Impacts of Calcium Addition on Humic Acid Fouling and the Related Mechanism in Ultrafiltration Process for Water Treatment

Humic acid (HA) is a major natural organic pollutant widely coexisting with calcium ions (Ca(2+)) in natural water and wastewater bodies, and the coagulation–ultrafiltration process is the most typical solution for surface water treatment. However, little is known about the influences of Ca(2+) on HA fouling in the ultrafiltration process. This study explored the roles of Ca(2+) addition in HA fouling and the potential of Ca(2+) addition for fouling mitigation in the coagulation-ultrafiltration process. It was found that the filtration flux of HA solution rose when Ca(2+) concentration increased from 0 to 5.0 mM, corresponding to the reduction of the hydraulic filtration resistance. However, the proportion and contribution of each resistance component in the total hydraulic filtration resistance have different variation trends with Ca(2+) concentration. An increase in Ca(2+) addition (0 to 5.0 mM) weakened the role of internal blocking resistance (9.02% to 4.81%) and concentration polarization resistance (50.73% to 32.17%) in the total hydraulic resistance but enhanced membrane surface deposit resistance (33.93% to 44.32%). A series of characterizations and thermodynamic analyses consistently suggest that the enlarged particle size caused by the Ca(2+) bridging effect was the main reason for the decreased filtration resistance of the HA solution. This work revealed the impacts of Ca(2+) on HA fouling and demonstrated the feasibility to mitigate fouling by adding Ca(2+) in the ultrafiltration process to treat HA pollutants.