Μελέτη υδραυλικών και λειτουργικών χαρακτηριστικών συνδυασμένου συστήματος επίπλευσης/μικροδιήθησης

Abstract

Membrane filtration is a popular solid/liquid (S/L) separation process used in water and wastewater treatment, since it plays a key role in removing from water complex components such as dissolved and particulate matter contaminants. Membrane filtration processes are used as an alternative to other more traditional separation methods, such as simple filtration, centrifugation, sedimentation etc., since they offer the advantages of a highly selective operation, separation with few auxiliary materials, ambient temperature operation, usually no phase changes; the process is also economical in small units, it is modular in construction and may be easily integrated in existing production processes, requiring relatively low capital and running costs. Fouling is the major factor limiting membrane lifetime. Fouling is a generic term, which describes the gradual deterioration of membrane performance in terms of permeation flux and selectivity, mainly due to the accumulation of solids on its surface and/or inside its pores. Several techniques have been implemented to prevent the particles reaching the membrane surface, which include the use of abrasives, filtration aids, and coagulants. On the other hand, feed pre-treatment, ultrasonic fields, periodic backflushing, and gas sparging are used to remove particles that already have been deposited in the membrane pores or on its surface. Flotation is another S/L separation process, in which the lifting force of bubbles (usually of air) is used for the removal of suspended matter from dispersions. The process is widely applied to the removal of organics from water, the removal of metal ions, oils, powders, and residual reagents from wastewater etc., as well as for the beneficiation of mineral particles in minerals processing. A combination of membrane filtration and air sparging integrated into a single unit could reduce fouling by removing a large part of the solid particles: the rising gas bubble swarm acts as a surface scrubber, preventing the solid particles from being deposited on the membrane surface and also scrubbing the membrane surface and removing the deposited cake. This combined process could be further improved if a surface-active agent was added to the suspension, making the solid particles hydrophobic: most of the solid particles would then be removed by flotation at the top of the cell by the rising bubbles. The innovative hybrid flotation/microfiltration system consists of a membrane module submerged inside a flotation column. A suspension of fine-sized adsorbent particles was fed into the hybrid cell, where the solid particles were partially removed by flotation, while clean water was obtained from the membrane module. Some of the solid particles remaining in the dispersion were deposited on the surface of the membrane, forming a cake that gradually caused membrane fouling. The rising gas bubbles were used for the flotation process, while at the same time acting as membrane-surface scrubbers, effectively removing the deposited cake and countering the fouling problem. Process parameters investigated were: the nature, size, and concentration of solid particles (zeolite, hydrotalcite, Zn(OH)2 precipitate and iron colloidal particles), the type and concentration of selected ions (Zn2+, P-PO4 3-), the type and concentration of flotation reagents, the effect of gas flow rate and of solid-liquid dispersion flow rate, and the frequency and length of backflushing, which was required in order to reduce the fouling of membranes and to extend their useful working time, among others.