Improvements in permeate flux by aluminum electroflotation pretreatment during microfiltration of surface water

Cathodic production of fine hydrogen bubbles over relatively long durations during aluminum electrochemical treatment of natural waters was empirically observed to induce floc flotation. Such electrochemically assisted flotation, termed electroflotation, was employed for microfiltration (MF) pretreatment by skimming off the surficial floc layer and drawing water from near the bottom of the electroflotation cell. This approach significantly increased permeate fluxes during surface water dead-end MF by reducing both the cake mass and the cumulative hydraulic resistance. These results were compared with a closely related process – electrocoagulation – wherein the entire destabilized suspension was sent to MF. Electrocoagulation pretreatment also improved MF fluxes compared with raw water, but not as much as electroflotation. In both pretreatment scenarios, amorphous Al(OH)3 was dominant as revealed by X-ray photoelectron spectroscopy. The absence of an intermediate particle removal step in electrocoagulation contributed to a higher cake mass and greater total cake resistance even though it reduced the specific resistance by forming larger aggregates. In addition to improving MF flux during forward filtration, electroflotation pretreatment appears to form largely reversible fouling layers that may lead to more effective MF backwashing. In contrast, nitrogen and silicon detected on membranes, even after cake removal suggests a greater extent of irreversible MF fouling following electrocoagulation pretreatment. Significant improvements in flux and potential improvements in backwashing effectiveness following aluminum electroflotation of surface water in laboratory-scale experiments points to the need for larger-scale evaluations before a hybrid electroflotation-MF process can be implemented for drinking water treatment.