Enhancing pollutant removal efficiency through multi-flow cascade flocculation and flotation reactor: a detailed flow field analysis

The treatment of oilfield wastewater, characterized by high oil content and complex composition, presents significant challenges in environmental protection. This study developed a novel multi-stage cascade flocculation and flotation reactor (MCFR) to enhance pollutant removal from oilfield wastewater. Particle image velocimetry was used to investigate the internal flow fluid distribution within the reactor. Results show that inlet flow rates of 100 and 150 L h^-1 create a high velocity and energy mixing environment near the inlet, facilitating thorough interaction between flocculants and wastewater. This promotes the rapid formation of small flocs and the coalescence of oil droplets. Under the influence of evenly distributed vortex generators, both flocs and oil droplets increase in size, with large oil droplets separated by flotation and dense flocs through sedimentation. In flocculation experiments, the MCFR, operating at 70 mg L^-1 of polymerized ferrous sulfate (PFS), 0.6 mg L^-1 of polyacrylamide (PAM), and an inlet flow rate of 100 L h^-1, achieved turbidity and oil removal rates of 95% and 94%, respectively. In comparison, a traditional stirred flocculation reactor achieves 82% and 78% removal rates for turbidity and oil, respectively, but requires a longer treatment time of up to 21 minutes. Additionally, the MCFR operates continuously with a treatment time of less than 1 minute, offering a faster and more efficient solution for gas and oil field wastewater treatment. These findings provide critical insights for designing advanced flocculation-flotation systems for the complex wastewater treatment needs of the oil and gas industry.