Filters; what can they do?
There are many types of filters available in the market place today. I will try to group them by the method they use to filter water. Almost everyone has seen the ads for the filter that fits on the end of your kitchen sink or bathroom spigot. These filters usually use two basic types of filtration: a filter ‘pad’ catches the large (usually over 25 micron in size) particles or ‘chunks’ , and a small amount of carbon to adsorb organics and/or chlorine. The main problem here is the flow rates at which they are expected to work at. The consumer expects to turn the tap on as normal and draw “filtered” water. To remove free chlorine, for instance, standard engineering practices set the maximum flow rate at 10 gallons per minute per square foot (144 square inches) of surface area of the carbon, *if* you are using a standard 30″ bed depth. To remove chloramines or organics, the maximum flow rate is set at 5 gallons per minute per square foot of surface area. If your spigot will provide a flow of 1.5 gallons per minute, what size filter do you need hanging on the end of that spigot to insure that the chlorine and organics will not be swept past through the filter, into your glass? If you purchase this type of filter, make sure it has a way of limiting the rate at which water passes through it.
Next comes the cartridge type filter. Most common are the 10 1/2 or 20 inch long filters. This type filter will usually have a removable housing, into which different types of “elements” can be placed. A sediment filter cartridge element can be manufactured to remove certain size particles and larger. Most elements for home use will indicate 30 or 50 micron and larger removal. More expensive elements, usually for industrial use, may indicate a particle size (in microns) and add the words “Absolute” after it. No, it isn’t Vodka, it simply means that if it says 5 micron absolute, it means it! Very few particles larger than 5 microns will pass through the filter. The regular filter may say 25 microns, meaning that *most* of the particles 25 microns and larger will be caught by the filter. Remember, there filters actually get better, or more effective, as they are used. The ‘junk’ in the water collects on the surface of the filter and becomes a part of the filter as well. As it builds up, progressively smaller and smaller particles are trapped, and the flow rate through the filter slowly diminishes. This slowing of the flow rate can be a source of problems to water using appliances in your home. If you use such a filter, regular changing of the filter element is very important. Elements for these filters can also be carbon (block or granular, or powdered), can be manufactured for use in hot water, can be ceramic, pleated as well as many other configurations. Some manufacturers are mixing a small amount of silver into the carbon to help prevent any bacteria growth in them. This has yet to be a proven methodology. In fact, make sure that such a filter doesn’t give off more silver than is allowed, if not rinsed thoroughly prior to use, especially after a prolonged period of non-use. Remember, all filters, carbon especially, trap organics that bacteria feed on, and as the water sits without moving, they can multiply rapidly. Always change the elements on a regular, frequent basis.
A relative newcomer to the market, some small filters now contain resins that only remove specific things from the water, such as Nitrates, Fluoride or Lead. Technology is rapidly changing in this area; If you have a need for such a device, you should ask for supporting test results from an independent testing lab to verify that the unit will perform as advertised. Many states now have legislation that requires such data be provided to you prior to purchase.
Used mainly in labs, manufacturing processes, or for serious aquarium owners, DI filters are actually more complex than a filter. True filters, unlike the selective resin and DI units, work on a mechanical basis: they just ‘catch’ the particles that are too large to fit through the spaces between the filter media. (Well, I fibbed a little; but who wants to know about the Van Der Waals or Coulomb forces?) DI works by ion exchange, just like a water softener. Just as a water softener exchanges sodium for hardness minerals, a DI unit will have two types of resin in it: Cation and Anion. Basically, the Cation resin (like in a water softener) removes the ions with a positive charge, while the Anion resin removes those ions with a negative charge. Instead of using salt as a regenerant, acid and caustic are used. Some small DI cartridges are sold as “throw-aways”, others can be returned for regeneration and reuse. These small units can treat only small amounts of raw, city water. Usually, it is much more economical to pretreat the water feeding a DI system with reverse osmosis water.
One of the oldest methods for cleaning water is distillation. Simply put, you boil water, catch the steam, and condense it back into water. Theory is, the minerals stay behind in the boiling chamber, and only *pure* water ends up in your container. In the real world, most of those things do happen; but if you do not perform preventative maintenance on your still, you can get very poor results. Distillation will kill bacteria, viruses, cysts as well as remove heavy metals, organics, radionuclides, inorganics and particulates if properly maintained. One thing you must watch out for is VOC’s (volatile organic chemicals). These chemicals have a lower boiling point than water (like benzene), and can vaporize and mix with the steam, carrying over into the product water. Some stills today have a volitle gas vent — a small hole at the top of the condensing coil that allows the venting of such substances. Many distillers have a carbon filter to “polish” the product water before use and to remove any VOC’s that may carry over. The energy used to treat a gallon of water is usually about 3,000 watts, or about 25 cents per gallon (average) in the US. This treatment method requires that you ‘plan ahead’ and make and store water for use, which makes it somewhat less appealing. The more elaborate units will make and store water automatically, but raise the initial investment and maintenance of the equipment.
This is a process that is often described as filtration, but it is far more complex than that. We sometimes explain it as a filter because it is much easier to visualize using those terms. We should remember that osmosis is how we feed each cell in our bodies: As our blood is carried into the smallest of capillaries in our bodies, nutrients actually pass through the cell wall to sustain it’s life. Reverse osmosis is just the opposite: We take water with “nutrients” (in this case, junk) in it, and apply pressure to it against a certain type of membrane, and, presto — out comes “clean” water. Lets review the basics: If you take a jar of water and place a semi-permeable membrane (like a cell wall? or a piece of skin?) in it, dividing the jar into two sections, then place water in both sides to an equal level, nothing happens. But, if you place salt (or other such substance) into one side of the jar, you will notice that, after awhile, the water level in the salty side begins to rise higher as the unsalted side lowers. This is osmotic pressure at work: The two solutions will continue to try to reach the same level of salt in each side by the unsalted water passing through the membrane to dilute the salty water. This will continue until the “head” pressure of the salt water overcomes the osmotic pressure created by the differences in the two solutions. On the other hand, researchers have discovered that if we take that membrane and feed water with sufficient pressure to overcome the osmotic pressure of the two waters, we can ‘manufacture’ clean water on the side of the membrane that has no pressure. We sometimes say we “filter” the water through the membrane. Depending on the membrane design, and the material it made from, the amount of TDS (total dissolved solids) reduction will range from 80 to over 95 per cent. Different minerals have different rejection rates, for instance, the removal rate for the membrane I am looking at now is 99.5% for Barium and Radium 226/228; but only 85.9% for Fluoride and 94.0% for Mercury. Removal rates are very dependant on feedwater pressures, and some membranes are not tolerant to high or low pH. For home use, it is important to make sure you get an RO *System*; ie, a sediment prefilter, a carbon prefilter, membrane, storage tank and post carbon filter. Some of these filters may be combined into one, ie, the prefilter may be a particulate and carbon both. A lot of comments have been made concerning the *wasting* of water by an RO. True, the old style units with the early type membranes were more prone to becoming plugged, or fouled by the “junk” they removed from the water. To help keep this from happening, a small amount of water was allowed to run across the membrane to help carry away those impurities to drain. Early designs only recovered 1 gallon of good water for every 4-8 gallons used to keep the membrane clean. Even worse, when your storage tank was full, water still ran to the drain because the early membranes were made of a material that the little bugs in your water supply (no, not pathogens, or dangerous to you in small numbers) loved to eat! So to prevent that, we just let the water run so they couldn’t have time to stop and eat. :>) Now membranes are made to not only recover a much higher percentage of the feedwater, but the bugs don’t eat them! Newer systems not only recover more, they can have a shut off device that stops all water flow when the storage tank is full. Actual recovery rate is dependant on several factors, including the TDS, and just what the TDS is composed of, in your feedwater. Temperature, pressure also have a big effect on the amount of product water you can make in a given period. Remember, all RO units are normally rated using a feedwater temperature of 77 degrees F — is your feed water temperature that high?