System observability and nonlinear parameter identification of nonylphenol biodegradation kinetics

Addition of surfactants, detergents and emulsifiers has been successfully applied for cleanup of petroleum-contaminated sites. However, a certain groupof widely used alkylphenolethoxylates (APEs) surfactants was recently banned in Europe because scientists discovered that APE breakdown products are estrogenic and highly toxic to aquatic organisms. Nonylphenol is one of the very toxic breakdown products. The process of nonylphenol biodegradation is very important to many scientists because of its potential effectiveness as a treatment tool for pollution. However, very little information is available on the biodegradation kinetics of nonylphenol. Kinetic information is necessary for predicting the fate of pollutants. We start with Monod's model for nonylphenol biodegradation which is based on a coupled system of nonlinear differential equations. We prove that the states of the system and the parameters of Monod's model are locally observable. This enables us to perform a meaningful parameter estimation analysis. By using nonlinear least-squares optimization, we obtain the biodegradation kinetics and verify physical feasibility on independent datasets.