Systematic study on calcium–alginate interaction in a hybrid coagulation-nanofiltration system

Hybrid coagulation-nanofiltration has been shown to be advantageous in removing organic substances and reducing the footprint of treatment plant when implemented. In this study, hybrid coagulation-nanofiltration of sodium alginate solution using a cross-flow filtration configuration in the presence of calcium ions was investigated. Permeate flux trends, rejection of sodium alginate, mono- and divalent ions, and resistances in systems containing sodium alginate, calcium, alum and their combination were analyzed. It was found that coagulation-nanofiltration in cross-flow filtration of sodium alginate could be advantageous when calcium ions were present in the solution, due to the lower resistances. In particular, the interaction between calcium ions and sodium alginate was systematically studied in order to explain the different outcomes between permeate flux trends during filtration of sodium alginate and calcium by observing the change in permeate flux, solution viscosity, floc size, and resistances at different calcium concentrations. We have found that alginate viscosity is dependent upon calcium concentration and ionic strength, which could translate into different trends of permeate flux. It is further suggested that different filtration configurations – dead-end and cross-flow configurations – could be responsible for the discrepancies by creating different conditions at the boundary layer. Finally, the interaction between calcium and alginate at different concentration ratios and ionic strength was proposed.