Preparation of apyrogenic water
In the manufacture of sterile products, the water used is water for injection (WFI) which is free from pyrogens and particulate matter. In industry, water is obtained from various sources like lakes and streams. Such water contains particulate matter; it may be hard water and may contain pyrogens. It is treated to convert it into WFI. The general scheme of preparation of WFI is given as :
(Cation exchange resins)
(sodium meta bisulfate)
Reverse osmosis systems
Multiple effect still or
Sand filters remove particulate material of upto certain size.
Water softeners are used to soften the water, i.e. to remove excessive ions which render the water hard; they remove ions like calcium and magnesium.
Chlorination of water removes any viable cells which are present in the water.
De-chlorination removes the excess of dissolved chlorine by chemical reaction.
Reverse Osmosis (RO):
As the name suggests, reverse osmosis implies reversing of the osmotic flow, i.e. the flow a liquid across a semi permeable membrane down the osmotic pressure gradient. In this process, a high pressure of the order 200-400 psig (pounds per square inch gauge pressure)  is applied across the membrane to drive the liquid in the opposite direction of the actual osmotic flow.
The membranes used for this process are made up of cellulose esters or polyamides . These are very resilient to high pressure and have very restricted pore sizes of the order upto 0.0006 microns . This enables the membranes to retain even the smallest of the bacteria, viruses and other viable cells and their metabolic products namely pyrogens. They also retain organic solutes like sugars. However, they cannot retain ions like sodium, chloride and bicarbonate. 
All reverse osmosis systems are thoroughly validated for consistent performance. Popular RO systems are provided by AMSCO and Millipore .
Distillation is the process of converting source water called distilland to its vapour state and then condensing it back again to its liquid state. The water obtained after re condensation is called distillate. The distillate so obtained is free from volatile as well as non-volatile impurities. The non-volatile components include organisms and pyrogens. Thus, water obtained by distillation is free from pyrogens. There are two types of distillation processes in the industry. They are:
- Compression distillation:
In this method, compressed vapour at high temperature (212o F)  is used to heat up the distilland. The compressed vapour loses its temperature and heats up the distilland and in the process condenses to liquid which retains the non-volatile impurities. The distilland which is now converted to vapour rises up the still. Special design of the head space of the still enables the removal of volatile impurities. Thus, the vapour which condenses into the collecting chamber is free from both the volatile and non-volatile impurities.
Compression distillation units are capable of capacities upto 50-2800 gallons/hour. Popular stills are manufactured by Aqua-chem, Barnstead and Meco. 
- Multiple effect still:
As the name suggests, there is more than one distillation still. A series of distillation stills are arranged so that high pressure steam enters the first still, it heats up the distilland in the next still and in the process loses its temperature and condenses. Similarly, the water in the second still which is converted to vapour now heats up the water in the third still and vaporises it. Thus, the process continues until the last still, where the pure distillate is obtained.
Multiple effect stills are capable of capacities upto 50-7000 gallons/hour. 
Freshly prepared WFI might not be used immediately in the production plant. It may be needed for future use. The prepared WFI can be stored to retain its properties. The USP dictates certain guidelines for the storage of WFI to ensure that. The WFI that is produced is first passed through membrane filters fixed to the storage vessels at high pressure. The storage vessels are made up of 316L grade of stainless steel electroplated on the inner surface with tin. It is maintained at a constant 80o C. The WFI thus stored can be used 48 hours after storage. 
"This book page does not contain plagiarized material"
- Remington: The science and practice of pharmacy; 21st edition, Volume I; pages: 807-809. [accessed on 14th August, 2010]
- https://www.waterfiltersonline.com/reverse-osmosis-explaination.asp [accessed on 14th August, 2010]