Water Treatment Options
Pre-treatment &
Filtration
Filtration is one of the most widely known concepts in water treatment but is unfortunately one of the most misunderstood. The word filtration implies that water is passed through smaller and smaller openings until all of the unwanted particles are caught in the filter and only water is able to get through. This is not quite how most filters work. This concept is mostly correct for filters such as some cartridge filters, reverse osmosis filters and ultrafiltration filters but for the most part filters usually work based on a different concept. In fact filters such as carbon filters leave considerably larger openings in between the media then the particles it used to remove. How then do these filters work? It all goes back to chemistry and what is being filtered. Iron filters rely on the iron first being chemically changed to a precipitate form than being passed through media that is specifically designed to attract this form of iron. Whereas carbon filters rely on the same intramolecular forces that make 3M’s adhesive command hooks possible.
Before we dive into the specifics of how exactly these filters work it may be beneficial to step back and examine the basic kinds and parts of filters. Granulated media filters generally work by forcing the water to be filtered through a media bed that is designed to capture the desired particles via intramolecular forces. This media is held in place by layers of gradated subfill that allow the water to pass down to the post filtration collection point but not the smaller media. Typically these filters will need to be cleaned every so often, this is accomplished via backwashing the filters. Backwashing involves reversing the direction of flow through the filter and thusly suspending the filter media in water. Due to this suspension and reverse direction of flow the filter media will then tend to brush up against each other interrupting the adhesive properties that had been keeping the filtered particles out of the water. After this cleaning cycle the filter can used again.
The specific chemistry of the particle/particles that are to be purified from a water source determine the media that will act to take it out of solution. For iron and other dissolved ions this requires the additional step of pretreatment chemical modification. This works by adding in a chemical such as hydrogen peroxide, or another oxidizing agent, which can oxidize the iron from an aqueous Fe2+ ion to a precipitate Fe3+ ion. This change then allows the media in the filters to more adequately bond to the now precipitated iron and take it out of solution.
For other organic substances that are dissolved in water a carbon filter may be needed. Carbon filters work by taking advantage of non-polar attractive forces between the filter media and the dissolved organic molecules that are to be filtered out of the water. These are referred to as Van Der Waals forces and help selectively target the polar bonds in the organic molecules that are to be filtered out. Further this activated carbon has an extensive surface area of up to 32,000 ft/sq per gram. This surface area not only maximizes the area that contaminates may come into contact with, but also helps to trap the contaminates within their extensive pore system. It is this pore systems vast surface area, coupled with Van Der Waals attractive forces that entrap many organic contaminates.
Ion Exchange
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Ion exchange has been widely used in the treatment of groundwater. So much so that it is quite easy to use one every day and not know it, this is because this technology is the backbone of most water softeners. However, the application of ion exchange for water treatment extends much farther than that.
Ion exchange works by loading a specially designed resin with an easily displaceable ion. When water is passed though the resin it then displaces these ions with other ions that are in the water. The resin may eventually be exhausted and run out of adequate open binding spots for the target ions, when this happens the resin must be recharged by running a high concentration of the weakly binding ions through the system to knock off the target ions and wasting the resulting water. Ion exchange devices can be broadly separated into two categories; ones that deal with positively charged ions such as calcium and magnesium (cation exchange, and ones that deal with negatively charged ions such as nitrates and sulfates (anion exchange).
So what problems can this treatment type address?
Cation exchange resin is commonly used to remove calcium and magnesium (hard water), as well as iron and manganese. Anion exchange is typically used for the removal of contaminates such as fluoride, mercury, nitrates, arsenic, uranium and some organic compounds such as sulfates.
Example of an Ion Exchange Column
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RO & Membrane treatment
Reverse osmosis (RO) and other membrane treatments are the most intuitive water treatment technologies relying on the same basic concept as a kitchen strainer (sieve), but instead of removing pasta these membranes are removing material up to 100 times smaller then the width of a human hair. Furthermore, in order to force the water through these pores, the membrane requires the incoming water to be pressurized. These treatment technologies are able to remove nearly all molecules from a water supply, thusly remove almost all contaminates commonly found in water except volatile organic compounds (VOCs).
Although these technologies rely on an intuitive concept, don't behave exactly as may be expected. One part of the operation of membranes may not be at first obvious, looking at the concept they rely on as they cannot filter all of the water that is feed to them. To explain this go back to the pasta analogy when the sieve is used and all of the water is passed though you are left with a clump of pasta in the strainer (sieve). If this happened to the membrane it would be left with a crystalline structure of all the contaminates that it removed, which would leave the filter completely useless. But unlike the pasta, which was never actually dissolved in the water, the compounds that are being removed with membranes are things that are dissolved in water (solutes) like salt (NaCl). These solutes have bonds that connect them to other molecules and cause them to be removed from the water need to be in enough water that they never reform those bonds, if however the concentration of solutes becomes high enough that this dose happen the solute can fall out of solution. This can destroy the membrane either leading to holes in the membrane far larger than the necessary the effective width, or clogging the membranes pours completely. So both scaling and membrane perforations (holes in the membrane cause by these contaminates falling out of solution with 1200 PSI behind them) are huge problems that can destroy the effectiveness of the membrane. Thusly only around half of the water that is feed to the system should ever make it through and the water that is left on the other side of the filter must be constantly circulated.