Causes of negatively charged meso-colloids formed in the coagulation process: Implication of the origin of foulants in the coagulation–membrane filtration process

Abstract

Tiny colloids with a size similar to that of membrane pores are responsible for irreversible fouling in the pre-coagulation microfiltration membrane filtration process for drinking water treatment. Such colloidal particles are defined here as meso‑colloids, and the charge neutralization of meso‑colloids is demonstrated to be a key to controlling irreversible fouling. However, meso‑colloids remain negatively charged at neutral pH, the reason for which is still unclear. To increase the efficiency of membrane operation, additional knowledge about the causes and behaviors of meso‑colloids during pre-coagulation is indispensable. Therefore, in this study, meso‑colloids are fractionated after a series of jar tests, and their exact composition and charge properties are characterized. Two natural water samples, the adjusted water consisting of meso‑colloid fraction separated from one of the natural water samples and additional inorganic chemicals, and the adjusted water by the addition of appropriate inorganic chemicals into pure water are used for jar tests, which are conducted with and without the addition of the coagulant polyaluminum chloride (PACl). After the jar tests using two natural water samples, all of the meso‑colloids exhibit a negative charge under the conditions applied for the jar tests, indicating that charge neutralization is difficult. The composition of the meso‑colloids is found to be completely different depending on the water source used. Organic-rich water tends to generate meso‑colloids with a low Al/C (mass ratio of aluminum and organic carbon) ratio. In contrast, organic-poor water tends to produce meso‑colloids with a high Al/C ratio. From the results of the jar tests using two kinds of adjusted water samples, it is found challenging to neutralize meso‑colloids by PACl at neutral pH, because the overdose and underdose of PACl result in negatively charged biopolymer or negatively charged aluminum species. Therefore, the development of a new coagulant for specific use in the coagulation membrane filtration process is proposed, which can minimize the formation of negatively charged species even at neutral pH.