Optimization, upscaling and kinetic study of famine technique in a microalgal biofilm-based photobioreactor for nutrient removal

Abstract

Microalgal wastewater treatment systems are promoted as promising, while harvesting costs, low removal efficiency, and long process time are major obstacles. The present study aims to introduce a comprehensive solution to overcome the difficulties by combining two methods of depletion and biofilm in the culture of Scenedesmus sp. using synthetic wastewater. For this purpose, two factors of depletion periods (one, two, three, and five days) and types of depletion (nitrogen, phosphorus, and both) were optimized by Taguchi design for the wastewater nutrient removal rate in lab-scale. The analysis of factors showed that a five-day complete depletion of nitrogen and phosphorus had the most significant effect on the removal of nitrogen compounds and phosphate, respectively. Optimization of factors led to 35, 8, 28 percent increases in ammonium, nitrate, and phosphate removal efficiencies, respectively and about 50 percent decrease in the process time in flasks. Lab-scale results were validated in cylindrical photobioreactors with external recycling (CPBERs) that achieved 73.2, 76.6, and 65.3 percent in ammonium, nitrate, and phosphate removal efficiencies, respectively. Finally, the comparative kinetic study of starved and non-starved processes led to a remarkable change in concentration dependency from first- to second-order after depletion. The optimized system that was provided can be of benefit for further researches in this area.