Kinetic modelling of the anaerobic digestion of wastewater derived from the pressing of orange rind produced in orange juice manufacturing

A simplified kinetic model for studying the anaerobic digestion of wastewater derived from the pressing of orange rind as a result of orange juice production was proposed on the basis of the experimental results obtained. The process was conducted in a laboratory-scale completely stirred tank reactor operating in batch mode at mesophilic temperature (35 °C), with COD loads in the range of 2–5 g COD. The following simplified three-step reaction scheme was proposed: (1) hydrolysis and conversion of complex organic compounds into intermediate products of lower molecular weight; (2) conversion of these intermediates to volatile fatty acids (VFA); and (3) methanization of the VFA by methanogenic microorganisms. A mathematical model based on four segregated differential equations was formulated assuming that a fraction of this substrate is non-biodegradable and the above-mentioned steps follow first-order kinetics. It was found that the kinetic constants corresponding to these three stages (K0, K1 and K2) decreased markedly with the load added to the reactor, showing the occurrence of an inhibition process. In addition, it was observed that the methanogenic step was the slowest in the overall anaerobic process. Finally, the model was validated by comparing the theoretical curves obtained with the corresponding experimental data of organic matter, VFA and methane. The deviations obtained (less than 20%) in most cases demonstrated the suitability of the mathematical model proposed and suggested that the parameters obtained represent and predict the activity of the microorganisms involved in the anaerobic digestion process of this wastewater.