Phosphate removal from water by red mud using crossflow microfiltration

Red mud (which is a waste material formed during the production of alumina and primarily contains ferric and aluminium oxides) was used as an adsorbent for the removal of phosphate ions from water using crossflow microfiltration. It is shown that phosphate ions act as a coagulant for red mud particles, forming a compressible cake with a compressibility index of unity. As shown also by dead-end filtration experiments, the cake resistance decreases with increasing phosphate concentration. The phosphate rejection is a strong function of feed dispersion pH, phosphate and red mud concentration ratio as well as the concentration of co-ions such as sulphate ions used to adjust the dispersion pH. Under certain conditions, especially when pH=5.2, steady state permeate flux and phosphate rejection reach a maximum with 100% rejection achievable. The permeate also contain metal ion impurities originating from red mud and their concentrations increase with increasing red mud concentration and decreasing pH. The effects of other process variables, crossflow velocity and membrane pore size on phosphate rejection and permeate flux are also studied.