Silica and silicate precipitation as limiting factors in high-recovery reverse osmosis operations

This paper investigated silica and silicate precipitation as limiting factors in high-recovery membrane operations. For this purpose a cation exchange pretreatment is proposed to reduce Ca2+, Ba2+, Mg2+ levels to prevent scaling of salts containing these ions during subsequent NF and RO filtration, in which RO is fed with NF concentrate. In a pilot plant experiments were carried out at total (NF+RO) water recovery of 91, 94, 96 and 98% with locally available tap water which contains 20 mg/L of silica as feed water. Autopsy studies were performed with the RO membranes after each experiment in which the fouling layer was studied using SEM–EDX to determine the structure and the composition of the fouling deposits. A thin cake layer was observed which covered approximately half of the membrane surface after operating for 20 days at 91 and 94% recovery. At 96 and 98% recovery the fouling layer was thicker and completely covered the membrane surface. EDX analysis indicated that the fouling layer was mainly composed of Si, Al, Fe and O, most likely due to the presence of iron oxides, iron hydroxides, silica and aluminosilicates. To be able to work at these high recoveries for an extended period, further measures need to be taken to prevent silica and aluminosilicate scaling.