Thesis (M. Eng.) - Asian Institute of Technology, 1995

A thesis submitted in partial fulfillment of the requirements for the Degree of Master of Engineering.

The goal of this research was to investigate the possibility of coupling Crossflow Microfiltration (CFMF) with Dissolved Air Flotation (DAF) for fiber separation. In the first stage of experiments, a Jar test was conducted to obtain optimum Alum and polyelectrolyte dosage required to produce floes. In the second stage, DAF batch experiments were conducted to investigate the effects of saturation pressure, recycle ratio and A/S ratio on the suspended solids removal. In the third stage CFMF runs were conducted to optimize the parameters which effect the flux in CFMF operations. During this stage the following parameters were examined. Crossflow velocity (2.0-6.0 m/s), feed concentration (100-2000 mg/L), transmembrane pressure (50-300 kPa), membrane pore size (0.2-5.0 micron). Once the optimum parameters were found they were tested for both flocculated and unflocculated feed. At the fourth stage, coupling DAF with CFMF was done inorder to observe the effects the introduction of air bubbles would have on permeate flux. In these runs saturation pressure of 400-800 kPa and recycle ratio of 8-30 % was adopted. From the stage one runs, it was found that an alum and polyelectrolyte dose of 140 and 0.5 mg/L was required at pH = 6.0 to fully destabilize fiber and to form floes . From stage two, it was found that with increasing recycle ratio there was an increased suspended solids removal. Here 97 % suspended solids removal rate was achieved at an A/S ratio of 0.075. Results from stage three show that permeate flux increased with increased crossflow velocity upto 4.0 mis and thereafter further increases in velocity decreased flux. Increases in transmembrane pressure increased flux , whereas there was an inverse relationship between feed concentration and flux. The addition of alum and polyelectrolyte decreases permeate flux by near! y 100 % . In stage four, with the introduction of air bubbles, flux increased beyond the flux obtained by stage three runs only beyond a bubble number concentration of 2.3*105 No./mL. for flocculated feed. However, for unflocculated feed this was not observed.