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Reverse Osmosis, also known as hyper filtration, was first
developed in the early 1960's with the successful synthesis
of semi-permeable membrane. This semi-permeable
membrane affectively rejects salt molecules but allows the
passage of water molecules, and is therefore truly a separation
process. Originally, the application for this technology was designed for seawater desalination as an alternative technology to the energy intensive, thermal-based separation. With the progress of the space race, NASA also invested heavily into membrane technology as the solution for providing drinkable water during space flight.
Today, aboard the space shuttle, Reverse Osmosis forms a critical component to recycle waste water and provide drinking water to the astronauts. Currently this technology is used for purification of many water sources - brackish well water, reuse of agriculturally contaminated ground sources, production of drinking water from seawater, and ultra-purification of water for microelectronics and pharmaceutical manufacturing.
In the early 1970s, the first commercial low pressure semi-permeable membrane was developed and was capable of producing 1 to 5 gallons per day of clean, safe drinking water for small households. Today, with technology advancements, the capacity of membrane systems produces enough water for entire communities. However, one issue that has remained constant is scaling. Scaling is the physical process that occurs when sparingly soluble salts come out of solution and form a solid 'scale' on the membrane. This occurs in membrane filtration due to the fact that as 'dirty' or 'salty' water enters the membrane, most of the water molecules are removed, leaving behind many salts in a reduced amount of water. As fresh water sources are reduced and the demand for higher recovery systems increase, the likelihood for scaling of the membrane also increases. Following a similar timeline (to that of membrane technology), pretreatment of water for separation by membranes enjoyed numerous advancements as well, notably the use of scale inhibitors.
Originally, pH adjustment was used to 'prevent scale; i.e., control the precipitation of sparingly soluble salts. The original membrane was cellulose acetate and required pH adjustment to create an acidic environment and introduction of an oxidant to prevent hydrolysis of the membrane. This pretreatment was fairly straight forward.
In the late 1970's when polyamide membrane (also know as thin film composite - TFC) was introduced into membrane separation applications, Sodium hexametaphosphate (SHMP) was introduced to control the potential scaling. Due to the limitations of SHMP (high dosage rates, degrades quickly, limited control of carbonate scale and degrades to P which is a bacterial food source), liquid antiscalants made primarily of polyacrylic acid ('borrowed' from cooling tower technology) were introduced and were found to be much more stable then 'sodium hex.' However, these chemistries also had limitations (limited control of carbonate scale and precipitation in the presence of metal hydroxides).
In the mid-to-late 80's, phosphate-based antiscalants were introduced to the application, and many formulations used today are derived from phosphate-based materials. These, too, carry many limitations (overdosing can exacerbate phosphate scaling, and have limited control over silica precipitation), along with increasing prohibitions by several United States' municipalities and many countries. Phosphates are the primary ingredient in many fertilizers and therefore can greatly increase the risk of algal blooms and bacterial proliferation of the discharge stream.
The new generation of antiscalants is unique in molecular structure. Phosphate-free and made in concentrated form, Dendriplex® technology is an evolutionary step in the science of RO operation. This technology eliminates the concern of fouling caused by overdosing and for most discharge regulations. Additionally, Dendriplex® technology chemistries such as SpectraGuard™ are stretching the limits of solubility and recovery as the membrane industry has yet to see. Welcome to the latest technology in membrane pretreatment chemicals.
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