(598m) Optimal Control of a System of Wastewater Treatment Ponds

Wastewater treatment processes constitute a main issue to minimize major adverse impacts on freshwater and coastal ecosystems asssociated to urban and industrial growth. Activated sludge processes are currently the most widely used biological processes. Stabilization ponds, in turn, are large lagoons where wastewater is stored for long periods to allow a wide range of microorganisms to break down organic matter and sludge is not returned. In these ponds, there are aerobic, facultative and anaerobic zones, and different chemical and biochemical processes take place within the different zones, including mutualistic relationships between microalgae, heterotrophic bacteria and fungi that greatly influence the pond efficiency in biological wastewater treatment. A few models have been proposed for waste stabilization ponds describing hydrodynamic (Manga et al., 2004, Shilton and Mara, 2005) or biochemical processes (Kayombo et al., 2000; Dochain et al., 2003). However, the different pollutant removal processes have yet not been entirely understood.

We address the formulation of a detailed mechanistic model for a system of stabilization ponds (aerobic and facultative) for control purposes within a control vector parameterization framework (PSEnterprise, 2011). Dynamic mass balances are formulated for bacteria, yeast, main groups of phytoplankton, nitrogen, phosphorus, dissolved oxygen and biochemical demand of oxygen. Main model parameters have been estimated in previous work (Iturmendi et al., 2011), with data colleted from a juice plant in Argentina. In this work, we formulate an optimal control problem considering electrical motor power for mixers and nutrient addition rate as control variables. As the specification on biochemical demand of oxygen 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 six months. Numerical results provide useful information on the complex relationship among microorganisms, nutrients and organic matter concentration, as well as optimal management of the system of ponds.

References

Dochain, D., Gregoire, S., Pauss, A., Schaegger, M., 2003. Dynamic modelling of a waste stabilization pond. Bioprocess and Biosystems Engineering 26, 19–26.

Iturmendi, F., V. Estrada, P. Ochoa, P.M. Hoch, M.S. Diaz, Biological Wastewater Treatment: Dynamic Global Sensitivity Analysis and Parameter Estimation in a System of Waste Stabilization Ponds, ESCAPE 22, 22th European Symposium on Computer Aided Engineering, London, UK,17-20 June, 2012

Kayombo, S., Mbwette, T.S.A., Mayo, A.W., Katima, J., Jorgensen, S.E., 2000. Modelling diurnal variation of dissolved oxygen in waste stabilization ponds. Ecol.Modelling 127, 21–31.

Manga, J.G., Molinares Nelson, R., Orlando Soto, E., Arrieta, J., Escaf Germa, J., Hernandez Gustavo, A., 2004. Influence of inlet-outlet structures on the flow pattern of a waste stabilization pond. In: Proceedings 6th International Conference of Waste Stabilization Ponds. Avignon, France.

PSEnterprise, 2011,  gPROMS User guide.

Shilton, A.N., Mara, D.D., 2005. CFD (computational fluid dynamics) modelling of baffles for optimizing tropical waste stabilization pond system. Water Science &Technology 51,103–106.