Optimization of Microalgae Harvesting and Separation Process by Electrical Coagulation in Biodiesel Production Cycle Using Response Surface Methodology

Abstract

Introduction Algae have demonstrated to be an efficient bio energy source because in contrast to sugarcane, soybean, canola and oil palm, algae are not edible, they are less expensive to produce, grow faster, allow higher yield and production rate per hectare, do not require clean water to grow, and have the potential of reducing carbon emission. Because of their small size (typically a few micrometer) and low concentration in the culture medium (0.5–2 gL-1), harvesting microalgae biomass is a major challenge. The main goal of this study was to demonstrate the proof of principle for harvesting of microalgae using electro-coagulation-flocculation and to investigation the influence of several important variable on the efficiency of the electro-coagulation-flocculation in harvesting and separating Dunaliella salina microalgae from the culture medium. This is a native species and halophyte microalgae with a different culture medium from the fresh water in terms of salinity and electrical conductivity. Materials and Methods In order to investigate the effects of five control variables (independent) was included: material of the electrodes on both levels of aluminum and iron; current intensity in the range of 300 to 1000 mA; time for electro-coagulation-flocculation, 5 to 20 minutes; the electrode gap, 1 to 3 cm; stirring speed between 0 to 400, on the recovery efficiency as the response variable (dependent) experiments based on multi factors response surface method (combining categorical with numeric factors) was designed. In this study, the experiments were made inside a batch reactor with an effective volume of 250 mm which is made of Pyrex glass. Two electrodes with dimensions of 5 × 5 cm and a surface area of 25 cm2 with distance 2 cm from bottom of the reactor in vertically state and in different stages were placed inside the reactor with distance of 1, 2 and 3 cm. The Voltage and required current in the reactor were provided with a digital DC power supply. The main pilot in shape of cubic rectangular which is made of plexiglass with dimensions 35 × 28 × 18 cm and the effective volume of 14 liters was designed and built, in order to test the results of optimal experiments. For designing an experiment, statistical analysis and optimization was used from the software Design-Expert. Results and Discussion In this study, the modified quadratic model was used to fit the microalgae recovery efficiency data obtained from each batch test. The coefficients of determination (R2), adjusted and predicted were respectively more than 0.98, 0.96 and 0.90, which indicated that the modified quadratic model could describe the microalgae recovery efficiency in the batch tests of this study successfully. The results indicated that the linear effect of control variable on the recovery efficiency is very statistically significant. Moreover with increasing the electric current intensity variable and ECF time, or reduce the distance between the electrodes, the recovery efficiency has increased significantly. Also by increasing stirrer speed from 0 to 200 rpm the amount of recovery efficiency is increased, and by increasing stirrer speed from 200 to 400 rpm the amount of recovery efficiency has decreased. The results showed that aluminum electrodes on the recovery of microalgae from the culture medium are more efficient than iron electrodes. In this study, were searched the optimal operating conditions with aims of maximization of the microalgae recovery efficiency. The maximum microalgae recovery efficiency of 98.06% was obtained at the current intensity of 999 mA, the time of 20 min, the electrode gap of 1.39 cm, the stirring speed of 222 rpm and with aluminum as electrode material Conclusion In this study was examined the effect of five control variables (independent) including: current intensity, electrode gap, ECF time, stirring speed and electrode material on the response variable (dependent) the recovery efficiency of Dunaliella salina microalgae from the culture medium. The modified quadratic model was used to fit the microalgae recovery efficiency data obtained from each batch test. The experimental results in different stages of our study indicated that the harvesting efficiency of the ECF process could be improved with optimized settings in different stages. If you want to achieve the maximum efficiency with considering economic factors, energy and environment, the second part of an article by the same research group that focuses on this topic is recommended. However, as the ECF process is complicated on a large scale, a pilot study is required to further adjust the harvesting efficiency and make alterations in current density and electrode plate distance in the ECF harvester so as to develop such technology and make commercial use of it in the future.