Probing the application of a zirconium coagulant in a coagulation–ultrafiltration process: observations on organics removal and membrane fouling

Abstract

Much attention has been paid to coagulation as a pretreatment for controlling membrane fouling. Until now, however, there has been no literature available on the application of Zr-based coagulants in the coagulation–ultrafiltration process. The aim of this work is therefore to investigate the coagulation behavior of ZrOCl₂ and its influence on membrane fouling, in comparison with that of the commonly used Al-based coagulants, Al₂(SO₄)₃ and PACl. Floc formation kinetics were studied via ‘a turbidity fluctuation technique’ (PDA measurement). A variation in the pH and temperature of raw water was also considered in order to test the performance of the three targeted coagulants. The results indicated that the best coagulation efficiency was achieved with the Zr coagulant, under neutral pH conditions and at a room temperature of 25 °C. This coagulant resulted in minimum turbidity (0.72 NTU) and the lowest concentration of the supernatant at UV₂₅₄ (0.005 cm⁻¹); the corresponding values for Al₂(SO₄)₃ were 2.79 NTU and 0.011 cm⁻¹, respectively, while those for PACl being 6.78 NTU and 0.015 cm⁻¹. The flocs obtained by the addition of this Zr coagulant were larger in size and had superior strength, recovery potential and settleability. In addition, membrane fouling was apparently alleviated through use of the Zr coagulant, especially in terms of the irreversible fouling, followed by PACl and Al₂(SO₄)₃. The superior removal efficiency of low-medium MW HA may be a plausible explanation for this phenomenon. Particularly, this work describes a novel Zr coagulant as an important potential candidate for application to the coagulation–ultrafiltration process in the future.