|M.Sc Student||Borisover Elina|
|Subject||Colloid Deposition and Release from Nanofiltration|
|Department||Department of Civil and Environmental Engineering||Supervisor||Assistant Professor Ramon Guy|
|Full Thesis text|
Biofouling in membrane separation processes is a complex problem, and its prevention and management is a long standing theme of research. Many studies use brown algae alginate as a model foulant since it resembles the polysaccharides found in the biofilm matrix. These polymers are an important component of the biofilm structure that contributes to its unique, microorganism growth-sustaining properties. Various studies have reported that the presence of calcium ions creates calcium bridging with alginate chains, which contributes to alginate-gel rigidity and ability to withstand deformation. When a cross-linked alginate gel layer is exposed to NaCl cleaning solution, ion-exchange between sodium and calcium ions takes place, resulting in the breakup of the gel network.
In this study, a direct observation method was used to monitor alginate fouling and cleaning with the aid of fluorescent marker beads. These markers are trapped in the alginate layer and their motion is hindered due the viscous medium. Observation of these marking particles, trapped at various distances from the membrane, enables to follow variations and kinetics in this medium.
The first experimental section describes the fouling and cleaning of the markers, fluorescent 1 µm polystyrene particles. Their deposition rate and colloid irreversibility degree, defined as the ratio of particles deposited on the membrane before and after deionized water rinse, was examined. Their dependence on permeate flux, ionic strength and crossflow velocity were tested. Higher deposition rate and colloidal fixation were observed for increased permeate flux and ionic strength and decreased cross flow velocity. This is consistent with the theory as higher permeate flux induces increased drag force on the particle and higher ionic strength decreases the repulsive electric double layer. Lower crossflow velocity decreases the crossflow lift.
The second experimental section describes alginate fouling and removal experiments. Fouling experiment have shown that indeed the calcium ions form a more rigid alginate layer, which results in faster permeate flux decline and hydraulic resistance increase. Removal experiments examined which factor is more dominant for triggering colloid detachment - permeate or trans-membrane pressure shut-off. Additionally, these experiments also examined the impact that calcium-sodium ion exchange have on alginate removal from the membrane surface. Results suggest that permeate flux had little effect on triggering alginate detachment. Successful cleaning was achieved by shutting off trans-membrane pressure. An existence of another acting force, which is caused by the pressure, was proposed. This force changes the net force balance acting on the alginate and even though the permeate drag force does not exist (since there is no permeation) this force prevents the alginate from detaching. Calcium ions presence formed a rigid layer, which was easily cleaned from the surface after addition of sodium ions to the solution.
Future work should include a detailed examination of alginate cleaning kinetics with the factors discussed above. Other factors can be examined, e.g. the impact of adding a chelating agent as well as pH variation, on alginate cleaning kinetics.