Effect of micelles-forming surfactant on performance of anaerobic digestion of synthetic municipal wastewater

Several papers have been published in the literature about the effect of surfactants on anaerobic digestion process and results have shown that surfactants had contradictory effect in different concentrations. To understand the reason of this contradictory effect, the surfactant behavior was studied during the anaerobic digestion process in this research. Firstly, laboratory-scale experiments were conducted using 200 cc reactors at mesophilic temperature to determine the optimum inoculum-substrate ratio and several parameters were measured such as chemical oxygen demand, total solids, volatile solids, pH, dissolved oxygen, electrical conductivity, total dissolved solids and alkalinity. Results have shown that optimum inoculum-substrate ratio was four. In the second part of this research, batch experiments were conducted with four Sodium Dodecyl Benzene Sulfonate concentrations namely 0, 100, 200, 400 under mesophilic condition lasted for 50 days. It was observed that a fundamental change in anaerobic digestion occurred at the surfactant concentration of 400 ppm in such a way that surfactant played the role of an obstacle in the anaerobic digestion. To study the effect of surfactant concentration on anaerobic digestion, electrical conductivity and turbidity of tab water and synthetic wastewater were plotted versus surfactant concentration. Results of these graphs showed that electrical conductivity of synthetic wastewater was raised sharply after critical micelle concentration unlike tab water due to the interaction between charged species in synthetic wastewater with formed micelles. In addition, at concentrations higher than 400 ppm, the number of produced micelles increased exponentially in synthetic wastewater. The inhibition of growth rate of microorganism could be attributing to the micelle-forming surfactant which induced curvature stress in cell membranes and led to disordering and then cell lysis.