There's a lot of attention being given to water availability and purity, with good reason. Obtaining enough water is a struggle in some areas. In others, water is tainted with contaminants. It is therefore predictable that regulations and economics will force careful water treatment and stress water re-use. Fortunately, there are water treatment methods that can help you meet the water quality challenges, and related water supply challenges, you face. Among these is crossflow filtration, a technology that has emerged as one of the most effective.

      Membrane filtration is the separation of the components of a pressurized fluid, effected by polymeric or inorganic membranes (generally man-made). The openings in the membrane material (pores) are so small that a significant fluid pressure is required to drive the liquid through them; the pressure required varies inversely with the size of the pores (basically classical orifice theory). There are now four commonly accepted categories or "classes" of membrane, defined based on the size of the material they will remove from the carrier liquid. Moving from the smallest to largest pore size, these are Reverse Osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF), and Microfiltration (MF).

      Reverse Osmosis (RO) membranes will reject dissolved and suspended materials including monovalent salts. Since essentially all dissolved and suspended material is rejected by the membrane, the RO permeate is pure water.

• Retains salts and organics
• Passes essentially only water
• Passes molecules in the range of 1 Angstroms (0.0001 micron)

      For example, water can be softened with a nanofiltration (NF) membrane that rejects 85% of salt (sodium chloride) but 99% of the hardness ions (calcium and magnesium). The highest salt rejection rates (99.7% or higher), which can be provided by RO membranes, are required for seawater desalination.

      RO membrane are designed for cross-flow separation, where a feed stream is introduced into the membrane element under pressure and passed over the membrane surface in a controlled flow path. A portion of the feed passes through the membrane and is called permeate. The rejected materials are flushed away in a stream called the concentrate. Cross-flow membrane filtration uses a high cross flow rate to enhance permeate passage and reduce membrane fouling.

      RO is a moderate to high pressure (80-1200 psig) driven process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane. This process is carried out by flowing a process solution along a membrane surface under pressure. Retained solutes (such as particulate matter and dissolved salts) leave with the flowing process stream and do not accumulate on the membrane surface. The amount of salt and other impurities is often referred to as TDS, or total dissolved solids. The higher the TDS, the more feed pressure required.

      Nanofiltration is a low to moderately high pressure (typically 50 - 450 psig) process in which monovalent ions will pass freely through the membrane but highly charged, multivalent salts and low molecular weight organics will be rejected to a much greater degree. Typical NF applications include water softening, desalination of dyestuffs, acid and caustic recovery and color removal.

• Retains divalent salts and organics
• Passes monovalent salts, water, acid and alkaline compounds
• Passes molecules in the range of 10 Angstroms (0.001 micron), Pore sizes ranging between UF and RO

      UF is a low pressure (5 - 150 psig) process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane.

• Retains oils, particulate matter, bacteria and suspended solids large macromolecules and proteins
• Passes most surfactants, water, acid and alkaline compounds
• Pore sizes ranging from 0.005 – 0.1 micron
• Permeate is clear (non-turbid) solution void of suspended solids

      MF is a low pressure (10-100 psig) process for separating larger size solutes from aqueous solutions by means of a semi-permeable membrane. This process is carried out by having a process solution flow along a membrane surface under pressure. Retained solutes (such as particulate matter) leave with the flowing process stream and do not accumulate on the membrane surface.

• Retains large suspended solids
• Passes some suspended solids and all dissolved material
• Pore ranges from 0.1 micron to 3 micron

      Membrane separation technology removes substances ranging in size from ionic to molecular. These substances are so small they typically are measured in Angstroms (1 Angstrom = one 10 billionth of a meter) or molecular weight (MW). Membranes have been developed with mass transfer properties and pore sizes such that ionic, molecular and organic substances measuring between 1 and 1000 Angstroms (MW between 100 and 500,000) are removed or rejected.

      A key difference between each membrane type is in the size of the pores. RO membrane pores are the smallest, measuring between 1 to 15 Angstroms.

      * 0 percent rejection is valid for a 30,000 parts per million (ppm) solution in mixtures with other icons.  The rejection for a pure 30,000 ppm solution of the ion in question is in the 5 percent to 15 percent range.  The higher rejection figure is valid for dilute solutions and the actual rejection may vary from 15 percent and up depending on the composition of the feed and the membrane characteristics. A standard RO membrane will generally reject 99 percent or more of dissolved salts.

      While each of the four membrane types have similarities, they each perform very different functions in varying applications. In general, RO and NF membranes are capable of separating substances as small as ions from feed streams while UF and microfiltration (MF) membranes typically separate larger molecules. All four membrane types allow water to pass.

      For example, RO membranes typically reject most of the ionic and organic species from the feed stream, allowing only water to pass. NF membranes are usually used to reject high percentages of multivalent ions and divalent cations while allowing monovalent ions to pass.

      UF and MF membranes reject molecules on the basis of size. UF membranes retain particles larger than about 15 to 200 Angstroms and MF membranes retain particles from about 200 to 1000 Angstroms. UF and MF membranes are typically rated in terms of pore size, or porosity, while RO and NF membranes are rated by terms of percent salt rejection and flow.

• Understanding Crossflow Filtration
• Membranes, the Finest Filtration
• Drinking water Via Reverse Osmosis (R.O)

       Natural Sunlight is a perfect sterilizer and certain wavelengths of the Ultra-violett light spectrum actually destroy or disable most micro-organisms, which can be found in water. Nowadays most bottled water companies use this additionally man-made duplication of natural sunrays to ensure proper sterilization of their products. It is a most natural way and does not leave any residual effect in the water, it just acts as a natural sterilizer.

      Ozone, which is a natural occuring gas, has a high desinfection rate, is instant and dissipates very fast, once it is released. You can smell ozone, when a flash of lightening has struck near you or you stand next to a waterfall. This ozone is produced by means of a combination of electrical arcing in conjunction with ultraviolet light, where the ozone is formed. Ozone is not a stable gas and will be absorbed by the air around it. For sterilization purposes in the bottled water industry, ozone is quite commonly used to ensure safe drinking water. Water bottlers usually choose between UV or Ozone for sterilization of their products.

       Another method of ensuring a basic good and reasonably safe production of drinking water is the use of various filters. Fine sediment filters at about 5 Micron will take out most of the loosely floating dirt of the water and some bacteria larger then 5 micron, additional carbon filtration will remove most chemicals, which usually are added to your municipal water supply, or chemicals from farm run-offs or other points of pollution. Water bottlers like this simple method because it is cheap and only requires the change of new filters, when these are exhausted. But as a lot of micro-organisms are much smaller then 5 micron, the bottlers usually add harmful chemicals again, like chlorine, to ensure a proper desinfection standard and shelf-life for their bottled water. This is of cause defeating the whole objective of producing clean, pure and healthy water. Water with added chlorine should be avoided,

       At high temperatures, water evaporates and forms steam which rises into the air. This steam are miniature waterdroplets (like rainclouds) and as they are so light, they can only carry their own weight and leave any dirt, bacteria or chemicals behind as residue. The steam is then cooled down again and pure water is formed in liquid form. Destilled water is usually about 99.5% pure and just about comparable to rain-water without the industrial pollution effects. As this water is boiled heavily, some people claim that it is "overcooked" like you may do with vegetables and that it looses its life force. Also people claim that it tastes kind of flat and shallow. Destilled water though is one of the safest way to produce high quality and guaranteed safe drinking water from nearly any water supply. Some water bottlers are using this method. The cost to produce destilled water is quite high though, as a lot of energy (electricity mostly) is needed to bring the water to a boil (even under the new vacuum process) and then a cooling system to bring it back to its liquid stage. The average cost is at about 25 US cents to produce one gallon (4 litres)