Temperature oscillations in a biological reactor with recycle

The oscillatory behavior of temperature in a wastewater treatment is studied in this work. This process is composed of a fixed bed reactor, where the anaerobic digestion of distillery vinasses takes place, and a heat exchanger, which allows to hold the bioreactor temperature around 308.5 K. The heat exchanger is basically controlled by a proportional-integral-derivative feedback (PID control) while the bioreactor is controlled by a feedback law with uncertainties estimator. Both bioreactor and heat exchanger are interconnected by recycle streams. The bioreactor temperature in the wastewater system displays oscillatory behavior under interconnection. By using time series analysis of the bioreactor temperature (Poincaré maps, maximum Lyapunov exponents and power spectrum density), we show that recycling induces aperiodic oscillatory behavior of the temperature within the bioreactor, which does not necessarily imply inexpedient operation.