Filtered water obtained from the Dual Media Filter is transferred to Ultra-Filtration (UF) for further treatment. Ultra-Filtration is a pressure-driven process that uses semi-permeable synthetic membranes to separate certain chemicals and materials from water.
Membrane separation technology is based on molecular size. The semi-permeable membrane in an ultra-filtration system has pore sizes in the range of 0.01 to 0.025 microns. The pores at the membrane surface are so small that they allow only water and small dissolved chemicals to pass through while stopping larger molecules and particles. Due to the very small pore size, water flows through the membrane only by applying pressure. Generally, a minimum pressure of approximately 1 kg/cm² (15 psi) is required.
Pressure is applied to one side of the membrane so that water and low molecular weight compounds in the feed stream flow through the pores as permeate, while the larger molecules and suspended solids flow across the membrane and become part of the concentrate.
In an ultra-filtration system, water flows parallel to the membrane surface, unlike the perpendicular flow of ordinary filtration. This cross-flow motion allows high filtration rates to be maintained continuously, whereas the constant buildup of solids along the filter surface can cause blockage in a perpendicular filtration system.
A phenomenon in ultra-filtration known as concentration polarization occurs when chemical contaminants build up at the membrane surface as water passes through to the other side. This condition is not permanent and can be removed by flushing the membrane with either more dilute feed, clean water, or permeate.
