Reverse osmosis (RO) is currently used in various applications ranging from small, undersink drinking water units to large industrial systems. The technology is widely used and accepted as it removes both dissolved ionic and organic impurities. This article will focus on the reverse osmosis technology in industrial applications and will discuss factors that differentiate industrial RO from commercialgrade systems.
RO installations are sometimes unsuccessful due to the system design and application. The approach and recommendations in this article are based on 16 years of experience in system design and application, as well as design experience and operating observations of successful membrane systems. The goal of this paper is to present factors that will contribute to reliable operation for many years. Alternate designs are sometimes used.
A natural process Osmosis is the natural phenomenon that occurs as pure water flows across a semi-permeable membrane to the side that contains a higher concentration of dissolved solids. The higher the dissolved solids concentration, the more pure water (dilute) will flow to the concentrated side. The osmotic pressure that occurs results in a higher level of concentrate water than dilute water (Figure 1). An externally applied pressure, higher than the osmotic pressure, drives water in the opposite direction, producing a higher volume of pure water. This process is called reverse osmosis.
The most common RO membranes are spiral wound (Figure 2). Several layers of flat-sheet membranes, concentrate chambers (feed spacers) and dilute chambers (permeate carriers) are wound around a center permeate tube that collects the dilute water. The membrane element is sealed around the outer edges with a glue line. The design of spiral-wound membranes is such that feedwater enters on the feed side of the membrane at a high pressure. Reverse osmosis occurs and low TDS water is collected in the permeate tube, then exits the center of the membrane element. The feedwater becomes more and more concentrated as it flows across the membrane and exits as the concentrate reject at much higher TDS levels than the feed flow. Since membranes are reported to have pores as small as 0.005 microns, the majority of organics (dissolved and undissolved) and other suspended solids cannot pass through the membrane. They are therefore typically rejected with the concentrate.