matter in the effluent of the treatment plant.

The model takes into account dynamic mass balances of biomass of algae, the main groups of bacteria: heterotrophic bacteria, autotrophic bacteria, fermenting bacteria, acetotrophicsulphate reducing bacteria and acetotrophicmethanogenic bacteria. Also, mass balances for organic load are formulated, such as slowly biodegradable particulate COD, inert particulate COD, fermentation products, inert soluble COD, and fermentable readily biodegradable soluble COD. For nutrients, ammonium and ammonia nitrogen, nitrate and nitrite nitrogen, sulphatesulphur and dissolved oxygen. Finally, molecular nitrogen and methane emissions are considered in the model.

The results provide useful information on the complex relationship among micro-organisms, nutrients and organic matter concentration, as well as information about the impact of modification in the pond system that can be used to improve the control of the effluent composition.

Keywords:Optimal Control - Wastewater Treatment- Stabilization Ponds System – DAE