8/14/2023 0 Comments Reverse ag ratioThe shut down procedure for non-working hours should assure that minimum flow and operating pressures are continued with a timed internal flush cycle. A membrane module lasts two to three years on the average. The spiral wound construction is less susceptible to fouling than that of the hollow fiber unit. A flush cycle is often used to reduce build up. Two RO modules in series should be considered for parenteral solutions principally for the reduction of monovalent ions and bacterial contaminants.įIGURE 5 FLOW DIAGRAM OF A REVERSE OSMOSIS SYSTEMĪ major problem in operating RO systems is concentration polarization or fouling which is the gradual build up of rejected solute on the feed side, immediately adjacent to the membrane. Since water conditions may vary from time to time, adequate pretreatment must be provided so that the dissolved solids and the bacterial level of the feed water, after prefiltration, can be controlled within the designed limits. In practice, the feed water may go through any one or a combination of several of the following pretreatments: sand bed, chlorinator and retention tank, anthracite filter, activated charcoal filter, degasifier, microfilter, neutralizer, and deionizer, depending on the condition of the feed water and the desired quality of the product water. Most RO units with a cellulose acetate membrane are designed to operate between 55 F - 86 F. The product water is then pumped to a storage tank and the concentrate is drained (6). Feed water is passed through a prefilter after pH adjustment, and is pumped to the membrane modules at the designed pressure. Schematics of these two types of membrane modules (permeators) are shown below (Figures 3 and 4).Ī typical reverse osmosis system is shown in the flow diagram (Figure 5) (image size 11KB). Either of these modules is assembled into a pressure housing. spiral wound -sheets of membrane sandwiched with mesh spacers are connected and wound around a permeate tube and 2. Two types of RO construction are commonly used: 1. The skin is the active barrier and primarily allows water to pass through. The membrane consists of a skin about 0.25 microns and a support layer about 100 microns. Many other kinds of membrane made of a single polymer or a copolymer are also available for specific purposes. The majority of the commercially manufactured RO membranes are made from cellulose acetate, polysulfonate, and polyamide. The nominal rejection ratio of common ionic salts is 85 - 98%. Monovalent ions such as chloride ions will not be rejected as efficiently as, for example, divalent sulfate ions. Ions are repelled by dielectric interactions ions with higher charges are repelled to a greater distance from the membrane surface. Salt ions, on the other hand, are rejected by a mechanism related to the valence of the ion. Most organic substances with a molecular weight over 100 are sieved out, i.e., oils, pyrogens and particulates including bacteria and viruses (13). The water molecules that enter the membrane by hydrogen bonding can be pushed through under pressure. Water molecules can form hydrogen bonds in the RO membrane and fit into the membrane matrix. If enough counter pressure is applied to the concentrated solution to overcome the osmotic pressure, the flow of water will be reversed (Figure 2). When two aqueous solutions of different concentrations are separated by a semi-permeable membrane, water passes through the membrane in the direction of the more concentrated solution as a result of osmotic pressure (Figure 1). Reverse osmosis is a process which uses a membrane under pressure to separate relatively pure water (or other solvent) from a less pure solution. This ITG will focus on the chemical and microbiological quality of water produced by reverse osmosis. In addition to these applications, RO is capable of producing water of sufficient purity to be used as Water For Injection (WFI) and for the preparation of parenteral solutions (9,10,11,12). In recent years, RO has been used increasingly in making processed water for dialysis in hospitals and for certain cosmetics and drugs by pharmaceutical manufacturers (7,8). Because RO operates at a comparatively low temperature and is relatively energy efficient, it is employed in various applications, e.g., desalination, treatment of waste water, reclamation of minerals, concentration of whey and other food products, and purification of water (5,6). Reverse osmosis (RO) has been known for more than a century, but it did not become a commercial process until the early sixties when a special membrane was developed (1,2,3,4).
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