Effect of chitosan coagulant on virus removal and water quality change: a pretreatment option for gravity-driven microfiltration

Abstract

Gravity-driven membrane (GDM) filtration has received increasing attention as a low-energy, decentralized drinking water treatment technology. Microfiltration (MF) membranes are widely available and can be used for GDM, however, its low removal performance for viruses can limit the widespread application of MF-based GDM technology. To increase the virus removal, this study proposed and assessed a chitosan coagulation pretreatment for the MF-based GDM. Laboratory-scale coagulation and GDM experiments were carried out to better understand the i) optimal dose of chitosan for virus removal, ii) chitosan-induced water quality changes, and iii) effect of chitosan pretreatment on the permeation rate and fouling layer characteristics in GDM filtration. Several water matrices were tested containing different levels of kaolinite (inorganic turbidity), MS-2 (virus), E. coli (bacteria), and humic acid (natural organic matter). The GDM filtration was operated at 73 mbar for one month with and without the chitosan pretreatment. At the chitosan doses of 0.5, 1 and 10 mg/L, the virus removal levels were more than 3.65-log10 reduction in a lake water. Water turbidity and E.coli did not influence the virus removal while humic acid significantly lowered the virus removal efficacy at the tested conditions. The chitosan dose of 1 mg/L is thus sufficient for more than 3-log10 reduction of virus in most condition unless the waters contained high levels of humic acid (e.g., > 3 mg/L). The chitosan coagulation at 0.5 – 10 mg/L doses could also achieve more than 75% removal of turbidity and 4-log10 reduction of E.coli. The presence of kaolinite (22.9 NTU) negatively affected the E.coli removals by chitosan. Significant DOM removal (55%) was observed only at the relative high chitosan dose (10 mg/L). The flux stabilization was observed for the both GDM alone and chitosan/GDM operations. The stabilized flux in the chitosan/GDM operation was 3.47(±0.20) L/(m2.hr) (p > 0.001) which was not significantly different from the GDM alone (=3.51(±0.20) L/(m2.hr)) (p > 0.05). The mass and ATP content in the membrane fouling layer of the chitosan/GDM were lower than those of GDM alone by 12% and 38%, respectively, even though there was little difference in the feed water quality (e.g., turbidity and DOC) for the two systems. The total cell concentration (TCC) in the feed water for the chitosan/GDM system was relatively lower by 63% (=0.44-log10) due to its removal by the chitosan coagulation. The spiked E.coli and MS-2 were removed by 4.9-log10 and 4.1-log10, respectively, in the chitosan/GDM while the MS-2 removal was only 1.5-log10 for the GDM alone treatment. Overall, this study demonstrates that a chitosan coagulation pretreatment at low chitosan dose (1 mg/L) can significantly enhance the virus removal efficacy of MF-based GDM filtration with minimal influence on the flux stabilization.