Optimizing the flocculation of powdered activated carbon in wastewater treatment by dosing iron salt in single- and two-stage processes

For the removal of anthropogenic micropollutants from wastewater, adsorption onto powdered activated carbon (PAC) is increasingly applied in wastewater treatment. To prevent desorption of adsorbed micropollutants into natural water bodies, the PAC is mostly separated from treated water by combined flocculation and filtration. While the adsorption efficiency of PAC has been thoroughly investigated in the past, no studies on its flocculation behavior and recommendations for operating large-scale PAC flocculation processes are available up to now. In this study, flocculation experiments were performed using secondary treated wastewater samples, a commercially available PAC product and an iron-based coagulant. Single- and two-stage flocculation processes were simulated in laboratory scale under variation of coagulant dosage, stirring rates and retention times in each process stage. In order to obtain real-time quantitative results, the flocculation process was continuously monitored by Focused Beam Reflectance Measurement and evaluated with regard to the reduction of a fine particle fraction sized < 10 mu m. The flocculation kinetics were modelled by exponential curves, taking into account mass transport limitations in the first phase of the process as well as equilibria between floc growth and break-up at longer retention times. On this basis, optimum operating parameters for PAC flocculation were determined and strategies to meet fluctuating process conditions were deduced. By operating a fast mixing stage at velocity gradients of 50-90/s for 10 min and, subsequently, a slow mixing stage at 20/s for 10 min, the concentration of particles sized < 10 mu m could be reduced by around 99%.