REVERSE OSMOSIS AND DEMINERALISATION

WATER PLANT PROCESS

Presented by:A.Venu

M.PharmDepartment of pharmaceutics

Under the guidance ofDr. D. Karthikeyan

Srikrupa Institute of Pharmaceutical Sciences

(vill):velkatta,(man):Kondapak,

(Dist):medak.

In the pharmaceutical industry, water forms part of the product itself, production processes as well as cleaning. Altogether, this makes heavy demands on water quality. Utility water is insufficient because the water contains salts, organic matter, particles, and microorganisms that pollute or even destroy products and processes.

Process water for the pharmaceutical industry must be of high quality and at the same time meet the regulatory requirements of the pharmacopoeia governing the water treatment equipment.

Softened and demineralized water is used in many processes to ensure uniform water quality of the end product and to facilitate the final validation.

Osmosis: Osmosis is a special case of diffusion in which the molecules are water and the concentration gradient occurs across a semipermeable membrane. The semipermeable membrane allows the passage of water, but not ions (e.g., Na+, Ca2+, Cl-) or larger molecules (e.g., glucose, urea, bacteria). Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the 'concentrated' side of the membrane.

Osmosis is a special case of diffusion in which the molecules are water and the concentration gradient occurs across a semipermeable membrane. The semipermeable membrane allows the passage of water, but not ions (e.g., Na+, Ca2+, Cl-) or larger molecules (e.g., glucose, urea, bacteria).

Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the 'concentrated' side of the membrane.

Osmosis:

Reverse Osmosis:Reverse osmosis occurs when the water is moved

across the membrane against the concentration gradient, from higher concentration to lower concentration.

To illustrate, imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous solution on the other side.

If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution.

In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.

Reverse Osmosis

Basic components common of a Reverse Osmosis System:

Cold Water Line Valve:   Valve that fits onto the cold water supply line. The valve has a tube that attaches to the inlet side of the RO pre filter. This is the water source for the RO system.

Pre-Filter (s): Water from the cold water supply line enters the Reverse Osmosis Pre Filter first. There may be more than one pre-filter used in a Reverse Osmosis system. The most commonly used pre-filters are sediment filters. These are used to remove sand silt, dirt and other sediment.

Additionally, carbon filters may be used to remove chlorine, which can have a negative effect on TFC (thin film composite) & TFM (thin film material) membranes. Carbon pre filters are not used if the RO system contains a CTA (cellulosetriacetate)membrane.

Reverse Osmosis Membrane:   The Reverse Osmosis Membrane is the heart of the system. The most commonly used is a spiral wound of which there are two options: the CTA (cellulose tri-acetate), which is chlorine tolerant, and the TFC/TFM (thin film composite/material), which is not chlorine tolerant.

Post filter (s):   After the water leaves the RO storage tank, but before going to the RO faucet, the product water goes through the post filter (s). The post filter (s) is generally carbon (either in granular or carbon block form). Any remaining tastes and odors are removed from the product water by post filtration.

Automatic Shut Off Valve (SOV):  To conserve water, the RO system has an automatic shutoff valve. When the storage tank is full (this may vary based upon the incoming water pressure) this valve stops any further water from entering the membrane, thereby stopping water production. By shutting off the flow this valve also stops water from flowing to the drain. Once water is drawn from the RO drinking water faucet, the pressure in the tank drops and the shut off valves opens, allowing water to flow to the membrane and waste-water (water containing contaminants) to flow down the drain. Check Valve:   A check valve is located in the outlet end of the RO membrane housing. The check valve prevents the backward flow or product water from the RO storage tank. A backward flow could rupture the RO membrane.

Flow Restrictor:   Water flow through the RO membrane is regulated by a flow control. There are many different styles of flow controls. This device maintains the flow rate required to obtain the highest quality drinking water (based on the gallon capacity of the membrane). It also helps maintain pressure on the inlet side of the membrane. Without the flow control very little drinking water would be produced because all the incoming tap water would take the path of least resistance and simply flow down the drain line. The flow control is located in the RO drain line tubing.

Storage Tank:   The standard RO storage tank holds up to 2.5 gallons of water. A bladder inside the tank keeps water pressurized in the tank when it is full.

Faucet:   The RO unit uses its own faucet in areas where required by plumbing codes an air-gap faucet is generally used.

Drain line:   This line runs from the outlet end of the Reverse Osmosis membrane housing to the drain. This line is used to dispose of the impurities and contaminants found in the incoming water source (tap water). The flow control is also installed in this line.

Advantages:Reverse osmosis systems have plenty of advantages.

They are friendly to the environment, as they do not produce or use any harmful chemicals during the process. These systems also require a minimal amount of power. Reverse osmosis systems work well in home filtration systems because they are typically small in size.

Taste of the purified water is another distinct advantage. Reverse osmosis removes dissolved minerals and other contaminants that cause water to smell unpleasant, taste poorly and take on unusual colors.

Removal of dissolved minerals, metals and other particles benefits plumbing systems. There is nothing in the water to corrode pipes or collect as sediment

Disadvantages:Reverse osmosis treatments require an enormous

amount of water. Such systems typically return as little as 5 to 15 percent of the water pushed through the system, which means it also takes a long time to properly treat the water. What's left then exits the system as waste water.

This amount of wastewater can burden home septic systems. Water entering the reverse osmosis system should also be free of bacteria. While reverse osmosis systems do remove nearly all microorganisms, the risk of contamination through tiny leaks or deteriorating parts prevents reverse osmosis systems from being used to remove bacteria.

All natural waters contain, in various concentrations, dissolved salts which dissociate in water to form charged ions. Positively charged ions are called cations; negatively charged ions are called anions.

Ionic impurities can seriously affect the reliability and operating efficiency of a boiler or process system. Overheating caused by the buildup of scale or deposits formed by these impurities can lead to catastrophic tube failures, costly production losses, and unscheduled downtime.

Hardness ions, such as calcium and magnesium, must be removed from the water supply before it can be used as boiler feedwater. For high-pressure boiler feedwater systems and many process systems, nearly complete removal of all ions, including carbon dioxide and silica, is required. Ion exchange systems are used for efficient removal of dissolved ions from water.

Demineralisation

As there are cations and anions in the water, we must use two different types of resins: a cation exchanger and an anion exchanger. This combined arrangement produces pure water, as presented in the general introduction. Demineralisation is also calleddeionisation. The cation resin is used in the hydrogen form (H+) and the anion resin in the hydroxyl form (OH–), so that the cation resin must be regenerated with an acid and the anion resin with an alkali.

A degasifier is used to remove the carbon dioxide created after cation exchange when the water contains a significant concentration of bicarbonate.

The cation resin is usually located before the anion resin: otherwise if the water contains any hardness, it would precipitate in the alkaline environment created by the OH— form anion resin as Ca(OH)2 or CaCO3, which have low solubility.

Layout SAC – (DEG) – SBALet us first consider a simple deminineralisation system comprising a

strong acid cation exchange resin in the H+ form, a degasifier (optional) and a strong base anion exchange resin in the OH– form. The first step is decationisation as shown above:

RSAC-H + Na+   RSAC-Na + H+

With calcium insead of sodium (also valid for magnesium and other divalent cations):

2 RSAC-H + Ca++   (RSAC)2-Ca + 2 H+

In the second step, all anions are removed with the strong base resin:RSBA-OH + Cl–   RSBA-Cl + OH–

The weak acids created after cation exchange, which are carbonic acid and silicic acid (H2CO3 and H2SiO3) are removed in the same way:

RSBA-OH + HCO3–   RSBA-HCO3

– + OH–

And finally, the H+ ions created in the first step react with the OH– ions of the second step to produce new molecules of water. This reaction is irreversible:

H+ + OH–   H2O

Raw water Decationised water Decat + degassed water

Decat + degassed water Demineralised water

Demineralised water is completely free of ions, except a few residual traces of sodium and silica, because the SAC and SBA resins have their lowest selectivity for these.

RegenerationThe SAC resin is regenerated with a strong acid, HCl or H2SO4: R-Na + H+   R-H + Na+

And the SBA resin is regenerated with a strong alkali, NaOH in 99 % of the cases:

RSBA-Cl + OH–   RSBA-OH + Cl–

Layout WAC/SAC – DEG – WBA/SBABecause weakly acidic and weakly basic resins offer a high

operating capacity and are very easy to regenerate, they are used in combination with strongly acidic and strongly basic resins in large plants. The first step with the WAC resin is dealkalisation (removal of bicarbonate hardness), and the second step with the SAC removes all the remaining cations. A WAC resin is used when both hardness and alkalinity are present in large relative concentrations in the feed water.

WBA resins remove only the strong acids after cation exchange. They are not capable of removing the weak acids such as SiO2 and CO2. In the regenerated, free base form, they are not dissociated, so no free OH– ions are available for neutral anion exchange. On the other hand, their basicity is enough to adsorb the strong acids created after cation exchange:

RWBA + H+Cl–   RWBA.HClIn the last step, a SBA resin is thus required to

remove the weak acids, as shown in the preceding section:

RSBA-OH + HCO3–   RSBA-HCO3

– + OH–

Cation exchange beginning with dealkalisation followed by the removal of all remaining cations:

Raw water Decarbonated waterDecationised water

WAC (H)

SAC (H)

Anion exchange begins with the removal of strong acids after degasification:

Decat + degassed water

Partially demineralised Demineralised water

A full demineralisation line is shown below, with a cation exchange column (WAC/SAC), a degasifier, an anion exchange column (WBA/SBA), and a polishing mixed bed unit. The use of a weakly acidic resin and the degasifier column are conditioned by the presence of hardness and alkalinity in the feed water, as explained.

demineralisation

RegenerationRegeneration is done in thoroughfare, which means that

the regenerant first goes through the strong resin, which requires an excess of regenerant, and the regenerant not consumed by the strong resin is usually sufficient to regenerate the weak resin without additional dosage.

The cation resins are regenerated with a strong acid, preferably HCl, because H2SO4 can precipitate calcium. The anion resins are regenerated with caustic soda.

Regeneration of the demineralisation

The quality obtained is the same as in the simple SAC-SBA layout, but because the weak resins are practicallly regenerated "free of charge", the regenerant consumption is considerably lower. Additionally, the weak resins have a higher operating capacity than the strong resins, so the total volume of ion exchange resins is reduced.

Advantages and LimitationsDemineralizers can produce high-purity water for nearly every

use. Demineralized water is widely used for high pressure boiler feedwater and for many process waters. The quality of water produced is comparable to distilled water, usually at a fraction of the cost. Demineralizers come in a wide variety of sizes. Systems range from laboratory columns that produce only a few gallons per hour to systems that produce thousands of gallons per minute.

Like other ion exchange systems, demineralizers require filtered water in order to function efficiently. Resin foulants and degrading agents, such as iron and chlorine, should be avoided or removed prior to demineralization. Anion resins are very susceptible to fouling and attack from the organic materials present in many surface water supplies. Some forms of silica, known as colloidal, or non-reactive, are not removed by a demineralizer. Hot, alkaline boiler water dissolves the colloidal material, forming simple silicates that are similar to those that enter the boiler in a soluble form. As such, they can form deposits on tube surfaces and volatilize into the steam.

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