Electrochemical Pretreatment for Microfiltration of Surface Water: Mechanisms of Fouling and Its Control

Aluminum electrochemical pretreatment was evaluated for fouling control during surface water microfiltration (MF) using bench-scale galvanostatic batch electrolysis experiments followed by constant pressure, dead-end MF. In all cases, electrochemical aluminum production quantitatively obeyed Faraday’s law with a 3-electron transfer at nearly 100% efficiency. Underlying physical and chemical mechanisms were delineated using a variety of characterization methods including Attenuated total reflectance – Fourier-transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), electron microscopy, light scattering, and colorimetry. Fouling of a commercial polymeric MF membrane following electrocoagulation was found to (1) be lower at pH 6.4 compared with 7.5, (2) decrease only up to an intermediate aluminum dosage, and (3) exacerbate with increasing transmembrane pressure. In all experiments, cake filtration was the predominant flux decline mechanism for the entire duration of MF. Fouling was controlled by antagonistic effects of adding more and more aluminum coagulant; increasing total mass loading of colloidal foulants (higher total filtration resistance) and creating larger and more porous flocs (decreasing specific cake resistance). Cathodic production of hydrogen bubbles induced floc flotation over relatively long durations of electrolysis. Such electrochemically assisted flotation - “electroflotation,” was employed for MF pretreatment and this approach increased permeate fluxes better than electrocoagulation (i.e., flotation was not utilized) by reducing both the cake mass and the cumulative hydraulic resistance. Physically irreversible fouling and fouling control after electroflotation pretreatment was investigated using ATR-FTIR and XPS. Hydrophobic molecules primarily appeared to initiate physically irreversible fouling during untreated raw water MF. Electroflotation pretreatment reduced fouling by decreasing foulants pore penetration and accumulation of hydrophobic, proteinaceous, carbohydrate-like, acidic, and siliceous molecules. Differences in physical and chemical characteristics between various MF feed water colloids including untreated raw water and flocs generated during electrochemical and chemical coagulation were used to rigorously deduce their contributions to observed differences in cake compressibility. (Electro)coagulation generated cakes that compacted significantly at higher pressures compared with raw water. Cake compression in pretreated waters included elastic contributions from gelatinous and fluffy, amorphous Al(OH)3(S) solids (as determined by XRD and microscopy). Electrocoagulated flocs were more porous (due to hydrogen bubbles attachment) and fragile (from greater presence of hydrophobic compounds and lower amounts of amides and acidic organics) compared with those generated by alum coagulation. Hence, they are expected to be more prone to break and rearrange at high pressures and contribute inelastically to cake collapse and compressibility.