Removal of phosphates from water by a hybrid flotation– membrane filtration cell

A new hybrid process for treating wastewater, combining membrane microfiltration and flotation, was investigated. The hybrid process combines the advantages of both membrane separation and flotation: the flotation cell removed a large proportion of suspended solid particles while the membrane module produced a clean water permeate effluent. The hybrid process was investigated using an aqueous suspension of iron colloidal particles produced in situ. Process parameters investigated were: effect of gas flow rate, which was necessary for the flotation process, and the frequency and length of backflushing, required in order to reduce the fouling of membranes and to extend their useful working time. The results indicated that the increase of airflow rate positively affected the hybrid cell performance, up to a critical value. Backflushing, on the other hand, resulted in an improvement of the hybrid cell performance.

Membrane filtration technologies are increasingly used for solid–liquid separation purposes in water and wastewater treatment plants. One of the major problems encountered during their operation is fouling by solid particles, causing a gradual degradation in process efficiency. It was illustrated that it is possible to prevent fouling by gas sparging, which prevents solid particles from depositing on the surface of the membranes or from entering and blocking the membrane pores. The bubble stream may be used at the same time to remove the solid particles from the dispersion by flotation; this apparently results in the membranes being subjected to only a part of the initial feed concentration of solid particles. The combination of the two sub-processes in a hybrid cell with the membrane module submerged inside the flotation cell also achieves energy savings since no additional energy to that needed for flotation was necessary for countering the fouling of the membranes. The impact of operating conditions on the performance of the hybrid process treating phosphate solutions with iron ions for pure water production was also examined. The gas flow rate and the backflushing were major operating variables and affected the efficient operation of the hybrid cell. The increase of air velocity affected positively the hybrid cell operation, but up to a critical value, beyond which any additional increase resulted in a decrease of the cell performance. As far as the backflushing effect is concerned, the variation of frequency and duration did not significantly affect the hybrid cell performance, which may be due to the particular nature of iron flocs.