Bacteria and Virus Control by Electrochemical Coagulation and Microfiltration
Tanneru, Charan Tej
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Bench-scale experiments were performed to evaluate microorganism control by electrochemical coagulation and membrane microfiltration. Natural organic matter (NOM) present in natural waters appears to reduce the effectiveness of iron electrocoagulation pretreatment to microfiltration for MS2 virus control by complexing ferrous ions generated at the sacrificial anode during electrolysis. This inhibits (i) Fe2+ oxidation, precipitation, and virus destabilization and (ii) virus inactivation through reactive oxygen species intermediates or by direct interactions with Fe2+ ions. In contrast, higher reductions in MS2 virus concentrations were obtained when aluminum was electrochemically added to surface water. Sweep flocculation was the primary virus destabilization mechanism with secondary contributions from charge neutralization. Direct evidence for virus enmeshment in flocs was provided by two independent methods: quantitative elution using beef extract at elevated pH and quantitating fluorescence from labeled viruses. Monotonically increasing adhesion force between viruses immobilized on AFM tips and floc surfaces with increasing electrocoagulant dosage was measured by atomic force microscopy, which was accompanied by decreasing magnitude of the zeta potential (→ 0) and increasing NOM removal. Hence, virus uptake mechanisms also include charge neutralization and hydrophobic interactions with NOM on floc surfaces. Evidence for virus inactivation was also obtained during iron and aluminum electrocoagulation of synthetic water spiked with viruses. Free chlorine was produced during aluminum electrolysis of saline solutions via oxidation of chloride ions, which inactivated MS2 viruses. Capsid protein modifications probed using Fourier transform infrared spectroscopy (FTIR) revealed significant oxidative modification in amide I and II (1700-1500 cm-1) region. Evidence for genome damage was obtained using quantitative real time polymerase chain reaction (q-RT-PCR). Hence, alterations of capsid proteins and loss of genome structural integrity both contributed to inactivation. Separate experiments were performed to examine the rejection of spherical silica colloids and viruses as well as capsule-shaped bacteria by clean microfiltration membranes. Modeling efforts (performed by Prof. Ruth Baltus’ group at Clarkson University) focused on incorporating the convective hindrance factor for a capsule shaped particle in a cylindrical pore into predictions of the rejection coefficient. Short-term MF experiments were performed at the University of Houston to measure rejection of three Gram negative bacteria, two spherical viruses, and several spherical silica particles by a number of track-etched membranes with near cylindrical pore geometry in a stirred cell before the onset of fouling. Experimental rejections of spherical viruses, and particulate silica and several rod-shaped Gram negative bacteria with aspect ratio from 2 to 5 by clean track-etched membranes were in general agreement with theoretical predictions.