Reverse osmosis membrane life

Prediction Method for the Life of Reverse-Osmosis Membranes... 

The rapid expansion of reverse osmosis technology during the past two decades has resulted in the development of a variety of new membranes. Unique polymer systems and fabrication methods have led to the production of membranes with significantly improved performance and reliability. In spite of these developments little is known about chemical sensitivity or life expectancy of reverse osmosis membranes used in desalting applications. Manufacturers are consequently reluctant to guarantee their products for long runs especially in unique chemical environments. 

The chemical sensitivity or life expectancy of reverse osmosis membranes is very important for manufacturing application. Thus chlorine is the most well known reagent for water disinfection. Glaster et al. 61 inspected the influence of halogens on the performance and durability of reverse osmosis membranes. Cellulose acetate was unresponsive to halogen agents but polyamide-type membranes deteriorated rapidly when exposed to halogens. 

Where the silica concentrations in the raw water are high, reverse osmosis has been most effective. Even in trace quantities, humic and fulvic acids have been responsible for impairing the life of anionic resins and affecting performance of ion exchange columns. These organics are readily removed by reverse osmosis membranes. 

Although these composite fibers were developed for reverse osmosis their acceptance in the desalination industry has been limited due to insufficient selectivity and oxidative stabiUty. The concept, however, is extremely viable composite membrane fiat films made from interfacial polymerisation (20) have gained wide industry approval. HoUow fibers using this technique to give equivalent properties and life, yet to be developed, should be market tested during the 1990s. 

The success of a reverse osmosis process hinges direcdy on the pretreatment of the feed stream. If typical process streams, without pretreatment to remove partially some of the constituents Hsted, were contacted with membranes, membrane life and performance would be unacceptable. There is no single pretreatment for all types of foulants. Pretreatment methods range from pH control, adsorption (qv), to filtration (qv), depending on the chemistry of the particular foulant. Some of the pretreatment methods for each type of foulant are as foUow (43—45) ... 

Tangential crossflow filtration Process where the feed stream sweeps the membrane surface and the particulate debris is expelled, thus extending filter life. The filtrate flows through the membrane. Most commonly used in the separation of high-and-low-molecular weight matter such as in ultrapure reverse osmosis, ultrafiltration, and submicron microfiltration processes. 

Reverse osmosis plant are always subject to an insidious and gradual loss of permeate volume output or quality deterioration due to membrane fouling. The rate of decline is strongly influenced by the input RW quality. Therefore, any and all features, such as those above, that can be employed to delay the onset and degree of fouling and extend membrane life are to be recommended. 

Membrane processes such as ultrahltrahon or reverse osmosis have been proposed as oil removal processes. Laboratory tests have indicated favorable oil removal, although relatively low flux rates, membrane fouling, and membrane life problems have presented concerns for the practical applicahon of membrane processes to oil removal. 

These facts shift cost estimation to much safer side, resulting into over-investment, which may reduce the advantage of reverse osmosis process over other processes. For the fiirther prosper of reverse osmosis, we definetely need more accurate and easier method to predict membrane life which may be determined by decrease in either flux through it or separation of solute. 

To stop the osmosis occurring, the pressure P, in Figure 8.3, can be applied to the left-hand side. This pressure will be equal to the osmotic pressure exerted by the solution in the opposite direction. If the external applied pressure, P, is greater than the osmotic pressure then reverse osmosis occurs and molecules can be forced to pass from the stronger to the weaker solution. In this process, the semi-permeable membrane acts as a molecular filter to remove the solute particles. In some areas of the world this process is used for desalination of sea water, i.e. getting rid of salts from water. It is also used in emergency life raft survival kits to enable drinking water to be made from sea water. 

The traditional membrane separation processes (reverse osmosis, micro-, ultra- and nanofiltration, electrodialysis, perva-poration, etc.), already largely used in many different applications, are today combined with new membrane systems such as CMRs and membrane contactors. Membranes are applied not only in traditional separation processes such as seawater desalination but also in medicine, bioengineering, microelectronics, the life in the space, etc. 

A reverse osmosis unit has been proposed for the recovery of organic acids [4]. The plant under consideration was assumed to have a cellulose feed rate of 44,000 kg (dry) of municipal solid waste per hour. The flow to the unit consisted of 3 % short-chain organic acids (usually referred to as VFA for volatile fatty acids). It was also estimated that the unit would concentrate 36,500 kg of VFA to a 25 % solution. The total estimated capital cost for the reverse osmosis unit was 26.5 million dollars, assuming a membrane life time of 2 years. This estimated price was 0,378 per kg of concentrate leaving the system. 

However, with more stringent guidelines and since Whitestone was discharging Its waste after treatment directly to a stream, it was decided to install an RO system on the final aeration pond. A pilot test of approximately 1000 hours showed the usefulness of reverse osmosis and allowed Osmonics to specify the membrane elements and system layout with sufficient certainty to guarantee a reasonable life and operating characteristics to Whitestone. 

Reverse osmosis uses membranes that allow water to pass through but not soluble salts. Chemical engineers are working on improved membrane materials that will allow higher pressures to improve efficiency and membrane life. 

In some cases, particularly when high volumes of water are required, the ion exchange resin columns are preceded by a water softener or reverse osmosis (RO) system, which increases the life of the exchange resins. Reverse osmosis uses a semipermeable membrane (pore size of 10 to 10 microns) which rejects salts, dissolved solids (90-98%), and organics (99%), but does require 130psi (or higher) feedwater and about 60% of the water is flushed away and does not enter the purification train. 

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